Air-sea Interaction from Space - Recent Results

[W. Timothy Liu]{.smallcaps}

*MS 300-323, Jet Propulsion Laboratory, 4800 Oak Grove Drive,*

*Pasadena, CA, 91109, USA.*

### Tel: (1) 818-354-2394; fax: (1) 818-393-6720; email:liu@pacific.jpl.nasa.gov

# Abstract

The status of satellite remote sensing of ocean-atmosphere exchanges is
reviewed. Three on-going studies in air-sea interaction with potential
impact are introduced. They include observation of intense biological
bloom driven by typhoon with inference on global carbon cycle, new
hypothesis on air-sea coupling without deep convection, and long-term
trend in wind magnitude and meridional Ekman heat transport.

# 

# Keywords: winds, hurricanes, biological-productivity, carbon-cycle, heat-transport

# Ocean-atmosphere Exchanges from Space

![](media/image1.png){width="3.9299212598425197in"
height="2.6791338582677167in"}The ocean is forced at the surface largely
through the exchanges of water, heat, and momentum. The exchanges drive
the transport and change the storage of heat, water, and greenhouse
gases, and thus moderate the world\'s climate. The ocean feedback to
climate changes must be manifested through these exchanges, without
which the Earth would be a more hostile habitat. We need to study
air-sea exchanges to understand natural global changes and to discern
the anthropogenic effects. The ocean and the atmosphere are
under-sampled turbulent fluid with non-linear interactions; processes at
one scale affect processes at other scales. Adequate observations at
significant temporal and spatial scales can only be achieved from the
vantage point of space.

The atmospheric forcing and oceanic responses observed from space are
summarized in Fig. 1. The momentum exchanges or the dynamic forcing of
the ocean is the results of wind shear. The spacebased scatterometer is
the only proven means of measuring wind speed and direction under clear
and cloudy conditions night and day \[Liu, 2002\]. The dynamic response
as ocean current is being measured by radar altimeters \[Fu, 2001\]. The
thermal exchanges or heat flux can be divided into four components --
the sensible heat flux resulting from the temperature gradient, the
latent heat carried by evaporation, the short-wave radiation from the
sun, and the long-wave radiation from the atmosphere and the ocean.
Solar heating and evaporative cooling are the larger variable
components. The variability of global surface net short-wave radiation
(SR) is largely controlled by the variability of clouds, and most of the
recent computations of SR over the ocean make use of data with high
resolution sampled by geostationary satellites, through the
International Satellite Cloud climatology Project (ISCCP). The Clouds
and Earth's Radiant Energy System (CERES) on the Tropical Rain Measuring
Mission (TRMM) and NASA's Earth Observing System (EOS) Platforms will
advance the computation of the radiation forcing. Latent heat flux is
usually computed from wind speed and sea-air humidity differences
through bulk parameterization models or directly from the radiances
measured by microwave radiometers such as the TRMM microwave imagers
(TMI) and Advanced Microwave Scanning Radiometer (AMSR). New methods
using neutral network to derive both sensible and latent heat fluxes
have been attempted \[Bourras et al., 2002 a, b; Jones et al., 2002\].
The hydrologic forcing is the difference between precipitation (rain)
and evaporation. TRMM, with its low-inclination orbit (higher sampling
ate), opened up a new era of estimating both surface rainfall and rain
profile \[Kummerow et al., 1998\]. Multiple AMSR on various platforms
will extend TRMM rainfall measurements to higher latitudes.

![](media/image2.png){width="3.253543307086614in"
height="2.08740157480315in"}![](media/image3.png){width="3.27992125984252in"
height="2.1098425196850394in"}![](media/image4.png){width="3.1736220472440944in"
height="1.7543307086614173in"}

A review of the remote sensing of ocean-atmosphere exchanges was given
by Liu and Katsaros, 2001\] and addressed by many of experts in this
conference. In this report I will give three examples of breakthrough
results in air-sea interaction.

# 

Wind Forcing on Biogeochemical Balance

There has been intense search for new sources of external nutrient
supply that contribute to sustaining the primary biological production
in nutrient-poor (oligotrophic) surface waters, where traditional
accepted mechanisms are insufficient. The vertical pumping of nutrient
by transient tropical cyclones, with its intense wind vorticity, has
been speculated as a possibility, but undocumented by observations.
Tropical cyclones are non-stationary and violent; the oceanic responses
cannot be tracked by ship or moorings. The all weather sea surface
temperature (SST) from TMI has caused new revelation of the cold wakes
left by hurricanes and typhoon. But there is almost no successful
observational correlation between tropical cyclones and biological
productivity

Typhoon Kai-Tak, with moderate intensity, was observed by QuikSCAT to
linger at a near stationary slow speed in northern South China Sea
(SCS), 5-8 July 2000. The slow movement and intense winds caused strong
mixing and upwelling, with estimated Ekman pumping velocity reaching 0.4
cm/s at the typhoon location (Fig. 2a). TMI observed a 9°C cooling of
SST immediately after the passage of the typhoon on 9 July 2000 (Fig.
2b), and 100 time increases (from 0.1 to 10 mg/m3) in chlorophyll-a
(indicating biological activities), 5 day later (Fig. 2c). The Kai-Tak
passage was found to generate 2-4% of the annual new production in SCS.
This amount is highly significant, considering that there are , on
average, 14 tropical depressions/ cyclones passing SCS annually. More
comprehensive results and analysis are presented by Lin et al.
\[2002a\].

# Air-sea coupling without penetrative convection

At the major inter-tropical convergence zone (ITCZ) north of the
equator, deep convection and surface wind convergence are co-located
with high SST. Here the northerly and southerly trade winds, driven by
surface pressure gradient, come together. Liu and Xie \[2002\] found
weaker and year-round convergence zones south of the equator in parallel
with the northern ITCZ, in both the Atlantic and Pacific, which are
sustained by a different mechanism. The weaker ITCZ over cooler water is
caused by the deceleration of the surface winds as they approach the
cold upwelling water near the equator. Decreases in vertical mixing and
increases in vertical wind shear in the atmospheric boundary layer are
suggested to be the causes of the deceleration of the trade-winds as
they move from warmer to colder water. The peak convergence does not
co-located with local SST maximum. This model of tropical ocean
atmosphere coupling through vertical mixing was first used by Xie et al.
\[1998\] to explain the coherence between SST and surface wind in the
tropical instability waves (TIW)-the westward propagating temperature
front of the cold tongue. Liu et al. \[2000\] validated this model with
rawindsonde measurements of a research cruise across the TIW, and with
the phase difference between the wind components measured by QuikSCAT
and SST from TMI. The same model was implied in a number of studies of
TIW, including Wentz et al. \[2000\] who tested the model with the
boundary parameterization of Liu et al. \[1979\], and Hashizume et al.
\[2002\] who related boundary layer structure to pressure gradient using
rawindsonde soundings.

![](media/image5.png){width="5.159842519685039in"
height="3.6547244094488187in"}Similar relation between SST and surface
winds appears to be much more prevalent. It was observed in the cold
patches left behind typhoon passages \[Lin et al., 2002b\] and even over
Gulf Stream rings \[Park and Cornillon, 2002\]. The vertical mixing
mechanism appears to be applicable over a broad spectrum of temporal and
spatial scales over different regions. Xie et al. \[2002\] demonstrate
similar relation in the Asian marginal seas. Fig. 3a shows a strong warm
tongue stretching from Cheju Island of South Korea, through the Yellow
Sea to the Bohai Bay. It is separated from a similar warm tongue in East
China Sea to the south by a cold tongue. The warm and cold tongues are
associated with high and low winds. Fig. 3b shows that the warm tongues
lie over deep channels. Under intense winter cooling, deep-water cools
much slower because of thermal inertia, than shallow water, and hence
stay warmer. Vertical mixing is a strong determining factor. Satellite
data revealed ocean-atmosphere interaction trigger by submerged ocean
topography during the winter cooling.

![](media/image6.png){width="5.94015748031496in"
height="4.455511811023622in"}

# Intradecadal Weakening of Tradewinds and Ekman Heat Transport

Ocean plays a major role in moderation of the world's climate through
the meridional transportation of heat; a major portion of which, the
Ekman heat flux (EHF), is driven by wind. El Nino (interannual anomalous
warming in the equatorial Pacific) is believed to be caused by the
collapse of the trade-winds. The trade-wind characteristics associated
with El Nino have been largely described in terms of the zonal wind
component near the equator measured at moored buoys and islands. The
trade-wind system as a whole, which cover a large area in the Pacific,
in relation to El Nino, has not been well-monitored

The long-term variation of the trade-wind system was examined by Liu and
Tang\[2001\]. Fig. 4 shows an example of significant decreasing trend
between the end of 1996 and the beginning of 1997, In this case, all
three measurements, ERS scatterometer, the microwave radiometers on
Special Sensor Microwave Imagers, and moorings, agree in the central
tropical pacific. In somepart of the oceans, however, wind speed
measurements by ERS and SSM/I do not show the same trend and there is no
buoy measurements to compared. Using ERS scattereometer winds, Sato et
al. \[2002\] show weakening of the magnitude of Ekman heat transport
over global oceans during the same period of time, which would have
significant climate impact.

## Acknowledgments

This study was performed at the Jet Propulsion Laboratory, California
Institute of Technology, under contract with the National Aeronautics
and Space Administration (NASA). It was jointly supported by the
Physical Oceanography Program, the TRMM Project, and QuikSCAT Project of
NASA. Wenqing Tang, Xiaosu Xie, Olga Sato, I-I Lin, and Shangping Xie
contribute to various components of the study.

## References

Bourras, D., L. Eymard, and W.T. Liu, 2002: A neural network to extimate
the latent heat flux over oceans from satellite observations. *Int. J.
Remote Sensing*, **23**, 2405-2423**.**

Bourras, D., L Eymard, W.T. Liu, H. Dupuis, 2002: Instantaneous near
surface air temperature and sensible heat fluxes fields during the
SEMAPHORE experiment using satellite data. *J. Appl. Meteor*, **41**,
241-252.

Fu, L.-L.2001: Ocean circulatinand variability from lsatellite
altimetry. *Ocean Circulation and Climate*. G. Siedler. J. Church, and
J. Gould (eds), **Ch. 3.3**, 141-172, Academic Press, New York.

Hashizume, H., S.-P.Xie, M. Fujiwara, M. Shiotani, T. Watanabe, Y.
Tanimoto, W.T. Liu, K. Takeuchi, 2002: Direct observations of
atmospheric boundary layer response to slow SST variations over the
eastern equatorial Pacific, *J. Climate*, in press.

Jones, C. P. Peterson, C. Gautier, and W.T. Liu, 2001: Satellite
observations of specific humidity and air temperature over the tropical
ocedans. *Remote Sensing of Environment,* submitted

Kummerow, C., W. Barnes, T. Kozu, J. Shiue, and J. Simpson, 1998: The
tropical rainfall measuring mission (TRMM) sensor package. *J. Atmos.
Oceanic Tech.,* **15**, 809-817.

Lin, I-I, W.T. Liu, C.-C. Wu, G. Wong, C. Hu, Z. Chen, W.-D. Liang, and
K.-K. Liu, 2002a: The fertilization of the sea by a tropical cyclone.
*Nature*, submitted.

Lin, I-I, W.T. Liu, C.-C. Wu, J.C. Chiang, and C.-H. Sui, 2002b:
Space-based observations of modulation of boundary layer winds by
typhoon-induced upper ocean cooling. *Geophys. Res Lett*., in press.

Liu, W.T., 2002: Progress in scatterometer application, *J. Oceanogr*.,
**58**, 121-136.

Liu, W.T., and K.B. Katsaros, 2001: Air-Sea Flux from Satellite Data.
*Ocean Circulation and Climate*. G. Siedler. J. Church, and J. Gould
(eds), **Ch. 3.4**, 173-179, Academic Press, New York.

Liu, W.T. and W. Tang, 2001: Monitoring ocean surface wind vector from
space. *Eos, Trans. AGU*, **82**, F309.

Liu, W.T. and X. Xie, 2002: Double Intertropical Convergence Zoens -- a
new look using scatterometer. *Geophys. Res. Lett*., in press.

Liu, W.T., K.B. Katsaros, and J.A. Businger, 1979: Bulk parameterization
of air-sea exchanges of heat and water vapor including the molecular
constraints at the interface. *J. Atmos. Sci.*, **36**, 1722-1735.

Liu, W.T., X. Xie, P.S. Polito, S. Xie, and H. Hashizume, 2000:
Atmosphere manifestation of tropical instability waves observed by
QuikSCAT and Tropical Rain Measuring Missions. *Geophys. Res. Lett*.,
**27**, 2545-2548.

Park, K-A., and P. Cornillon, 2002: Stability-induced modification of
sea surface winds over Gulf Stream rings. *Geophys. Res. Lett.*, in
press (2002GL014236)

Sato, O., P.S. Polito, and W.T. Liu, 2002: Interannual and intra-decadal
variability in the Ekman heat flux from scatterometer winds. *Geophys.
Res. Lett*., in press.

Wentz, F.J., C. Gentemann, D. Smith, D. Chelton, 2000: Satellite
measurements of sea surface temperature through clouds. *Science*,
**288**, 847-850.

Xie, S.-P., M. Ishiwatari, H. Hashizume, and K. Takeuchi, 1998: Coupled
ocean-atmosphere waves on the equatorial front. *Geophys. Res. Lett.*,
25, 3863-3866.

Xie, S.-P., J. Hafner, Y. Tanimoto, W.T. Liu, H. Tokinaga, and H. Xu,
2002: Bathymetric effect on the winter climate of the Yellow and East
China Sea*., Geophys. Res. Lett.*, submitted.