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Ocean Surface Topography from Space
SCIENCE
The dynamics of short-term ocean climate variability

Figure 1

Authors:


D.B. Chelton and R.A. De Szoeke
Oregon State University, USA

CORRESPONDING AUTHOR:
Dudley B. Chelton
College of Oceanic and
Atmospheric Sciences
104 Oceanography Administration Building
Oregon State University, Corvallis
OR 97331-5503 - USA
chelton@oce.orst.edu


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Abstract:

The objectives of this research are to develop more complete descriptions and dynamical understandings of phenomena observed in the TOPEX/POSEIDON (T/P) and Jason-1 altimeter data. This will be accomplished through combined analyses of the altimeter data and analytical and numerical modeling. Idealized analytical models will be used to explore first-order dynamical explanations for the observed features. This will provide guidance in the construction of more realistic numerical experiments. The emphasis will be on investigations of tropical instability waves and midlatitude Rossby waves.

Tropical instability waves

At periods shorter than 50 days, there are very energetic westward propagating signals
in the sea surface height (SSH) field at low latitudes (equatorward of about 10°) in
the Pacific. The variability is concentrated in two bands that are symmetric about the
equator along 5°N and 5°S between about 100°W and 160°E (see figure 1).

The amplitudes of the SSH anomalies are several times larger north of the equator.
The variability along these two latitudes is often highly coherent, although the
phase relation varies from year to year. These SSH signals are largest from June
through January. They are small or absent altogether in March and April and were
absent during the 1997-98 El Niño. The SSH anomalies along 5°N are the well-known
tropical instability waves (TIWs) that are generated by shear instabilities in the
equatorial current system. The coherence with SSH anomalies along 5°S suggests a
latitudinal structure of TIWs that has not previously been noted. This latitudinal
structure as well as the dispersive characteristics of these transequatorial TIWs
and the reasons for the annual and interannual modulations of their amplitudes will
be studied from a combination of additional data analysis and modeling.


Figure 2

An important question that can now be addressed from satellite data is the horizontal
eddy flux of heat associated with TIWs. Measurements by the TRMM Microwave Imager
(TMI) since December 1997 provide all-weather observations of sea surface
temperature (SST) simultaneous with the most recent two years of T/P observations
of SSH. The SST signatures of TIWs on both sides of the equator are clearly evident
in the TMI data [Chelton et al., 2000]. An example of the superposition of SSH
contours on a map of SST for the 3-day period centered on 9 December 1998 is
shown in figure 2. There is an obvious, strong correspondence between the two
fields. There are also some interesting differences, for example the cusps with
no SSH counterparts east of about 110°W. These relations will be investigated in
detail from the combined altimeter and SST datasets.

The relationships between SST perturbations T' and SSH perturbations h' (and the
associated perturbation geostrophic velocity components u' and v') will be
investigated statistically. This will include an estimation of the surface zonal
and meridional eddy heat fluxes <u'T'> and <v'T'>. The total horizontal eddy heat
flux requires knowledge of the vertical distribution of <u'T'> and <v'T'>. The
vertical scale of the coherent eddy heat flux signals can be estimated from T'
and h'. Through the hydrostatic relation, h' is proportional to the perturbation
pressure field p' while T' is proportional to dp'/dz. The relation between SST
and SSH therefore provides an estimate of the vertical scale of SST anomalies.
This can be obtained from the slope of a straight-line fit to a scatter plot of
h' versus T'. Alternatively, the vertical scale can be estimated from the ratio
<h'T'>/<h'h'>. The geographical variation of this vertical scale estimate will
be compared to gross environmental parameters such as thermocline depth. The
combination of the surface eddy heat flux components and a vertical scale will
be used to estimate the total horizontal eddy heat flux.

Rossby wave dispersion


Figure 3

Rossby waves are the mechanism for transient adjustment of the ocean to atmospheric
forcing. They are therefore of fundamental importance to ocean circulation on a
wide range of time scales. T/P observations of SSH have revealed that Rossby waves
are present throughout much of the world oceans [Chelton and Schlax, 1996]. The
behavior of these waves is qualitatively but not quantitatively consistent with
the classical theory for baroclinic Rossby waves. The westward phase speeds outside
of the tropics are systematically higher than predicted (upper and middle left panels
of figure 3). Westward propagating SSH anomalies with very similar phase speed
characteristics are also evident in ocean general circulation models such as the
global model developed by the Parallel Ocean Program (POP) at the Los Alamos National
Laboratory (see upper and middle right panels of figure 3).

From analytical modeling for the case of a continuously stratified ocean, Killworth
et al. [1997] have shown that the effects of vertical shear and advection in the mean
circulation (estimated from historical hydrographic data) can account for much of
the baroclinic Rossby wave speedup observed in the T/P data and POP model simulation
(see bottom panels of figure 3). A simple 3-layer model [de Szoeke and Chelton, 1998]
suggests that the key features in the mean circulation that are responsible for the
speedup are the well-known mid-depth layers of homogeneous internal potential vorticity.


Figure 4

The long T/P data record provides the first opportunity for global analysis of the
frequency-wavenumber characteristics of the westward propagating signals observed
in the T/P data. The frequency-wavenumber spectra of SSH anomalies along 24°N and
24°S are shown in figure 4. Over the range of wavenumbers resolvable in SSH fields
constructed from T/P data, it is apparent that the dispersion relation for the extended
theory of Killworth et al. [1997] (black circles) is in much better agreement with the
observed spectra than is the dispersion relation for the classical theory (white circles).
The coarse ground track spacing of the T/P orbit does not resolve wavelengths shorter
than about 5° of longitude. The overlapping T/P and Jason-1 missions with an
interleaved ground track pattern will double the zonal spatial resolution. The
frequency-wavenumber spectral characteristics will be examined at these higher
wavenumbers (shorter wavelengths) to determine the adequacy of the Killworth et
al. [1997] theory for the dispersion of Rossby waves in the presence of a vertically
sheared background mean flow.

Midlatitude interannual Rossby waves

The 8.5 years of T/P data are beginning to reveal interannual signals associated
with the El Niño phenomenon. Jacobs et al. [1993] argued that Rossby waves
originating along the eastern boundary of the North Pacific in association with
the 1982-83 El Niño may have perturbed the Kuroshio Extension a decade later.
This hypothesis will be testable with unprecedented capability from the long,
continuous data record afforded by the merged T/P and Jason-1 datasets. The major
El Niño that occurred in 1997-98 has imprinted a strong signature on the sea surface
height field in the mid-latitude eastern Pacific. This signal will be tracked over
the next decade as the eastern boundary manifestation of this El Niño event propagates
westward toward the Kuroshio Extension. In addition to testing the Jacobs et al.
(1993) hypothesis for the longevity of El Niño generated Rossby waves, the systematic
westward propagation of the midlatitude SSH anomalies associated with the 1997-98
El Niño event will allow a further quantitative test of the faster phase speed
predicted by the Killworth et al. [1997] theory.

References

Chelton D.B., M.G. Schlax, 1996: Global observations of oceanic Rossby waves. Science, 272, 234-238.

Chelton D.B., F.J. Wentz, C.L. Gentemann, R.A. De Szoeke, M.G. Schlax, 2000: Satellite microwave SST observations of transequatorial tropical instability waves. Geophys. Res. Lett., 27, 1239-1242.

De Szoeke R.A., D.B. Chelton, 1997: The modification of long planetary waves by homogeneous potential vorticity layers. J. Phys. Oceanogr. (submitted).

Jacobs G.A., H.E. Hurlburt, J.C. Kindle, E.J. Metzger, J.L. Mitchell, W.J. Teague, A.J. Wallcraft, 1994: Decade-scale trans-Pacific propagation and warming effects of an El Niño anomaly. Nature, 370, 360-363.

Killworth P.D., D.B. Chelton, R.A. De Szoeke, 1997: The speed of observed and theoretical long extra-tropical planetary waves. J. Phys. Oceanogr., 27, 1946-1966.


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