Western Alboran Gyre Research Articles

Spatial pattern of sea surface temperature and chlorophyll-a trends in relation to hydrodynamic processes in the Alborán Sea

Environmental conditions such as temperature, planktonic biomass, and ocean currents play an important role in the development and distribution of marine species. This work aims to estimate, in a high spatial resolution, the actual trends of sea surface temperature (SST) and chlorophyll-a concentration (Chl-a) and to assess the relationship with the local hydrodynamic conditions in the Alborán Sea. To investigate these objectives, time series of SST and Chl-a of satellite sensor data were analyzed during 20 years from January 2001 to December 2020, using the Seasonal-Trend-Loess (STL) decomposition method and the Mann-Kendall seasonality test. The results, obtained with a 95% of confidence, showed that the Alborán Sea basin is subject to sea surface warming evaluated at 0.027 ± 0.008 ° C per year, related to the warming of the Atlantic water mass, which contributes to a decrease of productivity evaluated at -0.0024 ± 0.0003 μg /l per year of Chl-a concentration. These trends are not homogeneous over the entire basin area but show a large regional variation between different parts of the Alborán Sea due to the hydrodynamic process of the Atlantic Jet - Western Alboran Gyre system (AJ-WAG), which is more active especially in summer/autumn seasons and contribute largely to these changes by mixing the waters of the Atlantic Ocean and the Mediterranean Sea.

K nearest neighbors classification of water masses in the western Alboran Sea using the sigma-pi diagram

Different classification techniques of water masses have been developped using the potential temperature-salinity (θ-S) diagram and its volumetric analysis. In this study, we propose a new method to automatically classify water masses via a supervised machine learning algorithm based on the K nearest neighbors (Knn), in the potential density and potential spicity (σ-π) coordinates. This method is applied to temperature and salinity data collected in the western side of the Alboran Sea during a glider mission, dedicated to sample the Western Alboran Gyre (WAG) in late winter 2021. The water masses in the studied region were classified into five different categories following a supervised learning process, based on ocean profile databases available on the region of interest. The results corroborate previous studies of the spatial distribution of water masses in the Alboran Sea, inferred from traditional method based on the expert analysis of the (θ-S) diagram, and suggest that this methodology is efficient and reliable for water masses classification. Compared to a classical clustering computation (herein k-means), this method is more appropriate in a region where the characteristics of the water masses change considerably in both space and time.

Frontal Dynamics in the Alboran Sea: 2. Processes for Vertical Velocities Development

AbstractSignificant lateral density gradients occur throughout the year in the Alboran Sea, giving rise to two main fronts: the Western Alboran Gyre Front (WAGF) and Eastern Alboran Gyre Front (EAGF), where large vertical velocities often develop. To improve the understanding of the processes that underlie the development of the vertical velocities in the fronts, the periods of development were analyzed in the perspective of the frontogenesis, instabilities, non‐linear Ekman, and filamentogenesis mechanisms, using multi‐platform in‐situ observations and a high‐resolution realistic simulation in spring 2018. The spatio‐temporal characteristics of the WAGF indicate a wider, deeper, and longer‐lasting front than the EAGF. Additionally, the WAGF shows stronger and deeper upwelling and downwelling regions. The WAGF vertical velocities (up to |55| m/day) are amplified by an across‐front ageostrophic secondary circulation generated by: (a) frontal intensification explained by frontogenesis, which shows a sharpening of buoyancy gradients associated with the Atlantic Jet, (b) nonlinear Ekman effects, that are enhanced by the persistent western wind blowing along the frontal direction, and (c) submesoscale instabilities (symmetric and ageostrophic baroclinic instabilities). The EAGF vertical velocities (up to |38| m/day) are amplified by two asymmetrical ageostrophic cells developed across the front with a narrow upwelling region in the middle. The cell's circulation is explained by frontal intensification produced by filamentogenesis through a cold filament advection to the Mediterranean Sea interior, that is characterized by pointy isopycnals at the center of the filament. This mechanism is observed in both the model and glider observations.

Patterns of microphytoplankton biomass and diversity in the southern Alboran Sea

The southern Alboran Sea is a highly dynamic region in the Mediterranean. However, there is few data on microphytoplankton which is, an important component of the marine ecosystem. We therefore collected microphytoplankton samples and related the biomass and diversity patterns to ambient variability, considering cross-shore and longitudinal gradients. There was a general eastward decrease in both species richness and biomass, with Cape Three Forks as a transitional point. Diversity increased in coastal areas and decreased with depth. High chlorophyll-a concentrations corresponded to low temperature and low-salinity waters, indicating the Atlantic origin. Microphytoplankton biomass was low in comparison with total chlorophyll, suggesting a dominance of nano- and picophytoplankton. Biomass values increased in the stretch between Cape Three Forks and Al Hoceima Bay, an area highly influenced by upwelled water originating from the northern Alboran Sea. We suggest that the Western Alboran Gyre enhances the development of dinoflagellates while local upwelling enhances the development of diatoms. A statistical relationship was found between microphytoplankton biomass and diversity, especially when diversity was estimated as species richness. These results are crucial for understanding microphytoplankton dynamics and trends in an area that is undergoing climate-derived changes and biodiversity losses.

Revising the Effects of Local and Remote Atmospheric Forcing on the Atlantic Jet and Western Alboran Gyre Dynamics

AbstractThe main objective of the present study is to gain insight into the response of the Atlantic Jet (AJ) to the atmospheric forcing, in order to understand the hydrodynamic processes behind its disconnection from the Western Alboran Gyre (WAG) and the eastward displacement of the latter. An analysis was carried out based on observations of high frequency radar (HFR) derived currents and sea level, numerical model simulations and satellite imagery, with particular focus on the events of intense and endured easterlies. Among the main factors that characterize the atmospheric forcing linked to intense and persistent easterly wind events, the conjugation of the direct local wind forcing effect on the Atlantic Inflow (AI) on the eastern side of the Strait of Gibraltar and the response of the coastal waters in the northwestern (NW) corner of the Alboran Sea to the wind forcing over the Alboran Sea are of particular interest. In addition, the effect of the elevation surges in the Strait coming from the western Mediterranean Sea on reducing the AI intensity must be taken into account. These surges appear in coincidence with the establishment of persistent and intense easterlies over the Alboran Sea. The alteration of the AJ‐WAG system in response to this forcing causes important changes in the phytoplankton distributions in the Alboran Sea. After the events of persistent and intense easterlies the phytoplankton accumulations in the NW Alboran Sea are displaced offshore and the phytoplankton transported with the AJ is injected toward the central zone of the WAG.

Tidally-induced submesoscale features in the atlantic jet and Western Alboran Gyre. A study based on HF radar and satellite images

An analysis is presented of the submesoscale tidally-induced anticyclonic eddies formed at the eastern mouth of the Strait of Gibraltar and their role in explaining mesoscale features of the Atlantic Jet – Western Alboran Gyre (AJ-WAG) system. These eddies have been identified for the first time using an observational approach based on the analysis of High Frequency Radar (HFR) derived surface currents on the eastern mouth of the Strait which has been complemented with the analysis of Sea Surface Temperature (SST) satellite images. It is found that the major source of positive vorticity to the AJ is provided by its interaction with these anticyclonic eddies which in turn help maintain the water feeding of the upper branch of the WAG. This new role in maintaining the WAG is added to others previously recognized by other authors based on the supply of negative vorticity that these eddies provide to the whole WAG. In addition, these tidally-induced eddies may contribute to the destabilization of the AJ-WAG system when strong and persistent easterly winds coincide with periods of high tidal current amplitude. In these cases, meteorological forcing produces a weak and southward deflected AJ almost disconnected from a displaced eastward WAG. The WAG in turn is being flattened by an overdeveloped North Western Cyclonic Gyre (NWCG) that may be favored by the action of the mentioned eddies.

The Western Alboran Gyre: An Analysis of Its Properties and Its Exchange with Surrounding Water

AbstractOne of the largest and most persistent features in the Alboran Sea is the Western Alboran Gyre (WAG), an anticyclonic recirculation bounded by the Atlantic Jet (AJ) to the north and the Moroccan coast to the south. Eulerian budgets from several months of a high-resolution model run are used to examine the exchange of water across the Eulerian WAG’s boundary and the processes affecting the salinity, temperature, and vorticity of the WAG. The volume transport across the sides of the WAG is found to be related to vertical isopycnal movement at the base of the gyre. Advection is found to drive a decay in the salinity minimum and anticyclonic vorticity of the Eulerian WAG. Given the large contributions of advection, a Lagrangian analysis is performed, revealing geometric aspects of the exchange that are hidden in an Eulerian view. In particular, stable and unstable manifolds identify a stirring region around the outer reaches of the gyre where water is exchanged with the WAG on a time scale of weeks. Its complement is an inner core that expands with depth and exchanges water with its surroundings on much longer time scales. The 3D evolution of one parcel, or lobe, of water as it enters the WAG is also described, identifying a general Lagrangian subduction pathway.

Long-Term Monitoring of the Atlantic Jet through the Strait of Gibraltar with HF Radar Observations

The present work focuses on the long-term coastal monitoring of the Atlantic surface inflow into the Mediterranean basin through the Strait of Gibraltar. Hourly current maps provided during 2016–2017 by a High Frequency radar (HFR) system were used to characterize the Atlantic Jet (AJ) since changes in its speed and direction modulate the upper-layer circulation of the Western Alboran Gyre (WAG). The AJ pattern was observed to follow a marked seasonal cycle. A stronger AJ flowed north-eastwards during autumn and winter, while a weaker AJ was directed more southwardly during the middle of the year, reaching a minimum of intensity during summertime. A strong relationship between AJ speeds and angles was evidenced: the AJ appeared to be frequently locked at an angle around 63°, measured clockwise from the North. The AJ speed usually fluctuated between 50 cm·s−1 and 170 cm·s−1, with occasional drops below 50 cm·s−1 which were coincident with abrupt modifications in AJ orientation. Peaks of current speed clearly reached values up to 250 cm·s−1, regardless of the season. A number of persistent full reversal episodes of the surface inflow were analyzed in terms of triggering synoptic conditions and the related wind-driven circulation patterns. High sea level pressures and intense (above 10 m·s−1), permanent and spatially-uniform easterlies prevailed over the study domain during the AJ collapse events analyzed. By contrast, tides seemed to play a secondary role by partially speeding up or slowing down the westward currents, depending on the phase of the tide. A detailed characterization of this unusual phenomenon in the Strait of Gibraltar is relevant from diverse aspects, encompassing search and rescue operations, the management of accidental marine pollution episodes or efficient ship routing.

Modulation of frontogenetic plankton production along a meandering jet by zonal wind forcing: An application to the Alboran Sea

AbstractWe present a coupled physical‐biological modeling study to elucidate the changes in ageostrophic frontal dynamics and the frontogenetic plankton production characteristics of a meandering jet under the impacts of successive westerly/easterly wind events combined with seasonal variations in the upstream transport and buoyancy flux characteristics of the jet, using a case study for the Alboran Sea (Western Mediterranean). Their nonlinear coupling is shown to result in different forms of physical and biological characteristics of the background jet structure that follows a meandering path around two anticyclonic gyres in the western and eastern basins and a cyclonic eddy in between. The westerly, downfront wind events broaden the jet, and result in stronger cross‐frontal density contrast and intensify ageostrophic cross‐frontal secondary circulation. Thus, they improve the frontogenetic plankton production with respect to the no‐wind case. They also support higher production along the northern coast in response to wind‐induced coastal upwelling and spreading of resulting nutrient‐rich, productive water by mesoscale stirring. These features weaken gradually as the jet transport reduces. In contrast, stronger and longer‐lasting easterlies during the reduced jet transport phase weaken the currents and frontal density structure, change the circular Western Alboran Gyre to an elongated form, and shift the main axis of the jet towards the southern basin. Then, frontogenesis fails to contribute to phytoplankton production that becomes limited to the eddy pumping within cyclones. Apart from the frontogenetic production, eddy pumping, mesoscale stirring, and diapycnal mixing of nutrients support intermittent and localized phytoplankton patches over the basin.

Recent changes (2004–2016) of temperature and salinity in the Mediterranean outflow

AbstractTemperature and salinity series near the seafloor at Espartel Sill (Strait of Gibraltar) have been used to analyze the thermohaline variability of the Mediterranean outflow. The series shows temperature drops by the end of most winters/early springs, which are the remote response to Western Mediterranean Deep Water (WMDW) formation events in the Gulf of Lion that uplift old WMDW nearby the strait. This process distorts the seasonal cycle of colder/warmer water flowing out in summer/winter likely linked to the seasonality of the Western Alborán Gyre. The series shows positive trends in agreement with previous values, which are largely increased after 2013. It is tentatively interpreted as the Western Mediterranean Transition (WMT) signature that started with the very cold winters of 2005 and 2006. It was only after the large new WMDW production of 2012 and 2013 harsh winters that WMT waters were made available to flow out of the Mediterranean Sea.

Impacts of reprocessed altimetry on the surface circulation and variability of the Western Alboran Gyre

New altimetry products in semi-enclosed seas are of major interest given the importance of the coastal-open ocean interactions. This study shows how reprocessed altimetry products in the Mediterranean Sea from Archiving, Validation and Interpolation of Satellite Oceanographic data (AVISO) have improved the representation of the surface circulation over the 1993–2012 period. We focus on the Alboran Sea, which is the highest mesoscale activity area of the western Mediterranean. The respective impacts of the new mean dynamic topography (MDT) and mapped sea level anomaly (MSLA) on the description of the Western Alboran Gyre (WAG) are quantitatively evaluated. The temporal mean and variability of the total kinetic energy have been significantly increased in the WAG considering both the new MDT and MSLA (by more than 50%). The new MDT has added 39% to the mean kinetic energy, while the new MSLA has increased the eddy kinetic energy mean (standard deviation) by 53% (30%). The new MSLA has yielded higher variability of total (eddy) kinetic energy, especially in the annual frequency band by a factor of 2 (3). The MDT reprocessing has particularly increased the low-frequency variability of the total kinetic energy by a factor of 2. Geostrophic velocities derived from the altimetry products have also been compared with drifter data. Both reprocessed MDT and MSLA products intensify the velocities of the WAG making them closer to the in situ estimations, reducing the root mean square differences and increasing the correlation for the zonal and meridional components. The results obtained using refined coastal processing of altimetry products and new observational data are very encouraging to better understand the ocean circulation variability and coastal-open ocean interactions, and for potential improvements in other sub-basins, marginal seas and coastal global ocean.

Surface mesoscale pico–nanoplankton patterns at the main fronts of the Alboran Sea

The mesoscale (10–100km, days–weeks) plays a key role in the Ocean's ecosystem structure and dynamics. This work compares the pico–nanoplankton patterns observed in the Alboran Sea (Western Mediterranean) during three oceanographic cruises. We analyse its response to different expressions of mesoscale circulation associated with the three major hydrodynamic features in the basin; namely the Northwestern Alboran Front (NWAF, surveyed in OMEGA-1 cruise), the Almeria–Oran Front (AOF, surveyed in OMEGA-2 cruise) and the Western Alboran Gyre (WAG, surveyed in BIOMEGA cruise). The first two surveys were carried out under the most typical quasi-stationary twin gyre conditions of the Alboran Sea, whereas the third cruise was performed after an eastward migration of the WAG. The analysis of pico and nanoplankton populations was carried out using flow cytometry. The patchiness observed in the three cruises indicates an association of phytoplankton peaks with the main frontal structures: abundances were higher in the NWAF/upwelling area in OMEGA-1, at the Mediterranean side of the AOF in OMEGA-2, and at a tongue of recent Atlantic Water west of the WAG in BIOMEGA. However, a more detailed analysis reveals that different factors explain the origin of the phytoplankton biomass in each front/cruise. Mixing processes at the Strait of Gibraltar and the subsequent advection of water properties into the Western Alboran Sea were the mechanisms responsible for the abundances observed in the NWAF. The highest concentrations observed in the AOF were related to the intrusion of Mediterranean Surface Waters to the north of the front. During the migrating WAG the phytoplankton distribution was influenced by the formation of a new gyre. The relation between phytoplankton and mesoscale dynamics is further explored in terms of vertical velocity diagnosis. In all cases, intense vertical motion is negatively correlated with the abundance of phytoplankton populations. This resulted from the intense geostrophic background flow associated with large vertical velocities, which drove low residence times of water mass properties and hence a weak biological response. Fast-repeated surveys made during the OMEGA-1 and OMEGA-2 cruises reveal that the surveyed hydrographic features are subjected to significant temporal variability. In this case, the impact on the biology is most evident at taxa level.

Sea surface temperature variations in the western Mediterranean Sea over the last 20 kyr: A dual‐organic proxy (U K′ 37 and LDI) approach

A high-resolution sea surface temperature (SST) reconstruction of the western Mediterranean was accomplished using two independent, algae-based molecular organic proxies, i.e., the U-37(K) index based on long-chain unsaturated ketones and the novel long-chain diol index (LDI) based on the relative abundances of C-28 and C-30 1,13- and 1,15-diols. Two marine records, from the western and eastern Alboran Sea basin, spanning the last 14 and 20kyr, respectively, were studied. Results from the surface sediments suggest that the two proxies presently reflect seasons with similar SST or simply annual mean SST. Both proxy records reveal the transition from the Last Glacial Maximum to the Holocene in the eastern Alboran Sea with an SST increase of approximately 7 degrees C for U-37(K) and 9 degrees C for LDI. Minimum SSTs (10-12 degrees C) are reached at the end of the Last Glacial Maximum and during the last Heinrich event with a subsequent rapid SST increase in LDI-SST toward the beginning of the Bolling period (20 degrees C), while U-37(K)-SST remains constantly low (similar to 12 degrees C). The Bolling-Allerod period is characterized by a rapid increase and subsequent decrease in U-37(K)-SST, while the LDI-SST decrease continuously. Short-term fluctuations in U-37(K)-SST are probably related to the availability of nutrients and seasonal changes. The Younger Dryas is recorded as a short cold interval followed by progressively warmer temperatures. During the Holocene, the general lower U-37(K)-derived temperature values in the eastern Alboran (by approximately 1.5-2 degrees C) suggest a southeastward cold water migration by the western Alboran gyre and divergence in the haptophyte blooming season between both basins. Key Points High-resolution SST reconstruction is performed in the westernmost Mediterranean Two algae-based proxies show the temperature evolution for the last 20 kyr Derived SSTs suggest different growth seasons of alkenone and diol producers

The Alboran Sea mesoscale in a long term high resolution simulation: Statistical analysis

We analyze the Alboran Sea mesoscale variability in a 2-decade high resolution (2km) simulation. The circulation modes and eddies are described from a statistical perspective. The double-gyre quasi-steady state is confirmed as the most common circulation mode in the Alboran Sea (48% of the time), followed by single-gyre mode in about 24% of the situations. These persistent modes are compared in terms of structure, frequency of occurrence and seasonality, energetics, and their links to inflow variability and wind forcing. The double-gyre state is the most stable situation with higher kinetic and potential energy and lower eddy kinetic energy and seems to be limited by a critical value of the Rossby radius (∼12km). The transitions between the quasi-steady states are studied and typified with a particular focus on the Western Alboran Gyre migration which stands out as the major contributor for flow transitions. A typical sequence of flow types and transients is proposed: The double-gyre is usually canceled after a migration event occurring in late summer or autumn. The Western Alboran Gyre is not immediately replaced by a new born anticyclone. Instead, there is a cycle of migration-merging events with the Eastern Alboran Gyre which eventually force the latter to move westward and form a single-gyre situation. The generation of a second Eastern Alboran Gyre in spring-early summer sets the beginning of a new double-gyre stable period. An analysis of coherent mesoscale eddies is conducted and their statistics, sites and processes of generation are described. Cyclones (anticyclones) mean radius are in the range of ∼12km (∼15km), they are much more frequent in winter time and cyclones significantly outnumber the anticyclones. The link of mesoscale processes and transients (in particular of migrations) with the inflow is analyzed, and no clear relationship of transient events with inflow magnitude is disclosed. On the other hand, the large majority of transitions (specially migrations) are associated with a clear shift of the angle and latitude of the Atlantic Jet to the south. The stratification and wind variability are also important suggesting a significant role of eddy dynamics and intrinsic variability in determining the stable modes and transitions. Despite the seasonality of the flow types sequence, there are periods of long stability with the double-gyre blocking situations resisting the winter periods. The time variability is analyzed and compared with observations.

What does cause the collapse of the Western Alboran Gyre? Results of an operational ocean model

The stability of the mean state of the surface circulation of the Western Alboran Sea, characterized by a jet of Atlantic water (AJ) surrounding the Western Alboran Gyre (WAG), is investigated with a submesoscale-resolving operational ocean model. It is shown that this circulation state collapses through the interaction between the AJ-WAG system and a growing cyclonic eddy that arises close to the Spanish coast. This eddy develops as the WAG partially blocks the positive potential vorticity (PV) flux coming from the Strait of Gibraltar. It is found that tides increase the positive PV flux with respect to a non-tidal simulation, thus making the destabilization of the mean circulation state more likely. Atmospheric pressure driven flows are also shown to have the potential to destabilize the circulation by rising dramatically the PV flux within a time scale of some days. This provides an explanation as to why the circulation of the Western Alboran Sea exhibits more variability toward winter time, when meteorological fluctuations are enhanced.

The Western Alboran Gyre helps ventilate the Western Mediterranean Deep Water through Gibraltar

Variable properties of the Mediterranean outflow and the variability of the Western Alboran Gyre are analyzed by means of 5-year long time series of near bottom potential temperature at Espartel sill in the Strait of Gibraltar and altimetry data in the Alboran Sea. Geostrophic velocity at the southern edge of the gyre and potential temperature at Espartel sill are significantly correlated (correlation coefficient 0.67), suggesting that the intensification of the Alboran Gyre favors the ventilation of Western Mediterranean Deep Water. The analysis of historical temperature profiles shows that Western Mediterranean Deep Water in the Alboran Sea can be suctioned from a layer between 500 and 700m depth for typical changes of the gyre intensification.

Meteorologically-induced mesoscale variability of the North-western Alboran Sea (southern Spain) and related biological patterns

Hydrographic mesoscale structures in the North-western Alboran Sea show a high variability induced by a number of different factors. One of the most important is the differences in atmospheric pressure over the Mediterranean basin when compared to the Gulf of Cádiz. This difference modulates the zonal wind field in the Alboran Sea and the intensity of the Atlantic inflow through the Strait of Gibraltar, also affecting the formation and extension of the Western Alboran Gyre (WAG). When westerly winds are dominant, lower atmospheric pressure in the Mediterranean enhances the inflow of Atlantic waters causing the Atlantic Jet to be located in the vicinity of the Spanish shore, creating a well-defined frontal zone in front of Estepona Cove. In this situation, the coastal upwelling is enhanced, leading to a minimum in sea surface temperature and a maximum of surface nutrient concentrations located in the coastal area. The vertical position of the chlorophyll maximum found in these circumstances appeared to be controlled by the nutrient availability. On the other hand, when easterly winds prevail, higher atmospheric pressure in the Mediterranean leads to a reduced inflow and the oceanographic and biological structures are clearly different. The Atlantic Jet moves southward flowing in a south-eastern direction, changing the structure of the currents, resulting in an enhanced cyclonic circulation extending throughout the North-western Alboran Sea basin. These physical alterations also induce changes in the distribution of biogeochemical variables. Maximum nutrient and chlorophyll concentrations are located further off the coast in the central area of the newly created cyclonic gyre. During these easterlies periods coastal upwelling stops and the distribution of phytoplankton cells seems to be mainly controlled by physical processes such as advection of coastal waters to the open sea.

Phytoplankton pigment distribution in the northwestern Alboran Sea and meteorological forcing: A remote sensing study

A set of weekly-composite SeaWiFS (Sea-viewing Wide Field of view Sensor) imagery (from 1998 to 2004) has been used to asses the temporal and spatial variability of the surface chlorophyll distribution in the Northwestern Alboran Sea. Empirical Orthogonal Function (EOF) analysis of the images series shows the existence of three different regions in which surface chlorophyll presents a homogeneous temporal behavior in the neighborhood of the Strait of Gibraltar. The temporal patterns of surface chlorophyll concentrations in each region have been characterized by studying the role of several forcing factors, mainly the zonal component of the wind field. The first identified region occupies the coastal area in front of Estepona and Malaga and shows the highest chlorophyll concentration in the entire region of study in absence of easterly winds. A second region is located far offshore, in the zone usually occupied by the Atlantic jet entering the Alboran Sea, showing high concentrations of chlorophyll when the easterlies blow. Finally, a third region has been situated in the normal location of the Western Alboran Gyre (WAG). The zonal wind field arises as a main forcing function in the modulation of the intensity and location of the upwelling processes in the area, while the entrance of the Atlantic jet seems to be the main agent feeding of all these processes.

Prediction of sea surface temperatures in the western Mediterranean Sea by neural networks using satellite observations

We use artificial neural networks (ANNs) to predict sea surface temperatures (SSTs) in the western Mediterranean Sea. The ANNs are trained with meteorological variables as input and concurrent satellite‐derived SSTs as target. The trained ANNs predict well both the seasonal and the interannual variability of SST in that region. We also reproduce the impact of the heat wave that occurred during the summer of 2003 on the SSTs of the western Mediterranean Sea. The ANN technique allows us to predict SST maps in the western Alboran Sea for time coordinates before SST satellite availability. The presence and later partial collapse of the western Alboran gyre throughout 1980 is detected with good agreement by both the ANN predictions and the concurrent results from a 3‐D circulation model. The same methodology is used to reconstruct incomplete SST satellite images.

Diagnosing Mesoscale Vertical Motion from Horizontal Velocity and Density Data

Abstract The mesoscale vertical velocity is obtained by solving a generalized omega equation (ω equation) using density and horizontal velocity data from three consecutive quasi-synoptic high-resolution surveys in the Alboran Sea. The Atlantic Jet (AJ) and the northern part of the Western Alboran Gyre (WAG) were observed as a large density anticyclonic front extending down to 200–230 m. The horizontal velocity uh in the AJ reached maxima of 1.2 m s−1 for the three surveys, with extreme Rossby numbers of ζ/f ≈ −0.9 in the WAG and +0.9 in the AJ (where ζ is the vertical vorticity and f is the Coriolis parameter). The generalized ω equation includes the ageostrophic horizontal flow. It is found that the most important “forcing” term in this equation is ( fζph + ∇hϱ) · ∇2huh, where ζph is the horizontal (pseudo) vorticity and ϱ is the buoyancy. This term is related to the horizontal advection of vertical vorticity by the vertical shear velocity, uhz · ∇hζ. Extreme values of the diagnosed vertical velocity w were located at 80–100 m with max{w} ⊂ [34, 45] and min{w} ⊂ [−64, −34] m day−1. Comparison with the quasigeostrophic (QG) ω equation shows that, because of the large Rossby numbers, non-QG terms are important. The differences between w and the QG vertical velocity are mainly related to the divergence of the ageostrophic part of the total Q vector (Qh ≡ ∇huh · ∇hϱ) in the ω equation.