Damià Gomis

Mesoscale Dynamics and Vertical Motion in the Alborán Sea

Abstract The inflow of Atlantic water through the strait of Gilbraltar usually exhibits a stationary wavelike pattern in the Alboran sea. We use an objective analysis technique for quantitative scale separation to investigate the interaction of scales in the region. The large scale is clearly dominated by an anticyclonic gyre. Smaller scale analysis shows the existence of several mesoscale cyclonic eddies along the northern boundary of the western Alboran sea anticyclonic gyre. The relationship between the large-scale/mesoscale variability and the induced ageostrophic vertical motion is established using the Q vector formulation. We find that on the macroscale, upward motion occurs upstream of the anticyclonic gyre (upstream of a wave crest) while downward motion takes place downstream (upstream of a trough). We also show that the vertical motion associated with the mesoscale eddies is an order of magnitude higher than the large-scale vertical motion. These patterns of large-scale and mesoscale vertical m...

Physical and chemical signatures of a developing anticyclonic eddy in the Leeuwin Current, eastern Indian Ocean

A multidisciplinary cruise aboard the R/V Southern Surveyor was conducted in May 2006 to sample a developing anticyclonic eddy of the Leeuwin Current off Western Australia. The eddy formed from a meander of the Leeuwin Current in mid-April 2006 and remained attached to the current until mid-August. In this study, a combination of satellite data (altimeter, sea surface temperature, and chlorophyll a) and shipboard measurements (acoustic Doppler current profiler and conductivity-temperature-depth) were used to characterize the physical and chemical signatures of the eddy. The temperature-salinity properties of the mixed layer waters within the anticyclonic eddy and on the shelf were both connected to that of the Leeuwin Current, indicating the water mass in the eddy is mainly derived from the Leeuwin Current and the modified Leeuwin Current water on the shelf. Above the salinity maximum near the eddy center, there was a regionally significant concentration of nitrate (>0.9 μmol L-1), and the maximum (2 μmol L-1) was at 150 in depth, below the photic zone. Nitrification within the eddy and/or local upwelling due to the forming eddy could be responsible for this high concentration of nitrate near the eddy center which potentially makes the eddy a relatively productive feature in the Leeuwin Current.

Seasonal changes of water mass structure and shelf slope exchanges at the Ebro Shelf (NW Mediterranean)

Among several field efforts undertaken in the framework of the EU MAST-III FANS project, three oceanographic cruises covering the Ebro shelf and slope regions (NW Mediterranean) were carried out on board R/V Garcia del Cid between November 1996 and July 1997. A major aim of these three cruises was to provide insight into the seasonal variability of the distribution of physico-chemical oceanographic parameters and the shelf/slope circulation. In this paper we discuss the observed changes in the hydrographic structure, the spatial distribution of nutrients, chlorophyll and suspended particulate matter and the local circulation in relationship to the seasonal variability of the Ebro river discharge rates and the water column stratification. The added effects of mesoscale circulation structures and of the exchanges across the shelf edge are also addressed.

Diagnostic analysis of the 3D ageostrophic circulation from a multivariate spatial interpolation of CTD and ADCP data

Abstract A multivariate (MV) optimal statistical interpolation method is applied to conductivity–temperature–depth (CTD) and ship-mounted acoustic doppler current profiler (ADCP) data from quasi-synoptic oceanographic surveys. MV analysis aims to improve the spatial interpolation of any particular variable (e.g., dynamic height) by including in the analysis observations of other physically related variables (e.g., current). The version used in this work also provides estimates of the non-divergent and irrotational components of the flow. The method is tested in a sharp frontal region to the north of the Western Alboran gyre. After deriving the optimal analysis parameters, we first show that MV statistical dynamic height analysis errors are significantly smaller than those derived from univariate (UV) analysis. In our region, this translates in a more realistic shape for the geostrophic relative vorticity and the vertical velocity field. The latter peaks at about 45 m/day (as given by the quasi-geostrophic omega equation), with a tendency for light water to be upwelled upstream of the gyre while denser water is downwelled downstream of the gyre. For the horizontal velocity we show the existence of large (up to 40 cm/s) ageostrophic velocities. These are mainly non-divergent and can be explained by the cyclostrophic acceleration induced by the anticyclonic gyre. The irrotational velocity component is of the order of 10 cm/s towards the dense side of the front. The robustness of the method is checked by means of several tests that evaluate the sensitivity of results with respect to the synopticity of the data, the analysis parameters, the reference level and the presence of tidal or inertial currents.

Multivariate analysis of the surface circulation in the Balearic Sea

Abstract This study gives a comprehensive view of the regional surface circulation of the Balearic Sea in early summer. Its spatial variability and dynamics are described from a multivariate analysis of AXBT, CTD and ADCP data collected during the mesoscale experiment FE-91 (May 22 – June 2, 1991). The results provide evidence of two energetic frontal jets that drive the circulation over the slopes: the southward Continental current (0.9Sv) and the northward Balearic current (0.5Sv). In the southern Balearic basin, to the north of the Ibiza sill, the analyses show that the slope circulation is drastically modified by a wide pool (≈70km) of Winter Intermediate Waters (WIW). In this region, both the Continental and Balearic currents leave the slopes and merge offshore. As a result, the Continental current veers cyclonically to the north and flows along the insular slope. Numerous mesoscale structures (eddies, meanders and filaments) are found in the centre of the basin, directly related to instabilities of the frontal jets often induced by sharp topographic irregularities and subsurface eddies, and involving significant offshore transport of slope waters. Assuming stationarity of the velocity field, a Lagrangian integration is performed to establish the trajectories and time scales associated with the analysed velocity field. The calculations indicate that a drifter released in the core of the Continental current at 22m depth drifts cyclonically around the Balearic basin and reaches the north of Mallorca in less than 40 days. This is consistent with data from an in situ surface drifter deployed in June 1993, and suggests that the velocity field obtained from FE-91 data is representative of the summer circulation of the Balearic Sea. All these results are discussed in the perspective of the general circulation of the Western Mediterranean.

Pressure-Forced Seiches of Large Amplitude in Inlets of the Balearic Islands

Large-amplitude harbor seiches usually occur in summer in the Balearic Islands. A significant correlation between sea level oscillations and atmospheric pressure disturbances has been found, though a proved physical mechanism to account for this atmosphere-ocean interaction is still missing. Using a flat bottom, shallow water model, we show that a direct coupling between atmospheric pressure and the free mode of an inlet is unlikely but that an oceanic wave of atmospheric origin can act as an intermediate mechanism and adequately force the inlet by resonance. The phase relationship derived from this mechanism is in good agreement with observations, provided the whole spectrum of oceanic waves is in opposite phase to the generating atmospheric disturbance. We also show that the very large oscillations observed at Ciutadella (an elongated, shallow inlet in the west coast of Menorca, Balearic Islands) can be explained in terms of the particular shape of this inlet.

Water masses and distribution of physico-chemical properties in the Western Bransfield Strait and Gerlache Strait during Austral summer 1995/96

In the framework of the FRUELA project, two oceanographic surveys were conducted by R/V Hesperides in the eastern Bellingshausen Sea, western basin of the Bransfield Strait and Gerlache Strait area during December 1995 and January 1996. The main hydrographic structures of the study domain were the Southern Boundary of the ACC and the Bransfield Front. The characteristics and zonation of local water masses are discussed in terms of temperature, salinity, dissolved oxygen, nutrient and inorganic carbon concentrations. Concentration intervals for water mass labelling, on the basis of chemical parameters in addition to the common θ/S-based classification, are defined. Silicate seems to be a very good discriminator for local water masses.

Comparing the sea level response to pressure and wind forcing of two barotropic models: Validation with tide gauge and altimetry data

The sea level output from two barotropic models, Hindcast of Dynamic Processes of the Ocean and Coastal Areas of Europe (HIPOCAS) and Dynamic Atmospheric Correction (DAC), are compared and evaluated on the basis of coastal tide gauges (TG) and TOPEX/POSEIDON measurements in the Mediterranean Sea and the NE Atlantic Ocean for the period 1993–2001. Both models reduce the observed sea level variance more than the classical inverted barometer correction. However, differences between the models arise for different regions and frequency bands. In coastal areas, Hindcast of Dynamic Processes of the Ocean and Coastal Areas of Europe (HIPOCAS) reproduces observed (TG recorded) sea level better than DAC (residual variance of 70.81 ± 0.69 cm2 versus 74.05 ± 0.68 cm2). This is particularly true in the Atlantic Iberian coasts (49.58 ± 1.09 cm2 versus 68.53 ± 1.12 cm2), where HIPOCAS is able to reproduce a wind-generated signal probably linked with seasonal upwelling. The exception is the northern Adriatic, where HIPOCAS gives higher residual variance than DAC (118.80 ± 0.60 cm2 versus 107.15 ± 0.60 cm2). At low frequencies (T > 20 days) the atmospherically induced coastal sea level is better reproduced by HIPOCAS in the entire domain (23.43 ± 0.34 cm2 versus 32.35 ± 0.33 cm2). At high frequencies (T < 20 days), DAC and HIPOCAS perform on average similarly (37 ± 0.5 cm2). In the open ocean, both corrections provide equivalent results (60 ± 5 cm2 residual altimeter variance). Our general recommendation would be to use either DAC or HIPOCAS for the correction of altimetry, and to use HIPOCAS for coastal studies aiming at separating the atmospheric contribution to sea level variability from the steric and mass contributions.

Mediterranean Sea level and barotropic flow through the Strait of Gibraltar for the period 1958–2001 and reconstructed since 1659

Sea level values from a two-dimensional model of the Mediterranean Sea forced by atmospheric pressure and wind are used to estimate the barotropic flow through the Strait of Gibraltar for the period 1958–2001. The Mediterranean mean sea level derived from the model ranges between ±20 cm with a standard deviation of 5.5 cm and is correlated to the North Atlantic Oscillation (NAO) index. Thus NAO historical data and reconstructions are used to derive the Mediterranean Sea level variability from 1659 until 2001. The accuracy of the reconstruction is estimated in 2.7 cm for monthly mean values, 0.41 cm for annual mean values, and 0.22 cm for decadal mean values (0.48 cm for decadal winter mean sea level). The barotropic flow through the strait is computed from the model output as the time derivative of the total volume of the basin. During the period 1958–2001 the estimated daily flow ranges between ±2.7 Sv, with a standard deviation of 0.56 Sv. The dominant periodicities are in between 1 and 2 weeks. At these scales the model successfully reproduces previously published flow estimates based on current meter observations, which confirms that atmospheric pressure and wind dominate the intraseasonal variability of the flow. For the annual cycle the variability of the atmospherically induced flow is similar to the variability of the flow induced by the evaporation-precipitation (E-P) budget (±0.025 Sv), though absolute values of the first are about a third of the latter. At longer timescales the atmospheric contribution is much smaller than the E-P induced flow.

Quasi-synoptic mesoscale variability in the Balearic Sea

Abstract The circulation and dynamics of the Balearic Sea, a sub-basin of the Western Mediterranean located between the Iberian peninsula and the Balearic Islands, are characterized by a significant temporal and spatial variability induced by two well defined shelf/slope fronts over the continental and island slopes. To establish the importance of the mesoscale features frequently observed, a quasi-synoptic AXBT survey of the Balearic Sea was carried out in only three days with a horizontal resolution of 18 km. An objective analysis technique for quantitative scale separation was used to investigate the spatial variability of the temperature field. At the sub-basin scale, the primary feature is the Balearic Front that separates cold Mediterranean waters from warmer waters of Atlantic origin. The front is located over the islands slope and is characterized by a well defined meandering structure that appears to be strongly correlated with bottom topography. Strong mesoscale activity is also found, mostly in the Balearic frontal region, with two small eddies embedded within the meanders and a warm filament overshooting towards the open sea. These energetic mesoscale features appear in the upper 150 m and strongly modify the almost permanent relatively weak sub-basin scale circulation. The relationship between these instabilities and local topography/coast irregularities is also suggested.