Upper ocean currents, Coriolis force, and Ekman Transport Gaspard-Gustave de Coriolis Walfrid Ekman

Upper ocean currents, Coriolis force, and Ekman Transport In the open ocean mixed layer: vertical structure was the key to biological productivity –mixing, light, stratification, critical depth In coastal regions and the equator, wind-driven horizontal currents can cause upwelling of water with high nutrients leading to sustained production over a season, or longer –Wind stress + Coriolis force give… Ekman currents and upwelling Seasonal input of new nutrients from winter convection

Time scales greater than a day introduce earth’s rotation in the dynamics – Coriolis force

From Lalli and Parson, “Biological Oceanography”

Some physics…

Coriolis force: In a fixed frame of reference the ball travels in a straight line (Newton’s laws) In a rotating frame of reference (on the table, or Earth), the ball appears to turn. In the example, the merry-go-round is turning clockwise and the ball turns toward the left. This is the Southern Hemisphere effect. In the Northern Hemisphere the local rotation is counter-clockwise, and Coriolis force deflects motion to the right. N S

Handout-Coastal-Upwelling.pdf

Figure 9.1 Inertial currents in the North Pacific in October 1987 (days ) measured by holey-sock drifting buoys drogued at a depth of 15 meters. Positions were observed times per day by the Argos system on NOAA polar-orbiting weather satellites and interpolated to positions every three hours. The largest currents were generated by a storm on day 277. Note: these are not individual eddies. The entire surface is rotating. A drogue placed anywhere in the region would have the same circular motion. From van Meurs (1998).

In the Northern hemisphere … Earth’s rotation is counter clockwise … and Coriolis force is to the right … of the direction of movement Coriolis force direction of flow In the absence of any other forces, Coriolis drives clockwise (NH) rotating inertial oscillations

Suppose a balance of forces between wind stress and Coriolis Coriolis force is to right of the direction of movement So direction of movement is to the right of the wind (in the northern hemisphere) Wind forceCoriolis force Current Wind force Current Wind force Current Wind force Current

Wind force Current

DEDE

DEDE V 0 is 45° to the right of the wind (in the northern hemisphere) V 0 decreases exponentially with depth as it turns clockwise (NH) At depth z = -D E the flow speed falls to e -π = 0.04 times the surface current and is in the opposite direction (typical D E is 20 to 40 m)

Progressive vector diagram, using daily averaged currents relative to the flow at 48 m, at a subset of depths from a moored ADCP at 37.1°N, 127.6°W in the California Current, deployed as part of the Eastern Boundary Currents experiment. Daily averaged wind vectors are plotted at midnight UT along the 8-m relative to 48-m displacement curve. Wind velocity scale is shown at bottom left. ( From: Chereskin, T. K., 1995: Evidence for an Ekman balance in the California Current. J. Geophys. Res., 100, ) wind water Eastern Boundary Current program Progressive vector diagram. Apr-Oct 1993

Equator-ward winds on ocean eastern boundaries Pole-ward wind on ocean western boundaries Pole-ward winds on ocean eastern boundaries Equator-ward wind on ocean western boundaries

Wind force Current

The magnitude of the Ekman transport is m 2 s -1 τ = wind stress (Pascals or N m -2 ) = water density (1027 kg m -3 ) f = Coriolis parameter = 2 Ω sin φ Ω = 2π/(24 hours) = 2 x Earth rotation rate x sin(latitude)

Wind speed and along-shelf currents at various depths along the continental shelf off northwest Africa Wind speed m s -1

Strong wind toward south Weak or no wind

Wind-driven currents and upwelling On timescales longer than a few days: Earth’s rotation introduces Coriolis force flow turns to the right (northern hemisphere) or left (southern hemisphere) wind stress balances Coriolis force = Ekman transport Oceanographer’s rule: Ekman transport is toward the right of the wind stress (in northern hemisphere) Adjacent to a coast… Alongshore wind produces Ekman transport across-shore … causes upwelling or downwelling of a few meters per day

Estimating the upwelling velocity from NJ coast data Wind data show southerly of 6 m s -1 over a few days To balance mass transport in a 2-dimensional (across-shelf/vertical) process, the average upwelling velocity (w) times the width of the upwelling zone must balance the Ekman transport. Over 1 day (86400 sec) this is 4.32 m day -1

Depth (z)

uEuE

uEuE Typical  = 0.1 Pa f = 5x10 -5 at 20N typical D E ~ 30 m u E ~ 0.1 / 1027 / 5x10 -5 / 30 = 6.5 cm s -1 Northwest Africa values:

50/8 = 6.25 km day -1 typical  = 0.1 Pa f = 5x10 -5 at 20N, D E ~ 30 m = 5.6 km/day

Alongshore flow shaded into page i.e. poleward Across-shore flow shaded to left (offshore) Density (kg m -3 ) 6 cm/s Upwelling favorable wind is out of the page

Wind-driven currents and upwelling On timescales longer than a few days: Earth’s rotation introduces Coriolis force flow turns to the right (northern hemisphere) or left (southern hemisphere) wind stress balances Coriolis force = Ekman transport Oceanographer’s rule: Ekman transport is toward the right of the wind stress (in northern hemisphere) Adjacent to a coast… Alongshore wind produces Ekman transport across-shore … causes upwelling or downwelling of a few meters per day