120-day Equatorial Forcing

We are using a model of the equatorial Pacific with realistic boundaries to study the interaction of wind forced waves and those reflected off the boudaries.

As a first step, the model was run with idealized wind forcing and three different basin configurations. In all cases, the model basin is from 70S to 35N, and includes representations of the Indian and Pacific Oceans. The grid spacing (C-grid) is 1 degree in longitude by 0.5 degrees in latitude, and it extends from 70S to 35N (closed poleward of these two), and from 20E to 54W, and is cyclic in the southern extreme (basically flow that exits the model Pacific ocean to the east in the high latitudes enters the model Indian Ocean at the same lat). The model is reduced-gravity, 1.5 layer (one active layer). The initial upper layer thickness (ULT) is 300m in each case, and the value for g' is 0.0229 m/s2, giving a value of 2.62 m/s for c. The applied wind stress is purely zonal, and is Gaussian, centered on the equator at 150E. The wind stress has a simple 120-day period (60 days easterly followed by 60 days westerly). The three basin configurations include a realistic run (includes land in the western Pacific; CLOSE), a run with an open western boundary (all land from 20S to 20N in the western Pacific is treated as ocean; OPEN), and a run with a flat eastern boundary (BOX). In each experiment the model was run for 5 years, starting from rest.

I have made the following links to animations of the model ULT from each experiment:

For an animation of the box boundary (no western boundary, flat eastern boundary), click BOX

For an animation of the realistic eastern boundary but open western boundary, click OPEN

For an animation of the realistic boundary, click CLOSE

In each animation, an upper panel shows the wind stress (purely zonal) and the middle panel is upper layer thickness variations (from the initial value). The upper layer thickness variations are sampled along the equator (yellow line) and shown in the lowest panel; ULT was also sampled along 150E, 180 and 140W (green, blue, magenta lines) and are shown in the right panel. Each frame is a 3-day snap-shot from the model output.

To compare the effects of the leaky western boundary, I have made an animation of the OPEN and CLOSE experiments, along with their difference; click CLOSE-OPEN to view this. The figures below show the model ULT variations from the CLOSE experiment and from the OPEN experiment (shading) for the last two years of model integration. The lower panel is the ULT along the equator (Figure 1) and along 4N (Figure 2).

Figure 1. Model ULT variations along the equator from the fourth and fifth year of the model integrations are shown. The shading in the upper panel is the ULT from the experiment with an open western boundary (OPEN), and the contours are the ULT variations from the experiment with a realistic western boundary. The lower panel shows the ULT from each experiment at day 576 (24 September year 5, shown with the horizontal line in the upper panel).

Figure 2. Model ULT variations along 4N from the fourth and fifth year of the model integrations are shown. The shading in the upper panel is the ULT from the experiment with an open western boundary (OPEN), and the contours are the ULT variations from the experiment with a realistic western boundary. The lower panel shows the ULT from each experiment at day 576 (24 September year 5, shown with the horizontal line in the upper panel).