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Abstract
The 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 (
Abstract
The 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 (
Abstract
Data from five moorings deployed in the Bay of Campeche during November 2007–July 2008 are used to analyze subinertial motions of waters below 1000-m depth. To the authors’ knowledge, this is the first time such a comprehensive observational program of direct deep-current measurements has been carried out in the region. The mean currents are in agreement with a cyclonic circulation at 1000-m depth; however, this cyclonic pattern is not so clearly defined at deeper levels. Only at the deepest mooring, located at 3500-m depth, are the mean currents uniform all the way to the bottom. Over the Bay of Campeche’s smooth western slope, currents show features compatible with topographic Rossby waves having vertical trapping scales thicker than 700 m, periods between 5 and 60 days, and horizontal wavelengths of 90–140 km. In contrast, the eastern slopes are characterized by rough topography, and motions with periods longer than 28 days decrease toward the bottom, suggesting a substantial reduction in the low-frequency topographic Rossby wave signal. Velocities from one of the two neighboring moorings located over the eastern rough slope have a strong 3-day period signal, which increases toward the bottom and has a vertical trapping scale of about 350 m. These higher frequency motions are interpreted in terms of edge waves.
Abstract
Data from five moorings deployed in the Bay of Campeche during November 2007–July 2008 are used to analyze subinertial motions of waters below 1000-m depth. To the authors’ knowledge, this is the first time such a comprehensive observational program of direct deep-current measurements has been carried out in the region. The mean currents are in agreement with a cyclonic circulation at 1000-m depth; however, this cyclonic pattern is not so clearly defined at deeper levels. Only at the deepest mooring, located at 3500-m depth, are the mean currents uniform all the way to the bottom. Over the Bay of Campeche’s smooth western slope, currents show features compatible with topographic Rossby waves having vertical trapping scales thicker than 700 m, periods between 5 and 60 days, and horizontal wavelengths of 90–140 km. In contrast, the eastern slopes are characterized by rough topography, and motions with periods longer than 28 days decrease toward the bottom, suggesting a substantial reduction in the low-frequency topographic Rossby wave signal. Velocities from one of the two neighboring moorings located over the eastern rough slope have a strong 3-day period signal, which increases toward the bottom and has a vertical trapping scale of about 350 m. These higher frequency motions are interpreted in terms of edge waves.
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.
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.
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.
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.
Abstract
The f-plane reduced-gravity model has been extended with the parameterization of lateral friction in the momentum equations. The parameterization should preferably fulfill some requisites. One of them is that in the absence of external torques the change in angular momentum should be determined by boundary conditions alone. Internal torques should balance in the angular momentum budget. This requirement is fulfilled when the parameterization in the vertically integrated momentum equations is the divergence of a symmetric stress tensor. These equations solve for the mean transport, which includes implicitly the eddy-induced contribution. Another requirement on the parameterization of lateral stress follows from considering that it should imply kinetic energy dissipation. Both requirements fail with a commonly used parameterization and are fulfilled with the one proposed by C. Schär and R. B. Smith, which also is in near agreement with derivations of the shallow-water equations via vertical integrations of the Navier–Stokes equations. Here, the authors show two other parameterizations that are consistent with the angular momentum and energy requirements. One of the parameterizations follows from the symmetric component of a stress tensor in agreement with the parameterization shown by P. R. Gent to be energetically consistent. The other parameterization is related to the so-called biharmonic dissipation. In general, the difficulty for friction parameterizations is on the energy dissipation requirement, because the one on angular momentum is easily fulfilled.
Abstract
The f-plane reduced-gravity model has been extended with the parameterization of lateral friction in the momentum equations. The parameterization should preferably fulfill some requisites. One of them is that in the absence of external torques the change in angular momentum should be determined by boundary conditions alone. Internal torques should balance in the angular momentum budget. This requirement is fulfilled when the parameterization in the vertically integrated momentum equations is the divergence of a symmetric stress tensor. These equations solve for the mean transport, which includes implicitly the eddy-induced contribution. Another requirement on the parameterization of lateral stress follows from considering that it should imply kinetic energy dissipation. Both requirements fail with a commonly used parameterization and are fulfilled with the one proposed by C. Schär and R. B. Smith, which also is in near agreement with derivations of the shallow-water equations via vertical integrations of the Navier–Stokes equations. Here, the authors show two other parameterizations that are consistent with the angular momentum and energy requirements. One of the parameterizations follows from the symmetric component of a stress tensor in agreement with the parameterization shown by P. R. Gent to be energetically consistent. The other parameterization is related to the so-called biharmonic dissipation. In general, the difficulty for friction parameterizations is on the energy dissipation requirement, because the one on angular momentum is easily fulfilled.
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 suggest that part of the converged heat, which is not recirculated within the eddy, must escape from the eddy and flow upward along the isopycnals up to the surface layers. This is in good agreement with previous results that have suggested that an excess heat gained by the Gulf in the intermediate levels through exchanges with the Caribbean Sea must be exported to the upper layers by an upward mean heat flux.
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 suggest that part of the converged heat, which is not recirculated within the eddy, must escape from the eddy and flow upward along the isopycnals up to the surface layers. This is in good agreement with previous results that have suggested that an excess heat gained by the Gulf in the intermediate levels through exchanges with the Caribbean Sea must be exported to the upper layers by an upward mean heat flux.
Abstract
Variability 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 
Abstract
Variability 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
Abstract
The coupling between the upper (z < 1000-m depth) and deep (z > 1500 m) circulation in the western Gulf of Mexico (WGoM) driven by the arrival of Loop Current eddies (LCEs) is analyzed from moorings measuring horizontal velocity in the full water column during a 5-yr period (October 2008–October 2013). Nine LCEs crossing the mooring array are documented. A composite of these events shows that strong northward currents at depth having speeds of 0.1–0.2 m s−1 precede (~10–20 days) the strong northward near-surface currents (~0.5 m s−1) characteristic of the western rim of the LCEs. These deep northward flow intensifications are followed by southward deep flows coupled with the surface-intensified southward current of the eastern (rear) part of the LCEs crossing the array. These results are consistent with the existence of a deep anticyclone leading and a cyclone trailing the upper-layer LCEs. Objectively interpolated regional maps of velocities and vertical vorticity obtained from up to 30 moorings indicate the mean circulation at 100-m depth in the northern WGoM is mostly anticyclonic and enhanced by the arrival of the westward-propagating LCEs, while the southern part is dominated by the presence of a semipermanent cyclonic structure (Bay of Campeche cyclonic gyre). At 1500-m depth, the mean circulation follows the slope in a cyclonic sense and shows a cyclonic vorticity maximum on the abyssal plane consistent with the LCE deep flow composites. This suggests the LCEs strongly modulate not only the upper-layer circulation but also impact the deep flow.
Abstract
The coupling between the upper (z < 1000-m depth) and deep (z > 1500 m) circulation in the western Gulf of Mexico (WGoM) driven by the arrival of Loop Current eddies (LCEs) is analyzed from moorings measuring horizontal velocity in the full water column during a 5-yr period (October 2008–October 2013). Nine LCEs crossing the mooring array are documented. A composite of these events shows that strong northward currents at depth having speeds of 0.1–0.2 m s−1 precede (~10–20 days) the strong northward near-surface currents (~0.5 m s−1) characteristic of the western rim of the LCEs. These deep northward flow intensifications are followed by southward deep flows coupled with the surface-intensified southward current of the eastern (rear) part of the LCEs crossing the array. These results are consistent with the existence of a deep anticyclone leading and a cyclone trailing the upper-layer LCEs. Objectively interpolated regional maps of velocities and vertical vorticity obtained from up to 30 moorings indicate the mean circulation at 100-m depth in the northern WGoM is mostly anticyclonic and enhanced by the arrival of the westward-propagating LCEs, while the southern part is dominated by the presence of a semipermanent cyclonic structure (Bay of Campeche cyclonic gyre). At 1500-m depth, the mean circulation follows the slope in a cyclonic sense and shows a cyclonic vorticity maximum on the abyssal plane consistent with the LCE deep flow composites. This suggests the LCEs strongly modulate not only the upper-layer circulation but also impact the deep flow.
Abstract
The Gulf of Mexico (GoM) surface circulation variability is dominated by the Loop Current (LC) and the episodically released anticyclonic Loop Current eddies (LCEs). The Yucatan Current feeds the LC through the Yucatan Channel (YC), and its flow structure at the YC is hypothesized to affect the LC evolution critically. This study examines the impact of assimilating YC subsurface velocity observations from a tall mooring array across the YC on the GoM circulation. State estimates and forecasts of the LC circulation were produced using a regional implementation of the Massachusetts Institute of Technology general circulation model (MITgcm) and its adjoint-based four-dimensional variational (4DVAR) assimilation system. The estimates were constrained by combinations of the YC observations and satellite-derived sea surface height (SSH) and sea surface temperature (SST). The results show that assimilation of both moored and satellite data improves the model hindcasts and forecasts for all LC phases. Additionally, one realization of the state estimate that assimilates only moored data matches the LCE detachment timing with that of AVISO SSH. Observations from the moorings close to the Yucatan Peninsula significantly impact the LCE detachment. A finite-time Lyapunov exponent analysis reveals the differences among the assimilation experiments, such as eddylike structures intruding into the GoM through the YC, and its relation to the typical LC sudden growth. Finally, an adjoint sensitivity analysis is used to verify the dynamic link between the LC extension and the intrusion of eddylike structures into the GoM.
Abstract
The Gulf of Mexico (GoM) surface circulation variability is dominated by the Loop Current (LC) and the episodically released anticyclonic Loop Current eddies (LCEs). The Yucatan Current feeds the LC through the Yucatan Channel (YC), and its flow structure at the YC is hypothesized to affect the LC evolution critically. This study examines the impact of assimilating YC subsurface velocity observations from a tall mooring array across the YC on the GoM circulation. State estimates and forecasts of the LC circulation were produced using a regional implementation of the Massachusetts Institute of Technology general circulation model (MITgcm) and its adjoint-based four-dimensional variational (4DVAR) assimilation system. The estimates were constrained by combinations of the YC observations and satellite-derived sea surface height (SSH) and sea surface temperature (SST). The results show that assimilation of both moored and satellite data improves the model hindcasts and forecasts for all LC phases. Additionally, one realization of the state estimate that assimilates only moored data matches the LCE detachment timing with that of AVISO SSH. Observations from the moorings close to the Yucatan Peninsula significantly impact the LCE detachment. A finite-time Lyapunov exponent analysis reveals the differences among the assimilation experiments, such as eddylike structures intruding into the GoM through the YC, and its relation to the typical LC sudden growth. Finally, an adjoint sensitivity analysis is used to verify the dynamic link between the LC extension and the intrusion of eddylike structures into the GoM.
Abstract
Surface dispersion properties in the southwestern Gulf of Mexico are studied by using a set of 441 drifters released during a 7-yr period and tracked for 2 months on average. The drifters have a drogue below the surface Ekman layer, so they approximately follow oceanic currents. This study follows two different approaches: First, two-particle (or pair) statistics are calculated [relative dispersion and finite-scale Lyapunov exponents (FSLEs)]. Relative dispersion estimates are consistent with theoretical dispersion regimes of two-dimensional turbulence: an exponential growth during the first 3 days, a Richardson-like regime between 3 and 20 days (in which relative dispersion grows as a power law in time), and standard dispersion (linear growth) for longer times. The FSLEs yield a power-law regime for scales between 10 and 150 km but do not detect an exponential regime for short separations (less than 10 km). Robust estimates of diffusivities based on both relative dispersion and FSLEs are provided. Second, two different dispersion scenarios are revealed by drifter trajectories and altimetric data and supported by two-particle statistics: (i) a south-to-north advection of drifters, predominantly along the western shelf of the region, and (ii) a retention of drifters during several weeks at the Bay of Campeche, the southernmost part of the Gulf of Mexico. Dominant processes that control the dispersion are the arrival of anticyclonic Loop Current eddies to the western shelf and their interaction with the semipermanent cyclonic structure in the Bay of Campeche.
Abstract
Surface dispersion properties in the southwestern Gulf of Mexico are studied by using a set of 441 drifters released during a 7-yr period and tracked for 2 months on average. The drifters have a drogue below the surface Ekman layer, so they approximately follow oceanic currents. This study follows two different approaches: First, two-particle (or pair) statistics are calculated [relative dispersion and finite-scale Lyapunov exponents (FSLEs)]. Relative dispersion estimates are consistent with theoretical dispersion regimes of two-dimensional turbulence: an exponential growth during the first 3 days, a Richardson-like regime between 3 and 20 days (in which relative dispersion grows as a power law in time), and standard dispersion (linear growth) for longer times. The FSLEs yield a power-law regime for scales between 10 and 150 km but do not detect an exponential regime for short separations (less than 10 km). Robust estimates of diffusivities based on both relative dispersion and FSLEs are provided. Second, two different dispersion scenarios are revealed by drifter trajectories and altimetric data and supported by two-particle statistics: (i) a south-to-north advection of drifters, predominantly along the western shelf of the region, and (ii) a retention of drifters during several weeks at the Bay of Campeche, the southernmost part of the Gulf of Mexico. Dominant processes that control the dispersion are the arrival of anticyclonic Loop Current eddies to the western shelf and their interaction with the semipermanent cyclonic structure in the Bay of Campeche.
