SIO 210: Fall 2002

Thermohaline Circulation III: The Return of Abyssal Waters

Readings

• Broecker, Wallace S., The Great Ocean Conveyor. [Oceanography, v. 4:2, pp. 79-89, 1991.] in Electronic Reserves
• Munk, Walter and Carl Wunsch, Abyssal Recipes II: energetics of tidal and wind mixing. [Deep Sea Research I, Vol 45 pp 1997-2010, 1998.] in Electronic Reserves You don't have to follow all the math in section 3 and 4. Pay particular attention to sections 5-8.

Study Questions

• Why does the deep western boundary current in the North Atlantic go the opposite direction from the Gulf Stream?
• What are the dynamics governing the flow of water in the deep ocean?
• What is the "Oceanic Conveyor Belt"? In what ways is the original schematic sketched out by Broecker too simplistic?
• Why is no deep water formed in the North Pacific Ocean?
• What are the property characteristics of NADW? AABW?
• Why are the properties in the three southern hemisphere oceans fairly similar compared with those in the northern hemisphere?
• What are the two primary sources of mechanical energy for mixing the deep ocean?

Problems

1. It is thought that about 20 Sv of bottom water enters the South Pacific and upwells to mid-depths. Assuming the northward flow is distributed evenly over a 1000m thick layer, calculate that average northward velocity of the bottom water (take the basin width to be 10,000 km.) If this were the characteristic velocity of northward flow, how long would it take a parcel to traverse from the South Pacific (40S) to the far North Pacific (50N) [1 degree of latitude equals 111 km]. How does this compare with radiocarbon ages in the deep North Pacific? What simplifications are there in this flow model which could account for the difference?
2. The heat balance for upwelling cold water (such as in the deep Brazil Basin) can be expressed as [ wTA = KT_zA] , where
   • w: upwelling velocity
   • T: temperature of isotherm
   • A: surface area of isotherm
   • K: vertical diffusivity
   • T_z: Vertical gradient of temperature.
   For some deep ocean basin, current meter arrays have measured the net long-term inflow below the 1 degree isotherm to be 2 Sv. Hydrographic observations show the mean vertical temperature gradient to be 2 x 10^-3 C/m and mapping has revealed that the surface area of this isotherm is 60 x 10⁵ km². Calculate the mean vertical diffusivity. Suppose that all the vertical mixing was confined to a region over the broken slope of mid-basin ridge with dimensions 3000 km by 100 km. What would the effective vertical diffusivity in this region be?

Figures from class