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Marisol Osman and Carolina S. Vera

Abstract

The predictability and forecast skill of the models participating in the Climate Historical Forecast Project (CHFP) database is assessed through evaluating the representation of the upper-tropospheric extratropical circulation in the Southern Hemisphere (SH) in winter and summer and its main modes of variability. In summer, the predictability of 200-hPa geopotential height anomalies mainly comes from the ability of the multimodel ensemble mean (MMEM) to forecast the first three modes of interannual variability with high fidelity. The MMEM can reproduce not only the spatial patterns of these modes but also their temporal evolution. On the other hand, in JJA only the second and fourth modes of variability are predictable by the MMEM. These seasonal differences in the performance of the MMEM seem to be related to the role that the sea surface temperature (SST) anomalies have in influencing the variability of each mode. Accordingly, modes that are strongly linked to tropical SST anomalies are better forecast by the MMEM and show less spread among models. The analysis of both 2-m temperature and precipitation anomalies in the SH associated with the predictable modes reveals that DJF predictable modes are accompanied by significant temperature anomalies. In particular, temperatures at polar (tropical) latitudes are significantly correlated with the first (second) mode. Furthermore, these links obtained with observations are also well forecast by the MMEM and can help to improve seasonal forecast of climate anomalies in those regions with low skill.

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Carolina S. Vera and Paula K. Vigliarolo

Abstract

Midlatitude disturbances such as intense cold fronts and cutoff lows are a very important cause of severe-weather events over the southern part of South America, particularly during the austral winter months. Behind cold fronts, cold air from higher latitudes is forced to flow equatorward to the east of the Andes. Occasionally fronts might produce cold surges in tropical latitudes, with freezing temperatures (referred as to GEADAS in Portuguese) over crop-growing areas of southern, southeastern, and central Brazil.

The structure and dynamical processes of winter synoptic-scale waves associated with cold surges over South America are documented in this paper. It is shown that the rotated extended empirical orthogonal function technique is capable of extracting well-defined regional patterns from 850-hPa meridional-wind perturbation fields. Over South America, waves behave very differently in the lower and middle troposphere: while upper-level waves propagate northeastward as they cross the Andes, evolving in a manner consistent with the concept of Rossby wave dispersion, at lower levels, waves tend to conform to the shape of the mountain range, in agreement with the theory of topographic Rossby waves. However, it was found that cold surges do not seem to result from the generation of rotationally trapped waves.

Significant differences have been found between synoptic-scale patterns associated with cold surges over extratropical South America (NO GEADA composites) and synoptic-scale patterns associated with major cold-air outbreaks that produce extreme low temperatures at tropical regions (GEADA composites). While both surges are characterized by a long-wave pattern consisting of a cyclonic perturbation over South America and anticyclonic perturbation behind over the southern Pacific Ocean, at the early stages of GEADA composites, two additional features were identified: a subpolar and subtropical short-wave pattern. The presence of the upper-level subtropical cyclonic perturbation is associated with the subtropical jetstream location farther north, which would facilitate the equatorward penetration of frontal systems. In addition, this cyclonic perturbation enhances rising motion at subtropical and tropical latitudes that is associated with a well-defined secondary circulation with its descending branch over central Argentina. It is shown that this configuration favors the rapid and equatorward penetration of the subpolar short-wave trough, the temperature decrease over southern Brazil, and the anticyclone intensification over central Argentina. Thus the presence of the subtropical upper-level feature plays a key role on the cold-surge occurrence over tropical regions of South America.

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Ernesto H. Berbery and Carolina S. Vera

Abstract

The structure and evolution of the fluctuations in synoptic scales in the Southern Hemisphere (SH) during winter are discussed using six years of European Centre for Medium-Range Weather Forecasts analyses.

It is shown that patterns from unfiltered meridional wind series in the SH display all the features needed to represent the synoptic-scale waves. Typical periods and wavelengths are similar to those observed in the Northern Hemisphere (4 days, 4000 km), although over the Pacific Ocean they can be as high as 7–8 days and 4700 km, respectively. As in the Northern Hemisphere, tilts are not geographically fixed but change with the stage of the evolution of the wave. The phase speed of the waves agrees with the low-level winds in extensive areas of the middle latitudes and ranges from 12 m s −1 in the Indian Ocean to 6 m s−1 in the Pacific Ocean. The estimated group velocities achieve maximum values of about 38 m s −1, also in the Indian Ocean, and agree with the upper-level maximum winds, in accord with reported model results for the leading fringe of the wave packets.

The wave packets show a decay of upstream centers as new ones grow downstream, suggesting that down-stream development contributes to the evolution of the synoptic-scale waves in the SH storm track. This process is observed both in the subpolar and subtropical jets, but the sequence of centers developing downstream is more coherent in the latter, probably due to the weaker baroclinicity.

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S. Bibiana Cerne, Carolina S. Vera, and Brant Liebmann

Abstract

This note describes the physical processes associated with the occurrence of a heat wave over central Argentina during the austral summer of 2002/03, during which the South American Low-Level Jet Experiment (SALLJEX) was carried out. The SALLJEX heat wave that lasted between 25 January and 2 February 2003 was punctuated by extreme conditions during its last 3 days, with the highest temperature recorded over the last 35 yr at several stations of the region. It was found that not only the activity of synoptic-scale waves, but also the intraseasonal oscillation variability, had a strong impact on the temperature evolution during this summer. During the weeks previous to the heat wave development, an intensified South Atlantic convergence zone (SACZ) dominated the atmospheric conditions over tropical South America. Temperatures started to increase in the subtropics due to the subsidence and diabatic warming associated with the SACZ, as depicted by SALLJEX upper-air observations. An extratropical anticyclone that evolved along southern South America further intensified subsidence conditions. By the end of January the warming processes associated with SACZ activity weakened, while horizontal temperature advection began to dominate over central Argentina due to the intensification of the South American low-level jet. This mechanism led to temperature extremes by 2 February with temperature anomalies at least two standard deviations larger than the climatological mean values. Intense solar heating favored by strong subsidence was responsible for the heat wave until 31 January, after which horizontal temperature advection was the primary process associated with the temperature peak.

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Paula K. Vigliarolo, Carolina S. Vera, and Susana B. Díaz

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.

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Carolina S. Vera, Paula K. Vigliarolo, and Ernesto Hugo Berbery

Abstract

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 (∼30°S) and over the subpolar jet latitudes (∼60°S). 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 three-dimensional 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.

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Brant Liebmann, George N. Kiladis, Carolina S. Vera, A. Celeste Saulo, and Leila M. V. Carvalho

Abstract

Regional and large-scale circulation anomalies associated with variations in rainfall downstream of the South American low-level jet are identified and compared to those in the South Atlantic convergence zone (SACZ). Composites of precipitation associated with strong jets reveal an approximate doubling of the quantities one would expect from climatology, with an evolution of the rainfall pattern from south to north. The occurrence of extreme precipitation events follows a similar pattern. Meridional cross sections of composite wind reveal a distinct low-level jet near 20°S and a baroclinic development farther south that appears to force the jet. Geopotential height, temperature, and large-scale wind composites suggest that this developing disturbance is tied to a wave train that originates in the midlatitude Pacific and turns equatorward as it crosses the Andes Mountains. Similar composites based on SACZ rainfall reveal similar features, but of opposite sign, suggesting that the phase of the wave as it crosses the Andes Mountains determines whether rainfall will be enhanced downstream of the jet or in the SACZ. The alternate suppression or enhancement of rainfall in these adjacent regions results in a precipitation “dipole.” Many previous studies have found a similar out-of-phase relationship over many time scales. The phase of the Madden–Julian oscillation (MJO) is composited relative to anomalous precipitation events, revealing statistically relevant amplitudes associated with rainfall both downstream of the jet and in the SACZ. The MJO is a particularly interesting intraseasonal oscillation because it has some predictability. It is speculated that the slowly varying dipole that has been observed is a consequence of the preferred phasing of synoptic waves due to variations of the planetary-scale basic-state flow, which is at times associated with the MJO.

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Brant Liebmann, George N. Kiladis, Leila M. V. Carvalho, Charles Jones, Carolina S. Vera, Ileana Bladé, and Dave Allured

Abstract

Convectively coupled Kelvin waves over the South American continent are examined through the use of temporal and spatial filtering of reanalysis, satellite, and gridded rainfall data. They are most prominent from November to April, the season analyzed herein. The following two types of events are isolated: those that result from preexisting Kelvin waves over the eastern Pacific Ocean propagating into the continent, and those that apparently originate over Amazonia, forced by disturbances propagating equatorward from central and southern South America.

The events with precursors in the Pacific are mainly upper-level disturbances, with almost no signal at the surface. Those events with precursors over South America, on the other hand, originate as upper-level synoptic wave trains that pass over the continent and resemble the “cold surges” documented by Garreaud and Wallace. As the wave train propagates over the Andes, it induces a southerly low-level wind that advects cold air to the north. Precipitation associated with a cold front reaches the equator a few days later and subsequently propagates eastward with the characteristics of a Kelvin wave. The structures of those waves originating over the Pacific are quite similar to those originating over South America as they propagate to eastern South America and into the Atlantic.

South America Kelvin waves that originate over neither the Pacific nor the midlatitudes of South America can also be identified. In a composite sense, these form over the eastern slope of the Andes Mountains, close to the equator. There are also cases of cold surges that reach the equator yet do not form Kelvin waves.

The interannual variability of the Pacific-originating events is related to sea surface temperatures in the central–eastern Pacific Ocean. When equatorial oceanic conditions are warm, there tends to be an increase in the number of disturbances that reach South America from the Pacific.

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Brant Liebmann, George N. Kiladis, Dave Allured, Carolina S. Vera, Charles Jones, Leila M. V. Carvalho, Ileana Bladé, and Paula L. M. Gonzáles

Abstract

The mechanisms resulting in large daily rainfall events in Northeast Brazil are analyzed using data filtering to exclude periods longer than 30 days. Composites of circulation fields that include all independent events do not reveal any obvious forcing mechanisms as multiple patterns contribute to Northeast Brazil precipitation variability. To isolate coherent patterns, subsets of events are selected based on anomalies that precede the Northeast Brazil precipitation events at different locations. The results indicate that at 10°S, 40°W, the area of lowest annual rainfall in Brazil, precipitation occurs mainly in association with trailing midlatitude synoptic wave trains originating in either hemisphere. Closer to the equator at 5°S, 37.5°W, an additional convection precursor is found to the west, with a spatial structure consistent with that of a Kelvin wave. Although these two sites are located within only several hundred kilometers of each other and the midlatitude patterns that induce precipitation appear to be quite similar, the dates on which large precipitation anomalies occur at each location are almost entirely independent, pointing to separate forcing mechanisms.

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Brant Liebmann, Carolina S. Vera, Leila M. V. Carvalho, Inés A. Camilloni, Martin P. Hoerling, Dave Allured, Vicente R. Barros, Julián Báez, and Mario Bidegain

Abstract

Seasonal linear trends of precipitation from South American station data, which have been averaged onto grids, are examined, with emphasis on the central continent. In the period 1976–99, the largest trend south of 20°S occurs during the January–March season, is positive, and is centered over southern Brazil. From 1948 to 1975 the trend is also positive, but with less than half the slope. The trend is not due to a systematic change in the timing of the rainy season, which almost always starts before January and usually ends after March, but rather results from an increase in the percent of rainy days, and an increase in the rainy day average. The dynamic causes of the trend are not obvious. It does not appear to be accounted for by an increase in synoptic wave activity in the region. The precipitation trend is related to a positive sea surface temperature trend in the nearby Atlantic Ocean, but apparently not causally. The trend in the Atlantic seems to result from a decrease in mechanical stirring and coastal upwelling associated with a decrease in the strength of the western edge of the circulation associated with the South Atlantic high.

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