Abstract:
Satellite altimetric measurements of sea surface height, together with subsurface temperature and salinity (T/S) profiles and reference level velocities, form a dynamically complete description of the physical state of the ocean. We will combine TOPEX/POSEIDON and Jason-1 altimetric data with T/S profiles and reference velocities from the Argo Project as well as from ongoing high-resolution XBT/XCTD transects. Our goal is to study the role of the ocean in the climate system and to provide appropriate tests for data assimilation models that synthesize similar datasets.
Introduction and objectives
The height of the sea surface is determined by the mass of water at a given location and by the
water's density (a function of temperature, salinity and pressure). The altimeter measures changes
in sea surface height due to both of these factors - redistribution of mass and changes in
density (steric height). On seasonal to interannual time-scales, density changes are the largest
contributor to sea level variability. In the tropics they are the dominant one.
Our project is aimed at an improved understanding of sea surface height variability and its causes through studies of altimetric sea level in combination with subsurface data. Models are not yet capable of accurately decomposing sea level variability into its components or of describing the vertical structure of the underlying density changes. By studying altimetric data together with subsurface data we can:
- Understand the relative contributions of mass and steric height variability to sea level change over a broad range of space and time-scales and geographic regimes.
- Verify the performance of models that assimilate altimetric data through direct comparison of subsurface model fields with data.
- Produce combined datasets that have the high spatial and temporal resolution of the altimeter, but that include the subsurface structure of temperature and salinity.
These datasets will be used for studies of ocean circulation and dynamics, especially the role of the ocean in the climate system - including heat transport and storage and the hydrological cycle.
Subsurface datasets
High-resolution XBT/XCTD (HRX) transects
Throughout the TOPEX/POSEIDON era, temperature and salinity profile data have been collected along a set of repeating commercial ship tracks (figure 1) spanning the Pacific Ocean. Transects are obtained quarterly along each track by a scientist or technician, consisting of temperature profiles (XBT) to 800 m at 10 - 40 km spacing plus sparse salinity profiles (XCTD). These data allow comparison of steric height and altimetric height (figure 2) variability on seasonal to interannual time-scales and on spatial scales from those of fronts, eddies and boundary currents to the 10,000 km scale of the Pacific Ocean. A new 2000 m XBT with improved accuracy is being phased into use. We designed the Pacific HRX Network as a component of World Ocean Circulation Experiment (WOCE) and Climate Variability and Predictability Experiment (CLIVAR), and we continue to implement it in collaboration with international partners.
Argo
A broadscale global array of 3000 CTD profiling floats, Argo (figure 3), is being implemented as part of CLIVAR and the Global Ocean Data Assimilation Experiment (GODAE). Once complete, the array will provide approximately 100,000 high-quality temperature/
salinity profiles per year, distributed randomly over the global ocean.
It replaces the present broadscale XBT network with deeper profiles, much better space-time coverage and the addition of salinity as well as temperature. In addition to profile data, Argo will obtain velocity measurements at a mid-depth reference level. Through the geostrophic relationship, velocity is proportional to the horizontal gradient of pressure and hence it is a measure of the mass distribution above the reference level. Argo was designed to be the subsurface counterpart to the Jason-1 altimeter, and the Jason-1/Argo combination measures sea level and its subsurface causes. Argo implementation has been started in 2000 by an international consortium of 12 float-providing nations, and the array will be complete by 2004. D. Roemmich was a co-designer of the Argo network and is chairman of the international Argo Science Team.
Interim results
- Comparative studies of steric height and altimetric height have been carried out in both the North Pacific [Gilson et al, 1998] and South Pacific [McCarthy et al, 2000]. Steric height was shown to be the dominant component of sea level variability, with large-scale residuals that are due to the mass field.
A correlation technique was developed for estimating subsurface temperature from altimetric data.
- Eddies observed concurrently by the TOPEX/POSEIDON altimeter and the HRX dataset in the tropical North Pacific were analyzed [Roemmich and Gilson, 2000] to reveal their surface (figure 4) and subsurface characteristics. Subsurface tilt of eddy centers
was shown to result in substantial poleward heat transport and interannual variability in the equatorward transport of thermocline waters.
- Studies of the mean and interannual heat balance of the North Pacific [Roemmich et al., 2000a] and the tropical Pacific [Roemmich et al., 2000b] have been carried out using HRX data supplemented by TOPEX/POSEIDON altimetry.
The shallow overturning circulation, carrying warm water poleward and thermocline waters equatorward, is responsible for 1.2 pW of mean heat export from the tropical Pacific, with interannual variability of about 30%.
Approach and future plans
As Jason-1 and Argo come on line, we will continue to synthesize the combined altimetric and subsurface datasets. These studies will begin with, but not be
limited to, the Pacific Ocean. They will focus broadly on the mass, heat and freshwater budgets of the ocean and on the large-scale circulation of the upper 2000
m. Our approach is complementary to data assimilating models, relying on statistical tools for dataset combination and interpolation, but without specifying the
complete ocean dynamics. The combined datasets are valuable in their own right for scientific studies of ocean circulation and transport, and as a tool for testing
dynamical assimilations. With assimilating models in their present rapidly developing state, analyses that are independent of the model dynamics and not subject to
coarse model resolution are crucial for testing and verification of model results.
References
Gilson J., D. Roemmich, B. Cornuelle, L.L. Fu, 1998: The Relationship of TOPEX/POSEIDON altimetric height to the steric height of the sea surface. J. Geophys. Res., 103, 27947-27965.
McCarthy M., L. Talley, D. Roemmich, 2000: Seasonal to interannual variability from expendable bathythermograph and TOPEX/POSEIDON altimetric data in the South Pacific subtropical gyre. J. Geophys Res., 105, 19535-19550.
Roemmich D., J. Gilson, 2000: Eddy transport of heat and thermocline waters in the North Pacific: A key to interannual/decadal climate variability. J. Phys. Oceanog. (in press).
Roemmich D., J. Gilson, B. Cornuelle, R. Weller, 2000a: The mean and time-varying meridional heat transport at the tropical/subtropical boundary of the North Pacific Ocean. J. Geophys. Res. (in press).
Roemmich D. et al., 2000b: Interannual variability in Pacific Ocean basin-scale circulation and transport during the 1990s. Invited Abstract, The WOCE Variability Workshop, Fukuoka, Japan, October 2000.
