OGAWA ET AL.: IMPACT OF OCEAN FRONTS ON THE ATMOSPHERE Geophysical Research Letters, 2011GL048516

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1 Research Center for Advanced Science and Technology, University of Tokyo, Tokyo, Japan. 2 Frontier Research Center for Global Change, JAMSTEC, Yokohama, Japan. 3 Earth Simulator Center, JAMSTEC, Yokohama, Japan Copyright 2011 by the American Geophysical Union. Paper number 2011GL048516.

Climatological dependence of the tropospheric zonal-mean circulation and

transient eddy activity on the latitude of a midlatitude oceanic front Fumiaki Ogawa,1 Hisashi Nakamura,1,2 Kazuaki Nishii,1 Takafumi Miyasaka,1 and Akira Kuwano-Yoshida3

[1] Major atmospheric “storm tracks”, where migratory cyclones and anticyclones recurrently develop, are observed around midlatitude oceanic frontal zones into which meridional gradient of sea-surface temperature (SST) is concentrated. A set of atmospheric general circulation model experiments is performed with zonally uniform SST prescribed at the model lower boundary. The latitudinal SST profile for a given hemisphere is characterized by a single front. Latitudinal dependency of the thermal influence of the SST front is investigated by varying its latitude systematically from one experiment to another. These idealized experiments reveal an obvious tendency for a low-level storm track to be organized along or slightly poleward of the SST front if located in the midlatitudes or subtropics. As a surface manifestation of an eddy-driven polar-front jet (PFJ), a surface westerly jet tends to form systematically poleward of the front. Though somewhat less obvious, qualitatively the same tendency is found for an upper-level storm track and a PFJ, indicative of their anchoring as an influence of the SST front. To the SST front at a subpolar latitude, in contrast, these tendencies are no longer applicable, as both the storm track and PFJ form equatorward of the front. Rather, their axial latitudes are close to those simulated with a particular SST profile from which frontal gradient has been removed, suggesting that thermal forcing of a subpolar SST front to anchor the storm track and PFJ is overshadowed by atmospheric internal dynamics.

1. Introduction [2] A midlatitude oceanic frontal zone is a confluent region of warm and cool ocean currents, characterized by strong meridional gradient in both sea surface temperature (SST) and surface air temperature (SAT). Nakamura et al. [2004] pointed out that the core regions of the major “storm tracks”, zonally elongated regions where migratory cyclones and anticyclones recurrently develop, are observed along or just downstream of the major oceanic frontal zones. They hypothesized that this collocation arises from the anchoring of a surface baroclinic zone through effective restoration of SAT gradient across the SST front via differential heat supply from the ocean (“oceanic baroclinic adjustment”), which has been verified by numerical studies by Nakamura et al. [2008], Nonaka et al. [2009], Taguchi et al. [2009], Sampe et al. [2010], and Hotta and Nakamura [2011]. In addition to the moisture supply from warm ocean currents [Hoskins and Valdes, 1990; Minobe et al., 2008], the oceanic baroclinic adjustment can be important not only for anchoring a storm

track but also for an associated polar-front jet (PFJ), which is driven by westerly momentum transport by transient eddies from a subtropical jet (STJ) [Lee and Kim, 2003; Nakamura et al., 2004]. Since those eddies amplify baroclinically in transporting heat poleward, a PFJ accompanies the surface westerlies, driving ocean currents [Trenberth et al., 1990], whose heat transport acts to confine sharp SST gradients into narrow frontal zones [Nakamura et al., 2004]. [3] Those previous studies have suggested a potential importance of midlatitude SST front, which may invoke the study for understanding the extratropical atmospheric general circulation as a coupled system with the underlying ocean. In fact, Nakamura et al. [2008] and Sampe et al. [2010] showed through their numerical experiments that the anchoring effect of a storm track would be reduced substantially without frontal SST gradient in suppressing the activity of transient eddies. Brayshaw et al. [2008] revealed notable sensitivity of the formation of a storm track and PFJ to the latitude of the strongest SST gradient. The influence of SST gradients on transient eddy activity and westerly jets has also been examined by [Chen et al., 2010]. The present study is an extension of these previous modeling studies. We use an atmospheric general circulation model (AGCM) with globally prescribed zonally uniform SST as its perpetual lower boundary condition for assessing the sensitivity of the climatological-mean state of the activity of tropospheric disturbances and zonal-mean winds to the latitudinal position of an SST front.

2. Experimental Design [4] The AGCM we used is called AFES [Ohfuchi et al., 2004], with 56 vertical levels up to the 0.09hPa level. The horizontal resolution (T79; equivalent to ~150km grid interval) is sufficient for resolving the effect of an oceanic front on the large-scale atmospheric circulation. The lower-boundary condition of the AGCM is set hypothetically as the global ocean with six different latitudinal profiles of zonally uniform SST (Figure 1a). With this idealized “aqua-planet” setting without any landmass, we can eliminate planetary-scale atmospheric stationary waves forced by land-sea thermal contrasts and topography as observed in the Northern Hemisphere (NH). As in Sampe et al. [2010], one of these SST profiles was taken from the OI-SST data1 for the South Indian Ocean [60~80°E], where the warm Agulhas Return Current is confluent with cool Antarctic Circumpolar Current to maintain frontal SST gradient throughout the year. The profile for austral winter [Jun.-Aug.] was assigned for the model Southern Hemisphere (SH) and the corresponding summertime profile [Dec.-Feb.] for the model NH. With this profile characterized by midlatitude SST front at 45° latitude in both hemispheres and the SST maximum in the NH tropics, the AGCM was integrated for 60 months for obtaining robust

1The NOAA Optimum Interpolation SST V2 is available at http://www.cdc.noaa.gov/cdc/data.noaa.oisst.v2.html.

Received 15 June 2011

OGAWA ET AL.: IMPACT OF OCEAN FRONTS ON THE ATMOSPHERE Geophysical Research Letters, 2011GL048516

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Figure 1. Figure 1. Latitudinal profiles of (a) SST and (b) its meridional gradient, prescribed as the lower-boundary condition for the AGCM experiments. Black lines represent observed profiles for the South Indian Ocean as an average between 60°E and 80°E, characterized by a midlatitude SST front at 45° in each of the hemispheres. The winter profile is assigned in the model SH and the summer profile in the NH. Lines with other colors indicate the profiles for sensitivity experiments, where the latitude of the SST front is displaced to other latitudes (30°, 35°, 40°, 50°, 55°) while the same magnitude of SST gradient is kept fixed.

statistics under the insolation fixed to its solstice condition. The model SH (NH) thus corresponds to the winter (summer) hemisphere. Five 60-month integrations were then repeated with SST profiles for which the latitude of the SST front had been displaced artificially from 45° southward to 30° or poleward to 55° with 5° intervals with the intensity of the frontal gradient (Figure 1b). The SST profile equatorward of 25° latitude remains the same for the six experiments so as to keep the same thermal forcing on the model Hadley cells and thereby STJs [cf. Brayshaw et al., 2008]. Our experiments can thus give some insight into the sensitivity of the atmospheric general circulation to the latitudinal position of a midlatitude SST front. 3. Axial latitude of surface baroclinic zone and midlatitude low-level storm track [5] A measure of the mean-flow baroclinicity can be given by the Eady growth rate σ [Eady, 1949], which is proportional to the meridional temperature gradient [Hoskins and Valdes, 1990]. The latitudinal profiles of near-surface σ, which is of critical importance for baroclinic development of synoptic-scale eddies, have been evaluated from the climatological zonal-mean temperature for the 850hPa and 1000hPa levels. Figure 2a shows the meridional profiles of the near-surface σ in the winter hemisphere for the six experiments. Each of the profiles exhibits a distinct peak (dots) collocated with the SST front, which reflects the anchoring of a surface baroclinic zone through effective

restoration of SAT gradient across the SST front via oceanic baroclinic adjustment. [6] Activity of baroclinically developing transient eddies can be measured by poleward eddy heat transport [v’T’], where primes denote quantities that have been exposed to high-pass filtering with a half-power cutoff period of eight days and the bracket denotes zonal averaging. For each of the experiments, the formation of a single “storm track” is evident in the winter hemisphere as represented as a well-defined peak of the climatological-mean [v’T’] at the 850hPa level (dot in Figure 2b). As summarized in Figure 2c for the winter hemisphere, the storm track (green line) shows a strong tendency to form in the immediate vicinity of the surface baroclinic zone (red line) anchored by the SST front (dashed line) whose latitude is 40° or higher, while the storm track forms slightly poleward of the SST front whose latitude is 35° or lower. Compared to the winter hemisphere, the poleward displacement of the low-level storm track relative to the surface baroclinic zone anchored by the SST front is more ubiquitous in the summer hemisphere (Figure 2d). Despite this displacement, the storm track shows obvious tendency to follow the latitudinal position of the SST front as long as it is 50° or lower, but the storm track forms equatorward of the front if located at 55°.

Figure 2. (a-b) Meridional profiles of the winter-time clima-tological mean states of (a) Eady growth rate [σ] estimated for the 1000 - 850 hPa layers, and (b) 850-hPa poleward eddy heat flux ([v’T’], K*m/s) associated with subweekly disturbances. Based on the 60-month AGCM integrations for the SST profiles shown in Figure 1, with SST front whose latitudinal position is indicated as colored triangles. (c) Diagram showing the climatological-mean latitudes (ordinate) of the maximum [σ] (red) and [v’T’] (green) for the winter hemisphere as function of the latitude of SST front (ab-scissa; also dotted line) prescribed for the individual experiments. (d) As in (c), but for the summer hemisphere.

OGAWA ET AL.: IMPACT OF OCEAN FRONTS ON THE ATMOSPHERE Geophysical Research Letters, 2011GL048516

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4. Axial latitudes of the surface westerlies and their eddy forcing [7] As a surface manifestation of a PFJ, midlatitude surface westerlies are maintained through the downward transport of westerly momentum from the upper-tropospheric core of the PFJ associated with poleward eddy heat transport [Lee and Kim, 2003; Nakamura et al., 2004]. In the transformed Eulerian mean (TEM) framework, the eddy forcing of the surface westerlies can be estimated as the divergence of the Eliassen-Palm (E-P) flux near the surface [Andrews and McIntyre, 1976]. Figure 3a and 3b show the latitude of the maximum climatological-mean eddy forcing for the winter and summer hemispheres, respectively, in terms of at 925hPa zonal-mean westerly acceleration ∂[U]/∂t derived from the E-P flux divergence based on the high-pass filtered quantities [Edmon et al., 1980]. The meridional and vertical components of the E-P flux are proportional to the meridional eddy transport of westerly momentum [u’v’] and heat [v’T’], respectively. [8] In the winter hemisphere (Figure 3a), the latitude of the maximum ∂[U]/∂t follows the axes of the SST front and low-level storm track (Figure 2c), while displaced systematically poleward from the frontal axis. The displacement reflects the slight poleward tilting of the maximum [v’T’] in the meridional plane (not shown). In a manner consistent with the eddy acceleration, the axial latitude of the climatological-mean near-surface westerlies [U] forms systematically poleward of the SST front. The [U] axis tends to be farther poleward of the eddy acceleration maximum if the SST front is located at 40° or a lower latitude. For the SST front at a subpolar latitude (50° or 55°), in contrast, surface westerlies exhibit double jet structure with another axis around 40° latitude. Driven by eddies away from the SST front, the presence of the secondary branch of surface [U] implies the importance of atmospheric internal dynamics that is not directly related to the thermal influence of the frontal SST gradient. [9] In summer hemisphere (Figure 3b), the surface westerly jet forms systematically poleward of the SST front as long as it is situated at 45° or lower latitude. This poleward displacement is consistent with the positions of the storm track and associated eddy forcing. Unlike in the winter hemisphere, a single westerly jet forms around 40° latitude even when the SST front is at a subpolar latitude (50° or 55°). This jet axis almost coincides with the corresponding midlatitude branch of the westerlies in the winter hemisphere. This result suggests a large contribution from internal dynamics to the maintenance of the surface westerlies particularly in the summer hemisphere. The particular contribution tends to be comparable to or even dominant over the anchoring effect on the surface westerly jet to the SST front when located at a subpolar latitude, in agreement with Brayshaw et al. [2008]. Interestingly, the midlatitude jet axis almost coincides with the jet axis simulated in the NF experiment by Nakamura et al. [2008] and Sampe et al. [2010], where the frontal SST gradient is artificially removed in their SST profile. This implies that the thermal forcing of a subpolar SST front on the lower-tropospheric general circulation tends to be less effective than that of a subpolar or subtropical SST front and thus dominated by internal dynamics.

5. Axial latitudes of upper-level storm track and PFJ [10] Figure 3c shows the climatological-mean axis of an upper-level storm track in the winter hemisphere simulated in each of the experiments, defined as the peak latitude of 300hPa eddy meridional wind variance [v’v’]. Though sub-stantially weaker than near the surface, the storm-track axis still exhibits a tendency to follow the latitudinal displacement of the SST front, if located in the midlatitudes or subtropics. For the SST front at a subpolar latitude (50° or 55°), in con-trast, the storm track forms near 45°, away from the SST front. Figure 3c also shows a tendency for the upper-level secondary jet in midlatitudes to coincide with the storm track, reflecting its characteristics of an eddy driven PFJ. In fact, the latitude of the maximum convergence of [u’v’] tends to be located a few degrees poleward of the secondary jet. Meanwhile, the strongest divergence of [u’v’] occurs on the poleward flank of a STJ, indicating systematic poleward transport of westerly momentum by eddies from the STJ toward the midlatitude PFJ located poleward of the SST front. As observed in the SH [Nakamura and Sampe, 2002], a secondary storm-track forms along the STJ without any notable baroclinic eddy growth (Figure 3c), featuring the role of the STJ as a wave-guide.

Figure 3. (a) As in Figure 2(c), but for the latitudes of maximum [U] (blue) and eddy westerly acceleration (orange); derived from E-P flux divergence at 925 hPa level. (b) As in (a), but for the sum-mer hemisphere. (c) As in (a), but for the axial latitudes of the up-per-tropospheric storm-track activity ([v’v’], green) and westerly jet ([U], blue), in addition to the maximum eddy westerly acceleration (solid orange) and deceleration (dashed orange) based on [u’v’] convergence and divergence, respectively. (d) As in (c), but for the summer hemisphere.

OGAWA ET AL.: IMPACT OF OCEAN FRONTS ON THE ATMOSPHERE Geophysical Research Letters, 2011GL048516

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[11] In the summer hemisphere (Figure 3d), the sensitivity of the axial latitudes of the midlatitude storm track and PFJ to the latitudinal position of the SST front is qualitatively the same as its wintertime counterpart, but the particular sensitiv-ity of the upper-level jet and storm-track tends to be dimin-ished if the SST front is located at a subpolar latitude (50° or 55°). Located a few degrees equatorward of the latitudes of storm track and maximum eddy acceleration in a systematic manner, the summertime midlatitude jet is essentially a PFJ.

6. Summary and discussion [12] Though performed under the idealized setting, our AGCM experiments have revealed certain sensitivity of the climatological-mean positions of a midlatitude storm track and an eddy-driven PFJ to the latitudinal position of an SST front. The sensitivity, which is unambiguous for the SST front in the subtropics or midlatitudes, is manifested as the tendency for the axial latitudes of the storm track and associated PFJ to follow the latitudinal displacement of the SST front. This ten-dency arises probably from the anchoring effect of the SST front on the storm track and associated PFJ by maintaining the surface baroclinic zone along the SST front, via “oceanic baroclinic adjustment” and by supplying moisture to individu-al storms and thereby energizing them. The particular sensi-tivity tends to be stronger in the lower troposphere than in the upper troposphere, while it is found more obvious in the win-ter hemisphere with an intensified STJ than in the summer hemisphere. If the SST front is situated at a subpolar latitude (50° or 55°), however, both the PFJ and upper-level storm track are situated in midlatitudes far from the SST frontal position in either hemisphere. Rather, their positions corre-spond to those realized without frontal SST gradient [Naka-mura et al., 2008; Sampe et al., 2010]. In other words, the an-choring effect of the SST front, if located at a subpolar lati-tude, tends to be overshadowed by atmospheric internal dy-namics, especially in the upper troposphere [Brayshaw et al., 2008]. Though idealized, our experiments provide some in-sight into the fundamental nature of the extratropical atmos-pheric general circulation. Our analysis in the present study is nevertheless limited to the climatological-mean state. Our in-vestigation is underway on how sensitive the structure and amplitude of the annular mode is to the latitudinal position of the SST front. [13] Acknowledgments. We used the Earth Simulator in support of JAMSTEC. We thank the AFES/CFES working team of JAMTEC, Y. Kosaka, T. Sampe and A. Goto for their advices for our experiments. This study is supported by the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) through the Grant-in-Aid for Scientific Research #22340135 and that on Innovative Areas #2205 and by the Japanese Ministry of Environment through the Global Environment Research Fund (S-5). References Andrews, D. G. and M. E. McIntyre (1976), Planetary waves in hori-

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F. Ogawa, Research Center for Advanced Science and Technology, University of Tokyo, Tokyo, 153-8904, Japan (fogawa@atmos.rcast.u-tokyo.ac.jp).

H. Nakamura, Research Center for Advanced Science and Tech-nology, University of Tokyo, Tokyo, 153-8904, Japan (hisashi@atmos.rcast.u-tokyo.ac.jp).

K. Nishii, Research Center for Advanced Science and Technology, University of Tokyo, Tokyo, 153-8904, Japan (nishii@atmos.rcast.u-tokyo.ac.jp).

T. Miyasaka, Research Center for Advanced Science and Technol-ogy, University of Tokyo, Tokyo, 153-8904, Japan (miyasaka@atmos.rcast.u-tokyo.ac.jp).

A. Kuwano-Yoshida, Earth Simulator Center, JAMSTEC, Yokohama, 236-0001, Japan (akiray@jamstec.go.jp).