On long baroclinic Rossby waves in the tropical North Atlantic observed from profiling floats

摘要

Argo float data (subsurface tracks and temperature profiles collected from March 2004 through May 2005) are used to detect signatures of long Rossby waves in the velocity of the currents at 1000-m depth and temperature, between the ocean surface and 950 m, in the zonal band of 4°N–24°N in the tropical North Atlantic. Different types of long Rossby waves (with the characteristic scales between 1000 and 2500 km) are identified in the western [west of the Mid-Atlantic Ridge (MAR)] and eastern [east of the MAR] subbasins. Along-shore wind fluctuations and an equatorially forced coastal Kelvin wave were found to be responsible for the excitation of annual- and semiannual-propagating Rossby waves in the eastern subbasin. These waves are transmitted along a waveguide formed by the African shelf and the MAR. The speed of their propagation varies in magnitude and direction because of bottom topography and irregularity of the coastline. Unstable standing Rossby waves with annual and semiannual periods are shown in both the subbasins. All unstable waves, decaying, radiate shorter free Rossby waves propagating both westward and northwestward, with speeds of up to 10 cm/s. The standing Rossby waves are probably excited by the wind-driven Ekman pumping alone or in combination with linear and nonlinear resonance mechanisms. The additional analysis of subsurface float tracks from May 2005 through May 2006 supports the obtained results.