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Wind Stress Curl Forcing of the Coastal Ocean Near Point Conception, California*

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Wind Stress Curl Forcing of the Coastal Ocean Near Point Conception, California* JUNE 2000 MUÈ NCHOW 1265 Wind Stress Curl Forcing of the Coastal Ocean near Point Conception, California* ANDREAS MUÈ NCHOW Institute of Marine and Coastal Sciences, Rutgers, The State University of New Jersey, New Brunswick, New Jersey (Manuscript received 18 March 1999, in ®nal form 8 July 1999) ABSTRACT Near Point Conception, California, the atmospheric ¯ow separates from the coast and a large wind stress curl results. Direct spatial wind ®eld observations from 20 aircraft over¯ights in the spring of 1983 suggest that Ekman pumping of on average 4 m day21 contributes to the local dynamics. During strong and persistent upwelling events the curl-driven Ekman pumping reaches up to 20 m day21. A single complex empirical orthogonal function explains more than 72% of the spatial and temporal wind stress variance. It reveals large wind stress curl at the center of the western Santa Barbara Channel entrance. This dominant mode correlates strongly (r2 5 0.79) with the wind stress observed at a moored buoy. The location of largest Ekman pumping in the ocean indicated by this mode coincides with the location of a laterally sheared, cyclonic ¯ow. Cold upwelled waters enter the Santa Barbara Channel along its southern perimeter while warm, and thus buoyant, waters from the Southern California Bight exit the channel along its northern perimeter. Buoy wind stress and lateral current shear at 30-m depth correlate signi®cantly at periods of about 3.5 and 6 days with phase lags of about 0.5 and 2 days, respectively. Hydrographic observations from 1983 do not, however, indicate effects of Ekman pumping on the internal mass ®eld as winds vary in speed and direction at daily timescales. This contrasts with 1984 observations that do indicate strong doming of isopycnals over the center of the channel at its western entrance. Winds prior to and during hydrographic observations in the spring of 1984 were both stronger and more steady than they were in 1983. Cyclonic shears reach 0.4 f at the entrance of the Santa Barbara Channel near Point Conception; here f is the planetary vorticity. The thermal wind balance explains the lateral shear of the alongchannel surface ¯ow rather well. 1. Introduction ing velocity from successive density observations along with mass conservation, Sverdrup (1938) presented the The study of wind-forced motions in the coastal ocean ®rst velocity section across the California shelf and has a long history that probably began with Fridtjof slope seas. He concluded that ``. the processes are far Nansen who, during the 1893±1896 Norwegian expe- more complicated than assumed on the basis of earlier dition to drift with the ice across the Arctic Ocean, data.'' Ongoing and planned observational research on discovered peculiar ice motions to the right of the wind. eastern boundary currents off California and elsewhere He subsequently attributed this motion to the earth's attests to this statement. rotation (Nansen 1902). His qualitative conjecture mo- Just 50 km to the south of Sverdrup's 1937 sections, tivated Walfried Ekman to study this effect using the the California coastline changes its orientation at Point laws of motion that Bjerknes (1901) had just formulated Conception by 90 degrees from north±south to east± for the ocean and atmosphere. A landmark paper re- west. Several large observational experiments took sulted (Ekman 1905), that, I believe, marked the be- place near Point Conception during the last two decades. ginning of coastal physical oceanography. Three de- Early work focused either on Point Conception or on cades later, another Arctic explorer, Harald Sverdrup, the Santa Barbara Channel while more recent work re- was the ®rst to quantify wind-forced upwelling along solved the coupling between these two domains. Brink the California coast. Using an inferred vertical upwell- et al. (1984) described the local atmospheric velocity and the oceanic density ®elds during the spring 1981 transition. They found a complex three-dimensional * Institute of Marine and Coastal Sciences Contribution Number density ®eld that responded to the local upwelling-fa- 98-26. vorable winds. The study commented on the spatial var- iability of the wind ®eld, but wind observations were insuf®cient to statistically analyze its spatial variability. Corresponding author address: Andreas MuÈnchow, The Graduate College of Marine Studies, University of Delaware, Robinson Hall, During the spring of 1983 several investigators col- Newark, DE 19716-3501. lected data with ships and aircraft in an extensive study E-mail: [email protected] of the meteorology and oceanography near Point Con- q 2000 American Meteorological Society Unauthenticated | Downloaded 09/26/21 06:43 PM UTC 1266 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 30 ception. Atkinson et al. (1986) outline the experiment netic energy is often trapped near such regions. The and discuss the hydrographic variability. Brink and wind ®eld along the coast near Point Conception has Muench (1986) and Auad et al. (1998) dynamically an- been analyzed by Caldwell et al. (1986) with time series alyze velocity measurements from this and a subsequent data. They emphasize temporal variability, but never- 1984 experiment for coastally trapped oscillating mo- theless ®nd a wind ®eld that varies along the coast. Their tions at subtidal timescales. MuÈnchow (1998) describes data, however, are inadequate to study the effect of the diurnal and semidiurnal tidal currents. Barth and Brink local wind stress curl since most of their wind stations (1987) study oceanic dynamics with data from a vessel- are on land. The data thus do not resolve the wind ®eld mounted acoustic Doppler current pro®ler. Emphasizing over the ocean where most of the spatial variability data from one particular upwelling event, they concep- occurs. In a different study, Eddington et al. (1992) tualize the oceanic ¯ow near Point Conception as an model the winds near Point Conception with a layer upwelling center with a cold plume that extends more model that features an active lower marine layer and a than 80 km offshore. More recently Strub et al. (1991) stagnant atmosphere above. Their process study suc- interpret such plumes near capes off California as me- cessfully reproduces both a wind maximum south of anders of the California Current system. Furthermore, Point Conception and the large horizontal wind shears the cold plume near Point Conception frequently enters across the western entrance of the Santa Barbara Chan- the Santa Barbara Channel instead of moving offshore. nel. Similar observational and theoretical studies are Barth and Brink (1987) thus describe only one of many reported for the CODE region off Northern California responses of the coastal ocean near Point Conception by Enriquez and Friehe (1995) and Samelson (1992). to the generally upwelling-favorable winds. Using more Here I investigate the spatially variable wind ®eld at recent data, Harms and Winant (1998) and Dever et al. the western entrance of the Santa Barbara Channel near (1998) study low-frequency surface circulation in the Point Conception. More speci®cally, I will discuss cir- Santa Barbara Channel with current meter mooring and cumstantial evidence to present the hypothesis that the drifter data, respectively. They ®nd a complex circu- wind stress curl near Point Conception contributes to lation that they classify in terms of reoccurring synoptic the oceanic circulation in the western Santa Barbara patterns suggested by empirical orthogonal functions Channel. This study statistically analyzes wind data (EOFs). They both comment on the forcing by the local from aircraft over¯ights in the spring of 1983, dem- winds, associated upwelling, as well as barotropic and onstrates a strong correlation between wind and wind baroclinic pressure gradients. stress curl, shows that atmospheric conditions in the Halliwell and Allen (1987) review the atmospheric spring of 1983 are similar to those in 1984, and, ®nally, circulation along the entire U.S. West Coast for the pe- interprets oceanic conditions in the spring of 1984 with riod of 1981±82 while Dorman and Winant (1995) sta- the wind patterns of 1983 in mind. The wind stress curl tistically analyze wind signals for the period of 1981± near Point Conception will emerge as one of several 90. Each winter passing cyclones and anticyclones cause potentially important forcing mechanisms for the cir- coastal winds that vary with time. In contrast, during culation in the western Santa Barbara Channel. the spring and summer seasons an almost stationary high pressure system over the subtropical Paci®c and a low 2. Data sources and study area pressure system over the southwestern deserts of the United States cause strong and persistent southward Aircraft wind observations constitute the main data winds along the coasts of California. The transition from source of this study. In April and May of 1983 the the winter to the spring/summer regime generally occurs NCAR aircraft Queen Air surveyed the wind ®eld near within a few days in March or April (Strub et al. 1987). Point Conception. Friehe et al. (1984) describe the in- The largest wind velocities occur at the height of a strumentation aboard the aircraft and its capability to temperature inversion that caps the cool and moist air measure winds over the coastal ocean off northern Cal- of a well-mixed marine layer from hot and dry air of ifornia. The appendix presents details of the wind cal- the upper atmosphere (Beardsley et al. 1987). The thick- ibration, the estimation of wind stresses, and the errors ness of the marine layer decreases from about 2000 m in derived quantities. Figure 1 shows the track of each near Hawaii to less than 200 m off California and ap- of 20 airborne wind ®eld surveys that took about 3±4 pears to be in thermal wind balance (Beardsley et al. hours to complete. The sea breeze in the study area can 1987).
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