Midlatitude Baroclinic Waves Research Articles

Investigating the role of tropical and extratropical waves in the Hadley circulation via present‐day Earth‐like to globally uniform sea‐surface temperature forcing

AbstractThe tropical overturning circulation is examined in a moist aquaplanet general circulation model forced using a non‐interactive sea‐surface temperature (SST) distribution that varies between a present‐day Earth‐like profile and one that is globally uniform. A Hadley cell (HC)‐like flow is observed in all experiments along with the poleward transport of heat and angular momentum. In simulations with non‐zero SST gradients, deep convection near the Equator sets up a deep tropical cell; midlatitude baroclinic Rossby waves flux heat and angular momentum poleward, reinforcing the thermally direct circulation. As the imposed SST gradient is weakened, the HC transitions from a thermally and eddy‐driven regime to one that is completely eddy‐driven. When the SST is globally uniform, equatorial waves concentrate precipitation in the Tropics and facilitate the lower‐level convergence necessary for the ascending branch of the HC. Midlatitude Rossby waves generated near the surface become very weak, but upper‐level baroclinicity generates waves that cause equatorward transport of heat and poleward transport of momentum. Moreover, these upper‐level waves induce a circulation that opposes the time‐mean HC, thus highlighting the role of tropical waves in driving an overturning circulation that looks similar to the present‐day Earth‐like case, even for the case with globally uniform SSTs. In all cases, anomalies associated with the tropical waves closely resemble those that sum to give the upper‐level zonal mean divergent outflow. Through their ability to modulate tropical rainfall and the related latent heating, equatorial waves cause considerable hemispheric asymmetry in the HC and impart synoptic and intraseasonal variability to the tropical overturning circulation.

The study of the impact of polar warming on global atmospheric circulation and mid-latitude baroclinic waves using a laboratory analog

The study of the impact of polar warming on global atmospheric circulation and mid-latitude baroclinic waves using a laboratory analog

Seasonal predictability of baroclinic wave activity

Midlatitude baroclinic waves drive extratropical weather and climate variations, but their predictability beyond 2 weeks has been deemed low. Here we analyze a large ensemble of climate simulations forced by observed sea surface temperatures (SSTs) and demonstrate that seasonal variations of baroclinic wave activity (BWA) are potentially predictable. This potential seasonal predictability is denoted by robust BWA responses to SST forcings. To probe regional sources of the potential predictability, a regression analysis is applied to the SST-forced large ensemble simulations. By filtering out variability internal to the atmosphere and land, this analysis identifies both well-known and unfamiliar BWA responses to SST forcings across latitudes. Finally, we confirm the model-indicated predictability by showing that an operational seasonal prediction system can leverage some of the identified SST-BWA relationships to achieve skillful predictions of BWA. Our findings help to extend long-range predictions of the statistics of extratropical weather events and their impacts.

Wind-Forced Variability of the Remote Meridional Overturning Circulation

AbstractThe mechanisms by which time-dependent wind stress anomalies at midlatitudes can force variability in the meridional overturning circulation at low latitudes are explored. It is shown that winds are effective at forcing remote variability in the overturning circulation when forcing periods are near the midlatitude baroclinic Rossby wave basin-crossing time. Remote overturning is required by an imbalance in the midlatitude mass storage and release resulting from the dependence of the Rossby wave phase speed on latitude. A heuristic theory is developed that predicts the strength and frequency dependence of the remote overturning well when compared to a two-layer numerical model. The theory indicates that the variable overturning strength, relative to the anomalous Ekman transport, depends primarily on the ratio of the meridional spatial scale of the anomalous wind stress curl to its latitude. For strongly forced systems, a mean deep western boundary current can also significantly enhance the overturning variability at all latitudes. For sufficiently large thermocline displacements, the deep western boundary current alternates between interior and near-boundary pathways in response to fluctuations in the wind, leading to large anomalies in the volume of North Atlantic Deep Water stored at midlatitudes and in the downstream deep western boundary current transport.

The Effect of Midlatitude Transient Eddies on Monsoonal Southerlies over Eastern China

Abstract The strengthening of monsoonal southerlies over East Asia is associated with the westward intensification of the North Pacific subtropical high. Previous work has shown that the seasonal-mean position and strength of subtropical highs are affected by tropical and subtropical diabatic heating. Here it is shown that the synoptic-time-scale strengthening of southerlies over eastern China is dynamically tied to extratropical eddy activity. Composite analysis based on strong southerly wind events highlights an antecedent baroclinic wave train propagating southeastward into eastern China from extratropical central Asia. This wave train generates quasigeostrophic ascent over eastern China that is associated with heavy precipitation. The anomalously cold upper-tropospheric conditions associated with the wave train decrease static stability throughout the lower and middle troposphere in eastern China, while low-level moistening enhances equivalent potential temperature. It is proposed that the resulting reductions in dry and moist static stability intensify the eddy-induced precipitating ascent. These results illustrate how East Asian monsoon circulation and precipitation can be enhanced by the interaction of midlatitude baroclinic waves with the moist subtropical monsoon region.

A Semi-Implicit Version of the MPAS-Atmosphere Dynamical Core

Abstract An important question for atmospheric modeling is the viability of semi-implicit time integration schemes on massively parallel computing architectures. Semi-implicit schemes can provide increased stability and accuracy. However, they require the solution of an elliptic problem at each time step, creating concerns about their parallel efficiency and scalability. Here, a semi-implicit (SI) version of the Model for Prediction Across Scales (MPAS) is developed and compared with the original model version, which uses a split Runge–Kutta (SRK3) time integration scheme. The SI scheme is based on a quasi-Newton iteration toward a Crank–Nicolson scheme. Each Newton iteration requires the solution of a Helmholtz problem; here, the Helmholtz problem is derived, and its solution using a geometric multigrid method is described. On two standard test cases, a midlatitude baroclinic wave and a small-planet nonhydrostatic gravity wave, the SI and SRK3 versions produce almost identical results. On the baroclinic wave test, the SI version can use somewhat larger time steps (about 60%) than the SRK3 version before losing stability. The SI version costs 10%–20% more per step than the SRK3 version, and the weak and strong scalability characteristics of the two versions are very similar for the processor configurations the authors have been able to test (up to 1920 processors). Because of the spatial discretization of the pressure gradient in the lowest model layer, the SI version becomes unstable in the presence of realistic orography. Some further work will be needed to demonstrate the viability of the SI scheme in this case.

Two modes and their seasonal and interannual variation of the baroclinic waves/storm tracks over the wintertime North Pacific

In this study, a newly developed method, termed moving empirical orthogonal function analysis (MEOF), is applied to the study of midlatitude baroclinic waves over the wintertime North Pacific from 1979 to 2009. It is shown that when the daily, high-pass filtered (2–10 days) meridional wind at 250 hPa is chosen as the variable of the MEOF analysis, typical features of baroclinic waves/storm tracks over the wintertime North Pacific can be well described by this method. It is found that the first two leading modes of the MEOF analysis, MEOF1 and MEOF2, assume quite different patterns. MEOF1 takes the form of a single wave train running in the east-west direction along 40°N, while MEOF2 is a double wave train pattern running in the east-west direction along 50°N and 30°N, respectively. The shift composites of various anomalous fields based on MEOF1 and MEOF2 assume typical baroclinic wave features.

Spatiotemporal development of irreversible mixing in midlatitude baroclinic wave life cycles: Morphology, energetics, and nonisentropic mixing activity

AbstractThe significance of nonisentropic irreversible mixing processes is diagnosed for idealized simulations of synoptic‐scale baroclinic wave life cycles along the subtropical jet stream, using a nonhydrostatic, anelastic, mesoscale model subject to a free‐slip surface boundary condition. A variety of morphological features of mixing is identified such as a mesoscale columnar vortex associated with the onset of frontal fracture, episodic overturnings along the surface fronts, “wrinkling” of the tropopause, and injection of tropospheric air into the stratosphere. The evolution of the degree of nonisentropic irreversible mixing is first analyzed by computing the change of the “base” component of potential energy that cannot be converted into kinetic energy. The structure of the mixing activity is also diagnosed through inspection of spatiotemporal changes of entropy to demonstrate that the surface fronts are by far the most active regions of such activity. This activity is found to be primarily longitudinal, extending from the surface to the lower stratosphere, to form a three‐dimensional spiral in the synoptic‐scale cyclone and along the fronts. However, an exceptional region also exists along the warm front, where the structure becomes primarily transverse in the mature phase of frontal development in a model including an explicit representation of small‐scale turbulence. In all simulations, the net transfer of mass and heat across the tropopause is from troposphere to stratosphere. The maximum transfer occurs when the observed climatological level of stratification contrast is assumed between stratosphere and troposphere. The same climatological choice leads to a minimum net irreversible mixing, which occurs primarily at Earth's surface.

How do Tropical Temperate Troughs Form and Develop over Southern Africa?

Abstract This paper aims at separating the respective influences of tropical and midlatitude variability on the development and life cycle of tropical temperate troughs (TTTs) over southern Africa in austral summer (November–February). Cluster analysis is applied to 1971–2000 40-yr ECMWF Re-Analysis (ERA-40) daily outgoing longwave radiation (OLR) anomalies to identify TTTs and monitor tropical convection. The same analysis applied to the zonal wind stretching deformation at 200 hPa (ZDEF) characterizes midlatitude transient perturbations. Results based on the comparison between these two classifications first confirm that midlatitude baroclinic waves are a necessary condition for TTT development, but they are not sufficient. Roughly 40% of those occurring in austral summer are associated with a TTT. They tend to be stronger than the baroclinic waves not associated with TTT development. In the tropics, additional conditions needed to form a TTT consist of an excess of latent energy over the Mozambique Channel, mostly because of moisture advections and convergence from the Atlantic and Indian Oceans. Taken together, these conditions are highly favorable for deep atmospheric convection over and near southern Africa and seem to explain a large fraction of TTT variability.

Energetics of super cyclone ‘GONU’ and very severe cyclonic storm ‘SIDR’

This paper discusses the energetics aspects of two tropical cyclones formed over the north Indian Ocean during 2007, viz., the Super Cyclonic Storm (GONU) and the Very Severe Cyclonic Storm (SIDR). From the analysis of various energetics terms such as the Eddy Available Potential Energy (AE), Zonal Available Potential Energy (AZ), Zonal Kinetic Energy (KZ), Eddy Kinetic Energy (KE) and their generation and inter-conversions i.e., G(AE), G(AZ), C(AE, KE), C(AZ, KZ), C(KZ, KE) and C(AZ, AE) have been computed on day to day basis during the periods of their intensifications over the domain 5° N to 25° N, 55° E to 75° E in respect of ‘GONU’ and 5° N to 25° N, 77° E to 97° E for ‘SIDR’. Besides the above, the area averaged value of s (Sigma), the vertically averaged Moist Static Energy (MSE), has also been computed on each day. Day-to-day evolution of these parameters is mapped and described. Some of the distinguishing features in the energetic of these two intense vortices which formed in entirely different climatological settings have been brought out. It is noticed that in the case of ‘GONU’, though both barotropic and baroclinic energy conversions have taken place during the life cycle, the intensification phase is characterized by an enhancement in AE, KE and vertically integrated Moist Static Energy. Enhancement in AE can be attributed to the generation of AE, which may again be attributed to the asymmetric latent heat of condensation associated with the asymmetric rainfall in the cyclone field. Enhancement in KE may be attributed to the enhancement in both barotropic and baroclinic conversion into KE. Though most of these observations made for ‘GONU’ are found to be attributable to ‘SIDR’ as well, the intensification of ‘SIDR’ appears to have more similarity to that of a typical growing mid-latitude baroclinic wave. In this case, the enhancement in AE, could also be attributed to positive C(AZ,AE), which is mainly due to interaction with mid-latitude baroclinic westerly wave. The energetics analysis also indicates that GONU had helped in the enhancement of seasonal mean meridional circulation where as the SIDR had inhibited the enhancement of seasonal mean meridional circulation.

Energy Dispersion in African Easterly Waves

Abstract The existence of an upstream (eastward) group velocity for African easterly waves (AEWs) is shown based on single-point lag regressions using gridded reanalysis data from 1990 to 2010. The eastward energy dispersion is consistent with the direction of ageostrophic geopotential flux vectors. A local eddy kinetic energy (EKE) budget reveals that, early in the life cycle of AEWs, growth rate due to geopotential flux convergence exceeds baroclinic and barotropic growth rates. Later in the life cycle, EKE decay due to geopotential flux divergence cancels or exceeds baroclinic and barotropic growth. A potential vorticity (PV) budget is used to diagnose tendencies related to group propagation. Although both upstream and downstream group speeds are possible because of the reversal in the mean meridional PV gradient, upstream propagation associated with the positive poleward PV gradient dominates wave packet evolution. Analogous to the concept of downstream development of midlatitude baroclinic waves, new AEWs develop preferentially upstream of the older ones, thus providing a mechanism for seeding new waves. It is suggested that these results are also relevant to AEW intermittency and storm-track structure.

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.

Simulations of Subtropical Cyclones in a Baroclinic Channel Model

Abstract The present study considers a variety of cyclone developments that occur in an idealized, baroclinic channel model featuring full condensation heating effects over an ocean with prescribed sea surface temperature variation. The geostrophic basic-state jet is specified by the tropopause shape, and horizontal shear is included by specifying the meridional variation of zonal wind on the lower boundary. The horizontal shear induces anticyclonic wave breaking of baroclinic waves. Normal mode perturbations are computed using a “fake-dry” version of the model but integrated forward using full physics. Low-latitude moist convection is particularly strong in simulations with strong surface easterlies that destabilize the troposphere through water vapor fluxes from the ocean surface. Deep convection produces a locally elevated dynamic tropopause and an associated anticyclone. This modified zonal flow supports moist baroclinic instability. The resulting cyclones, identified as subtropical cyclones, occur in deep westerly vertical wind shear but are nearly devoid of lower-tropospheric baroclinicity initially. These systems are distinguished from baroclinically dominated secondary cyclones that also form at relatively low latitudes in the simulations. For weak jets and strong subtropical surface easterlies, subtropical cyclone development dominates formation on the midlatitude jet. For strong westerly jets or weak horizontal shear, the situation is reversed and the midlatitude baroclinic wave can help or hinder the ultimate intensification of the subtropical cyclone. The similarity of this cross-latitude influence to the extratropical transition of tropical cyclones is noted.

Physical mechanisms of the seasonal, subseasonal, and high‐frequency variability in the seasonal cycle of summer precipitation in Korea

Three distinct physical mechanisms in the seasonal cycle of the 120 day (19 May to 15 September) summer precipitation in Korea (126°E–130°E, 33°N–38°N) were identified using the 1979–2008 observed precipitation records at 61 Korea Meteorological Administration stations. Detailed space‐time structures of the physical mechanisms of precipitation variability were derived using the daily National Center for Environmental Prediction/National Center for Atmospheric Research reanalysis data over Asia (80°E–180°E, 0°–60°N). The seasonal cycle of summertime precipitation in Korea exhibits three principal temporal scales (seasonal, subseasonal, and high‐frequency components) of variability, each with distinct physical mechanisms. The seasonal component represents the variability associated with the evolution of the Asian summer monsoon, specifically the East Asia summer monsoon, governed primarily by large‐scale circulation as a result of changes in sea level pressure contrasts between the Asian continent and the surrounding oceans. The arrival and the duration of a monsoon front primarily shape the seasonal evolution of precipitation in Korea. The bimodal peaks are due to the low‐level circulation change as a result of redistribution of temperature and, subsequently, of sea level pressure during summer. The subseasonal component has characteristic time scales of 10–30 days and is associated with eastward‐moving upper‐level disturbances at ∼40°N. The upper‐level disturbances affect the meridional circulations, resulting in low‐level convergence/divergence not only underneath but also to the south and to the north of the disturbance. From mid‐July to mid‐August, the subseasonal component is more clearly observable, and the period of oscillations is generally shorter, than during early or late summer. The high‐frequency component with time scales of less than 10 days is associated with midlatitude baroclinic Rossby waves; synoptic‐scale variations of upper‐level geopotential height and low‐level moisture convergence are well correlated with the high‐frequency variability of precipitation in Korea. Positive precipitation anomalies over Korea correspond to an upper‐level divergence with anticyclonic wind anomalies caused by baroclinic Rossby waves with axes around ∼40°N. The vertical structures of key physical variables are well explained in the context of baroclinic instability with the geopotential height anomaly field connecting the upper‐level divergence and the lower‐level convergence tilted westward with height and the temperature anomaly field slanting slightly in the opposite direction.

Synoptic-Scale Variability and Its Relationship with Total Ozone and Antarctic Vortex Displacements

Abstract The main synoptic-scale circulation anomaly pattern over extratropical South America during the austral spring (September–November) is identified by means of rotated extended empirical orthogonal function techniques, applied to the meridional wind perturbation time series at 300 hPa. The dataset is based on 15 spring seasons (1979–93) of meteorological data from the National Centers for Environmental Prediction–Department of Energy Atmospheric Model Intercomparison Project version-2 daily averaged reanalyses, given in 17 vertical levels from 1000 to 10 hPa. The total-ozone daily measurements for the same period are from the Total Ozone Mapping Spectrometer instrument (version 7). The principal synoptic-scale anomaly pattern is associated with an anticyclone–cyclone pair evolving eastward along subpolar latitudes (and hence it is termed the subpolar mode), with a typical length scale of 5000 km and a phase velocity of 8 m s−1. The subpolar-mode waves, which display the main characteristics of midlatitude baroclinic waves, typically maximize near or above the tropopause and propagate upward into the lower stratosphere, showing large amplitudes even at 50 hPa and above. Subpolar-mode-related circulation anomalies are found to be responsible for large total-ozone daily fluctuations near southern South America and nearby regions. In the positive phase of the subpolar mode, total-ozone fluctuations, which are negative, adopt a sigmoid structure, with a zonal scale as large as the anticyclone–cyclone pair. Moreover, it is herein shown that the associated anticyclone produces a local ozone-column decrease to the north and east of its center, due to adiabatic uplift of air parcels in the upper troposphere and lower stratosphere. At the same time, the downstream cyclonic disturbance is responsible for large negative total-ozone anomalies to the west and south of its center. As the cyclone develops in the lower stratosphere, it promotes the northward incursion of the Antarctic vortex up to about 55°S, along with air masses of highly depleted ozone levels.

Vertical Structure of Midlatitude Analysis and Forecast Errors

Abstract The dominant vertical structures for analysis and forecast errors are estimated in midlatitudes using a small ensemble of operational analyses. Errors for fixed locations in the central North Pacific and eastern North America are selected for comparing errors in regions with relatively low and high observation density, respectively. Results for these fixed locations are compared with results for zonal wavenumber 9, which provides a representative sample of baroclinic waves. This study focuses on deviations from the ensemble mean for meridional wind and temperature at 40°N; these quantities are chosen for simplicity and because they capture dynamical and thermodynamical aspects of midlatitude baroclinic waves. Results for the meridional wind show that analysis and forecast errors share the same dominant vertical structure as the analyses. This structure peaks near the tropopause and decays smoothly toward small values in the middle and lower troposphere. The dominant vertical structure for analysis errors exhibits upshear tilt and peaks just below the tropopause, suggesting an asymmetry in errors of the tropopause location, with a bias toward greater errors for downward tropopause displacements. The dominant vertical structure for temperature analysis errors is distinctly different from temperature analyses. Analysis errors have a sharp peak in the lower troposphere, with a secondary structure near the tropopause, whereas forecast errors and analyses show a dipole straddling the tropopause and smooth vertical structure, consistent with potential vorticity anomalies due to variance in tropopause position. Linear regression of forecast errors onto analysis errors for the western North Pacific is used to assess the nonseparable zonal-height structure of errors and their propagation. Analysis errors near the tropopause rapidly develop into a spreading wave packet, with a group speed that matches the mean zonal wind speed of 31 m s−1. A complementary calculation for the regression of 24-h forecast errors onto analysis errors shows that forecast errors originate from analysis errors in the middle and upper troposphere. These errors rapidly expand in the vertical to span the troposphere, with a peak at the tropopause.

Experiments to study baroclinic waves penetrating into a stratified layer by a quasi-geostrophic potential vorticity equation

The upward penetration of the mid-latitude baroclinic waves has been well known to influence the air motion in the stratosphere. To study this in the laboratory, we present a new type of rotating fluid annulus experiment where a radial temperature difference is imposed on water near the flat bottom to produce a baroclinic wave in a lower layer and the water surface is heated to create a stratified upper layer. To test this model, we carried out experiments to study baroclinic waves penetrating into the stratified layer by a quasi-geostrophic potential vorticity equation in the atmospheric dynamics. The baroclinic waves produced in the lower fluid layer were observed to penetrate into the stratified upper layer. As expected from the equation, the vorticities of the flows for wavenumbers 3, 4 and 5 decayed exponentially with height and showed a weaker decay for lower wavenumbers.

Moisture Variability over the Tropical Western Pacific Ocean

In this paper, we study the characteristics of moisture variability over the tropical western Pacific warm pool region using sounding data obtained through the use of research vessels. Data used here were collected in both boreal summer and winter seasons over a nine-year period from 1993 through 2001. Sounding data are categorized into five zonal wind vertical shear modes, and then examined for common and singular features of moisture variability. The results show frequent occurrences of high relative humidity centered around 945 hPa, and 560 hPa, for any wind shear conditions, as previously found in the Tropical Ocean and Global Atmosphere/ Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) studies. We also found another small peak at 845 hPa in low-level easterly conditions. The degree of these features is modulated by wind shear mode differences. An interesting feature is that the variation of total precipitable water is well represented by the synoptic scale mid-tropospheric moisture variation. However, a close relation between the total moisture budget and the moisture variation in the boundary layer is also obtained for a time scale of less than a few days (especially the diurnal cycle). We investigated extremely dry air conditions as one factor modulating mid-tropospheric moisture. Backward air parcel trajectories show that there are two major routes for the intrusion of dry air over the warm pool. In the first, the air parcel could be traced back to higher latitudes and the upper troposphere. For this type, anticyclonic equatorward flow associated with developing mid-latitude baroclinic waves, such as Rossby wave breaking, is responsible for injecting subtropical dry air into the Tropics—as proposed by Yoneyama and Parsons for TOGA COARE dry events. In the second type, the air parcel could be traced back to the upper troposphere, but within lower latitudes in the central-eastern Pacific. For this case, the air parcel originates from the trade winds that passed over the region where dry air was injected by the anticyclonic equatorward flow associated with the baroclinic wave development in the subtropical region. The relationship between dry air conditions and large-scale features is also discussed.

Cold Season Synoptic-Scale Waves over Subtropical South America

The most active winter synoptic-scale wave patterns over South America are identified using an extended empirical orthogonal function (EEOF) technique and are physically diagnosed using composite methods. Results show that the leading modes of short timescale variability propagate along two main paths: over the subtropical jet latitudes (;308S) and over the subpolar jet latitudes (;608S). This research focuses on the subtropical mode and its evolution over South America. The observed structure of the systems associated with the subtropical mode resembles that of midlatitude baroclinic waves. Both cyclonic and anticyclonic perturbations display significant modifications in their threedimensional structure as they evolve over extratropical and subtropical South America. While the upper-level perturbations are mostly unaffected when moving eastward, the lower-level perturbations advance following the shape of the Andes Mountains and exhibit an abrupt equatorward migration at the lee side of the mountains. As a result of such detachment, smaller eddy heat fluxes are observed in the vicinity of the orography and consequently a weaker eddy baroclinic growth is observed. Once the upper-level system is on the lee side, the perturbations acquire a more typical baroclinic wave structure and low-level intensification of the system occurs. The latter is largest around 1000 km east of the orography, where enhanced moisture transports from tropical latitudes along the eastern portion of the low-level cyclone favor precipitation occurrence over southeastern South America. Those precipitation processes seem to provide a diabatic source of energy that further contributes to the strengthening of the low-level cyclone. In addition, an intensification of the cyclone once over the ocean was found in 60% of the situations considered, which is consistent with previous research suggesting an additional source of moisture and heat flux due to the warm waters of the Brazil Current.

Role of tropical mid-latitude interaction in the genesis of a monsoon depression - A detailed case study

During northern summer, a monsoon stationary wave which maintains as part of its baroclinic structure three well-defined troughs, one each in the region of the Arabian sea, the Bay of Bengal and South China sea, frequently interacts with the mid-latitude baroclinic waves which amplify during their eastward passage with profound influence on the development of the monsoon troughs. The paper discusses the mechanism of this wave-wave interaction as suggested by the temporal evolution of the themla1 and wind fields associated with the waves and reports the findings of a detailed study of a case of tropical-mid latitude interaction in which the development of a monsoon trough led to the birth of a westward-propagating monsoon depression over South China.