Julio Sheinbaum

Geostrophy via potential vorticity inversion in the Yucatan Channel

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 (PV) 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 PV-free anomalies. The boundary conditions and the retention of PV 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 inflow into the Gulf of Mexico and 8 Sv of outflow into the Caribbean Sea. The highest velocities are 2.0 m sec 1 into the Gulf of Mexico near the surface on the western side of the channel, decreasing to 0.1 m sec 1 by 400 to 500 m depth. Beneath the core of the Yucatan Current a countercurrent, with speeds close to 0.2 m sec 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 flow 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 outflow through the Florida Straits. The most significant southerly flows do not occur in the deepest portion of the channel, but at depths around 1000 m.

Seasonal heat balance in the upper 100 m of the equatorial Atlantic Ocean

The variability of sea surface temperature (SST) in the equatorial Atlantic is characterized by strong cooling in May-June and a secondary cooling in November-December. A numerical simulation of the tropical Atlantic is used to diagnose the different contributions to the temperature tendencies in the upper ocean. Right at the equator, the coolest temperatures are observed between 20°W and 10°W due to enhanced turbulent heat flux in the center of the basin. This results from a strong vertical shear at the upper bound of the Equatorial Undercurrent (EUC). Cooling through vertical mixing exhibits a semiannual cycle with two peaks of comparable intensity. During the first peak, in May-June, vertical mixing drives the SST while during the second peak, in November-December, the strong heating due to air-sea fluxes leads to much weaker effective cooling than during boreal summer. Seasonal cooling events are closely linked to the enhancement of the vertical shear just above the core of the EUC, which appears to be not driven directly by the strength of the EUC but by the strength and the direction of the surface current. The vertical shear is maximum when the northern branch of the South Equatorial Current is intense. The surface cooling in the eastern equatorial Atlantic is not as marked as in the center of the basin. Mean thermocline and EUC rise eastward, but a strong stratification, caused by the presence of warm and low-saline surface waters, limits the vertical mixing to the upper 20 m and disconnects the surface from subsurface dynamics.

Variational Assimilation of XBT Data. Part 1

Abstract A variations method based on the adjoint equation technique is used to assimilate data in a relatively simple linear reduced gravity model of the tropical Pacific. Real XBT data are used by identifying the depth of the 16°C isotherm depth with the model layer depth. It is shown that the XBT data contain large scale information that corrects the model first guess. However, the model is not capable of fitting the data in the eastern Pacific for the whole assimilation period. Regions not seeded by the data are explicitly shown and the impact of data from different times on the initial state is also discussed.

Variational Assimilation of XBT Data. Part II. Sensitivity Studies and Use of Smoothing Constraints

Abstract A linear reduced-gravity model of the tropical pacific is used to assimilate XBT data. The model cannot fit the data in the eastern equatorial Pacific for the whole assimilation period. Several experiments with real and simulated data are performed to investigate the source of this deficiency, which may be in the model or the wind stress used to force the model. It is shown that on the basis of the simple model physics we cannot unambiguously partition the error between model and forcing in the real data assimilation experiments although simulated data experiments do permit discrimination between model and forcing errors. Because the data is incomplete and does not permit a unique determination of the initial state, the use of prior information in the form of first-guess fields and/or smoothing constraints is examined. The filtering characteristics of the optimization algorithm are also discussed by looking at the evolution of the initial conditions as a function of the iteration number.

Seasonal Modes of Surface Cooling in the Gulf of Guinea

A numerical simulation of the tropical Atlantic Ocean indicates that surface cooling in upwelling zones of the Gulf of Guinea is mostly due to vertical mixing. At the seasonal scale, the spatial structure and the time variability of the northern and southern branches of the South Equatorial Current (SEC), and of the Guinea Current, are correlated with the timing and distribution of turbulent heat fluxes in the Gulf of Guinea. Through modulation of the velocity shear at the subsurface, these surface currents control the vertical turbulent exchanges, bringing cold and nutrient-rich waters to the surface. This mechanism explains the seasonality and spatial distribution of surface chlorophyll concentrations better than the generally accepted hypothesis that thermocline movements control the nutrient flux. The position of the southern SEC explains why the cold tongue and high chlorophyll concentrations extend from the equator to 4°S in the southeastern part of the basin.

Upper‐Layer Circulation in the Approaches to Yucatan Channel

The Yucatan Current originates where the Cayman Current turns northwards off the Mayan coast. The latitude at which the current meets the coast fluctuates every few months from the Mexico-Belize border to the island of Cozumel, modulating the currents near the coast. A two-year measurement program recorded the Yucatan Current transporting close to 23 Sv, smaller than the classical value for the Florida Current off Miami, so parts of the Florida Current must flow through Windward Passage or bypass the Caribbean entirely. The velocity fluctuations of the Yucatan Current are larger than the mean everywhere except in its core, and have been attributed mostly to the passage of eddies. About one fourth of the Yucatan Current flows through Cozumel Channel, where significant ageostrophic episodes have been recorded, related to the curvature of the currents entering the channel, and caused possibly by the passage of large mesoscale features. The current also appears to shift laterally, including reversals of the southerly Cuban Countercurrent on the eastern side of the channel. The proper observation of eddies passing offshore might shed better light on the nature of current fluctuations and on the triggering of Loop Current eddies, but they remain elusive even in their bulk integral characteristics, as both the instrumentation deployed and remote sensors appear insufficient to resolve them. A construction of pseudo-streamlines from moored observations is used to signal large eddies passing through the channel.

Seasonal and Interannual Modulation of the Eddy Kinetic Energy in the Caribbean Sea

AbstractVariability of the mesoscale eddy field in the Caribbean Sea is analyzed over the period 1993–2009 using geostrophic anomalies derived from altimeter data and a high-resolution regional model. The Colombia Basin presents the largest values of eddy kinetic energy (EKE) and its semiannual cycle, with a main peak in August–October and a secondary peak in February–March, is the dominant feature in the whole Caribbean EKE cycle. The analysis of energy conversion terms between low-frequency currents and eddies explains these peaks by enhanced baroclinic and barotropic instabilities, in response to seasonally varying currents in the region of the Guajira Peninsula. The semiannual acceleration of the atmospheric Caribbean low-level jet intensifies the southern Caribbean Current (sCC) twice a year in this region, together with its vertical and horizontal velocity shears. The asymmetry of the EKE seasonal cycle in the Colombia Basin is explained by a summer peak in the annual cycle of the whole sCC. Numeric...

Heat Balance and Eddies in the Caribbean Upwelling System

AbstractThe upper-ocean heat budget of the Caribbean upwelling system is investigated during the onset of the Atlantic warm pool (June–September) using high-resolution observations of sea surface temperature and a high-resolution (°) regional model. Vertical mixing is found to be the major cooling contribution to the mixed layer heat budget in the nearshore and offshore Colombia Basin. Numerical results show that intense mesoscale eddies in the Colombia Basin significantly shape the turbulent cooling and may participate in the maintenance of cooler temperature in this region compared to surrounding areas. Indeed, increased mixing at the base of the mixed layer occurs below energetic surface jets that form on the downstream side of the eddies. These jets generally flow offshore and may arise from the deformation of the surface mesoscale field. It is shown that significant contribution of horizontal advection to the mixed layer heat budget is limited to a radius of 300 km around the Guajira and Margarita up...

Vertical Velocity and Vertical Heat Flux Observed within Loop Current Eddies in the Central Gulf of Mexico

Abstract Sixteen months of observations from a surface-to-bottom mooring in the central Gulf of Mexico show that acoustic Doppler current profilers (ADCPs) are useful for directly measuring the vertical velocity within mesoscale anticyclonic eddies, such as those shed from the Loop Current; and combining simultaneous temperature measurements, vertical heat flux can also be estimated (as a covariance of both variables). There is evidence of significant and coherent signals of vertical velocity ∼2–3 mm s−1 and vertical heat (temperature) transport ∼10−3 °C m s−1 during the presence of three anticyclones. A simple analysis shows downward flow near the eddies’ centers above 350 m and essentially upward flow in the peripheries, but below 700-m depth the pattern is indeed the opposite; however, further study is necessary to determine the eddies’ interior structures. The observations also suggest the existence of a vertical convergence of heat somewhere around 600-m depth, and estimations of adiabatic heat flux ...

Canek: Measuring Transport in the Yucatan Channel

The Yucatan Channel is one of the key restrictions of the North Atlantic surface circulation, and also a privileged location to understand the circulation within the Gulf of Mexico and the Caribbean Sea. From September 1999 to June 2000 a set of eight instrumented moorings measured currents and temperatures across the Yucatan Channel. The main result of such measurements, published elsewhere, is that the mean transport amounts to only 23 Sv instead of the 28–30 Sv believed until recently to be a robust value. This result implies the need of an overall review of the other Gulf Stream’s sources. Here we show that the second period of measurements with essentially the same array, from July 2000 to May 2001 is in good agreement with the first period’s mean transport. The correlation functions of both velocity components, in the subinertial band, show 500 m in the vertical and 70 km in the horizontal as the characteristic scales. An array of only fourteen currentmeters, slightly optimized in position, scattered through the section allows an adequate estimation of transport and fluctuations, with a standard error of 0.3 Sv. But an array of even more time series from only three moorings, each consisting of a near-surface, upward-looking ADCP plus two single point currentmeters distributed below, degrades the skill down to 2.5 Sv; this is because each ADCP measures highly correlated series, and the other meters are too far apart, thus producing a poor coverage.