OC4331-Mesoscale Oceanography
Final Project Summary
Topic Area
Identifying Ocean Fronts and Eddies with Satellite Altimetry
Project Team Members
ENS Robert Holt, USNR
ENS Timothy Ray, USNR
Major Findings
The first paper examines the seasonal surface circulation in the California Current System using satellite sea height and temperature fields.A major feature of the California Current System is the equatorward flowing jet. This jet migrates seasonally from 200 km to 800 km from the western coast of the United States.Altimeters can monitor this migration by calculating eddy kinetic energy (EKE) values from crossover points of satellite altimetry tracks. EKE is largest eastward of the jet, where filaments and upwelling help to create eddies with wavelengths of about 300 km. At about 600-800 km offshore, eddy sizes and wavelengths are typically 100-150 km, which is typical of the less dynamic Pacific Ocean. When the EKE of the California Current System is largest, the jet has migrated offshore, and likewise, as the jet migrates back toward the coast, the EKE of the system decreases.
The second paper analyzes the variability of the Azores Current using a combination of TOPEX/POSEIDON (T/P) and ERS-1 altimetry data. Satellite data is also compared to in situ hydrographic and Lagrangian measurements. Combining T/P and ERS-1 data is difficult due to the different sampling methods of the two satellites, yet when the altimetry data is brought together an excellent sea surface height field is produced. The T/P altimeter resolves a finer temporal scale of about 20 day periods, yet is limited by a spatial resolution of 540 km between tracks. In contrast, the ERS-1 altimeter only resolves to a period of 70 days, but also to the relatively small scale of 160 km between tracks. When combined the T/P and ERS-1 altimeters are accurate to about 2-3 cm, which is comparable and in some cases preferable to in situ measurements.
Finally, the third paper that we reviewed concerns itself with analyzing eddy kinetic energy in the California Current System. In particular, seasonal variations, as well as variations in both time and space were sought.To achieve their goal, the authors utilized multiple means of garnering surface geostrophic velocities for the region:drifters released in 1993-94, TOPEX/POSEIDON altimeter data from 1993 to 1995, as well as moored current meters.The drifters (62 in all) were deployed in two ways: an incoherent array (fixed grid) to study the overall flow of the CCS, and several coherent arrays designed to monitor eddies and eddy-like features. Collocated measurements were legion within the study area (982 altimeter and drifter data points separated by less than two days in time and 20 km in space).The moored ADCP is in the center of the studied rectangle. Needless to say, several filters and corrections needed to be run on both the altimeter data and the drifter data to remove noise and aliases such as tides, outliers, and incomplete profiles.Several factors made easy interpretation of the relationship between the drifter data and the altimeter difficult.The altimeter has a subtrack spacing of three degrees of longitude, giving it a wide but coarse sampling area.The velocity field from the drifters is biased toward the inevitable loss of velocity to be found in those coherent arrays deployed within the eddies, as opposed to the entire study region.To minimize the errors in aliasing in the altimeter, along –track values of Sea Surface Height (SSH) were used to determine the cross-track geostrophic velocity. With this value, it is possible to compare the velocity acquisition of drifter and the altimeter.An anomaly occurs in the measurement of the EKE values, as the drifters are 13% higher than the altimeter. Several factors could be responsible for this, though the authors believe that the cause is either the ageostrophic component in the drifters’ velocities, or the amount of low-pass filtering. The EKE computed at the ADCP mooring supported the validity of using the three different approaches (i.e. altimeter, drifter, and ADCP) to determine the EKE for a region.
The most important aspects of using Satellite Altimetry to find ocean fronts and eddies are:
- Drifter, altimeter, and ADCP measurements all show similar seasonal patterns in EKE generation, with the max values being reached in the late summer or early fall.
- The radar altimeter is highly correlated with the observations of sea surface height made by the drifters, ADCP, ships of opportunity, and other in situ methods.
- There is a direct connection between EKE and the large-scale SSH field, whereas, no such connection can be made between the winds and the EKE.
- Altimeter analysis of Sea Surface Height provides us with evidence of mechanisms such as Sverdrup balance, Ekman Convergence, and geostrophic flow.
Figure 1. Significant Wave Height from the TOPEX/POSEIDON Satellite.
Figure 2. El Nino Sea Surface Anomalies as taken by the ERS satellite.
References
Journals:
2Hernandez, F. and P.Y. Le Traon, 1995: “Mapping mesoscale variability of the Azores Current using TOPEX/POSEIDON and ERS-1 altimetry, together with hydrographic and Lagrangian measurements.” J. Geophys. Res.,
Vol 100, NO. C12, 24,995-25006.
3Kelly, K.A., Beardsley, R.C., Limeburner,
R., Brink, K.H., Paduan, J.D., and T.K Chereskin, 1998:
“Variability of the near-surface eddy kinetic
energy in the California Current based on altimetric, drifter, and moored
current
data.” J. Geophys. Res., Vol. 103, NO. C6, 13,067-13,083.
1
Strub, P.T. and C. James, 2000: “Altimeter-derived variability of surface velocities in the California Current System: 2. Seasonal circulation and eddy statistics.” Deep-Sea Research II47, 831-870.
Web Based:
El Nino photos http://www.deos.tudelft.nl/ers/enso/
TOPEX/POSEIDON Satellite Information http://topex-www.jpl.nasa.gov/mission/topex.html
|
This is a government-maintained internet site. Please read the U.S. Navy web page disclaimer and the disclaimer regarding external links. |
|