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Yale University (2017)

Thermodynamic and Energetic Views of Sahel Monsoon Rainfall and Shallow Meridional Flow

Shekhar, Ravi

Titre : Thermodynamic and Energetic Views of Sahel Monsoon Rainfall and Shallow Meridional Flow

Auteur : Shekhar, Ravi

Université de soutenance  : Yale University.

Grade : Doctor of Philosophy (PhD) 2017

Résumé partiel
Rainfall over the African Sahel region exhibited great interannual and interdecadal variability over the late twentieth century. This has been previously linked to the global distribution of sea surface temperature (SST), the shallow meridional circulation over the Sahara desert, and other local factors such as surface albedo. Some previous studies have suggested the association of Sahel precipitation variability with remote SST forcing can be explained through the vertically integrated atmospheric energy budget. Others take the convective quasi-equilibrium (CQE) view that the thermodynamic coupling between the near-surface subcloud layer and the upper troposphere can be used to diagnose Sahel precipitation. The Saharan shallow meridional circulation (Saharan SMC) has also been implicated as a cause of Sahel precipitation variability at a broad range of time scales. In this thesis, through three separate studies, I explore the atmospheric energy budget, CQE, and Saharan SMC variability with Sahel precipitation, in an idealized, zonally periodic model of Africa, and in atmospheric reanalyses.

In the first study, I examine CQE and the atmospheric energy budget in an idealized model of West Africa forced with a variety of SST and land albedo anomalies. The energy budget shows mixed success, demonstrating a strong correlation between the ITCZ and the energy flux equator, but a poor correlation between cross equatorial energy transport and ITCZ location. Accounting for tropical and subtropical energy sinks to the atmosphere causing non-monotonicity in the meridional energy transport restores the validity of energy budget theory. CQE misdiagnoses the location of the ITCZ in this model, due to insufficient entrainment of dry environmental air in convective updrafts. Considering a deeper lower-tropospheric layer coupling to the upper troposphere restores the validity of CQE. For both CQE and the energy budget, knowing the forcing is insufficient to predict how the ITCZ will shift, as the feedbacks are large, unpredictable, and forcing dependent.

Présentation (ProQuest)

Page publiée le 28 mai 2021