Westerly Jet Stream Research Articles

Circumglobal Teleconnection in the Northern Hemisphere Summer*

AbstractAnalysis of the 56-yr NCEP–NCAR reanalysis data reveals a recurrent circumglobal teleconnection (CGT) pattern in the summertime midlatitude circulation of the Northern Hemisphere. This pattern represents the second leading empirical orthogonal function of interannual variability of the upper-tropospheric circulation. The CGT, having a zonal wavenumber-5 structure, is primarily positioned within a waveguide that is associated with the westerly jet stream. The spatial phases of CGT tend to lock to preferred longitudes. The geographically phase-locked patterns bear close similarity during June, August, and September, but the pattern in July shows shorter wavelengths in the North Pacific–North America sector. The CGT is accompanied by significant rainfall and surface air temperature anomalies in the continental regions of western Europe, European Russia, India, east Asia, and North America. This implies that the CGT may be a source of climate variability and predictability in the above-mentioned midlatitude regions.The CGT has significant correlations with the Indian summer monsoon (ISM) and El Niño–Southern Oscillation (ENSO). However, in normal ISM years the CGT–ENSO correlation disappears; on the other hand, in the absence of El Niño or La Niña, the CGT–ISM correlation remains significant. It is suggested that the ISM acts as a “conductor” connecting the CGT and ENSO. When the interaction between the ISM and ENSO is active, ENSO may influence northern China via the ISM and the CGT. Additionally, the variability of the CGT has no significant association with the Arctic Oscillation and the variability of the western North Pacific summer monsoon. The circulation of the wave train shows a barotropic structure everywhere except the cell located to the northwest of India, where a baroclinic circulation structure dominates. Two possible scenarios are proposed. The abnormal ISM may excite an anomalous west-central Asian high and downstream Rossby wave train extending to the North Pacific and North America. On the other hand, a wave train that is excited in the jet exit region of the North Atlantic may affect the west-central Asian high and, thus, the intensity of the ISM. It is hypothesized that the interaction between the global wave train and the ISM heat source may be instrumental in maintaining the boreal summer CGT.

Interannual variation of North China rainfall in rainy season and SSTs in the equatorial eastern Pacific

The rainfall in North China during rainy season (July and August (JA)) exhibits a strong interannual variability. In this study, the atmospheric circulation and SST anomalies associated with the interannual variation of JA North China rainfall are examined. It is found that on the interannual timescale, the JA North China rainfall is associated with significant SST anomalies in the equatorial eastern Pacific, and the North China rainfall and SST anomaly in the equatorial eastern Pacific correspond to the similar variation of the upper-level westerly jet stream over East Asia. A possible mechanism is proposed for the influence of the SST anomalies in the equatorial eastern Pacific on the North China rainfall.

Relationship between stationary planetary wave activity and the East Asian winter monsoon

The variability of both the stationary planetary wave activity and the East Asian winter monsoon is strongly associated with the thermal contrast between oceans and landmasses. In this study, we explore the interannual relationship between the monsoon and the wave activity defined by an index of the difference in the divergence of Eliassen‐Palm flux between 50°N at 500 hPa and 40°N at 300 hPa. It is found that, compared to the winters of low wave activity, the equatorward propagation of planetary waves in the middle and upper troposphere is stronger in the high wave activity winters. During these high activity winters, the upward wave propagation from the troposphere into the stratosphere becomes weaker. This is accompanied by a smaller perturbation in the polar vortex, which tends to be colder and stronger. In the meantime, the East Asian westerly jet stream, the East Asian trough, the Siberian high, and the Aleutian low all become weaker apparently. In particular, the weakening of the Siberian high and the Aleutian low decreases the northeasterly wind over East Asia, leading to a warming condition in the region especially in northeastern Asia. A further analysis reveals that the zonal wavenumber‐2 pattern of planetary waves contributes dominantly to the variability of the East Asian winter monsoon.

The Climatological Rossby Wave Source over the STCZs in the Summer Northern Hemisphere

Using the twice-daily data for the 10-year period from 1989 to 1998, from the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis, the authors analyze the climatological Rossby wave sources at 200 hPa in summer, focusing on the uppertropospheric circulation related to the subtropical convergence zones (STCZs) in the Northern Hemisphere, namely, the North Pacific convergence zone (NPCZ, including the Meiyu/Baiu front) and the North Atlantic convergence zone. In the June-July-August (JJA) period, a Rossby wave sink clearly appears along the uppertropospheric westerly jet streams over East Asia and the extratropical North Pacific, but is very weak in the extratropical North Atlantic. For the JJA mean, or in early summer (June), the Rossby wave sink over East Asia, and the extratropical North Pacific, is much stronger than that in the tropical western North Pacific. The latter sink, however, becomes stronger in late summer (August), corresponding to the enhancement and poleward shift of the atmospheric convection, and becomes comparable with the former sink. The components of the Rossby wave source, namely, vortex stretching and advection of absolute vorticity by divergent flow, are also examined and used to judge the roles of tropical and extratropical heatings. Despite the integrated appearance of the sink along the NPCZ, this sink results dominantly from the advection of absolute vorticity by divergent flow in East Asia, and the western Pacific, and from vortex stretching over the mid-Pacific. These results reveal that the tropical heating is responsible for the maintenance of upper-tropospheric circulation in East Asia, and the extratropical western North Pacific, through the advection of absolute vorticity by divergent flow, while the diabatic heating in the NPCZ plays a dominant role in maintaining the upper-tropospheric circulation over the mid-Pacific.

Associations among the Components of the East Asian Summer Monsoon System in the Meridional Direction

The East Asian summer monsoon is characterized by strong interactions among its components in the meridional direction. The atmospheric convection over the Philippine Sea (PSAC), the western North Pacific subtropical high (WNPSH) and the East Asian westerly jet stream (EAJ) are all closely related to the summer rainfall in East Asia. In this study, we examined the relationship on the interannual timescale among these factors, by using the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Re-analysis data and satellite-observed outgoing long-wave radiation (OLR) data for the 20-year period from 1979 to 1998. It is found that the PSAC-EAJ relation is weak in June, but strong in July and August. Similar difference exists in the relationship between the PSAC and convective activity along the East Asian summer rainy belt. Corresponding to enhanced PSAC, the EAJ is weakened in July and strengthened in August, and tends to exhibit a slight poleward displacement in both months. All these variations in the EAJ intensity and meridional displacement, on the other hand, correspond to suppressed convection along the East Asian summer rainy belt. Finally, the monthly difference in the PSAC-EAJ relation is interpreted by the role of vertical shear of zonal wind. The easterly shear in July and August over the Philippine Sea excites external modes, which are necessary for the tropical-extratropical teleconnection mechanism according to previous numerical studies, but the neutral shear in June is inefficient in exciting external modes.

El Niño-related variations in the southern Pacific atmospheric circulation: model versus observations

Our objective is two-fold: (1) to study the influence of tropical Pacific sea surface temperature (SST) anomalies associated with the El Nino – Southern Oscillation (ENSO) on the southern Pacific atmospheric circulation using observations; and (2) to assess the ability of a fully comprehensive GCM to reproduce the observed ENSO-related variations. The observed circulation features were derived from the National Center for Environmental Prediction (NCEP) reanalyses of observations. The GCM used is the atmospheric component of the United Kingdom Meteorological Office (UKMO) unified model (UM). A detailed study of the impact of the ENSO on the seasonal mean circulation and on the intraseasonal variability on synoptic time scales has been carried out using both the model and NCEP data for austral summer season. We have also investigated the relationship between the high frequency (HF) transients and the quasi-stationary mean circulation during selected ENSO years. A regional Hadley cell intensity in the tropical Pacific has been used as an El Nino index to classify the warming and cooling years for our composite analysis. During El Nino years the regional Hadley cell acts as a medium through which the enhanced equatorial convection forces a train of Rossby waves to produce the quasi-stationary circulation anomalies in extratropics. The Hadley cell weakens during La Nina years, and the equatorial convection moves further westwards. The Rossby wave response to this situation is to reverse the phase of the quasi-stationary circulation anomalies by 180°, as expected. These features are fairly well simulated by the UM, though the extratropical circulation anomalies in response to the equatorial convection is found about 30° longitude further eastwards than observed during El Nino years. The model confirms the observational evidence that the storm track variations in the southern subtropical Pacific are largely driven by variations in the South Pacific Convergence Zone (SPCZ) associated with the El Nino and La Nina events. However the model struggles to reproduce the significant storm track activity, present in the NCEP data, related to the westerly jet stream during La Nina years. Finally, the relationship between the HF transients and the quasi-stationary anomalies found in both the model and observations supports the role of the HF transients in forcing the time-mean circulation, as reported by Hoerling and Ting 1994 for the Pacific – North American (PNA) region.

Observed and SST‐forced seasonal rainfall variability across tropical America

AbstractThree experiments starting from different initial conditions have been made with the ECHAM‐4 atmospheric General Circulation Model (GCM) integrated at T30 resolution forced with the observed sea‐surface temperature (SST) over the period 1960–1994. The tropical America modes of seasonal rainfall anomalies whose time variation is most accurately simulated by the GCM have been searched for using Singular Value Decomposition Analyses (SVDA) and Canonical Correlation Analysis (CCA) between observed and model fields. The leading modes revealed by SVDA and CCA are highly similar, even though the ordering of the modes showed some fluctuation. A first skilful rainfall anomaly mode has weights of the same sign almost everywhere in tropical America, except along the western coast and the sub‐tropical margins. This mode appears in all of the four seasons assessed. A second major skilful mode is usually a bipolar north–south (N–S) rainfall anomaly pattern (clear in December–March, DJFM; March–May, MAM; and June–September, JJAS).A large portion of the skill of the first rainfall anomaly mode (same sign anomalies across tropical America except small patches along the western coast) is through variance that is in common with the Southern Oscillation Index (SOI). In addition to forcing from the central/eastern tropical Pacific SST, there also appears a contribution from contrasting SST anomalies in the tropical Atlantic. This rainfall mode is usually a regional portion of a more large‐scale mode encompassing at least the whole tropical zone (especially in DJFM, MAM and September–November, SON). Analysis of the relationship of this mode with GCM circulation features reveals that a rainfall deficit (respectively excedent) over the main rainbelt of the tropical America region is associated with strengthening (respectively weakening) of the sub‐tropical westerly jet streams, a global warming (respectively cooling) of the tropical atmosphere, an anomalous divergence (respectively convergence) in the lower levels and an anomalous convergence (respectively divergence) in the upper levels over tropical America and in the region of the Atlantic Inter‐tropical Convergence Zone (ITCZ). Such global features are not so apparent for the dominant mode of JJAS, even though the correlations with El Niño–Southern Oscillation (ENSO) indicators (as SOI or NINO3 SST index) are as high as for the other seasons.The bipolar N–S rainfall anomaly mode in tropical America is mostly related to anomalous N–S gradient of SST anomalies in the tropical Atlantic. The atmospheric circulation anomalies emphasize changes in 850 hPa meridional winds in the tropical Atlantic. However, there is also interannual variance of this rainfall mode in both the model and observations that is unexplained by tropical Atlantic SSTs, but which is explained by central/eastern tropical Pacific SSTs and, potentially, SSTs from other tropical and extratropical areas. This is especially true in MAM.Some differences in the details of the model and observed teleconnection patterns are noted. Such differences can be used to statistically adjust the model simulations using the CCA or SVDA modes as basis patterns. Both statistical approaches have been applied and the results are consistent between the two. The increase of skill is stronger when temporal correlation (the pattern correlation) between the model and observed pattern is high (low) as for JJAS. The skill is moderate to high around the whole Amazon basin, but remains relatively low inside the Amazon basin, though reliability of the observations themselves may influence this result. Averaged over all the seasons, about 15–35% (35–55%) of the interannual grid‐box (regional) seasonal rainfall variance is skilfully simulated from the observed SST forcing. Copyright © 2001 Royal Meteorological Society

Seasonal cycle of convective spells over southern Africa during austral summer

The seasonal progression of summer wet spells over southern Africa is investigated. The November to March rainy season is modulated at the intraseasonal scale, with major wet spells every 20-30 days and internal evolution at higher frequencies. Corresponding circulation patterns for each wet spell illustrate the variable dynamics and physics as the season progresses. Early summer rainfall is associated with enhanced baroclinic westerly shear from the upper troposphere of the tropical Atlantic Ocean, whereas low-level moisture is advected from the central Indian )cean. Extratropical westerly wave action is an important influence during early summer. As the summer season progresses, heating causes a retreat of the subtropical westerly jet stream. A quasi-stationary line of confluence along 20°S develops in the low levels during late summer, and extends from the Mozambique Channel to Angola. This trough is supported by an influx of moisture from the Congo and southern Mozambique Channel. Composite differences between early and late summer wet spells are evaluated numerically for the first time in this presentation.

Preliminary climatology of southern Africa extreme weather: 1973?1992

Preliminary climatological statistics of extreme weather events over the southern Africa region are established through analysis of daily weather maps of the South African Weather Bureau for a twenty year peiod, 1973–1992. Influences of global warming and inter-annual variability imposed by El Nino events, amongst others, are sought. Notable trends include a decrease in the frequency of station days with rainfall > 70 mm and an increase in station days with temperature > 38°C. Correlations offer some insights to extreme climate associations. Lows over the land in the west and over the sea to the east display consistent interannual variability, despite opposing rainfall regimes. Agreement in extreme temperature statistics in all regions suggest that drought is widespread over southern Africa. Rainfall in the north is negatively related to lows over the sea to the south. A potential mechanism underlying the inverse relationship between midlatitude and sub-tropical storminess is the development of a vorticity dipole associated with the westerly jet stream. This is investigated in an El Nino-influenced case study.

Interaction of topography and tropospheric flow ? A possible generator for the Red Sea Trough?

Numerical investigation of the nature of one of the most typical Eastern Mediterranean atmospheric circulation phenomena — the Red Sea Trough is undertaken. The role of interaction of typical atmospheric flow systems with the local topography of the North African region is analyzed with the help of idealized numerical simulations employing the Penn State and the National Center for Atmospheric Research (NCAR) MM4 modeling system. The simulations are designed, based on results of a climatological evaluation of the 250 hPa wind field. Idealized initial data sets corresponding to typical transient and winter period positions of the upper tropospheric westerly jet stream are constructed. The data for the analysis are from the National Center for Environmental Prediction (NCEP) 25 y (1965–1989) objective analysis archive. It was found that the primary factor in the Red Sea trough generation is the interaction of the mid-tropospheric westerlies with the terrain in the area of the Red Sea.

夏季モンスーン循環と45日周期擾乱とのインタラクション

The influence of the low-level mean summer monsoon flow on 45-day transient eddies through barotropic interaction is diagnosed by a scalar product between the horizontal shear vector and Eliassen-Palm (E-P) flux, while the dry (moist) baroclinic interaction is measured by a vector product between the vertical wind shear and the eddy sensible (latent) heat transport, which represents the vertical component of E-P fluxes. Activity of 45-day transient eddies was monitored by the evolution of kinetic energy during the 9-year period of 1985-1993. The 45-day wave activity is relatively weak over the SEAM (Southeast Asian monsoon) domain. Absence of geographically fixed forcing makes the WNPM (western North Pacific monsoon) more violent, with the peak 45-day wave activity occurring in late August, the time of frequent development of vigorous tropical cyclones. Near the WNPM updraft center (15°N, 140°-150°E), the E-P wave energy fluxes are oriented in a direction which causes weakening of the sheared mean zonal and meridional winds, implying barotropic amplification of 45-day waves. The weak mean temperature gradient relegates the dry baroclinic instability to that of secondary importance over the WNPM domain. This is contrasted with the significant moist baroclinic process due to an approximate perpendicular relationship between the eddy moisture transport and the strong vertical shear of the monsoon flow. Curiously, 45-day waves are quite active near Japan even during August's dry, hot climate. Neither barotropic nor dry (moist) baroclinic instability can account for late-summer disturbance activity. Evidence is presented that 45-day convective oscillations occuring over the WNPM domain induce a strong 45-day response near Japan and further eastward, possibly indicating an intraseasonal Rossby wave dispersion along the westerly jet stream. Near the Aleutian Islands, a pronounced poleward eddy sensible heat flux is directed down the mean temperature gradient, resulting in dry baroclinic amplification of mid-latitude 45-day waves via action of the Coriolis torque. An import of moisture from the WNPM source region brings about a moist baroclinic instability of extratropical 45-day waves poleward of the Pacific High.

Modern eolian dust deposition in central Asia

Glaciers around the globe provide a unique medium for investigating the temporal and spatial variation of dust deposition. Here we investigate the spatial and seasonal distribution of eolian dust deposition in central Asia through the analysis of concentrations and size distributions of insoluble particles (over the size range 1–22 μ m diameter) in snow and ice samples collected from eight glaciers in the mountains of central Asia. The sample locations cover a wide geographical area and fall within 4 distinct landscape/climate zones: the southern slopes of the eastern Himalaya; the Karakoram; south eastern Tibetan Plateau; northern/western Tibetan Plateau. Snow samples from each of the 4 landscape/climatic zone displays distinct dust deposition characteristics which are dependent upon the physiographic location of the sampling site with respect to source regions of dust and atmospheric transport pathways. The southern slopes of the eastern Himalaya show very low particle concentrations and low annual dust deposition. This region is relatively free from the influence of Asian dust. The Karakoram appears to be a major sink for particles less than 2 μ m in diameter which have been transported long distances with the westerly jet stream. Glaciers in the south eastern Tibetan Plateau record the influx of dust generated in the arid and semi-arid regions to the west. Glaciers on the northern and western margin of the Tibetan Plateau lie adjacent to vast arid and semi-arid regions of western China and provide dust depostion records mainly of local significance. Our results suggest that high elevation glaciers in the eastern′ Himalaya, Karakoram and the south eastern Tibetan Plateau are those most likely to provide longer term dust deposition records which reflect regional to hemispheric signals. DOI: 10.1034/j.1600-0889.1994.t01-2-00005.x

Nonlinear structures with multivalued (q, ψ) relationships—exact solutions of the barotropic vorticity equation on a sphere

Abstract Finite-amplitude solutions are sought to the Barotropic Vorticity Equation (BVE) on a sphere in the presence of realistic zonal flows. The new solutions are characterized by multivalued (q, Ψ) relationships, and can be constructed analytically. The resulting zonal flows and the (q, Ψ) scatter plots can be very close to climatology. Many observed flow features of the persistent anomalies, like the midlatitude westerly jetstreams and splits as well as the well-known dipole-blocking structures can be well described in terms of these new solutions. It is suggested that the observed persistent anomalies in the atmosphere may be a manifestation of these solutions being resonantly excited through thermal forcing.

SAGE II measurements of early Pinatubo aerosols

SAGE II satellite measurements of the Mt. Pinatubo eruption cloud in the stratosphere during June, July, and early August 1991 show that aerosols in the tropics reached as high as 29 km altitude with most of the cloud between 20 and 25 km. The most optically thick portions of the cloud covered latitudes from 10°S to 30°N during the early part of this period. By late July, high stratospheric optical depths were observed to at least 70°N, with the high values north of about 30°N from layers below 20 km. High pressure systems in both hemispheres were observed to be correlated with the movement of volcanic material at 21 km into the westerly jet stream at high southern latitudes and similarly to high northern latitudes at 16 km. By August, the entire Southern Hemisphere had experienced a 10‐fold increase in optical depth relative to early July due to layers above 20 km. Initial mass calculations using SAGE II data place the aerosol produced from this eruption at 20 to 30 megatonnes, well above the 12 megatonnes produced by El Chichon.

On the Maintenance of Short-Term Subtropical Wind Maxima in the Southern Hemisphere during SOP-1, FGGF

The extent to which divergent circulations, induced by tropical heating, help to maintain westerly maxima in the Southern Hemisphere subtropics during the SOP-1 of FGGE is explored using Level III-b analyses from the Goddard Laboratory for Atmospheres. The contribution of the divergent wind component to the total ageostrophic flow in the subtropics is examined, as are the roles of other forcing terms in the localized Eliassen-Palm (1961) flux zonal momentum equation. Throughout the summertime subtropics, it is generally found that the dominant term in the zonal momentum budget is the Coriolis force applied to the diabatically driven meridional circulation. The largest positive tendencies due to this term are found in the entrance regions of the subtropical westerly maxima, and divergent circulations account for nearly all of the total ageostrophic flow. In the SPCZ region, however, it is found that transient eddies play an important role by partially offsetting the strong Coriolis acceleration in the entrance region of the local jet, and they help accelerate the westerly flow in the exit region through both barotropic and baroclinic processes. Energetically, the dominant term in the rotational kinetic energy budget throughout the subtropical belt is the conversion of divergent to rotational kinetic energy. The evidence supports the view that tropical heating in transient events drives or enhances local meridional overturning in the atmosphere, which, in turn, strengthens the summer subtropical westerly jet stream.

Pluvial climatic episodes in North America and North Africa: types and correlation with global climate

Differences in Late Quaternary pluvial chronologies among the world's deserts can be related to fundamental differences in the climatology of those pluvial episodes. Paleoenvironmental records and climate model simulations for two Northern Hemisphere desert regions are used to illustrate two types of pluvial climatic regime. In the higher-latitude deserts of the American Southwest, pluvial conditions during the last glacial maximum were caused by the southward displacement of the westerly jet stream, in turn caused by the blocking effect of Laurentide Ice Sheet and a steepened arctic-equator pressure gradient. Winter seasonality of precipitation and substantially lower mean annual temperatures are reflected in the paleoenvironmental record, and simulated by models of 18,000 yr B.P. climate. A second pluvial climate type which strongly influences the lower-latitude deserts of North Africa is characterized by enhanced summer monsoons, and occurs during interglacials and, perhaps, interstadials. The primary forcing mechanism of the last pluvial episode in the Sahara and Libyan Deserts appears to have been increased solar radiation (Milankovitch forcing), causing thermally enhanced ocean-land pressure gradients and increased flow of maritime tropical air into the interior of the continent. Poleward displacement of the summer monsoon is consistent with floristic evidence for summer seasonality of precipitation, and replicated by model simulations of 9000 and 6000 yr B.P. climate. Chronologies from some deserts, such as the southern portion of the American Southwest, suggest the consecutive occurrence of both types of pluvial climatic regime. Pluvial conditions during the last glacial maximum were brought about by the equatorward displacement of the prevailing westerlies. A subsequent, more subdued pluvial period occurred later, between ∼ 12,500 and 8000 yr B.P., caused by the poleward displacement of summer monsoons. Characterization of the climatology of pluvial episodes generates explanations for their temporal distribution and, therefore, this approach may be useful in assessing the Quaternary chronology of pluvial episodes in other desert regions.

Isolated Anomalies in Westerly Jet Streams: A Unified Approach

Abstract A weakly nonlinear theory is presented that may explain the persistence of the two main types of low-frequency anomalies observed in the midlatitude jet stream by Dole and Gordon (1983). The theory describes how nonlinearity can balance dispersion effects for both split jet stream anomalies (which resemble blocking flows) and for jet intensification anomalies. It is shown that the variation of the potential or refractive-index function ≡ dq/dψ across the jet stream is crucial for determining which types of anomaly will tend to persist. Although the theory is only weakly nonlinear it is argued that the same dynamical mechanisms will remain important in the high-amplitude regime particularly for the intense-jet anomalies. In the split anomalies the potential vorticity contours can easily become closed at high amplitude hence trapping air parcels (this is the origin of the strongly nonlinear modon solutions). However, even for very strong intense-jet anomalies the potential vorticity contours may re...

The Effect of summer tropical heating on the location and intensity of the extratropical westerly jet streams

The role of the summer hemisphere tropical diabatic heating on the location and magnitude of the winter hemisphere subtropical and extratropical westerly jet streams is studied. In particular, the relationships between the extratropical zonal wind, the subtropical meridional wind and the tropical outgoing longwave radiation (OLR) patterns are examined. It is found that the heating field, the cross‐equatorial meridional wind, and the winter zonal wind maxima are closely linked on annual and interannual time scales. These factors indicate that the adjacent hemisphere heating is at least as important as the northern hemisphere orography in determining the location and magnitude of the westerly jet streams. Credence is added to this conclusion by noting the similar associations between the northern hemisphere heating pattern and the location and variations of the southern hemisphere jet streams which are less clearly associated with orography. Furthermore, interannual changes in the location and magnitude of the westerly jet streams are closely related to the El Niño/Southern Oscillation (ENSO) phenomenon. Thus, in addition to the well known relationship between ENSO and the near‐equatorial or symmetric zonal circulation, there is a strong association on the same time scale in the cross‐equatorial flow or asymmetric components about the equator.

The Three-Dimensional Structure of Perturbation Kinetic Energy and Its Relationship to the Zonal Wind Field

Abstract We seek relationships between the perturbation kinetic energy (PKE), the zonal wind, and outgoing longwave radiation. It is found that the mean low-level PKE extratropical maxima are located at the “atmospheric centers of action” (e.g., the Aleutian and Iceland lows), which lie beneath the high-level PKE-maxima downstream of the westerly jet streams. As expected, this means that the PKE maxima in the middle latitudes are closely related to the propagating disturbances. In the tropics, however, the high-level PKE maxima and minima are located in the strongest upper tropospheric westerlies and easterlies, respectively, while the low-level PKE maxima and minima are located in the monsoon systems and the easterly trade winds, respectively. It is also found that the maxima and minima of the PKE in the middle latitudes, at least in the Northern Hemisphere, are in phase in the vertical but completely out of phase in the tropics. These features are closely linked to the structure of the zonal wind compon...

FGGE冬期における熱帯循環のエネルギー変換

FGGE level-IIIb data produced by the European Centre for Medium Range Weather Forecasts (ECMWF) for the DJF 1978-79 is used to analyse the kinetic energy budget over the global tropical belt. A detailed energetics of the mean and transient fluctuations, in particular, the generation, the exchange between the mean flow and the transients and the dissipation of kinetic energy are estimated for the tropics and compared with the earlier studies. It is found that about 60% of the kinetic energy generated in the tropics is dissipated by the frictional mechanisms and remaining 40% is transported to the middle latitudes of both the hemispheres. The subtropical westerly jet stream of the winter hemisphere plays a dominant role in the process of kinetic energy flux transport. It is observed that the inter-hemispheric exchange of kinetic energy is negligible. Further, the study confirms the fact that the tropical circulation is mainly maintained by the mean flow while in the extra tropics (beyond 30° latitude) the transients are quite important.