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Discussion papers
https://doi.org/10.5194/acp-2018-1316
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-1316
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 03 Jan 2019

Research article | 03 Jan 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

A Double ITCZ Phenomenology of Wind Errors in the Equatorial Atlantic in Seasonal Forecasts with ECMWF Models

Jonathan K. P. Shonk1, Teferi D. Demissie2, and Thomas Toniazzo2 Jonathan K. P. Shonk et al.
  • 1National Centre for Atmospheric Science, University of Reading, Reading, UK
  • 2Uni Research, Bjerknes Centre for Climate Research, Bergen, Norway

Abstract. 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 coupled models share common atmosphere and ocean components, although at different versions.

We examine the sequence in which different biases appear during the development of the westerly bias in early April, which is marked by a rapid transition from a wintertime bias pattern with an equatorial cold tongue and an easterly wind bias to a springtime westerly bias regime displaying a marked double ITCZ. The transition is a seasonal feature of the model climatology (independent of start date), and is associated with the seasonal increase in rainfall around the start of April and the consequent enhancement of the southern branch of a double ITCZ, which generates excess off-equatorial convergence and redirects the trade winds away from the equator.

There is no evidence of remote influences on the biases at the time of the transition. By contrast, there appears to be an association with a persistent dry bias north of the equator. Based on our analysis, a possible contribution to the springtime development of the double ITCZ and the westerly equatorial wind bias is a failure to correctly represent the meridional cross-equatorial flow, which can instigate a meridional rainfall bias pattern across the equator.

Jonathan K. P. Shonk et al.
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Jonathan K. P. Shonk et al.
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Latest update: 25 Mar 2019
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Short summary
Modern climate models are affected by systematic biases that harm their ability to produce reliable seasonal forecasts and climate projections. In this study, we investigate causes of biases in wind patterns over the tropical Atlantic during northern spring in three related models. We find that the wind biases are associated with an increase in excess rainfall and convergence in the tropical western Atlantic at the start of April, leading to the redirection of trade winds away from the equator.
Modern climate models are affected by systematic biases that harm their ability to produce...
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