A New Method for Tracking Ocean Dynamics

T he Kuroshio Current transports large amounts of warm and salty water from the tropics northward, whereas the Oyashio Current transports cold and fresh water from the Bering Sea and the Sea of Okhotsk southward. Where they meet in the western North Pacific, they form the Kuroshio Extension and the Oyashio Front, a region of very complicated currents and eddies. Large decadal variations that strongly affect Pacific climate have been found in the area. What happens in these currents is, therefore, of particular interest for climate study and climate prediction. It is difficult in this region to chart in any detail mesoscale features that evolve over several weeks, and information about them has been sparse. This lack of observations has been an obstacle to gaining an understanding of how such features might determine or affect Pacific Ocean climate. Mitsudera at the IPRC have developed a method that makes the detection and tracking of significant features easier, not only for this region, but also for the rest of the world's oceans. Figure 4 illustrates the kinds of information that can be obtained by applying their method, showing the evolu-tions of mean transport velocity (contours) and temperature anomalies (color) in the Kuroshio Extension at a depth of 236 m, every nine days from July 20 to September 21, 1997. The sharp temperature difference between the warm Kuroshio (red shades) and the subarctic ocean (blue shades) is clearly visible in the figure and as large as 12˚C in some places. The sequence of eight panels shows intense eddy activity. A cold meander (yellow shades) at the Kuroshio Extension front is present in the top, left panel at 150–152˚E and 32–33˚N. This meander turns into a mid-sized cold-core eddy as it moves westward at a speed of 4 cm/s. Anomalously warm water reaches the latitude of 35˚N by the end of August and interacts with the cold eddy. The warm anomaly detaches itself in the second half of September at 34˚N, 151˚E, forming an eddy with a warm core. Both the temperature and streamfunction fields show this event. The detachment of the eddy from the western warm-water tongue was possibly triggered by the cold core anomaly further south at 30–32˚N, 147–148˚E. Qualitative analyses of the flow dynamics beyond those shown above reveal a current that sometimes exceeds 1.2 m/s (nearly 3 miles per hour) and that advects density anomalies. These results were obtained by …