Document Type


Publication Date




Publication Title

Journal of Physical Oceanography








The WOCE Community Modeling 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 5 degrees and 11 degrees N, 35 degrees and 55 degrees W. Oscillations of the meridional component of velocity, with a period of about 50 days, appear as the seasonal meander pattern of the North Equatorial Countercurrent starts to break down in December. They appear first near 35 degrees W and are advected westward. They have a westward phase velocity of about 0.1 m s(-1), wavelength of about 600 km, and a very slow eastward group velocity. Their period, phase speed, and wavelength agree with recent observations. Calculation of the leading 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 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 retroflecting North Brazil Current produces Rossby waves with very slow eastward group velocity that are advected eastward by the NECC until they reach 35 degrees W, where they dissipate. A standing wave pattern is established for several months, while the NECC is active. Once it slows, the waves are advected westward and disappear totally by May. Neither wind forcing nor barotropic instability were considered to be responsible for the oscillations in the model.

Original Publication Citation

McClean, J.L., & Klinck, J.M. (1995). Description and vorticity analysis of 50-day oscillations in the western tropical region of the CME model. Journal of Physical Oceanography, 25(11), 2498-2517. doi: 10.1175/1520-0485(1995);2