OCEAN DYNAMIC TOPOGRAPHY IMAGES

	Dynamic ocean topography is a measure of sea level relative to Earth's
geoid, a surface of constant gravitational energy that would represent the sea
surface if the ocean were not in motion.  Therefore, oceanographers can use maps
of ocean topography to calculate the speed and direction of ocean currents in
the same way meteorologists use maps of atmospheric pressure to calculate the
speed and direction of winds.  TOPEX/Poseidon is the first space mission that
allows scientists to map ocean topography with sufficient accuracy to study the
large-scale current systems of the world's oceans.

	The monthly variations in dynamic ocean topography are illustrated by
the digital image files described below.  The top frame of the image shows that
the topographic relief ranges over about 2 meters from high-to-low.  The color
scale is such that the reds represent the high, the blues the moderate, and the
greens the low features.  Yellow and dark green represent the extreme high and
the extreme low features respectively.  The contour interval is 20 centimeters.

	The maximum sea level is located in the western Pacific Ocean and the
minimum sea level circumscribes Antarctica.  In the northern hemisphere, ocean
currents flow clockwise around the highs of ocean topography and
counterclockwise around the lows; this process is reversed in the southern
hemisphere.  These highs and lows are the oceanic counterparts of atmospheric
circulation systems.  These images reveal most of the current systems that have
been identified by shipboard observations collected over the past 100 years.
For instance, major current systems such as the Kuroshio (western North
Pacific), the Gulf Stream (western North Atlantic), and the Antarctic
Circumpolar Current, among others, are clearly visible.

	Note that the basic structure of these ocean systems is about the same
from month-to-month.  To illustrate the monthly variations in more detail,
anomalies in dynamic ocean topography relative to an annual mean are shown in
the bottom frame of the images.  The contour interval is 3 centimeters.

DESCRIPTION OF FILES AND DATA

	Files with the prefix "TPX" display dynamic topographic heights observed
by the TOPEX/Poseidon altimeter measurements and the corresponding sea level
anomalies in GIF format.  They were developed by scientists at the Center for
Space Research (CSR) at the University of Texas, Austin and are based on
altimeter data gathered within any given month.  The naming convention for these
files is TPXyymm.GIF, where yy is the last two digits for the year and mm is the
month during which the data was gathered.

	Media and instrument corrections (ionosphere, wet and dry troposphere,
and electromagnetic bias) and geophysical corrections (tides and inverted
barometer) have been applied to the measurements.  The altimeter measurements
are then reduced to sea surface topography heights using a precise orbit
computed at CSR using the available Satellite Laser Ranging (SLR) and DORIS
doppler tracking data.  The topography heights were computed using 10-second
altimeter normal points and consist of surface spherical harmonics complete to
degree and order 25.  All the topography heights in the grids are referenced to
the geoid computed from the JGM-2 gravity field model and corrected to refer to
the mean ocean geoid.

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