Midlatitude Baroclinic Rossby Waves in a High-Resolution OGCM Simulation

Abstract

Abstract The effects of background baroclinic zonal flow and bottom pressure decoupling on midlatitude oceanic Rossby wave dynamics using a high-resolution OGCM simulation are investigated. To examine these effects, the phase speed and vertical structure of the simulated wave are compared with each of the different linear Rossby wave solutions obtained for two different circumstances (with or without background flow) and two different boundary conditions (a flat bottom or a bottom pressure decoupling condition). First, a frequency–wavenumber spectrum is examined for depth anomaly of the permanent thermocline (27.0σθ surface) along 32°S. Most of the energy is distributed along the theoretical dispersion curve including the effects of background flow and bottom pressure decoupling. The authors focus on a secondary dominant peak (appearing at a frequency greater than 1 cycle per year) at which the differences between the dispersion curves are large enough to discuss the relation between the spectral peak and the dispersion curves. The phase speed of this peak is nearly 1.5 times larger than that of the standard long-wave theory (flat bottom and no background flow), which is similar to results from previous observational studies. The extended long-wave theory including background flow and bottom pressure decoupling effects overestimates the phase speed. However, taking into account finite wavelength effects, this theory provides a phase speed much closer to that of the secondary dominant peak. The vertical structure corresponding to the wave of the secondary dominant peak extracted by composite analysis is intensified in the surface layer, a result similar to that from the theory including background flow and bottom pressure decoupling effects. The authors also compare the latitudinal distribution of midlatitude phase speed estimated by the frequency–wavenumber spectrum with theoretical results. The theory including background flow, bottom pressure decoupling, and finite wavelength effects reproduces the latitudinal distribution well, suggesting that these effects are important for explaining Rossby wave speed. The dominant factor enhancing the phase speed is bottom pressure decoupling related to rough bottom topography, while north of 30°N the background flow makes a strong contribution to the phase speed enhancement.