Driving Forces and Flow Mechanisms of the Atlantic Ocean Currents

Kari Hänninen

doi:10.13189/eer.2020.080101

Abstract

The aim of the study is to clarify the driving forces and flow mechanisms of ocean currents. The primary driving forces of the surface currents are the thrust by the trade winds and westerlies. For the undercurrents, the driving force is the thrust of descending salty water (DSW) formed in the Arctic and Southern Oceans by brine ejection from ice. The annual production of DSW in the Arctic Ocean is 21.7 Sv and in the Southern Ocean 26 Sv. The annual average downward thrust created for the outflowing undercurrents in both oceans is 44,000 N/m2. Most of the DSW outflow from the Arctic Ocean occurs via the Labrador Sea towards the coast of northwest Africa as the North Atlantic Undercurrent (NAUC). The direction of the flow is towards the suction at the starting point of the North Equatorial Current. The flow and the fluid dynamics of the NAUC follow Pascal's law and Bernoulli's equation. Upwelling provides an unobstructed passage for the continuous flow of an undercurrent. The water molecules moving along streamlines trade speed for height or for pressure. Due to this exchange, the flow of the NAUC slows and the flow bed widens. Subsequently, it upwells along the 3,500 km of coastal area from A Coruna (NW Spain) to Dakar (NW Africa). In the abyssal oceans, the mixing of water is relatively weak and intermittent, driven by double diffusive convection (DDC). The phenomenon known as salt fingering is typical for the DDC. However, the DDC is vigorous where the temperature difference between warm water and the cold water below is great, like in the confluence of the Brazil vs. Malvinas currents. Then salt fingers which are several hundred meters deep and several hundred kilometres wide are formed. In these salt fingers, the water of the warm current downwells and the water of the cold current upwells. This slows down the horizontal movement of the water molecules in both currents.

Highlights

Warm surface and cold undercurrents neutralize the temperature difference between the oceans and adjacent continental areas
The aims of this meta-study are as follows: (1) apply basic ice physics to calculate the annual amount of descending salty water (DSW) formed by brine ejection from ice in the Arctic/Southern Oceans and the formation of undercurrents thereof, (2) calculate the thrust of the DSW as a driving force of the cold undercurrents, (3) apply the basic laws of hydraulics, Pascal’s law of transmission of fluid pressure and Bernoulli’s equation of fluid dynamics to understand the flow mechanisms of cold undercurrents, (4) clarify the importance of double diffusive convection (DDC) in the generation of the salinity profiles of oceans (5) understand mechanisms in confluences of warm surface vs. cold undercurrents, and (6) demonstrate that upwellings are important indicators of cold undercurrents
Where a is the salt increment which causes the sinking of b (=1 l) of surface water, c is the total amount of salt ejected and d is the annually formed amount of DSW in cubic meters

Summary

Introduction

Warm surface and cold undercurrents neutralize the temperature difference between the oceans and adjacent continental areas. A typical oceanic-atmospheric phenomenon is the desiccation of the nearby continental climate by the upwelling waters of the cold undercurrents. In the equatorial Atlantic Ocean, over a distance of 4,500 km, trade winds create a constant pressure gradient which induces the westward flowing North Equatorial (NEC) and South Equatorial (SEC) currents. Those currents are the backbone of the water circulation in the Atlantic Ocean. In the South Atlantic Ocean, there is a counter-clockwise surface gyre, the southern branch of which is the South Atlantic Current (SAC). These “anticyclonic” circulations dominate the low- and mid-latitude portions of the Atlantic Ocean [1]

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