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Regional observational studies in the North Atlantic have noted significant hydrographical shifts in 1997-98 because of the episodic drop in the North Atlantic oscillation (NAO) during 1996. Investigation using a basin-scale model finds that, although the western North Atlantic (WNA) witnessed unusually low-salinity water by 1997, the eastern North Atlantic (ENA) simultaneously evidenced intrusions of high-salinity water at intermediate depths. This study shows that a major source of high salinity in the ENA is from the northward penetration of Mediterranean Outflow Water (MOW) that occurred concurrently with a westward shift of the subpolar front. The authors confirm that the low-salinity intrusion in the WNA is from enhanced Labrador Current flow. Results from climatological high- and low-NAO simulations suggest that the NAO-induced circulation changes that occurred in 1997-98 are a characteristic North Atlantic basin response to different forcing conditions during characteristic high- and low-NAO periods. [ABSTRACT FROM AUTHOR]

Regional observational studies in the North Atlantic have noted significant hydrographical shifts in 1997-98 because of the episodic drop in the North Atlantic oscillation (NAO) during 1996. Investigation using a basin-scale model finds that, although the western North Atlantic (WNA) witnessed unusually low-salinity water by 1997, the eastern North Atlantic (ENA) simultaneously evidenced intrusions of high-salinity water at intermediate depths. This study shows that a major source of high salinity in the ENA is from the northward penetration of Mediterranean Outflow Water (MOW) that occurred concurrently with a westward shift of the subpolar front. The authors confirm that the low-salinity intrusion in the WNA is from enhanced Labrador Current flow. Results from climatological high- and low-NAO simulations suggest that the NAO-induced circulation changes that occurred in 1997-98 are a characteristic North Atlantic basin response to different forcing conditions during characteristic high- and low-NAO periods. [ABSTRACT FROM AUTHOR]

The multiscale feature models (MSFMs) developed for the circulation of the western North Atlantic (Part I) have been used for initialization in this study to forecast the Gulf Stream meanders and rings. The Harvard primitive equation model, which was calibrated and verified for the statistics of the synoptical dynamics in this region (Part II), provides the basis for these simulations. Three 2-week-long synoptical dynamical hindcasts are presented. These hindcasts are carried out in a forecast mode without assimilating any future information. In general, when compared against SST-derived frontal location and ring-stream interactions, 2-week-long forecasts are found to be statistically superior than persistence and dynamically consistent. These forecasts are also compared against similar forecasts based on the U.S. Navy's Optimum Thermal Interpolation System (OTIS) initializations. It is found that the MSFM-initialized simulations provide a better predictive capability than OTIS-initialized simulations over 2 weeks. Quantitatively, in terms of a statistical measure called the average offset of the axis of the stream, the former did better than persistence in all three cases over the 2-week periods. However, OTIS has two inherent characteristics, namely, a longitudinal distribution of temperature-salinity from climatology, and a warm pool at the core of the stream, which should be incorporated in the MSFM scheme to further enhance its predictive capability. The usefulness of multiscale kinematic synthesis in the initialization of the calibrated primitive equation dynamical model for forecasting the region over a 2-week period brings a closure to this three-part study of circulation and dynamics of the western North Atlantic. [ABSTRACT FROM AUTHOR]

A multiparameter kinematic synthesis of multiscale feature models for the circulation of the western North Atlantic was developed in Part I of this study. The dispersion characteristics and dynamical behavior of this circulation model are presented here in detail. Insertion of the kinematically synthesized features into a numerical dynamical model dynamically adjust the features and provide the basis for a multiparameter sensitivity study with respect to the reasonable range of parameter variation consistent with observations. Two primary characteristics of the Gulf Stream meandering behavior, namely, the wave-growth characteristics and the ring formation and absorption statistics are studied via both quasigeostrophic and primitive equation dynamics. In achieving realistic dispersion characteristics, a comprehensive methodology for dynamical model tuning and validation in this limited region ocean is developed. The realistic regimes of parameter variation are identified on the basis of observational growth rate and phase speed of the Gulf Stream meanders. Long-term simulations within these realistic regimes provide statistics of ring production and interaction behavior. The observed range of transport variability of the Gulf Stream system further constrains the parameter selection. Final tuning of parameters is obtained through an extensive study of the response from the dynamical models to the changes of parameters of the circulation model. Usefulness of the circulation model for initializing, forecasting, and updating in real synoptic cases will be the focus of Part III of this series of studies. [ABSTRACT FROM AUTHOR]

This is the first part of a three-part study on the circulation, dynamics, and mesoscale forecasting of the western North Atlantic. The overall objective of this series of studies is threefold: 1) to present a methodology for deriving a dynamically balanced regional climatology that maintains the synoptic structure of the permanent fronts embedded in a mean background circulation, 2) to present a methodology for using such a regional climatology for calibrating and validating dynamical models, and 3) to use similarly derived synoptic realizations as initialization and assimilation fields for mesoscale nowcasting and forecasting. In this paper, a data-based, kinematically balanced circulation model for the western North Atlantic is developed and described. The various multiscale synoptic and general circulation structures in this region are represented by analytical and analytical/empirical functions based on dynamical considerations and using observational datasets. These include the jet-scale currents, namely, the Gulf Stream and the deep western boundary current, the subbasin-scale recirculating gyres called the southern and the northern recirculation gyres, and the slope water gyre. The inclusion of subbasin-scale gyres as the background circulation for the energetic jet and mesoscale activity in any limited oceanic region is a new paradigm of this multiscale regional modeling study. A generalized kinematical constraint that links the multiscale structures is derived in terms of their interaction scales. For synoptic realizations, the currents and gyres are distorted from their mean state with mass conserving constraints, and mesoscale structures are added thereon. The kinematically balanced linked system is then adjusted via quasigeostrophic dynamics and a regional water-mass model to obtain three-dimensional circulation fields to be used for initialization and assimilation in primitive equation models. [ABSTRACT FROM AUTHOR]