The role of barotropic and baroclinic waves in oceanic teleconnections

Abstract

Buoyancy forcing in the high latitudes and variability in North Atlantic Deep Water formation leads to weakening of the Meridional Overturning Circulation (MOC). Perturbations at the high latitudes of the North Atlantic are adjusted via coastally trapped waves, equatorial Kelvin waves and westward propagating Rossby waves. This thesis works towards understanding the importance of oceanic teleconnections in transmitting variability through wave mechanisms. The effect of mesh resolution on off-shore decay characteristics and phase speed of baroclinic Kelvin waves is examined analytically and through a series of numerical simulations performed with a reduced-gravity Finite Element Shallow Water Model. The mesh resolution is refined down to 5 km at the coast and 20 km at the equator to resolve first mode of the baroclinic Kelvin waves. A parameter delta is defined to be the ratio of horizontal mesh resolution and Rossby radius. A stable off-shore decay structure is found for any delta for coastal Kelvin waves in a finite element shallow water model with unstructured triangular P1-P1 mesh. For delta << 1, the off-shore decay structure of a Kelvin wave resembles the off-shore decay structure of a classical Kelvin wave. For delta > 1, the off-shore decay structure broadens with increasing delta, however, the overall adjustment via westward propagating Rossby wave is not strongly affected. The phase speed of the Kelvin wave is independent of delta on uniform meshes if consistent mass matrices are used. The numerical experiments also show that the Kelvin wave characteristics are hardly disturbed for a reasonable range of lateral viscosity. Results show that the finite element method with unstructured triangular grid is a convenient tool to represent wave dynamics in an ocean model. The time scale and amplitude of response to variability in MOC due to buoyancy forcing at high latitudes is analysed. Barotropic and baroclinic dynamics operating at different frequency ranges is also assessed using the Barotropic-Baroclinic Interaction (BarBI) model. It is found that there is an overestimation of the amplitude and time scale of response in experiments conducted on reduced gravity setups compared to the amplitude and time scales on BarBI which includes the interaction of waves with topography and background mean circulation. There is a significant reduction in amplitude of response, and increase in the time scales upon the interaction of waves with topography and a mean background circulation. It is also found that the response is highly dependent on the frequency of forcing. Barotropic dynamics dominate the high frequency regime while baroclinic dynamics dominate the low frequency regime. The response through barotropic dynamics is limited to the basin where the forcing was applied. Low frequency baroclinic adjustment is mainly responsible for propagation of variability into adjacent basins. Furthermore, wave adjustment due to low frequency atmospheric variability over the North Atlantic, North Atlantic Oscillation, and over the South Atlantic, Southern Oscillation on ocean circulation is analysed via a series of numerical simulations. It is found that the magnitude of impact is significant in the hemisphere where the perturbation is applied and weakens significantly before reaching the opposite hemisphere, or another ocean basin.