Abstract:
The highly variable flows along ocean boundaries are of particular interest due to the extensive use of these regions by
humans and marine species. We combine two types of satellite observations, altimeter sea surface heights and velocities
derived from tracking thermal patterns, to provide regular maps of sea surface currents. Preliminary results show the
regular formation of cyclones and anticyclones in the East Australian Current.
Content:
Currents near ocean boundaries are often energetic, changing quickly over short distances
and times. The accurate estimates of sea surface height from altimeter data, such as
TOPEX/POSEIDON (T/P) and Jason-1, provide valuable insight into flows in these regions,
but only along the widely-spaced satellite ground tracks at intervals of 10 days or longer.
We aim to improve measurements of currents along ocean boundaries by combining altimeter-derived
sea surface height anomalies with current velocities derived from tracking thermal features
in radiometer images.
Observations
Strong boundary currents, such as the East Australian Current, bring water of very different
temperatures together (figure 1). Velocities can be estimated by tracking patterns between
successive thermal images using the maximum cross correlation (MCC) technique [Tokmakian
et al., 1990, Emery et al., 1992]. A 10-day composite of MCC velocities in August 1997
(figure 2a) shows a triangular cyclonic feature, corresponding to the area of colder water
in the thermal image, located directly south of where the warmest waters turn away from the
East Australian coast. The persistent southward current flowing along the northern coast
is also obvious. The altimeter sea surface height anomalies for the same 10 days (comprised
of both T/P and ERS-2 tracks) show the cyclone as a distinct low in the sea surface height
(figure 2b).
Interpolation method
The heights and velocities are optimally interpolated to a stream function on a regular
grid in time and space using a method similar to Chereskin and Trunnell [1994]. The optimal
interpolation requires a priori knowledge of the covariance functions of the signal and
measurement noise. Covariances were estimated by computing binned, lagged correlations of
MCC velocities and altimeter sea surface heights from the East Australian Current between
1996 and 1999. The covariances show the mesoscale features have length scales of about 200
kilometres and evolve on a time scale of approximately 10 days. The sea surface height
anomalies and MCC velocity anomalies (three-year mean removed) were mapped into an anomaly
stream function. Velocities derived from the stream function and the 3-year mean MCC
velocities (figure 3) are an optimal merging of the complementary data sets.
The East Australian Current
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A three year time series of velocities in the East Australian Current was mapped from
the observations. The strongest variability in the current is an approximate 100-day
oscillation most evident near 153°E and 34°S. Two snapshots of temperature and velocity
separated by 50 days show the formation of a train of cyclonic and anticyclonic circulations
(figure 4). Although variability at this frequency has been noted previously in the region
[Walker and Wilkin, 1998; Mata et al., 1998], the currents from the interpolation provide
an unprecedented coverage in space and time. Measurements from the altimeters over the south
Pacific are also essential in investigating the hypothesis that planetary waves propagating
across the ocean force the mesoscale activity in the current [Nilsson and Cresswell, 1981].
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References
Chereskin T.K., M. Trunnell, 1996: Correlation scales, objective mapping, and absolute geostrophic flow in the California Current. J. Geophys. Res., 101, 22619-22629.
Emery W.J., C.W. Fowler, C.A. Clayson, 1992: Satellite image derived Gulf Stream currents. J. Atmos. Ocean. Tech., 9, 285-304.
Mata M.M., S. Wijffels, M. Tomczak, J.A. Church, 1998: Direct measurements of the transport of the East Australian Current: A Data report from the WOCE Pacific Current Meter Array 3, Technical Report 16, Flinders Institute of Marine and Atmospheric Sciences, Adelaide, Australia.
Nilsson C.S., G.R. Cresswell, 1981: The formation and evolution of East Australian Current warm-core eddies. Prog. in Oceanography, 9, Pergamon, 133-183.
Tokmakian R.T., P.T. Strub, J. McClean-Padman, 1990: Evaluation of the maximum cross-correlation method of estimating sea surface velocities from sequential satellite images. J. Atmos. Ocean. Tech., 7, 852-865.
Walker A.E., J.L. Wilkin, 1998: Optimal averaging of NOAA/NASA Pathfinder satellite sea surface temperature data. J. Geophys. Res., 103, 12869-12883.
