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.

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.

Abstract

Summertime atmospheric circulation over the midlatitude western North Pacific (WNP) is influenced by anomalous convective activity near the Philippines. This meridional teleconnection, observed in monthly anomalies and known as the Pacific–Japan (PJ) pattern, is characterized by zonally elongated cyclonic and anticyclonic anomalies around the enhanced convection center and to its northeast, respectively, in the lower troposphere, with an apparent poleward phase tilt with height. The authors’ idealized two-layer linear model, whose basic state consists of a zonal subtropical jet and a pair of a monsoon system and a subtropical anticyclone, can simulate a PJ-like response against diabatic heating located between the pair. Each of the observed and simulated patterns can gain energy through barotropic and baroclinic conversions from the zonally varying baroclinic mean flow, in an efficiency comparable with that of energy generation due to the anomalous diabatic heating, indicating a characteristic of the pattern as a dry dynamical mode. In fact, the conversion efficiency is sensitive to the location of the anomaly pattern relative to the climatological-mean flow. Furthermore, the second-least damped mode identified in the idealized model bears certain resemblance with the observed PJ pattern, indicating its modal characteristics as well as a critical importance of these features in the mean field for the pattern. In addition to the PJ pattern, another meridional teleconnection pattern with high efficiency for its energy conversion is identified observationally in association with anomalous convection near the Bonin Islands.

The anomalous circulation of the PJ pattern, in turn, can intensify the anomalous convective activity near the Philippines through enhancing evaporation and moisture convergence and dynamically inducing anomalous ascent. It is thus hypothesized that the PJ pattern can be regarded as a moist dynamical mode that sustains itself both via dry energy conversion and interaction with moist processes.

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.

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.

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.

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.

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.

Abstract

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.

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.