Mesoscale subduction at the Almeria–Oran front: Part 1: Ageostrophic flow

This paper presents a detailed diagnostic analysis of hydrographic and current meter data from three, rapidly repeated, fine-scale surveys of the Almeria–Oran front. Instability of the frontal boundary, between surface waters of Atlantic and Mediterranean origin, is shown to provide a mechanism for significant heat transfer from the surface layers to the deep ocean in winter. The data were collected during the second observational phase of the EU funded OMEGA project on RRS Discovery cruise 224 during December 1996. High resolution hydrographic measurements using the towed undulating CTD vehicle, SeaSoar, traced the subduction of Mediterranean Surface Water across the Almeria–Oran front. This subduction is shown to result from a significant baroclinic component to the instability of the frontal jet. The Q-vector formulation of the omega equation is combined with a scale analysis to quantitatively diagnose vertical transport resulting from mesoscale ageostrophic circulation. The analyses are presented and discussed in the presence of satellite and airborne remotely sensed data; which provide the basis for a thorough and novel approach to the determination of observational error.     

Composite front maps for improved visibility of dynamic sea-surface features on cloudy SeaWiFS and AVHRR data

Novel techniques have been developed for increasing the value of cloud-affected sequences of Advanced Very High Resolution Radiometer (AVHRR) sea-surface temperature (SST) data and Sea-viewing Wide Field-of-view Sensor (SeaWiFS) ocean colour data for visualising dynamic physical and biological oceanic processes such as fronts, eddies and blooms. The proposed composite front map approach is to combine the location, strength and persistence of all fronts observed over several days into a single map, which allows intuitive interpretation of mesoscale structures. This method achieves a synoptic view without blurring dynamic features, an inherent problem with conventional time-averaging compositing methods. Objective validation confirms a significant improvement in feature visibility on composite maps compared to individual front maps. A further novel aspect is the automated detection of ocean colour fronts, correctly locating 96% of chlorophyll fronts in a test data set. A sizeable data set of 13,000 AVHRR and 1200 SeaWiFS scenes automatically processed using this technique is applied to the study of dynamic processes off the Iberian Peninsula such as mesoscale eddy generation, and many additional applications are identified. Front map animations provide a unique insight into the evolution of upwelling and eddies.     

Mesoscale high-frequency variability in the Alboran Sea and its influence on fish larvae distributions

This work analyses a multidisciplinary data set including hydrological and meteorological data, satellite images, and fish larvae abundance from a high-frequency experiment conducted along a north–south transect across the Western Alboran Sea anticyclonic gyre. Four consecutive occupations of the transect, crossing the frontal area, evidenced the high-frequency variability of hydrological structures and its influence on the latitudinal and vertical distribution of fish larvae in a period of a few days. The influence of dynamical processes on fish larvae depends on the location of the spawning as well as on the larval fish position in the water column. Wind induced upwelling and/or the southward drift of the Atlantic current transport larvae of neritic species, such as Sardina pilchardus and Engraulis encrasicolus to open sea areas. At the same time, these events bring about alterations in the latitudinal and vertical distributions of mesopelagic fish larvae. The species with a relatively surface distribution, as Ceratoscopelus maderensis, were transported and accumulated to the south of the Atlantic Jet (AJ), while those with a deeper distribution in the water column, as Maurolicus muelleri or Benthosema glaciale, would be upwelled and concentrated to the north. This study shows that it is difficult to extract conclusions about the mean or preferential distributions of fish larvae when one single/synoptic survey is considered and that time evolution of hydrological structures has to be considered in order to conclude.     

Algerian Eddies lifetime can near 3 years

The Algerian Current (AC) is unstable and generates mesoscale meanders and eddies. Only anticyclonic eddies can develop and reach diameters over 200 km with vertical extents down to the bottom (3000 m). Algerian Eddies (AEs) first propagate eastward along the Algerian slope at few kilometers per day. In the vicinity of the Channel of Sardinia, a few AEs detach from the Algerian slope and propagate along the Sardinian one. It was hypothesized that AEs then followed a counter-clockwise circuit in the eastern part of the basin. Maximum recorded lifetimes were known to exceed 9 months. Within the framework of the 1-year Eddies and Leddies Interdisciplinary Study off Algeria (ELISA) experiment (1997–1998), we exhaustively tracked two AEs, using mainly an 3-year time series of NOAA/AVHRR satellite images. We show that AEs lifetimes can near 3 years, exceeding 33 months at least. We also confirm the long-lived AEs preferential circuit in the eastern part of the Algerian Basin, and specify that it may include several loops (at least three).     

Northern Adriatic Sea surface circulation and temperature/pigment fields in September and October 1997

Satellite images of surface chlorophyll-a concentration measured by the sea-viewing wide field-of-view sensor (SeaWiFS) and of sea surface temperature derived from advanced very high resolution radiometer (AVHRR) measurements, combined with in-situ drifter measurements of surface currents, and ancillary wind, Po River discharge and surface salinity data, are used to describe the surface dynamics in the northern Adriatic during the period September–October 1997.The satellite observations revealed very complex mesoscale dynamics, with time scales of a day or two and length scales of about 10 km, including the meandering and instability of basin-scale currents (e.g., the western coastal layer), jets/filaments and eddies. In addition, the two typical patterns of the Po River plume are observed and qualitatively explained in terms of wind forcing. A basin-wide double gyre pattern spreads the rich runoff water across most of the northern Adriatic from mid-September to early October, following Bora wind events and under stratified sea conditions. In contrast, in late October the Po plume is confined to the coast due to weaker winds and de-stratified conditions. This variability in the Po River plume extension is also confirmed by in-situ salinity measurements.     

Review on the seasonal variation of the surface circulation in the Japan/East Sea

The seasonal variation of the surface circulation in the Japan/East Sea (JES) and the Tsushima/Korea Straits (TKS) is reviewed and discussed, focusing on mesoscale and submesoscale variabilities.The monsoon modified by coastal geographical features near Vladivostok generates a dipole of vortex off Vladivostok which induces dramatic changes in the surface circulation in the northwest JES, splitting the Subpolar Gyre into two smaller gyres by generating the Vladivostok Dome. Between these two smaller gyres, the Northwest Thermal Front is formed and current reversal is induced along the North Korean coast. The winter monsoon also induces a current reversal along the Sakhalin coast. The volume transport of the surface Subpolar Gyre has two maxima in January and August. The maximum in August is induced by the summer intensification of the Liman-North Korean Cold Current and the shallow and narrow surface coastal jet generated by the sea ice and snow melting. The maximum in January is induced by the northwest monsoon and associated cooling.Salient features in the TKS are the submesoscale variabilities. In the western channel, submesoscale eddies with length scale of about 80 km and time scale of 5–6 days develop in the cold period. On the lee side of the Tsushima Islands, Karman-like vortex pairs are generated in the warm period. Anticyclonic vortices generated at the northern tip of the Tsushima Islands have a time scale of 5 to 8 days, length scale of 35 to 60 km, and propagate toward the JES with a phase speed of 8 cm/s. Cyclonic vortices south of the anticyclonic counter part of the vortex pairs are rather stationary with intermittent occasional propagation toward the east. The development of stratification seems to be necessary for the development of Karman-like vortex pairs.Summarizing the results above, a schematic surface circulation with seasonal change is proposed.     

The use of digital filter initialization to diagnose the mesoscale circulation and vertical motion in the California coastal transition zone

A dynamical method of initializing the primitive equations is tested and used to diagnose the three-dimensional circulation associated with jets and eddies as found in the California coastal transition zone (CTZ). The initialization method, referred to as digital filter initialization (DFI), was recently developed by [Monthly Weather Review 120 (1992) 1019] for use in an intermittent data assimilation system in the atmosphere. The ability of DFI to recover the mesoscale ageostrophic circulation associated with finite amplitude jets and eddies in the ocean is first demonstrated using control data produced by simulations with a primitive equation model. The DFI method is then applied to synoptic hydrographic data collected during several California CTZ surveys in the summer of 1988. The diagnostic results indicate the existence of jets, eddies, and filaments in the CTZ domain with maximum horizontal currents of the order of 0.6 m/s at the surface. Currents associated with such jets and filaments are coherent to a depth of over 500 m. The surface currents associated with a prominent cool filament are generally confluent, and weakly convergent on average, along the 270 km offshore extent of the filament. Meanders in the jet display convergence and downwelling upstream of pressure troughs and divergence and upwelling downstream of the troughs. Maximum vertical velocities at 100 m are of the order of 10 m/day. This result is consistent with independent estimates of subduction rates made from biological studies in this and similar coastal filaments in the CTZ program.     

Improving the physics of a coupled physical–biogeochemical model of the North Atlantic through data assimilation: Impact on the ecosystem

Several studies on coupled physical–biogeochemical models have shown that major deficiencies in the biogeochemical fields arise from the deficiencies in the physical flow fields. This paper examines the improvement of the physics through data assimilation, and the subsequent impact on the ecosystem response in a coupled model of the North Atlantic. Sea surface temperature and sea surface height data are assimilated with a sequential method based on the SEEK filter adapted to the coupling needs. The model domain covers the Atlantic from 20°S to 70°N at eddy-permitting resolution. The biogeochemical model is a NPZD-DOM model based on the P3ZD formulation. The results of an annual assimilated simulation are compared with an annual free simulation.With assimilation, the representation of the mixed layer depth is significantly improved in mid latitudes, even though the mixed layer depth is generally overestimated compared to the observations. The representation of the mean and variance of the currents is also significantly improved.The nutrient input in the euphotic zone is used to assess the data assimilation impact on the ecosystem. Data assimilation results in a 50% reduction of the input due to vertical mixing in mid-latitudes, and in a four- to six-fold increase of the advective fluxes in mid-latitudes and subtropics. Averaged zonally, the net impact is a threefold increase for the subtropical gyre, and a moderate (20–30%) decrease at mid and high latitudes.Surface chlorophyll concentration increases along the subtropical gyre borders, but little changes are detected at mid and high latitudes. An increase of the primary production appears along the Gulf Stream path, but it represents only 12% on average for mid and high latitudes. In the subtropical gyre centre, primary production is augmented but stays underestimated (20% of observations). These experiments show the benefits of physical data assimilation in coupled physical–biogeochemical applications.     

Sea level and Eddy Kinetic Energy variability in the Bay of Biscay, inferred from satellite altimeter data

Twelve years (1993–2005) of altimetric data, combining different missions (ERS-1/2, TOPEX/Poseidon, Jason-1 and Envisat), are used to analyse sea level and Eddy Kinetic Energy variability in the Bay of Biscay at different time-scales. A specific processing of coastal data has been applied, to remove erroneous artefacts. Likewise, an optimal interpolation has been used, to create a series of regional Sea Level Anomaly maps, merging data sets from two satellites.The sea level presents a trend of about 2.7 mm/year, which is within the averaged values of sea level rise in the global ocean. Frequency spectra show that the seasonal cycle is the main time-scale affecting the sea level and Eddy Kinetic Energy variability. The maximum sea level occurs in October, whilst the minimum is observed in April. The steric effect is the cause of this annual cycle. The Northern French shelf/slope presents intense variability which is likely due to internal tides. Some areas of the ocean basin are also characterised by intense variability, due to the presence of eddies.The Eddy Kinetic Energy, in turn, is higher from December to May, than during the rest of the year and presents a weak positive trend from April 1995 to April 2005. Several documented mesoscale events, occurring at the end of 1997 and during 1998, are analysed. Altimetry maps prove to be a useful tool to monitor swoddy-like eddies from their birth to their decay, as well as the inflow of seasonal slope water current into the southeastern corner of the Bay of Biscay.     

Enrichment, concentration and retention processes in relation to anchovy (Engraulis ringens) eggs and larvae distributions in the northern Humboldt upwelling ecosystem

A Lagrangian model is used to simulate and quantify in the northern Humboldt upwelling ecosystem the processes of enrichment, concentration and retention, identified by Bakun [Bakun, A., 1996. Patterns in the ocean. Ocean processes and marine population dynamics. University of California Sea Grant, California, USA, in cooperation with Centro de Investigaciones Biologicas de Noroeste, La Paz, Baja California Sur, Mexico, 323 pp.] as being important for the survival and recruitment of early life stages of pelagic fish. The method relies on tracking the positions of particles within water velocity fields generated by a three-dimensional hydrodynamic model. Simple criteria for considering particles as participating to enrichment, concentration or retention are used to derive indices of the three processes. We analyse the spatial distribution of and seasonal variability in these indices. The results are discussed in relation to anchovy (Engraulis ringens) eggs and larvae distributions off Peru, and to a comparable study conducted in the southern Benguela upwelling ecosystem.