ALBERT

Description: Various current data, collected in the Strait of Otranto during the period 1994–2007, have been analysed with the aim of describing the characteristics of the tidal motions and their contribution to the total flow variance. The principal tidal constituents in the area were the semi-diurnal (M2) and the diurnal (K1), with the latter one predominant. The total flow was, in general, more energetic along the flanks than in the middle of the strait. Specifically, it was most energetic over the western shelf and in the upper layer along the eastern flank. In spite of the generally low velocities (a few cm s−1) of the principal tidal constituents, the tidal variance has a pattern similar to that of the total flow variance, that is, it was large over the western shelf and low in the middle. The proportion of non-tidal (comprising the inertial and sub-inertial low-frequency bands) to tidal flow variances was quite variable in both time and space. The low-frequency motions dominated over the tidal and inertial ones in the eastern portion of the strait during the major part of the year, particularly in the upper and intermediate layers. In the deep, near-bottom layer the variance was evenly distributed between the low frequency, diurnal and semi-diurnal bands. An exception was observed near the western shelf break during the summer season when the contribution of the tidal signal to the total variance reached 77%. This high contribution was mainly due to the intensification of the diurnal signal at that location at both upper and bottom current records (velocities of about 10 cm s−1). Local wind and sea level data were analysed and compared with the flow to find the possible origin of this diurnal intensification. Having excluded the sea-breeze impact on the intensification of the diurnal tidal signal, the most likely cause remains the generation of the topographically trapped internal waves and the diurnal resonance in the tidal response. These waves were sometimes generated by the barotropic tidal signal in the presence of summer stratification and the strong bottom slope. This phenomenon may stimulate diapycnal mixing during the stratified season and enhance ventilation of the near-bottom layers.

Description: An index of El Niño Southern Oscillation (ENSO) in the Pacific during pre monsoon is shown to account for a significant part of the variability of coastal Sea Surface Temperature (SST) anomalies measured a few months later within the wind driven southwest coast of India coastal upwelling region 7° N–14° N. This teleconnection is thought to result from an atmospheric bridge between the Pacific and north Indian Oceans, leading to warm (cold) ENSO events being associated with relaxation (intensification) of the Indian trade winds and of the wind-induced coastal upwelling. This ENSO related modulation of the wind-driven coastal upwelling appears to contribute to the connection observed at the basin-scale between ENSO and SST in the Arabian Sea. The ability to use this teleconnection to give warning of large changes in the southwest coast of India coastal upwelling few months in advance is successfully tested using data from 1998 and 1999 ENSO events.

Description: The interannual variability of the South Adriatic Gyre and its relation to the wind vorticity and the large-scale climatic pattern (North Atlantic Oscillation – NAO) was studied using the time series of satellite altimetric data and ocean surface wind products. The cyclonic circulation observed in the southern Adriatic area was partly sustained by the local wind forcing, as suggested by the positive correlation between the rate of change of the current vorticity and the wind-stress vorticity. Nevertheless, the influence of vorticity advection from the adjacent area (northern Ionian Sea) cannot be ignored and it is more significant during the anticyclonic phase of Adriatic–Ionian Bimodal Oscillation System. The geostrophic current vorticities of the southern Adriatic and northern Ionian seas are correlated with a time lag of 14 months, which approximately corresponds to an advection speed of  ∼  1 cm s−1. The different wind patterns observed during two NAO phases in the winter revealed a stronger positive vorticity during the negative NAO phase. Conversely, during the wintertime positive NAO phase the wind vorticity is characterized by lower positive or slightly negative values. Despite a statistically significant negative correlation between the NAO index and the wind vorticity, no unequivocal relationship between large climatic system and the interannual variability of the South Adriatic Gyre intensity was found due to additional effects of the vorticity advection from the Ionian. This can be explained by the fact that the Ionian circulation mode does not depend on the NAO variations. Therefore, the main result of this study is that the interannual variability of the southern Adriatic cyclonic circulation is a result of the combined influence of the vorticity advection from the Ionian and the local wind-curl effect.

Description: The interannual variability of the South Adriatic Gyre and its relation to the wind vorticity and the large-scale climatic pattern (North Atlantic Oscillation – NAO), was studied using the time-series of satellite altimetry data and ocean surface wind products. The cyclonic circulation observed in the South Adriatic area was mainly sustained by the local wind forcing, as suggested by the positive correlation between the rate of change of the current vorticity and the wind-stress vorticity. Nevertheless, the influence of vorticity advection from the adjacent area (North Ionian Sea) cannot be ignored and it is more significant during the anticyclonic phase of Adriatic–Ionian Bimodal Oscillation System. The geostrophic current vorticities of the South Adriatic and North Ionian Seas are correlated with a time lag of 15 months, which corresponds to an advection speed of ~1 cm s−1. The different wind patterns observed during the two NAO phases revealed a stronger positive vorticity during the negative NAO phase. Conversely, during the positive NAO phase the wind vorticity is characterized by lower positive values. Subsequently, the calculated positive linear correlation between the NAO index and the frequency of the cold and dry northerly wind suggests the strengthening of the winter convection, and of the consecutive deep water formation, during the positive NAO phases. As a consequence of the winter deep convection, Southern Adriatic area is characterized by the late winter/early spring algal blooms. Relationship between the spatially averaged surface chlorophyll concentrations and the northerly wind frequencies revealed that the two biological productivity regimes likely exist: the subtropical one and the subpolar one depending on the frequency of windy days. We also showed that the bloom timing is a linear function of the wind frequency and can vary within the range of almost two months. This study thus contributes to our understanding of the possible impact of climate change on the SAG circulation and its ecosystem.

Description: Adriatic and Ionian Seas are Mediterranean sub-basins linked through the Bimodal Oscillating System mechanism responsible for decadal reversals of the Ionian basin-wide circulation. Altimetric maps showed that the last cyclonic mode started in 2011 but unexpectedly in 2012 reversed to anticyclone. We related this "premature" inversion to extremely strong winter in 2012, which caused the formation of very dense Adriatic waters, flooding Ionian flanks in May and inverting the bottom pressure gradient. Using Lagrangian float measurements, the linear regression between the sea surface height and three isopycnal depths suggests that the southward deep-layer flow coincided with the surface northward geostrophic current and the anti-cyclonic circulation regime. Density variations at depth in the north-western Ionian revealed the arrival of Adriatic dense waters in May and maximum density in September. Comparison between the sea level height in the north-western Ionian and in the basin centre showed that in coincidence with the arrival of the newly formed Adriatic dense waters the sea level lowered in the north-western flank inverting the surface pressure gradient. Toward the end of 2012, the density gradient between the basin flanks and its centre went to zero, coinciding with the weakening of the anticyclonic circulation and eventually with its return to the cyclonic pattern. Thus, the premature and transient reversal of Ionian surface circulation originated from the extremely harsh winter in the Adriatic, resulting in the formation and spreading of highly dense bottom waters. The present study highlights the remarkable sensitiveness of the Adriatic–Ionian BiOS to climatic forcing.

Description: The importance of the Mediterranean Sea for the world ocean has long been recognized. First, the Mediterranean sea has a profound impact on the Atlantic ocean circulation and, consequently, on the global thermohaline conveyor belt. Maps of the Mediterranean salty water tongue exiting from the Gibraltar strait at intermediate depths and spreading throughout the Atlantic interior are well known since the 1950s. Through direct pathways to the Atlantic polar regions or through indirect mixing processes, the salty Mediterranean water preconditions the deep convection cells of the polar Atlantic. There the North Atlantic Deep Water is formed which successively spreads throughout the world ocean constituting the core of the global thermohaline circulation. Even more importantly, the Mediterranean Sea is a laboratory basin for the investigation of processes of global importance, being much more amenable to observational surveys because of its location in mid-latitude and its dimensions. Both the western and eastern basins in fact possess closed thermohaline circulations analogous to the global conveyor belt. A unique upper layer open thermohaline cell connects the eastern to the western basin and, successively, to the north Atlantic through the Gibraltar strait. In it, the Atlantic water entering into Gibraltar in the surface layer, after travelling to the easternmost Levantine basin, is transformed into one of the saltiest water masses through air–sea heat and moisture fluxes. This is the salty water which, crossing the entire basin in the opposite direction below the surface Atlantic water, finally exits from the Gibraltar strait at mid-depths. Both the western and eastern basins are endowed with deep/intermediate convection cells analogous to the polar Atlantic deep convection cells or to the intermediate mode water ones. Deep and intermediate water masses are therefore formed in different sites of the entire basin. Because of their easily accessible locations, these convection cells are much more amenable to direct observational surveys and mooring arrays. An ubiquitous, energetic mesoscale and sub-mesoscale eddy field is superimposed to and interacts with the sub-basin scale, wind-driven gyres that characterize the upper thermocline circulation. Three different scales of motion are therefore superimposed producing a richness of interaction processes which typify similar interactions in unexplored ocean regions. Both wide and narrow shelves are present separated by steep continental slopes from the deep interiors. Cross-shelf fluxes of physical as well biogeochemical parameters are crucial in determining the properties of the shallow versus deep local ecosystems and their trophic chain. Most importantly, the Mediterranean Sea is a basin of contrasting ecosystems, from the strongly oligotrophic deep interiors to the fully eutrophic northern Adriatic characterized by recurrent, anomalous algal blooms and related anoxia events. This review focuses on the identification of the major unresolved scientific issues and wants also to provide directions for future research which may lead to the formulation of interdisciplinary, collaborative implementation plans to address these issues both theoretically and observationally.

Description: Various current data, collected in the Strait of Otranto during the period 1994–2007, have been analysed with the aim of describing the characteristics of the tidal motions and their contribution to the total flow variance. The principal tidal constituents in the area were the semi-diurnal (M2) and the diurnal (K1), with the latter one predominant. The total flow was, in general, more energetic along the flanks than in the middle of the Strait. Specifically, it was most energetic over the western shelf and in the upper layer along the eastern flank. In spite of the generally low velocities (a few cm s−1) of the principal tidal constituents, the tidal variance has a pattern similar to that of the total flow variance, that is, it was large over the western shelf and low in the middle. The proportion of non-tidal (comprising the inertial and sub-inertial low-frequency bands) to tidal flow variances was quite variable in both time and space. The contribution of the low-frequency motions predominated over the tidal and inertial ones in the eastern portion of the strait during the major part of the year, particularly in the upper and intermediate layers. In the deep, near-bottom, layer the variance was evenly distributed between the low frequency, diurnal and semi-diurnal bands. A prominent exception was observed near the western shelf break during the summer season when the contribution of the tidal signal alone to the total variance reached 77%. This high contribution was mainly due to the intensification of the diurnal signal at that location in the proximity of both the surface and bottom layers (velocities of about 10 cm s−1). Local wind and sea level data were analysed and compared with the flow to find the possible origin of this diurnal intensification. Having excluded the sea-breeze impact on the intensification of the diurnal tidal signal, the most likely cause remains the generation of the topographically trapped internal waves and the diurnal resonance in the tidal response. These waves were sometimes generated by the barotropic tidal signal in the presence of summer stratification. The effect was seen only in the presence of the topographic slope change. This phenomenon may stimulate the diapycnal mixing during the stratified season and enhance ventilation of the near-bottom layers.

Description: Previous studies have demonstrated that the salinity in the Levantine depends on the intensity of the Atlantic Water inflow. Moreover, its spreading eastward or northward in the Ionian is determined by the Ionian circulation pattern, i.e. by the Adriatic–Ionian Bimodal Oscillating System mechanism. The aim of this paper is to relate salinity variations in the core of the Levantine Intermediate Water flowing through the Sicily Channel to the salt content in the Levantine and its possible impact on the Western Mediterranean Transition (i.e. the sudden salinity increase in the bottom layer of the Algero-Provençal sub-basin occurring since 2004). From the historical dataset MEDAR/MEDATLAS in the Levantine and Northern Ionian, we present evidence of decadal occurrences of extreme salinities associated with the varying flow pattern of Atlantic Water over the last 60 yr. Furthermore, we show that the salinity variations in the two sub-basins are out of phase. High-salinity events in the Levantine are a pre-conditioning for the potential occurrence of the Eastern Mediterranean Transient (EMT). However, there is no firm evidence of occurrences of EMT-like phenomenon prior to the one in the early 1990s. Cross-correlation between the salinity time series in the Levantine and in the Sicily Channel suggests that the travel time of the salinity signal is between 16 and 18 yr. From the timing of the Western Mediterranean Transition and the salinity maximum in the Levantine Intermediate Water core in the Sicily Channel we also conclude that the time interval needed for the signal propagating from the Levantine to reach the bottom of the Algero-Provençal sub-basin is about 27 yr.

Description: Previous studies have demonstrated that the salinity in the Levantine basin depends on the intensity of the Atlantic water (AW) inflow. Moreover, its spreading eastward (to the Levantine basin) or northward (to the Ionian Sea) is determined by the Ionian circulation pattern, i.e. by the Adriatic–Ionian Bimodal Oscillating System (BiOS) mechanism. The aim of this paper is to relate salinity variations in the Levantine basin to the salt content variability in the core of the Levantine Intermediate Water (LIW) passing through the Sicily Channel (SC) and its possible impact on the Western Mediterranean Transition – WMT (i.e. the sudden salinity and temperature increase in the deep layer of the Algero-Provençal subbasin occurring since 2004). From the historical data set MEDAR/MEDATLAS in the Levantine and northern Ionian, we present evidence of decadal occurrences of extreme salinities associated with the varying influx of AW over the last 60 yr. Furthermore, we show that the salinity variations in the two subbasins are out of phase. High-salinity episodes in the Levantine are a pre-conditioning for the potential occurrence of the events like the Eastern Mediterranean Transient (EMT). Cross-correlation between the salinity time series in the Levantine basin and in the SC suggests that the travel time of the LIW is between 10 and 13 yr. Comparing the timing of the salinity increase associated with the WMT and the salinity in the LIW core in the SC, we estimate that the total time interval needed for the signal propagating from the Levantine to reach the deep mixed layers of the Algero-Provençal subbasin is about 25 yr. We also showed that the extra salt input from the eastern Mediterranean contribute up to about 60% to the salt content increase in the bottom layer of the western Mediterranean.