AN OCEAN ANALYSIS SYSTEM FOR SEASONAL TO INTERANNUAL CLIMATE STUDIES

A dynamical model-based ocean analysis system has been implemented at the National Meteorological Center (NMC). This is used to provide retr ospective and routine weekly analyses for the Pacific and Atlantic Oce ans. Retrospective analyses have been performed for the period mid-198 2 to mid-1993. The analyses are used for diagnostics of past climatic variability, real-time climate monitoring, and as initial conditions f or coupled multiseason forecasts. The assimilation system is based on optimal interpolation objective analysis solved using an equivalent va riational formulation. Analysis errors are estimated by comparisons to independent datasets such as temperature data from moorings and sea l evel information from tide gauges. In the near equatorial zone rms err ors in thermocline depth are of order of 6-15 m. Comparisons of sea le vel estimates from the reanalyses with the records from tide gauges in dicate that the rms sea level errors for monthly analysis are of the o rder of 0.04-0.09 m. For the weekly analyses, which potentially have m ore accurate forcing fields, the rms sea level errors are about 0.02-0 .06 m. The analysis system can be used to infer the net heat flux at t he air-sea interface on mean annual and interannual timescales. Examin ation of the dominant components to the oceanic heat budget shows that advection, storage changes, and the net surface heat Bur can all be o f the same order of magnitude; however, frequently the net surface hea t flux is much smaller than the other components. The annual variation s in the components are as large or larger than the interannual variab ility. In the equatorial region interannual changes are of the order o f 50-100 W m(-2) and act as a negative feedback to the anomalous SSTs. In the subtropics the interannual variability is only in the order of 5-10 W m(-2). Principal component analysis of the monthly analyzed oc ean fields revealed an interannual sea level and SST empirical orthogo nal function that has an intradecadal timescale. This mode is characte rized by meridional adjustments of the thermal field. It is probably f orced by the changes in the curl of the stress caused by changes in th e intensity and location of the trade winds associated with the ENSO.