Next Article in Journal
Connected Variations of Meteorological and Electrical Quantities of Surface Atmosphere under the Influence of Heavy Rain
Next Article in Special Issue
Characteristics of Extreme Value Statistics of Annual Maximum Monthly Precipitation in East Asia Calculated Using an Earth System Model of Intermediate Complexity
Previous Article in Journal
Development of a Method to Determine the Fractional Deposition Efficiency of Full-Scale HVAC and HEPA Filter Cassettes for Nanoparticles ≥3.5 nm
Previous Article in Special Issue
The Role of Samalas Mega Volcanic Eruption in European Summer Hydroclimate Change
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Atmosphere
Volume 11
Issue 11
10.3390/atmos11111193
atmosphere-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Summer Westerly Jet in Northern Hemisphere during the Mid-Holocene: A Multi-Model Study

by
Chuchu Xu
1,
Mi Yan
1,2,3,*,
Liang Ning
1,2,3,4 and
Jian Liu
1,2,5
1
Key Laboratory for Virtual Geographic Environment, Ministry of Education, State Key Laboratory Cultivation Base of Geographical Environment Evolution of Jiangsu Province, Jiangsu Center for Collaborative Innovation in Geographical Information Resource Development and Application, School of Geography, Nanjing Normal University, Nanjing 210023, China
2
Open Studio for the Simulation of Ocean-Climate-Isotope, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266237, China
3
State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China
4
Climate System Research Center, Department of Geosciences, University of Massachusetts, Amherst, MA 01003, USA
5
Jiangsu Provincial Key Laboratory for Numerical Simulation of Large Scale Complex Systems, School of Mathematical Science, Nanjing Normal University, Nanjing 210023, China
*
Author to whom correspondence should be addressed.
Atmosphere 2020, 11(11), 1193; https://doi.org/10.3390/atmos11111193
Submission received: 17 August 2020 / Revised: 20 October 2020 / Accepted: 25 October 2020 / Published: 3 November 2020
(This article belongs to the Special Issue Climate Variability and Climate Extreme Events over Asia on Various Time-Scales since the Last Glacial Maximum)
Browse Figures
Versions Notes

Abstract

:
The upper-level jet stream, a narrow band of maximum wind speed in the mid-latitude westerlies, exerts a considerable influence on the global climate by modulating the transport and distribution of momentum, heat and moisture. In this study by using four high-resolution models in the Paleoclimate Modelling Intercomparison Project phase 3, the changes of position and intensity of the northern hemisphere westerly jet at 200 hPa in summer during the mid-Holocene (MH), as well as the related mechanisms, are investigated. The four models show similar performance on the westerly jet. At the hemispheric scale, the simulated westerly jet has a poleward shift during the MH compared to the preindustrial period. The warming in arctic and cooling in the tropics during the MH are caused by the orbital changes of the earth and the precipitation changes, and it could lead to the weakened meridional temperature gradient and pressure gradient, which might account for the poleward shift of the westerly jet from the thermodynamic perspective. From the dynamic perspective, two maximum centers of eddy kinetic energy are simulated over the North Pacific and North Atlantic with the north deviation, which could cause the northward movement of the westerly jet. The weakening of the jet stream is associated with the change of the Hadley cell and the meridional temperature gradient. The largest weakening is over the Pacific Ocean where both the dynamic and the thermodynamic processes have weakening effects. The smallest weakening is over the Atlantic Ocean, and it is induced by the offset effects of dynamic processes and thermodynamic processes. The weakening over the Eurasia is mainly caused by the dynamic processes.

1. Introduction

The westerlies are the planetary wind belts between the subtropical high-pressure belts and the subpolar low-pressure belts in the northern and southern hemispheres. As an important part of the circulation system, it exerts a considerable effect on the global climate by modulating the transportation and distribution of momentum, heat and moisture [1,2,3,4]. The upper-level jet stream is a narrow band of maximum wind speed near the tropopause in the westerlies. Even the location and intensity of the jet stream change slightly, it will have a great impact on the climate in the middle latitudes [5,6]. For example, the meridional position and zonal position of the jet stream could affect the intensity and seasonal variation of the East Asia monsoon [7,8,9,10]. Besides, the meridional movement of the jet stream is also closely related to the interannual variation of precipitation in China [5,8,9,11]. The North Atlantic jet stream, in conjunction with the midlatitude transient eddies, is closely associated with the North Atlantic Oscillation [12,13]. Moreover, recent studies have shown that the jet stream has a greater impact on the climate in the Asia Pacific region than that of the El Nino-Southern Oscillation (ENSO) [14,15]. As the jet stream is relevant to the location of the Hadley cell, the tropopause and the transient-eddy activity, a deep study on the westerly jet stream will improve the understanding of the dynamics of the atmospheric general circulation and the associated climate changes [16,17,18].
Many researchers used reanalysis datasets to study the changes of upper-level jet streams in the past decades, and found that the jet stream gradually moved poleward, causing some anomalous weather activities, such as rainfall, typhoon and hail [8,19]. Therefore, it is necessary to study the response of the jet stream in different climate states and the related mechanisms, not limited to the modern climate background. Due to the increased insolation, the mid-Holocene (ca. 6000 years ago, 6 ka) is a typical warm period during the Holocene in northern hemisphere, different from the glacial periods [20,21,22,23,24]. The Greenland Ice Sheet and Antarctic Ice Sheet in the mid-Holocene have melted, and the topography and coastlines are similar to the preindustrial period [25]. Zhang et al. pointed out that the future climate is also very similar to that in the mid-Holocene based on 13 ocean-atmosphere coupled models in Paleoclimate Modelling Intercomparison Project (PMIP) [25]. In the PMIP simulations, during the mid-Holocene, the earth’s orbital configurations differed from that of preindustrial period, with perihelion in boreal summer/autumn (implying greater seasonality of insolation in the northern hemisphere) and greater obliquity, implying higher summer (and annual) insolation in high latitudes [26]. The concentration of greenhouse gases and nitrogen dioxide was similar to that in the preindustrial period, but the methane gas had a quite different content. Compared with the preindustrial period, the mid-Holocene simulations are forced by altered astronomical parameters as well as prescribed greenhouse gases. Ice sheets have already melted to their preindustrial extents, making this a good period for exploring post-glacial climate changes.
Previous studies show that there are essentially two physical processes which could affect the upper-level jet streams: the thermodynamic mechanism caused by the tropical Hadley circulation [27], and the eddy-driven forcing resulting from the mid-latitude baroclinicity [28]. We can regard them as a thermodynamic factor and a dynamic factor. The meridional temperature gradient is considered to be one of the primary thermodynamic factors that steers the westerlies jet stream through the thermal wind relationship [9,29,30,31,32,33]. Because temperature is a fundamental factor, the movement of westerlies is connected to various temperature anomalies induced by climate variabilities, such as ENSO, tropical heating and the cooling of the troposphere bottom [34,35,36]. Besides, the baroclinicity can also cause the shift of jet streams, and the barocline anomalies derived from the changes of the South Asia high and the western Pacific subtropical high can generate the anomalies of the jet stream [8,36,37,38,39,40]. The jet stream over the East Asia simulated by the LASG/IAP coupled climate system model is assessed, and the mean state bias can be explained by the synoptic-scale transient eddy activity [41].
Studying the westerly jet stream based on the reconstruction and simulation data in modern times is insufficient to provide a complete climate state. Therefore, many researchers began to study the characteristics of the westerly jet during the Holocene [42], so as to improve the prediction ability of the westerly jet in the future. Observation and reanalysis suggest that entire extratropical climate zones are moving towards poles under climate change, affecting the westerly jets, storm tracks, cloud, precipitation and ocean circulation patterns [43,44,45,46,47,48,49,50]. In particular, this phenomenon is more prominent and zonally symmetric in the Southern Hemisphere [51]. By using pollen records, stalagmite and other reconstructed data, it is found that in Northern Hemisphere the mid-latitude westerly jet was intensified in the early Holocene and weakened since the mid-Holocene, and shifted southward compared with the early Holocene [8,31,32,33,34,35,36,37,38,39,40,41,42,52].
Besides, previous studies also focused on the westerlies during the last glacial maximum (LGM) [50,53,54,55,56], a typical period of cold climate, some suggested that the jet stream had a significant equatorward shift caused by the thermal wind and changes in the midlatitude baroclinic instability [12,57,58], while others found a poleward shift [56,59]. For the mid-Holocene, a typical warm period, some studies have shown that the temperature gradient is the main factor influencing the westerly jet. How the westerly jet performs during this period needs to be clarified.
In this study, based on the model simulations in the framework of the Coupled Model Inter-comparison Project phase 5 (CMIP5) or PMIP phase 3 (PMIP3), the northern hemisphere (NH) westerly jet in summer during the mid-Holocene is investigated, and the related mechanisms are analyzed. The remainder of this paper is organized as follows. The model and methods are described in Section 2. The position and intensity of NH summer westerly jet in the mid-Holocene are shown in Section 3. The thermodynamic and dynamic mechanisms of the westerly-jet changes are discussed in Section 4. Conclusions are provided in Section 5.

2. Model, Data and Methods

2.1. Model and Experiment

The results derived from the high horizontal resolution models are shown to be more accurate than those from low resolution ones [60]. Therefore, four models participated in CMIP5/PMIP3 with the resolution higher than 2° are used in this study, including CCSM4 from the National Center for Atmospheric Research of United States, CNRM-CM5 from the National Centre for Meteorological Research of France, MPI-ESM-P from the Max Planck Institute for Meteorology of Germany, and MRI-CGCM3 from the Meteorological Research Institute of Japan (Table 1). In order to obtain the multi-model mean results (MMM), we interpolate the results of the four models into the 0.5° (latitude) × 0.5° (longitude) grids using bilinear interpolation. The variables of monthly mean zonal and meridional wind, surface temperature, geopotential height, sea level pressure, and vertical velocity are used to analyze the changes of the NH summer westerly jet and the associated physical processes.
Two experiments are used in this study, including the mid-Holocene experiment (MH) and the preindustrial control experiment (PI). In order to be consistent in the length of time, only the last 100 years of PI are used. The boundary conditions (the orbital parameters, the trace gases and so on) are listed in Table 2. During the mid-Holocene, the orbital parameters are quite different from the preindustrial period, resulting in the changed insolation, particularly in mid-high latitudes in summer (Figure 1).
More information about the two experiments can be found on the PMIP3 website (Figure 1) [61] and in Braconnot et al. [62,63,64].
Furthermore, compared between the mid-Holocene and the pre-industrial period, the methane concentration has decreased slightly, but the concentrations of the carbon dioxide and the nitric oxide have not changed. The topography and coastlines are the same as those in the modern times. Therefore, the MH provides the opportunity to test the effects of orbital-induced insolation on the climate.

2.2. Method

The definition method of Rojas [65], although it was initially used to define the axis of westerly jet in Southern Hemisphere, is used to define the axis of the westerly jet and the location of the westerly jet, i.e., the locations of maximum wind speed at each longitude are linked up to form the westerlies axis. However, the westerly wind in the Northern Hemisphere is less zonally symmetric due to the land-sea distribution. To obtain the position and strength of the NH westerlies, we identify the latitude/longitude of maximum wind speed after the meridional/zonal running average at each longitude/latitude, and the relative maximum wind speed, as the position and intensity of westerly jet stream, respectively.
Since there are prominent anomalies over 140° E and 140° W, we divide the NH into three key regions, i.e., Eurasia (30° E–120° E), Pacific (130° E–120° W), and North America and North Atlantic (120° W–30° W). Because the jet axis is a line with directivity, in order to better describe the variation of the westerly jet over the three regions, the variation of the jet center is used to represent the variation of the jet stream.
In the mid-latitude regions, the prevailing westerlies are active in the troposphere, including the low-level jet stream at 850 hPa and the upper-level jet stream at 200 hPa. Unlike the low-level jet, the upper-level jet is not affected by topographical factors. Therefore, we will focus on the upper-level westerly jet at 200 hPa in this study.
In addition, to assess whether there are significant differences between MH and PI, Student t-tests were applied to the variables (ua, va, tas, zg, wap, ta and so on). The t-test is applied to not only each individual model but also MMM. That is to say, the results derived from MMM is tested individually using its own 100-year time series. These statistically significant values are presented as “dots” in the figures with 95% confidence level.

3. Results

According to the simulated results shown in Figure 2, the axis of the jet stream in the NH is located near 40º N and spanning the entire hemisphere. Figure 2a shows the simulation results based on the ensemble mean of the four models. Compared with the PI period (Figure 2f), the jet axis during the MH shifts northward in East Asia and the Pacific region near 30° N where the zonal wind is weakened. The jet axis in the west part of North America moves slightly northward, but the jet axis in the east North America and North Atlantic region features a smaller degree of shifting. During the mid-Holocene, the upper-level zonal wind is significantly weakened in Africa, the middle-low latitudes of Eurasia, the middle-latitude Pacific and part of the Atlantic regions, while the zonal wind is apparently strengthened in Europe, the middle and high latitudes of the Asian continent, the north Pacific and western North America. It is thus clear that in areas where the jet axis obviously moves northward, the westerly wind on the south side is weakened, whereas that on the north side is enhanced. For example, in Eurasia, the westerly wind to the south of the jet axis is significantly weakened, whereas that to the north of the jet axis is significantly strengthened, especially over the East Asia (Figure 2a).
The results from each individual model all show the negative anomalies of the zonal wind during the mid-Holocene over the western part of Africa, the southern part of Eurasia, the Pacific region near 40° N, the eastern tropical Pacific and the northwestern Atlantic, whereas positive anomalies are found in middle-latitude Eurasia, the Pacific region near 30° N and the eastern Atlantic near 30° N (Figure 2b–e). However, there are some differences among the four models. In MPI-ESM-P, the center of the positive anomaly is located in the western Eurasia; in the other three models, the center of the positive anomaly is located in the eastern Eurasia. The positive and negative anomalies over America and the Atlantic regions are smaller in CCSM4 and CNRM-CM5 than those in MRI-CGCM3 and MPI-ESM-P. Additionally, in CCSM4, there are no significant negative anomalies in North Africa (Figure 2d). Regarding to the movement of the jet axis, the results of MMM indicate that the westerly jet stream generally has a poleward shift (Figure 2a). The poleward shift over the Pacific region is the most obvious in all of the four models, while the shift over America and the Atlantic region is not so notable. The northward shift over Eurasia seems larger in MRI-CGCM3 than in the other three models.
To quantitatively reflect the changes of the westerly jet stream during the mid-Holocene compared to the PI period, we have calculated the differences between the two periods in terms of latitudinal position, longitudinal position and intensity, and the results are shown in Figure 3. The changes in the three regions are compared, i.e., Eurasia, Pacific, and North America and North Atlantic. Note that the changes derived from the MMM in Figure 3 are the quantified results from Figure 2a, instead of the arithmetic average of the changes derived from each individual model.
Considering the change of the latitudinal position, the results of MMM show that the latitudinal deviation is the largest in the Pacific region, which is about 1.4° to the north, followed by Eurasia, where the westerly jet stream moves 1.2° northward. The westerly jet stream over the North Atlantic is characterized by the smallest change with only 0.5° northward shifting (Figure 3a). Although there are some differences of amplitude among each individual model, the simulation results of all the models imply that the upper-level westerly jet stream has moved northward, with the deviation degree over the Pacific being relatively large and that over the North Atlantic being comparatively small. Over the Pacific, there are great differences between the simulation results of the four models. The changes of the latitudinal position of the westerly jet stream vary from the largest of 4.4° in MPI-ESM-P to the smallest of 0.8° in MRI-CGCM3. Over Eurasia, the differences between the models are relatively small. The changes range from 1.8° in MRI-CGCM3 to less than 0.1° in CNRM-CM5. For the upper-level westerly jet stream over the North Atlantic, the differences among the four models are smaller, with the deviation degree ranging from 0.8° to 0.4° (Figure 3a).
For the longitudinal position, the MMM of the simulated upper-level jet shows much large shift. The upper-level jet over Eurasia and the North Atlantic moves 8° and 6° westward, respectively. The jet stream over the Pacific region moves 4° to the east (Figure 3b). Despite the differences of magnitude among the four models, the simulated results of all models indicate the shifts of the jet stream in the same direction. The largest model difference remains over the Pacific region. The shift is the largest in CCSM4 (12°), whereas the difference among the other three models is relatively small (less than 2°). Over Eurasia, the westward shift spans from 11° in MPI-ESM-P to 7.5° in CCSM4. In North America and North Atlantic, the largest deviation degree is found in CCSM4, the simulated result of which shows that the jet stream moves 7° westward, and the smallest deviation degree (3.5°) is found in CNRM-CM5.
In terms of intensity, according to the MMM, the upper-level westerly jet stream is weaker during the mid-Holocene than during the preindustrial period. The zonal wind speed shows the largest weakening in the Pacific region with −6 m/s, followed by that in the Eurasian region, which is −4 m/s. The zonal wind speed in the Atlantic, which is less than −1 m/s (Figure 3c), is of the least anomaly. The weakening of the upper-level westerly jet, with the largest attenuation in the Pacific and the smallest attenuation in the Atlantic, is similar to the changes in the latitudinal position of the jet stream. The Pacific region still shows large model differences. The weakening ranges from 7.5 m/s in MRI-CGCM3 to 3 m/s in CCSM4. The variation of the wind speed over the Eurasia is relatively small, which is ranging from 3.5 m/s in CCSM4 to 1.5 m/s in CNRM-CM5. For the upper-level westerly jet stream over the Atlantic, the difference among the four models is comparatively the smallest, with a variation range within 1 m/s.

4. Mechanism Analysis

Previous studies have shown that the latitudinal position of the jet stream is affected by the meridional uneven heat transport [31,36,66], while the east-west movement of the jet is affected by the diabatic heating of the troposphere [6,10]. For the East Asian westerly jet, it has the westward movement due to the diabatic heating of the troposphere from the Tibetan plateau in summer [6,7]. It is also found that the northward shift of the westerly jet is accompanied by the westward movement [8]. Earlier studies also suggested that the westerly jet is closely related to the transport of momentum and vorticity, and its formation is related to sea-land thermal contrast and topography [66,67]. The simulation results in this study show that the westerly jet moves northward in three key regions. Over the Eurasian continent and the Atlantic the jet stream moves westward, but over the Pacific the westerly jet stream moves eastward. We will discuss the underlying mechanisms in the following.
Solar radiation is the direct cause of temperature rise in the NH during summer. During the mid-Holocene, solar radiation increased in the NH because of the changes in earth orbital parameters. Especially in the middle and high latitudes (Figure 1), the summer temperature increased significantly, with a concurrent large pressure change (Figure 4).
In general, based on the MMM, during the mid-Holocene, the surface temperature to the north of 30° N in the NH increases by more than 1° C, especially over the land. The temperature over the ocean at the middle-high latitudes tends to rise by about 0.5° C. The changes of the temperature over the land is mainly found in the middle-latitude Eurasia and the middle-high latitudes of the North American continent, with the temperature rising by over 1.5° C, which is consist with the reconstructions [23,31,68,69]. However, over the mid-low latitudes such as the south China and most of India and Africa, the temperature is decreased (Figure 4a). The decreased temperature might be caused by the release of latent heat induced by the enhanced monsoon precipitation. Wu and Liu [69] confirmed that during the mid-Holocene, the precipitation in the north of East Asia was decreasing, while that in the south was increasing. Sun et al. also found the precipitation increases most significantly in the North African [70]. Moreover, the reconstruction data also indicated a decline of temperature in the middle and low latitudes during the mid-Holocene [71,72]. The 200-hPa geopotential height anomaly (Figure 4b) also shows that there is a significant difference between the south and north regions, which is consistent with the surface temperature anomaly.
It is generally found that there are two physical processes related to the change of the westerly jet: the thermal forcing mechanism and the eddy-driven forcing mechanism [27,28]. The following discussion of the mechanisms about the change of the westerly jet will be based on these two factors. The meridional temperature gradient is considered as one of the primary thermodynamic factors that steers the westerly jet. The dynamical effect is related to the anomaly of transient eddy activities. Ren has pointed out that the dynamical connection between the zonal wind and the eddy is closely connected with the location and intensity variations of the westerly jet [72].

4.1. Positional Shifting

The changes in the position of the axis of the jet stream include north-south movement and east-west movement. The reasons of the east-west movement may be the small-scale events such as the thermal effect of the Qinghai Tibet Plateau and the onset time of the South China Sea summer monsoon. On the other hand, the northward movement of the jet stream is often accompanied by the westward movement, so we mainly focus on the mechanism of north-south movement.

4.1.1. Thermal Forcing

Figure 3a shows the northward shift of the westerly jet stream. The degree of deviation in the Pacific region is the largest, that in Eurasia is the second, and that in North America is the smallest. The jet stream has seasonal north-south shift following the seasonal movement of the general circulation, such as the Hadley circulation. According to the simulation results of the ensemble average, the associated stream function also presents significant northward-shift of the Hadley circulation in the middle and upper levels, which is consistent with the northward shift of the jet stream (figure omitted).
Wang et al. [59] found that the NH upper-level westerly jet also moved northward during the LGM. They suggested that the upper tropospheric cooling in the tropics, possibly due to reduced latent heat release, was expected to account for the poleward shift of the 200-hPa jet through the thermal wind relationship. Figure 5 displays the temperature anomalies between MH and PI at 850 hPa, 500 hPa and 200 hPa. It is shown that the low-latitude temperature in the upper level of the troposphere is declined while the high-latitude temperature is increased from lower to upper troposphere, and thus the meridional temperature gradient at high levels decreased. Therefore, the upper-level jet stream moved northward. Since the zonal wind connects temperature field through the thermal wind relationship, the westerly jet stream is located at the latitude where the thermal wind is at a maximum. The change of meridional temperature gradient directly affects the change of the meridional pressure, and thus affects the latitudinal position of the westerly jet.
In this study, during the mid-Holocene, the low-latitude temperature is decreased while the high-latitude temperature is increased in three key regions at lower to upper levels (Figure 5), leading to weakened meridional temperature gradient. The weakening of the temperature gradient in the upper-level might be an important factor leading to the northward shift of the upper-level westerly jet stream via pressure gradient.
The distribution of meridional pressure gradient indicates that the NH westerly jet is located in the region with the largest meridional pressure gradient (Figure 6a). According to the change of the meridional pressure gradient (Figure 6c), with 50° N as the boundary, the pressure gradient on the south side of the mid-latitude Eurasia and the Central Pacific is characterized by a positive anomaly (i.e., the negative pressure gradient is weakened), and the pressure gradient on the north side is characterized by a negative anomaly (i.e., the negative pressure gradient is strengthened), driving the westerly jet stream over the Eurasia and the Pacific to move northward. The deviation over the Eurasia is larger than over the Pacific. Over North America and North Atlantic, although the negative meridional pressure gradient on the south side is weakened, the enhanced negative meridional pressure gradient on the north side is also conducive to the northward movement of the jet stream. However, because of the smaller negative meridional pressure gradient in the northeastern part of North America and North Atlantic, the northward movement of the pressure gradient is suppressed, thereby reducing the northward movement of the westerly jet stream over these regions (Figure 3a). As a result, the simulated jet stream over the North America has the least northward movement. For the individual models, all of which show consistent northward shift of the jet stream, also show similar meridional pressure gradient changes but with different amplitudes. The different amplitudes have well explained the model difference of position changes. For example, the changes of the meridional pressure gradient over the eastern Eurasian and the Pacific regions show larger amplitudes in MRI-CGCM3 and CCSM4 than those in MPI-ESM-P and CNRM-CM5 (Figures not shown). This is consistent with the changes of the latitudinal position of the westerly jet stream, which show larger northward shift over the Pacific region in MRI-CGCM3 and CCSM4 (Figures not shown).
The change of the longitude position of the westerly jet stream shows that it shifts westward in Eurasian and Atlantic regions and shifts eastward in the Pacific region (Figure 3b). The jet center over the Eurasia is located on the east side of the negative zonal pressure gradient, whereas the jet center over the Pacific and the Atlantic is located on the west side of the positive zonal pressure gradient (Figure 6b). According to the zonal pressure gradient anomaly field (Figure 6d), with110° E as the boundary, the pressure gradient on the east side of the Eurasia features a positive anomaly (i.e., the negative pressure gradient is weakened), and the pressure gradient on the west side is characterized by a negative anomaly (e.g., the negative pressure gradient is enhanced), which causes the westerly jet stream over the Eurasia to move westward. Based on the anomaly field over the North America and North Atlantic region, with 60° W being the boundary, the pressure gradient on the west side is characterized by a positive anomaly (i.e., the negative pressure gradient is weakened), whereas the pressure gradient on the east side features a negative anomaly (i.e., the positive pressure gradient is weakened), thereby causing the westward movement of the jet stream centered over the Atlantic region. Over the Pacific region, the zonal pressure gradient anomaly indicates a strengthening of the positive gradient to the east of the jet stream center, which is conducive to the eastward movement of the jet stream center over the Pacific region. The experiment in this study also proves this point. There are small differences among the four models in simulating the changes of the longitudinal positions of the jet stream centers. The zonal pressure gradient also implies a small difference among the four models. The westerly jet stream over the Pacific region in CCSM4 has the largest eastward shift, which might be caused by the strong simulated negative anomaly at 140° W (Figures not shown). In contrast, the other models present small negative anomalies at 140° W.
Note that the westerly jet stream moves eastward in the Pacific region, which is in contrast with the aforementioned hypothesis, i.e., northward shift might be accompanied with westward shift. This requires further investigation.

4.1.2. Transient Eddy Forcing

The intensity of the transient eddy activity is mainly caused by the anomalies of the baroclinic instability that is dynamically related to the westerlies [20]. Changes of the midlatitude baroclinic instability are related to the jet stream in the upper troposphere through the anomalous eddy activity [28]. Hence, the transient eddy activity during the mid-Holocene is compared to that during the preindustrial period. Here, the transient eddy kinetic energy (EKE) is calculated to represent the transient eddy activity [73,74].
E K E = ( U ′ 2 ¯ + V ′ 2 ¯ ) ,
where U and V are respectively the monthly data of zonal and meridional full wind velocities in pressure coordinates. The overbar denotes a time average and the prime a deviation from this time average.
Figure 7 displays the distribution of EKE at 200 hPa in summer during PI and MH. Two distinct EKE maximum centers are found in the North Pacific and North Atlantic along the westerly jet stream axis during the two periods. The EKE maximum center over the North Pacific is in consistent with the jet center (Figure 2f). No distinct EKE center is found over the Eurasian continent. Meanwhile, the intensity of the EKE center over the North Pacific is larger than that over the North Atlantic. These conditions are similar in both of the periods. Xiao et al. [16] studied the relationship between the position of the westerly jet axis and the intensity of the eddy energy and found that the transient eddy activity whose southward shift was in conjunction with the displacement of the westerly jet in the same direction. They also found that there was no distinct EKE center over the Eurasian continent. In Figure 7, the two EKE maximum centers, corresponding to the jet stream, are both shifted northwards with a reduced intensity. It means that the impact of EKE centers on the westerly jet stream over the North Pacific and North Atlantic is northward shifting.
The absent EKE center over Eurasia indicates that over this region the westerly jet stream is much less relevant with the transient eddy activity and is mainly driven by the thermal forcing, which is consistent with Xiao et al. [16]. Therefore, with the combined effects of EKE forcing and the thermal forcing, the northward shift of the jet stream is larger over Western Pacific than over Eurasia.
However, the EKE forcing cannot explain the east-west shift of the jet stream, which needs further studies in the future.

4.2. Weakened Intensity

To learn the mechanisms of the position and intensity anomalies of the westerly jet, the thermal and dynamic factors are analyzed detailly in this section.

4.2.1. Thermal Forcing

During the mid-Holocene, the temperature increases in the middle and high latitudes of NH, whereas it decreases in the middle and low latitudes (Figure 4a), leading to a weakened meridional temperature gradient. Previous study showed that the temperature gradient is a critical factor influencing both the intensity and the location of the westerly jet [39]. Hence, the meridional temperature gradient and its changes (Figure 8) are presented in this study. There is positive temperature gradient over the Pacific region near 30° N during the mid-Holocene (i.e., the meridional temperature gradient is weakened). The temperature gradient to the southwest (20° N) and north (40° N) of 30° N are both characterized by negative anomalies. Over the North America regions, the meridional temperature gradient is characterized by a positive anomaly to the south of 50° N, and a negative anomaly to the north of 50° N. Over the North Atlantic, there is positive temperature gradient. Changes in the meridional temperature gradient over the Eurasia are relatively scattered (Figure 8b). The upper-level westerly jet stream is basically formed by the thermal wind, as well as the major atmospheric circulation systems. According to the thermal wind relation, the strongest part of the jet stream should be approximately located in the regions where the temperature gradient is the largest. The meridional temperature gradient anomalies in the Pacific, North America and North Atlantic regions are weakened between 30° N and 40° N, leading to a decrease in the meridional pressure gradient (Figure 5) and thus a weakened jet stream. However, changes in the meridional temperature gradient are unable to fully explain the intensity change of the westerly jet over Eurasia.

4.2.2. Dynamic Forcing

Bjerknes [75] stated that the strong Hadley circulation brought the low-latitude westerly angular momentum to the mid-latitudes, thereby strengthening the mid-latitude westerlies. Based on the changes of meridional circulation and vertical velocity, the ascending flow near 5–10° N and the descending flow near 30° N are weakened (Figure 9a), indicating a weakening of the Hadley circulation in NH during the mid-Holocene. The weakened Hadley cell then leads to the weakened westerly jet stream. Note that the changes of the Hadley circulation are different in different regions. The Hadley circulation is weakened the greatest in Eurasia (Figure 9b), followed by the Pacific region (Figure 9c), whereas it is enhanced in the North America and North Atlantic regions (Figure 9d).
By analyzing the changes in the meridional temperature gradient (thermodynamic factor) and the variation of the Hadley circulation (dynamic factor), the intensity of the westerly jet presents different results over the three key regions. The thermodynamic and dynamic effects both contribute to the weakening of jet stream over the Pacific region, which makes it the strongest weakening region among the three regions. Over Eurasia, the dynamic factor plays a key role in the weakening of the jet stream. Over the North America and North Atlantic regions, the thermodynamic factor tends to weaken the jet while the dynamic factor tends to strengthen the jet. This opposite effect then results in the intensity of the jet stream in the North America and North Atlantic regions to go through the smallest change among the three key regions (Figure 3c).

5. Conclusions

In this study, four high-resolution models participated in CMIP5/PMIP3 were used to investigate the characteristics and mechanisms of the position and intensity changes of the NH upper-level westerly jet in summer during the mid-Holocene. The main conclusions are as follows.
Compared with the PI period, the summer NH upper-level westerly jet during the mid-Holocene was generally shifted towards the polar region, with weakened intensity. The largest northward shift was found in the Pacific region, with its intensity decreasing to the greatest extent. The smallest northward shift occurred in the North America and North Atlantic regions, with its intensity weakened to the least extent. Zonally, in the Eurasia and North America and North Atlantic regions, the westerly jet center moved westward, whereas in the Pacific region, the westerly jet center moved eastward.
The change of the latitudinal position of the jet stream has been investigated. From the perspective of the thermal factor, the orbital-induced insolation change and the strengthened monsoon-rainfall latent-heat release could lead to the temperature increasing in the mid-high regions while decreasing in the mid-low latitudes. Moreover, the temperature changed more greatly over land than over the adjacent oceans. The temperature changes could result in the pressure changes, and thus affected the meridional and zonal pressure gradients in NH, which could make the westerly jet stream move northward. From the perspective of the dynamic factor, there were two EKE maximum centers over the North Pacific and North Atlantic, they were both shifted northward, which might lead to the northward shift of the westerly jet stream. No distinct EKE center was found over the Eurasian continent, indicating that the westerly jet stream over this region was much less relevant with transient eddy activity and was mainly driven by the thermal forcing. The mechanisms of the east-west shift of the westerly jet stream remains unknown and require further studies.
The changes of the jet intensity were related to the thermodynamic factor (meridional temperature gradient) and the dynamic factor (Hadley circulation). The greatest weakening of the upper-level jet in the Pacific region was caused by the joint effects of thermodynamic and dynamic factors, and the smallest weakening in the North America and North Atlantic regions was induced by the offset effects of thermodynamic and dynamic factors. Dominated by the dynamic factors, the westerly jet stream over Eurasia was moderately weakened among the three key regions.
Our study has revealed that during the warm mid-Holocene, the summer upper-level westerly jet weakened and shifted northward, which was to some extent opposite to the changes during the cold LGM. This indicates that the responses of atmospheric circulation to different climate backgrounds are quite complex, and we should be very careful when investigating the future change under global warming.

Author Contributions

Conceptualization, C.X., M.Y.; methodology, C.X., M.Y.; software, C.X., M.Y., L.N. and J.L.; validation, C.X., M.Y. and L.N.; formal analysis, C.X., M.Y.; resources, C.X., M.Y., L.N. and J.L.; writing—original draft preparation, C.X., M.Y.; writing—review and editing, C.X., M.Y. and L.N.; visualization, C.X., M.Y.; supervision, M.Y., L.N. and J.L.; project administration, M.Y., L.N. and J.L.; funding acquisition, M.Y., L.N. and J.L. All authors have read and agreed to the published version of the manuscript.

Funding

The simulation data was provided by the Paleoclimate Modelling Intercomparison Project Phase 3 (https://esgf-node.llnl.gov/search/cmip5/). This research was jointly supported by Strategic Priority Research Program of Chinese Academy of Sciences (Grant XDB40000000), the National Key Research and Development Program of China (grant No. 2016YFA0600401), the National Natural Science Foundation of China (grant nos. 41671197, 42075049, 41971021 and 41971108), Open Funds of State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, CAS (SKLLQG1930, SKLLQG1820), and the Priority Academic Development Program of Jiangsu Higher Education Institutions (PAPD, grant No. 164320H116).

Acknowledgments

Thanks for the two anonymous reviewers and Academic Editor for their great help to the manuscript. And thank Nanjing Hurricane Translation for reviewing the English language quality of this paper.

Conflicts of Interest

We declare no conflict of interest.

References

  1. Tao, S.Y.; Chen, L.X. A Review of Recent Research on the East Asian Summer Monsoon (in China); Oxford University Press: Oxford, UK, 1987; pp. 60–92. [Google Scholar]
  2. Tao, S.Y.; Wei, J. The westward, northward advance of the subtropical high over the West Pacific in summer. J. Appl. Meteorol. Sci. 2006, 17, 513–525. [Google Scholar]
  3. Sun, J.; Zhang, M.; Liu, T. Spatial and temporal characteristics of dust storms in China and its surrounding regions, 1960–1999: Relations to source area and climate. J. Geophys. Res. Atmos. 2001, 106, 10325–10333. [Google Scholar] [CrossRef]
  4. Nagashima, K.; Tada, R.; Tani, A.; Sun, Y.; Isozaki, Y.; Toyoda, S.; Hasegawa, H. Millennial-scale oscillations of the westerly jet path during the last glacial period. J. Asian Earth Sci. 2011, 40, 1214–1220. [Google Scholar] [CrossRef]
  5. Lu, R.Y. Associations among the components of the East Asian summer monsoon system in the meridional direction. J. Meteorol. Soc. Jpn. 2004, 82, 155–165. [Google Scholar] [CrossRef] [Green Version]
  6. Kuang, X.Y.; Zhang, Y.C. Impact of the position abnormalities of East Asian subtropical westerly jet on summer precipitation in middle-lower reaches of Yangtze River. Plateau Meteorol. 2006, 25, 382–389. (In Chinese) [Google Scholar]
  7. Chiang, J.C.H.; Swenson, L.M.; Kong, W. Role of seasonal transitions and the westerlies in the interannual variability of the East Asian summer monsoon precipitation. Geophys. Res. Lett. 2017, 44, 3788–3795. [Google Scholar] [CrossRef]
  8. Zhang, Y.C.; Kuang, X.Y. The Relationship between the Location Change of the East Asian Subtropical Westerly Jet and Asian Summer Monsoon Onset. Torrential Rain Disasters 2008, 27, 97–103. [Google Scholar]
  9. Zhang, Y.C.; Guo, L.L. Multi-model ensemble simulated changes in the subtropical westerly jet over East Asia under the global warming condition. Sci. Meteorol. Sin. 2010, 30, 694–700. [Google Scholar]
  10. Zhang, Y.-C.; Kuang, X.; Guo, W.; Zhou, T. Seasonal evolution of the upper-tropospheric westerly jet core over East Asia. Geophys. Res. Lett. 2006, 33. [Google Scholar] [CrossRef] [Green Version]
  11. Shao, P.C.; Li, D.L.; Chunxue, W. Spatial and Temporal Changes of Summer Rain in the Yellow River Basin and Its Relation to the East Asia Subtropical Westerly Jet in Last 50 Years. Plateau Meteorol. 2015, 34, 347–356. [Google Scholar]
  12. Woollings, T.; Hannachi, A.; Hoskins, B. Variability of the North Atlantic eddy-driven jet stream. Q. J. R. Meteorol. Soc. 2010, 136, 856–868. [Google Scholar] [CrossRef] [Green Version]
  13. Limpasuvan, V.; Hartmann, D.L. Eddies and the annular modes of climate variability. Geophys. Res. Lett. 1999, 26, 3133–3136. [Google Scholar] [CrossRef] [Green Version]
  14. Yang, S.; Lau, K.M.; Kim, K.M. Variation of the East Asian jet stream and Asian Pacific American winter climate Anomalies. J. Clim. 2002, 15, 306–325. [Google Scholar] [CrossRef]
  15. Liao, Q.H.; Gao, S.T.; Wang, H.J. Anomalies of the extratropical westerly jet in the north hemisphere and their impacts on east Asian summer climate anomalies. Chin. J. Geophys. 2004, 47, 11–18. (In Chinese) [Google Scholar] [CrossRef]
  16. Xiao, C.; Zhang, Y. Simulation of the westerly jet axis in boreal winter by the climate system model FGOALS-g2. Adv. Atmos. Sci. 2013, 30, 754–765. [Google Scholar] [CrossRef]
  17. Liang, X.Z.; Wang, W.C. Association between China monsoon rainfall and troposphere jets. Q. J. R. Meteorol. Soc. 1985, 124, 2597–2623. [Google Scholar] [CrossRef]
  18. Chiang, J.C.H.; Lee, S.-Y.; Putnam, A.E.; Wang, X. South Pacific Split Jet, ITCZ shifts, and atmospheric North–South linkages during abrupt climate changes of the last glacial period. Earth Planet. Sci. Lett. 2014, 406, 233–246. [Google Scholar] [CrossRef] [Green Version]
  19. Lucas, C.; Nguyen, H.; Timbal, B. An observational analysis of Southern Hemisphere tropical expansion. J. Geophys. Res. Atmos. 2012, 117. [Google Scholar] [CrossRef] [Green Version]
  20. Park, H.; Kim, S.-J.; Seo, K.-H.; Stewart, A.L.; Kim, S.-Y.; Son, S.-W. The impact of Arctic sea ice loss on mid-Holocene climate. Nat. Commun. 2018, 9, 4571. [Google Scholar] [CrossRef] [Green Version]
  21. Montaggioni, L.F.; Le Cornec, F.; Corrège, T.; Cabioch, G. Coral barium/calcium record of mid-Holocene upwelling activity in New Caledonia, South-West Pacific. Palaeogeogr. Palaeoclimatol. Palaeoecol. 2006, 237, 436–455. [Google Scholar] [CrossRef]
  22. Xu, H.; Zhou, K.; Lan, J.; Zhang, G.; Zhou, X. Arid Central Asia saw mid-Holocene drought. Geology 2019, 47, 255–258. [Google Scholar] [CrossRef]
  23. Lauterbach, S.; Witt, R.; Plessen, B.; Dulski, P.; Prasad, S.; Mingram, J.; Gleixner, G.; Hettler-Riedel, S.; Stebich, M.; Schnetger, B.; et al. Climatic imprint of the mid-latitude Westerlies in the Central Tian Shan of Kyrgyzstan and teleconnections to North Atlantic climate variability during the last 6000 years. Holocene 2014, 24, 970–984. [Google Scholar] [CrossRef] [Green Version]
  24. Baker, J.L.; Lachniet, J.L.B.M.S.; Chervyatsova, O.; Asmerom, Y.; Polyak, Y.A.V.J. Holocene warming in western continental Eurasia driven by glacial retreat and greenhouse forcing. Nat. Geosci. 2017, 10, 430–435. [Google Scholar] [CrossRef]
  25. Zhang, R.; Liu, X. An Analogy Analysis of Summer Precipitation Change Patterns between Mid-Holocene and Future Climatic Warming Scenarios over East Asia. Sci. Geogr. Sin. 2009, 29, 679–683. [Google Scholar]
  26. Joussaume, S.; Taylor, K.E. Status of the paleoclimate modeling intercomparison project (PMIP). In Proceedings of the First International AMIP Scientific Conference, Monterey, CA, USA, 15–19 May 1995; WCRP Report; pp. 425–430. [Google Scholar]
  27. Held, I.M.; Hou, A.Y. Zonal jet structure and the leading mode of variability. J. Atmos. Sci. 1980, 37, 515–533. [Google Scholar] [CrossRef] [Green Version]
  28. Held, I.M. Momentum transport by quasi geostrophic eddies. J. Atmos. Sci. 1975, 32, 1494–1497. [Google Scholar] [CrossRef] [Green Version]
  29. Lin, C. The Relationship between East Asian Summer Monsoon Activity and Northward Jump of the Upper Westerly Jet Location. Chin. J. Atmos. Sci. 2004, 28, 641–658. [Google Scholar]
  30. Huang, X.; Jing, J. The diagnostic analysis of the impact of ENSO events on East Asia subtropical westerly jet. Sci. Meterol. Sin. 2008, 28, 15–20. [Google Scholar]
  31. Routson, C.C.; McKay, N.P.; Kaufman, D.S.; Erb, M.P.; Goosse, H.; Shuman, B.N.; Rodysill, J.R.; Ault, T. Mid-latitude net precipitation decreased with Arctic warming during the Holocene. Nature 2019, 568, 83–86. [Google Scholar] [CrossRef]
  32. Lamy, F.; Kilian, R.; Arz, H.W.; Francois, J.-P.; Kaiser, J.; Prange, M.; Steinke, T. Holocene changes in the position and intensity of the southern westerly wind belt. Nat. Geosci. 2010, 3, 695–699. [Google Scholar] [CrossRef] [Green Version]
  33. Williams, G.P.; Bryan, K. Ice Age Winds. An Aquaplanet Model. J. Clim. 2006, 19, 1706–1715. [Google Scholar] [CrossRef] [Green Version]
  34. Cai, Q.Q.; Zhou, T.J.; Wu, B.; Li, B.; Zhang, L. The East Asian subtropical westerly jet and its interannual variability simulated b y a climate system model FGOALSg-l. Acta Oceanol. Sin. 2011, 33, 38–48. [Google Scholar]
  35. Kuang, X.; Zhan, Y. The Characteristics of the Coupling Variation between the Subtropical Westerly Jet and the Surface Heating Fields over East Asia. Chin. J. Atmos. Sci. 2007, 31, 77–88. [Google Scholar]
  36. Dong, M.; Yu, J. Relationship between the changes of the westerly jet and the tropical convective heating in East Asia. Chin. J. Atmos. Sci. 1999, 23, 62–70. [Google Scholar]
  37. Huang, R.H.; Sun, F.Y. Interannual variation of the summer teleconnection pattern over the Northern Hemisphere and its numerical simulation. Chin. J. Atmos. Sci. 1992, 16, 52–61. (In Chinese) [Google Scholar]
  38. Yang, L.; Zhang, Q. Climate Features of Summer Asia Subtropical Westerly Jet Stream. Clim. Environ. Res. 2008, 13, 10–20. [Google Scholar]
  39. Lorenz, D.J.; Hartmann, D.L. Eddy–zonal flow feedback in the Southern Hemisphere. J. Atmos. Sci. 2001, 58, 3312–3327. [Google Scholar] [CrossRef]
  40. Yang, L.; Zhang, Q. Anomalous Perturbation Kinetic Energy of Rossby Wave along East Asian Westerly Jet and Its Association with Summer Rainfall in China. Chin. J. Atmos. Sci. 2007, 31, 586–595. [Google Scholar]
  41. Enomoto, T.; Hoskins, B.J.; Matsuda, Y. The formation mechanism of the Bonin high in August. Q. J. R. Meteorol. Soc. 2003, 129, 157–178. [Google Scholar] [CrossRef]
  42. Herzschuh, U.; Cao, X.; Laepple, T.; Dallmeyer, A.; Telford, R.J.; Ni, J.; Zheng, Z. Position and orientation of the westerly jet determined Holocene rainfall patterns in China. Nat. Commun. 2019, 10, 1–8. [Google Scholar] [CrossRef]
  43. Fu, Q.; Lin, P. Poleward shift of subtropical jets inferred from satellite-observed lower-stratospheric temperatures. J. Clim. 2011, 24, 5597–5603. [Google Scholar] [CrossRef]
  44. Yin, J.H. A consistent poleward shift of the storm tracks in simulations of 21st century climate. Geophys. Res. Lett. 2005, 32, L18701. [Google Scholar] [CrossRef] [Green Version]
  45. Archer, C.L.; Caldeira, K. Historical trends in the jet streams. Geophys. Res. Lett. 2008, 35, L08803. [Google Scholar] [CrossRef]
  46. Norris, J.R.; Allen, R.J.; Evan, A.T.; Zelinka, M.D.; O’Dell, C.W.; Klein, S.A. Evidence for climate change in the satellite cloud record. Nature 2016, 536, 72–75. [Google Scholar] [CrossRef] [Green Version]
  47. Scheff, J.; Frierson, D. Twenty-first-century multimodel subtropical precipitation declines are mostly midlatitude shifts. J. Clim. 2012, 25, 4330–4347. [Google Scholar] [CrossRef]
  48. Yang, H.; Lohmann, G.; Lu, J.; Gowan, E.J.; Shi, X.; Liu, J.; Wang, Q. Tropical expansion driven by poleward advancing mid-latitude meridional temperature gradients. J. Geophys. Res. Atmos. 2020, 125, e2020JD033158. [Google Scholar] [CrossRef]
  49. Yang, H.; Lohmann, G.; Krebs-Kanzow, U.; Ionita, M.; Shi, X.; Sidorenko, D.; Gong, X.; Chen, X.; Gowan, E.J. Poleward shift of the major ocean gyres detected in a warming climate. Geophys. Res. Lett. 2020, 47, e2019GL085868. [Google Scholar] [CrossRef] [Green Version]
  50. Chen, G.; Lu, J.; Frierson, D.M.W. Phase Speed Spectra and the Latitude of Surface Westerlies: Interannual Variability and Global Warming Trend. J. Clim. 2008, 21, 5942–5959. [Google Scholar] [CrossRef]
  51. Barnes, E.A.; Polvani, L. Response of the midlatitude jets and of their variability to increased greenhouse gases in the CMIP5 models. J. Clim. 2013, 26, 7117–7135. [Google Scholar] [CrossRef]
  52. Chen, F.; Chen, J.; Huang, W.; Chen, S.; Huang, X.; Jin, L.; Jia, J.; Zhang, X.; An, C.; Zhang, J.; et al. Westerlies Asia and monsoonal Asia: Spatiotemporal differences in climate change and possible mechanisms on decadal to sub-orbital timescales. Earth-Sci. Rev. 2019, 192, 337–354. [Google Scholar] [CrossRef]
  53. Oster, J.L.; Ibarra, D.E.; Winnick, M.J.; Maher, K. Steering of westerly storms over western North America at the Last Glacial Maximum. Nat. Geosci. 2015, 8, 201–205. [Google Scholar] [CrossRef]
  54. Rojas, M. Sensitivity of Southern Hemisphere circulation to LGM and 4 × CO2 climates. Geophys. Res. Lett. 2013, 40, 965–970. [Google Scholar] [CrossRef]
  55. Chavaillaz, Y.; Codron, F.; Kageyama, M. Southern westerlies in LGM and future (RCP4.5) climates. Clim. Past 2013, 9, 517–524. [Google Scholar] [CrossRef] [Green Version]
  56. Sime, L.C.; Hodgson, D.; Bracegirdle, T.J.; Allen, C.; Perren, B.; Roberts, S.; De Boer, A.M. Sea ice led to poleward shifted winds at the Last Glacial Maximum: The influence of state dependency on CMIP5 and PMIP3 models. Clim. Past 2016, 12, 2241–2253. [Google Scholar] [CrossRef] [Green Version]
  57. Kaspar, F.; Spangehl, T.; Cubasch, U. Northern hemisphere winter storm tracks of the Eemian interglacial and the last glacial inception. Clim. Past 2007, 3, 181–192. [Google Scholar] [CrossRef] [Green Version]
  58. Kim, S.-J.; Jun, S.-Y.; Kim, B.-M. Sensitivity of southern hemisphere westerly wind to boundary conditions for the last glacial maximum. Quat. Int. 2017, 459, 165–174. [Google Scholar] [CrossRef]
  59. Wang, N.; Jiang, D.; Lang, X. Northern Westerlies during the Last Glacial Maximum: Results from CMIP5 Simulations. J. Clim. 2018, 31, 1135–1153. [Google Scholar] [CrossRef]
  60. Schenk, F.; Zorita, E. Reconstruction of high-resolution atmospheric fields for Northern Europe using analog-upscaling. Clim. Past 2012, 8, 1681–1703. [Google Scholar] [CrossRef] [Green Version]
  61. Taylor, K.E.; Ronald, S.; Meehl, G. A Summary of the CMIP5 experiment design. PCDMI Rep. 2007, 4. [Google Scholar]
  62. Braconnot, P.; Harrison, S.P.; Otto-Bliesner, B.; Abe-Ouchi, A. The Paleoclimate Modeling Intercomparison Project contribution to CMIP5. Clivar Exch. 2011, 56, 15–19. [Google Scholar]
  63. Krinner, G.; Lézine, A.-M.; Braconnot, P.; Sepulchre, P.; Ramstein, G.; Grenier, C.; Gouttevin, I. A reassessment of lake and wetland feedbacks on the North African Holocene climate. Geophys. Res. Lett. 2012, 39, 78–87. [Google Scholar] [CrossRef]
  64. Braconnot, P.; Harrison, S.P.; Kageyama, M.; Bartlein, P.J.; Masson-Delmotte, V.; Abe-Ouchi, A.; Otto-Bliesner, B.; Zhao, Y. Evaluation of climate models using palaeoclimatic data. Nat. Clim. Chang. 2016, 2, 417–424. [Google Scholar] [CrossRef]
  65. Rojas, M.; Moreno, P.; Kageyama, M.; Crucifix, M.; Hewitt, C.; Abe-Ouchi, A.; Ohgaito, R.; Brady, E.C.; Hope, P. The southern westerlies during the Last Glacial Maximum in PMIP2 simulations. Clim. Dyn. 2009, 32, 525–548. [Google Scholar] [CrossRef]
  66. Dong, M.; Zhu, W.; Xu, X. Variation of surface heat flux on the atmosphere over East Asia in early summer. Q. J. Appl. Meteorol. 2001, 12, 458–468. (In Chinese) [Google Scholar]
  67. Huang, R.H.; Gambo, K. The response of a hemispheric multilevel model atomosphere to forcing by topography and stationary heat sources. J. Meterorol. Soc. Jpn. 1982, 60, 78–108. [Google Scholar] [CrossRef] [Green Version]
  68. Wu, J.; Liu, Q.; Cui, Q.Y.; Xu, D.K.; Wang, L.; Shen, C.M.; Chu, G.Q.; Liu, J.Q. Shrinkage of East Asia winter monsoon associated with increased ENSO events since the Mid-Holocene. J. Geophys. Res. Atmos. 2019, 124, 3839–3848. [Google Scholar] [CrossRef]
  69. Wu, P.; Liu, Z. A simulation study on spatio-temporal asynchronism of East Asian summer’s precipitation variation since the mid-Holocene. Quat. Sci. 2013, 33, 1138–1147. [Google Scholar]
  70. Sun, W.; Wang, B.; Zhang, Q.; Pausata, F.S.; Chen, D.; Lu, G.; Yan, M.; Ning, L.; Liu, J. Northern Hemisphere land monsoon precipitation increased by the Green Sahara during mid-Holocene. Geophys. Res. Lett. 2019, 46, 9870–9879. [Google Scholar] [CrossRef] [Green Version]
  71. Liu, Z.; Harrison, S.P.; Kutzbach, J.; Otto-Bliesner, B. Global monsoon in the Mid-Holocene and oceanic feedback. Clim. Dyn. 2004, 22, 157–182. [Google Scholar] [CrossRef]
  72. Ren, X.J.; Zhang, Y.C.; Xiang, Y. Connections between wintertime jet stream variability, oceanic surface heating, and transient eddy activity in the North Pacific. J. Geophys. Res. 2008, 113, D21119. [Google Scholar] [CrossRef] [Green Version]
  73. Hoskins, B.J.; Hodges, K.I. New perspectives on the Northern Hemisphere winter storm tracks. J. Atmos. Sci. 2002, 59, 1041–1061. [Google Scholar] [CrossRef]
  74. Lau, N.; Oort, A.H. A Comparative Study of Observed Northern Hemisphere Circulation Statistics Based on GFDL and NMC Analyses. Part II: Transient Eddy Statistics and the Energy Cycle. Mon. Weather Rev. 1982, 110, 889–906. [Google Scholar] [CrossRef] [Green Version]
  75. Bjerknes, J. A possible response of the atmospheric Hadley circulation to equatorial anomalies of ocean temperature. Tellus 1960, 18, 802–829. [Google Scholar]
Atmosphere 11 01193 g001 550
Figure 1. Change of the insolation during the mid-Holocene compared to the preindustrial period. The insolation during the preindustrial period is plotted in contours, and the difference between the mid-Holocene and the preindustrial period is color-shaded (units in W m−2).
Figure 1. Change of the insolation during the mid-Holocene compared to the preindustrial period. The insolation during the preindustrial period is plotted in contours, and the difference between the mid-Holocene and the preindustrial period is color-shaded (units in W m−2).
Atmosphere 11 01193 g001
Atmosphere 11 01193 g002 550
Figure 2. Changes (color shading) of the 200-hPa zonal wind (mid-Holocene minus preindustrial); (a) refers to the arithmetic multi-model mean; (b–e) correspond to the four models, respectively. The red dashed line and the green solid line are the indicators of the jet axis in the preindustrial period and the mid-Holocene, respectively. (f) The climatology of zonal wind in PI at 200-hPa. The stippled areas in (a–e) indicate that the difference is statistically significant at the confidence level of 95%.
Figure 2. Changes (color shading) of the 200-hPa zonal wind (mid-Holocene minus preindustrial); (a) refers to the arithmetic multi-model mean; (b–e) correspond to the four models, respectively. The red dashed line and the green solid line are the indicators of the jet axis in the preindustrial period and the mid-Holocene, respectively. (f) The climatology of zonal wind in PI at 200-hPa. The stippled areas in (a–e) indicate that the difference is statistically significant at the confidence level of 95%.
Atmosphere 11 01193 g002
Atmosphere 11 01193 g003 550
Figure 3. (a,b) Location (unit: degree) and (c) intensity (unit: m/s) anomalies of the westerly jet at 200 hPa over three key regions—the Eurasia, the North Pacific, and the North Atlantic, respectively. The positive values in (a) refer to the northward movement of the center of the jet stream; in (b) refer to the eastward movement of the center of the jet stream; in (c) refer to the intensified jet stream. The values which are significant at 95% confidence level are marked with dots.
Figure 3. (a,b) Location (unit: degree) and (c) intensity (unit: m/s) anomalies of the westerly jet at 200 hPa over three key regions—the Eurasia, the North Pacific, and the North Atlantic, respectively. The positive values in (a) refer to the northward movement of the center of the jet stream; in (b) refer to the eastward movement of the center of the jet stream; in (c) refer to the intensified jet stream. The values which are significant at 95% confidence level are marked with dots.
Atmosphere 11 01193 g003
Atmosphere 11 01193 g004 550
Figure 4. The differences of the simulated (a) surface temperature and (b) geopotential height at 200 hPa between MH and PI. The stippled areas indicate that the difference is statistically significant at the confidence level of 95%.
Figure 4. The differences of the simulated (a) surface temperature and (b) geopotential height at 200 hPa between MH and PI. The stippled areas indicate that the difference is statistically significant at the confidence level of 95%.
Atmosphere 11 01193 g004
Atmosphere 11 01193 g005 550
Figure 5. The temperature anomalies between MH and PI at (a) 850 hPa, (b) 500 hPa and (c) 200 hPa, respectively. The stippled areas indicate that the difference is statistically significant at the confidence level of 95%.
Figure 5. The temperature anomalies between MH and PI at (a) 850 hPa, (b) 500 hPa and (c) 200 hPa, respectively. The stippled areas indicate that the difference is statistically significant at the confidence level of 95%.
Atmosphere 11 01193 g005
Atmosphere 11 01193 g006 550
Figure 6. The meridional and zonal pressure gradient at 200 hPa. (a) The meridional pressure gradient in PI. (b) The zonal pressure gradient in PI. (c) The difference of meridional pressure gradient between MH and PI. (d) The difference of zonal pressure gradient between MH and PI. The stippled areas indicate that the difference is statistically significant at the confidence level of 95%.
Figure 6. The meridional and zonal pressure gradient at 200 hPa. (a) The meridional pressure gradient in PI. (b) The zonal pressure gradient in PI. (c) The difference of meridional pressure gradient between MH and PI. (d) The difference of zonal pressure gradient between MH and PI. The stippled areas indicate that the difference is statistically significant at the confidence level of 95%.
Atmosphere 11 01193 g006
Atmosphere 11 01193 g007 550
Figure 7. Distributions of the transient eddy kinetic energy in summer (EKE, m2s−2; calculated by Equation (1)) and the westerly jet axis at 200 hPa for (a) PI and (b) MH. The red dashed line and the blue solid line are the indicators of the westerly axes in the preindustrial period and the Mid-Holocene, respectively.
Figure 7. Distributions of the transient eddy kinetic energy in summer (EKE, m2s−2; calculated by Equation (1)) and the westerly jet axis at 200 hPa for (a) PI and (b) MH. The red dashed line and the blue solid line are the indicators of the westerly axes in the preindustrial period and the Mid-Holocene, respectively.
Atmosphere 11 01193 g007
Atmosphere 11 01193 g008 550
Figure 8. The meridional surface temperature gradient. (a) The meridional surface climatology temperature gradient in PI. (b) The difference between MH and PI. The stippled areas indicate that the difference is statistically significant at the confidence level of 95%.
Figure 8. The meridional surface temperature gradient. (a) The meridional surface climatology temperature gradient in PI. (b) The difference between MH and PI. The stippled areas indicate that the difference is statistically significant at the confidence level of 95%.
Atmosphere 11 01193 g008
Atmosphere 11 01193 g009a 550Atmosphere 11 01193 g009b 550
Figure 9. Circulation anomalies in MH and PI. (a) Model-mean circulation anomalies showing annual-mean changes in meridional and vertical motion (vectors; the scale for vertical motion has been increased by 100 times to aid viewing) in the northern hemisphere and vertical motion (color-shaded; units in Pa/s). (b–d) Refer to the three key regions—Eurasia, the North Pacific, and the North Atlantic, respectively. The stippled areas indicate that the difference is statistically significant at the confidence level of 95%. The vectors are shown where the meridional wind is statistically significant.
Figure 9. Circulation anomalies in MH and PI. (a) Model-mean circulation anomalies showing annual-mean changes in meridional and vertical motion (vectors; the scale for vertical motion has been increased by 100 times to aid viewing) in the northern hemisphere and vertical motion (color-shaded; units in Pa/s). (b–d) Refer to the three key regions—Eurasia, the North Pacific, and the North Atlantic, respectively. The stippled areas indicate that the difference is statistically significant at the confidence level of 95%. The vectors are shown where the meridional wind is statistically significant.
Atmosphere 11 01193 g009aAtmosphere 11 01193 g009b
Table
Table 1. Basic information about the four CMIP5/PMIP3 models and the variables used in this study.
Table 1. Basic information about the four CMIP5/PMIP3 models and the variables used in this study.
Model IdentifierCountryAGCM Resolution
Lat × Lon Grids		Years Used in Analysis (0 ka)Years Used in Analysis (6 ka)Variables Used in Analysis
CCSM4United States288 × 192Last 100100ua va tas zg tos wap
CNRM-CM5France256 × 128Last 100100ua va tas zg tos wap
MPI-ESM-PGermany196 × 98Last 100100ua va tas zg tos wap
MRI-CGCM3Japan320 × 160Last 100100ua va tas zg tos wap
Table
Table 2. Comparison of the boundary conditions for the CMIP5/PMIP3 preindustrial and mid-Holocene experiments.
Table 2. Comparison of the boundary conditions for the CMIP5/PMIP3 preindustrial and mid-Holocene experiments.
ExperimentObliquityEccentricityAngular Precession
(°)		CO2
(ppmv)		CH4
(ppbv)		N2O
(ppbv)		Land-Sea Mask
Preindustrial23.4460.016724102.04280760270Modern
Mid-Holocene24.1050.0186820.87280650270Same as PI
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Share and Cite

MDPI and ACS Style

Xu, C.; Yan, M.; Ning, L.; Liu, J. Summer Westerly Jet in Northern Hemisphere during the Mid-Holocene: A Multi-Model Study. Atmosphere 2020, 11, 1193. https://doi.org/10.3390/atmos11111193

AMA Style

Xu C, Yan M, Ning L, Liu J. Summer Westerly Jet in Northern Hemisphere during the Mid-Holocene: A Multi-Model Study. Atmosphere. 2020; 11(11):1193. https://doi.org/10.3390/atmos11111193

Chicago/Turabian Style

Xu, Chuchu, Mi Yan, Liang Ning, and Jian Liu. 2020. "Summer Westerly Jet in Northern Hemisphere during the Mid-Holocene: A Multi-Model Study" Atmosphere 11, no. 11: 1193. https://doi.org/10.3390/atmos11111193

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop