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Mototaka Nakamura

Abstract

Simple parameterizations of the ice albedo feedback and its implicit effect on the high-latitude river runoff have been added to the coupled atmosphere-ocean box model developed by Nakamura et al. The additional parameterizations introduce four new feedbacks into the model. The first is a stabilizing feedback between high-latitude albedo and the thermohaline circulation through influence on the temperature. In terms of the Newtonian cooling concept, introduction of this feedback is equivalent to allowing changes in the “apparent equilibrium temperature.” The second and the third feedbacks are consequences of the first; the modified temperature profile affects the meridional transports of moisture and heat by atmospheric eddies, which form positive feedbacks with the thermohaline circulation as shown by Nakamura et al. The fourth is a stabilizing feedback between the thermohaline circulation and freshwater flux into the high-latitude ocean due to ice freezing or melting. The experiments show that the net effect of the four feedbacks is stabilizing in this idealized model when salinity or temperature are perturbed. However, it is destabilizing when the solar forcing is perturbed. The results suggest that accurate representation of the response of high-latitude hydrological paths to changing temperatures may be crucial to modeling the sensitivity of the thermohaline circulation.

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Mototaka Nakamura

Abstract

A sudden change in the reference Greenland Sea surface temperature (GSST) in 1979 is identified. It is found to be a part of complex changes in the northern North Atlantic seas. The GSST change, in particular, resulted in a major change in the near-surface baroclinicity in the region, in addition to a large change in the net surface heat flux at the air–sea boundary over the Greenland Sea. The differences in the atmospheric mean state between two periods, one before and the other after the GSST change in the late 1970s, resemble those between the high and low North Atlantic Oscillation (NAO) index states. In addition to the changes in the mean state, major changes in the interannual variability of the atmosphere are found. A particularly interesting change in the interannual variability is found in the relationship between July GSST and the NAO phase in the following February. There is a strong correlation between July GSST and the NAO phase in the following February before the late 1970s but not at all after the late 1970s. The characteristics of these changes suggest that they may be a part of the high-frequency details of the Atlantic multidecadal oscillation.

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Mototaka Nakamura

Abstract

The potential impact of the variability in the Agulhas Current system on the large-scale atmospheric state in the Southern Hemisphere is examined, using the monthly near-surface baroclinicity as the key parameter, for the period between September 1980 and August 2002. Dominant patterns of anomalous near-surface baroclinicity found from empirical orthogonal function (EOF) analyses in the region that includes most of the Agulhas Current system show a wide variety of anomaly patterns: some of which indicate spatial shifts in the position of the Agulhas Retroflection and/or Agulhas Return Current. Composited anomalies in various atmospheric fields, sea surface temperature, and the net surface heat flux at the air–sea boundary based on the signals in the EOFs suggest that sea surface temperature anomalies in the Agulhas Current system thermally force the atmosphere on the synoptic scale via modification of the near-surface baroclinicity in March and April and possibly in January and February as well.

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Mototaka Nakamura and Yi Chao

Abstract

Output of an eddy-resolving model of the North Atlantic Ocean is used to diagnose the eddy thickness diffusivity coefficient, κ, defined by Gent and McWilliams in their quasi-adiabatic parameterization for transports by mesoscale eddies. The results suggest that κ has large spatial and temporal variations, with negative values about half of the time. The order of magnitude of κ shows a wide range in the western North Atlantic, varying from 10 m2 s−1 to 107 m2 s−1. Also, the value of κ is considerably affected by the timescale used to define the high-frequency and low-frequency components. The results suggest that κ should be a diagnosed variable that reflects the strength of eddy mixing during a model integration.

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Mototaka Nakamura and Yi Chao

Abstract

Output of an eddy-resolving model of the North Atlantic is diagnosed in the vicinity of the Gulf Stream (GS), using quasigeostrophic potential vorticity (QGPV), quasigeostrophic potential enstrophy (QGPE), and modified divergent eddy potential vorticity flux, (Vhq*)d. A tongue or an elongated island of large mean QGPV along the model GS in the top 1000 m is associated with predominantly downgradient (Vhq*)d, suggesting that the horizontal eddy fluxes are balancing a sink of eddy QGPE in most of the tongue or island by converting the mean QGPE into eddy QGPE. Some large upgradient (Vhq*)d is observed to the north of the center of the tongue or island, however, suggesting that some of the eddy fluxes in the northern half of the tongue or island of high QGPV are balancing a source of eddy QGPE there by converting eddy QGPE into the mean QGPE. At the intermediate levels of the model, under the GS, eddy QGPE is small, and the role of eddies appears to be mixed; they are forcing the mean to some extent and dissipating the mean to some extent. At the deep levels, the eddies show predominantly a dissipative role, tending to convert the mean QGPE into eddy QGPE. In the region of large time-mean meanders in the model GS, eddies are found to be reinforcing the meanders in a way very similar to that found in diagnoses of atmospheric blocking, which is essentially a large quasi-stationary meander in the subtropical jet. It suggests that the problem of the excessive meander amplitude in the model may be due to an imbalance between eddy forcing in the vicinity of the separation point and zonal acceleration of the GS simulated by the model or due to an unrealistically strong topographic stationary wave forcing in the model.

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Mototaka Nakamura and Toru Miyama

Abstract

Impacts of a sea surface temperature front (SSTF) in the northwestern Pacific Ocean on the large-scale summer atmospheric state in the region are examined with a regional atmospheric circulation model developed at the International Pacific Research Center. Ensemble simulation experiments with various SSTF strengths and positions show that an SSTF does have strong impacts on the summer atmospheric circulation in the region. A meridional shift in the position of the SSTF generally shifts the tropospheric jet and brings temperature anomalies in the area affected by the shift.

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Mototaka Nakamura and Yi Chao

Abstract

Output of an eddy-resolving model of the North Atlantic is diagnosed in the vicinity of the Gulf Stream (GS), using quasigeostrophic potential vorticity (QGPV), Ertel’s potential vorticity (PV), and particle trajectories. Time series of QGPV show strong input of QGPV by the GS in the top 1000 m of the model ocean. Vigorous wave motions are observed in the vicinity of the model GS, mixing QGPV in the region. The time-mean horizontal QGPV structures show qualitative similarity to those of large-scale climatological PV calculated from hydrographic data by Keffer and that of Lozier. The top 1000 m of the model ocean is characterized by a tongue or an elongated island of high mean QGPV along the GS. It is demonstrated that the tongue is a product of strong QGPV input by the GS, vigorous mixing by eddies, and dissipation of QGPV along the path of the GS. At the intermediate depths, 1000–2500 m, a large region of nearly homogenized mean PV or weakly varying mean QGPV is found to the west of the Mid-Atlantic Ridge. It is located undernearth a region of strong near-surface eddy activity and is in qualitative agreement with a deep and large pool of nearly homogenized PV recently found by Lozier. Below the pool of nearly homogenized PV or weakly varying QGPV, the mean PV and QGPV show substantial horizontal gradient and some vertical gradient at deep levels. This structure is in qualitative agreement with results of idealized model experiments and a theory of baroclinic neutrality of the midlatitude atmosphere proposed by Lindzen that may well apply to this oceanic region of strong baroclinic wave activity.

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Mototaka Nakamura and Shozo Yamane

Abstract

Variability in the monthly-mean flow and storm track in the North Pacific basin is examined with a focus on the near-surface baroclinicity. Dominant patterns of anomalous near-surface baroclinicity found from empirical orthogonal function (EOF) analyses generally show mixed patterns of shift and changes in the strength of near-surface baroclinicity. Composited anomalies in the monthly-mean wind at various pressure levels based on the signals in the EOFs show 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 near-surface baroclinicity patterns exhibit, broadly speaking, structures anticipated from simple linear theories of baroclinic instability, and suggest a tendency for anomalous wave fluxes to accelerate–decelerate the surface westerly accordingly. However, the relationship between anomalous eddy fields and anomalous near-surface baroclinicity in the midwinter is not consistent with the simple linear baroclinic instability theories. Composited anomalous sea surface temperature (SST) accompanying anomalous near-surface baroclinicity often exhibits moderate values and large spatial scales in the basin, rather than large values concentrated near the oceanic fronts. In the midsummer and in some cases in cold months, however, large SST anomalies are found around the Kuroshio–Oyashio Extensions. Accompanying anomalies in the net surface heat flux, SST in the preceding and following months, and meridional eddy heat flux in the lower troposphere suggest active roles played by the ocean in generating the concomitant anomalous large-scale atmospheric state in some of these cases.

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Mototaka Nakamura and Shozo Yamane

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 = Bx i + By j. 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, structures 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.

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Mototaka Nakamura and R. Alan Plumb

Abstract

The relationship between the direction of the breaking of Rossby waves on an isolated jet and the cross-jet asymmetry of the flow is investigated. The flow structure of a circular jet is, under most circumstances, such that waves break uniquely outward, although the authors contrive a flow on which breaking is uniquely inward. The influence on the direction of the breaking of the location of critical lines in the undisturbed flow is discussed. On a straight jet with asymmetric shear, there are three wave amplitude regimes: weak waves do not break; waves of moderate amplitude break only toward the closer critical line; and waves of sufficiently large amplitude break both ways.

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