Summertime Response of the Tropical Atmosphere to the Indian Ocean Dipole Sea Surface Temperature Anomalies

Abstract

The anomalous response of the tropical atmosphere to the recently discovered Indian Ocean Dipole has been studied in the present article, using an Atmospheric General Circulation Model (AGCM), and the NCEP/ NCAR Reanalysis. Our AGCM study shows that the response of the atmosphere to the IOD is of dipole-like pattern in the circulation, and is baroclinic. An anomalous circulation in the zonalvertical plane is induced, with the subsidence over colder pole and the upward motion over the warmer pole, modulating the Walker circulation over the equatorial Indian Ocean, as observed. An anomalous circulation in the meridional-vertical plane from the eastern pole of the dipole towards India, and the Bay of Bengal is simulated, which affects the Indian summer monsoon rainfall. The model atmosphere is heated by the anomalous surplus of the latent heat, the net long-wave radiation, and sensible heat fluxes over the warmer pole of the IOD. This results in an excess of net vertically integrated moisture convergence, and excess latent heat release in the atmospheric column above the warm pole. This energy, together with the anomalous convergence of enthalpy, is converted into anomalous mechanical energy and leads to the divergence of mechanical energy flux. This anomalous divergence of the mechanical energy in the upper tropospheric part of the column causes propagation of the disturbances to the surrounding regions. The mean seasonal circulation during the boreal summer is crucial for maintenance of the anomalous energy distribution and propagation during the IOD event. Transient high frequency variability does not appear to contribute much to the energy conversions.

Indian Ocean Dipole Modulates the Number of Extreme Rainfall Events over India in a Warming Environment

“…100 mm of rain in a day represent 99.6 percentile of the rainfall distribution over this region. Interannual variability of summer monsoon rainfall in this region is more related to the Indian Ocean sea surface temperature anomalies (Guan et al 2003;Ashok et al 2004;Vecchi and Harrison 2004). This is also the region most vulnerable to inundating floods (Dilley et al 2005).…”

Influence of Indian Ocean Dipole on Poleward Propagation of Boreal Summer Intraseasonal Oscillations

“…The positive IOD starts in May with anomalous cooling in the southeastern equatorial Indian Ocean (Saji et al 1999;Guan et al 2003); the cooling reaches maximum in boreal fall. Southeastern equatorial Indian Ocean SST anomalies in boreal summer [JuneSeptember (JJAS)] in many positive (negative) IOD years are significantly below (above) normal in the peak monsoon time and have the potential to influence the monsoon.…”

A Look at the Relationship between the ENSO and the Indian Ocean Dipole.

“…The IOD events have a strong influence on the climate of not only the immediate neighboring regions such as East Africa and Indonesia ), but also the Indian summer monsoon region (Ashok et al 2001), East Asia (Saji and Yamagata 2002b;Guan et al 2002), the Mediterranean, Australia, and Brazil (Saji and Yamagata 2002b).…”