Intra-annual oscillations at Station ALOHA, north of Oahu, Hawaii

Inverted echo sounders have been used to make continuous measurements of dynamic height at a site chosen to be representative of the central Pacific subtropical gyre. These show that the dominant features in dynamic height are fluctuations having peak-to-peak amplitudes of up to 27 dyn cm and timescales of about 100 days. Empirical orthogonal function analyses of temperature and salinity profiles suggest that if the oscillations are due to vertical motion within the water column, the motion is not that of first baroclinic mode, but has much higher vertical wavenumber structure. Zonal wavenumbers associated with the oscillations indicate westward phase propagation, and meridional wavenumbers are close to zero. The dominant periods are too short for freely propagating Rossby waves. Two explanations of these observations are that the waves could be Doppler-shifted by the mean flow, or that the region could be an area of active forcing.