Interdecadal weakening of the cross-equatorial flows over the Maritime Continent during the boreal summer in the mid-1990s: drivers and physical processes

Xiaoxuan Zhao,Riyu Lu,Buwen Dong

doi:10.1007/s00382-021-05692-1

Abstract

In this study, the cross-equatorial flows (CEF) on both high and low level (HCEF/LCEF) troposphere over the Maritime Continent (MC) in boreal summer are found to have experienced an interdecadal weakening in the mid-1990s based on both JRA55 and NCEP reanalyses. The outputs of 8 coupled models in CMIP6 are used to investigate drivers and the corresponding mechanisms. Model results show that the role of external forcing is weak in the interdecadal weakening of CEF. By contrast, the observed interdecadal weakening of both HCEF and LCEF can be largely explained by internal variability associated with a negative phase of the interdecadal Pacific Oscillation (IPO). Associated with negative IPO are anomalous divergence (convergence), enhanced precipitation over MC and anomalous cyclonic (anticyclonic) circulations, reduced precipitation over western North Pacific (WNP) in the upper (lower) troposphere. Sensitivity experiments based on MetUM-GA6 further manifest that this IPO phase transition can lead to the interdecadal weakening of CEF, in which the central tropical Pacific (CTP) sea surface temperature (SST) anomalies play a dominant role. The cold SST anomalies in CTP lead to reduced local convection and trigger enhanced convection over MC through changes in the Walker circulation. The enhanced convection over MC leads to a change in local Hadley circulation over the western Pacific sector. This change is characterized by anomalous ascents over MC, southerlies in the upper troposphere, descents and reduced precipitation over WNP and northerlies in the lower troposphere, leading to the weakening of CEF. Meanwhile, positive SST anomalies over MC associated with negative IPO also make a contribution to the weakening of CEF by inducing a change in the Hadley circulation in the western Pacific sector through similar processes.

Highlights

In boreal summer, several channels of airflows across the equator from the Southern to Northern Hemisphere can be found within tropical areas in the lower troposphere (Shi et al 2007; Wang and Yang 2008; Li and Li 2014), which are named as the cross-equatorial flows (CEFs)
JRA55 and NCEP reanalyses show that the CEF at both upper and lower troposphere in boreal summer underwent a significant interdecadal weakening during 1975– 2014, characterized by weakening of upper tropospheric northerlies and lower tropospheric southerlies within the western tropical Pacific
Corresponding changes in sea surface temperature (SST) bear a similarity to negative phase of the interdecadal Pacific Oscillation (IPO), suggesting that phase transition of IPO might be responsible for the interdecadal weakening of CEF during the last 40 years in observations

Summary

Introduction

Several channels of airflows across the equator from the Southern to Northern Hemisphere can be found within tropical areas in the lower troposphere (Shi et al 2007; Wang and Yang 2008; Li and Li 2014), which are named as the cross-equatorial flows (CEFs). As one of important components of the East Asian Summer Monsoon (EASM) (Zeng and Li 2002), CEF plays a fundamental role in transporting moisture from the Southern Hemisphere to Northern Hemisphere (Wang and Li 1982; Lau and Li 1984). Strong CEF coincides with reduced (enhanced) rainfall in Yangtze and Huai River Basin (South China) (Han and He 2002; Zhu 2012). It can help maintaining the WNP monsoon trough (Lin and Chou 2014) and modulating the activities of tropical cyclones (Xu 2011; Zhao et al 2012; Feng et al 2017)

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