A Quasigeostrophic Analysis of a Meander in the Palamós Canyon: Vertical Velocity, Geopotential Tendency, and a Relocation Technique

Abstract

The quasigeostrophic geopotential tendency and omega equations are integrated to examine the dynamics of an upper-ocean coastal meander sampled during an intensive survey in the Palamós Canyon (northwestern Mediterranean Sea). Results for dynamic height tendency reveal that the meander is not growing or decaying but is propagating downstream at a velocity of about 4 km day Ϫ1 . This propagation speed implies a problem of synopticity in the field observations, which is solved through a data relocation scheme. The station relocation has important consequences on the magnitude of crucial dynamical variables such as the vertical velocity: maximum values of 20 m day Ϫ1 before the relocation reduce to 10 m day Ϫ1 after the relocation. The impact of bottom boundary conditions in the solution of the omega equation is also analyzed. Results indicate that for the stratification encountered during the survey, effects of topographic forcing are negligible above approximately 300 m.

Towards high resolution mapping of 3-D mesoscale dynamics from observations

“…On the other hand, simplified diagnostic models can be applied to retrieve the vertical velocity field from both 2-D and/or 3-D estimates of geostrophic currents and density fields (e.g. Tintoré et al, 1991;Allen and Smeed, 1996;Buongiorno Nardelli et al, 2001;Pascual et al, 2004;Isern-Fontanet et al, 2008;Ruiz et al, 2009).…”

“…As a second step, a quasi-geostrophic (QG) diagnostic numerical model (the Q vector formulation of the omega equation) has been used to estimate the vertical velocities (Pascual et al, 2004;Ruiz et al, 2009). The omega equation was applied to different MyOcean products (both model and observation-based) in order to quantify the differences/limitations in the diagnostic tools used and the impact of the spatial resolution on the retrieved velocity.…”

“…Carnes et al, 1994;Gavart and De Mey, 1997;Pascual and Gomis, 2003;Meinen and Watts, 2000;Watts et al, 2001;Mitchell et al, 2004). In fact, many of these methods are technically similar to some assimilation schemes (optimal interpolationlike), with the difference that the first guess used, i.e.…”

“…These methods analyze the steric height, temperature and/or salinity covariance and reconstruct the vertical profiles via a combination of a limited number of modes. Following an idea first proposed by Pascual and Gomis (2003), they include an approximation of the geopotential stream function (the steric height profile) in the status vector, thus more directly correlating physical-chemical parameter variability to dynamics. The application of these methods already yielded promising results, also compared to empirical methods such as the computation of the gravest empirical modes (Buongiorno Nardelli and Santoleri, 2005).…”

“…Though more complicated techniques such as the semi-geostrophic omega equation (Viúdez and Dritchel, 2004) have been proposed, the most used technique is based on the solution of the quasi-geostrophic (QG) omega equation (e.g. Tintoré et al, 1991;Buongiorno Nardelli et al, 2001;Pascual et al, 2004;Ruiz et al, 2009), which has already been shown to give reasonable estimates of the vertical velocities compared to primitive equation models (Pinot et al, 1996).…”