Abstract
Neglected High-frequency Variability events of the sea surface Wind Coupled with Precipitation (HV-WCP) events are revealed over the Kuroshio Sea Surface Temperature (SST) front in the East China Sea (ECS) during wintertime. This phenomenon is characterized as modulation events of the surface wind directions, whereby the southerly anomaly changes into a northwest anomaly, accompanied with increasing wind speed, and coupled with decreasing precipitation from morning to night on the day with the peak variability. The events are associated with significant climate effects on the rainfall anomaly over southeastern China and adjacent ocean areas of the ECS. To sustain these events, the large-scale circulation needs to exhibit declines in the upstream monsoon and weakening vertical mixing in the planetary boundary layer (PBL). The Kuroshio thermal effect on the underlying atmosphere becomes prominent due to weakening vertical mixing, which results in pronounced PBL height gradient over the Kuroshio SST front. More plentiful moisture transported from PBL to the free atmosphere, generates the initialization of rainfall and the cyclone anomaly. Meanwhile, the latent heat releasing caused by this precipitation forces positive vorticity anomaly within the PBL, which again strengthens the cyclone and the upward moisture transport. Therefore, a complement between local circulation and rainfall is developed over the Kuroshio region. Due to the significant effect of the Kuroshio SST front on the PBL structure, the HV-WCP intensity is able to maintain its strength over the Kuroshio region. However, because of the northwestern background monsoon, the HV-WCP events vanish quickly when the coupled system moves out of Kuroshio SST frontal region.
Highlights
In midlatitudes, the oceanic fronts with large horizontal gradients in sea surface temperature (SST) have been proved the essential areas where the forcing of the ocean onto atmosphere is pronounced (Chelton et al 2004; Tokinaga et al 2005, 2006; Minobe et al 2008; Small et al 2008; Bryan et al 2010; Chelton and Xie 2010)
We conducted a long-term analysis of atmospheric variations during wintertime, and we found that variabilities in significant high-frequency events of the surface wind field are coupled with large-scale precipitation anomalies over the Kuroshio SST front region
From 02:00 local time (LT) to 20:00 LT on the peak day with the dominant diurnal variabilities, the HV-WCP events have the features that the wind direction changes from southerly anomaly into northwest anomaly, accompanied with increases in wind speed and coupled precipitation which are gradually decreasing
Summary
The oceanic fronts with large horizontal gradients in sea surface temperature (SST) have been proved the essential areas where the forcing of the ocean onto atmosphere is pronounced (Chelton et al 2004; Tokinaga et al 2005, 2006; Minobe et al 2008; Small et al 2008; Bryan et al 2010; Chelton and Xie 2010). Over East Asia, many studies have shown that the initial oscillation mechanism is more effective to explain the diurnal variabilities of the low-level wind and the coupled precipitation events (Chen et al 2009, 2013; Du et al 2012; Du and Rotunno 2014, 2018). These questions have not been well solved This present study is designed to clarify the role of high-frequency variability in the atmospheric events over the Kuroshio SST front in the ECS during wintertime.
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