AOSN-II August 2003
The ESSE forecast for August 26, 0000 GMT was initialized from an error nowcast for August 24, 0000 GMT. The background ocean field on August 24, 0000 GMT is a HOPS forecast simulation which assimilates all calibrated data up to August 20, 000GMT. For the period August 20, 000GMT- August 23, 13GMT, only the data that could be ftp-ed and quality controlled has been assimilated.
The dominant 275 eigenvectors of the error covariance estimate for August 24, 0000 GMT was utilized to perturb the ocean fields on August 24, 0000 GMT. A white noise of an amplitude proportional to the estimated absolute and relative errors in the observations is added to this random combination, in part to represent the errors truncated by the error subspace. An ensemble of 2-day forecast simulations, each forced by forecast COAMPS atmospheric fluxes issued for Aug 24, was then carried out.
In the first table below, 15 forecast simulations are utilized to estimate the uncertainties on August 26, 0000 GMT (August 25, 5PM PDT): this forecast was issued on Aug 24. In the second table, 450 forecast simulations are employed: these additional ensemble members were computed in parallel during the night fo Aug 24 and completed on Aug 25, noon (EST).
| Full Domain | |||
| Temperature | Salinity | U velocity component | V velocity component |
| Monterey Bay Zoom | |||
| Temperature | Salinity | U velocity component | V velocity component |
| Barotropic Streamfunction (including Monterey Bay) | |||
| Full Domain | |||
| Temperature | Salinity | U velocity component | V velocity component |
| Monterey Bay Zoom | |||
| Temperature | Salinity | U velocity component | V velocity component |
| Barotropic Streamfunction (including Monterey Bay) | |||
Computations. The size of the ESSE ensemble is controlled by convergence criteria and by the computational resources available (Sun workstations). In the present case a similarity coefficient that measures the added value of new ensemble members is utilized for convergence criteria. The similarity coefficient compares the singular value decomposition of the most recent ensemble with that of a smaller ensemble (previously computed portion of the most recent ensemble), comparing the respective dominant singular vectors weighted by their singular values. Presently, the ensemble of 450 members is found to be similar at 86 percent to the ensemble of 225 members. Note that in this criteria, both the variances and covariances are taken into account. As shown by the error standard deviations in the above tables, the variances usually converge faster than the covariances.
Dynamics and uncertainties: In Monterey Bay, the forecast indicates the start of the re-establishment of a cyclonic circulation, mainly in response to upwelling favorable atmospheric forcings. The seeds of the cyclonic eddy are visible near the center of the Bay . In the T and S uncertainties, larger uncertainties are forecast around the Bay in surface (in relation to the uncertain strength of the cyclonic circulation) and more towards the center of the Bay at depth (in relation to the position of the core of the eddy). Near the center of the Bay, larger uncertainties are visible in the velocity errors (especially at 30m) and in the barotropic streamfunction errors (about 3.4-3.8e-2 Sv uncertainty in the center).Another larger source of uncertainty is related to a predicted meander of the coastal current which advects warm and fresh waters towards the Monterey Bay Peninsula. The position and strength of this meander are very uncertain. In fact, the analysis of the different ensemble members shows that the meander can be very weak (almost not present) or further north than the central forecast indicates.
In the full domain, uncertainties continue to be related to the position and strength of the different meanders and eddies of the coastal current and upwelling plumes system.