A “Dressed” Ensemble Kalman Filter Using the Hybrid Coordinate Ocean Model in the Pacific

The computational cost required by the Ensemble Kalman Filter （EnKF） is much larger than that of some simpler assimilation schemes, such as Optimal Interpolation （OI） or three-dimension variational as- similation （3DVAR）. Ensemble optimal interpolation （EnOI）, a crudely simplified implementation of EnKF, is sometimes used as a substitute in some oceanic applications and requires much less computational time than EnKF. In this paper, to compromise between computational cost and dynamic covariance, we use the idea of ＂dressing＂ a small size dynamical ensemble with a larger number of static ensembles in order to form an approximate dynamic covariance. The term ＂dressing＂ means that a dynamical ensemble seed from model runs is perturbed by adding the anomalies of some static ensembles. This dressing EnKF （DrEnKF for short） scheme is tested in assimilation of real altimetry data in the Pacific using the HYbrid Coordinate Ocean Model （HYCOM） over a four-year period. Ten dynamical ensemble seeds are each dressed by 10 static ensemble members selected from a 100-member static ensemble. Results are compared to two EnKF assimilation runs that use 10 and 100 dynamical ensemble members. Both temperature and salinity fields from the DrEnKF and the EnKF are compared to observations from Argo floats and an OI SST dataset. The results show that the DrEnKF and the 100-member EnKF yield similar root mean square errors （RMSE） at every model level. Error covariance matrices from the DrEnKF and the 100-member EnKF are also compared and show good agreement.

The cold pool of the Bay of Bengal and its association with the break phase of the Indian summer monsoon

During the summer monsoon season, strong coastal upwelling occurs along the southwest coast of India and at the southern tip of India, which cools the surface temperature of the waters around these regions. The summer monsoon current carries the upwelled cold waters into the Bay of Bengal and forms the ‘cold pool of the Bay of Bengal', with its core south of Sri Lanka and over the southcentral Bay of Bengal. The present study focuses on the intrusion of these cold waters into the south of the Bay of Bengal, its interannual variability, and its association with the surface wind during the break phase of the summer monsoon, when strong westerly surface winds flow south of 10°N.The authors hypothesize that the enhanced cooling in the cold pool region during monsoon spells is associated with the strong westerly wind stress there during the break spells of the monsoon.Seven cases of long break monsoon spells that occurred during the nine years from 2001 to 2009 are analyzed, and the results confirm our hypothesis.