Systematic Climate Model Rainfall Biases over Southern Africa: Links to Moisture Circulation and Topography

C Munday,R Washington

doi:10.1175/jcli-d-18-0008.1

Abstract

AbstractAn important challenge for climate science is to understand the regional circulation and rainfall response to global warming. Unfortunately, the climate models used to project future changes struggle to represent present-day rainfall and circulation, especially at a regional scale. This is the case in southern Africa, where models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) overestimate summer rainfall by as much as 300% compared to observations and tend to underestimate rainfall in Madagascar and the southwest Indian Ocean. In this paper, we explore the climate processes associated with the rainfall bias, with the aim of assessing the reliability of the CMIP5 ensemble and highlighting important areas for model development. We find that the high precipitation rates in models that are wet over southern Africa are associated with an anomalous northeasterly moisture transport (~10–30 g kg−1 s−1) that penetrates across the high topography of Tanzania and Malawi and into subtropical southern Africa. This transport occurs in preference to a southeasterly recurvature toward Madagascar that is seen in drier models and reanalysis data. We demonstrate that topographically related model biases in low-level flow are important for explaining the intermodel spread in rainfall; wetter models have a reduced tendency to block the oncoming northeasterly flow compared to dry models. The differences in low-level flow among models are related to upstream wind speed and model representation of topography, both of which should be foci for model development.

Highlights

The future evolution of the climate system under unprecedented human forcing is uncertain
A wide spread of model simulations around observations might be indicative of random error that will cancel in a multimodel mean (MMM), whereas a consistent direction of error implies some common structural error that will not cancel in an MMM (Sanderson and Knutti 2012)
To understand why this might be the case, we briefly review the circulation associated with the observed summer rainfall climatology and its variability

Summary

Introduction

The future evolution of the climate system under unprecedented human forcing is uncertain. Understanding the climate processes that underlie climate model errors is a first step toward assessing models’ reliability for projecting future change, and can help to guide future model development This is important in cases where an ensemble of models displays a Denotes content that is immediately available upon publication as open access. In subtropical southern Africa (208–308E, 208–308S), climate models from the most recent phase (phase 5) of the Coupled Model Intercomparison Project (CMIP5; Taylor et al 2012) systematically overestimate climatological summer rainfall (Fig. 1a). We hypothesize that the wetter than observed model climatology in southern Africa is linked to the drier than observed model climatology in Madagascar through a common mechanism To understand why this might be the case, we briefly review the circulation associated with the observed summer rainfall climatology and its variability. The drivers of the Madagascar rainfall maximum have not been fully demonstrated, with a combination of orographic forcing by the ;1500-m-high topography and the warm sea surface temperatures of the southwest Indian Ocean and Mozambique Channel likely to play a role

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Discussion

Conclusion