An objective definition of the Indian summer monsoon season and a new perspective on the ENSO–monsoon relationship

Abstract

ABSTRACT:The concept of an interannually varying Indian summer monsoon season is introduced here, considering that the duration of the primary driving of the Indian monsoon -the large-scale meridional gradient of the deep tropospheric heat source -may vary from one year to another. Onset (withdrawal) is defined as the day when the tropospheric heat source shifts from south to north (north to south). This physical principle leads to a new thermodynamic index of the seasonal mean monsoon. While the traditional measure of seasonal rainfall, averaged from 1 June to 30 September, indicates a breakdown of the ENSO-monsoon relationship in recent decades, it is argued that this breakdown is partly due to the inappropriate definition of a fixed monsoon season. With a new physically based definition of the seasonal mean, the ENSO-monsoon relationship has remained steady over the decades. El Niño (La Niña) events contract (expand) the season, and thus decrease (increase) the seasonal mean monsoon by setting up persistent negative (positive) tropospheric temperature (TT) anomalies over the southern Eurasian region. Thus, we propose a new pathway, whereby the Indian summer monsoon could be influenced by remote climatic phenomena via modification of TT over Eurasia.Diagnostics of the onset and withdrawal processes suggest that onset delay is due to the enhanced adiabatic subsidence that inhibits vertical mixing of sensible heating from warm landmass during the pre-monsoon months. On the other hand, the major factor that determines whether the withdrawal is early or late is the horizontal advective cooling. Most of the late (early) onsets and early (late) withdrawals are associated with El Niño (La Niña). This link between the ENSO and the monsoon is realized through vertical and horizontal advections associated with the stationary waves in the upper troposphere set up by the tropical ENSO heating.

Drying of Indian subcontinent by rapid Indian Ocean warming and a weakening land-sea thermal gradient

“…Many studies have shown that warm SST anomalies are accompanied by large variations in the lower and upper troposphere due to enhanced latent heating aloft from convection over the ocean [33][34][35] . Furthermore, these changes are highly correlated with the strength of the monsoon circulation 31,34 . A more recent study using a coupled model framework also points out that a spatial extension of Indian Ocean warm pool could enhance the convection over the ocean while introducing a dry bias over land, by modulating the meridional Hadley circulation 36 .…”

“…Once the monsoon rains set in on the South Asian subcontinent, the land surface cools down considerably. However, the troposphere above the land remains warm due to the latent heat release from the convective activity, keeping the thermal contrast functional 31 . This means that the tropospheric temperature is an ideal parameter for examining the thermodynamic forcing related to the monsoon 31 .…”

“…However, the troposphere above the land remains warm due to the latent heat release from the convective activity, keeping the thermal contrast functional 31 . This means that the tropospheric temperature is an ideal parameter for examining the thermodynamic forcing related to the monsoon 31 . However, unlike the surface temperature, robust records of air temperature at different levels of the troposphere are not available before the 1950s, which makes it difficult to examine the role of long-term tropospheric temperature trends on the monsoon.…”

Signatures of Tibetan Plateau heating on Indian summer monsoon rainfall variability

“…We used monsoon onset and withdrawal dates based on tropospheric temperatures by Goswami and Xavier [2005] and Xavier et al [2007]. For rainfall we used the gridded (1 Â 1 ) daily rainfall data over India developed by Rajeevan et al [2006].…”

Combining regional moist static energy and ENSO for forecasting of early and late season Indian monsoon rainfall and its extremes

“…These periods contain nearly all observed onset and withdrawal dates of monsoons defined in different ways [e.g., Ananthakrishnan and Soman, 1989;Fasullo and Webster, 2003;Goswami and Xavier, 2005;Joseph et al, 2006;Pai and Nair, 2009;Prasad and Hayashi, 2005;Taniguchi and Koike, 2006;Wang et al, 2004;Xavier et al, 2007;Zhang et al, 2002]. To consider forecasting of early and late season rainfall, which may be as important for crop yields and water resources as the total monsoon rainfall, we exploit two aspects of global climate with obvious connections to the Indian monsoon, local heating over the Indian subcontinent and its surroundings and the El Niño-Southern Oscillation, which correlate with onset and withdrawal dates [Goswami and Xavier, 2005;Rajagopalan and Molnar, 2012;Xavier et al, 2007]. Although Singh et al [2014] recently showed correlations between monthly rainfall variability and concurrent Pacific and Indian Ocean forcings that could potentially be exploited for improving skill, we found virtually no predictability of the peak monsoon season (mid-June to midAugust) rainfall with any of these large-scale climate features, but some success with the early and late seasons.…”