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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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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 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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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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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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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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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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Koutarou Takaya and Hisashi Nakamura

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Interannual variability of the East Asian winter monsoon is investigated through composite analysis applied to observational data for 50 recent years. Although the monsoon activity itself is confined into the lower troposphere, its midwinter variability tends to accompany upper-tropospheric geopotential height anomalies similar to the Eurasian (EU) and western Pacific (WP) teleconnection patterns. The “EU-like” pattern is characterized by a wavy signature over the Eurasian continent and the North Atlantic, with surface temperature anomalies over the Far East and North America. In the “WP-like” pattern, a meridional dipole of upper-level height anomalies is evident over the Far East.

These anomaly patterns related to the anomalous winter monsoon activity are found to accompany marked modulations of the climatological development of the upper-tropospheric planetary waves from late autumn to midwinter. Enhanced monsoon activity in January associated with the WP-like pattern involves anomalous seasonal development of a planetary wave ridge with enhanced positive height tendencies from November to January over eastern Siberia and Alaska, while the corresponding tendencies are anomalously negative under the weakened monsoon activity. The stronger monsoon also accompanies an enhanced seasonal decline of geopotential height over the midlatitude North Pacific, corresponding to the enhanced southeastward development of a planetary wave trough. Similar modulations of the planetary wave evolution are observed with the anomalous monsoon activity associated with the EU-like pattern. In addition, the anomalous midwinter activity of the monsoon is also accompanied by noticeable variability of the seasonal development of the planetary waves over the Euro-Atlantic sector.

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Yu Kosaka and Hisashi Nakamura

Abstract

A global survey is conducted for atmospheric anomaly patterns of meridional teleconnection over the summer hemisphere associated with anomalous tropical convection. The patterns may be akin to the Pacific–Japan (PJ) teleconnection pattern analyzed in detail in the companion paper. From the survey, meridional teleconnections are identified over five regions, namely, the western North Pacific and Central/North America in boreal summer, as well as the western South Indian Ocean, central South Pacific, and western South Atlantic in austral summer. All of the patterns are observed in the western peripheries of the summertime surface subtropical anticyclones over the individual ocean basins. Although all of the patterns can convert available potential energy (APE) efficiently from the vertically sheared subtropical westerly jets, the efficiencies of barotropic energy conversion from the mean flow and diabatic APE generation differ from one pattern to another. Still, all of the patterns gain energy as the net, to maintain themselves against dissipative processes. Both the anomalous moisture convergence near the surface and the midtropospheric anomalous ascent required for the vorticity and thermal balance act to sustain the anomalous tropical convection, while the wind-evaporation feedback contributes positively only to the PJ pattern over the western North Pacific. Examination of common features and discrepancies among the five teleconnection patterns with respect to their structures and energetics reveals that climatological background features, including the largest horizontal extent of the Asian monsoon system and the North Pacific subtropical anticyclone, in addition to particularly high SST over the Pacific warm pool, render the PJ pattern an outstanding mode of variability.

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Meiji Honda and Hisashi Nakamura

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Through analysis of observational data for the period of 1973–94, the late-winter formation of an interannual seesaw between the surface Aleutian and Icelandic lows (AL and IL, respectively) is shown to significantly impact the covariance structure of the leading mode of the interannual variability in the geopotential height field over the extratropical Northern Hemisphere. The tropospheric leading mode for early winter (November–January) is characterized by a polar–midlatitude dipole over the Euro–Atlantic sector with a high degree of the annularity, coupled with the anomalous lower-stratospheric polar vortex. Over the North Pacific, no significant anomalies are associated with this mode. After the formation of the AL–IL seesaw, however, the dipole no longer dominates in the upper-tropospheric variability. The dipole signature is masked in late winter (February–April) by the predominant combined signature of the so-called Pacific–North American pattern and a meridional dipole over the northwestern Atlantic as an upper-level manifestation of the seesaw. Though somewhat less pronounced, the leading mode of the near-surface variability is modified accordingly in late winter by the superposition of the distinct signature of the AL–IL seesaw. The annularity of the leading mode of the tropospheric variability is thus reduced in late winter, particularly at the upper levels. Nevertheless, because of the particular geographical alignment between the anomalous AL and IL, their seesaw changes the zonal wind coherently between the two ocean basins, yielding a strong projection on the meridional plane whose latitudinal profile is almost indistinguishable from the counterpart of the Arctic–midlatitude dipole.

It is argued that what is called the Arctic oscillation in some recent literature, defined as the leading mode of the sea level pressure variability for the entire cold season, may be interpreted as a superposition of the AL–IL seesaw upon a dominant signal of the Arctic–midlatitude dipole. The corresponding leading mode for the upper troposphere primarily represents the variability associated with the seesaw. It is also argued that the late-winter tropospheric variability over the North Atlantic may not necessarily be associated with the Arctic–midlatitude dipole. The remote influence of the North Pacific variability accounts for as much as 30%–50% of the variance in the vicinity of the IL for the data period considered.

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