The general circulation model precipitation bias over the southwestern equatorial Indian Ocean and its implications for simulating the South Asian monsoon
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Most of current general circulation models (GCMs) show a remarkable positive precipitation bias over the southwestern equatorial Indian Ocean (SWEIO), which can be thought of as a westward expansion of the simulated IO convergence zone toward the coast of Africa. The bias is common to both coupled and uncoupled models, suggesting that its origin does not stem from the way boundary conditions are specified. The spatio-temporal evolution of the precipitation and associated three-dimensional atmospheric circulation biases is comprehensively characterized by comparing the GFDL AM3 atmospheric model to observations. It is shown that the oceanic bias, which develops in spring and reduces during the monsoon season, is associated to a consistent precipitation and circulation anomalous pattern over the whole Indian region. In the vertical, the areas are linked by an anomalous Hadley-type meridional circulation, whose northern branch subsides over northeastern India significantly affecting the monsoon evolution (e.g., delaying its onset). This study makes the case that the precipitation bias over the SWEIO is forced by the model excess response to the local meridional sea surface temperature (SST) gradient through enhanced near-surface meridional wind convergence. This is suggested by observational evidence and supported by AM3 sensitivity experiments. The latter show that relaxing the magnitude of the meridional SST gradient in the SWEIO can lead to a significant reduction of both local and large-scale precipitation and circulation biases. The ability of local anomalies over the SWEIO to force a large-scale remote response to the north is further supported by numerical experiments with the GFDL spectral dry dynamical core model. By imposing a realistic anomalous heating source over the SWEIO the model is able to reproduce the main dynamical features of the AM3 bias. These results indicate that improved GCM simulations of the South Asian summer monsoon could be achieved by reducing the springtime model bias over the SWEIO. Deficiencies in the atmospheric model, and in particular in the convective parameterization, are suggested to play a key role. Finally, the important mechanism controlling the simulated precipitation distribution over South Asia found here should be considered in the interpretation and attribution of regional precipitation variation under climate change.
KeywordsGeneral circulation models South Asian monsoon Sea surface temperature gradient Precipitation
The authors would like to thank Chris Golaz for helpful discussions. We also thank Takeshi Doi and Matthew Harrison for reviewing an earlier version of the manuscript, as well as two anonymous reviewers for their comments.
- Adler RF, Huffman GJ, Chang A, Ferraro R, Xie P, Janowiak J, Rudolf B, Schneider U, Curtis S, Bolvin D, Gruber A, Susskind J, Arkin P, Nelkin E (2003) The version 2 global precipitation climatology project (GPCP) monthly precipitation analysis (1979-present). J Hydrometeorol 4:1147–1167CrossRefGoogle Scholar
- Goswami BN (2005) South Asian summer monsoon: an overview. In: Chang C-P, Wang B, Lau N-CG (eds) The global monsoon system: research and forecast. WMO/TD no. 1266 (TMRP report no. 70), pp 47–71Google Scholar
- Helsel DR, Hirsch RM (2002) Statistical Methods in water resources techniques of water resources investigations, book 4, chapter A3. U.S. Geological Survey, p 522. Available at the web address: http://pubs.usgs.gov/twri/twri4a3/
- Rajeevan M, Nanjundiah RS (2009) Coupled model simulations of twentieth century climate of the Indian summer monsoon. In: Platinum jubilee special volume of the Indian Academy of Sciences. Indian Academy of Sciences, Bangalore, pp 537–568Google Scholar
- Ramage CS (1971) Monsoon meteorology. Academic Press, New YorkGoogle Scholar
- Sobel AH (2007) Simple models of ensemble-averaged precipitation and surface wind, given the SST. In: Schneider T, Sobel AH (eds) The global circulation of the atmosphere. Princeton University Press, Princeton, pp 219–251Google Scholar
- Sumi A, Lau NC, Wang WC (2005) Present Status of Asian Monsoon Simulation. In: Chang C-P, Wang B, Lau N-CG (eds) The global monsoon system: research and forecast, WMO/TD no. 1266 (TMRP report no. 70), pp 376–385Google Scholar
- von Engeln A, Teixeira J (2010) A planetary boundary layer height climatology derived from ECMWF Re-analysis data. J Appl Met Clim (submitted)Google Scholar