Pierre Flament

Northern Adriatic Response to a Wintertime Bora Wind Event

During winters, the northern Adriatic Sea experiences frequent, intense cold-air outbreaks that drive oceanic heat loss and imprint complex but predictable patterns in the underlying waters. This strong, reliable forcing makes this region an excellent laboratory for observational and numerical investigations of air-sea interaction, sediment and biological transport, and mesoscale wind-driven flow. Narrow sea surface wind jets, commonly known as “bora,” occur when cold, dry air spills through gaps in the Dinaric Alps (the mountain range situated along the Adriatics eastern shore). Horizontal variations in these winds drive a mosaic of oceanic cyclonic and anticyclonic cells that draw coastal waters far into the middle basin. The winds also drive intense cooling and overturning, producing a sharp front between dense, vertically homogenous waters (North Adriatic Dense Water, or NAdDW) in the north and the lighter (colder, fresher), stratified waters of the Po River plume. Once subducted at the front, the NAdDW flows southward in a narrow vein following the isobaths (contours of constant depth) of the Italian coast. In addition to governing the basins general circulation, these processes also influence sediment transport and modulate biological and optical variability

Current measurements by SAR along-track interferometry from a Space Shuttle

We present one of the first studies on ocean current retrievals from interferometric synthetic aperture radar (InSAR) data acquired during the Shuttle Radar Topography Mission (SRTM) in February 2000. The InSAR system of SRTM was designed for high-resolution topographic mapping of the Earths land surfaces, using two SAR antennas on a Space Shuttle with a cross-track separation of 60 m. An additional along-track antenna separation of 7 m resulted in an effective time lag of about 0.5 ms between the two images, which could theoretically be exploited for target velocity retrievals. However, the feasibility of ocean current measurements with SRTM has been questionable, since the time lag was much shorter than the theoretical optimum (about 3 ms at X-band) and the signal-to-noise ratio over water was quite low. Nevertheless, some X-band InSAR images of coastal areas exhibit clear signatures of tidal flow patterns. As an example, we discuss an image of the Dutch Wadden Sea. We convert the InSAR data into a line-of-sight current field, which is then compared with results of the numerical circulation model KUSTWAD. For tidal phases close to the conditions at the time of the SRTM overpass; we obtain correlation coefficients of up to 0.6 and rms differences on the order of 0.2 m/s. Furthermore we find that SRTM resolves current variations down to spatial scales on the order of 1 km. This is consistent with predictions of a numerical InSAR imaging model. Remaining differences between SRTM- and KUSTWAD-derived currents can be attributed mainly to residual motion errors in the SRTM data as well as to a limited representation of the conditions at the time of the SRTM overpass in the available KUSTWAD results.

A meeting place of great ocean currents: shipboard observations of a convergent front at 2°N in the Pacific

Abstract We present a synthesis of physical, chemical and biological shipboard observations of a convergent front at 2°N, 140°W and its surrounding environment. The front was a component of a tropical instability wave generated by shear between westward-flowing equatorial waters to the south and warmer equatorial counter current water to the north. Surface waters on the cold side were undersaturated with oxygen, which suggests that the water had only been exposed at the sea surface for a period of a few weeks. Although the atmospheric exposure time was short, the effects of biological activity could be detected in enhanced concentrations of total (dissolved plus suspended particulate) organic carbon concentration, proving that TOC can be produced in the top centimeters of the changing environmental conditions. The front itself was dominated by the accumulation of a “patch” of buoyant diatoms Rhizosolenia castracanei concentrated in the top centimeters of the warm surface water north of the front, and elevated chlorophyll concentrations were observed from the air over a spatial scale of order 10–20 km northward from the front. The nitrogen budget and thorium data suggest that a significant fraction of the elevated POC, and virtually all of the PON, arrived in the patch waters as imported particles rather than in situ photosynthesis. Photosynthetic uptake of carbon appears to have occurred in patch waters, but without corresponding uptake of fixed nitrogen (an uncoupling of the usual Redfield stoichiometry). Solute chemistry of the patch appears to be controlled by turbulent mixing, which flushes out patch waters on a time scale of days

Lagrangian statistics in the central North Pacific

Abstract Lagrangian integral scales, diffusivities, dispersion and velocity spectra are calculated using surface drifter trajectories in the central North Pacific. The meridional integral time scale is relatively homogeneous throughout the region; a large increase in the zonal time and length scales south of Hawaii is attributed to meanders in the North Equatorial Current. Except in this current, the initial dispersion is consistent with Taylors Theorem. For lags of 20–120 days, the meridional dispersion can be modeled by a constant eddy diffusivity. Shear in the mean zonal currents magnifies the zonal dispersion at long lags. In the Lagrangian spectra, the energetic eddy band is at 3–20 days west of Hawaii, 10–40 days east and north of Hawaii, and 20–60 days in the North Equatorial Current. In the wake of Hawaii, energetic lee vortices produce sharp peaks in the cyclonic and anticyclonic rotary spectra.

The Surface Expression of Semidiurnal Internal Tides near a Strong Source at Hawaii. Part I: Observations and Numerical Predictions*

Abstract Observations of semidiurnal currents from high-frequency radio Doppler current meters and moored acoustic Doppler current profilers (ADCPs) in the Kauai Channel, Hawaii, are described and compared with two primitive equation numerical models of the tides. The Kauai Channel, separating the islands of Oahu and Kauai, is a site of strong internal tide generation by the barotropic tides flowing over Kaena Ridge, the subsurface extension of Oahu. The nature and impacts of internal tide generation in the Kauai Channel were intensively studied during the 2002–03 near-field component of the Hawaii Ocean Mixing Experiment. Comparisons of observed coherent (i.e., phase locked to the astronomical forcing) M2 and S2 surface currents with model predictions show good agreement for the phases, indicating propagation of internal tides away from the ridge. Although the predicted M2 and S2 surface currents are similar (except for their magnitudes), as expected for internal waves at periods closer to each other (12...

Evidence of inertially generated coastal‐trapped waves in the eastern tropical Pacific

Observations of coastal-trapped waves (CTW) are limited by instrumentation technologies and temporal and spatial resolutions; hence, their complete description is still limited. In the present work, we used measurements from high-frequency radio scatterometers (HFR) to analyze the subinertial dynamics of the Gulf of Tehuantepec in the Mexican Pacific, a region strongly influenced by offshore gap winds. The data showed subinertial oscillations that may be explained by poleward propagating CTWs. The oscillations showed higher coherence (95% confidence) with gap winds in the Gulfs of Papagayo and Panama than with local winds. Vertical thermocline oscillations, measured with a moored thermistor-chain, also showed subinertial oscillations coherent with Papagayo and Panama winds. The period of the observed oscillations was ∼4 days, which corresponds to the inertial period of the Gulf of Panama. This suggests that inertial oscillations generated by offshore wind outbursts over Panama may have traveled northward along the coastal shelf, and were detected as surface current pulses by the HFR installed approximately 2000 km further north in the Gulf of Tehuantepec. To further explore the presence of CTWs, the 4 day band-pass filtered currents measured by the HFR were analyzed using empirical orthogonal functions. We found that the first mode behaved like a CTW confined to the shelf break. Additionally, the observed oscillations were compared with baroclinic and barotropic CTW models. The results support the notion that nearly inertial baroclinic CTWs are generated in the Gulfs of Panama and Papagayo and then propagate toward the Gulf of Tehuantepec.

Towards an operational spaceborne system for high-resolution current measurements in coastal areas

Along-track interferometric synthetic aperture radar (along-track InSAR) is a new technology for imaging surface current fields from airborne or spaceborne platforms with accuracies of 0.1 m/s or better, spatial resolutions on the order of 10 to 1000 m, and swath widths of up to more than 100 km, depending on platform and instrument parameters. This is particularly attractive for the mapping and monitoring of current fields in coastal areas. The SRTM experiment on a Space Shuttle in early 2000 offered a first chance to demonstrate current measurements by InSAR from space. Although the SRTM configuration was not well suited for current measurements and the coverage of the ocean was very limited, some images of coastal scenes exhibit clear signatures of typical surface current patterns, which have been found to be in good agreement with theoretical predictions and to resolve current variations on spatial scales of about 1 km. The German satellite TerraSAR-X, which will be launched in 2005, will offer similar current measuring capabilities. Concepts for more specific, further optimized InSAR missions for oceanic applications are currently under investigation. We give an overview of these developments.

Aghullas ring trajectories and evolution from altimeter data

Oceanic vortices have been the subject of several experimental and theoretical studies. They play a key role in the energy budget of the global ocean. A better understanding of their physics, (e.g. interaction with the mean currents) would improved their knowledge of the ocean circulation. Satellite altimetry and numerical models are used to detect, track and analyze the strong anticyclonic eddies generated by the Aghullas retroflection. These rings have a long lifetime and can cross the South Atlantic basin. They could play a key role in the exchanges between the Indian and South Atlantic oceans. The LEGI QG model is a high resolution (1/6/spl deg/) eddy resolving model. By means of a simple nudging data assimilation procedure along track altimeter data are introduced into the model to control the simulation. Using a Gaussian eddy model and the smooth variation of eddy propagation speed, a method has been developed, based on the analysis of along track altimeter data in terms of eddy characteristics (amplitude, size) to estimate the eddy trajectory. This trajectory is then used as a frame of reference to estimate the bidimensional eddy structure. Three Aghullas rings, detected during a WOCE campaign are studied. Their trajectories, determined from altimetry, is compared to the numerical model ones. Their structure and their evolution are analyzed and compared to model and in situ data. The influence of bottom topography, mean currents, eddy/eddy interaction is analyzed.

CDs with satellite images available

Images from the GMS-4 geostationary satellite operated by Japan are now available on a pair of CD-ROMs in the form of compressed PostScript files. At ~140°E, the GMS-4 is optimally placed to observe the western Pacific. Its Visible and Infrared Spin Scan Radiometer scans the Earth hourly at 1.25-km resolution in the visible and 5.0-km in the thermal infrared. The telemetry from the GMS-4 was captured in Hawaii during the TOGA Coupled Ocean-Atmosphere Experiment from November 1992 to March 1993. The CD images will be useful in finding relationships between events in a survey or a time series and the general weather patterns in the vicinity of sampling platforms. Quantities that can be estimated include reflected short wave and outgoing long wave radiations, tropospheric winds, surface roughness, and rainfall. Since the images have been remapped to a common geographic grid, temporal statistics should be especially easy to construct.