Investigating Model Initial Drift in the Tropics in Seasonal Hindcasts

Abstract:

Despite several decades of development, general circulation models are still affected by persistent, unresolved systematic biases, particularly in the Tropics. The problems caused by these biases are especially acute in seasonal and decadal forecasting, where any drift in the model can dominate a forecast. Fixing these biases requires correction algorithms for which the physical basis is difficult to justify.

In this study, we identify the causes of systematic biases present in the tropical Pacific in the ECMWF’s seasonal prediction System 4 model, via the detailed analysis of the initial drift away from observations in retrospective forecasts (“hindcasts”). Specifically, we examine the chain of events that lead to the spurious northward displacement of the West Pacific intertropical convergence zone by a few degrees in operational seasonal hindcasts.

By comparing hindcasts both with a coupled ocean and sea-surface temperatures prescribed from observations, we are able to trace the source of this drift back to a pulse of easterly wind bias in the atmosphere component of the model that lasts for the first 40 days and extends over much of the western Pacific.