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Ageostrophic Circulation in the Ocean

Peter Niiler*


To understand ocean circulation from observations, large international projects have been undertaken in the past 50 years to measure the temperature, salinity and satellite sea level. 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).

Every ocean GCM (OCGM) in operation today can ‘smoothly’ assimilate these temperature, salinity and sea level data and produce a ‘geostropic’ circulation from a balance of local horizontal pressure gradient and the Coriolis force over most of the water column.

But theory and observations both demonstrate that the circulation is not in geostrophic balance along lateral ocean boundaries, in straits and overflows and in the upper 200 m nearly everywhere. In this upper ocean column where momentum and vorticity is imparted by the wind stress, the principal exchanges of thermal energy and fresh water takes place and where most of the biological productivity occurs, the circulation is quite different as implied by the geostrophic contours of Figure 1.

A ‘streak line’ map of the 15 m-depth circulation can be constructed from the integration or drifter motion (Figure 2) that shows dramatic

*

Scripps Institution of Oceanography, University of California, San Diego



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OCR for page 73
Ageostrophic Circulation in the Ocean Peter Niiler* To understand ocean circulation from observations, large interna- tional projects have been undertaken in the past 50 years to measure the temperature, salinity and satellite sea level. These data are used to com- pute 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 momen- tum balance during this computation a 50 km spatial scale distribution of dynamically balanced sea level, or geostrophic circulation can be com- puted (Figure 1). Every ocean GCM (OCGM) in operation today can ‘smoothly’ assimi- late these temperature, salinity and sea level data and produce a ‘geos- tropic’ circulation from a balance of local horizontal pressure gradient and the Coriolis force over most of the water column. But theory and observations both demonstrate that the circulation is not in geostrophic balance along lateral ocean boundaries, in straits and overflows and in the upper 200 m nearly everywhere. In this upper ocean column where momentum and vorticity is imparted by the wind stress, the principal exchanges of thermal energy and fresh water takes place and where most of the biological productivity occurs, the circulation is quite different as implied by the geostrophic contours of Figure 1. A ‘streak line’ map of the 15 m-depth circulation can be constructed from the integration or drifter motion (Figure 2) that shows dramatic * Scripps Institution of Oceanography, University of California, San Diego 3

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 OCEANOGRAPHY IN 2025 FIGURE 1 The 15 m-depth dynamic topography computed from application Niiler_Fig1.eps of the horizontal momentum balance to satellite sea level and drifter velocity bitmap image observations. The geostrophic currents flow along contours of constant sea level, contoured in 10 cm intervals. From Maximenko et al., 2009. departures from the geostrophic streamlines. 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. The shade indicated the speed on a logarithmic scale and black bitmap image arrows mark the direction of flow. Note the large scale convergent vortices on the both the northeast Pacific (a well known region of plastic accumulation) and a similar, and more stable vortex, in the southeast Pacific (from where no water property data has been garnered). From Maximenko et al., 2009.

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5 PETER NIILER velocity component to the south. The vertical structure of the ageo- strophic velocity is not well measured in general and each OGCM will produce a vertical structure depending upon its vertical and horizontal turbulence models and the spatial resolutions in which these are applied. The ageostrophic component of the horizontal circulation contains the largest horizontal divergence, and hence vertical velocities, and thus every model will present a different vertical circulation. Secondly, the greater part of the ocean contains both stationary and transient mesoscale features that commonly have local Rossby numbers of 0.1-0.2. 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). This verti- FIGURE 3 The 9-year mean sea level height and ageostrophic velocity at 15 m- depth of 5 km horizontal resolved Regional Ocean Model System (ROMS) of the Niiler_Fig3.eps California Current System (CCS) (left panel). As also observed, ROMS produces bitmap images four semi-permanent cyclonic meanders of the sea level, or standing cold ‘geo- strophic’ eddies in the CCS, even when driven with large scale COADS monthly mean winds. The ageostrophic velocity forms convergent and divergent patterns that are tied to the meanders. The right panel shows the ageostrophic surface ve- locity (black arrows) in a model of a symmetric cold eddy of the strength and ver- tical structure commonly observed in the CCS that is forced by a uniform wind. Note the similarity of the surface ageostrophic velocity in ROMS of the CCS and model of a single cold eddy-wind interaction. The contoured shades are changes in SST (Cº) caused by this interaction. From Centurioni et al., 2008.

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 OCEANOGRAPHY IN 2025 cal circulation, which extends to over 200 m depth, is produced from the ageostrophic horizontal velocity convergence. Model diagnostics show that this vertical circulation is a strong function of parameterizations of both horizontal and vertical mixing in the model. Oceanography of 2025 will require observations and realistic mod- eling 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 bal- ances maintain ageostrophic velocity structures and the associated verti- cal circulations. This task calls for development and implementation of continued new methods and instruments for direct velocity observations of the oceans. REFERENCES Maximenko et al. 2009 (in press). Mean dynamic topography of the ocean derived from satellite and drifting buoy data using three different techniques. Journal of Ocean and Atmospheric Technology. Centurioni et al. 2008. Permanent Meanders in the California Current System. Journal of Physical Oceanography. 38(8): 1690-1710.