Abstract
The simulation of Sahel rainfall and its onset during the West African Monsoon (WAM) remains a challenge for current state-of-the-art climate models due to their persistent biases, especially in the tropical Atlantic region. Here we show that improved representation of Atlantic Cold Tongue (ACT) development is essential for a more realistic seasonal evolution of the WAM, which is due to a further inland migration of the precipitation maximum. The observed marked relationship between ACT development and Sahel rainfall onset only can be reproduced by a climate model, the Kiel Climate Model (KCM), when sufficiently high resolution in its atmospheric component is employed, enabling enhanced equatorial Atlantic interannual sea surface temperature variability in the ACT region relative to versions with coarser atmospheric resolution. The ACT/Sahel rainfall relationship in the model critically depends on the correct seasonal phase-locking of the interannual variability rather than on its magnitude. We compare the KCM results with those obtained from climate models participating in the Coupled Model Intercomparison Project phase 5 (CMIP5).
Highlights
The simulation of Sahel rainfall and its onset during the West African Monsoon (WAM) remains a challenge for current state-of-the-art climate models due to their persistent biases, especially in the tropical Atlantic region
With a set of fully coupled climate model simulations, we show that a more realistic development of the Atlantic Cold Tongue (ACT) significantly improves the seasonal evolution of the West African Monsoon (WAM), and strongly influences the onset of Sahel rainfall and its intraseasonal variability
The influence of the slowly varying upper tropical Atlantic ocean dynamics on the atmosphere is shown in observational records[21,22] and potentially allows useful short-term forecasts of the timing of the WAM onset[23]
Summary
The simulation of Sahel rainfall and its onset during the West African Monsoon (WAM) remains a challenge for current state-of-the-art climate models due to their persistent biases, especially in the tropical Atlantic region. The maximum continental warming follows the solar heating northward, whereas the maximum cooling of the ocean surface, driven by meridional and zonal wind stress, is geographically fixed just south of the Equator These different physical processes combined with land-air interactions[3,4] lead to a more step-wise than gradual onset of the WAM5. Due to the strong coupling of tropical Atlantic SSTs to the meridional atmospheric pressure gradient and the seasonal evolution of the WAM, the biases severely limit the application of current coupled models for questions regarding Sahel rainfall, both for explaining the past and predicting the future[14]. Climate model projections of Sahel rainfall for the twenty-first century are highly uncertain, disagreeing even on the sign of future trends[15,16,17]
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