Some explanation of oceanographic terminology, techniques and principles

altimeter

A satellite altimeter measures the distance from the satellite to the sea surface directly below. Once the position of the satellite is known, the height (averaged over about 7km by 7km) of the sea surface can be determined to within several centimeters. Oceanographers use altimeters to measure highs and lows of the ocean surface, around which geostrophic currents flow. The Topex/Poseidon mission is in an orbit that takes 10 days to finish re-sampling a criss-crossing pattern of ground tracks which are about 250km apart at mid-latitudes. [play orbit movie; shows half a 10-d cycle] The ERS ground tracks are about 160km apart at mid-latitudes, but are re-sampled only every 35 days. [play movie of Topex/Poseidon and ERS along-track sea level measurements] For our present application, we have combined the track data from both satellites to make interpolated maps of the sea surface height. For further information, go to http://topex-www.jpl.nasa.gov/

AVHRR

Advanced Very High Resolution Radiometer. A sensor carried by the US National Ocean and Atmosphere Administration (NOAA) satellites which oceanographers use to measure Sea Surface Temperature (SST). The pixel size is about 1km and accuracy about +/-0.6° C. The NOAA satellites broadcast the data continuously, so any groundstation that can see the satellite can receive the data. Several satellite passes per day are tracked, received, processed and archived at WASTAC in Perth, CSIRO in Hobart and ACRES in Alice Springs.

Coriolis force

the `pseudo-force' on an object moving with respect to a rotating frame of reference. A cannon shell appears to veer left in the southern hemisphere, and right in the northern hemisphere.

Ekman velocity

A steady wind blowing over the open ocean causes the surface layer of water to move at a velocity (called the 'Ekman velocity') which is such that the Coriolis force on the layer balances the force of the wind. Hence, a wind from the south drives an Ekman velocity towards the west, in the southern hemisphere. The thickness of this layer is variable, with 30-50m being fairly typical. The Ekman velocity is additional to whatever velocity the surface layer was moving at before the wind commenced, and takes a day or two to settle down to being at right angles to the wind. The oscillations that follow a sudden change of the wind stress are called inertial oscillations.

geostrophic

balance of pressure gradient and the Coriolis force. In the southern hemisphere, a northward geostrophically balanced current has sea level higher to the west. Similarly, an anti-clockwise eddy has elevated sea level in the centre. Slowly-varying (ie over several days) currents are invariably close to being geostrophic, so by measuring either the current velocity or the slope of sea level, the other can be calculated.

SST

Sea Surface Temperature. Usually measured either continuously from ship at the depth of the engine water intake, or remotely by satellite (see AVHRR) or airplane.

upwelling

when water is drawn to the surface from great depth; significant because nutrients settling to the bottom then become available to phytoplankton living within the depth of penetration of sunlight. Various forces drive upwelling, the most common being alongshore winds which drive the surface waters away from the coast due to the Coriolis force.

windstress

The effect on the ocean of the wind increases greatly as the wind velocity increases. The windstress is a measure of this effect and is approximately proportional to the square of the wind velocity.