ORIGINAL PAPER Changes in the Walker and Hadley circulations associated with the Southern Annular Mode Nithin Viswambharan & K. Mohanakumar Received: 22 March 2013 /Accepted: 3 September 2013 # Springer-Verlag Wien 2013 Abstract Extratropical impacts on tropical climates are one of the most exciting areas of meteorological investigation in recent times. The present study elucidates the seasonal impact of the Southern Annular Mode (SAM) on predominant trop- ical circulations such as Hadley and Walker. The velocity potential at 200 hPa is used to understand the spatio- temporal variability in tropical circulations in the boreal sum- mer and winter seasons. The results show an intensification of seasonal velocity potential in the composite of the low phase of the SAM. The seasonal climatological values of velocity potential observed for the period from 1979 to 2012 are of lesser magnitudes than earlier findings. The convergence/ divergence locations of tropical circulation have shifted from their mean positions in the alternative phase of the SAM. The low-level convergence in the southern hemispheric Hadley circulation (HC) is enhanced in the composite of the positive phase of the SAM; however, the SAM’ s effect on the HC is no stronger in the summer. Another interesting feature noted in the present study is the weakening of the Walker circulation associated with the positive phase of the SAM, which can influence the basic state of the tropical Pacific Ocean. The SAM’ s interannual variability exhibits a significant positive trend in winter. The study reveals that the positive phase of the SAM could be a possible explanation for the recent changes in the tropical circulation patterns; however, the variability in tropical circulation anomalies associated with the SAM should be noted on seasonal and monthly scales to understand the dynamical mechanism behind the relationship. The impact of the SAM on tropical circulation may continue in future decades, as this southern extratropical vacillation is predicted to remain in a positive phase due to the increase in greenhouse gases and the variability in ozone. 1 Introduction In tropical regions, energy and momentum are primarily transferred through the Hadley and Walker circulations. The Hadley circulation (HC) is associated with the ascending branch in equatorial regions and the descending motion in the subtropics (Oort and Yienger 1996; Trenberth et al. 2000; Wallace and Hobbs 2006). The shifting of this meridional circulation from its mean position occurs in accordance with the seasonal migration of the Sun. On the other hand, the Walker Cell is an east–west circulation and the motion de- pends on the mean state of the sea surface temperature (SST) in the east and west Pacific. However, in recent years, significant changes have oc- curred in both the Hadley and Walker circulations (Kumar et al. 1999; Quan et al. 2004; Fu et al. 2006; Tretkoff 2011). As a result, a widespread variability in the tropical precipita- tion pattern has occurred (Lindzen and Hou 1988; Ropelewski and Halpert 1989; Meehl and Arblaster 2002; Kumar et al. 2004; Zhou et al. 2011). The primary cause for this unusual change in the circulation is thought to be as a contribution through anthropogenic activity (Houghton et al. 1990; Bhaskaran et al. 1995; Hartmann et al. 2000; Houghton et al. 2001; Marshall et al. 2004; Stott et al. 2006; Cai and Cowan 2007; IPCC 2007; Smith et al. 2007; Solomon et al. 2007). In addition, both natural and anthropogenic activities caused a climate shift in the Pacific during the mid-1970s and this shift modulated the east–west circulation (the Walker circulation) through the transition of colder to warmer SSTs (Meehl et al. 2009). In response to the change in greenhouse gas, a climate model of the poleward expansion of the HC was recently N. Viswambharan (*) : K. Mohanakumar Department of Atmospheric Sciences, Cochin University of Science and Technology, Cochin 682016, India e-mail: nithinviswam@gmail.com Theor Appl Climatol DOI 10.1007/s00704-013-1011-0