The accurate global measurements of the ocean tides by Topex/Poseidon
and Jason-1 allow a number of longstanding questions about tidal
dynamics to be addressed in a serious way. Among them are the
problems of tidal energy dissipation, the dynamics of long-period
tides, and the nature of internal tides. Ocean cotidal charts themselves
also continue to improve as the altimetry time series grows longer.
We intend to periodically release new global tide models of benefit to
other altimeter users and to the wider geophysical community.
Improved Tide Models
Improving our knowledge of the ocean tides continues to be
an important task for the Topex/Poseidon and Jason-1 missions.
The applications of improved tide models are widespread
throughout geodesy and oceanography, and, of course, none is
more crucial than the application to altimetry itself.
Following the rapid and marked improvements made early in the
Topex/Poseidon mission, later advances are understandably more
incremental. Our recent global solutions, dubbed GOT99.2,
GOT00.2, and TPXO.5, are nonetheless clearly more accurate than
our earlier global models, especially in shallow seas.
Figure 1 shows one recent solution for the principal lunar
diurnal tide O1.
A judicious use of data from ERS-1, ERS-2, GFO, and coastal tide
gauges, combined with hydrodynamic models, will serve as valuable
supplements to T/P and Jason-1 data. We expect that further
incremental improvements to global tide models will continue
throughout the Jason-1 mission.
The discovery of internal tide signals in satellite altimeter data
opens up an exciting new approach to the study of these waves.
Internal tides can be observed in altimetry as small (usually less than
2 cm), short-wavelenth (150 km) modulations in the tidal surface elevations.
They are therefore most easily observed in along-track tidal estimates.
Because of their tiny amplitudes, very long time series of sea level
observations are required to extract them from a background of larger
oceanographic signals and noise. Jason-1 will considerably help refine
our maps of internal tides. So too will the T/P and Jason tandem mission,
which will improve the spatial resolution in our estimates.
The advances in tidal mapping afforded by Topex/Poseidon have finally
allowed us to begin to answer some longstanding questions about tidal
energetics, specificically the nature and the location of tidal energy
dissipation. We have recently reported some initial findings
(Egbert and Ray, 2000). Analyses of the altimeter-derived cotidal
charts reveals that most tidal energy is dissipated in shallow seas,
as most oceanographers have long thought. However,
about 25 to 30% of the global energy dissipation,
or about 1 terawatt, occurs in the open ocean, generally near rugged
bottom topography. The mechanism at work is almost certainly the
scattering of surface wave energy into internal tides and other
baroclinic motions. It is thus conceivable that the dissipation of
tidal energy is intimately related to the vertical mixing of the ocean
and to its thermohaline circulation.
Figures 2 and 3 show two examples of deciphering the flow of energy
in the surface tide and internal tide, respectively. Such charts
can often immediately reveal the sources and sinks of tidal energy.
The barotropic energy fluxes of Figure 2 show considerable
energy flowing into various shallow seas; the major energy sinks
are clearly evident. The subtle deep-ocean dissipation is revealed
by computing the divergence of Figure 2 and subtracting the work
done by the moon on the ocean. As energy leaves the surface tide,
it can sometimes be seen to reappear in the internal tide, as Figure 3
shows for central North Pacific Ocean.
Internal wave energy is observed flowing both north and
south from the Hawaiian Ridge, sometimes in remarkably narrow
beam patterns. A very localized source along the Aleutian Ridge
generates waves that propagate southwards over 2000 km.
As the Topex/Poseidon time series grows longer and as it is
supplemented by the Jason-1 time series, these data only grow more
valuable for tide studies. We expect to see many new and
intriguing results over the next several years.
Egbert, G. D. and R. D. Ray, 2000: Significant dissipation of tidal
energy in the deep ocean inferred from satellite altimeter data,
Nature, 405, 775-778.
Ray, R. D. and D. E. Cartwright, 2001: Estimates of internal tide
energy fluxes from Topex/Poseidon altimetry: Central North Pacific,
Geophysical Research Letters, in press.
Ray, R. D. and G. T. MItchum, 1997: Surface manifestation of internal
tides in the deep ocean: observations from altimetry and island guages,
Progress in Oceanography, 40, 135-162.