Indian Ocean warming during 1958–2004 simulated by a climate system model and its mechanism

Lu Dong,Bo Wu,Tianjun Zhou

doi:10.1007/s00382-013-1722-z

Abstract

The mechanism responsible for Indian Ocean Sea surface temperature (SST) basin-wide warming trend during 1958–2004 is studied based on both observational data analysis and numerical experiments with a climate system model FGOALS-gl. To quantitatively estimate the relative contributions of external forcing (anthropogenic and natural forcing) and internal variability, three sets of numerical experiments are conducted, viz. an all forcing run forced by both anthropogenic forcing (greenhouse gases and sulfate aerosols) and natural forcing (solar constant and volcanic aerosols), a natural forcing run driven by only natural forcing, and a pre-industrial control run. The model results are compared to the observations. The results show that the observed warming trend during 1958–2004 (0.5 K (47-year)−1) is largely attributed to the external forcing (more than 90 % of the total trend), while the residual is attributed to the internal variability. Model results indicate that the anthropogenic forcing accounts for approximately 98.8 % contribution of the external forcing trend. Heat budget analysis shows that the surface latent heat flux due to atmosphere and surface longwave radiation, which are mainly associated with anthropogenic forcing, are in favor of the basin-wide warming trend. The basin-wide warming is not spatially uniform, but with an equatorial IOD-like pattern in climate model. The atmospheric processes, oceanic processes and climatological latent heat flux together form an equatorial IOD-like warming pattern, and the oceanic process is the most important in forming the zonal dipole pattern. Both the anthropogenic forcing and natural forcing result in easterly wind anomalies over the equator, which reduce the wind speed, thereby lead to less evaporation and warmer SST in the equatorial western basin. Based on Bjerknes feedback, the easterly wind anomalies uplift the thermocline, which is unfavorable to SST warming in the eastern basin, and contribute to SST warming via deeper thermocline in the western basin. The easterly anomalies also drive westward anomalous equatorial currents, against the eastward climatology currents, which is in favor of the SST warming in the western basin via anomalous warm advection. Therefore, both the atmospheric and oceanic processes are in favor of the IOD-like warming pattern formation over the equator.

Highlights

Oceans play a key role in the climate system as they absorb most of the excess solar heating trapped by the greenhouse gases (GHGs) due to their large heat capacity (Levitus et al 2005)
To quantitatively estimate the relative contributions of external forcing and internal variability, three sets of numerical experiments are conducted, viz. an all forcing run forced by both anthropogenic forcing and natural forcing, a natural forcing run driven by only natural forcing, and a pre-industrial control run
The model results are compared to the observations

Summary

Introduction

Oceans play a key role in the climate system as they absorb most of the excess solar heating trapped by the greenhouse gases (GHGs) due to their large heat capacity (Levitus et al 2005). The Indian Ocean SST has a great effect on the global climate change. Its warming trend is likely to influence internal modes of interannual variability such as Indian Ocean dipole (IOD) (Saji et al 1999). Atmospheric model simulations suggest that the Indian Ocean warming is related to the droughts in African Sahel (Giannini et al 2003), and influences the northern middle latitudes (Lu et al 2004; Hoerling et al 2004) and the extratropical teleconnections of ENSO (Lau et al 2006). The interdecadal transition of the East Asian climate, especially the East Asian summer monsoon circulation and rainfall is partly due to the atmospheric response to the Indian Ocean-western Pacific warming (Zhou et al 2009; Li et al 2010). Understanding the mechanism of Indian Ocean warming is of a crucial importance to the climate change studies

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