Geostrophy via potential vorticity inversion in the Yucatan Channel

Abstract

It has become common practice to measure ocean current velocities together with the hydrography by lowering an ADCP on typical CTD casts. The velocities and densities thus observed are considered to consist mostly of a background contribution in geostrophic balance, plus internal waves and tides. A method to infer the geostrophic component by inverting the linearized potential vorticity (P V) provides plausible geostrophic density and velocity distributions. The method extracts the geostrophic balance closest to the measurements by minimizing the energy involved in the difference, supposed to consist of P V-free anomalies. The boundary conditions and the retention of P V by the geostrophic estimates follow directly from the optimization, which is based on simple linear dynamics and avoids both the use of the thermal wind equation on the measured density, and the classical problem of a reference velocity. By construction, the transport in geostrophic balance equals the measured one. Tides are the largest source of error in the calculation. The method is applied to six ADCP/CTD surveys made across the Yucatan Channel in the springs of 1997 and 1998 and in the winter of 1998-1999. Although the time interval between sections is sometimes close to one inertial period, large variations on the order of 10 percent are found from one section to the next. Transports range from 20 to 31 Sv with a net average close to 25 Sv, consisting of 33 Sv of in ow into the Gulf of Mexico and 8 Sv of out ow into the Caribbean Sea. The highest velocities are 2.0 m sec 2 1 into the Gulf of Mexico near the surface on the western side of the channel, decreasing to 0.1 m sec 2 1 by 400 to 500 m depth. Beneath the core of the Yucatan Current a countercurrent,with speeds close to 0.2 m sec 2 1 and an average transport of 2 Sv, hugs the slopes of the channel from 500 to 1500 m depth. Our data show an additional 6 Sv of return ow within the same depth range over the abrupt slope near Cuba, which is likely to be the recirculating fraction of the Yucatan Current deep extention, unable to out ow through the Florida Straits. The most signi cant southerly ows do not occur in the deepest portion of the channel, but at depths around 1000 m.

Analysis of flow variability in the Yucatan Channel

“…Velocity data were interpolated on a grid of 105 vertical points (every 20 m) by 39 horizontal points separated by 0.05°of longitude. This objective interpolation method [Roemmich, 1983;Ochoa et al, 2001] uses two different scales of correlation for both longitude and depth. Large correlation scales of 150 km in the horizontal and 1500 m in the vertical serve to define the spatial structure of the mean flow, while smaller correlation scales of 70 km in the horizontal and 400 m in the vertical map the anomalies.…”

“…Large correlation scales of 150 km in the horizontal and 1500 m in the vertical serve to define the spatial structure of the mean flow, while smaller correlation scales of 70 km in the horizontal and 400 m in the vertical map the anomalies. The large scales are chosen based approximately on the size of the channel, whereas short scales are the result of correlation analysis of data anomalies [see Ochoa et al, 2001], although the primary goal was to obtain smooth and plausible maps. A Gaussian correlation function with a signal/noise ratio of 0.1 is used for the large scales and 0.05 for the smaller scales.…”

Simulation of velocities in the Yucatan Channel

“…2. There are in the model several cores of local maxima in flow speed, in locations similar to those found in the observations reported in [2] and [3]. The inflow into the Gulf is found near the surface on the western (Mexican) side of the Channel and has a maximum mean velocity of 1.48 m s -1 (Fig.…”

“…1., our knowledge of the structure and variability of the Channel's flow was limited in the past due to limited direct measurements in the Channel (e.g., [1]). However, recent observations, ( [2], [3]), show a considerably more complex flow field than previously thought, and suggest that the mean Channel transport (~24 Sv; 1Sv=10 6 m 3 s -1 ) is smaller than previous estimates (28-30 Sv; [4]). The new observations also allow a more detailed evaluation of model results than previously was possible.…”

Respuesta de la comunidad de fitoplancton y zooplancton al afloramiento de agua subterránea y surgencia costera en la península de Yucatán, México

“…La región del cabo, cuya profundidad varía entre 3.3 y 14 m, está influenciada por la corriente de Yucatán, que fluye muy cerca de la costa y domina las características oceanográficas de la región. Esta corriente proporciona la mayor parte del flujo al Golfo de México (Ochoa et al 2001). La presencia de un evento de surgencia costera estacional, en el periodo marzo-septiembre, está bien documentado (Merino 1997;ReyesMendoza et al 2015).…”