A double ITCZ phenomenology of wind errors in the equatorial Atlantic in seasonal forecasts with ECMWF models
<p>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 S...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
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Copernicus Publications
2019-09-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/19/11383/2019/acp-19-11383-2019.pdf |
| _version_ | 1811282860853690368 |
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| author | J. K. P. Shonk T. D. Demissie T. D. Demissie T. Toniazzo |
| author_facet | J. K. P. Shonk T. D. Demissie T. D. Demissie T. Toniazzo |
| author_sort | J. K. P. Shonk |
| collection | DOAJ |
| description | <p>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.</p>
<p>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.</p> |
| first_indexed | 2024-04-13T02:01:17Z |
| format | Article |
| id | doaj.art-1b6471591f534c298c45291be18f1968 |
| institution | Directory Open Access Journal |
| issn | 1680-7316 1680-7324 |
| language | English |
| last_indexed | 2024-04-13T02:01:17Z |
| publishDate | 2019-09-01 |
| publisher | Copernicus Publications |
| record_format | Article |
| series | Atmospheric Chemistry and Physics |
| spelling | doaj.art-1b6471591f534c298c45291be18f19682022-12-22T03:07:37ZengCopernicus PublicationsAtmospheric Chemistry and Physics1680-73161680-73242019-09-0119113831139910.5194/acp-19-11383-2019A double ITCZ phenomenology of wind errors in the equatorial Atlantic in seasonal forecasts with ECMWF modelsJ. K. P. Shonk0T. D. Demissie1T. D. Demissie2T. Toniazzo3National Centre for Atmospheric Science, University of Reading, Reading, UKNORCE Climate, NORCE Norwegian Research Centre AS, Bergen, NorwayCGIAR Research Program on Climate Change, Agriculture and Food Security (CCAFS) East Africa, Addis Ababa, EthiopiaNORCE Climate, NORCE Norwegian Research Centre AS, Bergen, Norway<p>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.</p> <p>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.</p>https://www.atmos-chem-phys.net/19/11383/2019/acp-19-11383-2019.pdf |
| spellingShingle | J. K. P. Shonk T. D. Demissie T. D. Demissie T. Toniazzo A double ITCZ phenomenology of wind errors in the equatorial Atlantic in seasonal forecasts with ECMWF models Atmospheric Chemistry and Physics |
| 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 |
| url | https://www.atmos-chem-phys.net/19/11383/2019/acp-19-11383-2019.pdf |
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