Eastward propagating MJO during boreal summer and Indian monsoon droughts

Abstract

Improved understanding of underlying mechanism responsible for Indian summer monsoon (ISM) droughts is important due to their profound socio-economic impact over the region. While some droughts are associated with 'external forcing' such as the El-Niño and Southern Oscillation (ENSO), many ISM droughts are not related to any known 'external forcing'. Here, we unravel a fundamental dynamic process responsible for droughts arising not only from external forcing but also those associated with internal dynamics. We show that most ISM droughts are associated with at least one very long break (VLB; breaks with duration of more than 10 days) and that the processes responsible for VLBs may also be the mechanism responsible for ISM droughts. Our analysis also reveals that all extended monsoon breaks (whether co-occurred with El-Niño or not) are associated with an eastward propagating Madden-Julian Oscillation (MJO) in the equatorial Indian Ocean and western Pacific extending to the dateline and westward propagating Rossby waves between 10°and 25°N. The divergent Rossby wave associated with the dry phase of equatorial convection propagates westward towards Indian land, couple with the northward propagating dry phase and leads to the sustenance of breaks. Thus, the propensity of eastward propagating MJO during boreal summer is largely the cause of monsoon droughts. While short breaks are not accompanied by westerly wind events (WWE) over equatorial western Pacific favorable for initiating air-sea interaction, all VLBs are accompanied by sustained WWE. The WWEs associated with all VLB during 1975-2005 initiate air-sea interaction on intraseasonal time scale, extend the warm pool eastward allowing the convectively coupled MJO to propagate further eastward and thereby sustaining the divergent circulation over India and the monsoon break. The ocean-atmosphere coupling on interannual time scale (such as El-Niño) can also produce VLB, but not necessary.

Intraseasonal-to-Interannual Variability of the Indian Monsoon Identified with the Large-Scale Index for the Indian Monsoon System (LIMS)

“…Active and break phases exhibit nearly symmetric spatial characteristics for all lags, consistent with Singh et al (2015). These events are clearly related to the northward propagation of convection accompanied by changes in circulation that are typically associated with boreal ISO (Krishnamurti and Ardanuy 1980;Lawrence andWebster 2001, 2002;Kripalani et al 2004;Joseph et al 2008;Hoyos and Webster 2007). Cyclonic (anticyclonic) intraseasonal anomalies begin to enhance (suppress) convective activity over southern India 10 days prior to the peak of wet (dry) regimes (Figs.…”

“…The Indian monsoon cycle is also strongly affected by the boreal intraseasonal oscillations (ISO) that often Madden and Julian 1994) is considered the most relevant mode of intraseasonal variability in the tropics with known impacts on the Indian summer monsoon (Knutson and Weickmann 1987;Jones et al 2004;Pai et al 2011;Joseph et al 2008;Suhas et al 2013). Depending on the timing and intensity of these events, they can cause serious economic problems for India.…”

Absorbing aerosols facilitate transition of Indian monsoon breaks to active spells

“…In addition, the following observational data sets have been used: daily OLR (Outgoing Longwave Radiation) measured from Advanced Very High Resolution Radiometer onboard National Oceanic and Atmospheric Administration polar orbiting spacecraft (Gruber and Kruger 1984) available from http://www.cdc.noaa.gov/ of Climate Diagnostics Center, Boulder at a resolution of 2.5°92.5°r esolution; daily circulation data from National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis (see Kalnay et al 1996, available from http://www.cdc.noaa.gov/); daily 1°91°gridded precipitation data from Global Precipitation Climatology Project (GPCP; Huffman et al 2001; http://ftp.ncdc.noaa.gov/pub/data/gpcp/1dd-v1.1); daily temperature, wind and specific humidity data set from Joseph et al (2009). Breaks (actives) are identified when the standardized OLR anomaly averaged over the Central Indian (CI) region (73°-82°E; 18°-28°N) is more (less) than 0.9 (-0.9) for consecutive 4 days and the average standardized anomaly over the region during the period exceeds (goes below) 1.0 (-1.0).…”

Can El Niño–Southern Oscillation (ENSO) events modulate intraseasonal oscillations of Indian summer monsoon?

“…The periodicity of a complete MISO cycle, comprising active, intermediate, and break phases, ranges from 30 to 60 days, with an average of 45 days. The active/break spells are conventionally identified based on criteria, such as rainfall or convection anomalies averaged over the central Indian region, exceeding or falling below a threshold [e.g., Annamalai and Slingo, 2001;Krishnan et al, 2000;Gadgil and Joseph, 2003;Joseph et al, 2009;Rajeevan et al, 2010]. Such definitions, in general, identify only the intense shades of the MISO, i.e., the most intense active or break phase.…”