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

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Contribution of Horizontal Advection to the Interannual Variability of Sea Surface Temperature in the North Atlantic 964 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 33 Contribution of Horizontal Advection to the Interannual Variability of Sea Surface Temperature in the North Atlantic 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 tem- perature 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 q 2003 American Meteorological Society Unauthenticated | Downloaded 09/25/21 12:28 PM UTC MAY 2003 VERBRUGGE AND REVERDIN 965 North Atlantic during the 1920s and 1930s based on dT 5 Q 2 (U9=T 1 U =T9) 2 (U =T) 1 R, (1) COADS data and hypothesized that the SST warming dt ig g ek along the Gulf Stream resulted from altered ocean cur- rents rather than local wind forcing. Different analyses with of ocean GCM simulations show that the current ¯uc- heat fluxes Q 5 . tuations in the western Atlantic and the North Atlantic i r 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 51TUUT9; Uek 51ek ek9 ; Ug 5 Willebrand 2001). These studies suggest that the chang- UUgg1 9); 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). els (Halliwell 1998; Visbeck et al. 1998). As discussed 9g= 9 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 ;10 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 18318 grid from 158 to 668N and discussing data and methods (section 2), we will ®rst 1008Wto208E (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 r 0CpH, where Cp is the speci®c heat of seawater, r 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 Dr 5 0.125 kg m23).
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