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Seasonal Variations of Sea Surface Height in the Gulf Stream Region*

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Seasonal Variations of Sea Surface Height in the Gulf Stream Region* VOLUME 29 JOURNAL OF PHYSICAL OCEANOGRAPHY MARCH 1999 Seasonal Variations of Sea Surface Height in the Gulf Stream Region* KATHRYN A. KELLY,1 SANDIPA SINGH, AND RUI XIN HUANG Department of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts (Manuscript received 2 October 1996, in ®nal form 12 March 1998) ABSTRACT Based on more than four years of altimetric sea surface height (SSH) data, the Gulf Stream shows distinct seasonal variations in surface transport and latitudinal position, with a seasonal range in the SSH difference across the Gulf Stream of 0.14 m and a seasonal range in position of 0.428 lat. The seasonal variations are most pronounced west (upstream) of about 638W, near the Gulf Stream's warm core. The changes in the SSH difference across the Gulf Stream are successfully modeled as a steric response to ECMWF heat ¯uxes, after removing the large SSH variations due to seasonal position changes of the Gulf Stream. A phase shift between predicted and observed SSH changes in the Gulf Stream suggests that advection may be important in the seasonal heat budget. Consistent with the interpretation of SSH variations as steric, comparisons with hydrographic data suggest that the fall maximum SSH difference is from the upper 250 m of the water column. The maximum volume transport is in the spring. Zonally averaged indices are used to quantify seasonal changes in the Gulf Stream, which are analogous to changes in the atmospheric jet stream. 1. Introduction inverted echo sounders suggest that the SSH ¯uctuations do re¯ect changes in upper-layer transport (Teague and Sea surface height (SSH), as measured by a radar Hallock 1990; Kelly and Watts 1994; Hallock and altimeter, contains the signature of several ocean pro- Teague 1993). However, seasonal SSH and even inter- cesses, including the steric response to the seasonal heat- annual variations may be dominated by the steric re- ing cycle and changes in ocean circulation, both baro- sponse to seasonal heating. clinic and barotropic. Changes in both the ocean's heat SSH maps can be interpreted in terms of circulation storage and the large-scale circulation are important to because the ocean is approximately in geostrophic bal- understanding climate issues; however, it is necessary ance, and the SSH h (relative to the earth's gravitational to distinguish between the two processes in SSH ob- geoid) gives the surface pressure, p95rgh. Gradients servations. An increase in the transport of the Gulf of SSH are related to geostrophic velocity ug at the Stream may signal an increase in the deep-water for- ocean surface by mation rate at high latitudes. Coupled ocean±atmo- sphere models suggest that changes in the meridional f k 3 ug 52g=h, (1) overturning circulation of the North Atlantic require changes in the Gulf Stream transport (e.g., Delworth et where f is the Coriolis parameter. A useful quantity in al. 1993). If changes in upper-ocean circulation can be describing the current ¯uctuations is the SSH difference inferred from altimetric SSH, changes in the overturning across the current (surface transport), which is the in- circulation could be monitored by satellite. On time- tegral of (one component of) the geostrophic velocity scales of weeks, comparisons of altimetric data with f Dh 5 u (y9) dy9, (2) g E g y where y is a line across the Gulf Stream. This quantity * Woods Hole Oceanographic Institution Contribution Number is insensitive to the orientation of y relative to the current 9367. direction. To infer volume transport from surface trans- 1 Current af®liation: Applied Physics Laboratory, University of Washington, Seattle, Washington. port, it is necessary to know the vertical structure of the current. In a barotropic ocean, for example, Dh would be proportional to the total transport of the cur- Corresponding author address: Dr. Kathryn A. Kelly, Applied rent system, or in a system characterized by one active Physics Laboratory, College of Ocean and Fishery Sciences, Uni- versity of Washington, 1013 N.E. 40th Street, Seattle, WA 98105- upper layer and a quiescent lower layer, it would be 6698. proportional to the transport of the upper layer. E-mail: [email protected] Seasonal heating produces changes in SSH, hsteric, due q 1999 American Meteorological Society 313 314 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 29 to the thermal expansion or contraction of the water particularly west of about 608W. The most robust sea- column, given by sonal variation is large-scale position changes: the four 0 years of altimeter data shown here are consistent with h 5 a T9 dz, (3) analyses of seven years of AVHRR data by Lee and steric E Tm Cornillon (1995) and with previous analyses summa- 2h m rized by Tracey and Watts (1986), showing the most whereT9m is the temperature anomaly, hm is the depth northerly position of the Gulf Stream occurs in the fall. 21 to which the anomaly penetrates, and aT 5]2r 0 r/]Tm In addition, Kelly (1991) and Qiu et al. (1991) pointed is the coef®cient of thermal expansion. Spatial gradients out that more northerly positions of the western bound- in hsteric will give rise to geostrophic currents as in (1), ary current are associated with larger surface transport. but these currents will be con®ned to the region above These Gulf Stream ¯uctuations are reminiscent of the shallow seasonal thermocline (Gill and Niiler 1973). changes observed in the atmospheric jet stream. Namias When the dominant source of SSH ¯uctuations is sea- (1950) described the ``zonal index cyle'' in which the sonal heating, then the surface transport anomaly Dh9 jet stream transitions in late winter or early spring from is more logically interpreted as a heat content anomaly a northerly straight path with a large eastward velocity than as a volume transport anomaly. (high index) to a more southerly, convoluted path with There have been numerous studies of altimetric SSH accompanying cyclones and weak eastward velocity in an attempt to characterize the seasonal surface trans- (low index). The zonal index, de®ned as the zonally port of western boundary currents. Studies using data averaged eastward velocity in a ®xed latitude range, was from the Geosat altimeter in both the Gulf Stream and ®rst introduced by Rossby (1939) and was widely used the Kuroshio Extension showed that the maximum in in synoptic studies of the atmospheric circulation in the SSH difference occurs in the fall (Tai 1990; Zlotnicki 1950s and 1960s. Namias argued that changes in the 1991; Kelly 1991; Qiu et al. 1991). Recent studies of index cycle were tied to the seasonal heating cycle and surface transport using the TOPEX/Poseidon data (Qiu that the abrupt change from a high index to a low index 1995; Wang and Koblinsky 1995), including the analysis in the early spring corresponded to a release of the res- presented here, show a fall surface transport maximum. ervoir of cold air from winter cooling, which was con- Because hydrographic data show a spring or early sum- ®ned to northern latitudes by the strong jet stream. mer volume transport maximum (Worthington 1976; To quantify the seasonal changes in the SSH, we de- Sato and Rossby 1995), seasonal heating has been sus- ®ne several zonally averaged indices. To understand the pected of creating the fall surface transport maximum. vertical structure of seasonal variations, we compare the Seasonal heating causes SSH variations of the right observed SSH variations with dynamic height derived magnitude, but the maximum in SSH difference across from hydrographic data. Finally, we estimate the effects the Gulf Stream requires that the regions north and south of seasonal heating on the SSH difference across the of the Gulf Stream front have seasonal steric responses Gulf Stream using the net surface heat ¯ux, and discuss that differ by more than 0.1 m. Using the Levitus (1982) some implications of the seasonal heating cycle. The data, Zlotnicki (1991) suggested that the seasonal cycle processing of the data ®elds used in this analysis is in steric height difference was too small by a factor of described in section 2. Maps of total SSH from the 2 to explain the Geosat SSH difference. Using the newer TOPEX/Poseidon altimeter were made by adding the version of the Levitus data (Levitus and Boyer 1994), SSH anomaly to a mean SSH (Fig. 2), which is also Wang and Koblinsky (1996) showed that the seasonal discussed in section 2. The analyses of the data ®elds steric height variations were consistent with seasonal are presented in section 3, followed by a discussion of variations in TOPEX/Poseidon SSH in the region of the results in section 4 and conclusions in section 5. Gulf Stream recirculation. However, based on a buoy- ancy budget using heat ¯uxes from the European Centre 2. Data ®elds for Medium-Range Weather Forecasts (ECMWF), they concluded that the heat ¯ux gradients over the Gulf Collinear height pro®les from the TOPEX/Poseidon Stream were 2±3 times too small to explain the observed altimeters [the Archiving Validation and Interpretation SSH gradients. Here we show that the inconsistency of Satellite Oceanographic data (AVISO) Merged Geo- with the ECMWF heat ¯ux is primarily the result of physical Data Record] cycles 4 through 80 (October neglecting the SSH variations due to the north±south 1992±November 1994) were processed using programs seasonal Gulf Stream position migration. described in Caruso et al. (1990) to obtain pro®les of There are other low-frequency variations in the Gulf residual SSH. Cycles 1 through 3 were discarded be- Stream besides the seasonal surface transport (Fig. 1), cause of poor data quality. All of the suggested correc- some of which are similar to those in the atmospheric tions from the CD-ROMs were applied and no orbit jet stream.
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