Spatio‐temporal variability in a mid‐latitude ocean basin subject to periodic wind forcing

Abstract

The mid-latitude ocean's response to time-dependent zonal wind-stress forcing is studied using a reduced-gravity, 1.5-layer, shallow-water IntroductionThe wind-driven, basin-scale circulation in the mid-latitude upper ocean exhibits variability on different time scales. This variability could be due to changes in the external atmospheric forcing or to the system's intrinsic instability and nonlinearity. The latter explanation was first put forward by Veronis (1963Veronis ( , 1966, but the former has enjoyed greater popularity in the oceanographic community until fairly recently. A systematic application of the methods of dynamical systems theory to the wind-driven circulation problem has yielded several physical mechanisms for the observed lowfrequency variability, on the time scale of several months to several years . This circulation has been studied in the last decade with a hierarchy of models that include rectangular-basin, as well as more realistic configurations. Pedlosky (1996) reviewed work that used time-independent, single-gyre forcing. Single-gyre results of particular interest since then include Berloff's (1997a, 1997b) work that emphasizes the role of basin size in this problem, and Sheremet et al.'s (1997) classification of instabilities for the single gyre. Chang et al. (2001) reviewed double-gyre results and compared the physical mechanisms for intrinsic variability found in this case with those obtained in the single-gyre problem. We only review here, therefore, the results that are most relevant to the present work. These emphasize the double-gyre problem. Jiang et al. (1995; JJG hereafter) studied the double-gyre wind-driven circulation in a mid-latitude rectangular basin on a β-plane using a 1.5-layer, reduced-gravity, shallow-water (SW) model. They used a time-constant zonal-wind profile, symmetric about the basin's mid-latitude axis. Their results showed that when non-linear processes come into play, multiple steady state solutions satisfying identical boundary conditions can arise for sufficiently strong wind-stress forcing. Two stable solutions were found to co-exist over a certain range of parameters. One had a stronger subpolar gyre and the other a stronger subtropical gyre, with overshooting of the western boundary currents to the north and south of the symmetry axis, respectively. Their periodic solutions had periods of several years and several weeks. Following JJG, Speich et al. (1995) analyzed the dependence of the stationary solutions on the SW model's nondimensional parameters such as the amplitude of the forcing, the Ekman number, the Rossby number, the non-dimensional β parameter, and the bottom drag coefficient. Using pseudo-

Interannual Variability in the North Atlantic Ocean’s Temperature Field and Its Association with the Wind Stress Forcing

“…Still, the statistical evidence of equivalent-barotropic behavior in this region is entirely consistent with the gyremode hypothesis, such as confirmed by several studies with more or less idealized double-gyre ocean models (Jiang et al 1995;Speich et al 1995;Simonnet and Dijkstra 2002;Simonnet et al 2003aSimonnet et al ,b, 2005Sushama et al 2007). Across a hierarchy of models -from simple rectangular-shaped ocean basins and 1.5-layer reducedgravity models (Jiang et al 1995;Speich et al 1995;Chang et al 2001;Simonnet et al 2003a) to more realistic North Atlantic-basin shapes, several layers and eddypermitting resolutions (Simonnet et al 2003b(Simonnet et al , 2005 -it was found that the idealized double-gyre flow becomes unstable to two types of barotropic modes: basin modes and gyre modes.…”

Dynamics of turbulent western-boundary currents at low latitude in a shallow-water model

“…The domain extends further northward than southward, as our research is directed towards studying the SC and NBC, ranging within the most energetic structures in the world's oceans and both occurring north of the Equator. The model is comparable to those used in idealized configuration to study mid-latitude gyres (see, e.g., Jiang et al, 1995;Sushama et al, 2007;Speich et al, 1995) and low latitude WBCs (see, e.g., Edwards and Pedlosky, 1998a;Edwards and Pedlosky, 1998b;Fox-Kemper, 2005). A Cartesian grid is used and the Coriolis parameter varies linearly with latitude; this geometry is usually referred to as the equatorial β-plane.…”

“…The coarse resolution in space and time of satellite data do not allow for a detailed study of these small-scale structures. Such flanking vortices are also present in the laboratory experiments of geophysical fluid dynamics (see, e.g., Van Heijst and Flor, 1989) and are also clearly visible in fine-resolution realistic simulations of the ocean dynamics.…”

The Atmospheric Circulation over the North Atlantic as Induced by the SST Field

“…The gyre mode was shown to be robust with respect to forcing by the seasonal cycle in wind stress (Sushama et al 2007) and had been found in a century of SST data for the North Atlantic (Moron et al 1998). Several other authors, going back to Veronis (1963Veronis ( , 1966 Further evidence that the 7-8-yr atmospheric mode is indeed induced by the neardecadal oscillations of the Gulf Stream front in the CHR and GBR regions is provided by three additional experiments.…”