Relative role of premonsoon conditions and intras

Authors: Rohit Ghosh Arindam Chakraborty Ravi S Nanjundiah

Publish Date: 2016/10/27

Volume: 131, Issue: 1-2, Pages: 319-333

PDF Link

Abstract

The aim of this paper is to identify relative roles of different landatmospheric conditions apart from sea surface temperature SST in determining early vs late summer monsoon intensity over India in a high resolution general circulation model GCM We find that in its early phase June–July JJ premonsoon landatmospheric processes play major role to modulate the precipitation over Indian region These effects of premonsoon conditions decrease substantially during its later phase August–September AS for which the interannual variation is mainly governed by the low frequency northward propagating intraseasonal oscillations This intraseasonal variability which is related to mean vertical wind shear has a significant role during the early phase of monsoon as well Further using multiple linear regression we show that interannual variation of early and late monsoon rainfall over India is best explained when all these landatmospheric parameters are taken together Our study delineates the relative role of different processes affecting early versus later summer monsoon rainfall over India that can be used for determining its subseasonal predictabilitySummer monsoon precipitation plays a major role in the economics of the Indian subcontinent Gadgil et al 2004 where agriculture is the vital source of growth However an accurate prediction of the interannual variations of Indian Summer JuneSeptember JJAS Monsoon Rainfall ISMR at least a season in advance remains challenging It is more difficult to predict the extremes of monsoon precipitation—the most important aspect of interannual variation Webster et al 1998 The poor predictability of the seasonal mean ISMR could be due to the fact that the unpredictable internal dynamics plays a major role in determining its interannual variationInterannual variation of sea surface temperature SST is considered as the primary forcing modulating the interannual variation of monsoon over India Goswami 1998 Several studies show that there is a link between ElNiño and Southern Oscillation ENSO and the intensity of ISMR eg Pant and Parthasarathy 1981 Sikka 1980 Rasmusson and Carpenter 1983 Droughts are more probable during the warmer east Pacific SST phase ElNiño On the other hand excess rainfall is more probable during the cooler phase of eastern Pacific SST LaNiña However a strong monsoonENSO relationship came under doubt after some striking failures of the proposed linkage eg during 1994 1997 summer monsoon Kumar et al 1999 claimed that the relationship had been weakening since the last two decades Moreover there is an equal chance of having deficit and excess rainfall with ElNiño Kumar et al 2006 Gadgil et al 2007 In the last decade another link was found related to SST variations in Equatorial Indian Ocean EIO which is termed as Equatorial Indian Ocean Oscillation EQUINOO Gadgil et al 2004 It was shown that ENSO and EQUINOO together can explain more extremes of ISMR as compared to ENSO alone However all the factors related to SST can explain at most 50 of the variability in monsoon rainfall Goswami and Xavier 2005 Hence there could be many other reasons which are governing the rest of the 50 of monsoon variabilityOther than the impact of SST there are premonsoon landatmosphere processes which can play vital role in determining the interannual variation of monsoon over the Indian region The Eurasian Snow cover is one of the most wellknown among them Barnett et al 1989 Vernekar and Zhou 1995 Fasullo 2004 Ye and Bao 2005 Peings and Douville 2010 The premonsoon soil moisture SM is supposed to have an impact over the monsoon rainfall too Shukla and Mintz 1982 Douville et al 2001 Douville 2002 Eltahir 1998 Fennessy and Shukla 1999 Joseph and Srinivasan 1999 have shown that the 200 hPa meridional wind anomaly in May has a very prominent impact over the monsoon rainfall The premonsoon surface moist static energy MSE also has relation with the upcoming monsoon rainfall Chakraborty et al 2006 2014 However a comprehensive study examining the relative impacts of these landatmosphere parameters in determining summer monsoon over India in absence of interannual variations of SST forcing has not been reported so far Therefore it is necessary to separate the impact of interannual variation of SST from other parameters to understand the relative role of the later in determining interannual variation of ISMR Such a study is possible using a general circulation model GCM forced by SST that does not vary interannuallyThe seasonal mean of ISMR depends on its relative intensity during early June–July JJ climatological contribution 52 and late August–September AS climatological contribution 48 phases However it has been shown by Terray et al 2003 that the early and late ISMR intensities are uncorrelated and it might be associated with the evolution of ENSO Boschat et al 2011 2012 The nonexisting relationship between the anomalies of early and late Indian summer monsoon could also be related to the fact that premonsoon conditions like the SM are more likely to impact the early part of the season Saha et al 2010 as compared to the later part when after the onset the internal dynamics is more likely to play a major role Such changes in the dependence of monsoon rainfall on landatmospheric conditions can have profound influence on the predictability of longrange as well as short to mediumrange forecasts which shows different rates of error growth over land as compared to the ocean Chakraborty 2010In this paper we try to understand the relative role played by different important landatmosphere parameters in determining the early and late ISMR intensities using a high resolution global GCM We force the model with seasonally varying climatological SST conditions to eliminate the impact of the interannual variation of SST on monsoon rainfall A brief description of the model used is given in Section 2 The experimental details are provided in Section 3 and the results are discussed in Section 4 Section 5 illustrates the use of a Multiple Linear Regression to find relative impacts of different parameters on Indian monsoon followed by the summary and conclusions of the studyThe atmospheric component of CCSM4 Community Climate System Model 4 Gent et al 2011 the Community Atmospheric Model 40 CAM4 Neale et al 2011 developed by National Center for Atmospheric Research NCAR is used for this study The model has a finite volume dynamical core Lin 2004 The horizontal grid has 384 × 576 latitude/longitude divisions resulting in a high resolution of 045∘ latitude and 0625∘ longitude spacing over the globe The mass flux scheme by Zhang and McFarlane 1995 is used for deep convection parameterization The shallow convection scheme by Hack 1994 is used in the model The model has 26 vertical layersWe perform a 50year long simulation of the model The boundary conditions over the ocean SST are forced from the climatological monthly mean values of Hadley Centre which is constructed by the Met Office Hadley Centre for Climate Prediction and Research Rayner et al 2003 Therefore the model saw seasonally varying SST However these values do not change from year to year The CAM4 is coupled to a land surface model CLM4 Lawrence et al 2011a which allows it to interact with the atmospheric model throughout the simulation

Keywords: