|Special Topic: Kuroshio
|Satellite Observations Reveal
Kuroshio's Influence on Winds
||Dr. Masami Nonaka and Dr. Shang-Ping Xie, both from International
Pacific Research Center, and Dr. Tanimoto of Climate Variations
Research Program have discovered that the sea surface temperature
distribution of Kuroshio current around southern coast of Japan
has strong effect on the sea wind distribution from the satellite
data analysis. Including this effect in climate models is expected
to increase their accuracy. Drs. Nonaka and Tanimoto explain
this research and their current effort to extend it.
|Frontier Research System for Global Change
International Pacfic Research Center
Frontier Research System for Global Change (FRSGC)
Climate Variations Research Program
The Traditional View of Air-sea Interaction
Ocean plays a key role in Earth's climate, transporting huge amounts
of heat away from the equator in its poleward currents. Climate
researchers know now much about how sea surface temperatures in
the tropics impact the atmosphere and climate. Also, there are many
media coverage on how El Niño, La Niña, or the Indian
Ocean Dipole influence Japan's climate.
In the midlatitudes, large-scale measurements over the ocean show
that wind speeds are generally higher over cooler waters than over
warmer waters, and the typical view among scientists has been that
the midlatitude ocean responds passively to the atmosphere. The
same way that blowing over coffee to cool it down, while it has
no effect back on its breath, high winds were thought to cool the
midlatitude ocean surface without any feedback on the winds.
Discovery of the Kuroshio's Effect on Local Winds
We had a hunch that this view was not the full picture. To see whether
the midlatitude ocean may influence the atmosphere, we analyzed
satellite observations of temperature fronts-sharp horizontal temperature
gradients-on the ocean surface. These fronts occur where currents
transporting tropical warm water poleward meet currents from polar
regions transporting cool water equatorward. Examples are such fronts
where the warm Kuroshio and the cold Oyashio currents meet in the
Northwestern Pacific, where the Gulf Stream meets cold water from
the Arctic, and where the Brazil current off Argentina meets cold
water from Antarctica.
Studying the Kuroshio-Oyashio front, we found something surprising:
Wind speeds are higher over the warm Kuroshio and lower over the
adjacent cool waters. Figure 1 shows Tropical Rain Measuring Mission
(TRMM) satellite observations of the distributions of sea surface
temperatures and wind speeds along the southern coast of Japan and
eastward during 1998 and 2001. In 2001, the Kuroshio took an offshore
path, and temperatures off Tokai region (black arrow in Fig. 1)
and east of Japan (white arrows in Fig. 1) were about 3ºC lower
than in the immediate vicinity. Over these cool waters, winds were
about 1 m/s lower than over the warmer areas. Similarly, when the
Kuroshio meanders around 34ºN, 146ºE in 1998, and cool
water extends southward, wind speeds over the cool area are lower
by 1 to 2 m/s. Analyzing satellite data over the Gulf Stream and
off the coast of Argentina, we found the same relationship: stronger
winds over warm and weaker winds over cool water.
These observational results, showing a different relation between
wind speed and ocean surface temperature to what was previously
assumed, strongly suggest the ocean does in fact make feed back
to the atmosphere. This does not mean that the traditional relationship
is wrong, but rather that in these midlatitude regions, a two-way
relationship between the atmosphere and ocean exists.
||Sea surface temperature and sea surface wind velocity as observed
by the Tropical Rainfall Measuring Mission Satellite (TRMM).
Sea surface temperature (a) and sea surface wind speed (b) averaged
for April to June 1998 (left) and 2001 (right). The warm water
is a signature of the Kuroshio Current from the tropics. The
Kuroshio Current was flowing far off of Tokai region, and there
was cold water near the coast in 2001 ((a) under the arrow).
Winds are stronger over the warm water and weaker over the cold
water ((b) under the arrow).
The following is the proposed mechanism for how ocean produces this
effect on the winds: Because of the drag by the slow-moving ocean
surface, winds at the sea surface are weaker than in the upper sky
so that wind speeds generally increase according to the atmospheric
height (Fig. 2). Over cool water, the air at the sea surface is cooled
and does not readily mix with air aloft (right panel in Fig. 2). In
contrast, over warm water, the air at the surface is warmed, rises,
and mixes with the upper air mass moving at higher wind speeds (left
panel in Fig. 2). This mixing brings the stronger winds down from
aloft, accelerating air speed at the sea surface.
Fig. 2 Sea surface temperature (SST) effect on local winds
|Implications and Future Work
Our analyses of in situ observational data, showing that winds blowing
over the sea surface are strongly influenced by differences in ocean
and air temperatures at the surface, are consistent with the aforementioned
mechanism. In order to verify whether this is indeed the feedback
mechanism, we need to analyze vertical profiles of air temperature
and winds in response to sea surface temperature changes. Satellites,
however, measure the strength of microwaves from the sea surface and
cannot see the winds and temperature aloft. We must, therefore, measure
the vertical profiles directly using radio-sondes sent from ships.
The plan for us is to carry out such observations in this winter 2003/4.
If we can verify our mechanism, the next step is to see whether including
this ocean-to-atmosphere feedback in numerical models improves the
accuracy of predicting climate variability. (with Dr. Gisela Speidel