Variability and predictability of the Northeast India summer monsoon rainfall

Abstract

The mesoscale orography over one of the wettest regions of the world makes the Northeast India (NEI) vulnerable to hydrological disasters while sustaining a biodiversity “hotspot.” The monsoon rainy season over the NEI is known to be longer than June–September (JJAS), but an objective definition has been lacking. Understanding the drivers and predictability of rainfall variability over the region is key for sustainable development planning and adaptation to increasing disasters in the backdrop of a warming climate but remained poor due to lack of a working definition of “summer monsoon season” over NEI. Here, using the relationship between rainfall over the region and winds at 850 hPa over North Bay of Bengal (BoB), we provide an objective definition of the monsoon season over NEI and argue that the “summer monsoon rainy season” over the NEI (NEIR) is from May to September (MJJAS). In contrast to a significant negative relationship between JJAS Central India (CI) rainfall and a JJAS El Niño–Southern Oscillation (ENSO) index, the MJJAS rainfall over the NEI has no relationship with MJJAS or JJAS ENSO. Instead, we unravel that (a) the tropical Northwest‐Pacific (TNWP) sea surface temperature (SST), (b) the South Equatorial Indian Ocean Dipole (SEIOD) and (c) the Atlantic Zonal mode (AZM) are potential drivers of the NEIR variability. Using a causal inference algorithm, namely the Peter and Clark Momentary Conditional Independence (PCMCI) method, we show that SEIOD directly influences NEIR while TNWP SST has a two‐way connection with the NEIR. While “internal variability” may be higher over the NEI, significant modulation of the variances of the subseasonal fluctuations by predictable drivers like the Atlantic Niño and North Atlantic water temperature provides optimism for seasonal prediction of the NEIR.

A Pre‐Monsoon Signal of False Alarms of Indian Monsoon Droughts

“…It is noteworthy that Sharma et al. (2023)'s finding of considerable contribution from lengthy rain spells to the total May rainfall over NE (their Figure S12 in Supporting Information ) is consistent with our Figure S3 in Supporting Information . We note that TRUE cases exhibit a barotropic convergence over NE India (Figure 4), consistent with what was reported by Sharma et al.…”

“…We note that TRUE cases exhibit a barotropic convergence over NE India (Figure 4), consistent with what was reported by Sharma et al. (2023). The black geopotential height contours in Figure 4 depict topography (500 m contour emphasized in thick magenta contour).…”

“…Nevertheless, the consistency of our results with the results of Sharma et al. (2023) is intriguing. We also performed some additional analysis, comparing excess and deficit composites of NE premon to check the robustness of our analysis (Robustness analysis in Supporting Information ).…”

“…This mechanism is not applicable in our study due to two reasons: (a) our results are about cases (i.e., seasons) that are about non-drought years that are exceptions given cold north Pacific SST anomalies as per this mechanism; and (b) we find the maximum correlation for the current year and not with north-Pacific SST leading by 1 year (Figures S7 and S8 in Supporting Information S1). We shall argue that a mechanism unraveled by Sharma et al (2023) very recently is relevant here. We adopted a compositing approach to distill a possible mechanism.…”

“…Sharma et al. (2023) found that May rainfall over NE comes from the interaction of the large‐scale circulation with the local orography. The extra‐tropical low‐frequency waves drive a barotropic convergence interacting with the local orography.…”