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Dominant Anomaly Patterns in the Near-Surface Baroclinicity and Accompanying Anomalies in the Atmosphere and Oceans

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Dominant Anomaly Patterns in the Near-Surface Baroclinicity and Accompanying Anomalies in the Atmosphere and Oceans 880 JOURNAL OF CLIMATE VOLUME 22 Dominant Anomaly Patterns in the Near-Surface Baroclinicity and Accompanying Anomalies in the Atmosphere and Oceans. Part I: North Atlantic Basin MOTOTAKA NAKAMURA AND SHOZO YAMANE* Frontier Research Center for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokohama, Kanagawa, Japan (Manuscript received 24 October 2007, in final form 6 August 2008) ABSTRACT Variability in the monthly mean flow and storm track in the North Atlantic basin is examined with a focus on the near-surface baroclinicity, B 5 Bxi 1 Byj. Dominant patterns of anomalous B found from empirical orthogonal function (EOF) analyses generally show patterns of shift and changes in the strength of B. Composited anomalies in the monthly mean wind at various pressure levels based on the signals in the EOFs display robust accompanying anomalies in the mean flow up to 50 hPa in the winter and up to 100 hPa in other seasons. Anomalous eddy fields accompanying the anomalous Bx patterns exhibit, broadly speaking, struc- tures anticipated from linear theories of baroclinic instabilities and suggest a tendency for anomalous wave fluxes to accelerate/decelerate the surface westerly accordingly. Atmospheric anomalies accompanying By anomalies have patterns different from those that accompany Bx anomalies but are as large as those found for Bx. Anomalies in the sea surface temperature (SST) found for the anomalous patterns of Bx often show large values of small spatial scales along the Gulf Stream (GS), indicating that a meridional shift in the position of the GS and/or changes in the heat transport by the GS may be responsible for the anomalous Bx and concomitant tropospheric and lower-stratospheric anomalies. Anomalies in the net surface heat flux, SST in preceding months, and meridional eddy heat flux in the lower troposphere support this interpretation. 1. Introduction in a zonally homogeneous steady mean state, where U is the mean zonal flow, f is the Coriolis parameter, and N Baroclinicity in the lower atmosphere in classic the- is the Brunt–Va¨isa¨la¨ frequency. Charney’s formula is ories of atmospheric stability is defined by the effect of slightly different from the Eady’s but still incorpora- the horizontal temperature gradient, in combination tes the same effects. Lindzen and Farrell (1980) first with the Coriolis force, on the vertical shear of the applied the Eady parameter to atmospheric data to horizontal velocity and the vertical stability of the at- successfully estimate the maximum growth rate of bar- mosphere (Charney 1947; Eady 1949). In its original oclinic instability in the troposphere. Hoskins and form, the Eady maximum growth rate for baroclinic Valdes (1990) used its localized version (i.e., U, N, and f instability B is defined by GRMax are all local Eulerian mean values) as the central pa- jjf ›U rameter in their study of the Northern Hemispheric B [ 0:31 GRMax N ›z storm tracks. This local version, or its simplified version, has been used successfully as an indicator of baroclinic wave generation in diagnostic studies of storm tracks in * Current affiliation: Science and Engineering, Doshisha Uni- recent years as well (Nakamura and Sampe 2002, 2004; versity, Kyotanabe, Kyoto, Japan. Nakamura et al. 2004). In our study, we define the baroclinic vector, B 5 Bxi 1 Byj, where g ›u Corresponding author address: Mototaka Nakamura, Japan Bx 5 À Agency for Marine-Earth Science and Technology, Yokohama, uN ›y Kanagawa-Pref, 236-0001, Japan. E-mail: [email protected] and DOI: 10.1175/2008JCLI2297.1 Ó 2009 American Meteorological Society Unauthenticated | Downloaded 10/05/21 09:55 AM UTC 15 FEBRUARY 2009 N A K A M U R A A N D Y A M A N E 881 g ›u gradient of the atmospheric forcing scale that matters. By 5 uN ›x For example, a small patch of warm SSTA with a di- ameter of 50 km embedded in the subpolar gyre is likely in which u is the monthly mean potential temperature, to be ignored by the large-scale atmospheric dynamics. and use it as the central quantity of the diagnoses. Un- On the other hand, a 50-km-wide patch of warm SSTA less stated otherwise, ‘‘anomalies’’ refer to deviations that extends along a sharp frontal current, such as the from the climatology hereafter. Though its meridional GS, over several hundred kilometers or more may have component does not appear in any classic theory of a significant impact on the large-scale atmospheric dy- baroclinic instability, a theory that does incorporate the namics through its modification of B. The high sensi- effect of By shows its important role in enhancing bar- tivity of B to changes in the temperature contrast across oclinic wave generation locally to the east of the mean the front and changes in the width of the front, and the trough (Niehaus 1980). In the North Atlantic storm track uncertainty in the impact of small SSTAs on B make region, where the surface temperature gradient has a sub- assessing the effective B anomalies that are attributable stantial zonal component due to the northward-flowing to the SSTAs from the available data very difficult. downstream branch of the Gulf Stream (GS), By may Moreover, exactly how the SSTAs in the presence or play a significant role in the atmospheric variability. absence of the land surface temperature anomalies may Although B is an indicator of baroclinic wave gener- or may not produce B anomalies that are significant to ation, it is also a parameter of upper-level flow steering the atmosphere is uncertain. For example, anomalously induced by the horizontal temperature structure in the warm or cold land along the east coast of North underlying layers. In simple theories of general circu- America can alter B in the storm track without any lation with a zonally homogeneous basic state, the SSTA. Or, large SSTAs along the east coast of North available potential energy generated by Bx is essentially America can have no impact on B if the large-scale land the driving force of the middle latitude mean flow and surface temperature along the east coast of North synoptic-scale waves (e.g., Lorenz 1955). Anomalous B America has the same anomaly. This complicating role and =u are anticipated to accompany anomalous V played by the land surface in assessing the impact of through the thermal wind balance. If there is a 1000-m- SSTAs in the vicinity of a large landmass on the over- thick layer in which anomalous ›U=›z 5 10 msÀ1kmÀ1; lying atmosphere must be taken into account when for instance, the upper atmosphere would have a 10 m s21 studying potential roles of extratropical SSTAs in the anomaly in U if other layers have zero anomalies in ›U/ extratropical atmospheric anomalies. ›z One may anticipate anomalous baroclinic wave Lau (1988) investigated patterns of anomalous storm generation arising from anomalous B to efficiently track activity and associated low-frequency flows by suppress the effect of the B anomaly in the interior of computing empirical orthogonal functions (EOF) for the atmosphere, thereby minimizing the impact of the B high-frequency 500-hPa geopotential heights for the anomaly on the upper atmosphere. On the other hand, if Northern Hemisphere winters. He found that both the anomalous baroclinic wave generation is not suffi- North Atlantic and North Pacific storm tracks have a cient to suppress the effect of anomalous B and/or pattern of meridional shift and a pattern of increased or anomalous B triggers an atmospheric response that decreased eddy activity in the first two EOFs. He also yields an equivalent-barotropic structure, like that of found that these changes in the storm tracks have blocking, then the effect of B anomaly on the upper symbiotic relationships with the background flows and atmosphere can be significant. Furthermore, anomalous have substantial impacts on the mean flow. We have B may result in anomalous wave generation/dissipation attempted to examine this issue of variability in the that, in turn, acts on the larger-scale flow. Therefore, B mean flow and storm tracks from a perspective that anomalies may well be directly or indirectly involved in focuses on B, in hopes of finding a clear link between the the upper-atmospheric anomalies in various ways. extratropical atmospheric anomalies and underlying The sea surface temperature (SST) is an important oceanic anomalies along the fronts. In this regard, we factor in determining B in the storm-track regions (e.g., have chosen not to project our results onto the two Hoskins and Valdes 1990; Nakamura et al. 2004). SST major modes of variability that involve the North At- anomalies (SSTAs) around the front of the GS or lantic basin, the North Atlantic Oscillation and Arctic Kuroshio/Oyashio can have a profound impact on B Oscillation, so that our presentation and discussion are along the storm tracks. A subtle point that has to be mostly confined to the wave–mean flow dynamics on a considered carefully in this regard is the spatial scale monthly or shorter time scale. and the location of SSTAs with respect to the clima- Our approach to the search for a link between tology since it is the anomalous surface temperature anomalies in the GS and the overlying atmosphere is as Unauthenticated | Downloaded 10/05/21 09:55 AM UTC 882 JOURNAL OF CLIMATE VOLUME 22 follows: 1) identify dominant patterns in anomalous B in data (Rayner et al. 2003) to compile anomaly compos- the storm track for each month and identify years in ites accompanying anomalous patterns in B. In addition, which the anomaly fits the pattern well, 2) composite we used 6-hourly temperature and wind data from the anomalies in the monthly mean circulation and high- NCEP–NCAR reanalyses to compute various eddy frequency transients in the atmosphere to obtain a fields.
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