Climate Adjustments over Africa Accompanying the Indian Monsoon Onset

Abstract

Rainfall and circulation changes accompanying the Indian monsoon onset are examined, focusing on the African continent and neighboring areas. The Indian Meteorological Department official monsoon onset dates over Kerala (MOK; on average on 1 June) are used. Composites are formed at a pentad (5 days) time scale to compare pre-and postonset conditions. Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) data for 1979-2007 indicate that a substantial rainfall decrease over several parts of Africa is associated with MOK. Significant rainfall anomalies, after removal of the mean seasonal cycle, are found in eastern Africa and the nearby western Indian Ocean. Indian monsoon onset dates over the period 1958-2001 are correlated at 0.55 with the cessation dates of the March-May rainy season (the long rains) averaged over Kenya and northern Tanzania. The rainy season cessation leads the onset by 12 days, on average. An early cessation of the March-May rains tends to precede an early onset over India. Over East Africa, the rainfall decrease is associated with a strengthening of the southerly winds (Somali jet) shortly before MOK, enhancing wind divergence and wind shear.A weaker rainfall signal is noted over western Africa and the Gulf of Guinea. MOK coincides with a pause in the progression of the West African monsoon. The pause is associated with anomalous descent over the Sahel, culminating two to three pentads after MOK. Composite maps further indicate that MOK is followed over much of the African continent by a sea level pressure rise and, between India and North Africa, a westward propagation of easterly wind and positive geopotential height anomalies. Many of these signals are associated with MaddenJulian oscillations, but independent features, suggesting Rossby wave propagation over North Africa, are also found to connect MOK and West Africa. Overall, these results are indicative of a large-scale adjustment of the atmospheric dynamics across both eastern and western Africa in conjunction with the monsoon onset over India.

Climate and climatic variability of rainfall over eastern Africa

“…It generally coincides with a decrease in rainfall in equatorial Africa and eastern Ethiopia and an increase in rainfall in western Ethiopia [ Riddle and Cook , ]. The end of the long rains is strongly correlated with the onset of the Indian monsoon ( r = 0.55 over 44 years) [ Camberlin et al ., ]. The cessation of the rains is associated with the strengthening of the southerly winds (the Somali Jet), which enhances wind divergence and wind shear [ Camberlin et al ., ], and with the onset of active convection over the Arabian Sea [ Okoola , ].…”

“…Spatial coherence is highest at the peak of the short rains [ Camberlin et al ., ]. Station‐to‐station correlations of interannual time series are weaker in April and in May than for any other rainy months [ Camberlin et al ., ].…”

“…The end of the long rains is strongly correlated with the onset of the Indian monsoon (r = 0.55 over 44 years) [Camberlin et al, 2010]. The cessation of the rains is associated with the strengthening of the southerly winds (the Somali Jet), which enhances wind divergence and wind shear [Camberlin et al, 2010], and with the onset of active convection over the Arabian Sea [Okoola, 1998].…”

“…Spatial coherence and potential predictability via SSTs peak during March. Although more rainfall occurs in April and May, it is much less spatially coherent during those months [Camberlin et al, 2009[Camberlin et al, , 2010Moron et al, 2013]. The zonal circulation and subsidence aloft become progressively weaker from March to May [Nicholson, 2017], and the factors in predictability are notably different in each month [Nicholson, 2015b].…”

The West African Sahel: A Review of Recent Studies on the Rainfall Regime and Its Interannual Variability

“…The onset may be related to the Indian monsoon [46,47]. The associated convection north of 15 ∘ N forces a westward propagating Rossby wave that reaches West Africa some 7 to 15 days later, with the wave passage triggering convection in the Sahel.…”