Modeling of 33-day Barotropic Rossby Wave Radiation in the North Pacific

Analyses of satellite altimetry provide compelling evidence that Tropical Instability Waves (TIWs) radiate barotropic Rossby waves into the North Pacific. The ultimate fate of these waves is unclear, as are the processes that produce the observed sea surface height (SSH) patterns. To shed light on these issues, we compare the output of two simple barotropic models of the North Pacific: one with a flat bottom, and the other with realistic bathymetry. Each is forced at a 10N southern boundary by a pattern of normal velocity that is consistent with the periodicity, westward phase propagation and longitudinal modulation of amplitude typical of TIWs.

We show that patterns of SSH variability comparable to those seen in the observations derive from two effects. The longitudinal modulation of TIW amplitude produces a broad wavenumber spectrum in the forcing. The consequent wavenumber interference results in parallel ridges of SSH variability that are clear in the flat bottom model, and are also evident in the topographic model. Topographic refraction in the latter model further increases the patchiness of the SSH variability.

Although both models use the same bottom and horizontal friction parameterizations, the waves in the flat bottom model reach the northern boundary, reflect, and return to the southern boundary, while those in the topographic model are essentially attenuated before they reach the northern boundary. This is true for a wide range of friction coefficients, suggesting that topographic trapping provides the dominant sink for these waves.