Figure 6: The changes of the volume transports across section A, B and C with the wind strength. Southward transport is negative. (a) For nonlinear cases. (b) For nonlinear cases.
The sharpest increase is seen between the α=1.25 and the α=1.50 cases. As shown in Fig. 4, from the α=1.50 case, the Mindanao eddy starts to collide with Celebes island. We think this collision is associated with the jump in the transport across section B. This is discussed in section 5.
4. Instantaneous field
Although the forcings are steady, some cases show high frequency eddy activity in the MC with about 40 days period. Figure 7 shows typical process of the eddy detachment (in the α=1.00 case). In (Fig. 7(a)), the Mindanao eddy has already well developed. Then, the eddy is starting to break off (Fig. 7. The center of the eddy penetrates southwestward. The Halmahera eddy lengthens northward. As a result, the anticyclonic eddy is formed in the east of the Mindanao eddy and cyclonic eddy is formed in the further east. As a result, the North Equatorial Countercurrent becomes very wavy. Figure 7(c) shows the eddy has detached from the Mindanao eddy. This cyclonic eddy (Fig. 7(d)) propagates westward. Because of the eddy detachment, the MC itself becomes weak. Returning to Fig. 7(a), the eddy is seen to propagate further westward. The MC again becomes strong. As the cyclonic eddy propagated westward, the anticyclonic eddy is sandwiched between the cyclonic eddy and the Mindanao eddy.
This eddy activity is not seen in the cases where the MC is strong or weak. Figure 8 shows fluctuations of the transports across section B in the 21th year. Oscillation of the solid line corresponding to α=1.00 is manifestation of eddy activity. However, the MC does not show eddy activity in the α=0.25 case (when the MC is weak) and α=2.00 case (when the MC is strong). Compared with the nonlinear cases, linear cases never show eddy activity under any wind strength (not shown here). Nonlinearity is essential to this oscillation.
5. Mechanism of eddy detachment
The importance of nonlinearity indicates significance of relative vorticity in vorticity balance. Thus, we will explain the mechanism of the eddy detachment in terms of potential vorticity conservation. The same technique was applied to eddy shedding phenomenon of the Loop current in the Gulf of Mexico by Hurlburt and Thompson (1980, 1982)
