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Seven years of stable and unstable manifolds in the northeastern equatorial Pacific in OFES QUICKSCAT run

The finite-size Lyapunov exponents (FSLE) were computed for every 6 days for the northeastern equatorial Pacific using the OFES QUICKSCAT 3-day sea surface height (SSH) output. The integration was over 30 days, the initial distance is the initial model grid, 0.01° and the final distance is 1°.

The 7-year average for the stable and unstable manifolds are shown in Fig. 1. It shows that the Hawaiian Islands are embedded in a zonally-elongated tongue of high values of FSLE, higher than north of 30°N and south of 15°N. The tongue meridional width increases from east to west, starting near 110°W and 18°N; its southern limit stays remarkably at the same latitude near 18°while its northern limit moves poleward to the west, reaching 28°N near 170°W. The tongue is separated by regions of low FSLE from the Northern American continent to the east and the northern edge of the North Equatorial Countercurrent (NECC) to the south. The FSLE stay heterogeneous within regions A and B (Fig. 1a), and tend to be stronger west of 150°W. Unfortunately, it does not seem that this region of relatively high level of FSLE corresponds to a region of elevated chlorophyll (Fig. 2).

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Figure 1: 7-year averaged (a) stable and (b) unstable manifolds during 2000-2006 in the OFES QUICKSCAT model output.

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Figure 2: Jan. 1, 2008-Aug. 21, 2009 averaged chlorophyll from MODIS.

The time series of the FSLE averaged out over two regions (A: west of the Hawaiian Archipelago and B: East of the Hawaiian Archipelago, defined in Fig. 1a) are shown in Fig. 3. The region-averaged FSLE is dominated mostly by annual and interannual variability, with some secondary monthly variability. In particular, a clear annual cycle emerges with a tendency to high FSLE in spring, especially east of the Hawaiian Archipelago. I do not know the reason for this maximum. It would also be interesting to know if a similar annual cycle exists with respect to mesoscale eddies.

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Figure 3: 7-year time series of (a) stable and (b) unstable manifolds in the OFES QUICKSCAT model output: (a) west of the Hawaiian Archipelago (170°W-150°W; 15°N-30°N) and (b) East of the Hawaiian Archipelago (150°W-130°W; 15°N-30°N).