Abstract:
Altimetry is a key component of the remote sensing work at Southampton Oceanography Centre.
We have used data from most previous missions, and those from Jason-1 are an integral part of
our future plans. We will make use of all the geophysical parameters obtainable over the ocean
- sea surface height, wave height, wind speed and rain rate, both separately and in
combination. Our proposed work with Jason-1 will focus on four main areas — CALVAL,
global climatologies, long-term monitoring and comparison with other datasets and models.
CALVAL
In order to make the best possible use of the data, their quality must be evaluated by
comparison with other measurements. This is especially critical when studying long-term
climate change involving data from many different satellites. First, for wind and wave
validation, we will use data from the large set of wave buoys, as well as comparisons
with other altimeters. This work will also evaluate new algorithms giving wave period
as well as the latest wind speed algorithms. Second, in combination with colleagues
in La Spezia, we will be deploying tide gauges on the island of Capraia (in the Corsica
Channel), which lies close to both the TOPEX/Jason and ERS/ENVISAT ground tracks.
Third, we hope to position a number of acoustic rain gauges under the satellite track
to assess the dual-frequency rain algorithm.
Global climatologies
With suitable averaging of the data, we can form global or regional maps of wave height,
wind speed and rain rate (see figure 1). We will be examining changes in these on
seasonal-to-decadal time scales. Analysis of weather-ship data by Bacon and Carter
[1991] has shown a trend to increasing wave height in the northeast Atlantic, possibly
related to the high state of the North Atlantic Oscillation, that is, the large
wintertime north-south pressure difference that has been prevalent in the 1990s.
The rain rate data from TOPEX have shown that the 1997-98 El Niño was marked by an
increase in the surface area and intensity of the Pacific rain belt [Quartly
et al., 2000c]. Jason-1 will probably give us the opportunity to observe another
El Niño, and also to continue our analysis of the spatial scales of rainfall. The
climatologies of all three parameters will be compared to those derived from other
datasets.
Long-term monitoring of ocean features
The Jason track close to Capraia is ideal for monitoring the height difference
between the two basins at either end of the Corsica Channel. Flows inferred
from TOPEX/POSEIDON data for 1993-98 compare favorably with current meters in the
channel [Vignudelli et al., 2000], demonstrating how Jason-1 could be used in
monitoring critical flows.
The Agulhas region to the south of South Africa is an active area, with many
eddies contributing to the fluxes of heat and salt-water between the Indian
and Atlantic Oceans. The translation of eddies across altimeter tracks allows
us to estimate their number and size through observing the changes in sea surface
height profile (see figure 2). We are also interested in further regional studies
in the northeast Atlantic and western Mediterranean, where the altimeter data
will be used in combination with ship and satellite observations of temperature,
current velocity and chlorophyll content. Application of the technique developed
by Challenor et al. [1996] will allow us to determine absolute velocities for all
altimetric repeats of a surveyed track. We will use this, in combination with
models, to investigate the effects of physical variability on phytoplankton
growth.
Comparisons between datasets
Although altimeter data are a major resource on their own, there are important
gains to be made from using them in combination with data from other sensors.
Rossby waves are an important process by which the ocean transmits information
from east to west, possibly affecting the strong Western Boundary Currents.
Cipollini et al. [2001] have shown that Rossby waves, as well as having a sea
surface height signature detectable by altimeters, are detectable in ocean color
data (see figure 3), and we have begun to look at the phase relationship of the
height, temperature and color signatures [Quartly et al., 2000a] in order to
understand the mechanisms involved.
Altimeter data will also be compared to models to test how well they represent
reality. Differences in location of features and/or in the timing of events can
lead to large errors in simple root-mean-square comparisons, with little insight
gained. Instead, we will apply spatial analysis techniques that ascertain the
common modes of variability. An important test of models is the strength and
speed of their Rossby waves [see Cipollini et al., 2000], for they indicate
how well and how fast a system responds to changes elsewhere.
References
Bacon S., D.J.T. Carter, 1991: Wave climate changes in the North Atlantic and North Sea. Intl. J. Climat., 11, 545-558.
Challenor P.G., J.F. Read, R.T. Pollard, R.T. Tokmakian, 1996: Measuring surface currents in Drake Passage from altimetry and hydrography. J. Phys. Oceanogr., 26, 2748-2759.
Cipollini P., D. Cromwell, G.D. Quartly, P.G. Challenor, 2000: Remote sensing of oceanic extra-tropical Rossby waves. in "Satellites, oceanography and society", D. Halpern, ed. Elsevier, 99-123.
Cipollini P., D. Cromwell, P.G. Challenor, S. Raffaglio, 2001: Rossby waves detected in global ocean color data. Geophys. Res. Lett., 28, 323-326.
Quartly G.D., P. Cipollini, D. Cromwell, P.G. Challenor, 2000a: Synergistic observations of Rossby waves. ERS-ENVISAT Symposium, 16th-20th Oct. 2000, Gothenburg, Sweden, ESA, 8pp.
Quartly G.D, T.H. Guymer, M.A. Srokosz, 2000b: Wind, waves and rain: Changes in spatial distribution and intercorrelation. Proc. of PORSEC 2000, I, 305-309.
Quartly G.D, M.A. Srokosz, T.H. Guymer, 2000c: Changes in oceanic precipitation during the 1997-98 El Niño. Geophys. Res. Lett., 27, 2293-2296.
Vignudelli S., P. Cipollini, M. Astraldi, G.P. Gasparini, G. Manzella, 2000: Integrated use of altimeter and in situ data for understanding the water exchanges between the Tyrrhenian and Ligurian Seas. J. Geophys. Res., 105, 19649-19663.
