An Ocean Analysis System for Seasonal to Interannual Climate Studies

Abstract

A dynamical model-based ocean analysis system has been implemented at the National Meteorological Center (NMC). This is used to provide retrospective and routine weekly analyses for the Pacific and Atlantic Oceans. Retrospective analyses have been performed for the period mid-1982 to mid-1993. The analyses are used for diagnostics of past climatic variability, real-time climate monitoring, and as initial conditions for coupled multiseason forecasts. The assimilation system is based on optimal interpolation objective analysis solved using an equivalent variational formulation. Analysis errors are estimated by comparisons to independent datasets such as temperature data from moorings and sea level information from tide gauges. In the near equatorial zone rms errors in thermocline depth are of order of 6?15 m. Comparisons of sea level estimates from the reanalyses with the records from tide gauges indicate that the rms sea level errors for monthly analysis are of the order of 0.04?0.09 m. For the weekly analyses, which potentially have more accurate forcing fields, the rms sea level errors am about 0.02?0.06 m. The analysis system can be used to infer the net heat flux at the air?sea interface on mean annual and interannual timescales. Examination of the dominant components to the oceanic heat budget shows that advection, storage changes, and the net surface heat flux can all be of the same order of magnitude; however, frequently the net surface heat flux is much smaller than the other components. The annual variations in the components are as large or larger than the interannual variability. In the equatorial region interannual changes are of the order of 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 ocean fields revealed an interannual sea level and SST empirical orthogonal function that has an intradecadal timescale. This mode is characterized by meridional adjustments of the thermal field. It is probably forced by the changes in the curl of the stress caused by changes in the intensity and location of the trade winds associated with the ENSO.