Western Adriatic Current Research Articles

Linking sardine recruitment in coastal areas to ocean currents using surface drifters and HF radar: a case study in the Gulf of Manfredonia, Adriatic Sea

Abstract. Understanding the role of ocean currents in the recruitment of commercially and ecologically important fish is an important step toward developing sustainable resource management guidelines. To this end, we attempt to elucidate the role of surface ocean transport in supplying recruits of European sardine (Sardina pilchardus) to the Gulf of Manfredonia, a known recruitment area in the Adriatic Sea. Sardine early life history stages (ELHSs) were collected during two cruises to provide observational estimates of their age–size relationship and their passive pelagic larval duration (PPLD). We combine these PPLDs with observations of surface ocean currents to test two hypotheses: (1) ELHSs are transported from remote spawning areas (SAs) by ocean currents to the Gulf of Manfredonia; (2) sardines spawn locally and ELHSs are retained by eddies. A historical surface drifter database is used to test hypothesis 1. Hypothesis 2 is tested by estimating residence times in the Gulf of Manfredonia using surface drifters and virtual particles trajectories that were computed from high-resolution observations of surface currents measured by a high-frequency (HF) radar network. Transport to the Gulf of Manfredonia from remote SAs seems more likely than local spawning and retention given a mismatch between observed PPLDs of 30–50 days and relatively short (<10-day) average residence times. The number and strength of connections between the gulf and remote SAs exhibit a strong dependence on PPLD. For PPLDs of 20 days or less, the gulf is connected to SAs on the western Adriatic coast through transport in the Western Adriatic Current (WAC). SAs on the east coast are more important at longer PPLDs. SAs in the northern and central Adriatic exhibit weak connections at all PPLD ranges considered. These results agree with otolith microstructure analysis, suggesting that the arrival of larvae in the gulf is characterized by repeated pulses from remote SAs. This is the first attempt to describe the processes related to Lagrangian connection to, and retention in, the Gulf of Manfredonia that will be complemented in the future using validated numerical ocean models and biophysical models.

The role of forcing agents on biogeochemical variability along the southwestern Adriatic coast: The Gulf of Manfredonia case study

This study investigates how multiple forcing factors such as rivers, surface marine circulation and winds affect hydrology and biogeochemical processes in the Gulf of Manfredonia and the seas around the Gargano peninsula, in the south-western Adriatic Sea. The study adopted an integrated approach, using in situ and remote sensing data, as well as the output of current models. The data reveal variability in the area's hydrography induced by local freshwater sources, the Western Adriatic Current (WAC) flowing from the north along the Italian coast, and the current patterns under different wind regimes. Specifically, exchange with offshore waters in the gulf induces variability in salinity and biogeochemical content, even within the same season, i.e. winter, in our case. This strong dependence on physical and biogeochemical factors makes the Manfredonia-Gargano ecosystem vulnerable to climate change, which could compromise its important role as a nursery area for the Adriatic Sea.

Observed and modeled surface Lagrangian transport between coastal regions in the Adriatic Sea with implications for marine protected areas

Surface drifters and virtual particles are used to investigate transport between seven coastal regions in the central and southern Adriatic Sea to estimate the degree to which these regions function as a network. Alongshore coastal currents and cyclonic gyres are the primary circulation features that connected regions in the Adriatic Sea. The historical drifter observations span 25 years and, thus, provide estimates of transport between regions realized by the mean surface circulation. The virtual particle trajectories and a dedicated drifter experiment show that southeasterly Sirocco winds can drive eastward cross-Adriatic transport from the Italian coast near the Gargano Promontory to the Dalmatian Islands in Croatia. Southeasterly winds disrupt alongshore transport on the west coast. Northwesterly Mistral winds enhanced east-to-west transport and resulted in stronger southeastward coastal currents in the western Adriatic current (WAC) and export to the northern Ionian Sea. The central Italian regions showed strong connections from north to south, likely realized by alongshore transport in the WAC. Alongshore, downstream transport was weaker on the east coast, likely due to the more complex topography introduced by the Dalmatian Islands of Croatia. Cross-Adriatic connection percentages were higher for east-to-west transport. Cross-Adriatic transport, in general, occurred via the cyclonic sub-gyres, with westward (eastward) transport observed in the northern (southern) arms of the central and southern gyres.

Long-term variability and trends of relative geostrophic currents in the middle Adriatic

The paper documents dynamics of the middle Adriatic region, and is based on computation of relative geostrophic currents from temperature and salinity data taken along the Palagruža Sill transect between 1957 and 2010. The strongest westward current is found in the central part of the sill, corresponding to the offshore deflected Eastern Adriatic Current (EAC). The Western Adriatic Current (WAC) is, as expected, found in the southern part of the measured transect. The WAC is most significant during the summer season and exhibits stronger variability than the EAC due to the frequent growth of eddies within it. Long-term trends in both the central and southern parts of the sill oppose mean currents, indicating the weakening of the EAC and WAC and therefore of the entire Adriatic thermohaline cell. These opposing trends have not been reproduced by a climate model.

Provenance of surface sediments along the southeastern Adriatic coast off Italy: An overview

Multi-proxy studies are necessary to understand sediment composition and related provenance on continental shelfs. Here it is shown that the spatial distribution of geochemical composition and grain size for surface sediments along the southeastern Italian coast is related to provenance and mechanisms influencing sediment pathways. A northern Adriatic/Italian provenance can be distinguished from a southern Apennine river source. This is done independent of grain size using the element ratios Ce/Ni and Zr/Cr. Furthermore, the origin of organic matter is determined using bulk carbon isotopes and the C/N ratio. Integrating these results with those from complementary studies on δ18O and δ13C of Globigerinoides ruber (white), the BIT index, stable isotopes of plant waxes and dinoflagellate cyst distribution from the same set of samples reveals that:•Sediments from the northwestern Adriatic are transported as far southward as the Gallipoli shelf (eastern Gulf of Taranto) by the Western Adriatic Current (WAC)•Along the WAC, there is a consistent southward decrease in Po river/northern Apennines provenance and a concomitant decrease in terrestrial (soil) organic matter (OM), whereas the percentage of marine OM increases.•The provenance for Gallipoli shelf sediments is for ∼80% attributed to Po River/northern Apennines sources and for ∼20% to southern Italian sources.•OM in the eastern Gulf of Taranto contains more marine OM than other areas within the WAC, whereas OM and sediments from the western part of the Gulf of Taranto have a more local, riverine provenance.

Climate of the past 2500 years in the Gulf of Taranto, central Mediterranean Sea: A high-resolution climate reconstruction based on δ18O and δ13C of Globigerinoides ruber (white)

We present a high-resolution isotope stratigraphy based on Globigerinoides ruber (white) over the past 2500 years in the Gulf of Taranto, central Mediterranean. G. ruber (white) reflects summer conditions in the Gulf of Taranto but is influenced by two major surface water masses: the Western Adriatic Current (WAC) and the Ionian Surface Water (ISW) and their variations on a decadal to multicentennial scale. Our analysis of the δ13C and δ18O of G. ruber (white) allows the distinction of several climatic periods: the ‘Roman Warm Period’ (RWP) (450–0 bc), with relatively wet and warm conditions and a higher influence of the WAC; the ‘Roman Classical Period’ (RCP) (ad 1–200) characterized by salinity increase resulting from circulation changes; the ‘Dark Ages Cold Period’ (DCP) (ad 500–750), where wetter conditions in the Gulf of Taranto region are coherent with an increase dominance of the WAC; the ‘Medieval Warm Period’ (MWP), with wet and warm conditions in the first, and a gradual drying in the second half; and finally, the transition from the MWP to the ‘Little Ice Age’ (LIA), which is characterized by continuing dry conditions.

What do SST proxies really tell us? A high-resolution multiproxy (UK′37, TEXH86 and foraminifera δ18O) study in the Gulf of Taranto, central Mediterranean Sea

We present a multiproxy reconstruction of sea surface temperatures (SST) and coastal environmental changes covering the last 600 years on sediments from the Gulf of Taranto, central Mediterranean Sea. The reconstruction is based on UK′37 (alkenones from haptophytes), TEXH86 (membrane lipids of marine crenarchaeota), and δ18O and δ13C of Globigerinoides ruber (white and pink) and of Uvigerina mediterranea. The amplitudes of the temperature signals reconstructed from δ18O of G. ruber (white), TEXH86 and UK′37 exceed the amplitudes observed in other reconstructions of local and Northern Hemisphere temperature. UK′37-based SSTs reflect mainly winter/spring conditions with an additional influence of changing nutrient supplies related to water column mixing and runoff. TEXH86-based temperatures reflect SSTs of the oligotrophic summer season, while influences from near-coastal areas may complicate its interpretation. Co-variation between both lipid-based SST records suggests a common environmental mechanism during the last 600 years. δ18O of G. ruber (white) also reflects summer conditions and is amplified by changes in salinity and nutrient availability, which are caused by variations in the relative influence of the Western Adriatic Current (WAC) and of the Ionian Surface Waters (ISW). The combination of SSTTEXH86 and δ18O of G. ruber (white) shows that the circulation in the Gulf of Taranto underwent significant changes during the last 600 years.

Flux of nutrients between the middle and southern Adriatic Sea (Gargano-Split section)

The transect Gargano-Split at the borderline between the middle and southern Adriatic on the Palagruža Sill is exposed to the influences both from the northern and southern Adriatic and is a main transition point for the Adriatic. Thus, water and dissolved inorganic nutrients fluxes between the northern shelf area and the southern deep sea can be estimated at this point. During the Dynamic of the Adriatic in Real Time (DART) project, 12 bottom-mounted Acoustic Doppler Current Profilers (ADCPs) were deployed from November 2005 to September 2006 along the Gargano-Split section. During the same period four different oceanographic cruises were carried out (in November 2005, March, April and July 2006). Hydrographic stations were sampled with the CTD/Rosette to measure physical (temperature, salinity and density) and chemical (oxygen, orthosilicate, orthophosphate and nitrite+nitrate) parameters. Measurements of current velocities allowed estimation of water fluxes and the net nutrient transports were estimated from these and nutrient concentrations. The orthosilicate flux across the transect for inflow and for outflow was higher (except for March) than the nitrite+nitrate and the orthophosphate fluxes. The outflowing nutrient fluxes in the Western Adriatic Current (WAC) were much lower than the total nutrient flux outflow as a large portion of the nutrient flux towards the southern Adriatic was carried by other outflowing waters including North Adriatic Dense Water (NAdDW) flowing along the Italian bottom slope and Middle Adriatic Dense Water (MAdDW) flowing through the deepest passage in the center of the Sill. The July survey provided the first opportunity for direct measurements of total nutrient fluxes across a full basin transect of the central Adriatic. The measurements suggest that the nutrient import and nutrient export were roughly in balance. Modified Levantine Intermediate Water (MLIW) significantly contributed to a total nutrient influx across the transect that was strong enough to counter-balance the nutrient export by the WAC, NAdDW, MAdDW, and other sources.

Comparison of model‐simulated and observed currents in the central Adriatic during DART

Numerical simulations of the Adriatic Sea were performed with the Navy Coastal Ocean Model (NCOM) during the Dynamics of the Adriatic in Real Time (DART) Experiments conducted between October 2005 and September 2006. Grid resolution was 1 km. Model forcing included tides, surface fluxes from the Aire Limitée Adaptation Dynamique Développement International (ALADIN) atmospheric model, relaxation to a daily satellite sea surface temperature analysis, extensive river and runoff discharges, and open boundary conditions south of Otranto specified from a global model. Currents predicted by the model were compared with currents from 12 Acoustic Doppler Current Profiler (ADCP) moorings located along a line between the Gargano Peninsula, Italy, and Split, Croatia. The nontidal comparisons were performed with detided currents. Correlations between the model and ADCP currents were highest in the Western Adriatic Current (WAC), which flows southeastward along the Italian coast. Lowest correlations were in the interior of the Adriatic, likely because of instability processes. Correlations between the ALADIN winds and the model and ADCP currents at the mooring locations were also highest in the WAC. For November 2005 through August 2006, the model and ADCP mean WAC transports were 0.321 and 0.304 Sv, respectively, with a temporal correlation of 0.79. Comparison of current variance showed best agreement near the Italian and Croatian coasts. In the interior, the NCOM current variance compared fairly well with that of the ADCPs for November–January but decreased significantly relative to the ADCPs for February–August. Spectral analyses indicate most of the difference in variance to be at periods exceeding 2 days.

Relative dispersion from a high-resolution coastal model of the Adriatic Sea

Synthetic drifter trajectories computed from velocity data produced by a high-resolution NCOM model are used to investigate the scaling of relative dispersion and the distribution of finite-scale Lyapunov exponent (FSLE) fields in the Adriatic Sea. The effects of varying degrees of spatial and temporal filtering of the input Eulerian velocity fields on the Lagrangian statistics are investigated in order to assess the sensitivity of such statistics to model error. It is shown that the relative dispersion in the model Adriatic circulation is generally super-diffusive, scaling nearly ballistically in close agreement with Lagrangian observations from a limited set of drifters. The large-scale dispersion is dominated by persistent separation regions and the controlling influence of the Western Adriatic Current (WAC). Temporal filtering with averaging windows up to monthly time scales only affects the relative dispersion at scales smaller than 20 km without altering the overall scaling regime. In contrast, spatial smoothing at scales as small as 5 km significantly reduces relative dispersion at all scales up to 100 km. While basin-scale dispersion statistics are strongly dependent on spatial resolution of the model WAC, maps of FSLE fields over initial conditions indicate that the detailed geometry of the dispersion is determined to a large extent by the temporal resolution of the model. In addition, the degree of spatial heterogeneity in the flow field implies that the existence, or non-existence, of a distinct exponential regime in the FSLE at small scales is extremely sensitive to the details of particle pair sampling strategies.

Measurements of storm and nonstorm circulation in the northern Adriatic: October 2002 Through April 2003

Fifteen bottom‐mounted Acoustic Doppler Current Profilers were deployed from October 2002 through April 2003 in the northern Adriatic Sea. Average transport from the portion of the Western Adriatic Current (WAC) along the Italian slope was 0.1470 ± 0.0043 Sv, punctuated by bursts of more than twice that amount during storm events. Monthly means were calculated with times of strong wind‐driven circulation excluded. These suggest a 2002/2003 seasonal separation consisting of October, December through February, and March through April. An extreme Po River flood influenced November conditions making seasonal categorization difficult. October generally had more kinetic energy and more vertical structure than other months, and near‐inertial waves were more frequent in April and October. The Eastern Adriatic Current (EAC)/WAC (i.e. inflow/outflow) system was clearly present in the means for all months. The cyclonic gyre north of the Po River was present October through February. Generally, in the WAC, over 50% of kinetic energy came from vertically uniform monthly mean flows. Elsewhere, eddy kinetic energy was stronger than mean kinetic energy with 10–40% contributions for vertically uniform monthly mean flows, 40–60% for vertically uniform monthly varying flows, and 10–30% for vertically varying monthly varying flows. Mean currents for bora storms indicate enhancement of the EAC/WAC and the cyclonic northern gyre, a shift toward Kvarner Bay in EAC direction, a circulation null point south of the Po, and double‐gyre bifurcation of flow at Istria. Strengthening of both the EAC and WAC also occurs during sirocco storms.

Orographic influences on the Adriatic sirocco wind

Abstract. Differences between meteorological fields over the Adriatic Sea as predicted by the mesoscale meteorological model ALADIN/HR and global ECMWF model, for sirocco episodes between November 2002 and September 2003, are analysed. Results indicate that the orography, namely Gargano Mountains and Apennines, may have a significant effect on the sirocco airflow. A brief discussion is given on the impact of the sirocco wind curl on the Western Adriatic Current (WAC).

Surface drifter derived circulation in the northern and middle Adriatic Sea: Response to wind regime and season

More than 120 satellite‐tracked drifters were deployed in the northern and middle Adriatic (NMA) Sea between September 2002 and November 2003, with the purpose of studying the surface circulation at mesoscale to seasonal scale in relation to wind forcing, river runoff, and bottom topography. Pseudo‐Eulerian and Lagrangian statistics were calculated from the low‐pass‐filtered drifter velocity data between September 2002 and December 2003. The structure of the mean circulation is determined with unprecedented high horizontal resolution by the new data. In particular, mean currents, velocity variance, and kinetic energy levels are shown to be maximal in the Western Adriatic Current (WAC). Separating data into seasons, we found that the mean kinetic energy is maximal in fall, with high values also in winter, while it is significantly weaker in summer. High‐resolution Local Area Model Italy winds were used to relate the drifter velocities to the wind fields. The surface currents appear to be significantly influenced by the winds. The mean flow during the northeasterly bora regime shows an intensification of the across‐basin recirculating currents. In addition, the WAC is strongly intensified both in intensity and in its offshore lateral extension. In the southeasterly sirocco regime, northward flow without recirculation dominates in the eastern half of the basin, while during northwesterly maestro the WAC is enhanced. Separating the data into low and high Po River discharge rates for low‐wind conditions shows that the WAC and the velocity fluctuations in front of the Po delta are stronger for high Po River runoff. Lagrangian covariance, diffusivity, and integral time and space scales are larger in the along‐basin direction and are maximal in the southern portion of the WAC.

Circulation and horizontal fluxes in the northern Adriatic Sea in the period June 1999–July 2002. Part II: Nutrients transport

Nutrient concentrations (orthophosphate, orthosilicate and dissolved inorganic nitrogen—DIN) were measured on three transects in the northern Adriatic approximately monthly in the period June 1999–July 2002. The net nutrient transports across the three transects were estimated from these concentration values and calculated water fluxes [Grilli F, Paschini E, Russo A, Precali R, Supić N. Circulation and horizontal fluxes in the Northern Adriatic Sea in the period June 1999_July 2002. Part I: geostrophic circulation and current measurement. Sci Total Environ 2005—this issue], with the aim to verify their possible role in the mucilage phenomenon. The nutrient transports in the northern Adriatic were very variable, seasonally and from year to year, both in intensity and direction. Some differences were noticed also among the nutrient species. At the northernmost transect Po Delta–Rovinj minimal values and variable directions were recorded, probably due to the prevailing of eastward transversal transports, observed already in late winter, and generally dominant in spring and summer. Northward transport was often measured at the central transect Cesenatico–Cape Kamenjak, particularly in spring 2000 and 2002, and summer 2001, as well as for only DIN in summer 1999. In contrast, southward directions prevailed at the southernmost transect Senigallia–Susak Island. In October significant southward transports occurred at all transects and for all nutrients, particularly strong in 2000, when exceptionally high Po River discharges occurred, and the Western Adriatic Current (WAC) was well developed. However, the nutrient transports in the upper water column were low or northwards at both transect Cesenatico–Cape Kamenjak and Senigallia–Susak Island in June and July 2000, 2001, and 2002, when the mucilage phenomenon developed, but were opposite in 1999, a year without events. The results suggest that high variations of nutrient fluxes and their ratios, in conditions of reduced water dynamics, are essential for the development of the phenomenon, rather than the absolute amounts of the nutrient inputs.