Tropospheric Variability Research Articles

Relation between the onset and end of the South American summer monsoon and rainfall in subtropical South America

CR Climate Research Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials CR 21:141-155 (2002) - doi:10.3354/cr021141 Relation between the onset and end of the South American summer monsoon and rainfall in subtropical South America Marcela González1,*, Vicente Barros1,2, Moira Doyle1 1Department of Atmospheric Sciences, University of Buenos Aires 2 piso, Pabellón II, Ciudad Universitaria, 1428 Capital Federal, Buenos Aires, Argentina 2Consejo de Investigaciones Científicas y Técnicas (CONICET), Buenos Aires, Argentina *E-mail: gonzalez@at.fcen.uba.ar ABSTRACT: This paper explores the relation between the inter-annual variability of the austral South American monsoon onset and end dates and the inter-annual variability of rainfall in September and May in subtropical South America. Outgoing longwave radiation (OLR) was used to define the onset and end dates of the convective season. Precipitation records were taken from national or state agencies of the region. Other surface and tropospheric variables were taken from NCEP/NCAR reanalysis. A 5 d canonical correlation analysis between rainfall in subtropical South America east of the Andes and the tropical convection of the South American monsoon estimated by the OLR shows that these variables are related during the austral autumn and spring. In both seasons, the first modes are very similar and indicate that rainfall in southern Brazil is negatively correlated with convection over the eastern part of the tropical continent. Because a delay (an early advent) of the monsoon end increases (reduces) the period with convection over the tropics, while the contrary occurs with the onset, rainfall composites between months corresponding to cases with extreme dates at the onset or the end of the tropical convective season show an important rainfall signal in southern Brazil during May and September. During May the early (delayed) end date is associated with enhanced (decreased) rainfall in most of the subtropical region. In September, the early (delayed) onset is associated with decreased (enhanced) rainfall only in southern Brazil, while in Argentina and Uruguay there is an opposite signal, non symmetric with respect to the May case. In May, the mean low-level flow of moisture from the tropical continent over eastern subtropical South America is considerably greater in the cases with an advanced monsoon end than in those with a delayed end, while in September, the composite differences between years with an early and a delayed onset show enhanced moisture advection from the tropical continent over most of Argentina and Uruguay, and a reduction over southern Brazil and Paraguay. KEY WORDS: Subtropical South America · Rainfall · Monsoon · Tropical convection · Outgoing longwave radiation Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in CR Vol. 21, No. 2. Online publication date: June 14, 2002 Print ISSN: 0936-577X; Online ISSN: 1616-1572 Copyright © 2002 Inter-Research.

CIMAR-5: A Snapshot of the Lower Troposphere over the Subtropical Southeast Pacific

The extensive and persistent deck of stratocumulus (SCu) off the west coast of subtropical South America plays an important role in the regional and global climate, as well as in coastal weather. As in other subtropical regions, the SCu form at the top of a marine boundary layer (MBL) bounded by a relatively cold ocean and a large-scale subsidence inversion. Nevertheless, details of the structure and variability of the lower troposphere over this region are largely unknown. Ship-based meteorological observations taken along a transect at 27°S from the Chilean coast (71°W) to Easter Island (110°W) during the second half of October 1999 provide a preliminary description of the low-level circulation, thermodynamic structure, and cloudiness over the subtropical southeast Pacific. Three types of observations were made: (a) 15-min average of air temperature, relative humidity, solar radiation, pressure, and wind by an automatic weather station on the ship deck; (b) 15-min average of vertical reflectivity and cloud base by a laser ceilometer on the ship deck; and (c) twice-daily rawinsondes. Several cloud and radiation properties were derived from GOES-8 imagery and validated against the ship-based observations. A preliminary assessment of the ability of NCEP–NCAR reanalysis and scatterometer winds in representing the atmosphere over a largely in situ data-void area is also presented. Sea surface temperature and near-surface air temperature increase gradually westward, while near-surface relative humidity remains nearly constant at ~80%. A significant increase in the free-tropospheric moisture indicates an offshore decrease in the large-scale subsidence. Consistently, the MBL evolves from a shallow, well-mixed MBL topped by compact SCu near the coast; to a deeper, decoupled MBL with a cumuli rising into a patchy SCu deck near Easter Island, in a similar fashion to the transition from subtropical-stratus regime to trade-cumulus regime described elsewhere. In addition to these “climatological” features, the ship data also reveal the large sensitivity of the MBL-trade inversion structure to synoptic-scale disturbances over the subtropical Pacific. Cloud droplet effective sizes increase from the coast to open ocean. Furthermore, cloud fraction, cloud-top height, liquid water path, and optical depth all peaked during the morning and reached a minimum by midafternoon.

Eastern Equatorial Pacific SST Variability: ENSO and Non-ENSO Components and Their Climatic Associations

Using an updated Kaplan et al. global SST anomaly (SSTA) dataset (1870-1999) a canonical representation of El Nino-Southern Oscillation (ENSO) is constructed. When this canonical ENSO is subtracted from the data, a residual (non-ENSO) dataset for SSTA is left that includes interseasonal to multidecadal variability. Over the eastern equatorial Pacific (Nino-3) the canonical ENSO accounts for about 79% of the total SSTA variability, and the residual, dominated by decadal timescales, accounts for the rest. In particular, about 40%-50% of the amplitudes of the strong 1982-83 and 1997-98 El Nino events were accounted for by the residual variability. The non-ENSO variability is characterized by the known shift from cold to warm eastern tropical Pacific in the mid- to late 1970s as well as by a (nonstationary) interannual variance increase during the 1980s and 1990s. Composite maps of surface (SST, sea level pressure, and winds) and tropospheric (divergent winds, velocity potential, and vertical velocity) variables are used to compare the spatial patterns characterizing the canonical ENSO and the residual components of the Nino-3 variability. The residual composites are found to only share large-amplitude fluctuations of SST anomalies in the equatorial Pacific east of the date line. When these com- posites are separated into decadal and interannual components, the decadal part closely resembles the structure of the Pacific decadal oscillation. The major patterns of tropospheric variability associated with the ENSO and decadal non-ENSO components are very different. At low latitudes, they imply nearly opposite impacts on far- field regional climates, based on their respective warming (or cooling) phases within the Nino-3 region. This unexpected result for low-latitude climate associations runs contrary to the naive expectation (recently shown to be true for North America) that a decadally warm tropical east Pacific will reinforce the climate effects associated with ENSO alone. This indicates that, in the Tropics, climate outlooks may be more accurate if based on separately analyzed relationships between these SSTA components and their associated climate fluctuations.

Long-term tropospheric variations of ozone content caused by galactic cosmic ray influence

Galactic cosmic ray (GCR) fluxes measured by balloons in the troposphere and stratosphere at several points in Russia, and total ozone (TO) records have been used to detect cosmic signal by linear regression analysis. It was shown that TO response is in phase with decadal variations of GCR in contrast to the assumption about ozone destruction by GCR due to the nitrogen catalytic cycle intensification. 1-D photochemical model was used to understand the situation. The results of calculations show positive ozone response in the troposphere caused by additional production of NO by GCRs.

A model study of the dynamical connection between the Arctic Oscillation and stratospheric vacillations

The dynamical connection between the stratosphere and the troposphere in the Northern Hemisphere winter is investigated with general circulation model (GCM) simulations under perpetual January conditions. The variability in the stratosphere is strongly dominated by vacillations on a timescale of 100 days. One‐point correlation maps of the zonal mean zonal wind reveal the characteristic downward propagation of the stratospheric disturbances. The meridional structure of the stratospheric vacillations is well described by a few empirical orthogonal functions (EOFs) of geopotential height. The leading EOF describes a standing oscillation of the stratosphere, while the two next describe the vertical and horizontal propagation. In the troposphere the leading EOF of the surface pressure shows the characteristic circumpolar structure of the Arctic Oscillation. The covariance between the leading EOF of the surface pressure and the zonal mean zonal wind reveals a vertical structure similar to the observed below 10 hPa, while the sign of the covariance changes above 10 hPa where observations are scarce. The downward propagating stratospheric modes play a prominent role in the vertical coupling. Singular value decomposition of the cross‐covariance matrix between the stratospheric zonal mean zonal wind and the surface pressure shows that a significant part of the tropospheric variability can be related to the downward propagating anomalies. The leading pattern of surface pressure anomalies found in this way closely resembles the pattern of the Arctic Oscillation and describes 20–30% of the total variance. This result is confirmed by studying the covariance between the geopotential height fields and the stratospheric principal components.

Chaos, Quasiperiodicity, and Interannual Variability: Studies of a Stratospheric Vacillation Model

The parameter space of a stratospheric vacillation model is investigated numerically. The model is the quasigeostrophic b-plane model introduced by Holton and Mass with time-independent forcing at the lower boundary. Chaotic and quasiperiodic states, diagnosed by the values of the largest Lyapunov exponents, are found for realistic values of the parameters. The quasiperiodic state branches off the periodic vacillations observed in previous studies through a Hopf bifurcation, and chaotic states appear after additional bifurcations. Defining an appropriate variable, a circle map behavior is revealed and the route to chaos identified as an example of the Ruelle‐Takens‐Newhouse scenario. The existence of additional bifurcations with constant forcing indicates that the explanation of the stratospheric variability in the Northern Hemisphere does not need to include external factors such as tropospheric variations or variations in the equatorial quasi-biennial oscillation. The bifurcations produce low-frequency components with timescales long enough to interfere with the annual cycle.

Anomalous scaling of mesoscale tropospheric humidity fluctuations

Water vapor fluctuations are measured and analyzed at an unprecedented 10‐m resolution throughout the troposphere. Computation of structure functions shows that specific humidity variations observed by research aircraft over the Pacific Ocean exhibit anomalous scaling from about 50 m to 100 km in horizontal range. The scaling laws show different characteristics for the marine boundary layer, the tropical free troposphere, and the extratropical free troposphere. More specifically, boundary‐layer humidity fluctuations are less smooth and more stationary than those in the free troposphere, while the extratropical free tropospheric variations are less intermittent than those in the other two regions. The anomalous scaling results argue against passive advection by a spatially smooth flow (chaotic advection) at these scales.

Northern Hemisphere snow extent: regional variability 1972-1994

Snow cover is an important hydrologic and climatic variable due to its effects on water supplies, and on energy and mass exchanges at the surface. We investigate the kinematics and climatology of Northern Hemisphere snow extent between 1972 and 1994, and associated circulation patterns. Interannual fluctuations of North American and Eurasian snow extents are driven by both hemispheric scale signals, as well as signals from smaller ‘coherent’ regions, within which interannual fluctuations of snow extent are highly correlated. These regions cover only 2‐6% of the continental land area north of 20°N, yet during many months they explain more than 60% of the variance in continental snow extent. They are identified using Principal Components Analysis (PCA) of digitized snow extent charts obtained from the National Oceanic and Atmospheric Administration (NOAA). Significant month-to-month persistence is found over western North America and Europe during winter and spring. Geographically and seasonally dependent associations are identified between North American snow extent and atmospheric circulation patterns, surface air temperature, and snowfall. Over western North America, snow extent is associated with the longitudinal position of the North American ridge. Over eastern North America, snow extent is associated with a meridional oscillation in the 500-mb geopotential height field. These teleconnection patterns, derived using composite analyses, are associated with secondary modes of tropospheric variability during autumn and winter. During spring, snow extent becomes effectively decoupled from tropospheric dynamics. These results are useful for understanding the natural variability of the climate system, reconstructing pre-satellite era climate variability, evaluating climate models, and detecting climate change. Copyright © 1999 Royal Meteorological Society.

Modeling weather fronts to improve GPS heights: A new tool for GPS meteorology?

The precision of vertical position and atmospheric water vapor content determined by the Global Positioning System (GPS) is limited by errors due to tropospheric delay. One factor is the spatial and temporal variability in tropospheric refractivity caused by passing weather fronts. We can explain some of the temporal characteristics of estimated tropospheric delay in terms of a simple path delay model as a function of frontal parameters. These results suggest that GPS could be used to estimate the geometry and passage time of a frontal zone. We have developed indices which detect tropospheric variability from GPS data alone; the detection rate of fronts with this approach is up to 70%. Once detected, we eliminated days affected by fronts or other tropospheric variability from the time series of station height estimates, resulting in improved long‐term repeatability. The additional variance attributable to fronts is estimated to be up to 10 mm2 at Herstmonceux, England, where fronts occur every 2–3 days. The effect of fronts on the horizontal station component is up to 80% smaller than for the vertical. Studies in the field of GPS meteorology may be improved by estimating frontal parameters.

A Hough Spectral Model for Three-Dimensional Studies of the Middle Atmosphere

Abstract A three-dimensional framework is developed for studying the middle atmosphere in relation to upper-tropospheric structure. The numerical model is formulated from the primitive equations in isentropic coordinates, which directly characterize diabatic processes forcing the Brewer–Dobson circulation. It is anchored in observed tropospheric variability, so integrations provide middle atmospheric behavior that tracks observed variations in the upper troposphere. The numerical framework is versatile and computationally efficient. It achieves enhanced performance by incorporating eigenfunctions of the primitive equations to represent structure spectrally in all three coordinates. Scale-selective dissipation can then be applied entirely at sixth order, which leaves all but the shortest vertical scales undamped. This feature allows vertical diffusion to be made small enough to represent stratospheric transport as advective (rather than diffusive) for most of the scales carried in the integration. Transpor...

Tropospheric corrections of SAR interferograms with strong topography. Application to Etna

The accuracy of spaceborne geodetic techniques, including SAR interferometry, is limited by the time and spatial variation and altitude dependance of the propagation delay of electomagnetic waves in the lower troposphere, particularly in mountainous areas. In this paper, we use a 1D model developed for tropospheric corrections of GPS and DORIS measurements to correct SAR data. The differential tropospheric delay is computed at each pixel of the interferogram from ground temperature, humidity and pressure using two empirical parameters calibrated from several radio‐soundings acquired in various latitude and climate conditions. It is shown that with such a model, given the 3300 meters topography of Etna, tropospheric variations can generate up to 4π phase rotations between the top and the bottom of the volcano. In 16 out of the 20 interferograms processed with images acquired between August 1992 and October 1993, correction of the tropospheric effect reduces the number of fringes associated with the 1991–93 eruption from previous estimates. The remaining deformation is consistant with a deforming source located at a depth of 14±1 km. During the second half of the eruption, the subsidence rate at the top of the volcano is roughly stable at 13±3 mm/month. These values are in good agreement with tiltmeter data collected on Etna during the same period and with the estimated volume of erupted material. No significant deformation can be observed during the last month of eruption. Inflation of the volcano seems to resume immediately after the end of the eruption at a rate of 3 mm/month.

A Description of a 1260-Year Control Integration with the Coupled ECHAM1/LSG General Circulation Model

A 1260-yr integration generated by the ECHAM1/LSG (Large Scale Geostrophic) coupled atmosphere‐ocean general circulation model is analyzed in this paper. The analysis focuses on the climate drift and on the variations of the coupled atmosphere‐ocean system after the initial climate drift has essentially died out. The initial drift is induced, to a large extent, by the applied heat flux correction, which has very large spatially fixed values of upward heat flux in the polar regions, in particular along the Antarctic coast. The globally integrated freshwater flux becomes unbalanced during the integration, due to the changes in the snow accumulation rate over Greenland and Antarctica. The resulting net upward freshwater flux induces a linear trend in the salinity of the upper ocean. The drift of temperature and salinity in the deep ocean, which is essentially independent of the boundary condition variations during the coupled integration, is presumably related to the spinup of the deep ocean prior to the coupling. The analysis of the last 810 yr of the integration, which is free from the strong initial drift, suggests that the tropospheric variations are white on timescales longer than 1 yr. The dominant Northern Hemispheric mode resembles the western Atlantic pattern. The dominant tropical and Southern Hemispheric modes are essentially zonally symmetric. All these modes can be found on both short (1 yr) and long (15 yr) timescales. For the oceanic variations, the spatial distribution of the total variance and the dominant modes and the relationships between these modes are studied. For the horizontal barotropic streamfunction, the most dominant mode describes an anomalous westward (eastward) circumpolar flow together with clockwise (anticlockwise) circulation in the Southern Atlantic and southeast of South Africa and in the Southern Pacific. For the zonally averaged meridional circulations the most dominant modes of variability describe essentially recirculations within each basin.

Atmospheric effects in interferometric synthetic aperture radar surface deformation and topographic maps

Interferogram images derived from repeat‐pass spaceborne synthetic aperture radar systems exhibit artifacts due to the time and space variations of atmospheric water vapor. Other tropospheric variations, such as pressure and temperature, also induce distortions, but the effects are smaller in magnitude and more evenly distributed throughout the interferogram than the wet troposphere term. Spatial and temporal changes of 20% in relative humidity lead to 10 cm errors in deformation products, and perhaps 100 m of error in derived topographic maps for those pass pairs with unfavorable baseline geometries. In wet regions such as Hawaii, these are by far the dominant errors in the Spaceborne Imaging Radar‐C and X Band Synthetic Aperature Radar (SIR‐C/X‐SAR) interferometric products. The unknown time delay from tropospheric distortion is independent of frequency, and thus multiwavelength measurements, such as those commonly used to correct radar altimeter and Global Positioning System (GPS) ionospheric biases, cannot be used to rectify the error. In the topographic case, the errors may be mitigated by choosing interferometric pairs with relatively long baselines, as the error amplitude is inversely proportional to the perpendicular component of the interferometer baseline. For the SIR‐C/X‐SAR Hawaii data we found that the best (longest) baseline pair produced a map supporting 100 m contouring, whereas the poorest baseline choice yielded an extremely noisy topographic map even at this coarse contour interval. In the case of deformation map errors the result is either independent of baseline parameters or else very nearly so. Here the only solution is averaging of independent interferograms, so in order to create accurate deformation products in wet regions many multiple passes may be required. Rules for designing optimal data acquisition and processing sequences for interferometric analyses in nondesert parts of the world are (1) to use the longest radar wavelengths possible, within ionospheric scintillation and Faraday rotation limits, (2) for topography, maximize interferometer baseline within decorrelation limits* and (3) for surface deformation, use multiple observations and average the derived products. Following the above recipe yields accuracies of 10 m for digital elevation models and 1 cm for deformation maps even in very wet regions, such as Hawaii.

A mechanism for the low-frequency variability of the mid-latitude troposphere

A mechanism for the low-frequency variability of the mid-latitude troposphere

Atmospheric temperature variability in the Arctic as revealed in a TOVS data record

AbstractThe Earth's high-latitude regions are of critical importance in many climate-change scenarios, but a time continuous, spatially complete, and well-calibrated record of tropospheric temperatures is needed in order to assess past and future climate changes. Studies of recently compiled upper-air data sets show no evidence of CO2-induced warming, but the spatial pattern of tropospheric temperature variability in the Arctic has not been thoroughly examined. This study analyzes a 108-month segment of the data record from the TIROS Operational Vertical Sounder (TOVS) aboard NOAA polar-orbiting satellites to examine both the spatial and temporal variability of atmospheric temperature in the Arctic.Temperature retrievals based on clear-column radiances archived at NOAA/NESDIS were done using algorithms tuned to Arctic conditions. The retrieved temperatures compared well with Arctic rawinsonde data, and include lowlevel inversions that are often problematic for satellite retrievals. The amplitude of the seasonal cycle in 500 mbar temperatures from the TOVS, NMC, and rawinsonde data generally agreed, whereas the phase comparisons produced mixed results. Principal component analyses of the TOVS and NMC temperatures revealed both monopole and dipole spatial patterns in the component loadings. Spatial patterns of the correlation between the upper-air data and the TOVS retrievals were similar to the principal component loading patterns. Whereas no significant trends were found in the station data for the same period as the TOVS record, a significant negative trend could be seen in the first principal component scores of the TOVS retrievals.

Stratospheric Effects of ENSO-Related Tropospheric Circulation Anomalies

Abstract Using 30 years of Northern Hemisphere geopotential data, from 1000 to 10 hPa, the link between three fundamental modes of tropospheric variability [the Pacific/North America (PNA), Western Pacific Oscillation (WPO), and Tropical/Northern Hemisphere (TNH) patterns] and the extratropical wintertime northern stratospheric circulation is explored. These modes of variability are known to be influenced by El Nino/Southern Oscillation (ENSO) and may provide a mechanism for ENSO to influence interannual variability of the strato-spheric flow. Aside from any link to ENSO, these modes may be important in providing tropospheric wave forcing to the stratosphere. These modes may be characterized by three of the leading rotated empirical orthogonal functions of the wintertime 5OO-hPa height field. An index of the amplitude of each of the modes is given by time series of principal components of that mode. By examining composites of years with high-index values minus years with low-index values it is shown that ...

Observed Tropospheric Biennial Variability and Its Relationship to the Southern Oscillation

Abstract Tropospheric biennial variability in several components of the Southern Oscillation (SO) is defined and described through analysis of observational data from the Comprehensive Ocean-Atmosphere Data Set (COADS), as well as through investigation of several SO index time series. The analysis suggests that the temporal behavior of the SO can be described in terms of three components: 1) a pervasive biennial pulse, which appears to be strong in both the Indian Ocean and the west Pacific surface zonal winds as well as in several SO indices, 2) the annual cycle, which tends to set the phase of biennial variability for the major SO excursions, and 3) a low-frequency, or residual, variability, which may be associated with temporal scales between large SO episodes. This study also supports recent papers in suggesting that complete models of the SO must include the Indian Ocean basin.

Intraseasonal Tropospheric Circulation Variability and its Association with Concurrent Atmospheric and Oceanic Fields

Abstract Interseasonal variability for a portion of the Northern Hemisphere has been quantified by using grid-point standard deviations of five-day mean 700 mb height for individual seasons, standard deviations of empirical orthogonal function amplitude time series for individual seasons, and a specially derived variability index (VI). The various measures or intraseasonal variability of 700 mb height were used in stratification studies (i.e., using difference maps based on high versus low variability) or correlation analyses so that relationships between intraseasonal variability and seasonal mean circulation or sea-surface temperature anomalies could be determined. Approximately 39 years of data (1948–76) were employed. Principal findings include: 1) Intraseasonal variability, characterized by the VI, generally showed little if any persistence from one season to the next. The exception was the autumn-winter period when the autocorrelation was 0.52. The largest contribution to this persistence came from ...

The Colorado Wind-Profiling Network

Remote sensing instrumentation has advanced to the point where serious consideration is being given to a next-generation tropospheric sounding system that uses radars and radiometers to provide profiles of tropospheric variables continuously and automatically. A network of five wind-profiling radars has been constructed in Colorado. This network represents a significant step in the development of a new observing system for operational and research meteorology. The radars and their capabilities and limitations are described.

Relations between interdiurnal pressure and temperature variations in troposphere and stratosphere over North America. By B. Haurwitz and W. E. Turnbull. Canadian Met. Mem., 1, 1938, No. 3

Relations between interdiurnal pressure and temperature variations in troposphere and stratosphere over North America. By B. Haurwitz and W. E. Turnbull. Canadian Met. Mem., 1, 1938, No. 3