[1] Recent observations of the outflowing Mediterranean water collected near the bottom in key points of the Strait of Gibraltar show the existence of a seasonal cycle with warmer and lighter waters leaving the Mediterranean Sea in winter and cooler and denser waters in spring early summer. The amplitude of the signal is around 5 10 À2°C for potential temperature and 1.5 10 À2 for potential density, salinity hardly showing seasonal fluctuations. The outflow also shows a seasonal cycle with maximum volume transport in April, in coincidence with the minimum of the signal of potential temperature. A simple analysis of the composition of the outflow in terms of the main water masses of the western Mediterranean basin and its comparison with climate indicators suggests that the seasonal cycle follows the annual process of the Western Mediterranean Deep Water formation that replenish the deep portion of the basin by the end of winter and rises the level of the deep water reservoir, facilitating the suction of cooler and denser water from the Mediterranean over the sills of the Strait. From this time onwards, the data show a smooth warming that would be explained by the progressive fall of the level of the Western Mediterranean Deep Water as it is drained out the Mediterranean, which would leave warmer water available for suction. The process is asymmetric in the sense that the transition from high to low temperature is completed in a short period while the progressive warming spans a longer period.

The Mediterranean Sea shows a peculiar anomaly in its nutrient pattern compared to the global ocean, as there is decrease in nutrient concentration from west to east. This feature has been attributed to the antiestuarine circulation at the Strait of Gibraltar, where an eastward flow of Atlantic nutrient‐poor surface waters is compensated by a westward countercurrent of Mediterranean nutrient‐rich deep waters. This water exchange has been suggested as the ultimate cause for the oligotrophy of the Mediterranean basin, even though only a few studies have accurately examined the magnitude of the nutrient flux through the Strait of Gibraltar. In this work, data from the Gibraltar Fixed Time series (GIFT) between 2005 and 2008 were used to assess nutrient distributions. Applying a two‐layer model of water mass exchange and using the Mediterranean outflow recorded in situ, the net export of nutrients from the Mediterranean to the Atlantic was calculated as 139 and 4.8 Gmol yr−1 of nitrate and phosphate, respectively. The results also demonstrated that the Atlantic inflow is not nutrient depleted and in particular contains significant levels of phosphate, which is the limiting factor for biological productivity in the eastern Mediterranean. The distribution of the quasi‐conservative parameter N* in the western and eastern basins indicated that nitrate‐deficient surface waters are transformed into phosphate‐deficient bottom waters by internal cycling processes. Therefore, phosphate depletion in the Mediterranean does not have its origin in the entry of a phosphorus‐impoverished Atlantic inflow through the Strait of Gibraltar.

Abstract. The exchange of both anthropogenic and natural inorganic carbon between the Atlantic Ocean and the Mediterranean Sea through Strait of Gibraltar was studied for a period of two years under the frame of the CARBOOCEAN project. A comprehensive sampling program was conducted, which was design to collect samples at eight fixed stations located in the Strait in successive cruises periodically distributed through the year in order to ensure a good spatial and temporal coverage. As a result of this monitoring, a time series namely GIFT (GIbraltar Fixed Time series) has been established, allowing the generation of an extensive data set of the carbon system parameters in the area. Data acquired during the development of nine campaigns were analyzed in this work. Total inorganic carbon concentration (C T ) was calculated from alkalinity-pH T pairs and appropriate thermodynamic relationships, with the concentration of anthropogenic carbon (C ANT ) being also computed using two methods, the C* and the TrOCA approach. Applying a two-layer model of water mass exchange through the Strait and using a value of −0.85 Sv for the average transport of the outflowing Mediterranean water recorded in situ during the considered period, a net export of inorganic carbon from the Mediterranean Sea to the Atlantic was obtained, which amounted to 25±0.6 Tg C yr −1 . A net alkalinity output of 16±0.6 Tg C yr −1 was also observed to occur through the Strait. In contrast, the Atlantic water was found to contain a higher concentration of anthropogenic carbon than the Mediterranean water, resulting in a net flux of C ANT towards Correspondence to: I. E. Huertas

Atmospheric data from reanalysis, satellite, and experimental observations have been combined to calculate a four‐year time series of the Atlantic inflow through the Strait of Gibraltar. The net flow through the strait, estimated from the Mediterranean water budget, and the Mediterranean outflow, estimated from currentmeter observations in Espartel sill (western Strait of Gibraltar) from October 2004 to January 2009, made it possible to estimate the Atlantic inflow as the sum of both of them. The obtained mean net flow is 0.038 ± 0.007 Sv, with a seasonal cycle of 0.042 ± 0.018 Sv annual amplitude and maximum in September. The Mediterranean outflow shows a seasonal signal with annual amplitude of 0.027 ± 0.015 Sv peaking in April (in absolute value), and a mean value of −0.78 ± 0.05 Sv. The resulting Atlantic inflow has a mean value of 0.81 ± 0.06 Sv and a seasonal cycle with annual amplitude of 0.034 ± 0.011 Sv, peaking in September, and high interannual variability. The inflow seasonal cycle is the result of a barotropic forcing associated with the cycle of the net flow, driven by the evaporative cycle, and a baroclinic forcing linked to the seasonal cycle of the reduced gravity that drives the exchange.

Three‐yearlong time series of Acoustic Doppler Current Profiler (ADCP) observations at a single station in Espartel Sill (Strait of Gibraltar) were used to compute an outflow of Q2 = −0.82 Sv through the main channel. The cross‐strait structure of the velocity field or the outflow through a secondary channel north of the submarine ridge of Majuan in Espartel section is not captured by observations so that an improved version of a numerical model (CEPOM) has been used to fill the observational gap. Previously, the model performance has been checked against historical data sets by comparing harmonic constants of the main diurnal and semidiurnal constituents from observed and modeled data at different sites of the strait. Considering the great complexity of tidal dynamics in the area, the comparison is quite satisfactory and validates the model to infer the exchange at longer timescales. Using a “climatological” April in the simulation, extracting a “single station” from the model at the same position as the monitoring station and processing the data similarly, the model gives an outflow through the southern channel 13% higher than observations. The inclusion of the cross‐strait structure of velocity reduces the computed outflow through the southern channel, whereas the contribution of the northern channel brings the total outflow close to that computed using a single station (5% smaller). If the same correction is applied to observations, the total outflow would reduce to Q2 = −0.78 Sv. The paper also assesses the importance of eddy fluxes to the total outflow, their contribution being negligible (≤5%).