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Hailan Wang, Siegfried Schubert, Max Suarez, and Randal Koster

a useful framework for isolating and assessing the roles of the leading SST patterns of variability in forcing U.S. hydroclimate variations (e.g., Schubert et al. 2004a ; Wang et al. 2009 ). However, the exact physical mechanisms by which the leading patterns of SST variability affect the U.S. hydroclimate in the GCMs, and the issue of model dependence of the results has not been sufficiently addressed. A systematic investigation of these mechanisms in AGCM experiments is necessary to improve

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Daisuke Hotta and Hisashi Nakamura

mechanism of atmospheric baroclinicity (e.g., Stone 1978 ); however, it cannot explain the existence of such meridionally narrow baroclinic zones, as observed near the surface along the storm tracks. HV90 was the first to address this issue. Using a planetary wave model linearized about the observed wintertime zonal-mean flow with each of the transient eddy momentum and heat fluxes and diabatic heating imposed in the NH storm-track regions—namely, the North Atlantic and the North Pacific—as forcing

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Terrence M. Joyce, Young-Oh Kwon, and Lisan Yu

particularly identify the dynamical cause. Because the system is coupled, there are processes within the atmosphere and the ocean separately that will force an alignment between the two ( Hoskins and Valdes 1990 , Nakamura et al. 2008 ): storms can produce vorticity fluxes that enhance the midlatitude zonal jet, which can affect the location of the GS and KE, and then further influence the development of the storms. Recently Tanimoto et al. (2003) have examined decadal variability in SST and interannual

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Young-Oh Kwon, Michael A. Alexander, Nicholas A. Bond, Claude Frankignoul, Hisashi Nakamura, Bo Qiu, and Lu Anne Thompson

that the atmosphere forces the ocean, and experiments using atmospheric general circulation models (AGCMs) with specified extratropical sea surface temperature anomalies (SSTAs) generally yield atmospheric anomalies of modest amplitude and variable spatial structure ( Kushnir et al. 2002 ). Given the low-frequency response of WBCs to integrated basinwide atmospheric forcing, the large ocean-to-atmosphere heat fluxes over the WBCs, their proximity to the storm tracks, and the long time scale

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Claude Frankignoul, Nathalie Sennéchael, Young-Oh Kwon, and Michael A. Alexander

be rather persistent because of SST anomaly reemergence, sustained forcing from the tropics, and low-frequency changes in oceanic heat advection. The extratropical SST variability is large near the strong SST gradients along the oceanic fronts associated with western boundary currents and their extensions (hereafter WBCs), especially at decadal time scales ( Nakamura et al. 1997 ; Nakamura and Kazmin 2003 ; Kwon et al. 2010a ), when the changes in the oceanic circulation are particularly

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Mototaka Nakamura and Shozo Yamane

advects synoptic-scale disturbances as they enter and grow in the storm track. The characteristics of MR z850 , whose divergence out of and convergence into the planetary boundary layer are related to, respectively, acceleration and deceleration of the surface pseudo-westerlies when the horizontal flux convergence is negligible ( Plumb 1986 ), are roughly in accord with those of V  ′ θ ′ 850 , which is a primary component of MR z850 . The sense of the surface momentum forcing by MR z850 along the

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Lu Anne Thompson and Young-Oh Kwon

decadal SST variability. Schneider et al. (2002) , who also examined ECHO-2, concluded that the decadal time scale of the KOE variability results from the integration along Rossby waves trajectories of stochastic atmospheric forcing. They suggest a positive feedback such that anomalies of wind stress curl over the western North Pacific forced locally by KOE anomalies reinforces those anomalies. However, they found no negative feedback that would close the loop to create a true coupled mode of

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Kathryn A. Kelly, R. Justin Small, R. M. Samelson, Bo Qiu, Terrence M. Joyce, Young-Oh Kwon, and Meghan F. Cronin

vertical shear of the horizontal wind multiplied by f / N (where f is the Coriolis force). Indeed, Cione et al. (1993) used the horizontal thermal gradient between the air temperature at Cape Hatteras and SST at the GS north wall as a measure of baroclinicity with which to estimate the potential growth of storms passing that way. Nakamura and Shimpo (2004) found that within the core region of the Southern Hemisphere storm track and subpolar jet over the Indian Ocean, the lower-troposphere eddy

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Xujing Jia Davis, Lewis M. Rothstein, William K. Dewar, and Dimitris Menemenlis

. NPSTMW variability and atmosphere forcing The above correlation between NPSTMW volume and the PDO links NPSTMW to North Pacific decadal variations. Here, the dynamics and thermodynamics of NPSTMW variability, particularly on interannual scales, are sought by examining wind stress and surface heat flux. We carried out an empirical orthogonal function (EOF) analysis of the wind stress and of surface heat flux to better understand the relationship between NPSTMW variability, the PDO index (with SST as

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Akira Kuwano-Yoshida, Shoshiro Minobe, and Shang-Ping Xie

) show a high correlation between the surface wind convergence and 2 SLP in the ECMWF operational analysis data, suggesting the importance of a pressure adjustment mechanism ( Lindzen and Nigam 1987 ) under SST forcing over the Gulf Stream. The annual and seasonal variations of the correlation coefficients among surface wind convergence, 2 SLP, and − 2 SST in CNTL and SMTH are shown in Table 1 . These are based on the monthly data over the North Atlantic Ocean within 30°–48°N, 80°–40°W, following

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