An Offshore Eddy in the Cal i fornia Current S y s t e m Part II

Ship and satellite observations taken over the last thirty years show that mesoscale patterns of sea surface temperature (SST) in the California Current System are consistently found throughout the year and usually occur in approximately the same geographical locations. Typically, these patterns are more pronounced in fall/winter than in spring/summer. The temporal and spatial characteristics of these persistent features were examined with satellite infrared (IR) measurements during winter 1980-81. In January 1981, a ship surveyed the vertical structure of several physical, chemical, and biological parameters beneath one of these SST features centered near 32°N, 124°W. The surface IR pattern had a length scale of 200 km and a time scale of about 100 days. It disintegrated following the first two storms of the winter season. Motion studies of the pattern in late October indicated an anticyclonic rotation with maximum velocities of 50 cm s -1 at 50 km from the axis of rotation. As a unit, the pattern advected southward with an average speed of 1 cm s -1. Thermal fronts, determined from the satellite imagery, were strongest (0.4°C km -L) along the rim of the pattern and were advected anticyclonically with the pattern: their length scales were 20-30 km in the along-front direction and less than 10 km wide. The hydrographic data revealed a three-layer structure beneath the surface pattern: a 75 m deep surface layer, a cold-core region from 75 to 200 m depth, and a warm-core eddy extending from 250 to 1450 m. The anticyclonic motion of the surface layer was caused by a geostrophic adjustment to the surface dynamic height anomaly produced by the subsurface warm-core eddy. The IR pattern observed from space reflects the horizontal structure of the surface layer and is consistent with a theoretical model of a mean horizontal SST gradient perturbed by a subsurface density anomaly. Ship of opportunity SST observations collected by the National Marine Fisheries are shown to resolve mesoscale patterns. For December 1980, the SST pattern near 32°N, 124°W represented a 2°C warm anomaly compared with the 20-year mean monthly SST pattern. 1. I N T R O D U C T I O N T h e m e a n sea su r f ace t e m p e r a t u r e (SST) o f f Cal i forn ia inc reases (0 .005°C k m l ) in a s o u t h w e s t e r l y d i rec t ion . Th is g rad ien t resu l t s f r o m b o t h the s o u t h w a r d advec t ion o f cool sur face water by the Cal i forn ia C u r r e n t S y s t e m (CCS) a long t h e ea s t e rn b o u n d a r y o f t he Pacific and the upwel l ing o f cold wa te r a long the coast . E v e r y fall and win te r for t he last th i r ty years , s h i p b o a r d o b s e r v a t i o n s h a v e s h o w n tha t m e s o s c a l e pa t t e rn s o f w a r m SST i n t r u d e the coole r coastal water and p e r t u r b t h e m e a n g rad ien t ( A n o n y m o u s , 1963; Burkov and Pav lova , 1980; Wyll ie and L y n n , 1971; S i m p s o n , 1982). In part icular , a m e s o s c a l e f e a tu r e c e n t e r e d near 32°N, 124°W has cons i s t en t l y b e e n f o u n d in sh ip data s ince 1950 (Wyllie, 1966) and in satel l i te in f ra red (IR) data s ince 1975 (e.g., Be rns t e in , Breaker and W h r i t n e r , 1977). A de ta i led satel l i te and ship s tudy o f this f e a t u r e was m a d e d u r i n g t h e w in te r o f 1980-81. This paper c o m p a r e s r e m o t e l y s e n s e d m e s o s c a l e sur face 1 0 "V " " " 2 cean Research Dt ismn A-030, Scripps Institution of Oceanography, La Jolla, CA 92093, USA 3Present address: Code 921, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA 4Marine Life Research Group A-030, Scripps Institution of Oceanography, La Jolla, CA 92093, USA Department of Geological Sc ences, Un vers ty of Southern Ca fornia, LOS Angeles, CA 90007, USA 52 C.J. KOBLINSKY, J.J. SIMPSON and T.D. DICKEY brightness temperature patterns in the CCS to observed subsurface structure and processes. 2. OBSERVATIONS In this study satellite IR observations of the CCS were moni tored daily from September 1980 through March 1981. These observations were used to determine the t ime and length scales of several mesoscale surface IR brightness temperature (IRBT) patterns. The relationship between IRBT and SST is discussed below. A ship survey examined the physical (Simpson, Dickey, and Koblinsky, 1984), chemical (Simpson, 1984), and biological (Haury, 1984) properties of the ocean beneath the mesoscale IRBT pattern centered near 32°N, 124°W (feature A in Fig. 1). Fig. 1. An infrared image from the NOAA-6 AVHRR 11 #m band on December 11, 1980 at 0345 GMT. The 3000 m bathymetric contour, separating the continental shelf and slope regime from the deep ocean is shown in white. See the text for a discussion of the square region outlined in white and features labeled A-F. An Offshore California Current Eddy -11: Surface Manifestation 53 The satellite IR observations were made with the Advanced Very High Resolution Radiometer (AVHRR) on the polar orbiting NOAA-6 and TIROS-N satellites. This instrument senses the upwelled irradiance in the 0.7, 0.9, 3.7 and 11/zm bands of the electromagnetic spectrum at a distance of 833 km above the surface (Schwalb, 1978). The spatial resolution of the AVHRR is 1.1 km at nadir. The field of view of the instrument directly available at an earth receiving station is approximately 2000 km east/west (perpendicular to the flight path) and 4000 km north/south (parallel to the flight path), because the orbit inclination angle is about 99 degrees. NOAA-6 passes over California at approximately 0400 and 1600 GMT, whereas TIROS-N crosses at 1000 and 2200 GMT. The AVHRR on TIROS-N ceased to operate properly after November 1980, but AVHRR data from NOAA-6 were available for the entire study period. All AVHRR observations were directly received and processed at the Scripps Satellite Oceanography Facility in La Jolla, California. Routine processing of these observations included: digitization of the analog signal transmitted by the spacecraft; earth location and removal of instrument attitude and orbital variations (Legeckis and Pritchard, 1976); interpolation of the data to an equirectangular grid at 1.1 km resolution; and calibration of the 3.7 and 11 p,m channel data to IRBT (Lauritson, Nelson, and Porto, 1979). The details of these routines are given in Young (1981) and Young and Fahle (1981). The resulting temperatures were then linearly mapped to a range of 256 grey shades and displayed as a black and white image on a cathode ray tube with 512 x 512 picture elements (pixels). The images were contrast enhanced for feature identification by varying the linear grey shade mapping, and photographed (e.g., Fig. 1). All images shown in this paper were mapped so that warmer temperatures are associated with lighter grey shades; hence, clouds are black. Because they have been enhanced separately for contrast, intercomparison of IRBTs is not possible with the images displayed in this paper. Additional analysis of the image data included the estimation of SST gradients by simple differences and the measurement of velocity from feature displacement. These latter methods are described in more detail below. The absolute positional accuracy of the processed images is approximately __.2 pixels or _+ 2.2 km. The IRBT accuracy is about +_ 0.10°K (Bernstein, 1982a). The ship survey was conducted to examine the properties of the underlying ocean and to provide a dynamical interpretation of the satellite observations. From January 9th to 17th, 1981, R.V. New Horizon mapped the detailed vertical structure beneath the mesoscale feature A shown in Fig. 1. Vertical profiles of temperature and salinity were made to 1450 db with a Nell Brown CTD/O2 system. The vertical resolution was 0.5 db, which has been block averaged to 2.5 db. Profiles were made approximately every 20 km along north/south and west/east transects through the feature A. The station pattern is shown in Fig. 1 of Simpson, Dickey, and Koblinsky (1984) and further details of the sampling and data analysis are given in the same paper.