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Empirical Orthogonal Function Analysis of Ocean Surface Currents Using Complex and Real-Vector Methods

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  • 1 Oceanography Division, Naval Research Laboratory, Stennis Space Center, Mississippi
  • | 2 Remote Sensing Division, Naval Research Laboratory, Washington, D.C.
  • | 3 Department of Meteorology and Physical Oceanography, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida
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Abstract

Empirical orthogonal function (EOF) analysis has been widely used in meteorology and oceanography to extract dominant modes of behavior in scalar and vector datasets. For analysis of two-dimensional vector fields, such as surface winds or currents, use of the complex EOF method has become widespread. In the present paper, this method is compared with a real-vector EOF method that apparently has previously been unused for current or wind fields in oceanography or meteorology. It is shown that these two methods differ primarily with respect to the concept of optimal representation. Further, the real-vector analysis can easily be extended to three-dimensional vector fields, whereas the complex method cannot. To illustrate the differences between approaches, both methods are applied to Ocean Surface Current Radar data collected off Cape Hatteras, North Carolina, in June and July 1993. For this dataset, while the complex analysis “converges” in fewer modes, the real analysis is better able to isolate flows with wide cross-shelf structures such as tides.

Corresponding author address: James M. Kaihatu, Oceanography Division, Code 7320, Naval Research Laboratory, Stennis Space Center, MS 39529-5004.

Email: kaihatu@taurus.nrlsc.navy.mil

Abstract

Empirical orthogonal function (EOF) analysis has been widely used in meteorology and oceanography to extract dominant modes of behavior in scalar and vector datasets. For analysis of two-dimensional vector fields, such as surface winds or currents, use of the complex EOF method has become widespread. In the present paper, this method is compared with a real-vector EOF method that apparently has previously been unused for current or wind fields in oceanography or meteorology. It is shown that these two methods differ primarily with respect to the concept of optimal representation. Further, the real-vector analysis can easily be extended to three-dimensional vector fields, whereas the complex method cannot. To illustrate the differences between approaches, both methods are applied to Ocean Surface Current Radar data collected off Cape Hatteras, North Carolina, in June and July 1993. For this dataset, while the complex analysis “converges” in fewer modes, the real analysis is better able to isolate flows with wide cross-shelf structures such as tides.

Corresponding author address: James M. Kaihatu, Oceanography Division, Code 7320, Naval Research Laboratory, Stennis Space Center, MS 39529-5004.

Email: kaihatu@taurus.nrlsc.navy.mil

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