Near-Surface Shear Flow in the Tropical Pacific Cold Tongue Front*

Abstract

Near-surface shear in the Pacific cold tongue front at 28N, 1408W was measured using a set of five moored current meters between 5 and 25 m for nine months during 2004-05. Mean near-surface currents were strongly westward and only weakly northward (;3 cm s 21). Mean near-surface shear was primarily westward and, thus, oriented to the left of the southeasterly trades. When the southwestward geostrophic shear was subtracted from the observed shear, the residual ageostrophic currents relative to 25 m were northward and had an Ekman-like spiral, in qualitative agreement with an Ekman model modified for regions with a vertically uniform front. According to this ''frontal Ekman'' model, the ageostrophic Ekman spiral is forced by the portion of the wind stress that is not balanced by the surface geostrophic shear. Analysis of a composite tropical instability wave (TIW) confirms that ageostrophic shear is minimized when winds blow along the front, and strengthens when winds blow oblique to the front. Furthermore, the magnitude of the near-surface shear, both in the TIW and diurnal composites, was sensitive to near-surface stratification and mixing. A diurnal jet was observed that was on average 12 cm s 21 stronger at 5 m than at 25 m, even though daytime stratification was weak. The resulting Richardson number indicates that turbulent viscosity is larger at night than daytime and decreases with depth. A ''generalized Ekman'' model is also developed that assumes that viscosity becomes zero below a defined frictional layer. The generalized model reproduces many of the features of the observed mean shear and is valid both in frontal regions and at the equator.

The dynamics of waves interacting with the Equatorial Undercurrent

“…In contrast to the classical model of uniform vertical viscosity, due to Stommel (1960), we assume that ν(z) is depth-dependent. This assumption is consistent with measurements and was introduced by Cronin and Kessler (2009). Associated with (A.1) is the knowledge of the surface wind stress, τ 0 < 0,…”

“…We will also impose this condition, known to be realistic for the central Pacific region (cf. Cronin and Kessler 2009). Differentiating the first equation in (A.1) with respect to the z-variable and the second equation with respect to the x-variable, their difference yields (ν u z ) z z = 0 for −h < z < 0.…”

Remote sensing of ocean surface currents: a review of what is being observed and what is being assimilated

“…with A v (z) being the eddy viscosity (e.g., Polton et al, 2005;Cronin and Kessler, 2009;Wenegrat and McPhaden, 2016). It is common to rewrite Eq.…”

“…However, an alternative approach is obtained by differentiating Eq. (7) and manipulating it to obtain an equation for the turbulent stress τ (x, z) known as the generalized Ekman model (Bonjean and Lagerloef, 2002;Cronin and Kessler, 2009;Wenegrat and McPhaden, 2016) or the turbulent thermal wind balance (Gula et al, 2014;McWilliams et al, 2015):…”

A steady, purely azimuthal flow model for the Antarctic Circumpolar Current

“…The classical model of uniform eddy viscosity is due to [17]. We follow the more realistic approach with a depth dependent viscosity function as it was introduced in [8]. For solutions of the equatorial current system and their qualitative properties under the assumption of depth-dependent eddy viscosity we refer to [3,13,16] and the references therein.…”