Mototaka Nakamura

Destabilization of the thermohaline circulation by atmospheric eddy transports

Abstract Simple proem models have been developed to investigate the role of atmosphere-ocean feedbacks in the stability of the current mode of the thermohaline circulation in the North Atlantic. A positive feedback between the meridional atmospheric transport of moisture and the high-latitude sinking thermohaline circulation (EMT feedback) has been found to help destabilize the latter. The minimum perturbation required to shut off the high-latitude sinking is considerably smaller when this feedback is included. Also, the high-latitude sinking is shut off much faster with this feedback than without it, given a perturbation of the same magnitude. There is also a strong positive feedback between atmospheric heat transport and the thermohaline circulation, but this can be modeled accurately on the global scale by using a properly tuned Newtonian cooling law for the surface beat flux. Idealized flux adjustment experiments suggest that the sensitivity of the real climate is not represented well in coupled atmos...

Dominant Anomaly Patterns in the Near-Surface Baroclinicity and Accompanying Anomalies in the Atmosphere and Oceans. Part I: North Atlantic Basin

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 = Bxi + 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, 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...

On the Eddy Isopycnal Thickness Diffusivity of the Gent–McWilliams Subgrid Mixing Parameterization

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.

Impacts of the Oyashio Temperature Front on the Regional Climate

AbstractImpacts 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.

Diagnoses of an Eddy-Resolving Atlantic Ocean Model Simulation in the Vicinity of the Gulf Stream. Part I: Potential Vorticity

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-Atla...

Effects of Ice Albedo and Runoff Feedbacks on the Thermohaline Circulation

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 ...

Characteristics of three-dimensional quasi-geostrophic transient eddy propagation in the vicinity of a simulated Gulf Stream

Output of an eddy-resolving model of the North Atlantic is diagnosed in the vicinity of the Gulf Stream (GS), using the three-dimensional quasi-geostrophic transient wave activity flux diagnostic developed by Plumb [1986]. The model North Atlantic shows large transient wave activity along the model GS and along the edge of the Mediterranean Outflow. The fluxes show advection of wave activity by the GS from the separation point and radiation of wave activity from the GS in the top 1000 m. The wave radiation from the model GS is particularly clear to its south and points southwestward in general. The fluxes in and around the Mediterranean Outflow appear to be generated and dissipated locally. The vertical component of the transient wave activity flux is downward on average at all depths, suggesting conversion of baroclinic energy supplied near the surface into kinetic energy distributed throughout the depth. Eddy fields show that transient eddies tend to reinforce quasi-stationary meanders in the GS just downstream of the separation point. We also find evidence for eddy forcing of the northern recirculation generated in the model, in agreement with the available observations.

Quasigeostrophic Transient Wave Activity Flux: Updated Climatology and Its Role in Polar Vortex Anomalies

Abstract The climatology of transient wave activity flux defined by Plumb has been calculated for each calendar month, for high-frequency (HF) and low-frequency (LF) waves, using the NCAR–NCEP reanalyses for both hemispheres. Wave activity flux of both HF and LF waves shows upward propagation of waves from the lower troposphere into the upper troposphere, then into the lower stratosphere during the summer and at least up to the midstratosphere during other seasons. While the upward flux emanating from the lower troposphere is particularly large in the two storm tracks in the Northern Hemisphere (NH), it is large in most of the extratropics in the Southern Hemisphere (SH). The HF waves radiate equatorward most noticeably in the upper troposphere, whereas the LF waves do not show visible signs of equatorward radiation. The total horizontal flux is generally dominated by the advective flux that represents the eddy enstrophy advection by the mean flow and appears predominantly pseudoeastward. Divergence of th...

Greenland Sea Surface Temperature Change and Accompanying Changes in the Northern Hemispheric Climate

AbstractA 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 ...

Diagnoses of an Eddy-Resolving Atlantic Ocean Model Simulation in the Vicinity of the Gulf Stream. Part II: Eddy Potential Enstrophy and Eddy Potential Vorticity Fluxes

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, (V′hq′*)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 (V′hq′*)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 (V′hq′*)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 ...