Impact of the Ocean In-situ Observations on the ECMWF Seasonal Forecasting System

Abstract:

We evaluate the impact of the in-situ ocean observations on seasonal forecasts. A series of seasonal reforecasts have been conducted for the period 1993-2015, in which different sets of ocean observations were withdrawn in the production of the ocean initial conditions. By comparing the different reforecast sets, it is possible to assess the impact on the forecast of ocean and atmospheric variables.

Results show that the in-situ observations have a profound and significant impact on the mean state of forecast ocean and atmospheric variables, and can be classified into different categories: i) impact due to local air-sea interaction, a direct consequence of changes in the mixed layer in the ocean initial conditions, and visible in the early stages of the forecasts; ii) changes due to different ocean dynamical balances, most visible in the Equatorial Pacific at time scales of 3-4 months; iii) changes to the atmospheric circulation resulting from changes in large scale SST gradients; these are non-local, mediated by the atmospheric bridge, and depend on the differential impact of the observing system in different regions. Additional experiments indicate that there is another sort of changes, due to slow ocean dynamics, which are visible at timescales longer than one year.