A Climatological Study of Extreme Cold Surges along the African Highlands

Abstract

Equatorward-moving cold surges occur along the lee of high terrain during the cold season. Even though the east coast of Africa features high terrain, little research exists on cold surges along the African highlands despite the fact that these surges could have potentially large agricultural and societal effects. This paper examines a 5-yr climatology of the most extreme African-highlands cold surges spanning the 2008–12 period. During these years, 186 cold surges occurred to the lee of the African highlands, with 84 events extending between 30° and 35°S (type 1), 27 extending between 25° and 30°S (type 2), and 75 extending equatorward of 25°S (type 3) based on the 1000–850-hPa thickness pattern. This climatology reveals that extreme African-highlands cold surges have a climatological maximum in September. Cold surges of type 1 and type 2 tend to occur throughout the Southern Hemisphere winter and spring, whereas surges of type 3 are generally confined to the winter months. These cold surges can last from 2 to 8 days, with the highest frequency of events spanning a 3-day period. A typical cold-surge event features maximum 925-hPa meridional flow of 30.0–39.9 kt (1 kt = 0.51 m s−1) that most frequently advects cold Antarctic air to between 15.0° and 24.9°S and at times as far as the equator.

A Climatology of Extreme South American Andean Cold Surges

“…Cold surges are shallow features (often extending from the surface only up to 850 hPa) that are associated with a sharp decline in temperature, an increase in mean sea level pressure, and a shift in winds to an equatorward-directed component on the meso-a to synoptic scale (Colle and Mass 1995). Cold-surge events routinely occur in the lee of the Andes Mountains and Brazilian Highlands in South America (Garreaud 2000), the Rocky Mountains in North America (Colle and Mass 1995), and the Ethiopian Highlands in Africa (Wang and Fu 2004;Crossett and Metz 2017), among other locations. In South America, coldsurge occurrence is not only important economically through crop destruction (Pezza and Ambrizzi 2005), but also may represent a mechanism by which South Pacific Ocean synoptic-scale, midlatitude systems can modulate the troposphere deep into the tropics (Liebmann et al 2009).…”

Day‐To‐Day Quantification of Changes in Global Lightning Activity Based on Schumann Resonances

“…The reason for this imbalanced attention may arise because the Antarctic winter air is colder than Arctic air, and because the protection of coffee plantations during freeze events in Brazil is of substantial economic interest (Marengo et al., 1997). The longitudinally‐confined nature of the polar outbreaks results in lower‐latitude impacts that are sometimes confined to individual continental chimneys (America, Africa, Southeast Asia), with corresponding collections of events in Prince and Evans (2018), Crossett and Metz (2017), Murakami (1979), respectively, or to broader impacts affecting multiple chimneys (Metz et al., 2013) as the equatorward‐moving cold air also advects eastward.…”

July–September rainfall in the Greater Horn of Africa: the combined influence of the Mascarene and South Atlantic highs

“…Studies show variation in the MH region has a positive relationship with Ethiopian JAS rainfall (Degefu et al 2017;Segele et al 2009bSegele et al , 2015Korecha and Barnston 2007), suggesting that MH intensity modulates the cross equatorial flow and moisture flux into Ethiopia (Degefu et al 2017;Korecha and Barnston 2007), and the strength of the Somali Jet (Segele et al 2015;Korecha and Barnston 2007). However, Crossett and Metz (2017) and Vizy and Cook (2020) show that nearer to the surface cross equatorial flow variability can result in cold air surges along the East African coast that inhibit rainfall locally. Segele et al (2009a) also argue that a weakened MH, associated with warmer southern Indian Ocean SSTs, leads to reduced Ethiopian summer rainfall.…”