Scientific cruise report Elisabeth Mann-Borgese SUMMIX-MESO

Objectives: It was intended to investigate the meso-scale and sub-meso-scale dynamics of the upper layers (upper 80 m) in the central Baltic Sea, using towed instruments and acoustic profilers, to better understand the physical conditions for cyanobacteria blooms. Under optimal weather conditions, we intended to carry out 10 one-day quasi-synoptic surveys by cruising in large meandering patterns (see fig. 1) covering areas of 15 X 15 nautical miles or 8 X 8 nautical miles, depending on the survey mode, see below. This cruise was the meso-scale component of the two-ship SUMMIX experiment together with RV Meteor (Physical and biochemical exchange-, mixing- and transformation processes in the central Baltic Sea during summer stratification and their controls on the cyanobacterial summer bloom) which was intended to be located at a fixed position nearby RV Elisabeth Mann Borgese in order to survey the water column in high vertical, spatial and parameter resolution, including biogeochemical experiments on board. In addition to the physical parameters, also vertical and horizontal zooplankton net tows as well as water samples taken by CTD bottles were planned

Hydrography and circulation west of Sardinia in June 2014

In the frame of the REP14-MED sea trial in June 2014, the hydrography and circulation west of Sardinia, observed by means of gliders, shipborne CTD (conductivity, temperature, depth) instruments, towed devices, and vessel-mounted ADCPs (acoustic doppler current profilers), are presented and compared with previous knowledge. So far, the circulation is not well-known in this area, and the hydrography is subject to long-term changes. Potential temperature, salinity, and potential density ranges as well as core values of the observed water masses were determined. Modified Atlantic Water (MAW), with potential density anomalies below 28.72 kg m−3, showed a salinity minimum of 37.93 at 50 dbar. Levantine Intermediate Water (LIW), with a salinity maximum of about 38.70 at 400 dbar, was observed within a range of 28.72<σΘ/(kg m−3) < 29.10. MAW and LIW showed slightly higher salinities than previous investigations. During the trial, LIW covered the whole area from the Sardinian shelf to 7°15′ E. Only north of 40° N was it tied to the continental slope. Within the MAW, a cold and saline anticyclonic eddy was observed in the southern trial area. The strongest variability in temperature and salinity appeared around this eddy, and in the southwestern part of the domain, where unusually low saline surface water entered the area towards the end of the experiment. An anticyclonic eddy of Winter Intermediate Water was recorded moving northward at 0.014 m s−1. Geostrophic currents and water mass transports calculated across zonal and meridional transects showed a good agreement with vessel-mounted ADCP measurements. Within the MAW, northward currents were observed over the shelf and offshore, while a southward transport of about 1.5 Sv occurred over the slope. A net northward transport of 0.38 Sv across the southern transect decreased to zero in the north. Within the LIW, northward transports of 0.6 Sv across the southern transects were mainly observed offshore, and decreased to 0.3 Sv in the north where they were primarily located over the slope. This presentation of the REP14-MED observations helps to further understand the long-term evolution of hydrography and circulation in the Western Mediterranean, where considerable changes occurred after the Eastern Mediterranean Transient and the Western Mediterranean Transition

High-resolution observations in the Western Mediterranean Sea: The REP14-MED experiment

The observational part of the REP14-MED experiment was conducted in June 2014 in the Sardo-Balearic Sea west of Sardinia Island (Western Mediterranean Sea). Two research vessels collected high-resolution oceanographic data by means of hydrographic casts, towed systems, and underway measurements. In addition, a vast amount of data was provided by a fleet of 11 gliders, time series were available from moored instruments, and information on Lagrangian flow patterns were obtained from surface drifters and one profiling float. The spatial resolution of the observations encompasses a spectrum over four orders of magnitude from O(101 m) to O(105 m), and the time series from the moored instruments cover a spectral range of five orders from O(101 s) to O(106 s). The objective of this article is to provide an overview of the huge data set which is utilized by various ongoing studies, focusing on (i) sub-mesoscale and mesoscale pattern analyses, (ii) operational forecasting in terms of the development and assessment of sampling strategies, assimilation methods, and model validation, (iii) modeling the variability of the ocean, and (iv) testing of new payloads for gliders

Energy conversion in the Cape Verde Frontal Zone

Mechanical energy terms are calculated from moored current meter data in the Cape Verde Frontal Zone (about 20°N, 25°W) and compared with those derived from a mesoscale model of this frontal region. The model is of the Bleck and Boudra [1981] type with isopycnal coordinates. An initially zonal jet, representing the Canary Current, is allowed to develop under the influence of baroclinic and barotropic instability processes. We find reasonable agreement in magnitudes, somewhat smaller in the model, and similar distributions in the vertical. This leads to the conclusion that the energy transfer terms from the model can be expected to be sufficiently close to reality. Determination of the transfer terms confirms that the energy transfer in the zone is dominated by baroclinic instability processes while barotropic instability is of minor importance. Average baroclinic instability energy transfer terms reach values of 2–3 μW m−3 in the pycnocline. Peak layer mean values are of the order 10 μW m−3. It is shown that the spatial distribution of active transfer regions is closely related to the structure of the transient eddy field in the frontal zone and that strong instability processes are restricted to the pycnocline

High-resolution observations in the western Mediterranean Sea: the REP14-MED experiment

The observational part of the REP14-MED experiment was conducted in June 2014 in the Sardo-Balearic Basin west of Sardinia (western Mediterranean Sea). Two research vessels collected high-resolution oceanographic data by means of hydrographic casts, towed systems, and underway measurements. In addition, a vast amount of data was provided by a fleet of 11 ocean gliders, time series were available from moored instruments, and information on Lagrangian flow patterns was obtained from surface drifters and one profiling float. The spatial resolution of the observations encompasses a spectrum over 4 orders of magnitude from (10¹ m) to (10⁵ m), and the time series from the moored instruments cover a spectral range of 5 orders from (10¹ s) to (10⁶ s). The objective of this article is to provide an overview of the huge data set which has been utilised by various studies, focusing on (i) water masses and circulation, (ii) operational forecasting, (iii) data assimilation, (iv) variability of the ocean, and (v) new payloads for gliders

Can the Weak Surface Currents of the Cape Verde Frontal Zone Be Measured With Altimetry?

Three data types are compared in the low-current-velocity regime in the southeastern North Atlantic, between 12-degrees-N and 30-degrees-N, 29-degrees-W and 18-degrees-W: Geosat altimetric sea level and derived surface geostrophic velocities, shallow current meter velocities, and dynamic heights derived from hydrographic data from cruises 4, 6, and 9 of the research vessel Meteor. The four current meter daily time series, at depths around 200 m, were smoothed over 1 month; the altimetric geostrophic velocities were computed from sea surface slopes over 142 km every 17 days. The correlation coefficients between the current meter and altimetric geostrophic velocities range between 0.64 and 0.90 for the moorings near 29-degrees-N but between 0.32 and 0.71 for the two around 21-degrees-N; the associated rms discrepancies between the two measurement types range between 1.5 and 4.4 cm/s, which is 49% to 127% of the rms of the respective current meter time series. Dynamic heights relative to 1950 dbar for the months of November 1986 (d(M4)), November 1987 (d(M6)), and February 1989 (d(M9)) were computed from Meteor cruises 4, 6, and 9. Both dynamic heights and altimetric heights (h(M4), h(M6), h(M9)) were averaged over 1-degrees boxes for the duration of each cruise. Differences d(M4) - d(M6) and d(M9) - d(M6) were computed only at bins where at least one station from both cruises existed, Assuming that dynamic heights d in dynamic centimeters are equivalent to sea level h in centimeters, the standard deviation sigma of the differences ((h(M4) - h(M6)) - (d(M4) - d(M6))) and corresponding M9 - M6 values was 2.1 cm. This value (squared) is only 13% of the (5.8 cm)2 variance of the dynamic height differences and is indistinguishable from the 2.7- to 5.6-cm natural variability of sea level in the area expected between the times when the ship and the satellite sampled the ocean. The areally averaged discrepancy for M9 - M6 was only 0.7 cm, but the corresponding value for M4 - M6 was 5.2 cm. A systematic difference between the water vapor corrections used before and after July 1987 is responsible for the M4 - M6 difference. The average M4 - M6 discrepancy is only 0.1 cm using the Fleet Numerical Oceanography Center correction, with a standard deviation of 3.1 cm. In spite of the underlying differences in sampling and physics, including unknown barotropic components not included in our hydrographic dynamic heights, and in data errors, including water vapor, ionospheric, and orbital effects on the altimetry, consistent interannual changes of the mean sea level from the independently obtained altimetric and hydrographic data sets are obtained, and correlated seasonal changes in surface currents are observed with both altimetry and current meters

A view of the Canary Basin thermocline circulation in winter

During January and February 1989 the recirculation of the subtropical gyre in the eastern North Atlantic was surveyed with a three-ship experiment. The analysis of hydrographic measurements and velocity data from a shipboard acoustic Doppler current profiler reveals the synoptic-scale circulation patterns and water mass distributions in the Canary Basin. The geostrophic transport stream function estimated with a horizontally varying reference level of no motion highlights the major currents in three layers representing the vertical structure of the horizontal circulation. The classical circulation scheme is shown by the stream function in the upper 200 m: the Azores, Canary, and North Equatorial currents. Unlike the deep-penetrating Azores Current, the Canary Current and the North Equatorial Current are restricted to the upper 200 m. Both carry North Atlantic Central Water along the water mass boundary with South Atlantic Central Water. South Atlantic Central Water flows through the passage between the Cape Verde archipelago and Africa via narrow currents into the area north of 14.5°N. At the southern edge of the subtropical gyre we identify an eastward flow of Antarctic Intermediate Water between 700 and 1200 m