# Presentation: 373515

## Boundary Currents and Interbasin Flows

- Harry L. Bryden
- Southampton Oceanography Centre

- 1.  Boundary Current Measurements
- 2.  Issues
- 3.  What Sets the Transports

- Outline

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## Issues

- 1.  What causes fluctuation in boundary current transport?

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## Issues

- 1.  What causes fluctuation in boundary current transport?

- Barotropic velocities represent the favourite answer.  BUT there has not yet been a comprehensive study of the variability in baroclinic structure over weekly to annual time scales.

## Issues

- 2.  Can the transport be estimated from a synoptic section referenced to current meter measurement?

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## Issues

- 2.  Can the transport be estimated from a synoptic section referenced to current meter measurement?

- Yes, but it is a synoptic transport estimate that may have little to do with the long-term boundary current transport.

## Issues

- 3. Is there a reference level?

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## Issues

- 3. Is there a reference level?

- Yes but the reference level velocity is not necessarily a zero velocity surface.

## Issues

- 4. Are the boundary current transports useful for defining the basin-scale circulation?

## Baringer and Larsen (2001)

- Florida Straits Transport

## Issues

- 4. Are the boundary current transports useful for defining the basin-scale circulation?

- Yes, without question.

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## What Sets the Transports

- 1.  Thermocline Western Boundary Currents

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## What Sets the Transports

- 1.  Thermocline Western Boundary Currents

- Primary answer is Sverdrup transport--wind stress curl--which gives the right sign and right order of magnitude. But, there is information about ocean circulation in the amount by which western boundary current transports are larger or smaller than Sverdrup transport.

## What Sets the Transports

- 2. Northward Flow of Bottom Water

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## What Sets the Transports

- 2. Northward Flow of Bottom Water

- Area of ocean sets the ratios implying that mixing is the controlling factor.

## What Sets the Transports

- 3.  Indonesian Throughflow

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## What Sets the Transports

- 3. Indonesian Throughflow

- Wind stress and wind stress curl.

## What Sets the Transports

- 4. Antarctic Circumpolar Current

- .

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## What Sets the Transports

- 4. Antarctic Circumpolar Current

- Wind stress, square root of wind stress, wind stress curl.

## What Sets the Transports

- 5. Upwelling around Antarctica

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## What Sets the Transports

- 5. Upwelling around Antarctica

- Eddies ultimately set up by wind stress.

## What Sets the Transports

- 6. Northward Flow across the Equator

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## What Sets the Transports

- 6. Northward Flow across the Equator

- Pressure gradient > Wind Stress in Southern Hemisphere
- Pressure gradient < Wind Stress in Northern Hemisphere

## Request

- We would like the wind stress doubled or halved please.  And for a long time so we can see the baroclinic as well as the barotropic adjustment

## Summary

- WOCE boundary current transport measurements have defined the size of the circulation in the thermocline, bottom, and deep waters as well as some of the interbasin flows.
- Understanding what sets the sizes of these flows in the modern ocean is a central problem in ocean circulation.
- Long time series boundary current measurements at a few locations where wind and buoyancy forcing may be expected to vary over interannual time scales represent essential observations required to test our understanding of ocean circulation and how it would change under different climate conditions.