Ocean Model Validation and Downscaling for Subseasonal-to-Seasonal Prediction

Abstract

Subseasonal-to-seasonal (S2S) prediction is a global effort to forecast the state of the atmosphere and ocean with lead times between two weeks and a season. This thesis explores the feasibility of S2S prediction of the ocean using a variety of tools including statistical analysis, a statistical-dynamical mixed layer model, and regional, high-resolution ocean circulation models based on physical principles.

First, ocean predictability on S2S timescales is analyzed by compositing winter sea surface temperature (SST) anomalies in the North Atlantic with respect to the Madden-Julian Oscillation (MJO). It is found that statistically significant, large-scale SST changes, particularly along the eastern seaboard of North America, can be related to the MJO. This signal is shown to be driven by anomalous air-sea heat fluxes caused by atmospheric perturbations in response to the MJO.

Next, the suitability of a high-resolution model of the Gulf of Maine and Scotian Shelf (GoMSS) as a tool for S2S prediction is demonstrated through extensive validation with a focus on the mean state and low-frequency changes. The mean dynamic topography (MDT) predicted by GoMSS is shown to be in good agreement with novel observations of geodetically referenced sea levels from coastal tide gauges. It is shown that the alongshore tilt of MDT can be used to make inferences about coastal circulation, and also upwelling averaged over an adjacent offshore region. A new method is developed for evaluating model predictions of MDT in shallow, tidally dominated regions using observations of overtides and mean currents. This is useful in regions where no sufficiently long sea level records exist for application of the geodetic approach to estimating coastal MDT.

Finally, the validated GoMSS model is used to predict the mean ocean response to the MJO. The model is able to capture the observed relationship between the MJO and SST in the northwest Atlantic. It is also shown that the anomalous atmospheric circulation in response to the MJO leads to anomalous upwelling on the Scotian Shelf. Overall, this thesis demonstrates that it is feasible, and of value, to use regional ocean models for S2S prediction.