Expansion of Hadley Cell in Climate Change Scenario: A Study Using Reanalysis And Global Radiosonde Observations

Abstract:

Hadley circulation(HC) is a planetary scale circulation spanning one-third of the globe from tropics to subtropics and is responsible for the humid climate of the tropics and the parched climate of the subtropics.Of late,sinking latitudes of HC has gained importance because of the fluctuations they exhibit,mainly due to changes in the meridional temperature gradient arising in response to the changing concentrations of greenhouse gases and other anthropogenic sources.Even though there are differing theories in explaining the possible mechanisms for the observed expansion of HC,almost all the studies agree on its expansion.However,there are marked differences in the rate of expansion simulated by the global circulation models and observations,which calls for extensive investigations on the processes responsible for HC expansion and validating the reanalysis datasets(which are extensively used to study the HC dynamics) using observations.Such kind of a validation is necessary lest the biases in reanalyses have affected the rate of expansion.

In this regard, a detailed study on the Hadley cell edges and their expansion using 35 years of four reanalysis datasets(JRA55,MERRA,NCEP,ECMWF) has been taken up.An algorithm is developed to estimate meridional mass stream function(MSF) from which the HC edges are characterized.The results show a consistent pattern of northern hemispheric and southern hemispheric edges of HC,including its centre.For the first time an attempt is made to validate four reanalysis datasets,which includes the relatively new JRA55 data,using global radiosonde network observations obtained from IGRA.Following the subsampling technique for the validation of reanalysis dataset with observation,it has been found that reanalyses reproduce in-situ data to a large extent,thus validating their use for climate studies especially over the tropics.The interannual variations of HC edges observed in the present study clearly indicate the expansion of the cell,which has profound implications on the precipitation and hence climate,on global and regional scales.An attempt is also being made to verify the expansion by means of physically observable parameters.It is also envisaged to investigate the physical processes responsible for the observed expansion of HC width in near future.