Stefano Vignudelli

Hydrographic characteristics and interannual variability of water masses in the central mediterranean: a sensitivity test for long-term changes in the Mediterranean Sea

Abstract Hydrographic observations repeated from 1993 to 1999 in the central Mediterranean Sea, between the Sicily Strait and the Sardinia Channel, allowed us to define the water masses exchanged between the Eastern and the Western Mediterranean and the long-term variability of their properties. Besides the well known Modified Atlantic Water (MAW) and the Levantine Intermediate Water (LIW), other water masses are involved in this exchange: the waters of the upper deep layer in the Ionian Sea (transitional Eastern Mediterranean Deep Water (tEMDW)), which form a density current flowing at the bottom, and, from the Western Mediterranean, the Western Mediterranean Deep Water and a stream of LIW that has re-circulated in that basin (old LIW). The sub-surface water masses flow into the Tyrrhenian Sea, an intermediate basin, where they are subject to intense mixing, which modifies them or even makes them disappear. Accordingly, the outflow is formed by LIW and by the modified Tyrrhenian Deep Water (mTDW), both of them contributing to the exchange with a stream directed to the western Mediterranean. An interesting aspect indicated by the hydrographic time series is that, although the temperature and salinity of LIW in the Sicily Strait showed a prevailing trend towards lower values consistent with the changes produced by the recent climatic transient in the Eastern Mediterranean, the temperature and salinity of mTDW increased progressively throughout the whole period. This apparent anomaly was related to the behaviour of tEMDW in the Tyrrhenian Sea. While sinking at the Tyrrhenian entry, tEMDW represents a source of heat and salt for the colder and less saline resident waters at depth, thus progressively raising their temperature and salt content. The small, long-term tendency of this was intensified by the arrival of the new waters produced by the climatic transient. In our opinion, the effects of this process may add a component to the well known positive trend affecting the temperature and salinity of deep waters of the western Mediterranean.

Exploiting the potential of an improved multimission altimetric data set over the coastal ocean

Until now, most satellite altimetry studies of the coastal ocean have been based on along-track data from a single mission, whereas up to four missions were operative in 2002–2005. Here, to monitor the coastal ocean we have applied specialized corrections and dedicated processing strategies to compute a multimission data set at a mean distance of 32 km of the coast. The resulting altimetric data set is compared with sea level data from three in situ stations over a coastal zone of the northwestern Mediterranean. The mean rms difference between this data set and the sea level stations is 2.9 cm against 3.7 cm when using the AVISO altimetric product. Comparison of altimeter-derived geostrophic velocities with a mooring also shows that the spatial and temporal variability of the surface current field is well reproduced. The agreement with in situ measurements extends to intraseasonal time scales showing a significant improvement compared to previous studies in the 50 km coastal-band.

A possible influence of the North Atlantic Oscillation on the circulation of the western Mediterranean Sea

A current time series spanning from 1985 to 1996 in the Corsica Channel shows that seasonal and interannual variabilities are stable components of the circulation of the northern part of the Western Mediterranean Sea. For the first time these characteristics are related to the state of the North Atlantic Oscillation during winter. We have found that this large-scale control is mainly maintained through the air-sea interaction processes which take place in the Northern Basin. These results provide new elements for a consistent explanation of the major features of the basin circulation, such as the seasonal increase of the boundary current and Deep Water Formation.

Integrated use of altimeter and in situ data for understanding the water exchanges between the Tyrrhenian and Ligurian Seas

In this paper we examine the relationship between the seasonal and interannual variability observed in the water flow through the Corsica Channel and the sea level difference between the Tyrrhenian and Ligurian Seas. The steric contribution to the sea level difference, computed from historical hydrological data, is in good agreement with the stable presence of the seasonal signal in the water exchanges. We obtain the maximum steric difference in winter (∼16 cm) and the minimum in summer (∼2 cm). These values are consistent with the corresponding estimates of water volume transport (0.8 and <0.1 Sv in winter and summer, respectively). Also, TOPEX/Poseidon (T/P) satellite altimetry is shown to be capable of capturing the sea level difference anomaly between the two basins at seasonal and interannual scales. Accuracy of altimeter data in the study region has been checked using measurements from a bottom pressure recorder deployed in Capraia Island (rms difference is found to be ∼2 cm after application of a 30-day half-amplitude Gaussian filter). Because of the lack of an accurate geoid, the total water transport cannot be adequately monitored by satellite altimetry alone. However, the recovered signal can be directly related to the water transport anomaly through the channel. The resulting T/P signal can be considered as representing, to a great extent, the real steric variation induced by the net sea surface heat fluxes. The high correlation found between the water transport and sea level difference anomalies confirms the results obtained by the climatological hydrography and provides new evidence that interannual differences in the sea level and water transport can be related to the different heat flux balance in the Tyrrhenian and Ligurian Seas. Moreover, it demonstrates, for the first time in the Mediterranean, the potential of satellite altimetry for long-term monitoring of the water exchanges between adjacent basins.

Modeling Envisat RA-2 Waveforms in the Coastal Zone: Case Study of Calm Water Contamination

Radar altimeters have so far had limited use in the coastal zone, the area with most societal impact. This is due to both lack of, or insufficient accuracy in the necessary corrections, and more complicated altimeter signals. This letter examines waveform data from the Envisat RA-2 as it passes regularly over Pianosa (a 10-km2 island in the northwestern Mediterranean). Forty-six repeat passes were analyzed, with most showing a reduction in signal upon passing over the island, with weak early returns corresponding to the reflections from land. Intriguingly, one third of cases showed an anomalously bright hyperbolic feature. This feature may be due to extremely calm waters in the Golfo della Botte (northern side of the island), but the cause of its intermittency is not clear. The modeling of waveforms in such a complex land/sea environment demonstrates the potential for sea surface height retrievals much closer to the coast than is achieved by routine processing. The long-term development of altimetric records in the coastal zone will not only improve the calibration of altimetric data with coastal tide gauges but also greatly enhance the study of storm surges and other coastal phenomena.

Potential use of ERS-1 and Topex/Poseidon altimeters for resolving oceanographic patterns in the Algerian Basin

Two independent and concurrent (1992–1993) sources of data obtained from ERS-1 and Topex/Poseidon satellites, provide a first look at the spatial distribution of mesoscale sea surface variability in the Algerian Basin. Values higher than 6 cm rms were found in a narrow band extending along the Algerian slope between 0° and 4°E, where meander formation and eastward eddy propagation occurred. East of 4°E, patterns of higher variability were also seen in the interior of the basin resulting from detachment of eddies from the Algerian Current that drift seaward. Based on geostrophic velocity estimates at crossover points, the eddy field variability appeared to be anisotropic, especially in the vicinity of the Algerian Current. Two mesoscale features whose signatures were clearly identified from altimeter transects, were supported by cloud-free AVHRR infrared imagery. In particular, the path of an anticyclonic eddy (140 km diameter with drift of ∼3 cm s−1) was sufficiently coherent with the temporal and spatial sampling of altimeters to describe its kinematic behaviour. The results thus demonstrate the potential usefulness of satellite altimetry for monitoring oceanographic conditions in the Algerian Basin.

Patterns of the loop current system and regions of sea surface height variability in the eastern Gulf of Mexico revealed by the self‐organizing maps

The Self-Organizing Map (SOM), an unsupervised learning neural network, is employed to extract patterns evinced by the Loop Current (LC) system and to identify regions of sea surface height (SSH) variability in the eastern Gulf of Mexico (GoM) from 23 years (1993–2015) of altimetry data. Spatial patterns are characterized as different LC extensions and different stages in the process of LC eddy shedding. The temporal evolutions and the frequency of occurrences of these patterns are obtained, and the typical trajectories of the LC system progression on the SOM grid are investigated. For an elongated, northwest-extended, or west-positioned LC, it is common for the LC anticyclonic eddy (LCE) to separate and propagate into the western GoM, while an initially separated LCE in close proximity to the west Florida continental slope often reattaches to the LC and develops into an elongated LC, or reduces intensity locally before moving westward as a smaller eddy. Regions of differing SSH variations are also identified using the joint SOM-wavelet analysis. Along the general axis of the LC, SSH exhibits strong variability on time scales of 3 months to 2 years, also with energetic intraseasonal variations, which is consistent with the joint Empirical Orthogonal Function (EOF)-wavelet analysis. In the more peripheral regions, the SSH has a dominant seasonal variation that also projects across the coastal ocean. The SOM, when applied to both space and time domains of the same data, provides a powerful tool for diagnosing ocean processes from such different perspectives.

15 Years of Altimetry at Various Scales over the Mediterranean

The present article reviews the application of altimetry in the Mediterranean Sea, to study the circulation and sea surface height variability, both at basin scale and in specific regions. The improvements needed to fully exploit the 15-year record of data close to the coast are also discussed. These range from improved tidal models, to specialized atmospheric corrections, to ad hoc screening of data in proximity of the coast. Some of these improvements are already underway while others are the focus of forthcoming programs.

Exploiting satellite altimetry in coastal ocean through the ALTICORE project

Altimeter-derived information on sea level and sea state could be extremely important for resolving the complex dynamics of the coastal ocean. Satellite altimetry was not originally conceived with coastal ocean in mind, but future missions (AltiKa and CryoSat-2) promise much improved nearshore capabilities. A current priority is to analyze the existing, under-exploited, 15-year global archive of coastal altimeter data to draw recommendations for these missions. There are intrinsic difficulties in processing and interpretation of the data, e.g.: the proximity of land, control by the seabed, and rapid variations due to tides and atmospheric effects. But there are also unexploited possibilities, including higher along track data rates and multi-altimetry scenarios that need to be explored. There are also difficulties of accessing and manipulating data from multiple sources, many of which undergo regular revision and enhancement. In response to these needs, the ALTICORE (ALTImetry for COastal REgions - www.alticore.eu) project started in December 2006, funded for two-years by the European INTAS scheme (www.intas.be). The overall aim of ALTICORE is to build up capacity for provision of altimeter-based information in support of coastal ocean studies in some European Seas (Mediterranean, Black, Caspian, White and Barents). ALTICORE will also contribute to improved cooperation between Europe and Eastern countries and enhance networking of capacity in the area of satellite altimetry. This paper discusses the approach, summarizes the planned work and shows how the coastal community should eventually benefit from better access to improved altimeter-derived information.

Coastal Altimetry Products in the Strait of Gibraltar

This paper analyzes the availability and accuracy of coastal altimetry sea level products in the Strait of Gibraltar. All possible repeats of two sections of the Envisat and AltiKa ground-tracks were used in the eastern and western portions of the strait. For Envisat, along-track sea level anomalies (SLAs) at 18-Hz posting rate were computed using ranges from two sources, namely, the official Sensor Geophysical Data Records (SGDRs) and the outputs of a coastal waveform retracker, the Adaptive Leading Edge Subwaveform (ALES) retracker; in addition, SLAs at 1 Hz were obtained from the Centre for Topographic studies of the Ocean and Hydrosphere (CTOH). For AltiKa, along-track SLA at 40 Hz was also computed both from SGDR and ALES ranges. The sea state bias correction was recomputed for the ALES-retracked Envisat SLA. The quality of these altimeter products was validated using two tide gauges located on the southern coast of Spain. For Envisat, the availability of data close to the coast depends crucially on the strategy followed for data screening. Most of the rejected data were due to the radar instrument operating in a low-precision nonocean mode. We observed an improvement of about 20% in the accuracy of the Envisat SLAs from ALES compared to the standard (SGDR) and the reprocessed CTOH data sets. AltiKa shows higher accuracy, with no significant differences between SGDR and ALES. The use of products from both missions allows longer times series, leading to a better understanding of the hydrodynamic processes in the study area.