The transient response of the North Atlantic: Some model studies

Abstract

Four numerical experiments have been designed to clarify the role of stratification and topography on the transient response of the ocean to a change in wind forcing. The geometry and topography appropriate to the North Atlantic between the equator and 50°N are used to make the study more appropriate to a real ocean. In all four experiments, zonally symmetric wind stresses are ‘switched on’ at the upper surface of a resting model ocean. Two short experiments, 1 and 2, with a duration of 100 days, are first discussed. These are for a homogeneous ocean with and without topography. The response in the flat‐bottomed case can be described either in terms of planetary waves or basin modes, but when topography is present, no obvious wave propagation was identified. Higher‐frequency basin modes are detectable, but their amplitude is much lower than that in the flat‐bottomed case. They are damped out on a time scale of ∼50 days. Two longer experiments, 3 and 4, are then analyzed. These are the analogs of 1 and 2, but stratification was included. The introduction of stratification for the ocean with topography leads to a new, longer time scale, not just for the baroclinic modes, but also for the barotropic. Despite the presence of topography, modal analysis was found useful in analyzing the results. Propagation effects are analyzed, both on the moderately fast time scale of internal Kelvin waves and on the slow time scale of internal planetary waves. Kelvin waves are apparent along the equator, the northern boundary, and on the eastern coast in the Gulf of Guinea from the equator to 20°N. They are not clearly visible anywhere on the west coast. Planetary waves can be detected in the interior both in the presence and absence of topography. When topography is present without stratification, the transport of the Gulf Stream is reduced from 30 to 14 million tons per second. This is a well‐known result. With stratification there is no significant difference in transport between the case with or without topography.