Stochastic forcing of the wind-driven ocean circulation.

摘要

Stochastic variations in atmospheric forcing represent a significant source of ocean forecast uncertainty and have been observed to significantly influence the ocean circulation. In this work, the influence of stochastic forcing on the ocean and the most disruptive patterns of stochastic forcing are investigated using ocean models of the North Atlantic Ocean and the Intra-Americas Seas. The results of this thesis are far reaching especially in the realm of ocean forecasting and predictability.; To examine the influence of the stochastic component of the wind stress forcing on the North Atlantic ocean, a quasi-geostrophic model with variable resolution was forced with estimates of the stochastic component of wind stress curl obtained from the NCAR Community Climate Model. Analysis reveals that much of the stochastically-induced variability in the ocean circulation occurs in the vicinity of the western boundary and parts of the Caribbean Sea and Gulf of Mexico.; In order to elucidate the dynamics involved in the stochastically forced system, the ideas of generalized stability theory were used, and it was found that the patterns of wind stress curl that are most effective for inducing variability in the model have their largest projection on the most nonnormal eigenmodes of the system, and tend to be suboptimal patterns of forcing. The excited nonnormal eigenmodes are confined to the western boundary region and are composed of long Rossby wave packets that are Doppler shifted by the Gulf Stream to have eastward group velocity. Linear interference of these eigenmodes yields transient growth of stochastically-induced perturbations, and it is this process that maintains the variance of the stochastically-induced western boundary circulations. Further analysis revealed that a dominant component of the stochastic winds is associated with the North Atlantic Oscillation which accounts for about 31% of the stochastically-induced variability arising from the total stochastic winds.; Due to the significant influence of stochastic forcing in the Caribbean, the influence of variations in surface forcing on the passage transports in the Intra-Americas Sea was also examined using the adjoint of a state of the art ocean general circulation model, the Regional Ocean Modeling System, configured for the IAS. The passage transports are of interest in this region because they ultimately control the flow through the Florida Strait and hence the variability of the Gulf Stream, a significant contributor to regional and global climate. Results showed that the passage transports in the IAS were influenced most by direct local variations in surface wind stress and heat flux forcing and that remote forcing by wind stress curl was also important.