Patekar et al. (2020) showed that the influence of the Indo-western Pacific Ocean Capacitor (IPOC) mode — the coupled tropical Indian Ocean warming / western North Pacific anticyclone mode — on Indian summer monsoon (ISM) rainfall is not stationary: it is strong in the multi-decadal epochs 1895–1926 and 1976–2007 but collapses in 1932–1972, when persistent central/eastern equatorial Pacific warm anomalies and a North Pacific cyclonic anomaly squeeze, weaken, and northeastwardly displace the western North Pacific (WNP) anticyclone, disorganizing the inter-basin coupling and the Indian rainfall dipole. Three linked issues remain unresolved beyond this diagnostic mechanism: the physical determinants of differences in the decay phase of El Niño (what controls how long equatorial Pacific sea surface temperature (SST) anomalies persist into the following spring and summer), the hypothesized but untested contribution of Pacific Decadal Oscillation (PDO)-like decadal variability to the epochal disruption, and the need for causal confirmation through process-level diagnostics and targeted model experiments rather than observational covariation. Later work corroborates and neighbors this question — Coupled Model Intercomparison Project Phase 6 (CMIP6) evaluation confirms the modulation exists in models but attributes model failure to biases; Liu et al. (2021) show the Interdecadal Pacific Oscillation modulates El Niño–Southern Oscillation (ENSO) decay speed and Indian Ocean warming persistence; Xie and Wang (2020) document PDO-associated decadal WNP anticyclone variability — but the physical determinants of the epochal modulation of the IPOC–ISM relationship have not been established or causally confirmed. The problem remains open.

Background

The Indian summer monsoon (ISM) rainfall is modulated on interannual timescales by tropical Indo-Pacific sea surface temperature (SST) anomalies. A central player is the Indo-western Pacific Ocean Capacitor (IPOC) mode: during the summer following a mature El Niño, the tropical Indian Ocean (TIO) retains a basin-wide warming — the Indian Ocean basin (IOB) mode, which peaks in spring and persists into summer — while an anomalous anticyclonic circulation appears at low levels over the western North Pacific (the WNP anticyclone, WNPAC). The TIO warming forces an atmospheric equatorial Kelvin wave (an eastward-propagating wave that adjusts tropical winds to heating) into the western Pacific; the resulting Ekman divergence (wind-induced deflection of the near-surface flow) suppresses convection and maintains the WNPAC, which feeds back on the Indian Ocean — hence "capacitor": the Indian Ocean stores the El Niño signal and discharges it into the following summer's atmosphere (Xie et al. 2009). The WNPAC is the low-level node of the Pacific–Japan (PJ) teleconnection pattern, a meridional atmospheric wave train connecting tropical western Pacific convection to East Asian summer climate. Chowdary et al. (2019) showed that the IPOC mode induces a southwest–northeast rainfall dipole over India, with positive anomalies over the southern peninsula and negative anomalies over the eastern Indo-Gangetic plains. Patekar et al. (2020) asked whether this interannual influence is stationary over the instrumental record. Extracting the IPOC mode as the leading singular value decomposition (SVD) mode — a coupled-pattern analysis that finds pairs of spatial patterns maximizing the covariance between two fields — between TIO SST and WNP 850 hPa (a pressure level near 1.5 km altitude, representative of the low-level flow) relative vorticity for June–August of 1886–2014, they found pronounced multi-decadal modulation: the IPOC–ISM rainfall relationship is strong in 1895–1926 (epoch-1) and 1976–2007 (epoch-3), but weak in 1932–1972 (epoch-2). In epoch-2 the coupled mode is disorganized: warm SST anomalies persisting through spring and summer in the central/eastern equatorial Pacific, together with a cyclonic circulation anomaly over the North Pacific, squeeze the WNPAC between them, weakening it and shifting it northeastward; this disrupts the PJ pattern and the TIO–WNP inter-basin feedback, and the Indian rainfall dipole disappears. The epoch-dependent variance fractions of the leading SVD mode (about 59%, 53%, and 77% in epochs 1–3) and an independent coupling metric based on empirical orthogonal function (EOF) analysis — decomposition of a field's covariance structure into its dominant uncorrelated spatial patterns, each with an associated time series — support this diagnosis. The mechanism was, however, established diagnostically — through regressions and EOF/SVD decompositions — and what remains unresolved beyond it includes: (i) the physical determinants of differences in the decay phase of El Niño, i.e. what controls how long and how far central/eastern equatorial Pacific SST anomalies persist into the following spring and summer (candidate processes include changes in background state, wind forcing, upwelling, and slow ocean-wave adjustments); (ii) whether Pacific Decadal Oscillation (PDO)-like SST patterns — the leading decadal SST mode of the North Pacific, whose Pacific-wide manifestation is often called the Interdecadal Pacific Oscillation (IPO) — or other basin-scale low-frequency variability determine the Pacific anomalies that disrupt IPOC in low-correlation epochs; and (iii) causal confirmation of the hypothesized epoch-dependent chain, for which process-level diagnostics, targeted sensitivity experiments, or model perturbation studies offer evidence beyond covariation alone. Adjacent results frame but do not answer these questions: the interdecadal PJ pattern is forced by the IPO and the IOB mode and weakens after the 1960s (Wu et al. 2016); the leading decadal mode of WNPAC variability is PDO-associated, with era-dependent maintenance mechanisms and a phase-relationship shift around 1966 (Xie and Wang 2020); and the IPO phase modulates ENSO decaying speed — through zonal advective and thermocline (the sharp subsurface boundary between warm surface water and cold deep water) feedbacks — thereby controlling whether Indian Ocean basin warming persists into the decaying summer (Liu et al. 2021).

Problem Statement

Identify and causally confirm the physical mechanisms that produce the epochal (multi-decadal) modulation of the Indo-western Pacific Ocean Capacitor (IPOC) mode's influence on Indian summer monsoon rainfall documented by Patekar et al. (2020): what causes the coupled tropical Indian Ocean sea surface temperature (SST)–western North Pacific circulation mode to organize and drive the southwest–northeast Indian rainfall dipole in the epochs 1895–1926 and 1976–2007, but to be disrupted — through the weakening and northeastward displacement of the western North Pacific (WNP) anticyclone by persistent central/eastern equatorial Pacific warm anomalies and North Pacific cyclonic anomalies — in the epoch 1932–1972? Patekar et al. (2020) established the proximal squeeze mechanism diagnostically, from observations and reanalysis; the open target is the underlying physical determinants of why the disruptive Pacific anomalies characterize the low-correlation epoch. Central candidate determinants, whose relative roles must be discriminated, are Pacific Decadal Oscillation (PDO)/Interdecadal Pacific Oscillation (IPO)-like basin-scale decadal variability (or other low-frequency modes) as a driver of the Pacific anomalies that disrupt the IPOC mode in low-correlation epochs, and inter-event differences in the decay phase of El Niño — the processes controlling how long and how far central/eastern equatorial Pacific SST anomalies persist into the following spring and summer (including the roles of background-state changes, wind forcing, upwelling, and slow ocean-wave adjustments). The target is to establish the causal physical determinants of the epochal modulation, beyond the documented observational covariation, and to discriminate the relative roles of the candidate determinants named above.

References

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