Ocean Climate Change: Comparison of Acoustic Tomography, Satellite Altimetry, and Modeling
Bruce M. Howe (Applied Physics Laboratory, University of Washington, Seattle, WA 98105, USA)
email: howe@apl.washington.edu
ATOC Consortium: A. B. Baggeroer, T. G. Birdsall, C. Clark, J. A. Colosi, B. D. Cornuelle, D. Costa, B. D. Dushaw, M. Dzieciuch, A. M. G. Forbes, C. Hill, B. M. Howe, J. Marshall, D. Menemenlis, J. A. Mercer, K. Metzger, W. Munk, R. C. Spindel, D. Stammer, P. F. Worcester, and C. Wunsch.
N. B. This proceedings paper is a condensed version of the following paper: ATOC Consortium, Science, 281, 1327 (1998).
ABSTRACT
Comparisons of gyre-scale acoustic and direct thermal measurements of heat content in the Pacific Ocean, satellite altimeter measurements of sea surface height, and results from a general circulation model show that only about half the seasonal and year-to-year changes in sea level are attributable to thermal expansion. Interpreting climate change signals from fluctuations in sea level is therefore complicated. The annual cycle of heat flux is 150 ± 25 W/m2 (peak-to-peak, corresponding to a 0.2℃ vertically averaged temperature cycle); an interannual change of similar magnitude is also detected. Meteorological estimates of surface heat flux, if accurate, require a surprisingly large seasonal cycle in the advective heat flux.
INTRODUCTION
Changes in oceanic heat storage are a major expected element of future climate shifts. Testing the ocean component of coupled atmosphere-ocean models by direct observations is very difficult, however, because climate-scale variability is masked by mesoscale variability as well as higher frequency internal waves. Two recently developed observational methods, satellite altimetry and acoustic tomography, are well suited for detecting climate-scale changes because they provide large-scale averages. Altimetry depends on the travel time of radio waves reflected at the sea surface. Tomography relies on the travel time of sound waves through the (electromagnetically opaque) ocean interior. The high horizontal resolution of the altimetry complements the vertical and temporal resolution of acoustic tomography. 1 Here we demonstrate that the combined data can extract changes in ocean heat storage on the scale of an ocean basin, the northeast Pacific. Fifteen months of acoustic data from the Acoustic Thermometry of Ocean Climate (ATOC) project are used in combination with altimeter data from the TOPEX/POSEIDON mission.2 Data are compared to an oceanic general circulation model (GCM) and then used to constrain the model.
PHYSICAL SETTING
We use sea level change (itself of intrinsic interest) as a convenient measure for comparing the different ATOC observations. Sea level change can be inferred from the acoustic measurements (under certain assumptions) for comparison with the direct altimetry measurements, but it is not possible uniquely to do the opposite-to infer heat content from the altimetry measurements for comparison with the “direct” (neglecting salinity) acoustic measurements.
