Can Thermosteric Heights be used as causal indicators of Indian Summer Monsoon Onset?

The Indian Summer Monsoon (ISM) is an outcome of the Intertropical Convergence

Zone (ITCZ) which drives the subcontinent’s economy. There is a growing interest in

investigating the causal mechanisms underlying the monsoon onset. The positive SSH

(Sea Surface Height) anomalies over the West Tropical Indian Ocean (WTIO, 50 to 70°E

& 10°N to 10°S) and the core LLJ zone (low-level south-westerly jets) in the Arabian Sea

are significant contributors of an early monsoon onset. The thermosteric height difference

in the SSH values between the WTIO and the South-east Tropical Indian Ocean (SETIO,

90 to 110°E & 10°S to 0°N) provides an insight to statistically derive causal linkages

to understand how the Precipitable Water Vapor (PWV) is being affected by convective

patterns, which further explains the thresholds and relationships with respect to the

SSH values to accurately ascertain the start of ISM. The intrusion of easterlies into

WTIO from SETIO further tantamount to incremental evaporation owing to carriage of

comparatively warmer sea-water during the late pre-monsoon period (March-April-May),

and thus, have to be causally analysed to introspect its long-term effect in contributing

to the start of ISM over the south-west coast of Kerala (India). In this study, we perform

the causal analysis between thermosteric heights and ISM precipitation to understand the

onset of ISM. The PCMCI+ algorithm gives the causal strengths and lead-lag connections

between the thermosteric component of the SSH values and the ISM onsets to statistically

determine the parent cause behind early/delayed monsoons, emphasizing over the above-mentioned

region of interests. This gives a perception over the inconsistent active and

break phases in the ISM onset patterns over the mainland owing to the variabilities in the

hydrometeorological patterns that adversely affect the Cumulonimbus cloud formation,

and are mainly responsible for the monsoon rainfall over the major climate classes of

India.