Study on links between the tropical Atlantic and Indian summer monsoon on interannual timescales

Abstract

The Indian Summer Monsoon (ISM) is a large scale land-ocean-atmosphere coupled system dominating the climate of the Indian Ocean region (e.g., Webster et al., 1998). Among other things, it is manifest in the winds blowing across the Indian Ocean over to the Indian subcontinent and the associated distribution of precipitation during boreal summer (e.g., Wang, 2006a). It is the lifeline of millions of people of the Indian subcontinent, and critical for the food production, economy, etc. of the subcontinent (Gadgil et al., 1999; Gadgil and Gadgil, 2006). Although great strides have been made in understanding the ISM and its variability, and predicting the monsoon over the past decades, there is a great need for improvements (e.g., Nanjundiah, 2009; Rao et al., 2019). Although the Indian Summer Monsoon Rainfall (ISMR) exhibits variability on a spectrum of timescales, its year-to-year variations are of particular interest as they are associated with droughts and floods over India (e.g., Parthasarathy and Mooley, 1978; Webster et al., 1998; Gadgil, 2003). The interannual variability of ISMR is a result of both internal dynamics and the influence of external factors (e.g., Webster et al., 1998). The part of variability that is driven by external factors forms the basis for the seasonal prediction, given the decent lead prediction skills of the tropical ocean drivers of the monsoon such as the El Ni˜no-Southern Oscillation (ENSO). The ENSO is a coupled ocean-atmosphere phenomenon operating in the tropical Pacific (e.g., Philander, 1989; Wang and Picaut, 2004). It is a major driver of the ISM as well as the global climate (e.g., Keshavamurty, 1982; Ashok and Saji, 2007; Yeh et al., 2018). While a warm phase of ENSO (or an El Ni˜no) tends to reduce the monsoon rainfall over India, a cold phase of ENSO (or a La Ni˜na) favors the enhancement of the rainfall (e.g., Sikka, 1980; Keshavamurty, 1982; Ashok et al., 2019, for the detailed literature). Although the ENSO is a major external driver of ISM, it does not explain all the droughts or floods over India. Hence it is imperative to study the impacts of other external factors on the ISM, especially that of the tropical modes of interannual variability. Another coupled ocean-atmosphere mode existing in the tropical Indian Ocean called the Indian Ocean Dipole (IOD; e.g., Saji et al., 1999; Webster et al., 1999; Murtugudde et al., 2000) is also shown to affect the ISM. A strong positive (negative) IOD event can contribute to an enhancement (a reduction) of ISMR, and even reduce the impacts of a strong concurrent El Ni˜no (La Ni˜na) event (e.g., Behera et al., 1999; Ashok et al., 2004b; Ashok and Saji, 2007). The remaining tropical ocean, i.e., the tropical Atlantic hosts its xi own coupled phenomenon called the Atlantic Zonal Mode (AZM) but studies on the impacts of AZM on the interannual variability of the ISM are all recent and relatively less in number (e.g., Kucharski et al., 2008; L¨ubbecke et al., 2018).