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Ocean Model Studies of Upper-Ocean Variability at 0°N, 160°W during the 1982–1983 ENSO: Local and Remotely Forced Response

D.E. Harrison

NOAA, Pacific Marine Environmental Laboratory, 7600 Sand Point Way NE, Seattle, WA 98115

A.P. Craig

School of Oceanography, University of Washington, Seattle, WA 98195

Journal of Physical Oceanography, 23(3), 426-451 (1993)
Copyright ©1993 American Meteorological Society. Further electronic distribution is not allowed.

3. Local forcing

The effect of local forcing is examined via a model experiment (LOCAL hereafter) that is identical to the SADLER hindcast experiment in every way except the wind forcing is restricted to the vicinity of 160°W as follows: the stresses west of 180° and east of 130°W are maintained at their January 1982 values throughout the two-year period; between 170°W and 180° and between 150°W and 130°W, a linear interpolation was carried out. This wind stress field is shown in Fig. 5. The zonal winds at 160°W (see Fig. 4) in the LOCAL experiment are substantial easterlies until September 1982. They begin decreasing in August and become westerly in late October 1982, reaching their maximum westerly value in December. They increase to weak easterlies in January 1983 and return to something near their climatological easterly value in April 1983. At 170°W, the LOCAL wind field also has a relatively strong period of westerly wind stress from October 1982 to December 1982. The meridional wind stress is weakly southerly before November 1982, becomes northerly in November, and increases to a substantial northerly value by January 1983. A steady decrease begins in May 1983 and continues until there is little meridional stress by May.

Figure 5. Monthly mean zonal and meridional wind stress along the equator, 1982-1983 for the LOCAL experiment. The contour interval is 0.02 N m and 0.01 N m, respectively.

Figures 6 and 7 show the zonal velocity and temperature results from LOCAL and the difference (SADLER minus LOCAL) on the same scales as Figs. 1 and 2. A striking aspect of Fig. 6a is that many aspects of the oceanic response between June 1982 and March 1983 from Fig. 1 are present: the EUC decelerates (although much less rapidly than before) in the fall of 1982; there is significant near-surface acceleration in November and December 1982; somewhat weaker near-surface deceleration occurs during the first few months of 1983; and then conditions start returning toward a more normal state during the spring of 1983. Although there are quantitative differences almost as large as the observed current changes themselves (Fig. 6), so that the LOCAL hindcast has at best "marginal skill" in zonal velocity in the sense discussed by Harrison et al. (1989), the qualitative behavior of the zonal velocity changes can be accounted for by the local wind stress changes. Figure 7 shows that the same statement cannot be made for the temperature field. In LOCAL, the deepening and intensification of the thermocline and its abrupt shoaling are greatly reduced compared with SADLER. Point differences exceed 4°C, both warm and cold; the LOCAL hindcast has "no skill" in temperature in the sense discussed by Harrison et al. (1989).

Figure 6. Zonal velocity at (0°, 160°W) (a) from the LOCAL experiment, (b) the difference (SADLER - LOCAL). The contour interval is 20 cm s in both plots.

Figure 7. Temperature at (0°, 160°W) for the LOCAL experiment, as for Fig. 6. The contour interval is 2° and 1°C, respectively.

Two additional local experiments were carried out (LOCAL + EAST, LOCAL + EAST-TAUY). Again, the only changes from the SADLER and LOCAL experiments are modifications to the wind field. In the LOCAL + EAST experiment, the stresses west of 170°W are as in LOCAL, and the SADLER stresses are used everywhere east of 170°W. Comparison of this experiment with LOCAL will let us explore the importance of remote forcing from the east (i.e., propagation of forced Rossby waves). In the LOCAL + EAST-TAUY experiment, the zonal stress is identical to that in LOCAL + EAST, but the meridional stress is zero everywhere. This experiment will offer some perspective on the importance of meridional wind stress forcing at and east of 160°W, but does not address the role of remote meridional wind forcing west of the date line. The LOCAL + EAST winds are shown in Fig. 8.

Figure 8. Monthly mean zonal and meridional wind stress along the equator, 1982-1983 for the LOCAL + EAST experiment. The contour interval is 0.02 N m in both plots.

Zonal velocity and temperature time series for LOCAL + EAST are shown in Figs. 9 and 10. The differences between these results and the LOCAL results (Figs. 6 and 7) are not large, particularly when compared with the differences between either of these experiments and SADLER. The zonal flow rms difference is about 5 cm s, and the largest point difference is about 40 cm s at 100 m in September-October 1982 when in LOCAL + EAST the EUC actually comes to rest, while it diminishes simply to a weak eastward flow in LOCAL. The differences with SADLER (Fig. 9) are very similar to those of Fig. 6.

Figure 9. Zonal velocity at (0°, 160°W) (a) from the LOCAL + EAST experiment, (b) the difference [SADLER - (LOCAL + EAST)] experiment. The contour interval is 20 cm s in both plots.

Figure 10. Temperature at (0°, 160°W) for the LOCAL and EAST experiment as for Fig. 9. The contour interval is 2° and 1°C, respectively.

Zonal velocity and temperature time series for LOCAL + EAST-TAUY, and for the differences [SADLER - (LOCAL + EAST-TAUY)] are shown in Figs. 11 and 12. The most significant difference is again in the deceleration of the EUC in September and October 1982. In LOCAL + EAST-TAUY, the EUC does not come to rest as it does in LOCAL + EAST. It maintains a flow of about 20 cm s, similar in form to that of the LOCAL experiment. The thermal changes and other zonal current changes are almost the same as those in LOCAL + EAST. Evidently, meridional wind stress changes east of the date line play little role in forcing the variability at (0°, 160°W) during this period.

Figure 11. Zonal velocity at (0°, 160°W) (a) from the LOCAL + EAST-TAUY experiment, (b) the difference [SADLER - (LOCAL + EAST-TAUY)]. The contour interval is 20 cm s in both plots.

Figure 12. Temperature at (0°, 160°W) for the LOCAL + EAST-TAUY experiment as for Fig. 11. The contour interval is 2° and 1°C, respectively.


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