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Hisashi Nakamura

Abstract

Seasonal variations in baroclinic wave activity and jet stream structure in the Northern Hemisphere are investigated based upon over 20 years of daily data. Baroclinic wave activity at each grid point is represented for each day by an envelope function, the lowpass-filtered time series of the squared highpass-filtered geopotential height. Baroclinic wave activity over the Atlantic exhibits a single maximum in January, whereas in the Pacific it exhibits peaks in late autumn and in early spring and a significant weakening in midwinter, which is more evident at the tropopause level than near the surface. This suppression occurs despite the fact that the low-level baroclinity and the intensity of the jet stream are strongest in midwinter.Based on the analysis of 31-day running mean fields for individual winters, it is shown that over both the oceans baroclinic wave activity is positively correlated with the strength of the upper-tropospheric jet for wind speeds up to ∼45 m s−1. When the strength of the westerlies exceeds this optimal value, as it usually does over the western Pacific during midwinter, the correlation is negative: wave amplitude and the meridional fluxes of heat and zonal momentum all decrease with increasing wind speed. The phase speed of the waves increases with wind speed, while the steering level drops, which is indicative of the increasing effects of the mean flow advection and the trapping of the waves near the surface.

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Hisashi Nakamura

Abstract

Horizontal divergence in the upper troposphere associated with zonally isolated jet streams in the climatological-mean fold for the Northern Hemisphere winter is examined by using the wind fields obtained from the NMC operational analyses in the 1980s. Divergence is dominant over the jet exit regions and convergence over the entrance regions, which is found to be consistent with the vertical-motion field in the ECMWF analyses. The divergence pattern cannot be fully explained in the framework of quasigeostrophic scaling. The vorticity advection by the ageostrophic flow across the tight vorticity gradient associated with the jet streams is found to be as strong as the advection by the geostrophic flow, and these two advective effects are in balance with the vortex-tube stretching associated with the observed divergence.

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Koutarou Takaya
and
Hisashi Nakamura

Abstract

Intraseasonal amplification events of the surface Siberian high in winter are generally associated with blocking ridge formation in the upper troposphere. Composite analysis applied to the 20 strongest intraseasonal events of upper-level anticyclonic anomalies at every grid point over Siberia reveals that the blocking formation differs fundamentally between the east and west of the climatological upper-level trough over the Far East. To the west, what can be called “wave-train (Atlantic-origin)” type is common, where a blocking ridge develops from anomalies as a component of a quasi-stationary Rossby wave train propagating across the Eurasian continent under modest feedback forcing from transient eddies. To the east of the trough, what can be called “Pacific-origin” type dominates, where a blocking ridge forms in association with westward development of anticyclonic anomalies from the North Pacific under stronger feedback forcing from the Pacific storm track. Regardless of a particular type of blocking formation in the upper troposphere, a cold air outbreak tends to occur once anomalously cold air reaches the northeastern slope of the Tibetan Plateau.

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Koutarou Takaya
and
Hisashi Nakamura

Abstract

Mechanisms of intraseasonal amplification of the Siberian high are investigated on the basis of composite anomaly evolution for its strongest events at each of the grid points over Siberia. At each location, the amplification of the surface high is associated with formation of a blocking ridge in the upper troposphere. Over central and western Siberia, what may be called “wave-train (Atlantic-origin)” type is common, where a blocking ridge forms as a component of a quasi-stationary Rossby wave train propagating across the Eurasian continent. A cold air outbreak follows once anomalous surface cold air reaches the northeastern slope of the Tibetan Plateau.

It is found through the potential vorticity (PV) inversion technique that interaction between the upper-level stationary Rossby wave train and preexisting surface cold anomalies is essential for the strong amplification of the surface high. Upper-level PV anomalies associated with the wave train reinforce the cold anticyclonic anomalies at the surface by inducing anomalous cold advection that counteracts the tendency of the thermal anomalies themselves to migrate eastward as surface thermal Rossby waves. The surface cold anomalies thus intensified, in turn, act to induce anomalous vorticity advection aloft that reinforces the blocking ridge and cyclonic anomalies downstream of it that constitute the propagating wave train. The baroclinic development of the anomalies through this vertical coupling is manifested as a significant upward flux of wave activity emanating from the surface cold anomalies, which may be interpreted as dissipative destabilization of the incoming external Rossby waves.

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Koutarou Takaya
and
Hisashi Nakamura

Abstract

A new formulation of an approximate conservation relation of wave-activity pseudomomentum is derived, which is applicable for either stationary or migratory quasigeostrophic (QG) eddies on a zonally varying basic flow. The authors utilize a combination of a quantity A that is proportional to wave enstrophy and another quantity E that is proportional to wave energy. Both A and E are approximately related to the wave-activity pseudomomentum. It is shown for QG eddies on a slowly varying, unforced nonzonal flow that a particular linear combination of A and E , namely, M ≡ (A + E )/2, is independent of the wave phase, even if unaveraged, in the limit of a small-amplitude plane wave. In the same limit, a flux of M is also free from an oscillatory component on a scale of one-half wavelength even without any averaging. It is shown that M is conserved under steady, unforced, and nondissipative conditions and the flux of M is parallel to the local three-dimensional group velocity in the WKB limit. The authors’ conservation relation based on a straightforward derivation is a generalization of that for stationary Rossby waves on a zonally uniform basic flow as derived by Plumb and others.

A dynamical interpretation is presented for each term of such a phase-independent flux of the authors or Plumb. Terms that consist of eddy heat and momentum fluxes are shown to represent systematic upstream transport of the mean-flow westerly momentum by a propagating wave packet, whereas other terms proportional to eddy streamfunction anomalies are shown to represent an ageostrophic flux of geopotential in the direction of the local group velocity. In such a flux, these two dynamical processes acting most strongly on the node lines and ridge/trough lines of the eddy streamfunction field, respectively, are appropriately combined to eliminate its phase dependency. The authors also derive generalized three-dimensional transformed Eulerian-mean equations with the residual circulation and eddy forcing both expressed in phase-independent forms.

The flux may not be particularly suited for evaluating the exact local budget of M, because of several assumptions imposed in the derivation. Nevertheless, these assumptions seem qualitatively valid in the assessment based on observed and simulated data. The wave-activity flux is a useful diagnostic tool for illustrating a“snapshot” of a propagating packet of stationary or migratory QG wave disturbances and thereby for inferring where the packet is emitted and absorbed, as verified in several applications to the data. It may also be useful for routine climate diagnoses in an operational center.

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Takafumi Miyasaka
and
Hisashi Nakamura

Abstract

Three-dimensional structure and dynamics of the climatological-mean summertime subtropical highs over the North Pacific and Atlantic (i.e., the Azores high) are investigated. Each of the observed surface highs is accompanied by a meridional vorticity dipole aloft, exhibiting barotropic and baroclinic structures in its northern and southern portions, respectively, in a manner dynamically consistent with the observed midtropospheric subsidence. Each of the highs develops over the relatively cool eastern ocean, where a pronounced near-surface thermal contrast exists with a heated landmass to the east. The authors demonstrate through numerical experiments that those highs can be reproduced in response to a local shallow cooling–heating couplet associated with this thermal contrast, although the upper-level response is somewhat underestimated. The model experiments suggest that the near-surface thermal contrasts associated with those surface subtropical highs over the Pacific and Atlantic can act as sources of the observed planetary waves over the Western Hemisphere. In fact, a wave activity flux for stationary Rossby waves is distinctively upward and diverging toward downstream in the upper troposphere above each of the observed surface highs. The observed wave activity injection is significant into the Azores high but not at all into the Pacific high. Since each of the subtropical highs can be reproduced reasonably well, even for the premonsoon season (i.e., May), in response to a local shallow land–sea heating contrast, it is suggested that the monsoonal convective heating may not necessarily be a significant direct forcing factor for the formation of the summertime subtropical highs. In fact, the model response is quite weak if forced only by mid- and upper-tropospheric convective heating. The present study suggests the presence of a local land–sea–atmosphere feedback loop associated with a subtropical high and a continental low to its east, which may be triggered by increasing insolation over land from spring to summer.

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Hisashi Nakamura
and
Akihiko Shimpo

Abstract

Regional characteristics of the climatological seasonal variations in Southern Hemisphere (SH) storm tracks are examined based on a reanalysis dataset. When differences in vertical structure between the subpolar and subtropical jet streams (SPJ and STJ, respectively) are considered, the regional characteristics can be interpreted reasonably well from a potential vorticity perspective of baroclinic eddy growth and downstream development of a baroclinic wave packet.

Eddy activity in the upper and lower tropospheres is strongest in the core region of the SPJ over the eastern South Atlantic and Indian Ocean throughout the year, even in austral winter when the intense STJ forms over the Indian Ocean and South Pacific. Showing its eddy-driven nature, the SPJ accompanies the strong surface westerlies along a well-defined baroclinic zone above an intense oceanic frontal zone. In this core region of the storm track, low-level eddy activity is strongly correlated with the local near-surface baroclinicity, with its late- winter maximum and summer minimum, while upper-level eddy activity also depends on the incoming wave activity from upstream.

Over the South Pacific, storm track activity depends critically on the formation of the STJ. In the absence of the intense STJ in summer and autumn, a single well-defined circumpolar storm track forms along the SPJ. During winter and spring, in contrast, wave activity accumulated in the core region is dispersed mainly toward the STJ, along which vigorous baroclinic eddy growth is unlikely to occur. The South Pacific storm track in the upper troposphere thus bifurcates into two branches along the STJ and SPJ, while at lower levels the storm track forms only along the enhanced baroclinic zone along the SPJ. Thus, under the trapping effect of the intense STJ, the storm track activity over the South Pacific is suppressed in winter, despite the enhanced low-level baroclinicity.

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Ning Shi
and
Hisashi Nakamura

Abstract

Blocking flow configurations, which tend to accompany strong circulation anomalies and therefore can cause extreme weather conditions, have recently been studied in relation to large-scale wave breaking (WB). Although WB events have been detected often from an instantaneous morphology perspective, the present study proposes a new approach for the detection from a wave-activity perspective in focusing on its accumulation, saturation, and release. This evolution of wave activity is theoretically equivalent to anomalous potential vorticity (PV) flux with its sign changing from negative to positive, which is utilized in this study to detect WB events that accompany high-amplitude height anomalies and blocking flow configurations. As in previous studies, a given WB event is classified into a high pressure type or low pressure type depending upon the sign of the primary PV anomaly center and further into an eastward or westward type depending upon the longitudinal movement of that center. The new method applied to the wintertime Northern Hemisphere shows that a WB event with a blocking anticyclone is likely to accompany an eastward-moving PV anomaly center, occurring mostly under anticyclonic westerly shear. By contrast, a WB event with a strong cyclonic anomaly mostly accompanies the eastward-moving PV anomaly center under cyclonic westerly shear. Composite analysis confirms the consistency between the sign-changing anomalous PV flux and convergence/divergence of wave-activity flux of quasi-stationary Rossby wave trains around the WB region.

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Daisuke Hotta
and
Hisashi Nakamura

Abstract

The relative importance between the sensible heat supply from the ocean and latent heating is assessed for the maintenance of near-surface mean baroclinicity in the major storm-track regions, by analyzing steady linear responses of a planetary wave model to individual components of zonally asymmetric thermal forcing taken from a global reanalysis dataset. The model experiments carried out separately for the North Atlantic, North Pacific, and south Indian Oceans indicate that distinct local maxima of near-surface baroclinicity observed along the storm tracks can be reinforced most efficiently as a response to the near-surface sensible heating. The result suggests the particular importance of the differential sensible heat supply from the ocean across an oceanic frontal zone for the efficient restoration of surface baroclinicity, which acts against the relaxing effect by poleward eddy heat transport, setting up conditions favorable for the recurrent development of transient eddies to anchor a storm track. Unlike what has been suggested, the corresponding reinforcement of the near-surface baroclinicity along a storm track as the response to the latent heating due either to cumulus convection or large-scale condensation is found less efficient. As is well known, poleward eddy heat flux convergence acts as the primary contributor to the reinforcement of the surface westerlies, especially in the core of a storm track. In its exit region, a substantial contribution to the reinforcement arises also from a planetary wave response to the sensible heat supply from the ocean. In contrast, the surface wind acceleration as a planetary wave response to the latent heating is found to contribute negatively to the maintenance of the surface westerlies along any of the major storm tracks.

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Yu Kosaka
and
Hisashi Nakamura

Abstract

A set of multimodel twentieth-century climate simulations for phase 3 of the Coupled Model Intercomparison Project (CMIP3) is analyzed to assess the model reproducibility of the Pacific–Japan (PJ) teleconnection pattern. It is the dominant low-frequency anomaly pattern over the summertime western North Pacific (WNP), characterized by a meridional dipole of zonally elongated vorticity anomalies in the lower troposphere and by anomalous precipitation over the tropical WNP. Most of the models can reproduce the PJ pattern reasonably well as one of the leading anomaly patterns or their combination. The model reproducibility of the pattern tends to be higher for those models in which the climatological-mean state over the WNP is better reproduced. Furthermore, intermodel diversity in the summertime climatological-mean fields over the WNP, especially in the lower troposphere, is found to be large and projected most strongly onto the observed PJ pattern. Nevertheless, the multimodel ensemble (MME) mean of these climatological-mean states is close to the observations.

Projected future changes in the summertime climatological-mean state under the Intergovernmental Panel on Climate Change’s (IPCC) Special Report on Emission Scenarios (SRES) A1B also bear certain similarities with the PJ pattern, in a manner consistent with the aforementioned sensitivity of the model climate to that pattern. The MME projection indicates an overall increase in precipitation over the entire tropics, but it is overwhelmed locally by the effects of the enhanced tropospheric stratification over the tropical WNP. A resultant local reduction of the mean ascent is dynamically consistent with the anticyclonic projection around the Philippines and the cyclonic projection around Japan in MME, as in the observed anomalous dipole associated with the PJ pattern. However, the polarity and magnitude of the PJ-like projected change vary substantially from one model to another.

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