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Ocean Surface Topography from Space
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Improving understanding and prediction of climate variability and change in the Australian region

Figure1

Authors:


J. Church (CSIRO Marine Research, Australia), (Antarctic CRC, Australia),
R. Coleman (University of Tasmania, Australia), (CSIRO Marine Research, Australia), (Antarctic CRC, Australia),
I. Barton (CSIRO Marine Research, Australia),
N. Bindoff (Antarctic CRC, Australia),
G. Meyers (CSIRO Marine Research, Australia),
P. McIntosh (CSIRO Marine Research, Australia),
W. Mitchell (National Tidal Facility, Australia),
R. Morrow (LEGOS/GRGS, France),
K. Ridgway (CSIRO Marine Research, Australia),
S. Rintoul (Antarctic CRC, Australia), (CSIRO Marine Research, Australia),
A. Schiller (CSIRO Marine Research, Australia),
D. Webb (SOC, UK),
B. de Cuevas (SOC, UK),
N. White (CSIRO Marine Research, Australia), (Antarctic CRC, Australia),
J. Wilkin (NIWA, New Zealand),
J. Wolff (Universität Oldenburg, Germany)

CORRESPONDING AUTHOR:
Dr. John Church
CSIRO Marine Research
GPO Box 1538
Hobart, Tasmania, 7001 - Australia
John.Church@marine.csiro.au


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Abstract:

Australia has marked climate variability and is potentially sensitive to the impact of
climate change. Our project is a coordinated plan, consisting of a number of separate
projects that use satellite altimeter data to improve our understanding and prediction
of both climate change and variability, particularly in the Australian region. Our
studies are supported by in situ observational programs and the development of
numerical models.

Satellite altimeter verification

Satellite altimetry is the only practical way of measuring global sea level change. Calibration
and intercalibration of altimeters is essential if altimeters are to reliably compute
long-term variations in sea level. Resolving absolute bias and bias drift at accuracies
better than a centimetre and a mm/yr respectively requires continuous observations at
verification sites for at least a decade.

At a colocated permanent GPS and tide gauge site at Burnie, Tasmania, an absolute
calibration of the Jason-1 (and T/P, ERS/ENVISAT) altimeter will be conducted using
fixed GPS receivers, GPS buoys and an array of current meters, pressure gauges, and
meteorological instruments.

To study bias drift, a number of tide gauge sites throughout Australia, the South
Pacific (AusAid South Pacific Sea Level and Climate Monitoring Project) and the
eastern Indian Ocean will be used to compare against the Jason-1 altimeter data.
Land motion estimates at these sites will be estimated from permanent or episodic
GPS sites. This work is done in conjunction with the Australian Marine Science and
Technology Limited, the Australian National Tidal Facility (NTF) and the Australian
Surveying and Land Information Group (AUSLIG).

Understanding and predicting Australian seasonal-to-interannual climate variability

Australia experiences marked climate variability on interannual time scales. This
variability strongly impacts agricultural production, water supply and the frequency
and intensity of forest fires. Sea surface temperatures (SSTs) in the Pacific and
Indian Oceans are important controls for much of Australia, with Southern Ocean
temperatures being important on the southern margin of Australia.

The overall goal is to build a global coupled ocean-atmosphere model capable of simulating
Australian climate variability and providing realistic projections of the coupled system
as an input to climate forecasts up to a year in advance.

For accurate projections, the ocean component of the coupled model must be initialised
with a realistic temperature structure. To improve the spatial coverage beyond that
available from in situ data sets from eXpendableBathyThermographs (XBTs) and the Tropical
Atmosphere Ocean (TAO) Array, satellite sea surface height (SSH) and SST data are being
used to infer subsurface ocean temperatures (synthetic XBTs). A comparison of ocean
temperatures measured at the TAO array and inferred from the satellite data is shown
in figure 1.

To understand Australian climate variability, improve the model and to build confidence
in the model projections, both the coupled and the ocean-only models are currently
being run in hindcast mode.

Dynamics of the Antarctic Circumpolar Current

The Southern Ocean plays a critical role in decadal and centennial climate variability
and in determining the timing and regional impact of climate change. Its circumpolar extent
allows interbasin exchange and thus adds a global dimension to the ocean's vertical
overturning circulation that dominates ocean heat transport and storage.

This project aims to better understand the dynamics of the ACC, including assessing the
variability of the eastward volume, heat and freshwater transport of the ACC, understanding
the role of eddies in the meridional transport of momentum and heat, and the development
of ocean models of the ACC system.

Full depth hydrographic sections (completed annually from 1991 to 1996 and then once
every five years) and upper ocean XBT sections (six sections each summer season)
have been used to estimate the baroclinic transport of the ACC. The in situ data
have been used to define a relationship between SSH and the baroclinic transport
function such that the altimeter data can be used to derive an ongoing record of
the ACC transport. In situ current meter observations are also being used to estimate
poleward transport of heat and momentum for comparison with satellite observations.

Understanding and modelling the Australian Exclusive Economic Zone (EEZ)

To provide a basis for the management of the Australian EEZ, an Ocean Analysis and Prediction
System (Oceans EEZ) is being developed. The primary objective is to implement an operational
nowcast system for mesoscale ocean conditions.

Oceans EEZ integrates historical oceanographic observations, fast-delivery altimeter data,
SST and other remotely sensed products, and modelling activities over the continental shelf,
slope and deep ocean regions to provide a comprehensive description of surface currents,
temperatures, salinities, and nutrients. All historical hydrographic data within the region
0-60°S, 90-200°E have been used to produce a high resolution (1/4°) ocean climatology atlas
enabling a mean surface steric height field to be determined.


Figure 2

During March 1999, complementary oceanographic observations were made along T/P ground-tracks
within a few days of satellite overpasses. Figure 2 shows surface currents along the cruise
tracks derived from ADCP (Acoustic Doppler Current Profiler) measurements (arrows) and the
combined CARS/altimeter height anomaly field for the same period. Along section AB (lower
panel), the height fields from the CTD and CARS/altimeter also agree well.

This approach is being used to study major current systems and coastal processes around
Australia, and their biological impacts (e.g., rock lobster, larval dispersion) and their
management implications. Other applications involve search and rescue, pollution prediction,
ship routing and naval operations.

Other projects

Satellite altimeter data are also being used to support other projects. These include
projects aimed at understanding the basin to global scale circulation and variability,
the understanding of western boundary currents and, in collaboration with U.K. scientists,
testing of the high resolution OCCAM global ocean model. A study of the transport and
variability of the East Australian Current (EAC) has shown that the variability of the
EAC transport is related to instabilities that form in the western Tasman Sea, perhaps
triggered by anomalies propagating westward across the Tasman Sea. As these instabilities
propagate southward they grow in intensity. A further study will investigate the boundary
current on the eastern flank of the Kerguelen Plateau where numerical models show large
bottom pressure variations. This study will also be of interest
to the GRACE mission.


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