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Ocean Surface Topography from Space
SCIENCE
Investigation of the Seasonal-to-Interannual Variabilities of the Ocean by Assimilating Satellite Altimetry and Ancillary Data into Numerical Ocean Models
Figure 1



Authors:


I. Fukumori,
Y. Chao,
L.-L. Fu,
T. Lee,
W. T. Liu,
D. Menemenlis,
V. Zlotnicki
(Jet Propulsion Laboratory,
California Institute of Technology
USA)

CORRESPONDING AUTHOR:
Ichiro Fukumori
JPL Mail Stop 300-323
4800 Oak Grove Drive
Pasadena, CA 91109
U.S.A.
E-mail: if@pacific.jpl.nasa.gov




Abstract:

We study physical processes that underlie the changes in ocean
circulation that satellites and in situ measurements observe. Our
approach is to estimate the ocean's currents, temperature, salinity
and other properties throughout the ocean (the complete
three-dimensional state) by combining satellite altimeter
observations, other measurements, and numerical models of ocean
circulation. Mechanisms of the mean and time-varying ocean
circulation are examined, with particular focus in identifying
dominant processes that control seasonal-to-decadal changes of the
Pacific Ocean (e.g., El Niño/La Niña, Pacific Decadal Oscillation).

Main Body:

The heat and fresh water that the oceans exchange with the atmosphere
dictate the impact of the ocean on climate at seasonal-to-decadal
time-scales. Understanding how the ocean transports, stores, and
releases heat and water to the atmosphere is fundamental to advancing
our knowledge of the ocean's role in climate change. However, large
discrepancies exist among different estimates of ocean circulation for
two main reasons: incomplete observations, and the fact that almost
global observations at two different times differ by how the ocean
changes with time. For instance, interannual variability in the
Tropical Pacific Ocean in the 1980s appears to be different from those
in the early 1990s. The prolonged persistence of warm conditions over
the Tropical Pacific Ocean in the early 1990s may be due to influx of
anomalously warm waters from the Subtropical Pacific Ocean (Gu and
Philander, 1997). The El Niño event of 1997-98 appears to be
different from those in the early 1990s, but similar to that of
1982-1983. In particular, changes in sea level suggest that a
possible climate shift occurred in 1999 associated with the Pacific
Decadal Oscillation (http://sealevel.jpl.nasa.gov/science/elninopdo/pdo/).

Changes in sea level, to a large extent, reflect changes in vertically
integrated heat content, and therefore altimetric measurements can
shed much light on how heat is transported. Jason-1, together with
TOPEX/POSEIDON that has been operational since September 1992,
provides a continuous record of observations that offers an
unprecedented opportunity to examine the temporal changes of the
ocean's heat content, in particular, the seasonal-to-interannual
variability of heat transport over the last decade.

In spite of its global coverage, altimetric sea level measurements by
themselves cannot reveal the inherently three-dimensional nature of
ocean circulation. Numerical ocean circulation models can help
extrapolate information obtained from satellite observations into the
ocean interior and to combine them further with those from in situ
measurements. These models can be thought of as providing theoretical
relationships among various properties of the ocean. Assimilation of
observations with numerical models amounts to solving a set of
simultaneous equations, many of which are nonlinear and have enormous
dimensions owing to the ocean being a vast continuum (order hundreds
of millions of equations). Various approaches have been advanced to
efficiently solve such complex computational problems (Fukumori,
2000).


Figure 2

Data assimilation provides optimal descriptions of the complete state
of ocean circulation, allowing estimates of various properties
including those that are not directly measured. For instance, Figure
1 compares temporal changes in subsurface circulation estimates made
from altimeter data (Fukumori et al., 1999.) The impact of altimeter
data is evidenced by the assimilated estimates (blue) being in closer
agreement with independent in situ measurements (black) than the
unconstrained model estimates (red) are. Improved estimates of
temperature and velocity give better understanding of the ocean's mass
and heat transport and their balance. Figure 2 shows the variability
of zonally integrated northward heat flux estimated across the Pacific
Ocean. The large seasonal cycle reflects changes in wind-driven
surface circulation. Differences resulting from assimilation are due
to corrections of model errors at depth and amount to a fair fraction
of the total variability that would lead to significant differences in
how the ocean affects the atmosphere.


Figure 3

The comprehensive description of the ocean by models helps
quantitatively evaluate and understand processes in the ocean. Figure
3 shows an example comparing estimates of different processes
affecting anomalies of sea surface temperature in the eastern
equatorial Pacific ("Niño3" region). Sea surface temperature
dictates surface heat flux from the ocean to the atmosphere, and is
therefore a central quantity of interest. Figure 3 indicates that
advection (in particular, the east-west component) was the primary
cause of the positive sea surface temperature anomaly associated with
the 1997-1998 El Niño event. Such evaluation would be difficult
without a physically consistent description of the entire state of the
ocean.


Figure 4

Analyses of circulation pathways provide additional insight into
mechanisms of ocean circulation. Model-based solutions are
particularly valuable in such assessment owing to their high spatial
and temporal resolution. Figure 4 shows an example describing
pathways of water parcels estimated over a two-year period released in
January 1985 along 24°N and 24°S. Such analysis helps distinguish
different dynamic regimes that determine water mass evolution and
improves our understanding of how changes in one part of the ocean
might affect another. For example, the figure illustrates that water
reaching the equator originates in the the eastern half of the
subtropical basin (red and green) with most of it traversing the
energetic western boundary region before reaching the equator.

These analyses above will be expanded with the advent of Jason-1. The
inclusion of more observations together with further improvements in
modeling and data assimilation will lead to increasingly more accurate
and complete descriptions of ocean circulation. Advancements will
also allow assimilated analyses to be produced routinely so as to
provide a monitoring tool for events such as the El Niño. Routine
descriptions will enable predictability studies of the coupled
ocean-atmosphere system, by providing initial conditions of the
oceanic state. Estimates of oceanic advection and mixing will also
facilitate understanding tracer distribution that is critical in
modeling biogeochemical processes of the ocean.

References

Fukumori, I., R. Raghunath, L. Fu, and Y. Chao, 1999. Assimilation of

TOPEX/POSEIDON data into a global ocean circulation model: How

good are the results?, Journal of Geophysical Research, 104,

25,647-25,665.

Fukumori, I., 2000. Data Assimilation by Models, in Satellite

Altimetry and Earth Sciences: A Handbook of Techniques and

Applications, L.L. Fu and A. Cazenave, editors, Academic

Press, 463pp.

Gu, D. F., and S. G. H. Philander, 1997. Interdecadal climate

fluctuations that depend on exchanges between the tropics and

extratropics, Science, 275, 805-807.


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