Interannual Variability of Regional Hadley Circulation Intensity Over Western Pacific During Boreal Winter and Its Climatic Impact Over Asia‐Australia Region

Abstract

This study investigates interannual variability of boreal winter regional Hadley circulation over western Pacific (WPHC) and its climatic impacts. A WPHC intensity index (WPHCI) is defined as the vertical shear of the divergent meridional winds. It shows that WPHCI correlates well with the El Niño–Southern Oscillation (ENSO). To investigate roles of the ENSO‐unrelated part of WPHCI (WPHCIres), variables that are linearly related to the Niño‐3 index have been removed. It reveals that meridional sea surface temperature gradient over the western Pacific plays an essential role in modulating the WPHCIres. The climatic impacts of WPHCIres are further investigated. Below‐normal (above‐normal) precipitation appears over south China (North Australia) when WPHCIres is stronger. This is due to the marked convergence (divergence) anomalies at the upper troposphere, divergence (convergence) at the lower troposphere, and the accompanied downward (upward) motion over south China (North Australia), which suppresses (enhances) precipitation there. In addition, a pronounced increase in surface air temperature (SAT) appears over south and central China when WPHCIres is stronger. A temperature diagnostic analysis suggests that the increase in SAT tendency over central China is primarily due to the warm zonal temperature advection and subsidence‐induced adiabatic heating. In addition, the increase in SAT tendency over south China is primarily contributed by the warm meridional temperature advection. Further analysis shows that the correlation of WPHCIres with the East Asian winter monsoon (EAWM) is weak. Thus, this study may provide additional sources besides EAWM and ENSO to improve understanding of the Asia‐Australia climate variability.

Has the regional Hadley circulation over western Pacific during boreal winter been strengthening in recent decades?

“…In particular, the WPHC during boreal winter is much stronger over the Northern Hemisphere (NH) than that over the Southern Hemisphere (SH), generally consistent with previous studies (e.g. Wang 2002;Zeng et al 2011;Huang et al 2018).…”

“…The asymmetry of the WPHC between the winter and summer hemispheres corresponds to the findings of previous studies (e.g. 2002;Zeng et al 2011;Huang et al 2018). The similar results obtained from Figures 1 and 2 reveal that the approach of the meridional mass streamfunction, proposed by several researchers (e.g.…”

“…Some studies have suggested that there is a robust regional HC in the western Pacific (WPHC) during boreal winter (e.g. Wang 2002;Zeng et al 2011;Huang et al 2018), which could exert significant influences on climate anomalies over East Asia. For instance, Zeng et al (2011) indicated that the anomalously warm SST in the central-eastern tropical Pacific is accompanied by a weakened WPHC, related to a weaker East Asian winter monsoon event.…”

“…Furthermore, investigating the long-term trend of the WPHC intensity is crucial for understanding the trend of the weather and climate over the Asia-Australia region (e.g. Huang et al 2018). The structure of this paper is arranged as follows: The data and methods used are briefly described in section 2.…”

Regional Widening of Tropical Overturning: Forced Change, Natural Variability, and Recent Trends

“…But outgoing longwave radiation is affected not only by changes in the tropical circulation but also by thermodynamic changes and is thus a poor measure of variability and change in the tropical overturning circulation, at least in the zonal mean (Waugh et al, ). The SLP changes associated with the HC are strongest over oceans (Schmidt & Grise, ), even though the tropical overturning circulation has clear impacts over both land and ocean (Chen et al, ; Freitas et al, ; Huang et al, ; Lucas & Nguyen, ; Zhao & Moore, ). For example, all of the world's hot deserts occur in or near subtropics.…”