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A Moist Static Energy Budget–Based Analysis of the Sahel Rainfall Response to Uniform Oceanic Warming

Hill, Spencer A. and Ming, Yi and Held, Isaac M. and Zhao, Ming (2017) A Moist Static Energy Budget–Based Analysis of the Sahel Rainfall Response to Uniform Oceanic Warming. Journal of Climate, 30 (15). pp. 5637-5660. ISSN 0894-8755. https://resolver.caltech.edu/CaltechAUTHORS:20170803-071257164

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Abstract

Climate models generate a wide range of precipitation responses to global warming in the African Sahel, but all that use the NOAA Geophysical Fluid Dynamics Laboratory AM2.1 model as their atmospheric component dry the region sharply. This study compares the Sahel’s wet season response to uniform 2-K SST warming in AM2.1 using either its default convective parameterization, relaxed Arakawa–Schubert (RAS), or an alternate, the University of Washington (UW) parameterization, using the moist static energy (MSE) budget to diagnose the relevant mechanisms. UW generates a drier, cooler control Sahel climate than does RAS and a modest rainfall increase with SST warming rather than a sharp decrease. Horizontal advection of dry, low-MSE air from the Sahara Desert—a leading-order term in the control MSE budget with either parameterization—is enhanced with oceanic warming, driven by enhanced meridional MSE and moisture gradients spanning the Sahel. With RAS, this occurs throughout the free troposphere and is balanced by anomalous MSE import through anomalous subsidence, which must be especially large in the midtroposphere where the moist static stability is small. With UW, the strengthening of the meridional MSE gradient is mostly confined to the lower troposphere, due in part to comparatively shallow prevailing convection. This necessitates less subsidence, enabling convective and total precipitation to increase with UW, although both large-scale precipitation and precipitation minus evaporation decrease. This broad set of hydrological and energetic responses persists in simulations with SSTs varied over a wide range.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1175/JCLI-D-16-0785.1DOIArticle
http://journals.ametsoc.org/doi/10.1175/JCLI-D-16-0785.1PublisherArticle
ORCID:
AuthorORCID
Hill, Spencer A.0000-0001-8672-0671
Zhao, Ming0000-0002-4258-9517
Additional Information:© 2017 American Meteorological Society. This article is licensed under a Creative Commons Attribution 4.0 license (http://creativecommons.org/licenses/by/4.0/). Manuscript received 31 October 2016, in final form 6 March 2017. Published online 26 June 2017. We thank Bill Boos, Usama Anber, and Kirsten Findell for their insightful reviews of earlier drafts and three anonymous reviewers. We thank Leo Donner for scientific guidance, Spencer Clark for guidance on computational procedures, and Lucas Harris for guidance on numerical techniques and model conservation properties. S.A.H. was supported during the majority of this work by a Department of Defense National Defense Science and Engineering Graduate Fellowship at Princeton University and by a National Science Foundation Postdoctoral Research Fellowship during the final stages.
Funders:
Funding AgencyGrant Number
National Defense Science and Engineering Graduate (NDSEG) FellowshipUNSPECIFIED
NSF Postdoctoral FellowshipUNSPECIFIED
Subject Keywords:Energy transport; Monsoons; Precipitation; Climate change; Climate models; Convective parameterization
Issue or Number:15
Record Number:CaltechAUTHORS:20170803-071257164
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20170803-071257164
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:79808
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:05 Aug 2017 21:06
Last Modified:03 Oct 2019 18:24

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