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The North Atlantic Oscillation, Surface Current Velocities, and SST Changes in the Subpolar North Atlantic

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The North Atlantic Oscillation, Surface Current Velocities, and SST Changes in the Subpolar North Atlantic 15 JULY 2003 FLATAU ET AL. 2355 The North Atlantic Oscillation, Surface Current Velocities, and SST Changes in the Subpolar North Atlantic MARIA K. FLATAU,* LYNNE TALLEY, AND PEARN P. N IILER Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California (Manuscript received 24 April 2002, in ®nal form 30 January 2003) ABSTRACT Changes in surface circulation in the subpolar North Atlantic are documented for the recent interannual switch in the North Atlantic Oscillation (NAO) index from positive values in the early 1990s to negative values in 1995/96. Data from Lagrangian drifters, which were deployed in the North Atlantic from 1992 to 1998, were used to compute the mean and varying surface currents. NCEP winds were used to calculate the Ekman com- ponent, allowing isolation of the geostrophic currents. The mean Ekman velocities are considerably smaller than the mean total velocities that resemble historical analyses. The northeastward ¯ow of the North Atlantic Current is organized into three strong cores associated with topography: along the eastern boundary in Rockall Trough, in the Iceland Basin (the subpolar front), and on the western ¯ank of the Reykjanes Ridge (Irminger Current). The last is isolated in this Eulerian mean from the rest of the North Atlantic Current by a region of weak velocities on the east side of the Reykjanes Ridge. The drifter results during the two different NAO periods are compared with geostrophic ¯ow changes calculated from the NASA/Path®nder monthly gridded sea surface height (SSH) variability products and the Advanced Very High Resolution Radiometer (AVHRR) SST data. During the positive NAO years the northeastward ¯ow in the North Atlantic Current appeared stronger and the circulation in the cyclonic gyre in the Irminger Basin became more intense. This was consistent with the geostrophic velocities calculated from altimetry data and surface temperature changes from AVHRR SST data, which show that during the positive NAO years, with stronger westerlies, the subpolar front was sharper and located farther east. SST gradients intensi®ed in the North Atlantic Current, Irminger Basin, and east of the Shetland Islands during the positive NAO phase, associated with stronger currents. SST differences between positive and negative NAO years were consistent with changes in air±sea heat ¯ux and the eastward shift of the subpolar front. SST advection, as diagnosed from the drifters, likely acted to reduce the SST differences. 1. Introduction Bermuda high. Recently, Thompson et al. (2000) pos- This paper investigates the changes in the ocean sur- tulated that the NAO is a part of the more general Arctic face circulation in the subpolar North Atlantic associ- Oscillation (AO) pattern. The AO can be viewed as a ated with the interannual ``switch'' in the North Atlantic zonally symmetric ``seesaw'' in the atmospheric mass Oscillation (NAO) pattern from positive in the early over the polar cap and midlatitude zonal ring centered 1990s to negative in the fall of 1995/96. As a part of around 458N and is related to the strength of the strato- this investigation, the mean surface circulation is also spheric polar vortex. In this interpretation the zonally derived using surface drifter data and the anomalous symmetric part of NAO is related to the AO state and circulation from drifters is compared with that from al- the zonal asymmetries result from the land±sea contrast. timetry. While the NAO index is usually based on the local SLP The NAO is a major component of interannual to difference between Iceland and Portugal, the AO state interdecadal variability in the North Atlantic area. Its is de®ned by the leading mode empirical orthogonal atmospheric component is characterized by a sea level function of the monthly mean surface pressure north of pressure (SLP) dipole between the Icelandic low and 258N (Thompson and Wallace 1998). In the North At- lantic the ``high index'' NAO and AO are characterized by a larger meridional pressure gradient, stronger west- * Current af®liation: Naval Research Laboratory, Monterey, Cal- ifornia. erlies, and northeastward displacement of the storm track compared with a low index. The SST pattern that accompanies the positive NAO regime has a structure Corresponding author address: Dr. Lynne D. Talley, Scripps In- with low temperatures in the polar regions and south of stitution of Oceanography, University of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0230. 208N, and higher SSTs between 258 and 458N. E-mail: [email protected] The causes of the AO or NAO variability and the role q 2003 American Meteorological Society 2356 JOURNAL OF CLIMATE VOLUME 16 of the ocean are subject to vigorous investigation. Ob- The goal of this work is to assess the change in surface servations indicate that the midlatitude ocean responds currents associated with the relaxing of positive NAO to the surface ¯ux anomalies created by an atmospheric at the end of 1995. The area of interest is shown in Fig. circulation related to the NAO (Cayan 1992). Joyce et 1. Our primary dataset is the Lagrangian drifters de- al. (2000) observed the changes in the Subtropical Mode ployed in the Atlantic between 1992 and 1998. Much Water (STMW) related to atmospheric buoyancy ¯uxes of this dataset was included in the analysis by Fratantoni and hypothesized that a north±south shift of the Gulf (2001) of Lagrangian drifters from the 1990s. Our anal- Stream position associated with STMW modi®cation ysis provides more uniform gridded coverage, particu- could in¯uence the midlatitude storms in the atmosphere larly of the northeast Atlantic and regions of low ve- and lead to a coupled oscillation. There is also evidence locity, and also includes a separation into the Ekman (Sutton and Allen 1997) that the SST anomalies forced and non-Ekman (essentially geostrophic) ¯ows. The Na- by atmospheric circulation can propagate along the the tional Centers for Environmental Prediction (NCEP) Gulf Stream and the North Atlantic Current, in¯uencing winds were used to compute Ekman velocities, yielding the temperature in the North Atlantic about 7 yr after an estimate of the geostrophic ¯ow ®eld based on the their forcing by the original atmospheric disturbance. drifters. The circulation changes inferred from the drift- Forcing of the atmosphere by the ocean is harder to ers were compared to the geostrophic ¯ow anomalies observe. Nevertheless, Czaja and Frankignoul (1999) in calculated from satellite altimetry data and the temper- their analysis of the covariance between atmospheric ature anomalies were obtained from the Jet Propulsion circulation and SST anomalies in the North Atlantic Laboratory (JPL) Advanced Very High Resolution Ra- show maximum covariance at a 6-month lag (SST lead- diometer (AVHRR) weekly averaged, gridded SST. ing the atmospheric circulation). They suggest that the NCEP reanalysis data were used to asses the surface anomalous tropospheric circulation is driven by the ¯uxes. anomalous heat ¯ux related to SST. Observations of ice cover in the Arctic (Mysak and Venegas 1998) show 2. Atmospheric circulation patterns in 1992±98 that ¯uctuation of ice extent can also contribute to changes in the atmospheric pressure and NAO pattern The North Atlantic Oscillation index between 1992 by in¯uencing the surface heat ¯ux. and 1998 used in this paper was taken from NCEP. The In general, the ocean±atmosphere coupling mecha- NCEP index is based on principal component analysis nism may involve the generation of the SST±ocean cir- of the most prominent teleconnection patterns (Barnston culation anomaly by atmospheric anomalies, advection and Livezey 1987) and is calculated monthly. We de- of anomalous SST by the ocean currents, or triggering ®ned the state of the NAO using the value of this index the anomalous SST pattern by anomalous circulation averaged for the winter months (December to February; and response of the atmosphere to the new anomalous Fig. 2). The NAO index de®ned in this manner was SST ®eld. Because of all the processes involved, the predominantly positive in the 1990s following the trend description of anomalous ocean currents related to NAO that started in the 1980s. However, a short period of variability is quite important. reversal occurred between 1995 and 1998, when the The recent switch in the NAO index that occurred in index suddenly switched from a high positive value (0.9) 1995 was during a period of intense in situ ocean ob- in the winter of 1994/95 to a low value (20.4) the servations and global satellite coverage. This created following winter. In 1996/97 the index increased but the opportunity for extensive documentation of atmo- remained negative, and decreased again in 1997/98. The spheric and oceanic changes related to the NAO pattern positive NAO phase returned in winter 1998/99. The reversal. Using data from a World Ocean Circulation value of the index changed slightly if March or March Experiment (WOCE) hydrographic section between and April were included in the average, but the essential Greenland and Ireland, Bersch et al. (1999) noticed that characteristics of the NAO evolution, with the 1995/96 relaxation of the NAO positive phase in 1995/96 was switch and ensuring weaker but negative NAO, re- accompanied by decreased density in the Subpolar mained the same. Mode Water, westward retreat of the subpolar front in A single-variable NAO index, based on the pressure the Iceland Basin, and suppressed spreading of the Lab- difference between Lisbon, Portugal, and southwest Ice- rador Sea Water. Reverdin et al. (1999) documented the land (Hurrell 1995), showed a slightly different tem- changes in ocean temperature along line AX2 of WOCE poral pattern. In became negative in 1995/96, but pos- (between Iceland and Newfoundland). They noticed that itive (although small) values returned in the two fol- interannual changes observed between 1994 and 1998 lowing years.
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