August 2025:
New publication:
Cai, W., C. Reason, E. Mohino, B. Rodr铆guez de Fonseca, J. Malherbe, A. Santoso, X. Li,
H. Chikoore, H. C. Nnamchi, M. J. McPhaden, N. Keenlyside, A. Taschetto, L. Wu, B.
Ng, Y. Liu, T. Geng, K. Yang, G. Wang, F. Jia, X. Lin, S. Li, Y. Yan, J. Wang, L. Zhang,
Z. Li, W. Pokam, L Zhou, X. Zhang, F. Engelbrecht (2025). Impact of El Ni帽o-Southern
Oscillation on African climate. Nature Reviews Earth & Environment. 6, 503520.
.
Abstract:
The El Ni帽o鈥揝outhern Oscillation (ENSO) 鈥� describing shifts between warm El Ni帽o and cold La Ni帽a phases 鈥� has a substantial effect on the global climate. In this Review, we outline the mechanisms and climate impacts of ENSO in Africa, focusing on rainfall. ENSO鈥檚 influence varies strongly by season, region, phase, event and decade, highlighting complex dynamics and asymmetries. Although difficult to generalize, key characteristics include: anomalies across the Sahel in July鈥揝eptember, related to the tropospheric temperature mechanism; a strong dipole in anomalies between eastern and southern Africa during October鈥揇ecember (the short rain reason) and December鈥揊ebruary, linked to interactions with the Indian Ocean Dipole and Indian Ocean Basin mode, respectively; and anomalies over southern Africa (with possible indications of opposite anomalies over East Africa) during March鈥揗ay (the long rain season), associated with continuation of the Indian Ocean Basin mode. These teleconnections tend to be most pronounced for East Pacific El Ni帽o and Central Pacific La Ni帽a events, as well as during decades when interbasin interactions are strongest. Although challenging to simulate, climate models suggest that these impacts will strengthen in the future, manifesting as an increased frequency of ENSO-related dry and wet extremes. Given the reliance of much of Africa on rain-fed agriculture, resolving these relationships is vital, necessitating realistic simulation of regional circulations, ENSO and its interbasin interactions.
Mechanisms of impact of the El Ni帽o鈥揝outhern Oscillation (ENSO) on African climate
a, Schematic representation of the tropospheric temperature mechanism, whereby El Ni帽o triggers a Gill鈥揗atsuno response, the atmospheric Kelvin waves (eastward arrows) of which increase atmospheric stability outside the Pacific, suppressing atmospheric convection (and thereby reducing rainfall) over the Sahel (blue downward arrows). b, Schematic representation of the Indian Ocean influence in September鈥揙ctober鈥揘ovember (SON), wherein El Ni帽o weakens the Walker circulation (grey arrows), leading to a positive Indian Ocean Dipole (IOD) that enhances short rains in East Africa through increasing convergence toward East Africa. c, As in panel b, but during December鈥揓anuary鈥揊ebruary (DJF) when El Ni帽o-related Walker circulation weakening causes Indian Ocean Basin mode (IOB) warming that reduces the land鈥搊cean pressure gradient, and lowers the southward extension of seasonal flow convergence and the equatorward shift of the South Indian Convergence Zone equatorward compared with the climatology (light versus dark arrows), in turn, increasing rainfall in northeast southern Africa but decreasing it in southeast southern Africa. d, Schematic representation of the Atlantic Ocean influence in June鈥揓uly鈥揂ugust (JJA), whereby El Ni帽o-related weakening of the Walker circulation drives development of an Atlantic Ni帽a that shifts the tropical rain belt northward, decreasing rainfall over the Guinea coast but increasing rainfall over the Sahel; this increased Sahel rainfall competes with rainfall reductions via the tropospheric temperature mechanism in panel a. Thus, El Ni帽o influences African climate through troposphere temperature warming and concurrent modes of variability in the Indian Ocean and the Atlantic oceans.
July 2025:
New publication:
Liu, Y., M. J. McPhaden, W. Cai, Y. Zhang, J. Zhao, H. C. Nnamchi, X. Lin, Z. Li, J-.Y. Yang (2025). Basin-wide and coastal modes of north tropical Atlantic variability have distinct impacts onhurricanes. Communications Earth & Environment. 6, 549. .
Abstract:
Warm sea surface temperature anomalies in the north tropical Atlantic are conducive to increased intensity and frequency of Atlantic hurricanes. The period 2023-2024 saw two consecutive warm events but with distinct anomaly patterns. Here we use observations and model outputs over the past several decades to determine whether there exists inherent diversity in north tropical Atlantic surface temperature spatial structures and impacts. We find two distinctive modes of variability: a basin-wide mode and a coastal mode, underpinned by differing relationships between air-sea heat flux and sea surface temperature anomalies. The basin-wide mode has a stronger influence on Atlantic hurricane activity due to its more westward and persistent anomaly pattern. Since the 1990s, the well-known impact from El Ni帽o-Southern Oscillation on the north tropical Atlantic is felt through its influence on the basin-wide mode. Our results highlight the importance of distinguishing the two distinctive modes in assessing and predicting their impacts.
July 2025:
New publication:
Tian, Q., J.-Y. Yu, H. C. Nnamchi, T. Li, J. Li, L. Zhang, X. Li (2025). Unraveling the mystery of recent shortened response time of ENSO to Atlantic forcing. Nature Communications. 16, 5884.
Abstract:
The El Ni帽o-Southern Oscillation (ENSO) is known to respond to tropical Atlantic (TA) sea surface temperature (SST) forcing. However, the response time of ENSO to the TA SST forcing is not stationary but varies over decades, the reasons for which remain poorly understood. Here we show that decadal changes in ENSO鈥檚 response time to TA SST forcing are primarily influenced by the south-north shift of the dominant mode of TA SST variability itself. Before the mid-1980s, the southward-shifted TA mode prolongs the response time to ~20 months through an eastward-propagating mid-latitude teleconnection. In contrast, when the TA mode shifts northward after the mid-1980s, the response time decreases to 6鈥�9 months via a faster westward-propagating subtropical teleconnection. Our findings underscore the importance of considering the meridional shift of the TA mode when understanding the impacts of the TA SST variability on ENSO, which has profound implications for ENSO forecasting.
Schematic diagram illustrating the physical mechanisms driving the changes in the tropical Atlantic (TA)鈭扙l Ni帽o-Southern Oscillation (ENSO) relationship.
During 1960鈥�1984 (P1), when spring-to-summer TA mode moves southward, the South Tropical Atlantic (STA) warming can force South Pacific Oscillation (SPO)-like atmospheric circulation anomalies by an eastward-propagating Mid-latitude Rossby wave, which can then induce a South Pacific Meridional Mode (SPMM)-like sea surface temperature (SST) footprint during the winter of the first year. The SPMM interacts with the trade winds and extends negative SST anomalies equatorward into the equatorial Pacific through the wind鈥揺vaporation鈥揝ST (WES) feedback, leading to an eastern Pacific (EP)-type La Ni帽a event over the equatorial Pacific during the winter of the second year. During 1990鈥�2014 (P2), when spring-to-summer TA mode moves northward, the North Tropical Atlantic (NTA) warming can quickly initiate a central Pacific (CP)-type La Ni帽a event through a westward-propagating subtropical Rossby wave during the winter of the first year.
