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Oceanography in 2025 Proceedings of a Workshop (2009) / Chapter Skim
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Ageostrophic Circulation in the Ocean--Peter Niiler
Pages 73-76

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From page 73...
... These data are used to compute horizontal pressure gradients and the corresponding geostrophic circulation on 400 km scales over the globe and 1-10 km on scales locally. When direct near surface velocity observations are added to the momentum balance during this computation a 50 km spatial scale distribution of dynamically balanced sea level, or geostrophic circulation can be computed (Figure 1)
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From page 74...
... The most notable is that while the geostrophic circulation moves water from the mid-latitudes toward the equator, the streak lines move water toward the pole. In the subtropical North Atlantic and North Pacific the ageostrophic velocity component to the north is at least twice as strong as is the geostrophic FIGURE 2  Streak lines of the 15Niiler_Fig2.eps m-depth velocity derived from the Lagrangian motion of drifters.
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From page 75...
... Model calculations demonstrate that in such circumstances it is not appropriate to construct a map of upper ocean circulation from the arithmatic sum of a local Ekman Current and local geostrophic current. An ageostrophic current, or a secondary three-dimensional flow pattern, results from the non-linear interaction of the local wind-forced flow and the vorticity structure of the underlying mesoscale (Figure 3)
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From page 76...
... Oceanography of 2025 will require observations and realistic modeling of the circulation patterns that contain the vertical motion of the upper 200 m. Models will be compared not by how well they assimilate or replicate the sea level or reproduce the geostrophic velocity, but rather by how their internal vorticity and thermal energy and fresh water balances maintain ageostrophic velocity structures and the associated vertical circulations.
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Key Terms

  • ageostrophic velocity
  • geostrophic circulation
  • upper ocean
  • horizontal pressure
  • velocity observations

  • ocean circulation
  • vertical circulation
  • fresh water
  • vertical structure
  • thermal energy

  • cal circulations
  • ocean column
  • convergent vortices
  • arithmatic sum
  • corresponding geostrophic

  • underlying mesoscale
  • biological productivity
  • standing cold
  • surface velocity
  • level data

  • maintain ageostrophic
  • largest horizontal
  • momentum balance
  • velocity forms
  • strophic velocity

  • stable vortex
  • velocity structures
  • single cold
  • lines move
  • ekman current


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