System Earth 3b

Oceans I

Misha Velthuis

m.velthuis@uva.nl

Fri 20 Sept 2024

Live ocean

Test 1

Mean scores

Correlation

What makes the ocean move?

Three key forces

Tides

Wind stress

Thermohaline circulation

Tides

Simple model

"On the side of Earth closest to the moon, the inward gravitational force (Fg) is greater than the outward inertial force (Fr); the net resulting force (A) is directed towards the moon, and creates a bulge of water on the side facing the moon. On the side of Earth opposite the moon, the outward inertial force is greater than the inward gravitational force; the net resulting force (C) is directed away from the moon, creating a water bulge directed away from the moon."

Inward = towards the moon. Outwards = away from the moon.

F_g = Gravity pulls the Earth towards the moon: stronger on the side that is closest to the moon.
F_r = Stuff having the tendency to stay put: inertia. The same everywhere on Earth. A pseudo-force acting away from the centre of gravity of the moon.
Effective forces cause two bulges.

From: Webb, P. (2021). Introduction to oceanography. Roger Williams University.

High tide

Watersnoodramp, 1953

Watersnoodramp

Reality is more complex

Wind stress

Four steps

Step 1	Linking the atmosphere to the ocean
Step 2	Ekman transport
Step 3	Geostrophic current
Step 4	Boundary currents

Driving force atmosphere ≠ driving force ocean

Step 1: linking the atmosphere to the ocean

Ocean currents flow at an angle to the wind

Circular patterns: gyres

Gyres: latitudinal redistribution of energy

Step 2: Ekman transport

Ekman spiral: surface flow ≠ net flow

90° relative to wind or surface current?

N.B. confusion:

In text: Ekman transport = net flow 90° to wind direction.
In images: Ekman transport = net flow 90° to surface currents.

Webb (2021, p. 215/393):

"The net movement of the water within the spiral is 90o relative to the wind direction"

Bottom line/ main message:

The Coriolis effect "adds up" with depth, and the net flow of water will be substantially to the right of the surface currents (in the NH (northern hemisphere)).
This bottom line/main message is also more "robust" than defining Ekman transport relative to the wind, since the ocean currents do not always follow the (general) direction of the wind; for instance when currents "hit" a continent and are moved north/south. Ekman transport will then still take place, but relative to the surface current (rather than the wind).

Ekman transport causing coastal upwelling

Step 3: Geostrophic current

Water piles up in center of gyre

Coriolis force = pressure gradient force

Geostrophic current circles around centre of gyre

Sargasso sea

Plastic soup

Plastic soup?

Plastic clouds?

Step 4: boundary currents

West vs east

Asymmetrical flows

Western boundary currents = narrow, deep strong

Eastern boundary currents = wide, shallow, weak

Five main gyres, ten boundary currents

Western intensification 1

Western intensification 2

Boundary currents in real life

Peru current (Humboldt current)

Real time (modelled) flows (oceans and air)

Next session

El Niño Southern Oscillation

Thermohaline circulation