Diabatic heating governs the seasonality of the Atlantic Ni\xf1o

Hyacinth C Nnamchi,Joakim Kjellsson,Mojib Latif,Noel S Keenlyside,Ingo Richter

doi:10.1038/s41467-020-20452-1

Abstract

The Atlantic Niño is the leading mode of interannual sea-surface temperature (SST) variability in the equatorial Atlantic and assumed to be largely governed by coupled ocean-atmosphere dynamics described by the Bjerknes-feedback loop. However, the role of the atmospheric diabatic heating, which can be either an indicator of the atmosphere’s response to, or its influence on the SST, is poorly understood. Here, using satellite-era observations from 1982–2015, we show that diabatic heating variability associated with the seasonal migration of the Inter-Tropical Convergence Zone controls the seasonality of the Atlantic Niño. The variability in precipitation, a measure of vertically integrated diabatic heating, leads that in SST, whereas the atmospheric response to SST variability is relatively weak. Our findings imply that the oceanic impact on the atmosphere is smaller than previously thought, questioning the relevance of the classical Bjerknes-feedback loop for the Atlantic Niño and limiting climate predictability over the equatorial Atlantic sector.

Highlights

The Atlantic Niño is the leading mode of interannual sea-surface temperature (SST) variability in the equatorial Atlantic and assumed to be largely governed by coupled oceanatmosphere dynamics described by the Bjerknes-feedback loop
It is shown that the seasonal migration of the Inter-Tropical Convergence Zone (ITCZ) modifies the background conditions over the equatorial Atlantic, which strongly influences the development of SST anomalies in this region
The level of interannual equatorial Atlantic SST variability exhibits a marked seasonality with its maximum in boreal summer[1,2,4,11]

Summary

Introduction

The Atlantic Niño is the leading mode of interannual sea-surface temperature (SST) variability in the equatorial Atlantic and assumed to be largely governed by coupled oceanatmosphere dynamics described by the Bjerknes-feedback loop. 1234567890():,; The interannual climate variability in the equatorial Atlantic region is dominated by the Atlantic Niño, referred to as the zonal mode It exhibits a characteristic zonally asymmetric structure in sea surface temperature (SST) and wind stress fluctuations[1,2,3]. According to the Bjerknes-feedback loop[12,13,14], an initial SST anomaly in the eastern equatorial Atlantic alters the zonal SST gradient (dSST/dx), which in turn modifies the vertical profile of the atmospheric diabatic heating through changes in convection, water vapor, cloud cover, and precipitation[15,16] across the equatorial Atlantic. In response to the wind stress anomaly, the zonal slope of the equatorial thermocline is reduced (τx → dh/dx), which is associated with an increase in upper-ocean heat content in the east and a drop in the west. The seasonality of the Atlantic Niño is primarily set by the atmosphere

Methods

Results

Conclusion