Representation and prediction of the Indian Ocean dipole in the POAMA seasonal forecast model

Abstract

Predictive skill of the Indian Ocean Dipole (IOD) using the Bureau of Meteorology's POAMA seasonal forecast model is assessed in light of the model's ability to represent the coupled nature of the IOD. The analysis is based on a ten‐member ensemble of nine‐month forecasts that are initialized from observed ocean and atmosphere states once per month for the period 1982–2006. Some aspects of the IOD are well represented by the POAMA forecast model, including the seasonal phase locking of the development of the IOD but the IOD's horizontal structure is distorted and its amplitude weakens with increasing forecast lead time, while at the same time the IOD becomes spuriously overdependent on ENSO. These deficiencies in the representation of the IOD are attributed to model error (including horizontal resolution and development of biases in the mean state), which affects depiction of important coupled dynamics for the IOD as well as the role of remote forcing of the IOD by ENSO (which itself exhibits distortion of its spatial structure attributed to biases in the simulated mean state). In the light of these limitations of representing the IOD in the POAMA forecast model, skilful prediction of the peak phase of the IOD in September–November is limited to about a four‐month lead time, with a rapid drop in skill for forecasts initialized before June. Forecast skill for the IOD appears to be limited mainly by ability to predict development of SST anomalies in its eastern pole. Potential predictability estimates, assuming a perfect model, indicate great scope for improved prediction, although the IOD is much less predictable (potentially and in practice) than ENSO. The deficient depiction of the IOD at longer lead times means that estimates of potential predictability assuming a perfect model may be unreliable. Copyright © 2009 Royal Meteorological Society