Abstract
The Botswana High is a prominent mid-tropospheric system that modulates rainfall over subtropical southern Africa, but the capability of a global climate model (GCM) to reproduce it remains unknown. This study examines the capability of a GCM with quasi-uniform resolution (Model Prediction Across Scales, hereafter MPAS) in simulating the characteristics of the Botswana High. The MPAS is applied to simulate the global climate at 240 km quasi-uniform resolution over the globe for the period 1980–2010. The model results are validated against gridded observation dataset (Climate Research Unit, CRU), satellite dataset (Global Precipitation Climatology Project, GPCP), and reanalysis datasets (Climate Forecast System Reanalysis, CFSR; the National Oceanic and Atmospheric Administration, NOAA; and the European Centre for Medium-Range Weather Forecasts version 5, ERA5). In general, MPAS replicates all the essential features in the climatology of temperature, rainfall, 500 hPa geopotential height and vertical motion over southern Africa, reproduces the spatial and temporal variation of the Botswana High, and captures the influence of the Botswana High on droughts and deep convections over the sub-continent. In addition, the model reproduces well the anomalies in vertical motion over subtropical southern Africa during +ve and −ve phases of the Botswana High. However, the model struggles to reproduce the precipitation pattern associated with the positive and negative modes of the Botswana High. The results of this study have an application in understanding the characteristics of the Botswana High and in improving MPAS for seasonal forecasting over southern Africa.
Highlights
Subtropical highs are semi-permanent features over the subtropics during summer and are known to influence tropical cyclone tracks (Stowasser et al 2007; Wu et al 2005; Kasahara 1959) and play a major role in the formation of the worlds subtropical deserts and zones of Mediterranean climate (Miyasaka and Nakamura 2010; Wu et al 2009; Rodwell and Hoskins 2001)
This section discusses how well MPAS reproduces the climatology over southern Africa with a focus on the spatial distribution of temperature (Figs. 2a, 2c and 2e), rainfall (Figs. 2b, 2d and 2f) and 500 hPa geopotential height (Fig. 3)
The model shows a cold bias over the land, more especially over Botswana, Angola, Zambia and western parts of Tanzania. This bias, which features in ERA5 results, may be due to how the MPAS resolves the topography over these high elevation areas
Summary
Subtropical highs are semi-permanent features over the subtropics during summer and are known to influence tropical cyclone tracks (Stowasser et al 2007; Wu et al 2005; Kasahara 1959) and play a major role in the formation of the worlds subtropical deserts and zones of Mediterranean climate (Miyasaka and Nakamura 2010; Wu et al 2009; Rodwell and Hoskins 2001). Midlevel (~500hPa) subtropical highs are observed over the Southern Hemisphere subtropical landmasses. These semi-permanent anticyclones are typically located over Bolivia in South America, Botswana/Namibia in southern Africa, and over western Australia and are understood to be thermally induced by condensational heating from tropical regions of high rainfall located to their northeast (Reason 2016). The Botswana High, a prominent feature in the mid‐levels (~500 hPa) over subtropical southern Africa (in austral spring, summer and early autumn), is an example of these semi-permanent anticyclones. Reason (2016) showed that the Botswana High forms around August and strengthens throughout spring and reaches its peak in the late austral summer (January to March). Thereafter, the high becomes weaker in March and dissipates around April. Driver and Reason (2017) linked the changes in the Botswana High locations with variation in precipitation over the Congo Basin and the Intertropical Convergence Zone (ITCZ), which lie northeast of the high
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