A double ITCZ phenomenology of wind errors in the equatorial Atlantic in seasonal forecasts with ECMWF models
Modern coupled general circulation models produce systematic biases in the tropical Atlantic that hamper the reliability of long-range predictions. This study focuses on a common springtime westerly wind bias in the equatorial Atlantic in seasonal hindcasts from two coupled models – ECMWF System 4 a...
| Autores principales: | , , |
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| Formato: | Journal Article |
| Lenguaje: | Inglés |
| Publicado: |
Copernicus GmbH
2019
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| Acceso en línea: | https://hdl.handle.net/10568/103620 |
| _version_ | 1855515930620067840 |
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| author | Shonk, Jonathan KP Demissie, Teferi Dejene Toniazzo, Thomas |
| author_browse | Demissie, Teferi Dejene Shonk, Jonathan KP Toniazzo, Thomas |
| author_facet | Shonk, Jonathan KP Demissie, Teferi Dejene Toniazzo, Thomas |
| author_sort | Shonk, Jonathan KP |
| collection | Repository of Agricultural Research Outputs (CGSpace) |
| description | Modern coupled general circulation models produce systematic biases in the tropical Atlantic that hamper the reliability of long-range predictions. This study focuses on a common springtime westerly wind bias in the equatorial Atlantic in seasonal hindcasts from two coupled models – ECMWF System 4 and EC-Earth v2.3 – and in hindcasts also based on System 4, but with prescribed sea-surface temperatures.
The development of the equatorial westerly bias in early April is marked by a rapid transition from a wintertime easterly, cold tongue bias to a springtime westerly bias regime that displays a marked double intertropical convergence zone (ITCZ). The transition is a seasonal feature of the model climatology (independent of initialisation date) and is associated with a seasonal increase in rainfall where a second branch of the ITCZ is produced south of the Equator. Excess off-equatorial convergence redirects the trade winds away from the Equator. Based on arguments of temporal coincidence, the results of our analysis contrast with those from previous work, and alleged causes hereto identified as the likely cause of the equatorial westerly bias in other models must be discarded. Quite in general, we find no evidence of remote influences on the development of the springtime equatorial bias in the Atlantic in the IFS-based models. Limited evidence however is presented that supports the hypothesis of an incorrect representation of the meridional equatorward flow in the marine boundary layer of the southern Atlantic as a contributing factor. Erroneous dynamical constraints on the flow upstream of the Equator may generate convergence and associated rainfall south of the Equator. This directs attention to the representation of the properties of the subtropical boundary layer as a potential source for the double ITCZ bias. |
| format | Journal Article |
| id | CGSpace103620 |
| institution | CGIAR Consortium |
| language | Inglés |
| publishDate | 2019 |
| publishDateRange | 2019 |
| publishDateSort | 2019 |
| publisher | Copernicus GmbH |
| publisherStr | Copernicus GmbH |
| record_format | dspace |
| spelling | CGSpace1036202024-03-06T10:16:43Z A double ITCZ phenomenology of wind errors in the equatorial Atlantic in seasonal forecasts with ECMWF models Shonk, Jonathan KP Demissie, Teferi Dejene Toniazzo, Thomas climate change forecasting modelling food security agriculture Modern coupled general circulation models produce systematic biases in the tropical Atlantic that hamper the reliability of long-range predictions. This study focuses on a common springtime westerly wind bias in the equatorial Atlantic in seasonal hindcasts from two coupled models – ECMWF System 4 and EC-Earth v2.3 – and in hindcasts also based on System 4, but with prescribed sea-surface temperatures. The development of the equatorial westerly bias in early April is marked by a rapid transition from a wintertime easterly, cold tongue bias to a springtime westerly bias regime that displays a marked double intertropical convergence zone (ITCZ). The transition is a seasonal feature of the model climatology (independent of initialisation date) and is associated with a seasonal increase in rainfall where a second branch of the ITCZ is produced south of the Equator. Excess off-equatorial convergence redirects the trade winds away from the Equator. Based on arguments of temporal coincidence, the results of our analysis contrast with those from previous work, and alleged causes hereto identified as the likely cause of the equatorial westerly bias in other models must be discarded. Quite in general, we find no evidence of remote influences on the development of the springtime equatorial bias in the Atlantic in the IFS-based models. Limited evidence however is presented that supports the hypothesis of an incorrect representation of the meridional equatorward flow in the marine boundary layer of the southern Atlantic as a contributing factor. Erroneous dynamical constraints on the flow upstream of the Equator may generate convergence and associated rainfall south of the Equator. This directs attention to the representation of the properties of the subtropical boundary layer as a potential source for the double ITCZ bias. 2019-09-10 2019-09-11T13:11:08Z 2019-09-11T13:11:08Z Journal Article https://hdl.handle.net/10568/103620 en Open Access Copernicus GmbH Shonk JKP, Demissie TD, Toniazzo T. 2019. A double ITCZ phenomenology of wind errors in the equatorial Atlantic in seasonal forecasts with ECMWF models. Atmospheric Chemistry and Physics 19(17):11383–11399. |
| spellingShingle | climate change forecasting modelling food security agriculture Shonk, Jonathan KP Demissie, Teferi Dejene Toniazzo, Thomas A double ITCZ phenomenology of wind errors in the equatorial Atlantic in seasonal forecasts with ECMWF models |
| title | A double ITCZ phenomenology of wind errors in the equatorial Atlantic in seasonal forecasts with ECMWF models |
| title_full | A double ITCZ phenomenology of wind errors in the equatorial Atlantic in seasonal forecasts with ECMWF models |
| title_fullStr | A double ITCZ phenomenology of wind errors in the equatorial Atlantic in seasonal forecasts with ECMWF models |
| title_full_unstemmed | A double ITCZ phenomenology of wind errors in the equatorial Atlantic in seasonal forecasts with ECMWF models |
| title_short | A double ITCZ phenomenology of wind errors in the equatorial Atlantic in seasonal forecasts with ECMWF models |
| title_sort | double itcz phenomenology of wind errors in the equatorial atlantic in seasonal forecasts with ecmwf models |
| topic | climate change forecasting modelling food security agriculture |
| url | https://hdl.handle.net/10568/103620 |
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