Evidence for Large-Scale Eddy-Driven Gyresin the North Atlantic

M. Susan Lozier

Analysis of a recent climatological database for the North Atlantic has detailed anextensive large-scale recirculation system in the intermediate waters of the North Atlanticbasin. The pressure fields that define this recirculation, coupled with the potential vor-ticity fields associated with the recirculating flow, provide observational evidence forbasin-scale eddy-driven flow in the global ocean. The recirculations are intimately tiedto the waters carried southward by the Deep Western Boundary Current and are thereforelikely to affect the distribution of climatic anomalies in the North Atlantic.

The meridional thermohaline overturningcell in the Atlantic has been described as aconveyor belt, with surface waters trans-porting heat from the South Atlantic viaswift, narrow boundary currents to the deepwater formation sites in the northern NorthAtlantic. To complete the cycle, cold newlyventilated deep waters are exported fromthe high latitudes via the Deep WesternBoundary Current, which follows the west-ern boundary of the North and South At-lantic basins. Though attractive in its sim-plicity, this two-dimensional interpretationexcludes recirculations in the horizontalplane.

Recirculations in the upper thermoclinewaters of the North Atlantic have beenfairly well mapped compared with those inthe deeper ocean, primarily because of dif-ferences in data resolution between the sur-face and deep ocean. However, a recentclimatological database for the North At-lantic (1) has considerably improved theresolution of the intermediate and deep hy-drographic property fields in the North At-lantic over previous global databases (2),chiefly by choosing smoothing scales specif-ic to the relatively data-rich North Atlan-tic. One of the more intriguing features thathas emerged from this hydrographic data-base is the detailed signature of extensiverecirculations on surfaces of constant po-tential density anomaly (3), which aretermed isopycnals, below the main thermo-cline (1) (Fig. 1). Isobars in the upper ther-mocline (Fig. 1A) trace the large-scale, an-ticyclonic subtropical gyre that is driven bythe curl of the wind stress. The “bowl”formed by the isobars, with the depressionlocated in the western portion of the gyre, ischaracteristic of all surfaces lying in themain thermocline. At the base of the ther-mocline (Fig. 1B), the isopycnals are rela-tively flat south of the Gulf Stream, whichindicates the weakening of the direct windforcing. The distinct lack of vertical shear

in this area is consistent with the priorcharacterization (4) of this region as theweakly depth-dependent segment of theNorth Atlantic. Deeper in the water col-umn (Fig. 1C), at ;2200 dbar, the isobarsindicate a strong reappearance of recircula-tion with a pattern that is distinct from therecirculation in the upper thermocline (Fig.1A). Instead of one broad recirculation thatcharacterizes the wind-driven gyre in theupper thermocline waters, the pattern cre-ated by the isobars on this deep isopycnaldefines several recirculation cells situated inthe near vicinity of the Gulf Stream andNorth Atlantic Current. These individualrecirculations, separated by low-pressureridges, are linked to form a large-scale an-ticyclonic recirculation (5) that is in gen-eral agreement with a prior study based onquasi-synoptic data (6). As defined by the2000- to 2200-dbar contours (Fig. 1C), thewaters from the Gulf Stream are divertedinto the North Atlantic Current, flow east-ward over the Mid-Atlantic Ridge, recircu-late anticyclonically southwest of Ireland(7), and flow westward back over the ridge(8). The flow continues along the westernflank of the Mid-Atlantic Ridge, turningwestward near 37°N to rejoin the GulfStream, thus forming a strong, extensiveanticyclonic recirculation of the subtropicalwaters. These gyres are a persistent featurein the vertical, extending over ;1700 m ofthe water column. The horizontal extent ofthe gyres changes with depth; the maximumextent is on the surface shown in Fig. 1C.The recirculation shown by this pressuremap (Fig. 1C) links previously observedlocal gyres (9) into a large-scale gyre thatrecirculates subtropical waters and entrainssubpolar waters. In addition to the docu-mented entrainment of Deep WesternBoundary Current waters at these depthsinto the subtropical waters off of Cape Hat-teras (10), this gyre system identifies a newpathway for subpolar waters to enter thesubtropical gyre, namely from the southernbranches of the North Atlantic Current onboth sides of the Mid-Atlantic Ridge. A

recent study on the spreading of LabradorSea Water supports such a pathway (11).

The depth and structure of the interme-diate recirculation in Fig. 1C weigh againstwind forcing as the driving mechanism forthis flow. Instead, the gyres could be eddy-driven (12) or created by pressure torquesassociated with bottom topography in a ba-roclinic ocean (13). Though the lattermechanism has been shown to produce re-circulations at depth in non–eddy-resolvingmodels (14), the horizontal and verticalstructure of the gyres shown in Fig. 1Cargues against such a choice as the primarydriving mechanism (15). However, thealignment of the intermediate gyres withthe western boundary current system andthe gyres’ maximal extent at mid-depth areconsistent with eddy-driven flow. The forc-ing provided by eddy fluxes of potentialvorticity, which are present as a result of theinstability of the wind-driven flow, can pro-duce closed geostrophic contours at depth.According to a theory formulated 15 yearsago, potential vorticity is expected to beuniform within the regions of closedgeostrophic contours, with the gradients ofpotential vorticity confined to the edges ofthe gyre (16). Numerical experiments witheddy-resolving quasi-geostrophic generalcirculation models have provided strikingexamples of deep eddy-driven gyres andtheir coincident pools of homogenous po-tential vorticity (17). When the North At-lantic potential vorticity field was previous-ly examined for evidence of homogeniza-tion (18), a region of uniform potentialvorticity was found in the upper thermo-cline. It remains ambiguous whether theobserved uniformity is actually connectedto the process of potential vorticity homog-enization or whether it reflects the presenceof a single water mass in the region, createdby convective processes (19). These priorstudies included only those waters above;1100 m in the North Atlantic, leavingthe depths below the main thermoclinelargely unexplored.

To study the potential vorticity signalassociated with the noted recirculations,high-resolution fields of potential vorticity(20) were produced from the climatologicaldatabase described in (1) (Fig. 2). In theupper thermocline (Fig. 2A), the potentialvorticity contours trace the subtropical gyrepattern, but no homogenization is evident.The outcrop at the northern edge of thegyre provides ample ventilation to preventhomogenization. The field is marked by asharp northward gradient across the GulfStream, which reflects the sharp decrease inlayer thickness across that jet. A generallynorthward gradient of potential vorticityappears in the lower thermocline (Fig. 2B).This trend is interrupted at mid-latitudes by

Division of Earth and Ocean Sciences, Nicholas School ofthe Environment, Duke University, Box 90230, Durham,NC 27708–0230, USA.

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a tongue of relatively high potential vortic-ity, supplied by the deep Mediterraneanwaters that spread westward to ;50°W(21). Unlike the upper thermocline, thepotential vorticity gradient across the GulfStream is generally weaker than elsewhereon this isopycnal. As depth increases, theextent of this weakening increases until apooling of waters with nearly uniform po-tential vorticity results. The pooling reach-es its maximum spatial extent at a depth of

approximately 2200 dbar (Fig. 2C), wherethe potential vorticity underneath the GulfStream, the North Atlantic Current, andtheir surroundings is virtually uniform. Foreven deeper surfaces, the pooling recedesand weakens, yielding a potential vorticityfield that is generally dominated by plane-tary vorticity (22).

To show that the pool of nearly homog-enous potential vorticity is generally coin-cident with the large-scale recirculation, an

absolute velocity field (23) for the deepsurface was superposed on its potential vor-ticity field for the area of interest (Fig. 3).The region of homogenized potential vor-ticity, shaded yellow, resides within the se-ries of anticyclonic gyres west of the Mid-Atlantic Ridge (the recirculating gyres ofthe Sargasso Sea and the Newfoundlandbasin), lending credence to the suppositionthat the homogenization of potential vor-ticity is a consequence of the recirculation.

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Fig. 1. Pressure in decibars on three selected isopycnals, s0 5 26.50 (A),s1.5 5 34.60 (B), and s2 5 36.95 (C), which represent upper thermoclinewaters, lower thermocline waters, and deep waters of the subtropicalNorth Atlantic, respectively. Because the horizontal pressure gradient onan isopycnal is essentially equivalent to the slope of an isopycnal, a pres-sure map yields the approximate direction of the vertical shear. The hori-zontal pressure gradient imposed by sea surface topography is unknown;therefore the pressure gradient deduced from the density field alone (thatis, from the hydrography) yields information on the relative flow field only. Ifone assumes that the velocity field goes to zero at a location in the watercolumn (a level-of-no-motion assumption), the isobars approximate abso-lute flow lines. A deep level of no motion for the subtropical circulation,

including the Gulf Stream, its recirculations, and its extension into theNorth Atlantic Current, is used for the interpretation of these pressuremaps; thus high pressures are to the right of the flow, looking downstream.For s0 5 26.50 (A), the pressure is contoured at 50 dbar. This surfaceoutcrops in winter between the 50- and 100-dbar contour. For s1.5 534.60 (B), the pressure is contoured at 100 dbar (solid lines) except for the1450- and 1550-dbar contours (dashed lines). For s2 5 36.95 (C), thepressure is contoured at 50 dbar (solid lines); the recirculations are detailedwith 25 dbar contours from 2200 to 2325 dbar (dashed lines). Bathymetry,500 m is shaded dark gray for all surfaces, bathymetry ,1000 m isshaded medium gray in (B) and (C), and bathymetry ,2000 m is shadedlight gray in (C).

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Fig. 2. Potential vorticity on the same isopycnals as in Fig. 1. For s0 526.50 (A), the contour intervals are 100 (solid lines) and 200 (long-dashedlines) 3 10212 m21 s21 bar. The 150 3 10212 m21 s21 contour is desig-nated with a short-dashed line. For s1.5 5 34.60 (B), the contour intervalsare 2 (solid lines) and 50 (dashed lines) dbar. For s2 5 36.95 (C), the

contour intervals are 1 (solid lines), 10 (short-dashed lines), and 100 (long-dashed lines) 3 10212 m21 s21. Bathymetry is as in Fig. 1. The deeppotential vorticity field (C) preserves its uniformity even when contoured atthe level of the computed standard errors (0.2 to 0.4 3 10212 m21 s21 inthe western North Atlantic).

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Actually, the existence of the coincident,homogeneous, potential vorticity pool is it-self evidence for the existence of the recir-culation. The relatively low value of poten-tial vorticity within the pool reflects thetendency of western-intensified anticy-clonic flow to preferentially advect watersfrom the southern rim of the gyre into thegyre interior (24). Finally, the southern rimof the potential vorticity pool in the west-ern North Atlantic coincides with a regionwhere the necessary conditions for flow in-stability are met, thus suggesting a sourcefor mesoscale eddies in the flow field (25).

In contrast to the gyres west of the Mid-Atlantic Ridge, the anticyclonic gyre east ofthe ridge is characterized by a sizable east-west gradient of potential vorticity (Fig.2C). There is no difference in the standarderrors for the pressure fields east and west ofthe ridge, and the standard errors for poten-tial vorticity are only slightly larger(;0.6 3 10212 m21 s21) east of the ridgethan west of the ridge (;0.2 to 0.4 3 10212

m21 s21). Thus, there is little reason tosuspect that the fields east of the ridge areany less representative than those to thewest. Rather, it is hypothesized that theconditions required for potential vorticityhomogenization are not met in this locale.It is central to the theory of potential vor-ticity homogenization that there are no lo-cal sources of potential vorticity in the lo-cale; such a source would inhibit homoge-nization by essentially resetting the value ofpotential vorticity as the fluid circulatedabout the gyre. On the deep surface (Fig.2C), the highest value of potential vorticityoccurs in the northeast corner of the basin,

where the isopycnal rises rapidly to theupper water column, which is strongly strat-ified. Because these high-potential-vorticitywaters contrast sharply with the low-poten-tial-vorticity waters recirculating in thegyre, it is suggested that the concomitantstrong diffusive fluxes would prevent ho-mogenization of potential vorticity in thisregion (26).

On the basis of the work presented here,it is hypothesized that the signature of gyresin the intermediate North Atlantic watersis an expression of eddy-driven flow. Thishypothesis is based on the following rea-sons. First, the gyres appear at intermediatedepths isolated from the direct wind orbuoyancy forcing of the upper waters andthe strong topographic forcing of the abys-sal waters. Second, the gyres are stronglylinked to the eddy-rich Gulf Stream andNorth Atlantic Current. Using observa-tions at two mooring sites in the GulfStream, Hogg showed that eddy fluxes arestrong enough to force significant recircu-lation gyres in the deep ocean (27). Third,potential vorticity is homogeneous withinthe gyres, with the extent of the homoge-neous pool of potential vorticity maximizedat the depth at which the recirculation isstrongest, as measured by the local gradientof the pressure field. An examination of thepressure fields and potential vorticity fieldsshows that as the recirculation weakens, thepool recedes, which strongly suggests thatthese features are dynamically linked.

The particular eddy mechanism drivingthis flow cannot easily be deduced from theclimatological fields. One theory of eddy-driven flow predicts that baroclinic eddies,

acting on the large-scale Sverdrup flow,would create closed geostrophic contours atdepth (28). A second theory attributes theproduction of closed streamlines in theocean interior to the eddies transported by awestern boundary current, which is append-ed to the recirculating gyres (29). The ver-tical structure of the gyres and their prox-imity to the Gulf Stream and North Atlan-tic Current favors the second theory, butmore investigation is needed before eithertheory of eddy-driven flow can be validatedby these observations.

The large-scale recirculation at inter-mediate depths in the North Atlantic af-fects the propagation of climatic anoma-lies from the convective sites in the north-ern North Atlantic in two important ways.First, it provides a pathway other than theDeep Western Boundary Current for new-ly ventilated deep water to enter the sub-tropical gyre. Second, recirculations canmodulate the signal and increase the res-idence time for properties carried equator-ward from the convective sites. Evidencefor such an impact comes from observa-tions of tracers in the subtropical gyre andthe adjoining Deep Western BoundaryCurrent (30). In particular, a minimumin chlorofluorocarbon concentrations (atracer of newly ventilated waters) coin-cides with the depth range at which theseintermediate recirculations exist. Al-though the intermittency of deep waterformation in this depth range is also animportant factor in the determination ofthis signal, this coincidence suggests thatthese recirculations may contribute to thisminimum through the dilution of the trac-er signal. Finally, the existence of theserecirculations may have to be taken intoaccount in paleoceanographic reconstruc-tions of meridional circulation in theNorth Atlantic.

REFERENCES AND NOTES___________________________

1. M. S. Lozier, W. B. Owens, R. G. Curry, Prog.Oceanogr. 36, 1 (1995).

2. S. Levitus, NOAA Prof. Pap. 13 (Government PrintingOffice, Washington, DC, 1982); S. Levitus and T. P.Boyer, World Ocean Atlas 1994, vol. 4, Temperature,NOAA Atlas NESDIS 4 (National Oceanic and Atmo-spheric Administration, Washington, DC, 1994).

3. Potential density is the density of seawater calculat-ed with a reference pressure, rather than in situ pres-sure, in order to remove compressibility effects. Thepotential density anomaly (sn, in kilograms per cubicmeter) is defined as potential density 2 1000 kgm23. The reference pressure divided by 1000 dbar isdenoted by n. The depth of the potential densityanomaly surface is approximated by its pressure,because 1 dbar ; 1 m.

4. W. J. Schmitz, J. Mar. Res. 38, 111 (1980).5. Because the detailed signature of the recirculation

was created by isobars contoured between 25 and50 dbar, it is important to note that the standarderrors for the pressure field on this surface are ;10 to20 dbar for the western North Atlantic.

6. An earlier analysis of a collection of historical hydro-graphic lines [J. L. Reid, Prog. Oceanogr. 33, 1

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Fig. 3. Velocity field superposed on the potential vorticity field for the s2 5 36.95 surface showing thedomain of interest. The dark line at the bottom of the figure shows the location where the upper layerpotential vorticity gradient changes sign (Fig. 2A). Because this flow field was calculated with a deep levelof no motion for the entire region, the Deep Western Boundary Current (for which a shallow level of nomotion is appropriate) is reversed. Unless one makes some arbitrary choice as to where the level of nomotion should change, the two juxtaposed flows (the deep Gulf Stream and the Deep WesternBoundary Current) cannot be easily delineated. Bathymetry ,1000 m is shaded dark gray and bathym-etry ,2000 m is shaded light gray.

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(1994)] showed a large-scale anticyclonic gyre in thegeneral vicinity of the gyre shown in Fig. 1C. Howev-er, Reid’s gyre and the one depicted in Fig. 1C havesubstantial differences in their vertical structure andhorizontal substructure.

7. Southward flow of North Atlantic Deep Water on theeastern side of the Mid-Atlantic Ridge is supportedby P. M. Saunders [J. Mar. Res. 40, 641 (1982)], S.Gana and C. Provost [J. Mar. Syst. 4, 67 (1993)], andJ. Paillet and H. Mercier (Deep-Sea Res., Part I, inpress).

8. Topographic sill depths of ;1000 m are a barrier forflow of this density into the Nordic Seas.

9. L. V. Worthington, Johns Hopkins Oceanogr. Stud.6, (1976); R. A. Clarke, H. W. Hill, R. F. Reininger,B. A. Warren, J. Phys. Oceanogr. 10, 25 (1980);M. S. McCartney, Prog. Oceanogr. 29, 283 (1992);W. J. Schmitz and M. S. McCartney, Rev. Geophys.31, 29 (1993).

10. R. S. Pickart and W. M. Smethie, J. Phys. Oceanogr.23, 2602 (1993).

11. J. Paillet, M. Arhan, M. S. McCartney, in preparation.12. P. B. Rhines and W. R. Holland, Dyn. Atmos. Oceans

3, 289 (1979).13. W. R. Holland, Geophys. Fluid Dyn. 4, 187 (1973).14. R. Gerdes and C. Koberle, J. Phys. Oceanogr. 25,

2624 (1995).15. These gyres at intermediate depths are distinct from

the smaller scale gyres at abyssal depths that aremore clearly associated with the basin topography.

16. P. B. Rhines and W. R. Young, J. Fluid Mech. 122,347 (1982).

17. W. R. Holland and P. B. Rhines, J. Phys. Oceanogr.10, 1010 (1980).

18. S. McDowell, P. Rhines, T. Keffer, ibid. 12, 1417(1982); T. Keffer, ibid. 15, 509 (1985); J. L. Sar-miento, C. G. H. Rooth, W. Roether, J. Geophys.Res. 87, 8047 (1982).

19. J. Pedlosky, Ocean Circulation Theory (Springer-Verlag, New York, 1996).

20. Potential vorticity is calculated as f 1/so 3 ]sn /]z,where f is the planetary vorticity, z is the verticalcoordinate, and so is a constant potential density.The vertical derivative is computed locally over anominal depth of 100 m. In an effort to approximateneutral surfaces [ T. J. McDougall, J. Phys. Ocean-ogr. 17, 1950 (1987)], potential vorticity was alsocalculated as f/h, where h is the distance betweentwo locally referenced isopycnals. The differences inthe two methods were insignificant to the results ofthis study; that is, the region and extent of homoge-nization were the same with either calculation.

21. M. S. McCartney and L. D. Talley, ibid. 12, 1169(1982).

22. J. O’Dwyer and R. G. Williams (J. Phys. Oceanogr.,in press) report, from an analysis of the Levitus dataset, possible regions of homogenization at abyssaldepths in the western North Atlantic.

23. An absolute flow field was calculated for s2 5 36.95by differentiation of a modified [H.-M. Zhang andN. G. Hogg, J. Mar. Res. 50, 385 (1992)] Montgom-ery stream-function field [R. B. Montgomery, Bull.Am. Meteorol. Soc. 18, 210 (1937)] with s3 5 41.45(at ;3000 m) as the level of no motion.

24. D. L. Musgrave, J. Geophys. Res. 90, 7037 (1985).25. Surfaces shallower than s2 5 36.95 show counter-

rotating gyres separated by this instability region inthe western North Atlantic, which suggests the im-portance of eddy flux divergence in the forcing of thegyres.

26. J. Pedlosky, J. Phys. Oceanogr. 13, 2121 (1983).27. N. Hogg, Deep-Sea Res. Part A 30, 945 (1983).28. P. B. Rhines and W. R. Young, J. Mar. Res. 40, 559

(1982).29. P. Cessi, G. Ierley, W. Young, J. Phys. Oceanogr. 17,

1640 (1987); P. Cessi, ibid. 18, 662 (1988).30. W. J. Jenkins and P. B. Rhines, Nature 286, 877 (1980);

R. S. Pickart, Deep-Sea Res. Part A 39, 1553 (1992);W. M. Smethie, Prog. Oceanogr. 31, 51 (1993).

31. I thank J. Pedlosky for his aid in the interpretation ofthese fields and P. Rhines for his comments on themanuscript. Support from NSF (grant OCE- 9629489)is gratefully acknowledged.

20 February 1997; accepted 22 May 1997

Maximum and Minimum TemperatureTrends for the Globe

David R. Easterling,* Briony Horton, Philip D. Jones,Thomas C. Peterson, Thomas R. Karl, David E. Parker,

M. James Salinger, Vyacheslav Razuvayev, Neil Plummer,Paul Jamason, Christopher K. Folland

Analysis of the global mean surface air temperature has shown that its increase is due,at least in part, to differential changes in daily maximum and minimum temperatures,resulting in a narrowing of the diurnal temperature range (DTR). The analysis, usingstation metadata and improved areal coverage for much of the Southern Hemispherelandmass, indicates that the DTR is continuing to decrease in most parts of the world,that urban effects on globally and hemispherically averaged time series are negligible,and that circulation variations in parts of the Northern Hemisphere appear to be relatedto the DTR. Atmospheric aerosol loading in the Southern Hemisphere is much less thanthat in the Northern Hemisphere, suggesting that there are likely a number of factors,such as increases in cloudiness, contributing to the decreases in DTR.

The global mean surface air temperaturehas risen about 0.5°C during the 20th cen-tury (1). Analysis has shown that this risehas resulted, in part, from the daily mini-mum temperature increasing at a faster rateor decreasing at a slower rate than the dailymaximum, resulting in a decrease in theDTR for many parts of the world (2, 3).Decreases in the DTR were first identifiedin the United States, where large-areatrends show that maximum temperatureshave remained constant or have increasedonly slightly, whereas minimum tempera-tures have increased at a faster rate (4).Similar changes have been found for otherparts of the world as data have becomeavailable, allowing more global analyses (2,3). However, in some areas the pattern hasbeen different: In parts of New Zealand (5)and alpine regions of central Europe (6),maximum and minimum temperature haveincreased at similar rates, and in India, theDTR has increased as a result of a decreasein the minimum temperature (7). To eval-uate these varying results, we conducted anexpanded analysis on global and regionalscales.

Local effects such as urban growth, ir-

rigation, desertification, and variations inlocal land use can all affect the DTR (3);in particular, urbanized areas often show anarrower DTR than nearby rural areas (8).Large-scale climatic effects on the DTRinclude increases in cloud cover, surfaceevaporative cooling from precipitation,greenhouse gases, and tropospheric aero-sols (9, 10). Recent studies have demon-strated a strong relation between trends ofthe DTR and decreases in pan evaporationover the former Soviet Union and theUnited States (11), suggesting that theDTR decrease in these areas is influencedby increases of cloud amount and reducedinsolation (1). Furthermore, recent mod-eling studies have suggested that the de-crease in the DTR may be a result of acombination of direct absorption of infra-red portions of incoming solar radiation,aerosols, and water-vapor feedbacks, in-cluding surface evaporative effects (12).

We analyzed monthly averaged maxi-mum and minimum temperatures and theDTR at 5400 observing stations aroundthe world. Each time series from eachstation was subjected independently tohomogeneity analyses and adjustments ac-cording to recently developed techniques(13). In general, these homogeneity ad-justments have little effect on large-areaaverages (global or hemispheric), but theycan have a noticeable effect on smallerregions (14), particularly when comparingtrends at individual or adjacent grid boxes.

Our data covers 54% of the total globalland area, 17% more than in previousstudies (3). Most of the increases are inthe Southern Hemisphere, with the addi-tion of data for South America, New Zea-land, all of Australia, a number of Pacificislands, and Indonesia. Data were also in-

D. R. Easterling, T. C. Peterson, T. R. Karl, National Cli-matic Data Center, Asheville, NC 28801, USA.B. Horton, D. E. Parker, C. K. Folland, Hadley Center,Meteorological Office, Bracknell, Berkshire, UK.P. D. Jones, Climatic Research Unit, University of EastAnglia, Norwich, UK.M. J. Salinger, National Institute of Water and Atmo-spheric Research, Auckland, New Zealand.V. Razuvayev, All-Russia Research Institute of Hydro-meteorological Information, Obninsk, Russia.N. Plummer, National Climate Center, Bureau of Meteo-rology, Melbourne, Australia.P. Jamason, DynTel Inc., National Climatic Data Center,Asheville, NC 28801, USA.

*To whom correspondence should be addressed: E-mail:deasterl@ncdc.noaa.gov

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