Thomas H. Kinder

Exchange through the Strait of Gibraltar

To measure the exchange between the Atlantic and Mediterranean through the Strait of Gibraltar, an array of current meter moorings was deployed for a year in the Strait during 1985–1986. A novel aspect of these measurements is the inclusion of conductivity as well as temperature and pressure sensors on each current meter so that the salinity of the flows could be monitored continuously. These salinity measurements determine the water mass characteristics of the flows crossing the sill; they allow definition of the 37 psu isohaline as the interface between inflowing fresher Atlantic water and outflowing saltier Mediterranean water; and they enable time series to be developed for the depth of this interface, for the upper layer inflow, and for the lower layer outflow. From these measurements, the time-averaged outflow of Mediterranean water is estimated to be −0.68Sv (1Sv = 1 × 106m3s−1) and the outflow salinity transport, defined to be the outflow times the salinity excess above a basic Atlantic water salinity of 36.1 psu, is estimated to be −1.50 × 103m3s−1 (1Sv × 1psu = 1 × 103m3s−1) equivalent to a net evaporation over the Mediterranean basin of 52cm y−1. Extrapolated measurements of the inflow from current meters generally deployed below 100m depth yield an estimate for the time-averaged inflow of 0.93Sv, which is believed to be unrealistically high in view of the better measured outflow and net evaporation. Thus, a more realistic estimate of the inflow is 0.72Sv, equal to the sum of the outflow and net evaporation as required by the mass budget for the Mediterranean Sea. Such estimates of the exchange are smaller by almost a factor of 2 than previous values for the exchange by Lacombe and Richez (1982). The exchange across the Gibraltar sill is found to be due in nearly equal parts to the mean currents and to the tidal fluctuations. The mean currents are smaller than had been expected reaching a peak value of only about −60 cm s−1 in the deep outflow over the sill. The tidal exchange is due to a strong correlation over the tidal period between the depth of the interface and the strength of the inflowing currents. For the M2-tide at the sill, the amplitude of the interface depth is 51 m and the amplitude of the tidal currents is 1.2m s−1; furthermore, the inflow and interface depth have similar phases. As a consequence, the upper layer is deep on the inflowing tide so that a large slug of Atlantic water crosses the sill into the Mediterranean; on the outflowing tide, the interface is shallow so that a large slug of Mediterranean water crosses the sill into the Atlantic. Similar processes occur for the S2, O1 and K1 tides, though the amplitude are smaller. In this manner, tidal oscillations lead to a time-averaged exchange of water masses across the Gibraltar sill. The inflow and outflow, defined to be the instantaneous transports above and below the 37psu isohaline interface, exhibit M2-tidal amplitudes of 2.3 Sv and 1.3Sv respectively. Thus, the tides are large enough to reverse the mean upper layer inflow and lower layer outflow. Daily averaged inflow and outflow transports exhibit low-frequency fluctuations with standard deviations of 0.37Sv and 0.22Sv respectively. Such low frequency fluctuations have been shown previously to be associated with barotropic flows through the Strait of Gibraltar compensating for sea level variations over the Mediterranean due to atmospheric pressure fluctuations (Candela, Winant and Bryden, 1989). Finally, from these measurements there appear to be little fortnightly or annual period fluctuations in the exchange through the Strait of Gibraltar.

Steady two-layer exchange through the Strait of Gibraltar

Recent progress in steady hydraulic control modeling of the two-layer flow through the Strait of Gibraltar is reviewed. An application of the model is made to the specific physical configuration of the Strait using the most recent hathymetry. The maximal exchange solution resulting from the model must satisfy mass and salt conservation relations for the Mediterranean Basin. Hence, it is not possible to specify arbitrarily the density difference between the inflowing and outflowing layers, and the proper reservoir condition is a specification of the net evaporation over the Mediterranean Basin. For a specified evaporation, both the magnitude of the maximum two-layer exchange and the density or salinity difference between the inflowing Atlantic water and outflowing Mediterranean water can be determined from the combination of the hydraulic control model and mass and salt conservation statements. For net evaporation values between 50 and 70 cm y-1, the predicted inflow, outflow and salinity difference are in reasonable agreement with recent observations. In particular, for a net evaporation of 60 cm y-1, the steady hydraulic control model yields a predicted inflow of 0.92 × 106 m3 s-1, an outflow of 0.88 × 106 m3 s-1 and a salinity difference of 2.0‰. For comparison, recent observations exhibit an inflow of 0.95 × 106 m3 s-1, an outflow of 0.79 × 106 m3 s-1 and an effective salinity difference of 2.1‰.

The Atlantic inflow in the Western Alboran Sea

Abstract An extensive dataset collected in the Alboran Sea during the 1982 Donde Va? experiment is used to characterize the kinematics and dynamics of the inflow of Atlantic water into the Mediterranean Sea. The veering of the inflow toward the ENE after leaving the Strait of Gibraltar and the existence of an anticyclonic gyre that fills much of the western Alboran Sea in the upper 200 m are confirmed in the mean. Inflow variability with periods of 2 to 10 days is described. Particularly striking is one interval of about 9 days during which the gyre was not present and the inflow adopted a southerly course after leaving the strait. The observations are interpreted in terms of vorticity conservation and in the light of earlier theoretical and numerical results.

Correlations between seabirds and ocenic fronts around the Pribilof Islands, Alaska

Abstract Located on the extensive continental shelf of the Bering Sea, the Pribilof Islands, Alaska are the site of one of the largest breeding colonies of seabirds in the northern hemisphere. During summer these islands are surrounded by a front that separates vertically homogeneous waters from well stratified waters farther seaward. We studied the front with hydrographic data and the bird distributions with concurrent counts during summer 1977 and spring, summer and fall 1978. Murres (Uria lomvia and U. aalge) sitting on the water aggregated near the front during summer 1977 and probably during summer 1978. Other species, such as northern fulmars (Fulmarus glacialis) and auklets (Aethia pusilla and A. cristatella) were unaffected by the front. We hypothesize that the aggregation of the murres was related to an enhanced availability of their food near the front.

Low-Frequency Current Regimes over the Bering Sea Shelf

Abstract Using direct current measurements made during the period 1975–81, we describe the general circulation over the southeastern Bering Sea and differentiate it by regimes related to depth and forcing mechanisms. Three regimes are present, delineated by water depth (z): the coastal (z ≤ 50 m), the middle shelf (50 < z < 100 m), and the outer shelf (z ≥ 100 m). These are nearly coincident with previously described hydrographic domains. Statistically significant mean flow (∼1 to 10 cm s−1) exists over the outer shelf, generally directed toward the northwest, but with a cross-isobath component. Flow of similar magnitude (1–6 cm s−1) occurs in the coastal regime, paralleling the 50 m isobath in a counterclockwise sense around the shelf. Mean flow in the middle shelf is insignificant. Kinetic energy at frequencies < 0.5 cycle per day (cpd) is greater over the outer shelf than in the other two regimes, suggesting that oceanic forcing is important there but does not affect the remainder of the shelf. Kinetic...

Westward Intensification of the Mean Circulation on the Bering Sea Shelf

Abstract Most of the water that eventually flows northward through Bering Strait originates about 500 km south, seaward of the shelfbreak in the Bering Sea. Cumulative observational evidence supports the idea that most of this northward flow across the gently shoaling eastern Bering Sea continental shelf occurs as a western boundary current along the Siberian coast. A homogeneous rotating laboratory model and a barotropic numerical model each demonstrate this westward intensification of the mean flow. The intensification results from the well-known topographic β-effect: the combination of rotation and the depth decrease in the direction of flow acts in a similar fashion to the meridional gradient of the Coriolis parameter. For reasonable values of Bering Strait transport and shelf bottom friction, current speeds of 10–20 cm s−1 and a current width of ∼50 km are predicted.

Observation of a Baroclinic Eddy: An Example of Mesoscale Variability in the Bering Sea

Abstract Drift buoys with shallow (17 m) drogues, released during May 1977 and tracked by satellite, delineated an eddy in the southeastern Bering Sea. Located above complex topography having a depth range of 200 to 3000 m, the eddy had a diameter of about 150 km. Mean rotational speeds ∼50 km from the eddys center were 20 cm s−1, but speeds up to 50 cm s−1 were measured. A CTD survey during July defined the eddy from 200 to 1500 m depth in temperature and salinity distributions, but no hydrographic evidence for the eddy existed at the surface. A geostrophic calculation relative to 1500 m agreed qualitatively with drifter data, but was ∼5 cm s−1 less than mean drifter speeds. Examination of the T-S correlation showed that water masses at the eddys core were the same as those at its periphery, in contrast with a cyclonic ring observed nearby in July 1974. The last drifter left the eddy in October, and a second CTD survey in February 1978 showed that the eddy had either dissipated or moved. An earlier STD...

Aspiration of Deep Waters through Straits

In 1973 Stommel, Bryden and Mangelsdorf conjectured that high speed shallow flow in the Strait of Gibraltar is capable of sucking deep Mediterranean Water from the adjacent Alboran Sea directly up and over the sill into the Atlantic Ocean. This mechanism, which we call Bernoulli aspiration, has since been demonstrated in laboratory models, and the direct outflow has been confirmed by field experiment. Laboratory modeling, numerical experiment, and field measurement also have shown that the upstream path of the outflowing deep water is constrained by the combination of rotation and topography to form a narrow boundary current against the African coast. Data that was recently acquired during the Gibraltar Experiment (1985–1986) is used to describe the direct outflow of deep water and its distribution upstream of the sill.

Gibraltar Experiment: A Plan for Dynamic and Kinematic Investigations of Strait Mixing, Exchange and Turbulence.

Funding was provided by the Office of Naval Research under contracts no. N00014-82-C-0019, NR 083-004, and N00014-85-C-0001, NR 083-004.

Gibraltar experiment : summary of the field program and initial results of the Gibraltar experiment

Funding was provided by the Office of Naval Research through contract Numbers N00014-82-C-0019, N00014-85-C-0001, and N00014-87-K-0007.