Current boundaries in Norway (I)

The NOAA AVHRR image (left) and the ERS-1 SAR image (right) are obtained six hours apart on 3 October 1992, and cover a region of approximately 100x300 km off the west coast of Norway. The radar and radiometer manifestations of surface roughness and sea surface temperature are compared. The spatial resolution of the AVHRR image is 1 km compared with 100 m in the SAR image.

AVHRR images are frequently used in studies of water-mass distribution and ocean circulation, and several examples have demonstrated the capabilities in multisensor AVHRR and radar altimeter (measures sea surface slopes) studies of mesoscale ocean circulation. SAR images, on the other hand, have not until recently been systematically used in studies of mesoscale current features.

The multisensor IR-SAR comparison clearly favours the SAR imaging capabilities. There appears to be a remarkable relationship between the sea surface temperature and roughness field. In the IR image the surface temperature decreases from 14ºC (dark blue) in the Atlantic water offshore, implying the existence of typical weak temperature contrast at this time of the year. The maximum temperature gradient is about 0.75ºC/km.
Across the temperature fronts, corresponding salinity and hence density fronts are present which maintain a change in the current velocity. The structure of the sea surface temperature field with the curved paths of the temperature fronts, represents the mesoscale meandering pattern of the unstable Norwegian Coastal Current (NCC) with typical scales of about 50 km. Smaller, 10 km sized features are also observed.
The coastal islands and main land are grey, while clouds are masked as black in the lower part of the image.

The ERS-1 SAR image expresses frontal features with configuration and orientation in good agreement with those seen in the IR image, both at the 50 km scale and at smaller scales of about 10 km. The coastal landscape is seen along the right-hand boundary of the image with backscatter slightly different from the sea.

The SAR frontal boundaries are recognized with both dark and bright radar cross-section of varying cross frontal width, in particular the bright front in the central part of the image. Spatial variations of the radar cross-section are usually induced by larger scale features and processes such as long gravity waves, variable near surface wind field, air-sea temperature differences, and upper ocean circulation features including mesoscale meandering fronts and eddies.
Under moderate winds between 3 and 10 m/s it is shown that the SAR can manifest current boundaries including meanders and eddies. The corresponding imaging mechanisms are proposed to alternate between
(1) damping of short gravity waves by the presence of natural slicks aligned along the frontal boundary;
(2) short gravity wave-current interaction along shear and/or convergence zones within the front;
(3) changes in wind stress induced by strong gradients in the sea surface temperature; and
(4) long gravity-wave current refraction.
In order to rank these mechanisms, auxiliary data are useful.

Northerly winds of about 5 m/s and air temperature from 12 to 14ºC were reported along the coast from the analysed weather map. Furthermore, a northward near surface current of about 0.30 m/s, a temperature at 50 m depth of 14ºC, and a significant wave height of about 1 m were reported from one Seawatch buoy deployed about 20 km offshore (see the point in interpreted image.

Since the temperature fronts are relatively weak, and the air-sea temperature difference remained close to neutral, the SAR image expression is not significantly modulated by wind variations induced from changes in the boundary layer stratification. Moreover, salinity gradients are not reported to provide backscatter anomalies. Hence, we interpret the SAR image expressions to be a manifestation of short-gravity wave-current interaction along the current fronts.
As the short gravity waves are propagating into and/or across the current front, they change steepness and propagation direction as long as the wind is moderate. Consequently, the SAR is capable of detecting the current fronts. It is evident that this SAR image interpretation is strongly supported by the near coincident sea surface temperature field mapped in the cloud-free AVHRR image. The combined analysis therefore, supports the multisensor approach to improved utilization of the remote sensing data in mesoscale ocean studies.

NERSC, Bergen, Norway

Current boundaries in Norway (II)

Several frontal boundaries can be seen. The fronts undergo both bright and dark departures from surrounding mean radar intensity. Again the wavelengths are of the order of 50 km as in the previous example .
A weak wind front is found in the south-west corner of the image. Westerly winds of about 10 m/s from the south are reported at the time of SAR acquisition.

NERSC, Bergen, Norway

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry