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Identifying causes of Western Pacific ITCZ drift in ECMWF System 4 hindcasts

Shonk, J. K. .P., Guilyardi, E., Toniazzo, T., Woolnough, S. J. and Stockdale, T. (2018) Identifying causes of Western Pacific ITCZ drift in ECMWF System 4 hindcasts. Climate Dynamics, 50 (3-4). pp. 939-954. ISSN 0930-7575

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To link to this item DOI: 10.1007/s00382-017-3650-9

Abstract/Summary

The development of systematic biases in climate models used in operational seasonal forecasting adversely affects the quality of forecasts they produce. In this study, we examine the initial evolution of systematic biases in the ECMWF System 4 forecast model, and isolate aspects of the model simulations that lead to the development of these biases. We focus on the tendency of the simulated intertropical convergence zone in the western equatorial Pacific to drift northwards by between 0.5° and 3° of latitude depending on season. Comparing observations with both fully coupled atmosphere–ocean hindcasts and atmosphere-only hindcasts (driven by observed sea-surface temperatures), we show that the northward drift is caused by a cooling of the sea-surface temperature on the Equator. The cooling is associated with anomalous easterly wind stress and excessive evaporation during the first twenty days of hindcast, both of which occur whether air-sea interactions are permitted or not. The easterly wind bias develops immediately after initialisation throughout the lower troposphere; a westerly bias develops in the upper troposphere after about ten days of hindcast. At this point, the baroclinic structure of the wind bias suggests coupling with errors in convective heating, although the initial wind bias is barotropic in structure and appears to have an alternative origin.

Item Type:Article
Refereed:Yes
Divisions:Faculty of Science > School of Mathematical, Physical and Computational Sciences > NCAS
Faculty of Science > School of Mathematical, Physical and Computational Sciences > Department of Meteorology
ID Code:69818
Uncontrolled Keywords:Seasonal forecasting, model drift, western Pacific, ITCZ.
Publisher:Springer

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