Impact of tropical Atlantic sea-surface temperature biases on the simulated atmospheric circulation and precipitation over the Atlantic region: An ECHAM6 model study

Astrid Eichhorn,Jürgen Bader

doi:10.1007/s00382-016-3415-x

Abstract

As many coupled atmosphere-ocean general circulation models, the coupled Earth System Model developed at the Max Planck Institute for Meteorology suffers from severe sea-surface temperature (SST) biases in the tropical Atlantic. We performed a set of SST sensitivity experiments with its atmospheric model component ECHAM6 to understand the impact of tropical Atlantic SST biases on atmospheric circulation and precipitation. The model was forced by a climatology of observed global SSTs to focus on simulated seasonal and annual mean state climate. Through the superposition of varying tropical Atlantic bias patterns extracted from the MPI-ESM on top of the control field, this study investigates the relevance of the seasonal variation and spatial structure of tropical Atlantic biases for the simulated response. Results show that the position and structure of the Intertropical Convergence Zone (ITCZ) across the Atlantic is significantly affected, exhibiting a dynamically forced shift of annual mean precipitation maximum to the east of the Atlantic basin as well as a southward shift of the oceanic rain belt. The SST-induced changes in the ITCZ in turn affect seasonal rainfall over adjacent continents. However not only the ITCZ position but also other effects arising from biases in tropical Atlantic SSTs, e.g. variations in the wind field, change the simulation of precipitation over land. The seasonal variation and spatial pattern of tropical Atlantic SST biases turns out to be crucial for the simulated atmospheric response and is essential for analyzing the contribution of SST biases to coupled model mean state biases. Our experiments show that MPI-ESM mean-state biases in the Atlantic sector are mainly driven by SST biases in the tropical Atlantic while teleconnections from other basins seem to play a minor role.

Highlights

The majority of current coupled atmosphere-ocean general circulation models (AOGCMs) suffers from substantial biases in simulating sea-surface temperatures (SSTs) in the tropical Atlantic (TA) in terms of climatological seasonal cycle and climate mean state
As a result of TA SST biases, annual mean precipitation changes across the tropical Atlantic are mainly characterized by an eastward shift of the rainfall maximum, and expansion and southward movement of the zonal-mean Intertropical Convergence Zone (ITCZ)
The TA SST bias pattern leads to a substantial decrease in precipitation over the northwestern tropical Atlantic (NWTA), and an increase across the south-eastern tropical Atlantic (SETA) (Fig. 3)

Summary

Introduction

The majority of current coupled atmosphere-ocean general circulation models (AOGCMs) suffers from substantial biases in simulating sea-surface temperatures (SSTs) in the tropical Atlantic (TA) in terms of climatological seasonal cycle and climate mean state. The bias maximum often exceeds 5 K and is centered at the eastern boundary of the basin in the upwelling region of the NamibianAngolan coast (Richter et al 2012a; Toniazzo and Woolnough 2014; Voldoire et al 2014). The extension of this warm bias spreads towards the equator and covers a large fraction of the basin. In many cases the combination of the aforementioned erroneous SSTs leads to a reversal of the annual mean equatorial SST gradient (Davey et al 2002; DeWitt 2005; Richter and Xie 2008)

Methods

Results

Conclusion