filed by Dr. Craig Lee on 10 June 2003
The R/V Knorr sailed from Ancona, Italy on 26 June with a team of scientists from American, Italian, Croatian, and Austrian institutions. In contrast to the wintertime study of response to strong atmospheric forcing, springtime investigations focused on a period when riverine buoyancy flux (the spring freshette) normally dominates the northern and central Adriatic. As before, we utilized near-real time remotely sensed sea surface temperature and ocean color (provided by the Ocean Color Group at NRL Stennis Space Center and the Remote Sensing Group at OGS-Trieste) to direct sampling toward specific fronts and filaments (Figure 1). Specialized meteorological forecasts were provided by Dietmar Thaler (Austrian Military Weather Service) and Dr. Vlasta Tutis (Croatian Meteorological Service). This continuous information stream allowed us to refine sampling strategies and focus effectively on rapidly evolving mesoscale variability.
Forcing during the spring measurement period has been weak to non-existent. Climatologically, we expected to sample during the spring freshette, but recent data (supplied by Nello Russo, Univ. of Ancona, shortly after we sailed) place the discharge strength at approximately 30% of normal (for the spring freshette). Likewise, wind forcing has been extremely weak. We experienced a (weak) Sirocco about one week into the cruise, but for the most part it has been so calm that the sea is like glass. As expected, the entire basin is stratified, with very shallow (0-5 m) mixed layers and significant stratification throughout the water column. The upshot is that, without strong forcing to organize the circulation, we've been sampling weak, small scale flows dominated by internal dynamics. One striking find was the presence of small-scale T-S interleaving features in many parts of the basin.
We began with a couple of sections across the northern basin to shake out the various instruments and to scout the 'Istria front' (Figure 2). Remotely sensed SST provides little information this time of year, as the entire basin is surface capped. Ocean color is a bit better, though the Po plume dominates the variability. The April/May SACLANTCEN/NRL cruise found a subsurface front at the same location where we observed the 'Istria front' last winter, though the feature was considerably weaker. We observed a weak front in this region (Figure 3), probably formed by a filament of cooler water extending from Kvarner Bay.
Similar to wintertime conditions, the Mid-Adriatic Filament (MAF) did not appear in the remote sensing, though previous studies suggested that it might be present. We executed a 24-hour survey over the Jabuka Pit which revealed a distinct westward current (the MAF?) embedded in an energetic, highly variable flow field. The MAF turned westward while still over deep water, well before the bathymetry shoaled. Pierre's drifters and our own data suggest that inertial motions may be significant. With only weak wind forcing this seems a little odd, though even weak winds may be sufficient to accelerate the extremely thin surface layer. Given the potential importance of inertial motions and our concerns that the lateral scales might be quite small, we executed a rapidly repeated survey (~6 hours each with 6 occupations) spanning the MAF. This should allow us to assess MAF dynamics and provide a rough characterization of inertial variability. Within this region the optical distribution was characteristic of a seasonal subsurface chlorophyll maximum at about 75 meters (Figure 4), as well as a deep non-phytoplankton particle and CDOM layer apparently associated with northern Adriatic deep water formed during the winter. We also occupied a section running through the deepest regions of the Jabuka Pit, in an attempt to investigate the paths followed by North Adriatic Deep Water (Figure 6).
Following this, we transited to the Istrian coast, where we sampled the remnants of a filament that had extended from Kvarner Bay. Though there was a clear salinity front accompanied by optical variability (Figure 3), we couldn't avoid comparisons with the dramatic front we sampled here during the previous winter. There were weak increases in optical signals north of the front with both higher absolute absorption and also higher absorption/attenuation ratios indicative of increased phytoplankton abundance. However, throughout the area both attenuation and CDOM fluorescence increased near-bottom indicating a layer of elevated particle and CDOM concentrations.
Over the past few days, we've been working at the mouth of the Po, studying the outflow region during a period of weak discharge. Sampling has included two orthogonally-oriented TriSoarus surveys and an intensive daytime section of CTDs and optical profiles. The attenuation at 650 nm from the first Po survey is shown in Figure 5. Next, we intend to run an extended pattern, executing cross-shelf TriSoarus sections between the Po Delta and the Jabuka Pit. During this period, we will choose an instability somewhere along the Po plume (the best candidate at this time is a pair of filaments extending over the Jabuka Pit) for a final, focused three-day study.