Large hemispheric differences in the Hadley cell strength variability due to ocean coupling

Rei Chemke

doi:10.1038/s41612-021-00225-3

Abstract

By modulating the distribution of heat, precipitation and moisture, the Hadley cell holds large climate impacts at low and subtropical latitudes. Here we show that the interannual variability of the annual mean Hadley cell strength is ~ 30% less in the Northern Hemisphere than in the Southern Hemisphere. Using a hierarchy of ocean coupling experiments, we find that the smaller variability in the Northern Hemisphere stems from dynamic ocean coupling, which has opposite effects on the variability of the Hadley cell in the Southern and Northern Hemispheres; it acts to increase the variability in the Southern Hemisphere, which is inversely linked to equatorial upwelling, and reduce the variability in the Northern Hemisphere, which shows a direct relation with the subtropical wind-driven overturning circulation. The important role of ocean coupling in modulating the tropical circulation suggests that further investigation should be carried out to better understand the climate impacts of ocean-atmosphere coupling at low latitudes.

Highlights

The Hadley circulation plays a central role in controlling the meridional distribution of temperature, humidity, and precipitation at low latitudes
We start by assessing the different interannual variability in the annual mean Southern Hemisphere (SH) and Northern Hemisphere (NH) Hadley cell strength (Ψmax, Methods) over recent decades in the Coupled Model Intercomparison Project Phase 5 (CMIP5) and Reanalyses (Methods)
Note that here we focus on the annual mean Hadley cell variability, in order to eliminate the effects of the seasonal cycle on the relation between ocean coupling processes and the Hadley cell strength, to focus only on the effect of the interannual variability of the ocean on the Hadley cell’s variability in the two hemispheres

Summary

Introduction

The Hadley circulation plays a central role in controlling the meridional distribution of temperature, humidity, and precipitation at low latitudes. Changes in the strength and position of the Hadley cell have large climate impacts in tropical and subtropical regions. The interannual variability of the Hadley cell was argued to modulate the equatorial atmospheric heat transport, the location and strength of the Inter-Tropical Convergence Zone (ITCZ), and the activity of tropical cyclones[5,6,7]. On intraannual timescales dynamic ocean coupling (i.e., the effect of changes in ocean heat flux convergence, OHFC) was found to weaken the tropical circulation and to generate the strong asymmetry between the summer and winter tropical circulations and precipitation[9]. In response to the projected sea-ice loss, while thermodynamic ocean coupling was found to expand and weaken the Hadley cell, dynamic coupling was found to contract and strengthen the circulation[12,13,14]

Methods

Results

Conclusion