ANALYSIS OF THE NORTH-ATLANTIC CLIMATOLOGIES USING A COMBINED OGCM ADJOINT APPROACH/

An exact adjoint for the full-scale Bryan-Cox primitive equation model is applied to assimilate the North Atlantic climatologies. The invers e calculation aims at searching a steady state oceanic general circula tion consistent with observations, by controlling the model input para meters (such as the initial states and the upper thermal and haline bo undary conditions). Two climatological hydrographies (Levitus (1982) a nd Fukumori and Wunsch (1991)) are used for the assimilation. This ena bles the examination of the sensitivity of the assimilated results to data quality. In addition, the consistency between the climatological hydrography and fluxes is discussed by examining the fits between the optimally estimated surface fluxes and the fluxes calculated by Oberhu ber (1988). The efforts made in the study are directed toward assessin g the effectiveness of the combined OGCM/adjoint approach in estimatin g the state of the ocean from climatologies and identifying the associ ated problems. The major findings of the study include: (1) The result s show that the full OGCM dynamics substantially helps the model in be tter simulating the frontal structure of the Gulf Stream system and th e large-scale features of the velocity field, thus demonstrating the a dvantage of the full OGCM and its exact adjoint. (2) The study finds t hat the optimized temperature field has a systematic error structure i n the vertical-the upper ocean is cooler and the deep ocean is wanner compared to the climatology. Our analysis indicates that the cool surf ace layer is a correction imposed by the optimization to reduce large data misfits in the deep ocean due to the deep warming. This deep warm ing is an outcome of using the steady state assumption, the annual mea n climatology and the relaxation boundary condition at the model north ern boundary. The annual mean hydrography has a surface water warmer t han the observed winter surface water, and a deep ocean whose properti es are determined by the surface water at high latitudes. Due to the i mposed model northern boundary condition, the modeled deep waters are formed through the artificial sinking of surface waters with annual-me an temperature in the relaxation zone. This process leads to a warm de ep ocean and large model-data discrepancies in the vast deep layer. In order to reduce the misfits as required by the optimal procedure, the surface layer which is the source for the modeled deep water needs to be cooler. The strong and deep vertical mixing formed in the model pr ovides the means for an effective cooling. The results further show th at the surface cooling is stronger for the experiment assimilating the Fukumori and Wunsch hydrography because this climatology has an even warmer surface water due to the use of the summer-dominated data sourc e. (3) The experiments assimilating the Levitus hydrography illustrate two anomalous features, one is a strong zonally integrated upwelling in the midlatitude and the other a very noisy flux estimation. The ana lysis shows that both features are induced by the smeared representati on of the Gulf Stream frontal structure in the Levitus hydrography, wh ich indicates that data quality is one of the important factors in obt aining satisfactory results from the assimilation. (4) Although the re quirements for a global minimum are only partially satisfied, the expe riments show that, comparing with the Levitus hydrography, the Fukumor i and Wunsch hydrography is dynamically more compatible with the Oberh uber climatological fluxes.