Midlatitude Wave Research Articles

Mechanisms for the Holton-Tan relationship and its decadal variation

This study provides a mechanistic explanation of why the Holton-Tan (HT) effect, a phenomenon in which the strength of northern stratospheric winter polar vortex synchronizes with the equatorial quasi-biennial oscillation (QBO), was disrupted in the middle to late winters of 1978–1997. In line with recent reassessments of the HT effect, we find that an easterly QBO in the lower stratosphere leads to the formation of a midlatitude wave guide that enhances both the upward propagating planetary waves from the troposphere into the lower stratosphere (~35–50°N, 30–200 hPa) and the northward wave propagation in the upper to middle stratosphere (~35–60°N, 20–5 hPa). This enhanced poleward refraction of planetary waves results in a more disturbed polar vortex, causing the HT effect. The weakening of the HT effect in 1978–1997 was associated with a broader and strengthened polar vortex in November to January. The divergence of wave activity generated by eddies growing within the vortex provided the momentum source and allowed wave activity to propagate meridionally away from the vortex; this interfered with the QBO modulation of planetary wave propagation and led to a weakening of the HT effect during this period. The stronger than average polar vortex in 1978–1997 was associated with a vertically coherent cooling signature over northeastern Asia in the stratosphere. We suggest that a change of stratospheric circulation and/or a change of the stratosphere-troposphere coupling were the main causes for the disrupted HT effect in 1978–1997.

Extratropical forcing of tropical wave disturbances along the Indian Ocean ITCZ

The role of extratropical waves in the excitation of tropical waves on submonthly timescales is explored along the Indian Ocean Intertropical Convergence Zone (ITCZ) during the austral summer using Japanese Reanalysis products and NOAA outgoing longwave radiation data. The analysis period is DecemberŰFebruary of 1979/1980 to 2008/2009. The submonthly tropical waves are regarded as a type of Rossby wave propagating along the mean monsoon westerly flow. They play an important role in modulating the Indian Ocean ITCZ convection. The linkage between the tropical and extratropical waves, which is responsible for the formation and strengthening of tropical waves, is examined. Composite analysis results linked to the tropical wave train development show that the midlatitude Rossby wave train progresses eastward and northeastward from the South Atlantic into the subtropical Indian Ocean. As a trough and ridge that form part of the midlatitude wave train approach the southern Africa‐southwest Indian Ocean (SWIO) region, a southwest‐northeast‐oriented wave train is subsequently established, originating from this feature, and is strengthened across the tropical Indian Ocean. The midlatitude wave propagation toward the subtropics induces the growth of the trough and ridge over the SWIO. Wave activity flux diagnostics indicate that the amplified trough and ridge over the SWIO act as an energy source for northeastward amplification of the tropical waves through the wave energy dispersion process. The results suggest that the propagation of the midlatitude wave toward the SWIO is the fundamental mechanism behind the development of the tropical waves along the Indian Ocean ITCZ.

The interplanetary magnetic field influences mid-latitude surface atmospheric pressure

The existence of a meteorological response in the polar regions to fluctuations in the interplanetary magnetic field (IMF) component By is well established. More controversially, there is evidence to suggest that this Sun–weather coupling occurs via the global atmospheric electric circuit. Consequently, it has been assumed that the effect is maximized at high latitudes and is negligible at low and mid-latitudes, because the perturbation by the IMF is concentrated in the polar regions. We demonstrate a previously unrecognized influence of the IMF By on mid-latitude surface pressure. The difference between the mean surface pressures during times of high positive and high negative IMF By possesses a statistically significant mid-latitude wave structure similar to atmospheric Rossby waves. Our results show that a mechanism that is known to produce atmospheric responses to the IMF in the polar regions is also able to modulate pre-existing weather patterns at mid-latitudes. We suggest the mechanism for this from conventional meteorology. The amplitude of the effect is comparable to typical initial analysis uncertainties in ensemble numerical weather prediction. Thus, a relatively localized small-amplitude solar influence on the upper atmosphere could have an important effect, via the nonlinear evolution of atmospheric dynamics, on critical atmospheric processes.

Interaction of a tropical cyclone with a high‐amplitude, midlatitude wave pattern: Waviness analysis, trough deformation and track bifurcation

AbstractAn idealized scenario of extratropical transition (ET) is investigated, in which a tropical cyclone interacts with a high‐amplitude, upper‐level wave pattern and well‐developed surface cyclones. Early during the interaction, the external forcing of the upper‐level wave by theETsystem is quantified based on a metric for the waviness of the midlatitude flow. Local amplification of the wave pattern is diagnosed, associated prominently with the trough downstream ofET. This amplified trough, however, exhibits pronounced anticyclonic breaking and thus, in contrast to many previousETstudies, it is not clear that the amplification of the upper‐level wave propagates into the farther downstream region. Subsequently, theETsystem merges with the upstream cyclone. The upstream trough undergoes strong deformation and cyclonic breaking associated with straining due to the cyclonic circulation of theETsystem. With the decay of this trough, theETsystem weakens considerably and the upper‐level wave pattern changes locally to a zonal flow orientation. This zonal flow pattern then extends into the downstream region and promotes the decay of the downstream baroclinic systems.As in previous studies, the evolution ofETexhibits large sensitivity to the initial location of the tropical cyclone. Examining the steering flow's topology, i.e. identifying the stagnation points and the streamlines emanating from these points, helps to identify three different regimes: a no‐ETregime and twoETregimes reminiscent of the northwest and northeast patterns, respectively, introduced previously by Harret al.. A stagnation point located on the axis of the upstream trough governs the bifurcation into no‐ETandETregimes. A stagnation point located on the axis of the downstream ridge governs the bifurcation into northwest and northeast patterns.

The upper-level circulation anomaly over Central Asia and its relationship to the Asian monsoon and mid-latitude wave train in early summer

A large intraseasonal variation in geopotential height over the Central Asia region, where the Asian subtropical jet is located, occurs between May and June, and the most dominant variation has a wave-like distribution. This variation in geopotential height influences precipitation across South and Southeast Asia. In this paper, we use composite analysis to determine the causes of this intraseasonal variation over Central Asia. The wave train propagates from the northern Atlantic Ocean to Central Asia over a period of a week, and generates an anomaly in geopotential height over the region. The tropical disturbance, which is similar to the Madden–Julian oscillation, appears a few days before the maximum of the anticyclonic anomaly over Central Asia, and is accompanied by active convection over the Indian Ocean and suppressed convection over Central America. Results of numerical experiments using a linear baroclinic model show that the active convection over the northern Indian Ocean causes the anticyclonic anomaly over Central Asia. The wave train that extends from the northern Atlantic Ocean to Central Asia is generated by negative thermal forcing over Central America, and the phase distribution of this wave train is similar to that observed in the composite analysis. Central Asia is the region where the effects of the tropics and middle latitudes overlap, and it is an important connection point between the Asian monsoon and middle latitudes.

Linear interference and the Northern Annular Mode response to tropical SST forcing: Sensitivity to model configuration

AbstractInterannual variability in tropical sea surface temperatures (SSTs) associated with the El Niño–Southern Oscillation is linked to teleconnections with the Northern Annular Mode (NAM). Previous work highlighted that the sign and amplitude of the NAM response to tropical SSTs are controlled by the total wave activity entering the subpolar stratosphere, which depends on the linear interference of planetary wave anomalies with the climatological stationary wave field. This study uses multiple configurations of atmospheric general circulation models to assess the robustness of these linkages to details of the tropical SST forcing and model configuration. Across 23 cases with idealized SST forcing, the amplitudes of the tropical and extratropical wave responses are found to scale approximately linearly with forcing strength. But wave amplitude alone is not sufficient to predict the NAM response. Instead, the spatial structure of the wave response (and hence the linear interference) provides the best explanation of the NAM response in all cases. Linear interference explains most of the total wave activity response even in cases with stronger nonlinear contributions, due to consistent cancellation between quasi‐stationary wave nonlinearity and nonlinearity arising from transient waves. Within this limited set of experiments, there is no evidence for a consistent sensitivity of the NAM response to horizontal resolution or to vertical resolution in the stratosphere. These findings reveal that linear interference provides a robust and reproducible mechanism linking midlatitude wave responses to zonal mean circulation (NAM) responses across a wide variety of forcing cases.

The mechanism of mid-latitude Pi2 waves in the upper ionosphere as revealed by combined Doppler and magnetometer observations

Abstract. The interpretation of simultaneous ionospheric Doppler sounding and ground magnetometer observations of low-latitude Pi2 waves is revised. We compare the theoretical estimates of the ionospheric Doppler velocity for the same amplitude of the ground magnetic disturbances produced by a large-scale compressional mode and an Alfvén mode. The plasma vertical displacement caused by the wave electric field is shown to be the dominating effect. Taking into account the correction of the previous paper, the observations of low-latitude Pi2 in the F layer ionosphere by Doppler sounding and SuperDARN (Super Dual Auroral Radar Network) radars give consistent results. We suggest that the Doppler response to Pi2 waves is produced by the Alfvén wave component, but not the fast-mode component, whereas the ground magnetic signal is composed from both Alfvén and fast magnetosonic modes.

A Strengthened Influence of ENSO on August High Temperature Extremes over the Southern Yangtze River Valley since the Late 1980s

AbstractThe present study investigates the decadal change in the relationship between China high temperature extremes (HTEs) and El Niño–Southern Oscillation (ENSO). It is found that the relationship between the August HTEs in the southern Yangtze River valley (SYRV) and ENSO has strengthened since the late 1980s. Before the late 1980s, the relationship is weak, whereas, after the late 1980s, the August hot-day numbers in the SYRV region tend to be more than normal during El Niño decaying years. During 1988–2008, El Niño–induced August warm SST anomalies are mainly located in the eastern tropical and north Indian Ocean. As a response to the north Indian Ocean warming, the South Asia high extends eastward, and the SYRV is overlain by upper-level easterly anomalies. The cold horizontal temperature advection induced by upper-level easterly anomalies leads to anomalous descent, which is conducive to the occurrence of HTEs through adiabatic warming. During 1966–86, El Niño–induced August warm SST anomalies are mainly distributed in the equatorial central and southwest tropical Indian Ocean. Corresponding to the equatorial Indian Ocean warming, the ascending motion over the Arabian Sea is enhanced, which leads to an anomalous anticyclone over the Middle East through a Rossby wave–type response and in turn an anomalous cyclone over China through a midlatitude wave pattern. The SYRV is controlled by upper-level westerly anomalies, which is not conducive to the occurrence of HTEs since the corresponding horizontal temperature advection and anomalous vertical motion are weak. As such, the impact of ENSO on August SYRV HTEs is weak before the late 1980s.

What Percentage of Western North Pacific Tropical Cyclones Form within the Monsoon Trough?

Abstract It is frequently stated that 70%–80% of western North Pacific tropical cyclones form “within the monsoon trough,” but without an objective definition of the term. Several definitions are tested here. When the monsoon trough (MT) is defined as the contiguous region where long-term (1988–2010) mean July–November 850-hPa relative vorticity is positive, 73% of all July–November tropical cyclones form within the MT. This percentage varies interannually, however, from as low as 50% to nearly 100%. The percentage correlates with the Niño-3.4 index, with more storms forming within the MT during warm periods. When the MT is defined instead using long-term monthly mean ζ850, more than 80% of tropical cyclones form within the MT in all months except July and August, when more than 30% of storms form poleward of the MT. It is hypothesized that the known peak in the frequency of upper-tropospheric midlatitude wave breaking in July and August is responsible. It is argued that any long-term mean provides a suitable definition of the MT. Defining it on less than seasonal time scales, however, creates a lack of conceptual separation between the MT and other tropical disturbances such as the MJO, equatorial waves, and easterly waves. The term monsoon trough should represent a climatological feature that provides an asymmetric background state within which other disturbances evolve.

Impacts of multi-scale solar activity on climate. Part I: Atmospheric circulation patterns and climate extremes

The impacts of solar activity on climate are explored in this two-part study. Based on the principles of atmospheric dynamics, Part I propose an amplifying mechanism of solar impacts on winter climate extremes through changing the atmospheric circulation patterns. This mechanism is supported by data analysis of the sunspot number up to the predicted Solar Cycle 24, the historical surface temperature data, and atmospheric variables of NCEP/NCAR Reanalysis up to the February 2011 for the Northern Hemisphere winters. For low solar activity, the thermal contrast between the low- and high-latitudes is enhanced, so as the mid-latitude baroclinic ultra-long wave activity. The land-ocean thermal contrast is also enhanced, which amplifies the topographic waves. The enhanced mid-latitude waves in turn enhance the meridional heat transport from the low to high latitudes, making the atmospheric “heat engine” more efficient than normal. The jets shift southward and the polar vortex is weakened. The Northern Annular Mode (NAM) index tends to be negative. The mid-latitude surface exhibits large-scale convergence and updrafts, which favor extreme weather/climate events to occur. The thermally driven Siberian high is enhanced, which enhances the East Asian winter monsoon (EAWM). For high solar activity, the mid-latitude circulation patterns are less wavy with less meridional transport. The NAM tends to be positive, and the Siberian high and the EAWM tend to be weaker than normal. Thus the extreme weather/climate events for high solar activity occur in different regions with different severity from those for low solar activity. The solar influence on the midto high-latitude surface temperature and circulations can stand out after removing the influence from the El Nino-Southern Oscillation. The atmospheric amplifying mechanism indicates that the solar impacts on climate should not be simply estimated by the magnitude of the change in the solar radiation over solar cycles when it is compared with other external radiative forcings that do not influence the climate in the same way as the sun does.

Rapid transpacific transport in autumn observed by the A‐train satellites

Transpacific transport of dust and pollutants is well documented for spring but less so for other seasons. Here we investigate rapid transpacific transport in autumn utilizing the A‐train satellites. In three episodes studied as examples, SO2 plumes over East Asia were detected by the Ozone Monitoring Instrument aboard the Aura satellite and found to reach North America in 5–6 days. They were likely derived from anthropogenic sources, given that identical transport patterns of CO, a tracer for incomplete combustion, were simultaneously observed by the Aqua satellite. Trajectory analysis and meteorological data were employed to explore the meteorological circumstances surrounding these events: like many of their counterparts in spring, all three plumes were lifted to the free troposphere in warm conveyor belts associated with midlatitude wave cyclones, and their migration to the downwind region was regulated by the meteorology over the east Pacific. These cases provide further evidence that a fraction of SO2 could escape wet scavenging and be transported at much greater efficiency than could NOx (NO + NO2). An analysis of the SO2 and CO data from September to November during 2005–2008 found 16 SO2 long‐range transport episodes, out of 62 Asian outflow events. While the counts are sensitive to the choice of criteria, they suggest that the long‐range transport of Asian sulfur species occurs quite frequently and could exert strong impacts on large downstream areas. This study also highlights the importance of transpacific transport in autumn, which has thus far been rarely studied and deserves more attention from the community.

Abrupt Transition to Strong Superrotation Driven by Equatorial Wave Resonance in an Idealized GCM

Abstract Persistent superrotation is seen in the atmospheres of other terrestrial bodies (Venus, Titan) but not in that of present Earth, which is distinguished by equatorial easterlies. Nevertheless, superrotation has appeared in numerical simulations of Earth’s atmosphere, from two-layer models to multilevel comprehensive GCMs. Simulations of warm climates that generate enhanced tropical convective variability seem particularly prone to superrotation, which has led to hypotheses that the warmer atmospheres of the early Pliocene and Eocene may have been superrotating, and that the phenomenon may be relevant to future climate projections. This paper considers a positive feedback leading to superrotation based on an equatorial wave resonance that occurs in a westerly background flow. The authors present simulations with an idealized multilevel GCM forced with a zonally varying equatorial heating, which show abrupt transitions to strongly superrotating states. Linear shallow water theory is used to show that these transitions occur as the superrotating jet velocity approaches the phase speed of free equatorial Rossby wave modes, leading to a resonant amplification of the response to eddy heating and its associated equatorward momentum flux. The resonance and transition are most prominent in simulations where the meridional temperature gradient has been reduced, and hysteresis behavior is seen when the gradient is eliminated completely. No evidence is found in these simulations for the midlatitude wave feedback believed to drive abrupt transitions in two-layer models, and there is only a minor role for the axisymmetric feedback based on vertical advection by the Hadley circulation.

Impact of ice sheet induced North Atlantic oscillation on East Asian summer monsoon during an interglacial 500,000 years ago

Marine Isotope Stage (MIS) 13, an interglacial about 500,000 years ago, is unique due to an exceptionally strong East Asia summer monsoon (EASM) occurring in a relatively cool climate with low greenhouse gas concentrations (GHG). This paper attempts to find one of the possible mechanisms for this seeming paradox. Simulations with an Earth System model LOVECLIM show that the presence of ice sheets over North America and Eurasia during MIS-13 induces a positive phase of the winter North Atlantic Oscillation (NAO) like feature. The ocean having a longer memory than the atmosphere, the oceanic anomalies associated with NAO persists until summer. The signals of summer NAO are transmitted to East Asia to reinforce the monsoon there through the stationary waves excited at the Asian Jet entrance. The geopotential height shows clearly a mid-latitude wave train with positive anomalies over the eastern Mediterranean/Caspian Sea and the Okhotsk Sea and a negative anomaly over Lake Baikal. This reinforces the effect of the high-latitude wave train induced independently by the Eurasian ice sheet topography as shown in previous study. These features reinforce the Meiyu front and enhance the precipitation over East Asia. The results obtained from LOVECLIM are further confirmed by an atmospheric general circulation model, ARPEGE.

ENSO induced monthly oscillations of precipitation: the unique case of the south tropical Indian Ocean in austral summer

Seasonal (three-month average) climate forecasts have advanced due in large part to improved modeling of the ENSO phenomenon. Long-range monthly forecasts are more problematic because of internal atmospheric variability. Further, it is often assumed that monthly precipitation anomalies are representative of the overall seasonal anomaly. This is not always the case as, according to the Global Precipitation Climatology Project Version 2.1 data set, up to 20% of areas demonstrating some significant teleconnection to ENSO show El Nino minus La Nina differences of one sign in the middle month and the opposite sign in the adjacent months. Most interestingly, this maximum percentage occurs in December–January–February (DJF), a time when the ENSO boundary forcing is strongest. These oscillatory DJF seasons also cluster in space—with significant positive–negative-positive differences in the western South Tropical Indian Ocean (STIO) and negative–positive–negative differences in the far eastern STIO. Representative gauges confirm that these precipitation patterns have been associated with ENSO events since 1951, and pentad precipitation data confirm that they are confined to DJF and evolve at the monthly scale. The abrupt end of the Indian Ocean Dipole mode in January, an increase in the importance of local SST anomalies in February, and an ENSO-induced mid-latitude Rossby wave during austral summer combine to generate the cross-basin precipitation gradient around 15°S.

The role of cold fronts in the onset of the monsoon season in the South Atlantic convergence zone

AbstractThe onset of the South American monsoon season culminates with the abrupt establishment of the South American convergence zone (SACZ). The impact of cold fronts on the abrupt establishment of the SACZ is studied using an 11‐year composite analysis of the dynamic and thermodynamic structures, intensity and propagation of cold fronts that occur prior to, during, and after monsoon onset in the SACZ. A significant change in the structure and propagation of cold fronts is observed at the time of monsoon onset in the SACZ, with cold fronts suddenly stalling and becoming stationary in southeastern Brazil. It is proposed that this regime change of the structure and propagation of cold fronts causes the abrupt onset of the monsoon in the SACZ. A mechanism for this sudden change in cold front behaviour is suggested by analyzing changes in the observed upper‐level structure of midlatitude waves. Our observations show that, at the time of monsoon onset in the SACZ, midlatitude waves develop a ‘thinning trough’ behaviour, with upper‐level troughs becoming thinner, westward tilted, and sometimes forming cut‐off lows. This behaviour is characteristic of midlatitude waves that develop in the region of anticyclonic shear on the equatorward side of the upper‐level midlatitude jet. During the austral spring, the upper‐level midlatitude jet over South America migrates southward leaving subtropical South America in the equatorward, anticyclonic shear side of the jet. It is hypothesized that, as the meridional shear on the equatorward side of the upper‐level jet gradually becomes more anticyclonic, it suddenly crosses the critical threshold of horizontal shear that has been shown to produce a sudden transition to a ‘thinning trough’ midlatitude wave regime. At this time a regime change occurs whereby the character of frontal systems crossing South America changes abruptly, causing the onset of the monsoon season in the SACZ. Copyright © 2011 Royal Meteorological Society

Formation of the Philippine Twin Tropical Cyclones during the 2008 Summer Monsoon Onset

Abstract Stretched from Indochina, across the South China Sea, to the Philippine Sea, a monsoon cyclonic shear flow was formed by easterlies of the cold surge-like flow in the north and monsoon westerlies in the south before the onset of the tropical Southeast Asian monsoon on 12 May 2008. On this date, two named tropical cyclones (Halong and Matmo) evolved with a 12-h lag from a closed vortex adjacent to the coast of central Vietnam and another closed vortex near Palawan Island (Philippines) within this shear flow. These two cyclones, named the twin Philippine tropical cyclones, moved almost on the same track, along the anomalous shear line (departure from the climatological one) across the Philippines, and turned northeastward to the ocean south of Japan. It was revealed from synoptic analysis that the cold surge-like flow was coupled with the midlatitude eastward-propagating short wave in northeast Asia, and part of the monsoon westerlies were fed by the cross-equatorial flow, the downstream flow of easterlies around the northern rim of the Southern Hemisphere subtropical high. The environment favorable for the formation of the twin cyclones was developed from the tropics–midlatitude interaction between synoptic systems in these two latitudinal zones. Formations of these cyclones were a result of drastic spinups of the two closed vortices (within the monsoon shear flow) following the surge of monsoon westerlies, which coincided with those of easterlies of the cold surge-like flow, and the cross-equatorial flow originating from easterlies between the Southern Hemisphere subtropical high and the Southern Hemisphere shear flow.

Spatial Analysis of the Factors Contributing to the Relationship between the Transient, Meridional Eddy Sensible, and Latent Heat Flux in the Southern Hemisphere

In this paper principal component analysis (PCA) and singular value decomposition (SVD) are used to defne the importance of the variables contributing to the relationship between the transient latent and sensible heat fluxes and to show their temporal and spatial variation. SVD is ofered as an alternative means of isolating spatial and temporal structures in data with the advantage that it can depict simultaneous space-time variations that are aggregates of the results produced by PCA. Both methods of analysis produced two vey important uncorrelated modes of variability in Janua y and July, indicating that the transient heat fluxes are injuenced by fm controlling factors. We suggest that these modes of variability represent the influences of the meridional temperature gradient, atmospheric moisture, and activity within the source and sink regions of the transient heat fluxes. The physical relationships between the heat fluxes that appear to represented by the statistical modes of variability are discussed. The latitudinal imbalance of energy that arises from unequal radiative heating is in part redressed by the poleward transport of energy by the atmosphere. Poleward airflow transfers warmer, more moist air toward colder, drier regions. Much of this transport takes place during winter when the equator-pole gradient of temperature (and energy) is maximized. The flux of energy is therefore necessary to maintain existing global temperature distributions. Much of the energy transported is in the form of latent and sensible heat, and the main agents of transfer in the extratropics are the quasi-stationary eddies (semipermanent features of the general circulation, for example, the Icelandic and Aleutian Lows) and transient eddies (for example, baroclinic eddies, midlatitude wave This work was completed while Marilyn Raphael was on sabbatical at the National Center for Atmospheric Research, where Steve Cherry was a postdoctoral student. The authors thank the CAS group at NCAR for the funding and data that made this work possible. They also thank the three anonymous reviewers for their helpful comments on earlier drafts of this paper. Steve Cherry was supported by the NSF through the Geophysical Statistics Project at NCAR.

Using TIGGE Data to Diagnose Initial Perturbations and Their Growth for Tropical Cyclone Ensemble Forecasts

Abstract Ensemble initial perturbations around Typhoon Sinlaku (2008) produced by ECMWF, NCEP, and the Japan Meteorological Agency (JMA) ensembles are compared using The Observing System Research and Predictability Experiment (THORPEX) Interactive Grand Global Ensemble (TIGGE) data, and the dynamical mechanisms of perturbation growth associated with the tropical cyclone (TC) motion are investigated for the ECMWF and NCEP ensembles. In the comparison, it is found that the vertical and horizontal distributions of initial perturbations as well as the amplitude are quite different among the three NWP centers before, during, and after the recurvature of Sinlaku. In addition, it turns out that those variations cause a difference in the TC motion not only at the initial time but also during the subsequent forecast period. The ECMWF ensemble exhibits relatively large perturbation growth, which results from 1) the baroclinic energy conversion in a vortex, 2) the baroclinic energy conversion associated with the midlatitude waves, and 3) the barotropic energy conversion in a vortex. Those features are less distinctive in the NCEP ensemble. A statistical verification shows that the ensemble spread of TC track predictions in NCEP (ECMWF) is larger than ECMWF (NCEP) for 1- (3-) day forecasts on average. It can be inferred that while the ECMWF ensemble starts from a relatively small amplitude of initial perturbations, the growth of the perturbations helps to amplify the ensemble spread of tracks. On the other hand, a relatively large amplitude of initial perturbations seems to play a role in producing the ensemble spread of tracks in the NCEP ensemble.

Circumglobal wave train and the summer monsoon over northwestern India and Pakistan: the explicit role of the surface heat low

This study examines the influence of the mid-latitude circulation on the surface heat low (HL) and associated monsoon rainfall over northwestern India and Pakistan using the ERA40 data and high resolution (T106L31) climate model ECHAM5 simulation. Special emphasis is given to the surface HL which forms over Pakistan and adjoining areas of India, Iran and Afghanistan during the summer season. A heat low index (HLI) is defined to depict the surface HL. The HLI displays significant correlations with the upper level mid-latitude circulation over western central Asia and low level monsoon circulation over Arabian Sea and acts as a bridge connecting the mid-latitude wave train to the Indian summer monsoon. A time-lagged singular value decomposition analysis reveals that the eastward propagation of the mid-latitude circumglobal wave train (CGT) influences the surface pressure anomalies over the Indian domain. The largest low (negative) pressure anomalies over the western parts of the HL region (i.e., Iran and Afghanistan) occur in conjunction with the upper level anomalous high that develops over western-central Asia during the positive phase of the CGT. The composite analysis also reveals a significant increase in the low pressure anomalies over Iran and Afghanistan during the positive phase of CGT. The westward increasing low pressure anomalies with its north–south orientation provokes enormous north–south pressure gradient (lower pressure over land than over sea). This in turn enables the moist southerly flow from the Arabian Sea to penetrate farther northward over northwestern India and Pakistan. A monsoon trough like conditions develops over northwestern India and Pakistan where the moist southwesterly flow from the Arabian Sea and the Persian Gulf converge. The convergence in association with the orographic uplifting expedites convection and associated precipitation over northwestern India and Pakistan. The high resolution climate model ECHAM5 simulation also underlines the proposed findings and mechanism.

Studies of inertio‐gravity waves on midlatitude beta‐plane without the traditional approximation

AbstractThe role of the Rossby parameter β due to the meridional variation of the Coriolis parameter f is investigated for inertio‐gravity waves in midlatitudes based on a linear Boussinesq model without the traditional approximation, namely including the role of the horizontal component of the Coriolis vector of the Earth's rotation. Without the β‐effect, it is well established that the non‐traditional Coriolis effects give rise to an additional mode of inertio‐gravity (IG) waves whose frequencies are slightly smaller than the local Coriolis frequency f unlike the traditional IG‐mode whose frequencies are larger than f. The addition of the β‐effect modifies the properties of the sub‐inertial and super‐inertial IG‐modes. These modifications are examined by calculating the model normal modes for a given vertical profile of the buoyancy frequency N. Inclusion of the β‐effect yields the external and internal Rossby modes in addition to the IG‐modes. Presence of the external Rossby mode in this model is a new result, since it does not occur in the traditional version with β nor in the non‐traditional version without β. The normal‐mode properties, including wave frequencies and eigenfunctions, are presented for various values of N. The normal‐mode analysis is complemented by the initial‐value approach to study the evolution of waves generated by forcing. The results of numerical time‐integrations of wave generation in the ocean forced by atmospheric disturbances and by the interaction of tidal flows with bottom topography are presented to reveal a notable role of β for the IG‐waves. Application to atmospheric problems is proposed. Copyright © 2010 Royal Meteorological Society