The Indo-western Pacific Ocean Capacitor (IPOC) mode — the coupled mode linking tropical Indian Ocean (TIO) sea-surface-temperature warming and the western North Pacific (WNP) summer anomalous anticyclone in summers following El Niño (or a strong Indian Ocean Dipole) — drives a characteristic tripole of South Asian monsoon rainfall: positive anomalies over the western Ghats and over the Sundarbans–Bangladesh region separated by negative anomalies over the monsoon trough. A single-model study with the NCEP CFSv2 coupled climate model (Climate Forecast System version 2) attributed roughly 80% of the western Ghats anomaly to TIO warming and roughly 75% of the Sundarbans–trough anomalies to the WNP anticyclone, but with imposed SST perturbations an order of magnitude stronger than observed IPOC anomalies, a single neutral background year, and coarse orography. The open problem is to convert this into a model-robust, process-resolving account: decompose orographic versus moisture-convergence contributions at sufficient model resolution, establish the process-level causal chains by which the IPOC background state alters intraseasonal oscillation generation, propagation and active/break life cycles, and verify the fractional attributions across models and forcing amplitudes, including the coupled-model SST-bias limits on using IPOC diagnostics for operational monsoon prediction.

Background

The Indian summer monsoon (ISM) dominates South Asian rainfall, and its interannual variability is strongly controlled by the tropical Indo-Pacific ocean–atmosphere state. After a major El Niño event decays, the tropical Indian Ocean (TIO) stays anomalously warm through the following boreal summer, while an anomalous anticyclone appears over the western North Pacific (WNP). Xie and co-authors showed that the TIO warming charges and then discharges like a capacitor, exciting a Kelvin-wave response (an eastward-propagating equatorial atmospheric wave) that helps maintain the WNP anticyclone — the so-called Indian Ocean capacitor effect (Xie et al. 2009, doi:10.1175/2008JCLI2544.1). The jointly varying TIO–WNP system is called the Indo-western Pacific Ocean Capacitor (IPOC) mode; it can be extracted, for example, as the leading singular value decomposition (SVD) mode between TIO sea surface temperature (SST) and WNP 850 hPa relative vorticity. Reviews (Du et al. 2022, doi:10.3389/fclim.2022.1014138; Gnanaseelan and Chowdary 2021, doi:10.54302/mausam.v70i4.236) show the capacitor can also be triggered by a strong positive Indian Ocean Dipole (IOD), an east–west SST seesaw of the tropical Indian Ocean, without a strong El Niño. The IPOC mode is a leading source of seasonal predictability for the Asian summer monsoons, and skillful one-year-lead predictions of the Asian summer monsoon rely on simulating this post-El Niño–Southern Oscillation (ENSO) Indo–WNP response.

Chowdary, Patekar, Srinivas, Gnanaseelan and Parekh (2019, doi:10.1007/s00382-019-04850-w) established the regional signature of the IPOC mode over South Asia: a summer (June–August) tripole in precipitation, with positive anomalies over parts of the western Ghats (the orographic mountain barrier along India's west coast, 65–75°E, 8–18°N) and over the Sundarbans–Bangladesh region (77–86°E), separated by negative anomalies over the monsoon trough (the seasonal low-pressure zone along the Indo-Gangetic plains). Observations and sensitivity experiments with the NCEP CFSv2 coupled general circulation model (the National Centers for Environmental Prediction's Climate Forecast System version 2) showed that low-level convergence on the northern flank of the WNP anticyclone, acting together with orographic lift, enhances Sundarbans–Bangladesh rainfall; a westward-propagating atmospheric Rossby-wave response to suppressed WNP convection induces moisture divergence and subsidence over the monsoon trough; and TIO basin-wide warming, aided by easterly wind anomalies, feeds moisture convergence off India's west coast that sustains the western Ghats anomaly. Their experiments imposed strong idealized SST anomalies — cooling of −1 to −2 °C over the WNP (isolating the anticyclone effect) or warming of +1 to +2 °C over the TIO (isolating the warming effect) — on a single ENSO/IPOC-neutral background year (1990), yielding estimates that about 75% of the Sundarbans–monsoon-trough anomalies and about 80% of the western Ghats–eastern Arabian Sea anomaly are contributed by the WNP anticyclone and TIO warming, respectively. These imposed amplitudes are roughly an order of magnitude larger than the IPOC-regressed SST anomalies observed (~0.2–0.4 °C). On subseasonal timescales the ISM rainfall oscillates between active and break phases associated with the boreal-summer intraseasonal oscillation (ISO, the 10–60-day northward-propagating convective mode of the monsoon region, closely related to the Madden–Julian Oscillation); the same paper documented that the tripole is weak in June and developed by July–August and that composite ISO active/break circulation anomalies change orientation and propagation in positive-IPOC years, but from only four such years (1987, 1988, 1998, 2010) without significance testing.

Later work has multiplied the contingencies rather than resolved them. The IPOC–ISM rainfall relationship is epoch-dependent on interdecadal timescales (strong in 1895–1926 and 1976–2007, weak in 1932–1972; Patekar et al. 2019, doi:10.1007/s00382-019-05085-5), and an evaluation of 40 CMIP6 (Coupled Model Intercomparison Project phase 6) coupled models found that only about 30% reproduce the observed IPOC–ISM rainfall pattern, with the failure traced to persistent central-equatorial-Pacific SST anomalies and excess North Pacific cooling in the models (Darshana et al. 2022, doi:10.1007/s00382-021-06133-9). ENSO's influence on the ISO is documented regionally — ENSO events modulate ISM intraseasonal oscillations statistically (Joseph et al. 2011, doi:10.1029/2010JD015510), the preceding winter ENSO state controls the interannual meridional shift of East Asian–WNP ISO activity (Lin 2019, doi:10.1038/s41612-019-0088-2), and ENSO shapes WNP ISO intensity through the seasonal-mean background state (Wang and Wu 2020, doi:10.1007/s00382-019-05100-9) — but these are East Asian/WNP or statistical descriptions, not process-level IPOC causal chains for South Asia. A single-season case study of 2020 linked the unusual month-to-month ISM rainfall evolution to a strong WNP anticyclone maintained by TIO-warming-induced Kelvin waves, which CFSv2 predicted with the wrong July sign (Darshana et al. 2024, doi:10.1002/qj.4675), and the 2020 northeast-subcontinent orographically anchored rainfall extreme was tied to the Indo–northwest-Pacific anomalous anticyclone in a climate-projection framing (Tang et al. 2022, doi:10.1029/2021GL096377). The WNP circulation anomaly is increasingly treated as an independent subseasonal predictor of ISM rainfall beyond ENSO (Son et al. 2024, doi:10.1038/s41598-024-73269-z), and the IPOC mode's reach now extends to East Asian atmospheric rivers (Chen et al. 2023, doi:10.1007/s00382-023-07036-7). What no later study supplies is the robust, process-resolving quantitative account of the IPOC → South Asian rainfall link.

Problem Statement

Establish a model-robust, process-resolving quantitative account of how the Indo-western Pacific Ocean Capacitor (IPOC) mode forces the South Asian summer monsoon rainfall tripole. Concretely, determine: (1) the orographic contribution — using model experiments at resolution sufficient to represent the western Ghats and Sundarbans–Bangladesh orography, decompose the IPOC-forced positive rainfall anomalies there into orographic-amplification and moisture-convergence components — defined, respectively, as the sensitivity of the rainfall anomaly to removing the western Ghats and Sundarbans–Bangladesh orography and as the remaining convergence-driven anomaly — and determine how this split depends on model resolution and orographic representation; (2) the subseasonal interaction — establish the process-level causal chains by which the IPOC background state (TIO warming plus WNP anticyclone) alters intraseasonal-oscillation generation, propagation and active/break life cycles over South Asia, and quantify the resulting changes in subseasonal-to-seasonal predictability, going beyond the four-year untested composites of the source study; and (3) the robustness of the attribution — verify, in coupled models beyond a single configuration, the estimates that about 80% of the western Ghats–eastern Arabian Sea anomaly follows from TIO warming and about 75% of the Sundarbans–monsoon-trough anomalies from the WNP anticyclone, including their sensitivity to imposed perturbation amplitude and to background state, and characterize which coupled-model SST biases must be reduced before IPOC diagnostics can be used operationally for Indian summer monsoon prediction. These three components together serve one common objective — replacing a single-model, strong-perturbation, single-background-year account with a model-robust and process-resolved one.

References

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