Date of Award
Master of Science in Oceanography
D. Randolph Watts
Topographic Rossby Waves (TRWs) have been shown to be strikingly energetic and persistent in the Mid-Atlantic Bight, and a mounting observational evidence suggests that they might couple to the Gulf Stream (GS) in the region off Cape Hatteras, affecting GS meander development processes. This thesis presents a detailed study of TRW properties from a quasi-geostrophic (QG) framework with an aim to address the problem of TRW-Gulf Stream coupling.
First, a quasi-geostrophic theory is employed to find the dispersion properties of TRWs off Cape Hatteras, in the absence of a mean flow. A new tool is developed which allows to determine TRW dispersion and modal structure in a media with an arbitrary vertical stratification and bottom topography sloping in an arbitrary direction. This tool is based on a finite-difference solution to a corresponding 1-D eigenvalue problem in the vertical. The properties of the barotropic and the baroclinic Rossby waves are also recovered by this numerical technique if a bottom with zero slope is specified. A rigid-lid or a free-surface top boundary conditions can be used.
The described numerical tool allowed us to investigate an adjustment of vortical waves to sloping topography. First and foremost, a sloping bottom eliminates the orthogonality of the wave modal structure. The topographic (bottom-intensified) and the baroclinic waves are correlated over sloping bottom. on-zero correlation moments work to enable an interaction between these waves, under other favorable conditions. Secondly, the baroclinic and the barotropic waves adjust differently to sloping topography. The baroclinic waves move one of their nodes to the bottom, and thus, effectively minimize the effect of sloping topography on their dispersion properties. The barotropic wave become bottom-trapped and, thus, fully topographically controlled.
The environment off Cape Hatteras is found to be favorable for 40-day period TRWs. TRWs are most energetic within the deep layer, below the main thermocline, where stratification is the weakest. The baroclinic waves, on the contrary, have maximum velocity perturbations within the main thermocline, where stratification is the strongest. Our analysis confirms that TRWs off Cape Hatteras have a component of the wavenumber in the direction of Gulf Stream path, necessary for TRW-Gulf Stream coupling.
An impact of a mean flow on TRWs is investigated using the QG formulation for vortical waves in a zonal channel. A 2-D eigenvalue problem is solved to determine TRW dispersion and modal structure in the presence of a mean flow. TRW modal structure is suppressed by the QG PV gradient of a mean flow. TRWs adjust to a mean flow so that their dispersion properties are least affected. TRWs are correlated with mean-flow trapped waves and the correlation moments increase for longer waves.
All presented evidence suggests that TRW-Gulf Stream coupling is a dynamically plausible process.
Logoutov, Oleg G., "A STUDY OF TOPOGRAPHIC ROSSBY WAVES IN THE MID-ATLANTIC BIGHT AND THEIR POSSIBLE COUPLING TO GULF STREAM MEANDERS" (2000). Open Access Master's Theses. Paper 2392.