Theory 24/Earth, Climate & Energy /Monsoon dynamics
Monsoon Onset Is a Soil Gradient
Onset is forecast from ocean warmth. The land may decide when it starts.
Monsoon onset is set by the soil moisture gradient between land and ocean rather than by sea surface temperature, so a dry spring can advance onset and a wet one can delay it at the same ocean temperature.
01The evidence
Onset dates vary widely at similar sea surface temperatures, and the variability tracks pre-season soil moisture.
02The tension
Forecast models emphasise ocean conditions, which predict onset from the ocean's heat and treat land state as a boundary condition.
03The mechanism
The monsoon is a land-sea circulation driven by the pressure contrast. Dry soil heats faster and deepens the low, which strengthens the contrast and pulls the circulation north earlier.
That makes the soil gradient the determinant, and predicts onset should track spring soil moisture at fixed ocean temperature.
04What this is not
Those models predict from sea surface temperature. This predicts from land state, which explains variability at fixed ocean.
05Predictions
- P1Onset should track spring soil moisture.
- P2Dry springs should advance onset.
- P3Land state should predict better than ocean temperature.
06Refutation
Onset tracking ocean temperature at controlled soil state would restore the ocean model.
07What it changes
It would change seasonal forecasting.