Description and Vorticity Analysis of 50-Day Oscillations in the Western Tropical Region of the CME Model

The WOCB Community Modeling Effon (CME) general circulation model of the No~ Atlantic was used to investigate the behavior, nature, and dynamics of 50-day oscillations seen in the mend1on~ co~nent. of velocity between 5° and I l"N, 35° and 55"W. Oscillations of the meridional component of velocity, with a penod of about 50 days, appear as the seasonal meander pattern of the Nonh Equatorial Countercurrent stans to break down in December. They appear first near 35"W and are advected westward. They have a .westw8;1"d p~ase velocity of aboutO.I ms-•, wavelength of about 600 km, and a very slow eastward group velocity. The1rpenod, phase speed, and wavelength agree with recent observations. Calculation of the leading tenns ~~ 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 vonicity analysis on an undecomposed snapshot during the period of the retroflection revealed the NECC meanders also to be 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 December. The retroftecting North Brazil Current produces Rossby waves with very slow eastward group velocity that are advected eastward by the NECC until th~y ~ch 35"W, where they dissipate. A slanding wave pattern is established for several months, while the NBCC 1s active. Once it slows, the waves sre advected westward and disappear totally by May. Neither wind forcing nor barotropic instability were considered to be responsible for the oscillations in the model. VOLUME 25