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Ocean Surface Topography from Space
SCIENCE
Estimation of air-sea gas transfer using Jason-1 dual-frequency normalized backscatter

Figure 1

Authors:


N.M. Frew
(Woods Hole Oceanographic Institution, USA),
D.M. Glover
(Woods Hole Oceanographic Institution, USA),
E.J. Bock
(University of Heidelberg, Germany)

CORRESPONDING AUTHOR:
Nelson M. Frew
Department of Marine Chemistry and Geochemistry
Woods Hole Oceanographic Institution
360 Woods Hole Rd. Mail Stop 4, Woods Hole, MA 02543-1543 - USA
nfrew@whoi.edu


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Abstract:

The exchange rates of climatically- important gases between the ocean and the atmosphere are
determined from altimeter estimates of the roughness of the ocean surface. The intensity of
backscattered microwave radiation at two different wavelengths is used to measure the slope
or steepness of the small-scale ocean waves that promote gas exchange and to derive monthly
global maps of air-sea exchange rates and a multiyear climatology.

Introduction

Gas exchange flux is calculated from the product of a concentration gradient across the air-water
interface and the exchange coefficient or transfer velocity representing a parameterization of a
combination of important near-surface exchange mechanisms. We refer to k as the transfer velocity
(cm/h) and this is the parameter describing the important near-surface exchange mechanisms.
Transfer velocity fields are predicted by various parameterizations based on wind speed [Liss
and Merlivat, 1986; Wanninkhof, 1992; Nightingale et al., 2000] and lead to widely varying
estimates of zonal and global net CO2 flux. These are not sufficiently constrained to validate
global climate change models, which suggest a global uptake of 2±0.8 GtC/yr, or to shed light
on the apparent "missing sink" for anthropogenic CO2 (1.6 GtC/yr). The uncertainty is a
significant fraction of the total annual 3.5 GtC uptake by non-atmospheric sinks [Johnson, 1995].
Thus, the Intergovernmental Panel on Climate Change (IPCC, 1996) has identified uncertainty
in the gas exchange coefficient as a significant limitation in assessing the role of the ocean
in absorbing anthropogenic CO2 and has called for increased study of its global spatial and
temporal variations in order to help close the global carbon budget. What is needed, then,
is a direct measurement of the surface roughness expressed by the small gravity-capillary
wave portion (the gas-exchange-active portion) of the surface wave spectrum.

The goal of this project is to develop an algorithm for estimating air-sea gas transfer velocities
using the dual-frequency Jason-1 altimeter. The approach is based on parameterization of the gas
transfer velocity (k) using normalized radar backscatter as a direct measure of sea surface
roughness due to small-scale waves. The small-scale waves (order of 6 to 16 cm wavelength) have
an overall average slope (mean square slope: <s2>) that is a robust predictor of k. This mean
square slope can be estimated from nadir-looking microwave backscatter, such as that returned
by altimeters like the instrument on Jason-1. Since k is linearly related to <s2> and <s2> is
inversely related to backscatter, we have a basis for an algorithm to derive k from the backscatter
measured by altimeters. The differential scattering of the dual frequency altimeter
(Ku- and C-band) allows us to isolate the contribution of small-scale waves to mean-square
slope and gas transfer. The algorithm is used to construct monthly global maps of CO2 transfer
velocity, to estimate seasonal transfer velocity variations, and from a lengthy time series
of satellite data produce a climatology of k.

Results

Our work prior to the Jason-1 launch has focused on development of the algorithm using
the extended TOPEX/POSEIDON altimeter record. The feasibility of calculating gas transfer
velocity directly from altimeter estimates of sea surface roughness presents a unique
opportunity to look at seasonal and interannual variability in the transfer velocity field.
Using the extended TOPEX/POSEIDON MGDR-B data set, we have produced a six-year time series
(1993-1998) of TOPEX data processed into gas transfer velocity and have examined the
variability of these results in space and time. The seasonal and interannual variability
of the regional patterns yield insight into the sensitivity of the altimeter-based gas
transfer velocity to phenomena such as ENSO. We have also compared the results of this
time series to similar time series created through the application of more traditional
wind speed-gas transfer velocity parameterizations to the wind speed estimates made by
the National Center for Environmental Prediction reanalysis project for the same period.


Figure 2

From January 1993 to December 1998, we have computed gas transfer velocity k660. The transfer
velocities are normalized to Schmidt number Sc = 660, the value for CO2 in seawater at 20°C,
in order to remove temperature effects and facilitate comparison with other parameterizations.
Global climatological gas transfer velocity fields for the months of February and September
are shown in figure 1, representing the seasonal extremes in the Northern and Southern
Hemispheres. The backscatter-derived k660 fields are shown in figure 2 as the zonal averages.
The overall pattern of seasonal variation is clearly seen in figure 2, with the maximum
transfer velocities in each hemisphere's corresponding wintertime. Additionally there is
an anti-correlated period of low to very low transfer velocities along the equator. At
mid-latitudes (20° - 40°N) there is a period of low transfer velocities developing each
year in summertime. A similarly low austral summertime low, zonally-averaged, transfer
velocity does not appear in any year in the 20° - 40°S zone. Additionally, early 1997
has the highest zonally-averaged transfer velocities in the northern subpolar region.
The extremely low zonal averages along the equator are interrupted during two periods:
late winter-early spring in 1997 (El Niño) and in late autumn-early winter in 1998. Except
in the eastern equatorial Pacific, the El Niño signal is not apparent over most of the
globe in the zonal averages.


Figure 3

The data of figure 2 have been collapsed into a time series climatology, shown in figure 3a.
Averaged over the six years, the pattern of higher transfer velocities poleward displays
an asymmetry, highest between 50° - 60°S. This is not surprising since at those latitudes
the fetch is greatest. To the north, land intervenes, and to the south the seasonal ice-pack
covers the air-sea interface. We have further processed the data to produce a time series of
the monthly global average transfer velocities shown in figure 3b. Treating both hemispheres
together smooths out the seasonal pattern mentioned in figure 2. However, starting in mid-1996
there is a distinct increase in the global average transfer velocity peaking in January 1997,
approximately two to three months before the generally-recognized beginning of the last El Niño.
After decreasing later in 1997, the transfer velocities in 1998 return to a value higher than
the average obtained from the 1993-1995 period.

References

Liss P.S., L. Merlivat, 1986: Air-sea gas exchange rates: introduction and synthesis. In: The Role of Air-Sea Exchange in Geochemical Cycling, P. Buat-Menard, editor, Reidel, Dordrecht, 113-127.

Nightingale P.D., G. Malin, C.S. Law, A.J. Watson, P.S. Liss, M.I. Liddicoat, J. Boutin, R.C. Upstill-Goddard, 2000: In situ evaluation of air-sea gas exchange parameterizations using novel conservative and volatile tracers. Global Biogeochem. Cycles, 14(1), 373-387.

Wanninkhof R., 1992: Relationship between wind speed and gas exchange over the ocean. J. Geophys. Res., 97(C5), 7373-7382.


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