Wahl, Sebastian (2009) The Tropical Atlantic SST bias in the Kiel Climate Model. (PhD/ Doctoral thesis), Christian-Albrechts-Universität, Kiel, 116 pp.

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Abstract

Until today, the Tropical Atlantic Ocean is a region which is difficult to represent in numerical models. Most of the current coupled general circulation models (CGCM) show a strong warm bias in the eastern Tropical Atlantic and are unable to reproduce the observed variability especially directly along the equator. In this work various sensitivity experiments with the Kiel Climate Model (KCM) are described. A largely reduced warm bias and an improved seasonal cycle in the eastern Tropical Atlantic are simulated in one particular version of KCM. By comparing the stable and well-tested standard version with the sensitivity experiments and the modified version, mechanisms contributing to the reduction of the eastern Atlantic warm bias are identified and compared to what has been proposed in literature. The errors in the spring and early summer zonal winds associated with erroneous zonal precipitation seems to be the key mechanism, and large-scale coupled ocean-atmosphere feedbacks play an important role in reducing the warm bias. Improved winds in boreal spring cause the summer cooling in the eastern Tropical Atlantic via shoaling of the thermocline and increased upwelling, and hence reduced sea surface temperature (SST). Reduced SSTs in the summer suppress convection and favor the development of low-level cloud cover in the eastern Tropical Atlantic region. Subsurface ocean structure is shown to be improved, and potentially influences the development of the bias. The strong warm bias along the southeastern coastline is related to underestimation of low-level cloud cover and the associated overestimation of surface shortwave radiation in the same region. Therefore, in addition to the primarily wind forced response at the equator both changes in surface shortwave radiation and outgoing longwave radiation contribute significantly to reduction of the warm bias from summer to fall. The better representation of the mean annual cycle in the Tropical Atlantic also improves the variability in the Tropical Atlantic. The different steps of the Bjerknes feedback mechanism are more realistically simulated in those version of KCM that have an better mean state. The improved representation of equatorial Atlantic variability is believed to be responsible for better potential predictability. No predictability that is significantly above persistence is found when hindcasting equatorial Atlantic SST between 1971 and 2004 using different configurations of KCM.

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