Wagging the Pacific Dog by its Indian Tail? A west Indian Ocean Precursor to El Niño
El Niño and La Niña have widespread effects on weather, ecology and economy, and hence timely and accurate predictions are of great use to many communities. We are by now reasonably skilled in predicting the next winter's El Niño about 1/2 year in advance (which basically amounts to detecting events that are already growing), but El Niño forecasts at longer lead times remains a challenge: the Spring Predictability Barrier.
Can we sneak around the barrier by considering regions outside the tropical Pacific? Analysis of observational data suggests that the sea surface temperature in a region in the Southwest Indian Ocean - the Seychelles Dome region - is significantly negatively correlated to the El Niño index Nino3.4 a 1.5 years lead time. In other words, a cool Seychelles Dome in boreal summer is likely to be followed by El Niño in the winter 1.5 years later. Can we wag the mighty Pacific dog, El Niño, by its Indian tail?
First, we have to make sure we are not confusing cause and consequences. After all, El Niño is known to influence the Indian Ocean, so our correlation might well be a byproduct of the impact of El Niño on the Seychelles Dome. However, a specially designed statistical test suggests that this is not the case.
Second, we want to understand the underlying physical mechanisms. Further analysis of the observations suggests that a cool Seychelles Dome can cause easterly winds over the west Pacific via a convection feedback above Indonesia. These easterlies "push" warm water towards Indonesia, thereby creating a warm water reservoir which can be used to warm the East Pacific in the next season, thereby causing El Niño. Simulations with a simple model (an Indo-Pacific extension of the Zebiak-Cane model) confirm that this mechanism may work in principle, but suggest that the east Indian Ocean could have more influence on El Niño than the west Indian Ocean / Seychelles Dome. Output from a full climate model (CESM) likewise suggests a strong influence form the east Indian Ocean. This is in disagreement with our initial correlation analysis of the observations.
Did we hunt a ghost, an artifact of a misleading (e.g. too short) data set? Maybe not. CESM, like many climate models, has a bias in the east Indian Ocean which might cause an artificially strong impact of this region onto ENSO. We suggest that this bias should be reduced before studying the subtle Indo-Pacific interactions, and provide an analysis of the possible origin of the bias in CESM.