Date of Award

Winter 1993

Document Type


Degree Name

Doctor of Philosophy (PhD)


Ocean/Earth/Atmos Sciences



Committee Director

John M. Klinck

Committee Member

Larry P. Atkinson

Committee Member

Gabriel T. Csanady

Committee Member

John E. Kroll


The WOCE Community Modelling Effort (CME) general circulation model of the north Atlantic was used to investigate the behavior, nature and dynamics of 50-day oscillations seen in the meridional component of velocity between 35° and 55°W and between 5° and 11°N. Validation studies showed that the model reproduced the surface circulation in this area with a reasonable degree of accuracy, in particular, the characteristic seasonal variability. From June to December, the North Brazil Current (NBC) retroflects to form the western arm of the North Equatorial Countercurrent (NECC). Associated with the NECC is a standing meander pattern which extends from the retroflection region (50°W, 7°N) into the interior of the basin (35$\sp\circ$W, 5$\sp\circ$N), and has a wavelength of about 800 km. This meander pattern starts to break down in December and concurrently, oscillations of the meridional component of velocity appear with a period of about 50 days. They appear first near 35°W, and are advected westward. They have a westward phase velocity of 0.1 m s-1, wavelength of about 500 km and a very slow eastward group velocity. Their period, phase speed and wavelength agree with observations (Johns et al. 1990).

Calculation of the leading balance of terms from the full vorticity equation following a modal decomposition in the propagation region, showed that the oscillations were first and second mode baroclinic Rossby waves. Repetition of the vorticity analysis during the period of the retroflection revealed the NECC meanders also to be first and second mode baroclinic Rossby waves; the same as the 50-day oscillations. These findings, together with the time evolution of the individual flow components over an annual cycle, suggested that the 50-day oscillations were the westward advected residue of the NECC meander pattern that is released as the NECC slows in fall. The retroflecting NBC produces Rossby waves with very slow eastward group velocity that are advected eastward until they reach 35°W, where they dissipate. A standing wave pattern is established for several months, while the NECC is active. Once it slows, the waves retreat westward and disappear totally by May. Wind forcing was not considered to be responsible for the oscillations in this model.



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