Vertical current structures in the Deep Gulf using EOF analysis

Abstract

Abstract Empirical orthogonal function (EOF) analysis was performed on current meter records and model outputs in the deep regions of the Gulf of Mexico. The results of the EOF analysis were similar for model and observation stations located on the continental slope and rise. The vertical structure of modes, generally, shows a surface-intensified mode containing a high percentage of total variance and a bottom-intensified nearbarotropic mode beginning at roughly 800 m and extending to the bottom. Introduction We examined current meter records and model outputs of the University of Colorado Princeton Ocean Model (CUPOM) to construct empirical orthogonal functions (EOFs) which describe the vertical current structure in the deep Gulf of Mexico. EOF analysis, also known as principal component analysis, has become an accepted tool to quantify patterns of variability present in large sets of time-series data1. To facilitate analysis of the EOF modal time series, all EOFs done in this paper used singular value decomposition (SVD) to provide both the modal amplitudes and principal component time series. Further, both east-west (U) and north-south (V) current velocity EOFs were done simultaneously. Therefore, percentages of variance reported are based on the variance contained in both the U and V time series. Data Current meter observations. Analyzed were datasets that consisted of simultaneous point measurements of current velocity of 5 or more distinct depths and which lasted at least 4 months2. Initial EOF analysis concentrated on observations in water depths greater than 700 m during the period 1980-1988. The analysis was done on raw unfiltered data from the eastern, central, and western Gulf of Mexico. The analysis was performed using those data that were available at all depth levels, i.e., irrespective of any temporal gaps. CUPOM data. Model data were generated from a version of the Princeton Ocean Model adapted for the Gulf of Mexico. The model has 1/12° horizontal resolution and 24 vertical sigma levels. The model hindcast covered the years 1993-1998. EOF analysis of model output was performed using data from cross-and along-isobaths as well as from single locations. The across-isobath lines went from 200-3000 m total depth across the continental slope at longitudes 88°, 90°, 92°, 94°, and 96°W and across the slope meridionally in the eastern and western Gulf at 26°N. Along-isobath lines were in the central Gulf along the 500, 1000, and 2000-m isobaths between 89°W and 94°W. In this paper we concentrate on the along-isobath line at 2000 m and the cross-slope line at 90°W (Fig. 1). Results Fig. 2 illustrates the typical modal structure seen from slope locations around the northern Gulf of Mexico. The data used to construct the modes of Fig. 2 were from CUPOM model year 1993 and taken near 27.2°N and 89.9°W and at the edge of the Sigsbee Escarpment. Unfiltered hourly current velocity time series at all 24 sigma levels were used to construct the EOF modes.