Enhanced Energy Dissipation in the Equatorial Pycnocline in a Simple Model of a Tropical Ocean

Numerical experiments show that in a zonally symmetric model of a tropical ocean forced by transient wind stress both inertia-gravity wave (IGW) activity and the energy dissipation rate have a pronounced maximum in the pycnocline close to the equator regardless of the latitudinal distribution of the energy input into the ocean's mixed layer by the wind. We show that this equatorial enhancement is due to a combination of three factors: a stronger superinertial component of the wind forcing close to the equator, wave action convergence at turning latitudes for various equatorially trapped waves, and nonlinear wave-wave interactions between equatorially trapped waves. Another possible mechanism is wave-breaking during refraction of IGWs at the top of the pycnocline. We show that the latter mechanism can operate at any latitude, but is limited in its capacity to amplify the Froude number associated with propagating IGW packets and requires short (shorter than the local inertial period) energetic wind bursts to produce enhanced mixing.