Contribution of Horizontal Advection to the Interannual Variability of Sea Surface Temperature in the North Atlantic

964 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 33

NATHALIE VERBRUGGE Laboratoire d'Etudes en GeÂophysique et OceÂanographie Spatiales, Toulouse, France

GILLES REVERDIN Laboratoire d'Etudes en GeÂophysique et OceÂanographie Spatiales, Toulouse, and Laboratoire d'OceÂanographie Dynamique et de Climatologie, Paris, France

(Manuscript received 9 February 2002, in ®nal form 15 October 2002)

ABSTRACT The interannual variability of sea surface temperature (SST) in the North Atlantic is investigated from October 1992 to October 1999 with special emphasis on analyzing the contribution of horizontal advection to this variability. Horizontal advection is estimated using anomalous geostrophic currents derived from the TOPEX/ Poseidon sea level data, average currents estimated from drifter data, scatterometer-derived Ekman drifts, and monthly SST data. These estimates have large uncertainties, in particular related to the sea level product, the average currents, and the mixed-layer depth, that contribute signi®cantly to the nonclosure of the surface temperature budget. The large scales in winter temperature change over a year present similarities with the heat ¯uxes integrated over the same periods. However, the amplitudes do not match well. Furthermore, in the western subtropical gyre (south of the Gulf Stream) and in the subpolar regions, the time evolutions of both ®elds are different. In both regions, advection is found to contribute signi®cantly to the interannual winter temperature variability. In the subpolar gyre, advection often contributes more to the SST variability than the heat ¯uxes. It seems in particular responsible for a low-frequency trend from 1994 to 1998 (increase in the subpolar gyre and decrease in the western subtropical gyre), which is not found in the heat ¯uxes and in the North Atlantic Oscillation index after 1996.

1. Introduction 2000). This is further illustrated by Battisti et al. (1995), who simulate well the winter SST using a one-dimen- Earlier analyses of observations revealed that a large sional model of the upper North Atlantic Ocean during part of the sea surface temperature (SST) variability on the 1950±88 period. Using both an ocean mixed-layer seasonal to interannual timescales at middle and high model and a dynamical ocean general circulation model latitudes is driven by the atmospheric forcing through (OGCM) coupled with an atmospheric mixed-layer the air±sea heat exchange and/or the wind-driven cir- model, Seager et al. (2000) ®nd that a large part of the culation (Frankignoul 1985; Wallace and Jiang 1990; interannual to decadal variability of the North Atlantic Deser and Timlin 1997). Halliwell and Mayer (1996) SST between 1958 and 1998 can be explained as a show a large similitude between the SST response ob- response to changes in surface heat ¯uxes. tained from a theoretical stochastic forcing model and Cayan (1992), using COADS SST and heat ¯ux data, in the Comprehensive Ocean±Atmosphere Data Set suggests that the amount of intermonthly d(sst)/dt var- (COADS) in the westerly and trade wind latitude bands iance accounted for by the heat ¯ux anomalies during at periods of several months to a few years. The analysis winter months ranges from 10% to 40% in the North of COADS data presented in Kushnir (1994) suggests Atlantic with the largest values at the midlatitudes. also that the interannual SST anomalies are driven by Therefore, this analysis reveals a large atmospheric forc- the circulation anomalies. Models con®rm the impor- ing which controls the large-scale patterns of month-to- tance of the atmospheric forcing on the interannual SST month SST variability but also suggests that it is nec- variability but also at longer period (decadal and more) essary to consider other processes to explain SST chang- in the last four decades (Halliwell 1998; HaÈkkinen es. In the Gulf Stream region, Kelly and Qiu (1995) showed that both Ekman and geostrophic advection sig- Corresponding author address: Dr. Gilles Reverdin, LODYC, Case ni®cantly contributes to the SST changes between No- 100, University Paris VI, 4 Place Jussieu, Paris 75005, France. vember 1986 and April 1989. Deser and Blackmon E-mail: [email protected] (1993) observed a surface warming trend in the western

᭧ 2003 American Meteorological Society

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North Atlantic during the 1920s and 1930s based on dT (١T) ϩ R, (1 ١TЈ) Ϫ (U ϭ Q Ϫ (UЈ١T ϩ U COADS data and hypothesized that the SST warming dt ig g ek along the Gulf Stream resulted from altered ocean currents rather than local wind forcing. Different analyses with of ocean GCM simulations show that the current ¯uc- heat fluxes Q ϭ . tuations in the western Atlantic and the North Atlantic i ␳ cH current could result from subtropical gyre adjustments 0 p to the wind-driven circulation or to modi®cations in the In this equation, the prime and overbar refer to the intensity of the overturning on interannual to decadal anomalous and time-averaged parts of the considered timescales (Halliwell 1998; HaÈkkinen 2000; Eden and large-scale ®eld (T ϭϩTUUTЈ; Uek ϭϩekЈ ek; Ug ϭ Willebrand 2001). These studies suggest that the chang- UUggϩ Ј); H is an adequate depth over which the heat es in the oceanic circulation play an important role in and momentum ¯uxes are distributed, and the horizontal the decadal variability of the North Atlantic Ocean. current U is assumed to be the combination of a geo- Analyses of winter surface temperature during the strophic and Ekman components. 1950s to early 1990s also show the propagation of SST The ®rst term corresponds to the heat ¯uxes; the sec- anomalies from south of the Gulf Stream (GS) to the ond term on the right-hand side of the equation repre- subpolar gyre following more or less the major currents sents the horizontal temperature advection (ADV-sum) (GS and North Atlantic current; Hansen and Bezdek as a sum of the contributions by the geostrophic current 1996; Sutton and Allen 1997). This suggests a contri- anomalies (ADV-up) and by the mean oceanic currents bution of the horizontal advection of temperature anom- (ADV-um); the third term is the Ekman advection (EA); alies by the ocean currents to the North Atlantic SST R is a residual that corresponds to the terms we have decadal variability for that period, which is supported neglected in this analysis (in particular, vertical advec- by numerical simulations of low-resolution ocean mod- tion and horizontal transports by mesoscale structures, but also the U T term). Ј els (Halliwell 1998; Visbeck et al. 1998). As discussed Јg١ We estimate the oceanic horizontal advection at the in the Krahmann et al. (2001) ocean modeling study, ocean surface by using anomalous geostrophic currents the SST variability in the subpolar gyre could result derived from the TOPEX/Poseidon (T/P) altimeter sea from the competition or the association between the level, mean currents estimated from drifter data (Rev- horizontal advective effect and the thermodynamic at- erdin et al. 2002), Ekman drifts from the European re- mospheric forcing. In their study, the wind forcing is search satellite scatterometers [product from CERSAT limited to its regression pattern on the North Atlantic (French ERS processing and archiving facility product); Oscillation (NAO) index. For periods of wind forcing Bentamy et al. (1997)], and monthly gridded SST data up to 4 yr, they found that the subpolar response is from Reynolds and Smith (1994). In addition, air±sea dominated by the heat ¯ux forcing. For longer periods, heat ¯uxes are used and some of the results are com- the response integrates the in¯uence of the oceanic ad- pared to the winter (December±March) NAO index from vection and this term explains much of the subpolar Jones et al. (1997) and Osborn et al. (1999). SST variability. The TOPEX/Poseidon project provides high accuracy Up to now there has been no direct estimation of estimates of the sea level (Mitchum 1994) along the horizontal advection (Ekman and geostrophic) and of ground tracks which are repeated every ϳ10 days. The its potential contribution to the interannual variability data are referred as anomalies with respect to the Oc- of SST in the mid- and high latitudes of the North At- tober 1992±October 1999 period and have been linearly lantic Ocean. This paper is an attempt at that. After interpolated on a 1Њϫ1Њ grid from 15Њ to 66ЊN and discussing data and methods (section 2), we will ®rst 100ЊWto20ЊE (excluding data on shelves). Monthly present the dominant basin-scale structures of the SST averages are constructed for the October 1992±October variability between 1992 to 1999 (section 3a) and of the 1999 period. Alternatively, the Developing Use of Al- heat ¯uxes and oceanic advection forcing terms of win- timetry for Climate Studies (DUACS) sea level product ter SST interannual variability (section 3b). Last, the (Ducet et al. 2000) is used (see discussion in the ap- different contributions to these SST 1-yr variations are pendix). averaged in particular areas identi®ed on the EOF pat- The air±sea heat ¯uxes (positive downward) are from terns (section 4). The discussion (section 5) addresses the National Centers for Environmental Prediction±Na- the respective roles of the heat ¯uxes and the horizontal tional Center for Atmospheric Research (NCEP±NCAR) advection terms on the year-to-year SST changes. reanalysis (Kalnay et al. 1996). To evaluate their impact on SST, we divided them by ␳ 0CpH, where Cp is the speci®c heat of seawater, ␳ 0 is the average ocean surface 2. Data and methods density, and H is the mixed-layer depth obtained from the Levitus monthly climatological dataset (as the depth The temperature evolution equation we investigate presenting a density increase with respect to the sea can be written as surface of ⌬␳ ϭ 0.125 kg mϪ3). The same H is used

Unauthenticated | Downloaded 09/25/21 12:28 PM UTC 966 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 33 to estimate the Ekman currents from the CERSAT monthly wind stresses. When the heat ¯uxes and the Ekman advection are integrated from one winter to the next, instead of a monthly mixed-layer depth, we have chosen to use the maximum value of the climatological mixed-layer depth obtained from the Levitus dataset, in order to take into account the vertical redistribution of heat content in winter (Fig. 1). This calculation is more representative of the layer really affected by the heat ¯uxes for interannual variability, although this does not take into account the possible impact of the interannual variability of the winter mixed-layer depth or differ- FIG. 1. Max value of the mixed-layer depth obtained from the ential temperature advection through that layer during Levitus monthly climatological dataset (using ⌬␳ ϭ 0.125 kg mϪ3). the seasons when this layer is strati®ed. The spatial resolution is 1Њ for each dataset. The av- trade winds region and offshore of the European coasts. erage seasonal cycle during the period October 1992± It is strongly negative in the western subtropical gyre October 1999 has been removed from the advection, (west of 40Њ±45ЊW) and north of the Gulf Stream. Its heat ¯uxes, and SST ®elds. Then, a 5-month running value is near 0 in the Gulf Stream and the subarctic mean average has been applied to focus on the inter- front associated with the North Atlantic Current seems annual variability. We also remove small scales in the to be associated with a gradient in the amplitude of the geostrophic currents by applying a 2Њϫ3Њ spatial av- pattern. The areas where this mode captures a large erage. The effect of this ®ltering will be reviewed in percentage of the variability (not shown) are situated the discussion section. What we refer to as the winter south of Nova Scotia (38.5Њ±42.5ЊN, 60.5Њ±69.5ЊW), in season consists of the January±March average, so that the Labrador Sea and in the center of the subpolar gyre the winter SST anomalies are centered on February. (55ЊN, 38ЊW). The associated PC (Fig. 2b) presents Considering instead the SST anomalies centered at the large increases between mid-1994 and mid-1995, and end of winter before the ocean restrati®es (usually, between the end of 1996 and the end of 1997, which March) did not modify substantially the results. contributes to a positive trend between the winter 1994 Empirical orthogonal function (EOF) analysis is used to the end of winter 1998. to identify regions of coherent variability and ®lter in The second EOF (Fig. 2c) has large positive loadings space and time the signals to remove noise resulting between 30Њ and 40ЊN, in particular north of the GS and from errors. It consists in calculating the covariance near 30ЊW. This excludes nevertheless the vicinity of matrix of a zero mean ®eld in order to obtain a set of the Iberian Peninsula, maybe because SST variability is eigenvalues and corresponding eigenvectors. Each ei- somewhat controlled there by upwelling dynamics. In- genvector (EOF) can be viewed as a spatial pattern and terestingly, there are other signs of ocean dynamics on the fraction of the total ®eld variance explained by a this pattern. There is a band of positive values along given EOF is proportional to its associated eigenvalue. the slope of the southern Labrador Sea corresponding Then, in order to see how a given EOF evolves in time, to the location of the Labrador Current and winter ice the eigenvectors are projected onto the original ®eld to edge, and there is a negative extremum near the sub- obtain a time series (principal component, PC herein- arctic front along 51ЊN. PC2 is negative in early 1996 after). and to a lesser degree in mid-1997, after periods of negative winter NAO index. The positive value in 1999 also corresponds to positive NAO index (Fig. 2d). Near 3. Spatial structure of variability 30Њ±40ЊN, this is coherent with what is expected from a. SST a response of SST to the heat ¯uxes associated with NAO with stronger winds when the NAO index is neg- The period investigated is very short (7 yr) so the ative (Cayan 1992), although the signal expected from nature of SST variability differs somewhat from that of a response to NAO heat ¯uxes in the subpolar gyre is a longer period. However, some of the large-scale sig- not evident in the second EOF. nals characteristic of the different seasons are present The SST EOF 1 resembles the interannual composite in 1992±99. To illustrate this, we present the result of SST maps associated with the 50Њ±60ЊN SST average an EOF decomposition for all months on the low-passed of the Kushnir (1994) analyses and the SST EOF 2 SST anomalies. The ®rst and second EOFs represent resembles the one associated with the 30Њ±40ЊN SST respectively 43% and 18% of the total variance. These average. Kushnir obtained these composites by sub- associated PCs project strongly on the corresponding tracting the average winter conditions during cold years ones estimated from the 1981±2000 SST monthly ®elds to those during warm years for 1947 to 1987 for each from Reynolds and Smith (1994). The ®rst EOF (Fig. speci®c latitudinal SST average. The small differences 2a) has positive loadings in the subpolar gyre, in the with Kushnir's analysis, that is the maximum SSTA po-

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FIG. 2. EOFs of sea surface temperature in the North Atlantic (15Њ±66ЊN) from Reynolds SST for Dec 1992±Aug 1999. (a), (c) Spatial patterns of EOF 1 and 2 (thick line is zero contour, and contour interval is 0.2ЊC). (b), (d) Associated principal components (PC) normalized (line) as well as the winter NAO index (asterisks). sition in the western Atlantic shifted northward in our are remarkably consistent. Cayan's study differs slightly ®rst mode and the distinct subpolar structure in our sec- from ours in that he focused on the winter season and ond mode for instance, probably result from the different on monthly timescale instead of interannual. period and seasons in the two studies. Kushnir (1994) The interannual SST variability is investigated by cal- comments that these structures are likely to result mainly culating the difference from one winter to the next, from an ocean response to air±sea heat ¯uxes. We also which is compared to the heat ¯ux forcing term (positive ®nd, however, that parts of the structures identi®ed in downward) integrated over a year. These differences are the ®rst and second EOF here are likely to have an presented in Fig. 3. The pattern of the SST change be- oceanic source. We will now mostly concentrate on in- tween the winters of 1995 and 1996 has the opposite terannual winter variability of SST, which is easiest to sign to the one between the winters 1998 and 1999; that interpret. is, a subtropical cooling and subpolar warming between 1995 and 1996 and an opposite evolution between 1998 b. Interannual SST winter variability and 1999. These patterns resemble the SST tendency First, we will compare the SST variability to the heat signal captured in the second SST mode whose time ¯ux forcing term. Cayan (1992) showed from COADS function is similar to the NAO index (Fig. 2d). The data that the dT/dt variance explained by the heat ¯uxes associated heat forcing term in Fig. 3 usually overlays is in the range of 10%±40% during the winter months. well the pattern of SST change. Obviously, as in Cayan Furthermore, from canonical correlation analysis and (1992), there are similarities between the heat ¯ux terms composites based on atmospheric circulation anomaly and the basin-scale SST changes from one winter to the modes, he demonstrates that the heat ¯uxes (latent plus next. This corresponds to a usually signi®cant correla- sensible) and dT/dt covary with patterns that span the tion between the two sets of ®elds (based on the very ocean basins. He comments nevertheless that the vari- small sample of six ®elds). Nevertheless, there are areas ance of the SST anomalies accounted for by the ¯ux where the ®elds do not overlay well, for example near anomalies is modest even if the patterns of the two ®elds 50Њ±55ЊN between 1998 and 1999 or near 30Њ±40ЊN,

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20Њ±40ЊW between 1996 and 1997. Also, the two ®elds do not overlay well near the GS. The ratio of the standard deviation (STD) of one-year integrated forcing terms to the STD of SST winter-to- winter changes (⌬SST) con®rms the relative importance of the heat ¯uxes on the SST winter-to-winter anomalies (Fig. 4b). In the western and southern parts of the do-

main the ratio of STD(Qi) to STD(⌬SST) (Fig. 4b) is up to 0.3 and reaches 0.8 and more along the U.S. east coast and in the Caribbean Sea. In the eastern part of the basin, the ratio is low, the large amplitude of STD(⌬SST) south of 45ЊN (Fig. 4a) not being associated with large values of the heat ¯ux forcing term. This is an area of large interannual variability of the winter mixed-layer depth that is not taken into account in our estimation of the heat forcing term (the climatological

winter mixed layer depth was retained). Years when Qi is positive are associated there with weak winds (high NAO) (Cayan 1992) and therefore shallow trapping of the SST anomalies, so that we underestimate the variability of the heat ¯ux forcing term. The ratio of the standard deviations of ADV-sum and ⌬SST (Fig. 4c) reveals a signi®cant contribution of the horizontal geostrophic advection (ADV-sum) in the subpolar gyre as well as in the western part of the North Atlantic, particularly south of the Gulf Stream, where STD ADV-sum represents between 30% and 90% of STD(⌬SST). It is often as large or larger than the heat forcing term. Last, note that the large contribution of the horizontal Ekman advection (EA; Fig. 4d) is con- centrated in the areas of strong SST gradients north of the Gulf Stream and east of the Grand Banks of New- foundland as well as in the western part of the North Atlantic Current (with ratios often exceeding 0.9). These results suggest that the temperature horizontal advection terms also signi®cantly contribute to the SST interannual variability in the North Atlantic Ocean and that there is a strong spatial dependency of the processes responsible for SST interannual variability. However, the ratio of the STD of the sum of the advection and

Qi terms on the right-hand side of (1) over STD(⌬SST) (Fig. 4e) is often far from 1. This indicates that the budget is far from closed in large parts of the Atlantic, in particular in the eastern Atlantic, which we will comment further in the discussion section. A similar result is indicated by considering the STD of the difference of the two sides of (1) (not shown).

←

FIG. 3. SST changes (contours) and Qi (shading) integrated from

one winter (JFM) to the next during the 1993±99 period [Qi is es-

timated with Hmax the max mixed-layer depth (from Fig. 1)] (positive anomalies are gray shaded). The contour interval for the SST ®elds is 0.5ЊCyrϪ1 (dashed contours indicate negative anomalies, zero contour is identi®ed by a thick solid line).

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FIG. 4. Standard deviation (STD) of the winter-to-winter SST changes during the (a) 1993±99 period and ratios of the STDs of the 1-yr integrated forcing terms over the STD of the winter-to-winter SST changes (Eq. 1). These terms are (b) Qi, (c) geostrophic advection (ADV-sum), (d) Ekman advection (EA), and (e) the sum of these three terms. STD ratios are nondimensional and SST STD is in degrees Celsius (ЊC). The contours are at 0.3, 0.5, 0.9. The heavy white line indicates the mean path of the Gulf Stream from the 1992±98 T/P data (Frankignoul et al. 2001).

4. Regional heat budgets Stream (GSS), a region north of the Gulf Stream and south of Nova Scotia (NS), the subarctic front area [or To investigate further the contribution of temperature intergyre region (IG)], 48.5Њ±54.5ЊN, 20.5Њ±44.5ЊW,and horizontal advection in the upper ocean to SST vari- the center of the subpolar gyre (SPG). (Positions indi- ability, we will examine the budget integrated over one cated in Figs. 5a and 7a.) year in speci®c regions where the contributions of geostrophic advection or Ekman advection were not small. The choice of the regions is also based on the EOF a. Gulf Stream area decomposition of SST variability between 1992 and 1999 (Fig. 2) and on what we know about the currents. We will ®rst focus the analysis of SST variability in We focus on four regions: a region south of the Gulf the part of the western Atlantic straddled by the Gulf

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in 1994 and since 1998. The dominant EOF of the advection by the average currents ADV-um has a fairly similar PC1 (Fig. 5c) with a correlation of 0.7 at 0 lag. The SST PC1 leads ADV-um by 1 to 6 months. The ADV-um EOF 1 pattern is, however, quite different from the one of SST variability, being shifted toward the northeast along the average Gulf Stream path in comparison with the SST EOF 1 (Figs. 5a,b). This pattern clearly indicates the contribution of the mean current in sweeping downstream (to the northeast) the SST anomalies created south of Nova Scotia (see the gradient in the EOF pattern along the GS in Fig. 5a). The SST anomalies also indicate changes in the Gulf Stream path and current intensity, which can be seen in the change of geostrophic current during that period (Fig. 6, Joyce et al. 2000). To further analyze the heat budget, we will consider more speci®cally the box GSS that includes the western part of the southern recirculation of the Gulf Stream and the box NS centered over the maximum of the SST EOF 1 pattern north of the Gulf Stream in the slope water region. The 2Њϫ3Њ spatial smoothing applied to the geostrophic currents before estimating advection probably diminishes the intensity of the recirculation zone south of the Gulf Stream: This probably mimics the effect of mixing by eddies. In the GSS region between winter 1993 and winter 1994, there is a partial compensation between the 0.2ЊC warming by the air±sea heat ¯uxes and a small cooling by the advection terms (Fig. 7b). For this year interval, the advection term has the opposite sign to the SST change. However, for the other time intervals from 1994 to 1999, the geostrophic advection term (ADV-sum) represents between 50% and more than 100% of ⌬SST and has the same sign. The geostrophic term is often par- tially compensated by the EA, which often has the opposite sign to ⌬SST but is much smaller in this area. This is quite different from what was obtained by Kelly and Qiu (1995) for the November 1986±April 1989 Geosat Exact Repeat Mission period from a numerical model of the upper mixed layer that assimilates temperature and altimetric velocity. They suggest a large contribution of the Ekman advection near the Gulf FIG. 5. First EOF of (a) sea surface temperature and of (b) ADV- Stream and an opposite response of the geostrophic ad- um in the Gulf Stream area (32Њ±46ЊN, west of 55ЊW; Dec 1992± vection to reestablish the alignment between isotherms Aug 1999). Contour interval is 0.2ЊC for SST and the unit for the and geostrophic contours disrupted by the Ekman cur- grayscale is ЊC and ЊC monthϪ1 in (a) and (b), respectively. The heavy dashed line (white or black) indicates the mean path of the Gulf rents. These differences with our results are probably Stream from the 1992±98 T/P data (Frankignoul et al. 2001). The somewhat related to differences in the respective po- two black boxes in (a) indicate the regions used for the regional heat sition of the domain with respect to the Gulf Stream: budget analysis in the Gulf Stream area. (c) The ®rst principal com- the average position of the GS was also further north ponents associated with SST and ADV-um (thin and dashed line, respectively). during the recent period than in 1986±1989 (Joyce et al. 2000). The GSS region does not straddle the GS in the same way as in Kelly and Qiu (1995). Indeed, we Stream. The ®rst EOF of SST in the Gulf Stream area ®nd in our analysis that the contribution of Ekman ad- corresponds to 46.5% of the total variance. It is char- vection (Fig. 4b) is larger farther north. acterized by a strong anomaly north of the current and For the GSS region, we ®nd that the advection by centered at 64Њ±65ЊW (Fig. 5a). The associated PC1 is both the mean currents (ADV-um) and the anomalous a seesaw with low values in 1996±97 and high values currents (ADV-up) are important (Fig. 7b). The ADV-

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FIG. 6. Change in the geostrophic currents between 1996/98 and 1993/95 from T/P on the western Atlantic. A 2Њϫ3Њ spatial smoothing and a 5-month running mean have been applied to the current data. The average seasonal cycle has been removed. up contribution in the GSS could be linked to adjust- gion where the wintertime interannual to subdecadal ments to the wind-driven subtropical gyre circulation variability in SST was not well reproduced is the region (Fig. 6) forced by basin-scale atmospheric circulation off North America between Cape Hatteras and Nova anomalies as is suggested by Halliwell (1998) and Fran- Scotia. The analysis of the winter geostrophic current kignoul et al. (2001), but could also originate in oceanic anomalies (averaged on JFM months) reveals signi®cant processes not driven by the atmospheric anomalies. The variability in this region (Fig. 6). In this area, the di- comparison between the ADV-um and ADV-up inte- rection of the anomalous geostrophic currents changes grated over one year (not shown) and ⌬SST (Fig. 3) between the winters 1995 and 1996: they point toward con®rms that both terms contribute to the SST winter an east to northeast direction during the winter 1995 to winter changes south of the Gulf Stream. Finally, the and toward a western direction during the winter 1996. sum of the heat ¯uxes and horizontal advection terms Between the winters 1998 and 1999, their direction is (SUM) has usually the same sign as ⌬SST (except in reversed one more time. This is coherent with Frankig- 1998±99), but their values differ by up to 0.5ЊCyrϪ1. noul et al.'s (2001) detailed analysis of the Gulf Stream The strong SST variability observed north of the Gulf shifts, with a northern position before 1996 and a shift Stream and south of Nova Scotia remains poorly ex- to further south in 1996. Such lateral Gulf Stream po- plained by the present analysis (Fig. 7c; see also poor sition shifts could cause signi®cant temperature changes performance when using DUACS anomaly currents in at 200-m depth as well as at the ocean surface, as sug- the appendix); neither the sign of ⌬SST nor its mag- gested by Joyce et al. (2000). The impact on the SST nitude are reproduced by SUM. In the years with a large could come both from the isotherm displacements and change in SST (1.7ЊC cooling between 1995 and 1996, from the modi®cations of the air±sea interactions as- or 1.3ЊC warming between 1998 and 1999), they have sociated with the Gulf Stream position change. North the same sign. However, SUM is negative before 1995 of the Gulf Stream and for multiyear variability, Mol- and positive between 1996 and 1998, periods with less inari et al. (1997) observed a cooling from 1969 to 1993 SST change. All the terms contribute signi®cantly to and attributed it to an increase in the width of the strong- SUM with varying signs. Both ADV-up and ADV-um ly baroclinic portion of the current. This increased bar- contribute to the geostrophic advection term, often with oclinicity is characterized by a wider region of strongly an opposite sign. sloping isotherms. They also related the temperature The potential importance of the geostrophic currents changes in the Gulf Stream to the isotherm vertical dis- at the northern edge and in the northern recirculation placements. In their study, the 2Њ meridional spatial res- of the Gulf Stream has also been suggested in other olution prevents a de®nite conclusion regarding vertical studies. In a 1950±88 model simulation with no ad- movement or horizontal shift of the current. vected anomalies by Battisti et al. (1995), the only re- We have not estimated vertical advection. It is, how-

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Unauthenticated | Downloaded 09/25/21 12:28 PM UTC MAY 2003 VERBRUGGE AND REVERDIN 973 ever, unlikely to be a major contribution to the SST cantly through this large domain. Both in the northwest pattern in Fig. 5, which extends to the continental slope. corner area (46Њ±52ЊN, 40Њ±44ЊW) and in the anticy- Interestingly, Frankignoul et al. (2001) show that an clonic steady eddy (40Њ±44ЊN, 41Њ±47ЊW) observed in SST warming (cooling) peaking north of the Gulf the mean geostrophic currents (Reverdin et al. 2002), Stream precedes by one year the latitudinal shifts of the ADV-um, in 1994±95, and ADV-up, in 1995±96, con- Gulf Stream. There is also the possibility that vertical tribute largely to the SST warming (not shown). During motion at the base of the winter mixed layer or near the these two periods, ADV-um and ADV-up oppose each Gulf Stream north wall could entrain water away from other in such a way that ADV-sum becomes insigni®cant the path suggested by the surface current, introducing (ϳ0) in the northwest corner but remains three or four an error in the advection estimates. times as large as ⌬SST in the 42ЊN eddy. In this region with intense currents that cross the box- Some studies mention an in¯uence of surface water es in less than the year over which the budget is inte- from the Labrador Sea on the subpolar and intergyre grated, the information from these spatially averaged surface temperature (Dickson et al. 1988; Reverdin et budgets needs to be combined with the spatial distri- al. 1999; Belkin et al. 1998). This was further investi- bution of advection in Fig. 5. This suggests that the gated by Deser et al. (2002) who observed a slow east- mean geostrophic currents in the Gulf Stream contribute ward progression of negative SST from the Labrador to the SST anomalies propagation toward the northeast. Sea following high sea ice coverage in the Labrador The winter to winter integrated ADV-um patterns (not Sea. Such an event took place in 1990±91 and 1993. shown) also indicate that the SST one-year warming These studies suggest an advection of negative SST (cooling) in the Gulf Stream area is followed the year anomalies originating from slope currents by the mean after by positive (negative) ADV-um downstream. ¯ow. The present study indicates that both anomalous Ekman and geostrophic currents in the Labrador Sea act to reinforce the cold water intrusion along the Labrador b. Intergyre (IG) shelf and offshore Newfoundland between winter 1993 In the IG domain (48.5Њ±54.5ЊN, 20.5Њ±44.5ЊW), and 1994 (not shown). These, associated with the neg- which includes the subarctic front, the geostrophic ad- ative heat ¯ux anomalies, could explain the cooling ob- vection term opposes ⌬SST in four out of six years (Fig. served during this period (Figs. 7d,e). These results are 7d), as is particularly striking for 1995±96, which has coherent with the scenario presented in Drinkwater et the largest (0.7ЊC) surface warming. On the other hand, al. (1996), which is based on several in situ datasets. Ekman advection and the heat ¯ux forcing have often the same sign as SST, in particular in 1995±96 when ⌬ c. Center of the subpolar gyre (SPG) they contribute to a 0.25ЊC and 0.2ЊC warming, respectively. In this latitude band, the ®rst wind stress In the center of the SPG, ⌬SST is somewhat better EOF (not shown) presents intense westerlies before explained by SUM (same sign 5 out of 6 times) than 1995 and westerlies weakening between winter 1995 in the previous regions. The 1995±96 0.8ЊC warming is and winter 1996 with strong minima in January 1996 largely driven by the heat ¯uxes (more than 0.3ЊC) and (as well as in early 1998) (not shown). This ®rst mode by EA to a lesser extent (ϳ0.1ЊC; Fig. 7e). But between represents the classical wind stress NAO component 1996 and 1998, SST changes are mostly due to the (Krahmann et al. 2001). Intense (weak) westerly wind geostrophic advection terms (more than 50% of ⌬SST) stress is associated with intense heat loss (gain) for the with a small additional contribution of EA. The con- upper ocean and cooling (heating) by Ekman advection tribution of ADV-up is larger for those years than ADV- in the intergyre region and large parts of the subpolar um. Between the winters 1998 and 1999, the cooling is gyre domain [Ekman drifts toward the south (north)]. however dominated again by the heat ¯ux term. Intense westerly wind stress is also associated with large We expect that the anomalous geostrophic currents changes in the vertical mixing that can contribute to the are somewhat related to the anomalies of upper ocean SST changes, even in winter, but are not considered in temperature (from geostrophy), and therefore to surface this analysis. The differences between SUM and ⌬SST temperature in the subpolar gyre. Nonetheless, the cor- are quite strong, particularly in 1995±96 (Fig. 7d), and relation between the PC1s of SST and ADV-up (also the two terms do not bear much relation to each other. currents) in the subpolar gyre exceed 0.9 at 0 lag (Fig. The differences are less when smaller domains are re- 8). The pattern is also somewhat coherent with an an- tained, the geostrophic advection term varying signi®- ticyclonic circulation around a positive temperature

←

FIG. 7. Regional heat budget histograms calculated from one winter (JFM) to the next one during the 1993±99 period. (a) The map of the domain with the different regions. The isobaths 200 and 500 m are indicated by dashed contours. (b)±(e) Histograms for the different Ϫ1 regions (ЊCyr ). The different terms are explained in the text: SUM ϭ Qi ϩ EA ϩ ADV-sum, where the horizontal geostrophic advection term ADV-sum (5 on the histograms) is the sum ADV-up ϩ ADV-um.

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arctic Front and of the Irminger gyre, which allows the penetration of warm water from the east to the center of the subpolar gyre. The surface warming observed in the subpolar area extends to subsurface (Reverdin et al. 1999; Bersch et al. 1999). Reverdin et al. (1999) used the World Ocean Circulation Experiment (WOCE) line AX2 data (between Iceland and Newfoundland) during the 1993±98 period. They showed that the magnitude of the upper ocean heat content change (surface to 700 m) is much larger than what it is expected from the air± sea heat ¯uxes. In particular, the continuation of the warming in 1998 is not explained by the heat ¯uxes in their study. Their results suggest also a possible impact of the advection terms (either horizontal or vertical) on the upper ocean heat budget. Bersch et al. (1999) an- alyzed the WOCE hydrographic section A1E/AR7E (between Greenland and Island) during the 1991±96 period. They also noted that the air±sea heat exchanges are not suf®cient to explain the amplitude of the SST variations and hypothesized an important participation of the shift of the subarctic front and of the advection of the subtropical anomalies. This latter study does not cross the

FIG. 8. (a) First EOFs of sea surface temperature (gray shading) SPG domain but con®rms that the advection terms and geostrophic advection by the anomalous currents (thin black con- should probably be considered to explain a large portion tours with contour interval of 0.1ЊC) in the subpolar gyre (SPG) from of the upper-ocean temperature (and salinity) interan- Reynolds SST and T/P data for Dec 1992 to Aug 1999 [the SPG nual variability in this region. region (Fig. 7a) is centered near the maximum in ADV-up]. (b) First principal component for SST (solid line) and ADV-up (dashed line). 5. Discussion anomaly, suggestive of a baroclinic contribution to the We have shown that in 1992±99 the largest contri- surface current variability. After winter 1996, this re- bution to the interannual SST variability is usually the sults in ADV-up reinforcing the initial SST increase in air±sea heat ¯ux term. However, the present analysis SPG (and decreasing it farther east). ADV-up contrib- also shows that the advection terms signi®cantly con- utes to more than a 0.5ЊC increase, which is nearly 50% tribute to the upper-ocean temperature changes from one of the 1ЊC warming between winter 1996 and winter winter to the next in large parts of the western Atlantic 1998, whereas ADV-um has a relatively low impact and the subpolar gyre. during this period. We found that the upper-ocean temperature anomalies The difference between 1993±94 and 1996±98 in the created north of the Gulf Stream are quickly advected anomalous geostrophic currents (Fig. 9) illustrates that by the mean currents toward the northeast and that geo- the ADV-up positive anomalies in 1996±98 are related strophic advection is a key contributor to SST variability to the slowing down of the NAC branches after 1996. in this region. This is consistent with an active role of This probably corresponds to a weakening of the Sub- the ocean currents on SST variability between Cape

FIG. 9. Change between 1993±94 and 1996±98 in the anomalous geostrophic currents from T/P in the subpolar gyre (north of 45ЊN). A 2Њ ϫ 3Њ spatial smoothing and a 5-month running mean has been applied to the current data. The average seasonal cycle has been removed.

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Hatteras and the area east of the Grand Banks of New- are also likely to contribute to the error in SUM, al- foundland, as was suggested by Sutton and Allen though probably to a lesser extent. See Ferry et al. (1997), Visbeck et al. (1998), and Krahmann et al. (2000) for a recent analysis of the consistency of the (2001), whose analyses showed that SST anomalies seasonal heat ¯uxes with large-scale oceanic variability. move along the Gulf Stream toward the northeast. Czaja Other processes can also be invoked to explain the and Marshall (2001, manuscript submitted to J. Climate) discrepancies. For example, advection by boundary cur- also argue that on decadal timescales, SST variations rents around the rim of the subpolar gyre in ice-prone across the separated Gulf Stream are largely controlled regions is not properly resolved in this study (no alti- by anomalous geostrophic current. Joyce et al. (2000) metric current is produced where ice is present) but comment that meridional shifts of the GS could also could contribute to the variability in parts of the sub- in¯uence SST in the GS region. This would be through polar gyre. For instance, it could be necessary to con- the ADV-up term and would mostly in¯uence the region sider such pathways to explain the warming along the north of the Gulf Stream with a warming before 1995 Greenland eastern coast between 1997 and 1998 (Fig. and a cooling between mid-1995 and 1998 when the 3). In this area, SST variability is partly linked to the GS moves southward, which is consistent with the sign sea ice coverage changes (weak in 1997 and 1998 for of ADV-up that we ®nd. instance, returning strongly only during early 2000) and A major concern with our results is that we ®nd large therefore to the changes in water export from the Nordic differences between our estimate of the year-to-year in- Seas through Denmark Strait. This is not fully included tegrated SUM and the change in temperature. The issue in our estimate of advection (Ekman and geostrophic). is whether this arises from large uncertainties in the terms we estimated or from the terms we neglected 6. Conclusions [within R in (1)]. Various modeling studies (Halliwell 1998; Krahmann et al. 2001; HaÈkkinen 2000) simulated We have found for the investigated period 1993±99 with reasonable success the decadal and short-term in- that air±sea ¯uxes are the largest forcing of large-scale terdecadal winter surface temperature during a 40-yr interannual SST. However, the discrepancies between period with ocean circulation models driven by the at- the heat ¯ux forcing and the year-to-year SST changes mospheric circulation anomalies using distinct heat ¯ux were found to be large. Our analysis of the temperature formulations. Halliwell (1998) showed for a domain evolution differs from previous analyses based on data centered on 45ЊN, 40ЊW (westerly winds region) that (Wallace and Jiang 1990; Cayan 1992; Moisan and Ni- the SST anomalies are primarily driven by anomalous iler 1998) as it includes a direct estimate of the hori- local air±sea ¯ux forcing and, to a smaller extent, by zontal advection terms. So, we have quanti®ed hori- entrainment heat ¯ux and Ekman ¯ow. In the Gulf zontal advection by the geostrophic currents and the Stream region, he found that SST variability is domi- Ekman drift which signi®cantly contribute to the SST nated by changes in geostrophic horizontal heat advec- interannual variability in large parts of the western At- tion along with anomalous entrainment heat ¯ux. The lantic and subpolar gyre. latter is associated with adjustments in the vertical den- In the Gulf Stream area the advection by both anom- sity and temperature structure at and beneath the base alous and mean geostrophic currents largely contribute of the mixed layer. to the 1-yr ⌬SST. However, our budget is far from These results are in agreement with ours concerning closed, and having added advection to the heat ¯ux term heat ¯ux and horizontal advection, but they furthermore does not improve the ®t to the observed SST change. suggest that vertical processes, such as entrainment heat The situation is better south of the Gulf Stream than ¯ux, should also be considered to explain a larger part north of it in the slope water area, where we suspect of the SST interannual variability. Moreover, the vertical that vertical processes play an important role in inter- processes are expected to be important in the upwelling annual SST variability. regions (e.g., West African coast), in part of the subpolar In the subarctic front domain (48.5Њ±54.5ЊN), heat gyre, and in the deep and intermediate water formation ¯uxes and Ekman advection both contributed to the zone (Labrador and Irminger Seas, 18ЊC water forma- 1995±96 warming. In this region, the advection by geo- tion zone south of the Gulf Stream). So, part of the strophic currents is positive after 1996 and negative be- discrepancies between SUM and dT/dt could result from fore, and therefore more in phase with SST than with the fact that vertical processes were neglected. We have d(SST)/dt. However, this large region is not homoge- also not considered the mixed-layer depth interannual nous, with different budgets found in its western part variability which modulates the heat ¯ux and Ekman (off the Labrador Current) than in the central Atlantic term, an assumption that clearly results in a poor budget where the subarctic front is more zonal. closure in the eastern Atlantic. It is not obvious how to In the central subpolar gyre, the warming observed estimate a mixed-layer depth representative of the depth between 1994 and 1998 seems to be initiated by the of penetration of the surface signals over one year, and heat ¯uxes and to a smaller extent by the Ekman ad- furthermore the data are not suf®cient to map its inter- vection. After the winter of 1996, advection by anom- annual variability. Errors in the heat ¯uxes or wind stress alous geostrophic currents resulting from a weakening

Unauthenticated | Downloaded 09/25/21 12:28 PM UTC 976 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 33 of the subarctic front dominates and reinforces the initial warming. Advection contributes to an increase by more than one-half of a degree near 55ЊN between 1996 and 1998, at a time when the anomalies in the air±sea heat ¯uxes are small. We attribute this impact of the geostrophic currents to a possible weakening of the subarctic front. Ekman advection contributes also to an anomalous warming between the winters 1995 and 1998. Nevertheless, the 1-yr integrated sum of the heat ¯ux forcing term and horizontal advection terms differs largely from the observed change in SST. In the appendix, we comment that we will have to re®ne our estimate of horizontal advection. We also discussed the omission of vertical processes that will need to be taken into account in some regions and that we should clearly take more proper consideration of the changes in mixed- layer depth. We should also stress that the 7-yr time series we investigate is rather short to allow a robust statistical analysis, so that the results obtained may be strongly linked to the 1993±99 period. Clearly, adequate subsurface data and high-resolution simulation studies would be necessary to investigate the role of the other important processes controlling SST variability (mixed-layer depth variability, vertical advection, and mixing). This, as well as longer time series, are required to improve our results.

Acknowledgments. In addition to our analyses of sea surface height, we used maps produced by CLS Space Oceanography Division as part of the Environment and Climate EC AGORA (ENV4-CT9560113) and DUACS (ENV4-CT96-0357) projects and kindly provided by Pier- re-Yves Le Traon from CLS. This project was supported by the French ``Programme National d'Etude de la Dy- FIG. A1. Integrated geostrophic advection by anomalous currents namique du Climat.'' Comments by the reviewers and (ADV-up) from one winter to the next for the 1996±99 period and Lynne Talley contributed to improving the manuscript. the four regions presented in Fig. 7a (ЊCyrϪ1). (a) The T/P anomalous currents and (b) geostrophic currents from the DUACS composite APPENDIX sea surface height anomaly (SSH) from T/P and ERS-1/2 altimeters (Ducet et al. 2000). No additional spatial ®ltering has been applied to this product before estimating ADV-up. The average of the three Geostrophic Advection Uncertainties 1-yr estimates has been removed before plotting in order to remove part of the difference induced by the different period over which an The uncertainty in the estimated geostrophic advec- average seasonal cycle is estimated. tion by the variable currents ADV-up originates mostly from the sea level product we use. To estimate current deviations, we were obliged to smooth heavily (2Њϫ resolution (0.5Њϫ0.5Њ) than what we have used but 3Њ) the gridded data in order to remove the aliased signal there is no product between December 1993 and March resulting from the sampling of the eddy ®eld by the 1995, and therefore the average seasonal cycle is not altimeter along tracks repeated every 10 days (zonal estimated over the same period than in section 4. We separation between tracks at 40Њ±50ЊN is less than 200 therefore restrict the comparison of ADV-up to the km) and from the residual error in the altimetric data. 1996±1999 period. Geostrophic advection ®elds are cal- In order to get a sense of what this implies for the culated on the 1Њϫ1Њ grid from the combination of geostrophic advection, we compare the estimate used the DUACS geostrophic currents and the Reynolds SST for the budgets (ADV-up) with the one obtained from (DUACS-ADV). The estimates are then ®ltered by a 5- the DUACS composite sea surface height anomaly month running mean, but no additional spatial smooth- (SSH) based on T/P and ERS-1/2 altimeter data (Ducet ing is applied. et al. 2000). The DUACS product results from an op- The winter-to-winter integrated ®elds present similar timal interpolation of the data of the two satellites after structures in the two products but with larger amplitudes reduction of the ERS orbit errors. It has a better spatial in DUACS-ADV. To illustrate these differences, we av-

Unauthenticated | Downloaded 09/25/21 12:28 PM UTC MAY 2003 VERBRUGGE AND REVERDIN 977 erage ADV-up in the same four boxes (GSS, NS, IG, timescales in the North Atlantic and Paci®c. J. Climate, 10, 393± SPG). We ®nd that this estimate of ADV-up is often 408. ÐÐ, M. Holland, M. Timlin, and G. Reverdin, 2002: Decadal var- twice as large in the western Atlantic. The difference iations in Labrador sea ice cover and North Atlantic sea surface is still large in the intergyre area, but is much less in temperatures. J. Geophys. Res., 107, 3035, doi:10.1029/ the central parts of the subpolar gyre (Fig. A1). Note 2000JC000683. that we have removed shelf areas south of Nova Scotia Dickson, R. R., J. Meincke, S.-A. Malmberg, and A. J. Lee, 1988: The ``Great Salinity Anomaly'' in the northern North Atlantic in this estimate (Fig. 7a). In the region GSS (south of 1968±1982. Progress in Oceanography, Vol. 20, Pergamon, the Gulf Stream), the sign is different in 1997/98. 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