Climatology and Interannual Variability of Boreal Spring Wet Season Precipitation in the Eastern Horn of Africa and Implications for Its Recent Decline

Abstract

The 1981–2014 climatology and variability of the March–May eastern Horn of Africa boreal spring wet season are examined using precipitation, upper- and lower-level winds, low-level specific humidity, and convective available potential energy (CAPE), with the aim of better understanding the establishment of the wet season and the cause of the recent observed decline. At 850 mb, the development of the wet season is characterized by increasing specific humidity and winds that veer from northeasterly in February to southerly in June and advect moisture into the region, in agreement with an earlier study. Equally important, however, is a substantial weakening of the 200-mb climatological easterly winds in March. Likewise, the shutdown of the wet season coincides with the return of strong easterly winds in June. Similar changes are seen in the daily evolution of specific humidity and 200-mb wind when composited relative to the interannual wet season onset and end, with the easterlies decreasing (increasing) several days prior to the start (end) of the wet season. The 1981–2014 decrease in March–May precipitation has also coincided with an increase in 200-mb easterly winds, with no attendant change in specific humidity, leading to the conclusion that, while high values of specific humidity are an important ingredient of the wet season, the recent observed precipitation decline has resulted mostly from a strengthening of the 200-mb easterlies. This change in the easterly winds appears to be related to an increase in convection over the Indonesian region and in the associated outflow from that enhanced heat source.

Examining the role of unusually warm Indo‐Pacific sea‐surface temperatures in recent African droughts

“…We also plotted the correlation between our African precipitation time series and contemporaneous rainfall of the last 20 years (Figure d–f). We chose to use this shorter time period to guide our selection process because (a) our focus here is on informing improved forecasting and monitoring for the next several SA or EA droughts, thus we are primarily interested in predicting droughts under circumstances similar to current conditions, and (b) the EA MAM rainfall–SST relationship appears to have increased substantially over the past 20 years, presumably due to a stronger Indian Ocean branch of the Walker Circulation (Liebmann et al, ; ). Table lists 1977–1996 and 1996–2016 rainfall correlation values for Niño 3.4, IOD and western Pacific SST.…”

“…The results presented here challenge that assumption, finding strong relationships between WNP SST and recent droughts (Figure f) and decadal rainfall variations (Figure c). While valid arguments for a weak MAM teleconnection can be made based on the weak 1981–2016 correlation between EA MAM rainfall and SST, this correlation has increased substantially in recent years (Liebmann et al ., ), likely due to a strengthening of the Indian Ocean branch of the Walker Circulation (Funk et al, ; Williams and Funk, ; Liebmann et al, ). Another important aspect may be nonlinearities in EA MAM teleconnections; SST composites for the six 1981–2016 EA MAM wet seasons show no significant Indo‐Pacific anomalies.…”

“…Looking aloft, we also find an increase in upper‐level easterly winds. It has recently been advanced (Liebmann et al ., ; ) that an increase in the strength of the Indian Ocean branch of the Walker Circulation might account for the relatively recent increased teleconnections between EA MAM rainfall and Pacific SST (Table ). The increase in upper Figure f winds over the eastern Indian Ocean and EA would be consistent with this hypothesis.…”

“…A third perspective would acknowledge the important (dominant) role played by Pacific SST, but would interpret those SST variations as an interaction between anthropogenic warming in the western equatorial Pacific (WEP) and western North Pacific (WNP), El Niño/Southern Oscillation (ENSO) and PDV Hoell and Funk, ; Funk and Hoell, ; Hoell et al, ). Western Pacific warming is thought to increase influence of La Niña SST (Williams and Funk, ; Hoell et al, ; ; Hoell and Funk, ), and an associated increase in the strength of the Indian Ocean branch of the Walker Circulation may explain the recent marked increase in correlation between observed MAM rainfall and SST (Liebmann et al, ; ). WEP influences are thought to provide one mechanism linking consecutive poor OND and MAM rainy seasons (Hoell and Funk, ).…”

Dynamical and Thermodynamic Elements of Modeled Climate Change at the East African Margin of Convection

“…It should be noted that in the current climate, El Niño–Southern Oscillation (ENSO) does not show a consistent rainfall teleconnection to eastern Africa in MAM (Lyon, ). However, recent studies (Liebmann et al, ; Lyon & Vigaud, ) provide observational evidence that recent drying in East Africa during MAM was associated with an anomalous Walker circulation over the Indian Ocean, with enhanced convection in the western equatorial Pacific and subsidence over East Africa, suggesting that a similar (and opposite‐signed) link to the Pacific is at least a plausible mechanism in the CMIP5 models.…”