ALBERT

Description: This paper shows that the mean flow of an eddy-permitting model can be altered by assimilation of surface height variability, providing that information about the mean sea surface is included, using an adaption of a statistical–dynamical method devised by Oschlies and Willebrand. We show that for a restricted depth range (about 1000 m), dynamical knowledge can make up for the null space present in surface data whose temporal extent may be too short to distinguish between vertical modes. The lack of an accurate geoid has meant that most assimilation methods, while representing variability well, have been unable to modify the mean flow to any extent. However, we show that by including several approximate forms for the mean sea surface, the mean interior flow in the upper kilometer can be rapidly adjusted towards reality by the assimilation, with the location of major current systems moved by hundreds of kilometers.

Description: Seasonal changes in eddy energy are used to investigate the role of high-frequency wind forcing in generating eddy kinetic energy in the oceans. To this end, we analyze two experiments of an eddy-permitting model of the North Atlantic driven by daily and monthly mean wind stress fields, and compare results with corresponding changes in the variance of the wind fields, and related results from previous studies using altimeter and current meter data. With daily wind-stress forcing the model is found to be in general agreement with altimetric observations and reveal a complex pattern of temporal changes in variability over the North Atlantic. Observations and the model indicate enhanced levels of eddy energy during winter months over several areas of the northern and, particularly northeastern North Atlantic. Since the wind-generated variability is primarily barotropic, its signal can be detected mostly in the low-energy regions of the northern and north-eastern North Atlantic, which are remote from baroclinically unstable currents. There the winter-to-summer difference in simulated eddy kinetic energy caused by the variable wind forcing is 〈0.5 cm2 s2 between 30° and 55°N, and is 1–3 cm2 s2 north of 55°N. Seasonal changes in kinetic energy are insignificant along the path of the North Atlantic current and south of about 30°N. The weak depth dependence of the seasonal changes in eddy energy implies that the relative importance of wind-generated eddy energy is maximum at depth where the general (baroclinic) variability level is low. Accordingly, a significant correlation is found between the seasonal cycle in the variance of wind stress and the seasonal cycle in eddy energy over a substantially wider area than near the surface, notably across the entire eastern North Atlantic between the North Atlantic Current and the North Equatorial Current.

Description: Three different, eddy-permitting numerical models are used to examine the seasonal variation of meridional mass and heat flux in the North Atlantic, with a focus on the transport mechanisms in the subtropics relating to observational studies near 25°N. The models, developed in the DYNAMO project, cover the same horizontal domain, with a locally isotropic grid of 1/3° resolution in longitude, and are subject to the same monthly-mean atmospheric forcing based on a three-year ECMWF climatology. The models differ in the vertical-coordinate scheme (geopotential, isopycnic, and sigma), implying differences in lateral and diapycnic mixing concepts, and implementation of bottom topography. As shown in the companion paper of Willebrand et al. (2001), the model solutions exhibit significant discrepancies in the annual-mean patterns of meridional mass and heat transport, as well as in the structure of the western boundary current system. Despite these differences in the mean properties, the seasonal anomalies of the meridional fluxes are in remarkable agreement, demonstrating a robust model behavior that is primarily dependent on the external forcing, and independent of choices of numerics and parameterization. The annual range is smaller than in previous model studies in which wind stress climatologies based on marine observations were used, both in the equatorial Atlantic (1.4 PW) and in the subtropics (0.4–0.5 PW). This is a consequence of a weaker seasonal variation in the zonal wind stresses based on the ECMWF analysis than those derived from climatologies of marine observations. The similarities in the amplitude and patterns of the meridional transport anomalies betwen the different model realizations provide support for previous model conclusions concerning the mechanism of seasonal and intraseasonal heat flux variations: they can be rationalized in terms of a time-varying Ekman transport and their predominantly barotropic compensation at depth. Analysis for 25°N indicates that the net meridional flow variation at depth is concentrated near the western boundary, but cannot be inferred from transport measurements in the western boundary current system, because of significant and complex recirculations over the western half of the basin. The model results instead suggest that the main requirement for estimating the annual cycle of heat flux through a transoceanic section, and the major source of error in model simulations, is an accurate knowledge of the wind stress variation.