Sea Surface Wind and Rainfall Responses to Marine Heatwaves in the Northern South China Sea

Abstract

In this study, the properties and related anomalies in sea surface wind and rainfall associated with marine heatwaves (MHWs) in the northern South China Sea (SCS) were investigated. The intensity and frequency (duration) of MHWs are high (short) along the coast and decrease (increase) when moving toward the open sea. On the continental shelf of the northern SCS, the wind anomalies associated with MHWs move in a northeastward direction in seasons other than summer. In the summer, MHW-induced wind anomalies were found to be statistically insignificant. In response to MHWs, there is a notable negative rainfall anomaly observed during the spring and summer, whereas a positive anomaly is observed in the fall. In the winter, the MHW-induced rainfall anomalies were deemed insignificant. The presence of an El Niño event can delay the influence of MHWs on rainfall anomalies and attenuate the amplitude of MHW-induced sea surface wind anomalies.

1. Introduction

Marine heatwaves (MHWs) are discrete, prolonged, and anomalous warming events that have garnered significant attention due to their exceptional impacts on the structures and functions of ecosystems [1,2,3,4,5]. Numerous MHW events have been documented, and it has been observed that they influence the spatial distribution of species and the diversity of fisheries [5,6,7,8,9,10]. These events are observed globally, presenting various intensities and frequencies. MHWs can occur in marginal seas all over the world, and their durations are displaying increasing long-term trends [11,12]. Beyond responding to climate variability, discrete MHWs contribute to variability but also induce changes in ocean heat content and air–sea interaction dynamics [12]. Consequently, enhancing our understanding of MHWs’ characteristics and their influences on the associated air–sea interactions holds promise with respect to better estimating their overall impacts.

The South China Sea (SCS) is the largest semi-enclosed marginal sea in the northwestern Pacific, and it frequently experiences MHWs [13]. Recent studies have highlighted a notable increase in MHW frequency, intensity, and duration in the SCS in the recent past, in the present, and in the projected future [14,15,16]. The key contributors to MHW genesis in the SCS are air–sea heat flux and oceanic dynamic processes [17,18,19,20,21].

On the northern continental shelf of the SCS, MHWs manifest with higher intensity and frequency compared to the interior region of this sea [14,15,16,22,23]. Notably, the presence of robust slope flow and active cross-slope water exchange along the northern slope of the SCS significantly influences the decay of MHWs on the continental shelf [24,25,26,27,28]. These intricate oceanic dynamics play a pivotal role in shaping the characteristics of MHWs within this region, delineating their intensity and temporal behavior [21]. Therefore, in this study, we focus on the MHWs around the northern continental shelf of the SCS.

MHWs have a profound impact on the ecosystem and climate variability of the South China Sea (SCS). These events have led to disruptions in the mutualistic relationship between corals and algae, resulting in coral reef decline throughout the SCS [29,30,31]). Furthermore, the influence of extreme warming in the SCS extends to the modulation of adjacent climate variability [19,25].

The structure of this paper is organized as follows. Section 2 presents the data and methodology employed in this study. Section 3 delves into a discussion on Section 2. Details of the MHW properties observed in the northern SCS are also presented. Subsequently, Section 4 explores the wind and rainfall anomalies associated with MHWs. Finally, Section 5 encapsulates this research’s main findings and conclusions.

2. Data and Methods

2.1. Data

The daily sea surface temperature (SST), 10 m wind field, and rainfall data used in this study were sourced from the European Centre for Medium-Range Weather Forecasts Reanalysis Interim (ERA-5) reanalysis products. These datasets possess a horizontal grid spacing of 0.25°. For the purposes of this study, the period spanning from 1979 to 2021 has been selected due to the greater availability of data for this period and their relevance.

2.2. Definition of Indices

The definition of Marine Heatwaves (MHWs) used herein adheres to the criteria established by Hobday et al. (2016). In the definition process, the daily Sea Surface Temperature (SST) data first undergo an 11-day moving and smoothing process. Subsequently, the threshold is determined as the 90th percentile of all SST values for a specific day of the year. Finally, an MHW can be said to be identified when warming surpasses the threshold and endures for a minimum of five consecutive days. The duration of an MHW is defined as the continuous period during which the SST remains above the threshold, while the intensity is represented by the average (or highest) mean (maximum) SST anomaly throughout the MHW event.

These metrics are available in the form of software modules within the MATLAB programming environment and can be accessed via the following link: https://github.com/ZijieZhaoMMHW/m_mhw1.0 accessed on 1 January 2023. Notably, intensity and duration are pivotal indices employed to characterize MHWs and illustrate how oceanic dynamics can potentially influence air–sea interactions. Given their significance, the aforementioned indices have been chosen to effectively depict the attributes of MHWs.

A Niño-3.4 index higher than 0.58 °C for six consecutive months or longer is defined as an El Niño event. The Niño 3.4 data used herein were taken from the NOAA Physical Sciences Laboratory (https://psl.noaa.gov/gcos_wgsp/Timeseries/ accessed on 1 January 2023).

3. Properties of MHWs in the Northern SCS

Around the continental shelf of the northern South China Sea (SCS), the frequency of Marine Heatwaves (MHWs) surpasses two occurrences per year, displaying an escalation in frequency closer to the coastline (refer to Figure 1a). The durations of these MHWs predominantly fall within the 11- to 12-day range along the coast. Conversely, around the Dongsha Islands and southwest Taiwan, their durations extend to around 14 days (see Figure 1b). The spatial distribution of the mean intensity of MHWs forms a belt-like pattern parallel to the coastline, with an upward trajectory toward the shore (depicted in Figure 1c).

Adjacent to the coastline, the mean intensity of MHWs exceeds 1.6 °C and gradually declines to approximately 1.3 °C along the perimeter of the continental shelf. Comparatively, the average maximum intensity of MHWs surpasses the mean intensity by approximately 0.3 °C, displaying a parallel spatial arrangement (shown in Figure 1d).

Figure 2 presents the probabilities associated with various characteristics of Marine Heatwave (MHW) occurrences. Notably, years featuring either one or two instances of MHWs accounted for over 50% of the total sampled years (illustrated in Figure 2a). Remarkably, the highest number of MHW events in single year was 7, constituting 10% of the total sampled years.

As for MHW durations, more than half of the total occurrences lasted for fewer than 10 days, with over 80% of durations lasting fewer than 15 days (depicted in Figure 2b). An exceptional case with a duration of 55 days, constituting the longest MHW duration, was observed only once. Regarding MHW intensity, it was primarily concentrated around 1.0 °C and 1.5 °C. The lowest (largest) mean intensity was 0.75 °C (1.9 °C) (as shown in Figure 2c). The spatial distribution of the maximum MHW intensity closely aligned with that of the mean intensity yet exhibited greater SST anomalies and larger spans (depicted in Figure 2d).

Figure 3 illustrates the seasonal characteristics of MHWs in the northern SCS. In the winter, the mean intensity and duration of MHWs exhibited wider distributions, spanning from less than 0.5 °C to over 2 °C and from 5 days to beyond 40 days, respectively. In contrast, the distribution of MHW mean intensity and duration in the other seasons tended to be more concentrated. For instance, in the spring, the mean intensity is around 1.5 °C and the duration is approximately 10 days. Similarly, in the summer, the mean intensity is about 0.9 °C, with a duration of around 12 days. In the fall, the mean intensity hovers at around 1.0 °C, and the duration is roughly 11 days. Notably, the occurrence of MHWs in the winter is noticeably lower compared to the other seasons.

MHWs exhibit a high frequency and intensity on the continental shelf of the northern SCS, suggesting a potential impact on sea surface wind and rainfall patterns. With this in mind, the subsequent section delves into an exploration of the wind anomalies and rainfall anomalies connected with MHWs occurring near the continental shelf of the northern SCS.

4. Sea Surface Wind and Rainfall Responses to MHWs in the Northern SCS

4.1. Anomalous Sea Surface Wind and Rainfall

Given the seasonal variability of MHWs in the northern SCS, the sea surface wind and rainfall responses to these events exhibit variations throughout the different seasons (Figure 4). During winter MHW events, the wind direction shows a predominant direction anomaly toward the northeast (Figure 4a), with a stronger wind speed anomaly observed around the western continental shelf compared to the eastern region (Figure 4e). The northern SCS displays a similar northeast wind direction anomaly during spring MHW events (Figure 4b), but the wind speed anomaly is notably higher than that observed in the winter (Figure 4a,b), particularly along the eastern continental shelf. There are few distinct dominant wind direction anomalies in the northern SCS during summer MHW events, while significant wind anomalies connected to MHWs are mainly concentrated along the continental shelf’s edge (Figure 4c,g). The fall is associated with a prevalent northeast wind direction anomaly, and the wind speed anomaly is pronounced in both the eastern and western continental shelf areas while being comparatively weaker in the central region (Figure 4d,h).

In contrast to the wind patterns, the reaction of rainfall anomalies to MHWs is less evident in the winter (Figure 4i). The spring, however, showcases a significant negative rainfall anomaly closely associated with MHWs, encompassing much of the continental shelf (Figure 4j). In the summer, a significant negative rainfall anomaly was noticeable around the eastern continental shelf, while other areas did not exhibit significant changes (Figure 4k). Finally, in the fall, a scattered positive rainfall anomaly was observed across the entire continental shelf (Figure 4l).

The connection between MHW-induced sea surface wind anomalies and rainfall anomalies is depicted in Figure 5. Since the response of rainfall anomalies to MHWs is less pronounced during the winter and fall (Figure 4i,l), the association between sea surface wind and rainfall alterations is weaker (Figure 5a,d,e,h). In the spring, a robust positive correlation is evident between MHW-induced sea surface wind and rainfall around the central area of the continental shelf (Figure 5b,f). However, both the zonal and meridional wind anomalies are positive, while the rainfall anomaly is negative (Figure 4f,j). Consequently, the relationship between sea surface wind and rainfall is characterized not by positive feedback but rather a mutually inhibitory interaction. Throughout the summer, the rainfall anomaly displays a positive correlation solely with the meridional wind anomaly east of the continental shelf (Figure 5c,g). Analogous to the situation in the spring, meridional sea surface wind and rainfall influence each other in a mutually inhibitory manner.

During both the spring and summer, the MHW-induced wind anomalies primarily move in a northward direction (Figure 4f,g), which allows for the transport of moisture from the open sea to the continental shelf. This influx of abundant moisture facilitates positive rainfall anomalies. However, MHWs induce negative rainfall anomalies. Therefore, they influence each other in a mutually inhibitory manner.

4.2. Influence of Anomalous Sea Surface Wind and Rainfall Conditions on the Background

To assess their impact on the background of rainfall and sea surface wind, the anomalous sea surface wind and rainfall conditions were superimposed on a climatological time series, as depicted in Figure 6. In the winter and early spring months, the MHW-induced rainfall anomalies have a minimal influence on the climatological rainfall pattern (Figure 6a). However, in the late spring and summer periods, the MHW-induced rainfall anomalies significantly reduce climatological rainfall. Conversely, in the fall, the MHW-induced rainfall anomaly amplifies the climatological rainfall. In the absence of the El Niño effect, the MHW-induced dry period will begin earlier in the early spring and persist until the late summer. Moreover, the MHW-induced wet period commences earlier and exhibits a greater intensity in the early fall.

In seasons other than the summer, both the anomalous zonal and meridional sea surface winds responding to the MHW exhibit positive trends (Figure 6b,c). During the fall and winter, the anomalous sea surface winds contribute to the weakening of the winter monsoon while promoting the onset of the summer monsoon in the spring. Conversely, in the summer, the anomalous sea surface winds contribute to the attenuation of the summer monsoon. When accounting for the influence of El Niño, the MHW-induced sea surface wind anomaly’s impact on the climatological monsoon remains relatively consistent, with a slight decrease in intensity.

4.3. Large-Scale Atmospheric Background

To investigate the large-scale atmospheric background, we present the composite 850 hPa wind anomaly (m/s) and 500 hPa vertical velocity (Pa/s) for MHWs in Figure 7. During the winter, the winter monsoon anomaly weakens across the entire SCS, and this process is accompanied by a north–south dipole in the vertical movement anomaly (Figure 7a). In both the spring and summer, the wind field exhibits westerlies at lower latitudes and easterlies at relatively higher latitudes, generating shearing effects over the northern SCS (Figure 7b,c). Notably, the descending region covers the northern SCS during these seasons, contributing to dry conditions in the area. Moving into the fall season, anticyclonic anomalies extend over the western tropical Pacific and its northern periphery, exerting a remote influence on the air–sea interaction associated with the MHWs over the northern SCS (Figure 7d). This period sees alternating coverage of ascending and descending areas over the northern SCS.

5. Conclusions

Utilizing a comprehensive dataset of long-term SST and atmospheric variables, this study delved into the characteristics of marine heatwaves (MHWs) around the continental shelf of the northern South China Sea (SCS) along with their associated sea surface wind and rainfall patterns. Our investigation revealed that MHWs’ intensity and occurrence exhibit a notable degree of spatial differentiation along the continental shelf of the northern SCS, with higher values closer to the coast that begin lowering when moving toward the open sea. Additionally, the MHW duration showcased a distinct pattern: MHWs lasted for less time near the coast and became progressively longer towards the open sea.

On the continental shelf of the northern SCS, the composite anomalies of wind speed linked with MHWs display significant northeastward trends in seasons other than the summer, leading to a weakening of the winter monsoon. Conversely, the response of sea surface wind to MHWs in the summer is inconsequential. When accounting for the influence of El Niño, a subtle decline in the MHW-induced sea surface wind anomaly’s intensity becomes evident.

In terms of rainfall, MHWs bring about contrasting conditions, promoting dryness during the spring and summer while fostering wet conditions in the fall. The MHW-induced rainfall anomaly contributes to a reduction (augmentation) in climatological rainfall during the late spring and summer (fall). Moreover, when excluding El Niño’s impact, the precipitation anomalies’ response to MHWs exhibits an advanced pattern.

Although this study provides fresh insights into MHWs’ characteristics and repercussions on sea surface wind and rainfall conditions in the northern SCS, certain unresolved questions persist. The intricacies of the relationship between MHW-induced rainfall and sea surface phenomena, as well as the teleconnection mechanism between El Niño and MHWs in the northern SCS, remain unexplored. Further investigation into the interaction between MHW-induced sea surface wind anomalies and associated rainfall anomalies is imperative for future research endeavors.