Simulations of the observed ‘jump’ in the West African monsoon and its underlying dynamics using the MIT regional climate model
The observed seasonal migration of rainfall associated with the West African monsoon (WAM) is characterized by two regimes of relatively intense rainfall: an early, intense peak over the Guinean Coast during late May to early July; and a late, less-intense peak over the Sahel during mid-July to mid-...
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Wiley Blackwell
2018
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| Online Access: | http://hdl.handle.net/1721.1/117277 https://orcid.org/0000-0002-6148-7997 |
| _version_ | 1826197056398557184 |
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| author | Im, Eun Soon Eltahir, Elfatih A. B. |
| author2 | Massachusetts Institute of Technology. Department of Civil and Environmental Engineering |
| author_facet | Massachusetts Institute of Technology. Department of Civil and Environmental Engineering Im, Eun Soon Eltahir, Elfatih A. B. |
| author_sort | Im, Eun Soon |
| collection | MIT |
| description | The observed seasonal migration of rainfall associated with the West African monsoon (WAM) is characterized by two regimes of relatively intense rainfall: an early, intense peak over the Guinean Coast during late May to early July; and a late, less-intense peak over the Sahel during mid-July to mid-September. The transition between these two rainfall regimes occurs relatively quickly around the beginning of July. This quick transition can be described as a ‘jump’ of the WAM into the continent. Eltahir and Gong (1996) proposed a theory for the WAM whereby the solar radiation forcing during the summer shapes a distribution of boundary-layer entropy that peaks over the continent. By assuming a quasi-equilibrium balance between moist convection and the large-scale radiative forcing, the distribution of boundary-layer entropy can be linked to the absolute vorticity at the tropopause. According to this analytical theory, the onset of the monsoon, characterized by the ‘jump’, reflects of a nonlinear shift from a radiative-convective equilibrium regime to an angular momentum conserving regime that would only occur when the value of absolute vorticity in the upper troposphere approaches a threshold of zero. It is because, when the absolute vorticity is significantly different from zero, then the air as a rotating fluid is too rigid to exhibit a meridional overturning. Here, we use the MIT regional climate model (MRCM) to test this theory further and reach a couple of conclusions. First, MRCM succeeds in reproducing the main features of the observed rainfall distribution, including the ‘jump’. Second, analysis of the rainfall, vorticity, entropy, and wind fields simulated by the model reveals a dynamical picture consistent with the proposed theory. |
| first_indexed | 2024-09-23T10:42:08Z |
| format | Article |
| id | mit-1721.1/117277 |
| institution | Massachusetts Institute of Technology |
| last_indexed | 2024-09-23T10:42:08Z |
| publishDate | 2018 |
| publisher | Wiley Blackwell |
| record_format | dspace |
| spelling | mit-1721.1/1172772022-09-27T14:22:29Z Simulations of the observed ‘jump’ in the West African monsoon and its underlying dynamics using the MIT regional climate model Im, Eun Soon Eltahir, Elfatih A. B. Massachusetts Institute of Technology. Department of Civil and Environmental Engineering Parsons Laboratory for Environmental Science and Engineering (Massachusetts Institute of Technology) Im, Eun Soon Eltahir, Elfatih A. B. The observed seasonal migration of rainfall associated with the West African monsoon (WAM) is characterized by two regimes of relatively intense rainfall: an early, intense peak over the Guinean Coast during late May to early July; and a late, less-intense peak over the Sahel during mid-July to mid-September. The transition between these two rainfall regimes occurs relatively quickly around the beginning of July. This quick transition can be described as a ‘jump’ of the WAM into the continent. Eltahir and Gong (1996) proposed a theory for the WAM whereby the solar radiation forcing during the summer shapes a distribution of boundary-layer entropy that peaks over the continent. By assuming a quasi-equilibrium balance between moist convection and the large-scale radiative forcing, the distribution of boundary-layer entropy can be linked to the absolute vorticity at the tropopause. According to this analytical theory, the onset of the monsoon, characterized by the ‘jump’, reflects of a nonlinear shift from a radiative-convective equilibrium regime to an angular momentum conserving regime that would only occur when the value of absolute vorticity in the upper troposphere approaches a threshold of zero. It is because, when the absolute vorticity is significantly different from zero, then the air as a rotating fluid is too rigid to exhibit a meridional overturning. Here, we use the MIT regional climate model (MRCM) to test this theory further and reach a couple of conclusions. First, MRCM succeeds in reproducing the main features of the observed rainfall distribution, including the ‘jump’. Second, analysis of the rainfall, vorticity, entropy, and wind fields simulated by the model reveals a dynamical picture consistent with the proposed theory. 2018-08-06T15:45:26Z 2018-08-06T15:45:26Z 2018-03 2017-09 2018-08-01T13:28:57Z Article http://purl.org/eprint/type/JournalArticle 0899-8418 1097-0088 http://hdl.handle.net/1721.1/117277 Im, Eun-Soon and Elfatih A. B. Eltahir. “Simulations of the Observed ‘jump’ in the West African Monsoon and Its Underlying Dynamics Using the MIT Regional Climate Model.” International Journal of Climatology 38, 2 (July 2017): 841–852 © 2017 The Authors https://orcid.org/0000-0002-6148-7997 http://dx.doi.org/10.1002/JOC.5214 International Journal of Climatology Creative Commons Attribution-NonCommercial 4.0 International http://creativecommons.org/licenses/by-nc/4.0/ application/pdf Wiley Blackwell Royal Meteorological Society |
| spellingShingle | Im, Eun Soon Eltahir, Elfatih A. B. Simulations of the observed ‘jump’ in the West African monsoon and its underlying dynamics using the MIT regional climate model |
| title | Simulations of the observed ‘jump’ in the West African monsoon and its underlying dynamics using the MIT regional climate model |
| title_full | Simulations of the observed ‘jump’ in the West African monsoon and its underlying dynamics using the MIT regional climate model |
| title_fullStr | Simulations of the observed ‘jump’ in the West African monsoon and its underlying dynamics using the MIT regional climate model |
| title_full_unstemmed | Simulations of the observed ‘jump’ in the West African monsoon and its underlying dynamics using the MIT regional climate model |
| title_short | Simulations of the observed ‘jump’ in the West African monsoon and its underlying dynamics using the MIT regional climate model |
| title_sort | simulations of the observed jump in the west african monsoon and its underlying dynamics using the mit regional climate model |
| url | http://hdl.handle.net/1721.1/117277 https://orcid.org/0000-0002-6148-7997 |
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