Sensitivity of the atmospheric response to sea-surface temperature forcing in the South West Indian Ocean: A regional climate modelling study

The MM5 regional climate model has been used to investigate the sensitivity of the atmospheric response to sea-surface temperature (SST) forcing in the South West Indian Ocean. Two model runs were analysed and compared against each other; namely, one in which the model was forced by an observed warm SST anomaly during a summer season with above-average rainfall over southern Africa, and the other in which the model was forced with a smoothed representation of this anomaly but with the centre shifted closer to the east coast of South Africa. The latter experiment was motivated by correlation analyses between rainfall and SST and by previous experiments with coarser-resolution global circulation models, which suggest that the model response over the land is larger if the SST forcing is shifted closer to it. Analysis of the differences in the model response between the two runs suggests that, consistent with the global models, the MM5 response is indeed larger over southern Africa and more conducive to above-average rainfall in the experiment with the smoothed and westward shifted SST forcing. Increased evaporation over the South West Indian Ocean, local uplift and enhanced moisture flux westwards into southern Africa (as well as southwards over the land from the equatorial region) all play a role in enhancing the regional atmospheric conditions favourable for rainfall over a large area of southern Africa during the season simulate

Chapter 10 - Detection and attribution of climate change: From global to regional

This chapter assesses the causes of observed changes assessed in Chapters 2 to 5 and uses understanding of physical processes, climate models and statistical approaches. The chapter adopts the terminology for detection and attribution proposed by the IPCC good practice guidance paper on detection and attribution (Hegerl et al., 2010) and for uncertainty Mastrandrea et al. (2011). Detection and attribution of impacts of climate changes are assessed by Working Group II, where Chapter 18 assesses the extent to which atmospheric and oceanic changes influence ecosystems, infrastructure, human health and activities in economic sectors

Idealized SST anomaly regional climate model experiments: a note of caution

To date, a number of studies have focused on the influence of sea surface temperature (SST) on global and regional rainfall variability, with the majority of these focusing on certain ocean basins e.g. the Pacific, North Atlantic and Indian Ocean. In contrast, relatively less work has been done on the influence of the central South Atlantic, particularly in relation to rainfall over southern Africa. Previous work by the authors, using reanalysis data and general circulation model (GCM) experiments, has suggested that cold SST anomalies in the central southern Atlantic Ocean are linked to an increase in rainfall extremes across southern Africa. In this paper we present results from idealised regional climate model (RCM) experiments forced with both positive and negative SST anomalies in the southern Atlantic Ocean. These experiments reveal an unexpected response of rainfall over southern Africa. In particular it was found that SST anomalies of opposite sign can cause similar rainfall responses in the model experiments, with isolated increases in rainfall over central southern Africa as well as a large region of drying over the Mozambique Channel. The purpose of this paper is to highlight this finding and explore explanations for the behaviour of the climate model. It is suggested that the observed changes in rainfall might result from the redistribution of energy (associated with upper level changes to Rossby waves) or, of more concern, model error, and therefore the paper concludes that the results of idealised regional climate models forced with SST anomalies should be viewed cautiously

Modulation of the Southern Africa precipitation response to the El Niño Southern Oscillation by the subtropical Indian Ocean Dipole

The climate of Southern Africa, defined as the land area bound by the region 15°S–35°S; 12.5°E–42.5°E, during the December–March rainy season is driven by Indo-Pacific sea surface temperature (SST) anomalies associated with the El Niño Southern Oscillation (ENSO) and the Subtropical Indian Ocean Dipole (SIOD). The observed December–March 1979–2014 Southern Africa precipitation during the four ENSO and SIOD phase combinations suggests that the phase of the SIOD can disrupt or enhance the Southern Africa precipitation response to ENSO. Here, we use a large ensemble of model simulations driven by global SST and ENSO-only SST to test whether the SIOD modifies the relationship between Southern Africa precipitation and ENSO. Since ENSO-based precipitation forecasts are used extensively over Southern Africa, an improved understanding of how other modes of SST variability modulate the regional response to ENSO is important. ENSO, in the absence of the SIOD, forces an equivalent barotropic Rossby wave over Southern Africa that modifies the regional mid-tropospheric vertical motions and precipitation anomalies. El Niño (La Niña) is related with high (low) pressure over Southern Africa that produces anomalous mid-tropospheric descent (ascent) and decreases (increases) in precipitation relative to average. When the SIOD and ENSO are in opposite phases, the SIOD compliments the ENSO-related atmospheric response over Southern Africa by strengthening the regional equivalent barotropic Rossby wave, anomalous mid-tropospheric vertical motions and anomalous precipitation. By contrast, when the SIOD and ENSO are in the same phase, the SIOD disrupts the ENSO-related atmospheric response over Southern Africa by weakening the regional equivalent barotropic Rossby wave, anomalous mid-tropospheric vertical motions and anomalous precipitation