How Purely Wind-Driven Long Planetary Geostrophic Waves May Be Energized in the Western Part of Ocean Subtropical Gyres

Abstract

Abstract Satellite observations and idealized numerical studies reveal intensification of long-period (on the order of one cycle per year) waves in the western part of ocean basins. The authors explore the idea that the intensification is associated with the spatial growth of purely time periodic, but baroclinically unstable, motions. The framework is a simple idealized 2½-layer model in which only the upper layer is directly forced by the wind, a setting similar to the shadow zone of the Luyten–Pedlosky–Stommel (LPS) model. The upper two layers participate in the wave motion, which is driven by a large-scale wind stress fluctuating with the annual period, representing the seasonal cycle. Although possibly unstable solutions exist everywhere in the subtropical gyre on account of the nonzero meridional background flow, they are not seen in the eastern part of the basin in satellite observations nor are they excited there by model gyre-scale annual-period winds. Instead, energy injected into the model ocean at a fixed frequency and with zonal and meridional wavenumbers, such that the resulting flow perturbation is locally stable, refracts westward as it propagates through the spatially varying background flow without change of frequency and reaches distant regions where the spatial wavenumber becomes complex so that spatial growth occurs. This process results in spatially growing solutions of annual or near-annual frequency only in the southwestern part of the model subtropical gyre, thus explaining why the intensification is preferentially manifested in the southwestern subtropical gyre in published numerical model results. The paper concludes with a discussion of relevant satellite and in situ observations.