Lecture 10 – Surface and Abyssal Circulation

Ocean Surface Currents

Origin of ocean surface current: in a simplistic case, surface currents develop from friction between the ocean and the wind that blows across its surface. Only about 2% of the wind’s energy is transferred to the ocean surface, you can simulate this on a tiny scale simply by blowing gently and steadily across a cup of coffee. If there were no continents on Earth, the surface currents would generally follow the major wind belts of the world. If there were no continents on Earth, the surface currents would generally follow the major wind belts of the world. Other main factors that influence surface current patterns include gravity, friction, and the Coriolis effect.

The Equatorial Undercurrent

In mid-latitudes the gyres are existing in all the basins, and may be understood as the direct response to the curl of the wind stress. In the equatorial regions the currents also display some robust and distinctive features. The main features are:

• A shallow westward flowing surface current, typically confined to the upper 50 m or less, strongest within a few degrees of the equator, although not always symmetric about the equator. Its speed is typically a few tens of centimetres per second.
• A strong coherent eastward undercurrent extending to about 200 m depth, confined to within a few degrees of the equator. Its speed is up to a metre per second or a little more, and it is this current that dominates the vertically integrated transport at the equator. Beneath the undercurrent the flow is relatively weak.
• Westward flow on either side of the undercurrent, with eastward countercurrents poleward of this. In the Pacific the countercurrent is strongest in the Northern Hemisphere where it reaches the surface.

Ocean currents near the Equator: Although the details vary with longitude, the surface currents are generally aligned with the direction of the wind near the equator—this is known as the South Equatorial Current (SEC). North of the equator, the surface currents flow against the prevailing winds; this eastward-flowing current is called the North Equatorial Countercurrent (NECC), and it lies within the Intertropical Convergence Zone (ITCZ).
Beneath the surface, underneath the westward-moving SEC, there exists a strong eastward-flowing current known as the Equatorial Undercurrent (EUC). The EUC flows opposite to the direction of the trade winds, has a velocity of approximately 1 m/s, and is located within the thermocline. It is about 200 kilometers wide and 100 meters deep.
Heavy arrows below the ocean surface represent the following processes:

• upwelling along the equator
• poleward Ekman divergence on either side of the equator
• equatorward return flow at a few hundred meters depth to replenish the surface-diverging water

While the midlatitude ocean is warmest during the (Northern Hemisphere) summer and coldest in winter, the tropical ocean experiences its warmest period around March–April and its coolest period around September–October. Along the equator, deviations from the annual mean sea surface temperature (SST) clearly propagate westward, with most of the annual amplitude concentrated in the eastern third of the tropical Pacific Ocean.
The largest seasonal variations in SST occur in the eastern Pacific, where temperatures reach their lowest during the Northern Hemisphere fall and winter.

Squids Found in the Stomachs of Sperm Whales

Imprints or scars from squid suckers have been found on the skin of sperm whales and even in their stomach. Although only a few giant-squid beaks may be found in a sperm whale's stomach along with hundreds of beaks from other squid species, the sheer size of a single giant squid may take up a third of the volume of a whale's stomach. Battle of the Titans.

Antarctic Circumpolar Current

The Antarctic Circumpolar Current (ACC) is a strong nearly zonal flow in the Southern Ocean. The ACC is unique because:

• It has no zonal boundaries to inhibite the zonal flow.
• There are no meridional sidewalls to create frictions like in Munk's model.

Start from the zonal momentum balance, the zonal momentum equation integrated vertically: $$ \underbrace{-fv}_{\text{Coriolis}} = \underbrace{-\frac{1}{\rho_0} \frac{\partial p}{\partial x}}_{\text{Zonal pressure gradient}} + \underbrace{\tau^x_z}_{\text{Zonal stress (e.g., wind or bottom drag)}} $$ Integrate from $z = -D$ to $z = 0$: $$ \int_{-D}^{0} \left( \underbrace{-fv}_{\text{Coriolis}} \right) dz = - \frac{1}{\rho_0} \int_{-D}^{0} \underbrace{\frac{\partial p}{\partial x}}_{\text{Pressure gradient}} dz + \underbrace{\tau_S^x}_{\substack{\text{surface} \\ \text{wind stress}}} - \underbrace{\tau_B^x}_{\substack{\text{bottom} \\ \text{friction}}} $$ Assume that total meridional mass flux integrates to zero ⇒ no net $fV$ across the entire zonal band: $$ \Rightarrow -fV = -\frac{1}{\rho_0} \overline{P_x} + \tau_S^x - \tau_B^x \quad \text{and integrating zonally } \oint dx: \quad 0 = 0 $$ Mass conservation ⇒ balance of surface and bottom stress using a closed zonal path: $$ \oint (\tau_S^x - \tau_B^x) dx = 0 \Rightarrow \tau_S^x = \tau_B^x $$ The surface wind stress is balanced by bottom drag. Using a quadratic drag law: $$ c_D \rho u^2 = \tau_B^x \quad \text{with dimensionless drag coefficien } c_D \approx 10^{-3}, \quad \tau_B^x \approx 0.1\, \text{N/m}^2 $$ We estimate: $$ u \approx 0.3 \, \text{m/s}, \quad \text{and ACC transport } \approx 1000 \, \text{Sv} $$ This bottom stress closes the momentum budget in the ACC, especially since there are no sidewalls to provide lateral drag. Apply Leibniz rule to pressure gradient: $$ \int_{-D}^{0} \frac{\partial p}{\partial x} dz = \frac{\partial}{\partial x} \int_{-D}^{0} p \, dz + p_D \frac{dD}{dx} = P_x + \rho D D_x $$ This bottom slope term gives rise to the form drag: $$ \tau_S^x = \tau_B^x + \oint \rho D D_x \, dx $$ So the wind stress is not just balanced by local bottom friction but also by form drag from topography.

Bonus: Cookiecutter Shark

With large prey, the cookiecutter shark latches onto their body and spins, removing large plugs of tissue. This ice cream scoop-like action removes a very distinctive circular chunk of flesh from the larger ‘host’. Can lead to some anxiety.

Next: Project 6 – Southern Ocean Winds and Overturning