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## Abstract

Horizontal current and density data fields are analyzed in order to validate, from an experimental point of view, the contribution of the advective and Coriolis accelerations and the hydrostatic pressure gradient term to the balance of horizontal momentum. The relative importance of the vertical advection of horizontal velocity in this balance is estimated by solving the quasigeostrophic (QG) omega equation. The analysis of the balance of horizontal momentum is carried out using data from three consecutive high-resolution samplings of the Atlantic jet (AJ) and western Alboran gyre (WAG) on the eastern side of the Strait of Gibraltar.

The horizontal velocity reached maximum values of 1.30 m s^{−1} in the AJ at the surface. The ageostrophic velocity field reaches maximum absolute values of 30 cm s^{−1} at the surface, thus confirming the supergeostrophic nature of the AJ. At the surface the pressure gradient term reaches absolute values of 8–10 (×10^{−5} m s^{−2}), the Coriolis acceleration 10–12 (×10^{−5} m s^{−2}), and the advective horizontal acceleration 3 × 10^{−5} m s^{−2}. The vertical advection of horizontal velocity by the QG vertical velocity at 100 m is one order of magnitude smaller [*O*(10^{−6} m s^{−2})].

The *geostrophic imbalance* (difference between the pressure gradient term and the Coriolis acceleration) reaches 5 × 10^{−5} m s^{−2} at the surface. The *gradient imbalance* (defined as the difference between the pressure gradient term and the Coriolis plus advective accelerations) is smaller than the geostrophic imbalance (being of order 2.5 × 10^{−5} m s^{−2}) making gradient balance the best estimate of the balance of horizontal momentum given the characteristics (synopticity and experimental errors) of the analyzed dataset.

The gradient imbalance is not uniform in the horizontal but rather is larger in the AJ than in the WAG. From this result it is inferred that the AJ current experiences larger variations (larger local acceleration) than the WAG current.

## Abstract

Horizontal current and density data fields are analyzed in order to validate, from an experimental point of view, the contribution of the advective and Coriolis accelerations and the hydrostatic pressure gradient term to the balance of horizontal momentum. The relative importance of the vertical advection of horizontal velocity in this balance is estimated by solving the quasigeostrophic (QG) omega equation. The analysis of the balance of horizontal momentum is carried out using data from three consecutive high-resolution samplings of the Atlantic jet (AJ) and western Alboran gyre (WAG) on the eastern side of the Strait of Gibraltar.

The horizontal velocity reached maximum values of 1.30 m s^{−1} in the AJ at the surface. The ageostrophic velocity field reaches maximum absolute values of 30 cm s^{−1} at the surface, thus confirming the supergeostrophic nature of the AJ. At the surface the pressure gradient term reaches absolute values of 8–10 (×10^{−5} m s^{−2}), the Coriolis acceleration 10–12 (×10^{−5} m s^{−2}), and the advective horizontal acceleration 3 × 10^{−5} m s^{−2}. The vertical advection of horizontal velocity by the QG vertical velocity at 100 m is one order of magnitude smaller [*O*(10^{−6} m s^{−2})].

The *geostrophic imbalance* (difference between the pressure gradient term and the Coriolis acceleration) reaches 5 × 10^{−5} m s^{−2} at the surface. The *gradient imbalance* (defined as the difference between the pressure gradient term and the Coriolis plus advective accelerations) is smaller than the geostrophic imbalance (being of order 2.5 × 10^{−5} m s^{−2}) making gradient balance the best estimate of the balance of horizontal momentum given the characteristics (synopticity and experimental errors) of the analyzed dataset.

The gradient imbalance is not uniform in the horizontal but rather is larger in the AJ than in the WAG. From this result it is inferred that the AJ current experiences larger variations (larger local acceleration) than the WAG current.

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## Abstract

A study of mesoscale subduction at the Antarctic Polar Front (PF) is conducted by use of hydrographic data from a high-resolution, quasi-synoptic survey of the front. The geostrophic velocity and isopycnal potential vorticity (PV) fields are computed, and the ageostrophic flow diagnosed from the semigeostrophic omega equation. It is found that the ageostrophic circulation induced by baroclinic instability counteracts the frontogenesis and frontolysis effected by the confluence and difluence, respectively, of the geostrophic velocity field. Though the sense of the ageostrophic circulation is reversed repeatedly along the front, the existence of PV gradients along isopycnals leads to a net cross-front “bolus” transport. In response to a reversal of this gradient with depth (a necessary condition for the onset of baroclinic instability), the bolus transport is northward at the protruding temperature minimum layer that characterizes the PF, and southward above. This net cross-front overturning circulation acts to flatten the isopycnals of the front and results in a subduction of the temperature minimum layer as it progresses northward along isopycnals. Consistently, a net baroclinic conversion rate of approximately 1 cm^{2} s^{−2} d^{−1}, corresponding to a net subduction rate of *O*(20 m yr^{−1}), is calculated in the survey area. The similarity between the PV field of the PF and other Southern Ocean fronts suggests that the authors' results may also be applicable there. This has profound implications for the understanding of the zonation of the Antarctic Circumpolar Current.

## Abstract

A study of mesoscale subduction at the Antarctic Polar Front (PF) is conducted by use of hydrographic data from a high-resolution, quasi-synoptic survey of the front. The geostrophic velocity and isopycnal potential vorticity (PV) fields are computed, and the ageostrophic flow diagnosed from the semigeostrophic omega equation. It is found that the ageostrophic circulation induced by baroclinic instability counteracts the frontogenesis and frontolysis effected by the confluence and difluence, respectively, of the geostrophic velocity field. Though the sense of the ageostrophic circulation is reversed repeatedly along the front, the existence of PV gradients along isopycnals leads to a net cross-front “bolus” transport. In response to a reversal of this gradient with depth (a necessary condition for the onset of baroclinic instability), the bolus transport is northward at the protruding temperature minimum layer that characterizes the PF, and southward above. This net cross-front overturning circulation acts to flatten the isopycnals of the front and results in a subduction of the temperature minimum layer as it progresses northward along isopycnals. Consistently, a net baroclinic conversion rate of approximately 1 cm^{2} s^{−2} d^{−1}, corresponding to a net subduction rate of *O*(20 m yr^{−1}), is calculated in the survey area. The similarity between the PV field of the PF and other Southern Ocean fronts suggests that the authors' results may also be applicable there. This has profound implications for the understanding of the zonation of the Antarctic Circumpolar Current.