Deep Sea Research, Vol.32, No. 2, pp. 237 to 250, 1985. 019841149/85 $3.00 + 0.00 Printedin GreatBritain. ~ 1985 Pergamon Press Ltd. NOTE Gulf Stream variability for acoustic tomography PAOLA MALANOTTE RIZZOLI,* JOHN L. SPIESBERGERt* and MICHAEL CHAJES~ (Received 2 April 1984; accepted 29 August 1984) Abstract--Fluctuations of sound speed from average conditions in the Slope waters, Gulf Stream, and Sargasso Sea are investigated from historical measurements. The covariance of the sound speed fluctuations is computed as a function of horizontal distance from the north wall of the Gulf Stream in four different vertical layers. Above (below) 500 m the scales are greater (less) in the Slope water than in the Sargnsso Sea, indicating a strong anisotropy in the covafiance function. The empirical orthogonal functions (EOF's) of the vertical variability are also computed. The EOF's in the Sargasso Sea and Gulf Stream are similar but differ from the EOF's computed for the Slope water. For all three regions the first three EOF's describe about 95% of the variance. These statistical descriptions of sound speed variability are useful for mapping Gulf Stream variability using the techniques of ocean acoustic tomography. INTRODUCTION ACOUSTIC tomography was proposed by MUNKand WimSCH(1979) as a method for making measurements of ocean variability. The technique provides synoptic maps (not point measurements) for large regions that are difficult to map with conventional techniques. One such area, which is of fundamental oceanographic importance, is the Gulf Stream. The data obtained from a tomographic array are the travel times of sound pulses which are sent from sources to _rec~__'vers. These travel times are then compared with theoretically calculated travel times bued on the historical sound speed structure of the region. Through geophysical inversion techniques these data are used to map the interior fields of sound speed, temperature, and other variables of interest. A stochastic inverse has been successfully used to map the mesoscale sound speed anomaly field in space and time (OCEAN TOMOOgAPHY GROUP, 1982). This inversion scheme requires an a priori knowledge of the covariance of the sound speed fluctuations(Com~Jm.L~ 1983). The Gulf Stream region is characterized by extreme horizontal and vertical variability. Its constituents are the Gulf Stream jet, with its well-defined water properties, and two different water bodies, the Sargamo Sea and the slope water. The purpose of this note is to investigate the statistics of the Gulf Stream region on the basis of historical hydrographic data with a focus on inhomogeneities and anisotropies. * Massachusetts ln~tute of Technology, Department of Earth, Atmospheric and Planetary Sciences, Cam- bridge, MA 02139, U.S.A. ~"Woods Hole Oceanographic Institution, Woods Hole, MA 02543, U.S.A. :]: Summer Student Fellow, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, U.S.A. 237