Asymmetric Relationship Between Mid-latitude Eurasian Circulation and Summer Rainfall in Hong Kong in Different Phases of ENSO

Yana Li,Ho-Nam Cheung,Wen Zhou

doi:10.3389/feart.2021.642588

Yana Li, Ho-Nam Cheung + Show 1 more

Open Access

https://doi.org/10.3389/feart.2021.642588

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Abstract

During the period 1979–2019, the interannual variation of summer rainfall in Hong Kong (HK), located on the South China coast, is weakly correlated with tropical forcing, including the El Niño/Southern Oscillation (ENSO). Instead, HK summer rainfall is strongly correlated with the mid-latitude circulation over the Urals and the preceding spring sea surface temperature (SST) over the North Atlantic (SST-Atl). The above relationship is stronger in negative ENSO summers, where the SST-Atl anomaly tends to persist from spring to summer. The persistence of the warm SST-Atl anomaly is associated with a Rossby wave train propagating from the North Atlantic to East Asia, with a low over the Urals and a high over the high latitudes of Asia. Correspondingly, the upper-tropospheric westerly jet in East Asia becomes stronger and shifts southward toward South China. The enhanced westerly wind over South China is accompanied by an anomalous Philippine Sea anticyclone, which transports more water vapor to the South China coast and causes more rainfall in HK. On the other hand, during positive ENSO summers, HK summer rainfall is affected by variation in the subtropical westerly jet over South China, which is related to water vapor transport from the Indian Ocean and Bay of Bengal. This is also associated with a height anomaly over northeastern China and the spring sub-polar North Atlantic SST. Therefore, it is important to investigate the impact of mid-latitude forcing on summer rainfall on the South China coast.

Highlights

Hong Kong (HK) is a subtropical city with hot and humid weather in the boreal summer and cold and dry weather in the boreal winter
HK is located adjacent to the South China Sea, the interannual variation of HK summer rainfall during the period
From a forecasting perspective, HK summer rainfall cannot be well predicted by the El Niño/Southern Oscillation (ENSO) signal, which is an important precursor of East Asian summer monsoon intensity on interannual timescales

Summary

INTRODUCTION

Hong Kong (HK) is a subtropical city with hot and humid weather in the boreal summer and cold and dry weather in the boreal winter. Whereas the dipole anomaly over the North Atlantic associated with SST-Atl weakens and moves eastward, it persistently affects the East Asian circulation via the aforementioned two wave trains, especially the highlatitude wave train (Figures 9B–D) This corresponds to the persistence of a negative height anomaly near the Barents Sea and the Ural Mountains. Warmer spring SST-Atl-P is associated with slightly above-normal rainfall east of the Philippine Sea and a weak cyclonic flow over the western North Pacific and the South China Sea (Figure 8C) This suggests that the spring North Atlantic SST in positive Niño-3.4 years does not modulate HK summer rainfall via enhancing water vapor transport associated with the Philippine Sea anticyclone. The stratospheric signal cannot well explain the large-scale circulation anomalies associated with the HK summer rainfall anomaly (figures not shown)

SUMMARY AND DISCUSSION

Findings

DATA AVAILABILITY STATEMENT