Identifying fluctuations in the Norwegian Atlantic Slope Current by means of empirical orthogonal functions

Variability of the Norwegian Atlantic Slope Current off west-Norway are investigated based on current meter records from four moorings from April 1997 to April 1998. The along-slope and cross-slope velocity fluctuations are analyzed separately by using conventional Empirical Orthogonal Function (EOF) decomposition. The analysis shows that the first three EOF modes are unique and statistically significant, both for along- and cross-slope currents. (1) The leading EOF mode for the along-slope flow appears to represent a quasi-stationary forced response to the local along-slope wind component but delayed by about 12h relative to the forcing. (2) The combined effect of (lie second along-slope and first cross-slope EOF modes are in accordance with the first topographic wave mode. The associated principal components appear with a 90degrees phase shifts for periods t > 20 h, and maximum coherence for periods from 40 to 70 It. (3) Also but less conclusively, the combined effect of the third along-slope and second cross-slope EOF modes are in accordance with the second topographic wave mode. The associated principal components appear with a W phase shift for periods from 24 to I 10 h and maximum coherence for periods from 80 to 110 h. Validity of the EOF method is supported by results from rotary spectra of the original current records which are in good accordance with the expected rotary characteristic of the first and second topographic wave modes. (C) 2002 Elsevier Science Ltd. All rights reserved.