Cyclogeostrophic balance in the Mozambique Channel

Abstract

Three methods are proposed for the inclusion of inertia when deriving currents from sea surface height (SSH) in the Mozambique Channel: gradient wind, perturbation expansion, and an iterative method. They are tested in a model and applied to satellite altimetry. For an eddy of 25 cm amplitude and 100 km radius, typical of Mozambique Channel rings at 18°S, the error made with geostrophy is 40% for the anticyclones and 20% for the cyclones. Inertia could reach one third of the pressure gradient. Geostrophy underestimates subsurface currents by up to 50 cm s-1, resulting in errors of 30-40%. The iterative method results in errors of <5% for the most part of the structure. The error RMS in velocities based on 8 years of model SSH is in excess of 30 cm s-1 for geostrophy and reduces to about 10 cm s-1 for the gradient wind and iterative methods. The perturbation method is less accurate. Applied to satellite altimetry, the addition of inertia results in a significant increase in velocities for the anticyclones and a decrease for the cyclones. It induces a velocity increase of >50% in Mozambique Channel rings. Geostrophic EKE reaches 1400 cm2 s-2, while it attains 1800 cm 2 s-2 when inertia is added. Applied to the Gulf Stream, these methods confirm the hypothesis of Maximenko and Niiler [2006] that centrifugal accelerations should be the main cause for the difference observed between geostrophic and drifter EKE. This methodology should result in a net improvement for operational surface ocean currents. Key Points The centrifugal force is important in Mozambique Channel rings We propose to include inertia when deriving currents from sea surface height Inclusion of inertia results in a net improvement when computing ocean currents © 2014. American Geophysical Union. All Rights Reserved.