Circulation and Dynamics of the Western North Atlantic. Part I: Multiscale Feature Models

Abstract

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.