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Ocean Surface Topography from Space
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Jason-1 near real-time applications

figure 1

Authors:


G.H. Born (CCAR, USA)
R.R. Leben (CCAR, USA)
D.S. McCollum (CCAR, USA)
J.M. Wilczak 2 (NOAA Environmental Technology Laboratory, USA)

CORRESPONDING AUTHOR:
George H. Born
CCAR, Campus Box 431, Boulder,
Colorado 80309-0431 - USA
georgeb@ccar.colorado.edu


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Abstract:

The very successful TOPEX/POSEIDON and ERS-1 and -2 altimeter missions flown
in the 1990s demonstrated the utility of altimeter data for near real-time
ocean monitoring. One of the primary objectives of the Jason-1 mission is to
build on this success and initiate an era of operational oceanography that
will continue well into the 2000s. As a part of our Jason-1 research activities
we will continue to provide near real-time data products for research activities
and to work toward operational use of altimeter data by the public, industrial
and government sectors.

Content:

One application of altimetry is to provide observations over sparsely sampled
ocean regions to improve weather and climate forecasts. For example, improved
forecasts of severe landfalling winter storms on the U.S. West Coast have the
potential to prevent serious human and economic losses. It is likely that
altimetry-based upper ocean heat content estimates could aid in the refinement
of forecast and hindcast simulations of these storms by providing improved model
input at the ocean boundary. We have been investigating how well altimetry-based
heat content estimates compare with in situ heat content measurements taken from
airborne expendable bathythermographs (AXBTs) off the California coast. Preliminary
studies show very good agreement, both in space and time, and we are working
closely with the ongoing NOAA observational and modeling programs toward operational
applications of altimeter data in the region.

Introduction

The human and economic impact of severe landfalling winter storms on the U.S. West
Coast are comparable, on an annual average, to earthquakes. Like hurricanes, the
prediction of these storms is hindered because they develop over the ocean where
observations are sparse. An important operational application of Jason-1 will be
to provide additional ocean observations to improve forecasts of severe weather
and climate events.

During the NOAA California Landfalling Jets Experiment (CALJET) in the winter
of 1997-1998, data were collected in regions where additional and/or improved
observations could potentially help downstream forecasts. A region of sensitivity,
found by an adjoint analysis using the MM5 numerical weather model, was the
temperature field in the lower atmosphere 1000 km off the California coast.
As a result one of the objectives of CALJET was to determine the sensitivity
of landfalling storms to the upper-ocean thermal structure, which is still very
much a work in progress. In the winter of 1997-1998, NOAA P3 aircraft were used
to perform airborne expendable bathythermograph surveys in the region to profile
the upper ocean temperature in the region.

Current research and operational weather models rely on satellite-based sea
surface temperature (SST) measurements in combination with climatological SST
data sets to infer the upper ocean thermal structure and heat content. Our goal
is to develop a procedure whereby altimetry-based heat content estimates can be
used to accurately monitor the upper-ocean heat content off the West Coast of
the U.S. The ultimate objective is to develop a near real-time capability for
estimating the upper-ocean thermal structure from altimetry to use as input
into research and operational forecast models.

We have made preliminary comparisons of altimetry-based upper-ocean heat content
estimates with in situ measurements from the CALJET study. The extended heat content
time series in the study region spanning the entire TOPEX/POSEIDON (T/P) mission
is useful for putting into context the warmer conditions observed in the study region
that were associated with the 1997-1998 El Niño. We found that resolving the mesoscale
oceanographic component of heat content is required to adequately monitor the
upper-ocean heat content in the region. The results from this study are being used to
help plan additional ocean observations during the NOAA Pacific Landfalling Jets
Experiment (PACJET) [Ralph, 2000], a follow-on to CALJET, during the winter of 2000-2001.

Data and methods

Synoptic AXBT surveys were performed from aircraft to collect upper-ocean temperature
profiles off the West Coast of the U.S. during the winter of 1997-1998. Three sets of
altimetry-based sea surface height anomalies (SSHA) and the AXBT temperature
measurements were used to compute the heat content in the upper 350 meters of the
water column. The AXBT heat content was computed directly from the temperature
profiles. Altimetry-based heat content estimates were made using a linear relationship
between changes in the surface geopotential anomaly and the underlying heat content
variations using the World Ocean Atlas 1998 (WOA 1998) [Hendricks, 1996;
Chambers, 1997]. The climatological mean heat content also was used to reference
the heat content determined from AXBTs to a long-term mean.

Three altimetry data sets were used for comparison with the in situ measurements.
The first altimetric data set was based on TOPEX data alone. The second altimetric
data set was obtained by "blending" TOPEX and ERS-2 altimeter data with an emphasis
on retaining the longer wavelength oceanographic signals accurately measured by T/P,
and the mesoscale signal sampled by both the T/P and ERS-2 satellites (blended TOPEX/ERS).
The third data set is an operational data set that is produced by heavily filtering,
combining TOPEX with ERS-1 and ERS-2 altimetry data with an emphasis on retaining only
mesoscale structure (mesoscale TOPEX/ERS) [Lillibridge, 1997]. No in situ mesurements
of salinity were made during the aerial surveys. Since climatology-based salinity
corrections do not significantly improve altimetry-based heat content estimates,
none were applied [Sato, 2000].

Results


figure 2

Three analysis dates were chosen to compare the AXBT heat content with the
altimetry-based heat content. The three chosen dates each consisted of more
than thirty points and spatially covered the majority of the region. Altimetry
data were interpolated to the dates and geographic locations of the AXBT data.
The correlation between the AXBT and the blended TOPEX/ERS heat content is 0.72,
which means that over 50% of the variance in the AXBT heat content is represented
by the blended TOPEX/ERS heat content estimate. Figure 1 shows maps of heat content
estimates for January 20, 1998, comparing the altimetry-based heat content estimates
with the AXBT heat content for the upper 350 meters of the ocean in the region. Visual
inspection of the heat content anomaly maps reveals the very good correlation between
the blended TOPEX/ERS and AXBT heat content. The spatial mean heat content in the
region over six years, 1993 through 1998, is shown in Figure 2. The TOPEX heat content
varies the most, a result of under sampling mesoscale features. The TOPEX and the
blended TOPEX/ERS series capture seasonal changes well, including the 1997-1998 El Niño
and a smaller spike in heat content near the time of the 3 February 1998 storm that
initiated one of the PACJET surveys of opportunity.

References

Chambers D.P., B.D. Tapley, R.H. Stewart, 1997: Long-period ocean heat storage rates and basin-scale heat fluxes from TOPEX. J. Geophys. Res., Vol. 102, No. C5, pp. 10525-10533.

Hendricks J.R., 1996: Global sea level rise and upper ocean heat storage estimates from TOPEX/POSEIDON satellite altimetry. Doctoral Dissertation, University of Colorado at Boulder.

Lillibridge J., R. Leben, F. Vossepoel, 1997: Real-time altimetry from ERS-2. Proc. 3rd ERS Symp., Florence, Italy, March 1997.

Ralph M., 2000: The Pacific Landfalling Jets Experiment (PACJET) and a long-term effort to improve 0-24 Hour west coast forecasts. PACJET Program Document, http://www.etl.noaa.gov/programs/2001/pacjet/pacjet.shtml, January 27, 2000.

Sato O.T., P.S. Polito, W.T. Liu, 2000: Importance of salinity measurements in the heat storage estimation from TOPEX/POSEIDON. Geophys. Res. Lett., Vol. 27, No. 4, pp. 549-551.


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