Modulation of Spring Barents and Kara Seas Ice Concentration on the Meiyu Onset over the Yangtze–Huaihe River Basin in China

Abstract

Meiyu is a critical component of the summer rainy season over the Yangtze–Huaihe River Basin (YHRB) in China, and the Meiyu onset date (MOD), serving as a key indicator of Meiyu, has garnered substantial attention. This article demonstrates an in-phase relationship between MOD and the preceding spring Barents–Kara Seas ice concentration (BKSIC) during 1979–2023. Specifically, the loss of spring BKSIC promotes an earlier MOD. Further analysis indicates that decreased spring BKSIC reduces the reflection of shortwave radiation, thereby enhancing oceanic solar radiation absorption and warming sea surface temperature (SST) in spring. The warming SST persists into summer and induces significant deep warming in the BKS through enhanced upward longwave radiation. The BKS deep warming triggers a wave train propagating southeastward to the East Asia–Northwest Pacific region, leading to a strengthened East Asian Subtropical Jet and an intensified Western North Pacific Subtropical High in summer. Under these conditions, the transport of warm and humid airflows into the YHRB is enhanced, promoting convective instability through increased low-level warming and humidity, combined with enhanced wind shear, which jointly contribute to an earlier MOD. These results may advance the understanding of MOD variability and provide valuable information for disaster prevention and mitigation.

1. Introduction

Meiyu (also known as Changma and Baiu in South Korea and Japan, respectively) is a unique weather and climate phenomenon in East Asia, fundamentally governed by the seasonal dynamics of the East Asian summer monsoon [1,2,3]. The main rainy season, occurring from June to July, can extend from the Yangtze–Huaihe River Basin (YHRB) in China to South Korea and Japan [4,5,6,7].

In China, Meiyu rainfall constitutes a pivotal determinant of the general trend of drought and flood conditions during the rainy season over the YHRB region excluding the influence of tropical cyclones; thus, the total rainfall and duration of the Meiyu season are of critical significance [4,5,6,7,8,9]. The YHRB region is one of the most important industrial and agricultural centers in China, characterized by a dense population and a relatively high level of economic development. During the Meiyu period, the region is highly susceptible to prolonged heavy rainfall and intense storm events, which can trigger severe flooding disasters [1,2,3,9,10,11,12,13]. Given that these events pose substantial threats to the safety of residents, resulting in considerable losses to human lives and agricultural production, deeply understanding the variability of Meiyu and its corresponding physical processes over the YHRB region is of paramount importance for regional agricultural production, flood control, and drought mitigation strategies.

As a manifestation of the seasonal dynamics of the East Asian summer monsoon, previous studies have reported that Meiyu is identified to be intricately associated with several key atmospheric circulation systems, including the Western North Pacific subtropical high (WNPSH) [14,15,16,17], the South Asian high [18,19,20], Eurasian blocking activities [21,22,23,24], and the westerly jet stream across Eurasia [25,26,27,28]. The sea surface temperature (SST) anomalies [29,30,31], the Arctic sea ice anomalies [32,33], soil moisture anomalies [34], and snow cover anomalies [35] can exert pronounced effects on the Meiyu variability by regulating the aforementioned atmospheric circulation systems. For example, it is reported that the 2020 extreme Meiyu–Baiu event was partially attributable to the sea ice loss along the Siberian coast by affecting the East Siberian blocking [32].

Given its direct and substantial influence on agricultural production, the Meiyu onset date (MOD) has attracted considerable concern. Previous studies have revealed that the MOD serves as a crucial factor influencing both the total precipitation and the duration of the Meiyu season. It has been demonstrated a significant correlation between the MOD and the total precipitation and the duration of Meiyu [36,37,38]. Specifically, an earlier (later) MOD is typically associated with increased (decreased) total precipitation and a longer (shorter) duration of the Meiyu period [38,39]. Concurrently, in light of its pivotal role in predicting regional hydroclimatic conditions, the variability and its related physical processes of the MOD have gained more attention from the scientific community, policymakers, and the general public.

To date, a multitude of influential factors like the WNPSH [38,40], the South Asian high [40], the westerly jet stream [28,41], intraseasonal oscillation [38,41], atmospheric quasi-biweekly oscillation [42], and teleconnection patterns [43,44,45] have been identified and documented to modulate the MOD over the YHRB region. Moreover, previous studies demonstrated that the MOD variability is also closely associated with the changes in SST [40,43,46,47,48,49], atmospheric latent heating over the Southeast Asian [50], and the Tibetan Plateau heating [46]. For instance, the extremely heavy Meiyu event in the summer of 2020, characterized by its early onset and extremely late retreat over the YHRB region, is considered to be jointly associated with the thermal effect of the Tibetan Plateau, the warm SST observed in the oceanic regions to the east of the Philippines, and the tropical Indian Ocean [40,46]. Wang et al. [43] showed that the tropical Pacific SST anomalies were highlighted to affect the variability of the MOD over the YHRB region by regulating the East Asian–Pacific and Japanese–Pacific teleconnection patterns. A cold (warm) phase of Central Pacific El Niño–Southern Oscillation (CP-ENSO) in spring may tend to an earlier (delayed) MOD over the YHRB region due to the early tropical warm wet water vapor transport to the YHRB region. The tropical South Atlantic SST anomalies are also identified as a contributing factor to the variability of the MOD through their modulation on the anticyclonic circulation over the western North Pacific [48].

However, the preceding studies mostly paid attention to the impact of the SST anomalies of the three oceans (Pacific, Indian, and Atlantic Oceans) on the MOD. Few studies have discussed the potential role of the Arctic Sea ice anomalies in modulating the MOD across the YHRB region. This deficiency is partly attributable to the predominant focus of existing studies on elucidating how the Arctic Sea ice anomalies influence Meiyu anomalies in the YHRB region, with relatively scant attention devoted to exploring their relationship with the MOD. Therefore, the aims of the present study are to identify the impact of the Arctic Sea ice anomalies on the MOD variability and investigate the underlying mechanism. These findings are helpful to deepen our understanding of the role of the Arctic Sea ice in the MOD variability across the YHRB region and thus provide a scientific basis for the Meiyu prediction and regional drought mitigation strategies.

The remainder of this paper is structured as follows: Section 2 presents a detailed description of the data and methods used in this study. Section 3 reveals the variability of the MOD and examines the relationship between the MOD and the preceding spring Barents–Kara Seas (BKS) ice. The possible physical mechanism is also explored in Section 3. Section 4 provides the conclusions and discussions.

2. Data and Methods

2.1. Data

The daily precipitation data of the CN05.1 gridded dataset, which has a horizontal resolution of 0.25° × 0.25° and is based on data from more than 2400 stations in China [51], was employed to calculate the MOD for the period 1979–2023. The monthly mean sea ice concentration (SIC) with a horizontal resolution of 1.0° × 1.0° was downloaded from the Met Office Hadley Centre Sea Ice and SST dataset (HadISST1) [52] and covers the time period 1979–2023. The monthly mean SST data of the Extended Reconstructed SST version 5 (ERSST v5) [53] at a horizontal resolution of 2.0° × 2.0° is available from the National Oceanic and Atmospheric Administration (NOAA) for the 1979–2023 period. The monthly mean atmospheric reanalysis data with a horizontal resolution of 2.5° × 2.5° were provided by the National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) Reanalysis I [54] for the period 1979–2023. The monthly mean convective available potential energy data with a horizontal resolution of 0.25° × 0.25° from the ERA5 dataset [55] were downloaded from the European Centre for Medium-Range Weather Forecasts (ECMWF) for the 1979–2023 period.

2.2. Definition of the MOD Across the YHRB Region

According to the Meiyu Monitoring Indices issued by the China Meteorological Administration in 2017, the MOD criteria are formulated as follows: (a) Define a certain day as a rainy day in the YHRB region (28–34° N, 110–122.5° E) if more than one-third of the monitoring stations (a total of 277 stations, with one-third being 93 stations) in the monitoring YHRB region have a daily mean precipitation rate greater than or equal to 0.1 mm day⁻¹, and the average daily precipitation rate over the YHRB region is greater than or equal to 2.0 mm day⁻¹. (b) If the number of rainy days in the YHRB region from the first rainy day to the next 2 days, 3 days, and until 10 days accounts for 50% or more of the total number of days in the corresponding period, then the first rainy day is defined as the start date of the rainy period. There can be multiple rainy periods within the Meiyu period, and the start date of the first rainy period is defined as the MOD of the Meiyu period.

2.3. Methods

The horizontal wave activity flux (W) was used to diagnose the energy propagation of quasi-stationary Rossby waves, which can be calculated as the following formula [56]:

$$W={\displaystyle \frac{p}{2\left|U\right|}}\left[\begin{array}{c}U\left({\psi }_x^{{\prime }^2}-{\psi }^{\prime }{\psi }_{xx}^{\prime }\right)+V({\psi }_x^{\prime }{\psi }_y^{\prime }-{\psi }^{\prime }{\psi }_{xy}^{\prime })\\ U\left({\psi }_x^{\prime }{\psi }_y^{\prime }-{\psi }^{\prime }{\psi }_{xy}^{\prime }\right)+V({\psi }_y^{{\prime }^2}-{\psi }^{\prime }{\psi }_{yy}^{\prime })\end{array}\right]$$

in which, ψ′ represents the perturbation stream function; |U| represents the horizontal wind speed; and p represents the pressure divided by 1000 hPa. U and V represent the original zonal and meridional component of the basic flow for the whole period, respectively.

Additionally, regression and correlation analyses were also employed. The statistical significance of the results for regression and correlation analyses was determined by the two-tailed Student’s t-test. Since the Meiyu period mainly occurrs from June to July every year, June and July are defined as the Meiyu period in this study. Spring and summer refer to March–April–May (MAM) and June–July (JJ), respectively. The time period for analysis in this research is from 1979 to 2023. To eliminate the possible influence of global warming and facilitate the elucidation of underlying physical mechanisms, all data were detrended prior to analysis, unless otherwise specified.

3. Results

3.1. Variability of the MOD over the YHRB Region

According to the definition of the MOD over the YHRB region, as detailed in Section 2.2, the MOD is depicted and calculated in Figure 1. For the period from 1979 to 2023, there is a certain interannual variability of the MOD without significant linear trends (Figure 1a), which is consistent with previous studies [45,48]. The average MOD is approximately June 10 (Figure 1a). Among these years, the earliest occurrences of the MOD were on May 25, observed in both 1995 and 2016 (Figure 1a), representing a 16-day advance from the average onset date (Figure 1b). Conversely, the latest MOD was recorded on July 10, 1981 (Figure 1a), which is 30 days later than the average MOD (Figure 1b). To facilitate the analysis, we define the normalized MOD as the MOD Index (MODI), represented by the green line in Figure 2. The original and detrended MODIs demonstrate consistent interannual variations and exhibit no discernible trends or interdecadal variations throughout the observational period (Figure 2).

To investigate the changes in synchronous atmospheric circulations associated with the variability of the MOD over the YHRB region for the period 1979–2023, we present the simultaneous regressions of the key atmospheric circulations against the sign-reversed MODI, including the summer zonal wind at 200 hPa, eddy geopotential height at 500 hPa, and horizontal wind at 850 hPa (Figure 3). The background atmospheric circulations associated with the earlier-than-normal MOD are characterized by a quasi-barotropic structure extending from the lower to the upper troposphere (Figure 3). In the upper troposphere, the zonal wind anomalies over the East Asia–Northwest Pacific region exhibit a distinct meridional dipole pattern, with a significant westerly anomaly to the north of the YHRB region and a pronounced easterly anomaly to its south (Figure 3a). This distribution is indicative of an intensified East Asian Subtropical Jet Stream (EASJ), as the climatological position of the summer EASJ is usually centered around 40° N (Figure 3a). This configuration enhances the in situ wind shear, thereby contributing to the convective instability over the YHRB region [25,57]. In the middle and lower troposphere over the East Asia–Northwest Pacific region, a meridional dipole structure with a pronounced northwest–southeast tilt is evident, with its centers situated over the Northwest Pacific Ocean of southern Japan and the Inner Mongolian Plateau, respectively (Figure 3b). Under the atmospheric circulations, the strengthening of the WNPSH facilitates the northward transport of the warm and moist water vapor from the South China Sea and western North Pacific toward the YHRB region, thereby providing ample moisture conditions conducive to the MOD. Additionally, the increased air temperature at the lower troposphere leads to an enhanced lapse rate, which also promotes convective instability and is further favorable for the MOD [58,59,60].

3.2. Role of the BKS Ice in the Variability of the MOD over the YHRB Region

Several preceding studies have documented that Meiyu has been susceptible to the Arctic Sea ice conditions, especially in the region from the Barents Sea to the East Siberian Sea [4,32,58]. To address the potential impacts of the Arctic Sea ice on the MOD over the YHRB region, Figure 4a illustrates the spatial distribution of preceding spring SIC anomalies associated with the sign-reversed MODI for the period of 1979–2023, shown with a polar projection. Associated with the earlier-than-normal MOD, the evident sea ice loss is observed in the BKS region, which is similar to previous research on Meiyu [4,32,58]. To facilitate the following analysis, we define the area-averaged spring SIC over the BKS region (68–79° N, 30–70° E, indicated by the green box in Figure 4) as the spring BKSIC index. The original spring BKSIC index exhibits a clear decreasing trend, while the detrended index reveals interannual variability (Figure 2). This spring BKSIC index exhibits a positive correlation of 0.51 with the MODI over the entire study period (Figure 2b), which is statistically significant at the 99% confidence level.

A question arises naturally: how does the spring BKSIC exert effects on the summer MOD over the YHRB region? To address this issue, we investigate the thermal effects linked to the BKSIC loss. Specifically, we present regression analyses of spring net shortwave radiation flux and spring and summer SST with respect to the sign-reversed BKSIC index from 1979 to 2023 (Figure 4b–d), which have been confirmed to be modulated by the sea ice variation [61,62,63,64]. Corresponding to the sea ice loss over the BKS region shown in Figure 4a, the increased open water area leads to a decrease in sea ice albedo, thereby reducing the amount of solar radiation reflected by sea ice. As a result, more solar radiation is absorbed by the ocean, triggering a positive sea ice albedo feedback mechanism that causes an increase in SST (Figure 4b,c) [61,62]. Owing to the long memory of the SST anomalies [64,65], the warmer SST anomalies can persist from spring through to summer (Figure 4d). We define the BKSST index as the area-averaged SST over the same BKS region (68–79° N, 30–70° E, indicated by the green box in Figure 4). The correlation coefficient between spring and summer BKSST indices is 0.60, which is significant at the 99% confidence level. The significant correlation between spring BKSIC and summer BKSST is −0.62, confirming this close relationship, although the intensity of the two indices may vary (Figure 2b).

The warming of SST can elicit a response in regional thermal conditions [64,66]. Figure 5 illustrates the anomalies of the upward longwave radiation flux and air temperature at 850 hPa regressed against the summer BKSST index during the period 1979–2023. The warming BKSST in summer is associated with a corresponding enhancement of the upward longwave radiation flux (Figure 5a). This increase in upward longwave radiation flux leads to an increase in diabatic heating, which in turn warms the atmosphere over the BKS region (Figure 5b). As shown in Figure 6a, pronounced deep warming is observed over the BKS region around 68–79° N, characterized by significantly positive air temperature anomalies extending from 1000 hPa to 400 hPa throughout the troposphere. These changes in thermal conditions further affect atmospheric circulation. Specifically, changes in the meridional air temperature gradient result in zonal westerly anomalies to the north of the BKS region and zonal easterly anomalies to the south (Figure 6b). The zonal wind anomalies extend from the lower troposphere to the middle and upper troposphere, forming a quasi-barotropic atmospheric circulation structure (Figure 6b), thereby facilitating the development of a regional anticyclonic circulation over the BKS region (Figure 7a).

Previous studies have documented the role of SSTs in modulating the MOD variability [40,46,47,48,49]. Thus, the summer BKSST warming, resulting from the spring BKSIC loss, is expected to affect the MOD through influencing atmospheric circulation. To further investigate the impact of the summer BKSST on the atmospheric circulation, the summer BKSST-related simultaneous wave activity flux is examined (Figure 7a). Under the influence of the basic flow, a well-organized atmospheric wave train can be observed, propagating from the BKS region southeastward to the Inner Mongolian Plateau, and subsequently shifting toward the East Asia–Northwest Pacific region in the upper troposphere, characterized by alternating positive and negative anomalies in the geopotential height (Figure 7a). Notably, two huge vectors of wave activity flux are observed from the region near Lake Baikal toward the Korean Peninsula and Japan (Figure 7b), which may be associated with the pronounced westerly basic flow at mid-latitudes [56,67]. Consequently, positive geopotential height anomalies (anticyclonic circulation anomalies) dominate the Northwest Pacific region (Figure 7a), resulting in upper-level anomalous westerlies on the north of the YHRB region, whereas anomalous easterlies appear on the south side, implying the enhancement of the EASJ (Figure 7b). Meanwhile, an anomalous anticyclonic circulation is observed over the Northwest Pacific region in the mid- and lower troposphere, indicating an intensified WNPSH (Figure 7c). Accordingly, the summer BKSST-related atmospheric patterns over the East Asia–Northwest Pacific region closely resemble those associated with an earlier-than-normal MOD, including the significant intensification of the EASJ and the WNPSH (Figure 3 and Figure 7). These patterns are beneficial for the intrusion of warm and moisture airflows into the YHRB region, resulting in an increase in the specific humidity over the YHRB region (Figure 8a). At the same time, the enhanced WNPSH induces positive air temperature advection in the lower troposphere (Figure 8b), thereby warming the air of the lower troposphere (Figure 8c) and increasing the lapse rate, which consequently promotes convective instability over the YHRB region (Figure 8d) [58,59,60], together with the enhancement of the wind shear driven by the intensified EASJ (Figure 7b) [25,57]. Such an atmospheric circulation structure indicates that the summer atmospheric conditions and thermodynamic processes associated with the summer BKSST warming, which results from the preceding spring BKSIC loss, are conducive to an earlier MOD.

4. Conclusions and Discussions

This study has investigated the variability of the MOD and explored the potential role of the precedent spring BKSIC on the MOD for the period 1979–2023. The results indicate that the MOD is predominantly characterized by interannual variability, and a pronounced in-phase relationship is observed between the MOD and the preceding spring BKSIC. Specifically, the reduction in the spring BKSIC can facilitate the advance of the MOD, and vice versa. Furthermore, the strengthening of the EASJ and the WNPSH during summer are identified as the key atmospheric circulation systems that contribute to the earlier MOD.

Further analysis reveals that the BKSIC loss in spring leads to a decrease in the reflection of shortwave radiation, thereby allowing the ocean to absorb more solar radiation. This process results in a warming of the spring SST over the BKS region, which persists into the summer season. Through enhanced upward longwave radiation flux, this BKSST warming induces a significant deep warming over the BKS region during summer. This consequent deep BKS air warming gives rise to a pronounced regional anticyclonic circulation, which may induce the quasi-barotropic anticyclonic circulation over the East Asia–Northwest Pacific region by an atmospheric wave train that propagates southeastward from the BKS region towards the East Asia–Northwest Pacific region. The anticyclonic circulation over the East Asia–Northwest Pacific region facilitates the enhancement of the EASJ and the WNPSH, both of which are associated with an early MOD in the YHRB region. Furthermore, the intensified WNPSH promotes the transport of warm and humid airflows into the YHRB region, resulting in local low-level warming and increased humidity. Combined with enhanced wind shear driven by the intensified EASJ, these conditions promote convective instability and finally contribute to the earlier MOD. It is noteworthy that the additional composite analysis has also yielded results that are virtually identical to those obtained from the regression analysis. This serves as a robust corroboration of the above physical mechanism.

In summary, the results of this study provide valuable information for further understanding the variability of the MOD, and they also hold significant implications for disaster prevention and mitigation over the YHRB region. However, the contribution of spring BKSIC to the MOD and the underlying physical mechanisms necessitate further validation through targeted numerical simulations employing sensitivity experiments. Additionally, given that the spring BKSIC may partly account for the variability of the MOD, as suggested by the inconsistency between the original BKSIC and MOD trends (Figure 2a), the potential impacts of other factors on the MOD also need more in-depth investigation in future research.