Characteristics of atmospheric divergence and convergence in the Indian Ocean inferred from scatterometer winds Alvarinho J. Luis * , P.C. Pandey National Centre for Antarctic and Ocean Research, Polar Remote Sensing Division, Department of Ocean Development, Headland Sada, Vasco-da-Gama, Goa 403804, India Received 13 January 2005; received in revised form 29 March 2005; accepted 2 April 2005 Abstract Large-scale surface atmospheric convergence and divergence patterns in the Indian Ocean are mapped using high-spatial resolution, merged scatterometer wind vectors during 1991 – 2000. The convergence zone evolves to north of 15-S as a result of convection promoted by warm (> 28 -C) equatorial sea surface temperature (SST), and it exhibits strong intensity during boreal summer and winter. A divergence zone evolves to the south of 15-S as a result of subdued convection caused by colder SST (<24 -C) that reduces outgoing long-wave radiation; it exhibits enhanced intensity in the eastern Indian Ocean during boreal winter. The interannual variability shows that the divergence in the eastern Indian Ocean lags its western counterpart by 5 – 7 months. The convergence in the eastern Indian Ocean is stronger than its western counterpart during boreal summer. Relationship between Southern Oscillation Index and spatially averaged convergence time series indicate that the latter weakened during strong El Nin˜ o years 1994 and 1997. Spatially averaged divergence time series show a near-contemporaneous relationship with all-India rainfall, with a temporal lag of 1¨2 months. D 2005 Elsevier Inc. All rights reserved. Keywords: Indian Ocean; Atmospheric convergence/divergence; Monsoons; Rainfall; Scatterometer winds; SST 1. Introduction A characteristic feature of the atmospheric circulation in the tropics is the formation of large-scale convergence zones associated with a strong convective activity. These zones play a key role in regulating moisture and tropospheric heat, which in turn modulate the local precipitation pattern through hydrological forcing (Hsu et al., 1997). The intertropical convergence (ITCZ) zones, which form a zonal belt a few degrees to the north and south of the equator, are best examples of atmospheric convergences which drive the Hadley circulation. The position of these zones migrates seasonally and they exhibit interannual variations, which has a major effect on local precipitation (Philander, 1990). In particular, the north–south movement of the ITCZ in the equatorial Indian Ocean (IO) is vital to the modulation of Indian summer monsoon rainfall, while its east–west movement affects the local precipitation. In the Pacific and Atlantic oceans the interannual variability and the underlying mechanisms for ITCZ have been addressed elsewhere by using remote sensing data (cf. Liu & Xie, 2002). Using QuikSCAT winds, Liu and Xie (2002) provided evidence for the existence of ITCZ on the either side of the equator. They pointed out that convergence of northerly and southerly winds over warm water promotes strong ITCZ, while the trade winds moving from warmer water with well-mixed boundary layer towards the cold upwelling water near the equator decelerate due to an increase in the wind shear and leads to weak ITCZ. Hubert et al. (1969), Lietzke et al. (2001), Waliser and Gautier (1993), Zhang (2001) and Zheng et al. (1997) used rainfall distribution, vector winds from scatterometer flown on European Resource Satellite (ERS) series-1, highly reflec- tive clouds, satellite-based moisture soundings and cloud 0034-4257/$ - see front matter D 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.rse.2005.04.016 * Corresponding author. Fax: +91 832 2520877. E-mail address: alvluis@yahoo.com (A.J. Luis). Remote Sensing of Environment 97 (2005) 231 – 237 www.elsevier.com/locate/rse