NOAA GFDL - Geophysical Fluid Dynamics LaboratoryGFDL - Geophysical Fluid Dynamics Laboratory

Model Precipitation Bias over the Southwestern Equatorial Indian Ocean

June 1st, 2012

Key Findings

  • The precipitation bias over the southwestern equatorial Indian Ocean develops in spring and reduces during the monsoon season and is part of a dynamically-forced regional-scale pattern of precipitation and circulation biases.
  • The bias is related to enhanced near-surface meridional convergence over the ocean, suggested to be in large part modulated by the local meridional sea surface temperature (SST) gradient.
  • The enhanced equatorial precipitation is responsible for an anomalous Hadley-type meridional circulation whose subsiding branch to the north affects the subsequent simulated monsoon evolution over India.

Massimo A. Bollasina and Yi Ming. Journal: Climate Dynamics. DOI: 10.1007/s00382-012-1347-7

Summary

This study investigated the spatio-temporal features of the serious positive precipitation bias over the southwestern equatorial Indian Ocean, found in most current coupled and uncoupled general circulation models, and its links with the large scale monsoon circulation. The study attempted to answer two questions: Is there a common identifiable mechanism forcing the local bias? Does the local bias have an influence on the continental simulated monsoon precipitation?

Identifying the processes which lead to this large-scale precipitation anomaly may provide important insights into the factors controlling precipitation formation over the tropical Indian Ocean (and, possibly, over other basins) and the associated large-scale ocean-atmosphere coupling in models. In a broader sense, this analysis might also help to shed light on well-known difficulties in simulating the monsoon precipitation over South Asia.

These results indicate that improved simulations of the South Asian summer monsoon could be achieved by reducing the springtime model bias over the southwestern equatorial Indian Ocean. Deficiencies in the atmospheric model, and in particular in the convective parameterization, are suggested to play a key role in determining the model error over the Indian Ocean. Finally, this important mechanism controlling the simulated precipitation distribution over South Asia should be considered in the interpretation and attribution of regional precipitation variation under climate change.

This work is relevant to NOAA’s mission for advancing scientific understanding of climate variability and change, particularly in the hydrological cycle and at a regional scale. A better understanding of the deficiencies in current models is crucial, given the key role of models in the simulation and prediction of climate variability and change.

Fig. (a): Latitude-time plot of monthly observed GPCP precipitation (mm day-1, shaded), sea level pressure (hPa, black contours), and SST (°C, green contours) averaged over the oceanic grid-points between 50°-80°E. (b): as in (a) but for the AM3 bias in precipitation (mm day-1, shaded) and sea level pressure (hPa, black contours).
Fig. (a): Latitude-time plot of monthly observed GPCP precipitation (mm day-1, shaded), sea level pressure (hPa, black contours), and SST (°C, green contours) averaged over the oceanic grid-points between 50°-80°E. (b): as in (a) but for the AM3 bias in precipitation (mm day-1, shaded) and sea level pressure (hPa, black contours).