This work sits among studies linking ocean currents and heat to atmospheric circulation on icy moons, in the solar wind, and in Earth's oceans. Neighboring articles examine how rotation alters heat transport on Enceladus, how plasma waves carve out voids, and how a lava ocean affects hot spots. Here, the focus is on the tropical Indian Ocean: how subsurface waves, born from winds when La Niña coincides with a negative dipole, drive monsoon convection. This is closer to studies of Martian waves and the ocean conveyor, but with an emphasis on interannual variability and regional rainfall.
The study used IMD gridded rainfall data, NCEP/NCAR reanalyses for circulation, NOAA OLR as a convection proxy, HadISST for sea surface temperature, and ERA5 for subsurface fields. Years of La Niña (by the ONI index) and negative dipole (from the Australian Bureau of Meteorology) overlap were identified from 1974 to 2022. Anomalies were calculated after removing the linear trend, along with the D20 thermocline depth, heat content of the upper 700 m, and meridional heat transport. For verification, a 200-year control run of the IITM-ESM model without anthropogenic forcing was used.
In overlap years, especially 2010 and 2022, westerly wind anomalies prevail over the equatorial Indian Ocean. They excite downwelling Kelvin waves that propagate eastward, deepening the thermocline off Sumatra. The waves then round the southern tip of India and transform into westward-propagating Rossby waves, deepening the thermocline in the northern Arabian Sea. There, a center of low-level wind convergence forms, and a second center arises over the Maritime Continent due to the warm surface in the eastern equatorial region. Both centers draw in moisture and sustain convection. In 2022, the heat content of the upper 700 m in the northern Arabian Sea increased by tens of megajoules per square meter compared to the average, and meridional heat transport north of 8°N strengthened from July. In composite years 1974, 1998, and 2016, this mechanism was absent: convergence over the northern Arabian Sea was missing, and heat content grew more weakly.
Unlike neighboring studies that treat ocean waves as a response to local winds or as heat carriers in the global conveyor, here a chain is shown from equatorial winds through reflected Rossby waves to regional convergence and rainfall. This is the first quantitative link between subsurface dynamics and monsoon anomalies specifically for coinciding La Niña and negative dipole. The IITM-ESM model reproduces the mechanism, strengthening confidence in the conclusion.
One could test how often such a mechanism operates in other CMIP6 climate models and in forecasting systems. A separate question is how Rossby waves interact with intraseasonal monsoon oscillations. Long-term observations of the thermocline in the Arabian Sea, including Argo floats, would help refine the time lags between wind, waves, and rainfall.
The results will affect seasonal forecasting of the monsoon in India and Pakistan, as well as understanding regional climate risks for agriculture and water resources.
Verify the mechanism against independent Argo buoy data and in a CMIP6 model ensemble; assess how often the coincidence of La Niña and negative dipole leads to rainfall anomalies in other regions of South Asia.
For monsoon climatology, an unresolved question remains: why do some coinciding La Niña and negative dipole events produce strong anomalies while others do not. The work shows that the key may lie in subsurface heat content and reflected Rossby waves, not just surface temperature. This links the problem to the broader issue of predictability of interannual variability in the tropical ocean and atmosphere.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The study shows how, in years when La Niña coincides with a negative Indian Ocean Dipole, equatorial westerly winds excite Kelvin waves that, after reflecting off the southern tip of India, turn into Rossby waves and deepen the thermocline in the northern Arabian Sea. This strengthens wind convergence and sustains convection, explaining the rainfall anomalies of 2022. The analysis uses IMD data, NCEP/NCAR reanalyses, NOAA OLR, HadISST, and ERA5, along with a control run of the IITM-ESM model.
Where its strength is
The authors build a quantitative chain from subsurface ocean dynamics to regional rainfall for a specific combination of climate modes, which allows forecasts based on mechanism rather than statistics alone.
Where it could go next
Worth trying
Why this may matter
Understanding subsurface dynamics could improve seasonal monsoon forecasts in years when rare but important combinations of climate modes occur, affecting India's agriculture and water resources.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.58, against a usual 0.64. It looks like few have drilled nearby.
A Kelvin wave born at the equator takes several weeks to reach Sumatra, and the reflected Rossby wave returns to the Arabian Sea even more slowly. The full cycle takes about two months, comparable to the duration of the active phase of the monsoon.