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 (FRSGC)
International Pacfic Research Center
Dr.Masami Nonaka
Frontier Research System for Global Change (FRSGC)
Climate Variations Research Program
Dr.Youichi Tanimoto

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.


Fig.1 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).
Mechanism
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 of IPRC.)
Frontier Newsletter/No.24
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