The Role of Deep Mesoscale Eddies in Ensemble Forecast Performance

Abstract: Present forecasting efforts rely on assimilation of observational data captured in the upper ocean (< 1000 m depth). These observations constrain the upper ocean and minimally influence the deep ocean. Nevertheless, development of the full water column circulation critically depends upon the dynamical interactions between upper and deep fields. Forecasts demonstrate that the initialization of the deep field is influential for the development and evolution of the surface in the forecast. Deep initial conditions that better agree with observations have lower upper ocean uncertainty as the forecast progresses. Here, best and worst ensemble members in two 92-day forecasts are identified and contrasted in order to determine how the deep ocean differs between these groups. The forecasts cover the duration of the Loop Current Eddy Thor separation event, which coincides with available deep observations in the Gulf. Model member performance is assessed by comparing surface variables against verifying analysis and satellite altimeter data during the forecast time-period. Deep cyclonic and anticyclonic features are reviewed, and compared against deep observations, indicating subtle differences in locations of deep eddies at relevant times. These results highlight both the importance of deep circulation in the dynamics of the Loop Current system and more broadly motivate efforts to assimilate deep observations to better constrain the deep initial fields and improve surface predictions.