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The sensitivity of African easterly waves to eastern tropical Atlantic sea-surface temperatures

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Abstract

The results of two regional atmospheric model simulations are compared to assess the influence of the eastern tropical Atlantic sea-surface temperature maximum on local precipitation, transient easterly waves and the West African summer monsoon. Both model simulations were initialized with reanalysis 2 data (US National Center for Environmental Prediction and Department of Energy) on 15 May 2006 and extended through 6 October 2006, forced by synchronous reanalysis 2 lateral boundary conditions introduced four times daily. One simulation uses 2006 reanalysis 2 sea-surface temperatures, also updated four times daily, while the second simulation considers ocean forcing absent the sea-surface temperature maximum, achieved here by subtracting 3°K at every ocean grid point between 0° and 15°N during the entire simulation. The simulation with 2006 sea-surface temperature forcing produces a realistic distribution of June–September mean precipitation and realistic westward propagating swaths of maximum rainfall, based on validation against Tropical Rainfall Measuring Mission (TRMM) estimates. The simulation without the sea-surface temperature maximum produces only 57% of the control June–September total precipitation over the eastern tropical Atlantic and about 83% of the Sahel precipitation. The simulation with warmer ocean temperatures generates generally stronger circulation, which in turn enhances precipitation by increasing moisture convergence. Some local precipitation enhancement is also attributed to lower vertical thermal stability above the warm water. The study shows that the eastern tropical Atlantic sea-surface temperature maximum enhances the strength of transient easterly waves and broadens the spatial extent of associated precipitation. However, large-scale circulation and its interaction with the African continent, and not sea-surface temperatures, control the timing and trajectories of the waves.

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Acknowledgments

This research was supported by National Science Foundation grants AGS-0652518 and AGS- 0652518, National Aeronautics and Space Administration (NAMMA) grant NNX07A193G and the National Aeronautics and Space Administration Climate and Earth Observing System Program. TRMM data were acquired using the GES DISC Interactive Online Visualization and Analysis Infrastructure (Giovanni) as part of NASA’s Goddard Earth Sciences (GES) Data and Information Services Center (DISC). NCEP reanalysis 2 data were obtained online from the National Oceanographic and Atmospheric Agency/Earth System Research Laboratory (Physical Sciences Division). We gratefully acknowledge the three anonymous reviewers who made suggestions that contributed significantly to the final version of the paper.

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  1. Center for Climate Systems Research, Columbia University and the NASA/Goddard Institute for Space Studies, 2880, Broadway, New York, NY, 10025, USA

    Leonard M. Druyan & Matthew Fulakeza

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  1. Leonard M. Druyan
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  2. Matthew Fulakeza
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Correspondence to Leonard M. Druyan.

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Responsible editor: J. Fasullo.

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Druyan, L.M., Fulakeza, M. The sensitivity of African easterly waves to eastern tropical Atlantic sea-surface temperatures. Meteorol Atmos Phys 113, 39–53 (2011). https://doi.org/10.1007/s00703-011-0145-9

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