Tropical Indo-Pacific SST influences on vegetation variability in eastern Africa

In-Won Kim,Jong-Seong Kug,Jin-Soo Kim,Malte F Stuecker,So-Won Park,Elke Zeller,Axel Timmermann

doi:10.1038/s41598-021-89824-x

Abstract

Mechanisms by which tropical Pacific and Indian Ocean sea surface temperatures (SST) influence vegetation in eastern Africa have not been fully explored. Here, we use a suite of idealized Earth system model simulations to elucidate the governing processes for eastern African interannual vegetation changes. Our analysis focuses on Tanzania. In the absence of ENSO-induced sea surface temperature anomalies in the Tropical Indian Ocean (TIO), El Niño causes during its peak phase negative precipitation anomalies over Tanzania due to a weakening of the tropical-wide Walker circulation and anomalous descending motion over the Indian Ocean and southeastern Africa. Resulting drought conditions increase the occurrence of wildfires, which leads to a marked decrease in vegetation cover. Subsequent wetter La Niña conditions in boreal winter reverse the phase in vegetation anomalies, causing a gradual 1-year-long recovery phase. The 2-year-long vegetation decline in Tanzania during an ENSO cycle can be explained as a double-integration of the local rainfall anomalies, which originate from the seasonally-modulated ENSO Pacific-SST forcing (Combination mode). In the presence of interannual TIO SST forcing, the southeast African precipitation and vegetation responses to ENSO are muted due to Indian Ocean warming and the resulting anomalous upward motion in the atmosphere.

Highlights

Natural fluctuations in Africa’s vegetation are affected by rainfall variability[1,2,3,4,5,6,7]
Both the “Periodic” and the “Pacific” experiments (SST anomalies are only prescribed in the tropical Pacific) show pronounced Walker circulation changes between El Niño and La Niña events with anomalous ascending motion over the eastern Pacific region and anomalous descending motion during the peak El Niño-Southern Oscillation (ENSO) phase of December–January–February [D(0)JF(1)] (Fig. 1a,b)
The pattern of large-scale atmospheric anomalies in the Tropics experiment (Fig. 1c) is more consistent with the observations (Fig. 1d) than the Periodic and Pacific experiments (Fig. 1a, b). This suggests that Tropical Indian Ocean (TIO) warming affects the change of the large-scale atmospheric circulation around the African continent related to ENSO, as suggested previously[28]

Summary

Introduction

Natural fluctuations in Africa’s vegetation are affected by rainfall variability[1,2,3,4,5,6,7]. Regarding the LAI response to ENSO at this 16-18 months lag (that is, in May-June-July in year 2 after the ENSO event peak time: MJJ(2)), we find larger negative anomalies over Tanzania in the Pacific and the Periodic experiments (Fig. 2c,f), while they are much weaker anomalies in the Tropics experiment and the observations (Fig. 2i,l).

Results

Conclusion