Sistema Regional de Información
en línea para Revistas Científicas de América Latina,
el Caribe, España y Portugal

An evaluation of the Hadley Centre atmospheric general circulation model, HadAM2b, is presented, focusing on the ability of the model to simulate Southern Hemisphere (SH) transient disturbances. An assessment is also made of the effect of changing resolution. Standard resolution (2.5º latitude by 3.75º longitude) and high resolution (0.833º latitude by 1.25º longitude) AMIP (Atmospheric Model Intercomparison Project) runs in which the models are forced with observed monthly varying sea surface temperature (SST) are compared with NCEP and ERA re-analysis data for the period 1979-1988. We focus on the model’s simulation of selected variables representative of the mean state and eddy properties which characterise synoptic-scale activity of the SH. Both models reproduce the basic observed patterns of the pressure field and upper level jetstream quite well. Overall, the changes due to increasing resolution do not represent an improvement in the simulated climate. The transient eddy activity is reasonably well simulated, though overestimated, relative to the NCEP re-analysis, at both resolutions. This overestimation is much less when compared with ERA. The magnitudes of the eddy kinetic energy, momentum fluxes and heat fluxes in the latitudes of the storm tracks increase with resolution, though some regional features are better simulated with the high-resolution model. Both models capture the structure and time evolution of synoptic-scale disturbances very well, though the amplitude of the disturbances is larger than in the analysis. Both models also capture the north-eastward propagation of the waves in the vicinity of the Andes, though the NW-SE tilt of the disturbances on the lee side are slightly overestimated, in agreement with larger eddy momentum flux. The orographic influence of the Andes mountains is better resolved in the high-resolution simulation for the waves emanating from the polar branch of the Pacific storm-track.  

This paper analyzes the synoptic scale features of the upper and lower level tropospheric circulation over tropical South America during the first Atmospheric Mesoscale Campaign in the Wet Season (WETAMC) of the Large Scale Biosphere – Atmosphere Experiment in Amazonia (LBA). This campaign occurred in January and February, 1999 and concomitantly with the validation of the Tropical Rainfall Measuring Mission (TRMM) satellite over Brazil. National Centers for Environmental Predictions (NCEP) grid point data were used to analyze the winds and its associated divergence fields and define the prevailing patterns of the tropospheric circulation in that region. TRMM images were used to depict the rain patterns. The results show at least four distinct patterns in the upper air winds fields during the WETAMC-TRMM/LBA, involving mainly the Bolivian anticyclonic circulation and cyclonic vortices in the vicinity of Northeast Brazil. The results also suggest that, in general, the convective activity over vast areas of South America is dynamically supported by upper level divergence. Two out of a total of six vortices were observed to have anomalous displacements into the Amazonia during the study period. At lower levels, the South Atlantic Convergence Zone (SACZ), except for the first half of January, was not active but equatorward incursions of midlatitude frontal systems disturbed the convective activy in the Southwestern Amazon basin. Besides, during the period of the study, the low-level moisture divergence as derived from NCEP data does not show convergence where the TRMM rain field clearly reveals the existence of precipitation areas.

Emissions of nitrous oxide (N2O) were measured in agricultural lands used for farming wheat and maize during 1998 in the states of Hidalgo and Tlaxcala in Mexico. In an irrigated wheat field (El Teñhé, Hidalgo), an average flux of -10.85 µg N2O-N m-2h-1 was obtained for the total cycle (155 days between December and May). There, high negative values were observed with Water Fill Porous Space (WFPS) close to 70%. The average flux of the complete cycle (269 days between March and December) in an irrigated maize field (El Progreso, Hidalgo) was 37.43 µg N2O-N m-2h-1. In this case, more significant negative fluxes were found with WFPS close to 45% or less. These last results may have been influenced by the strong “El Niño”, which occurred in the middle of 1998. Twenty one percent of the measurements in the state of Hidalgo showed soil acting as a nitrous oxide sink. The samples from Tlaxcala showed that these fields acted as emitters. In the rain fed fields in the state of Tlaxcala, an average flux of 121 µg N2O-Nm-2h-1 was obtained for the wheat field. The farming season lasted 142 days, from July to December. In addition, for the maize field the averaged flux was 285.61 µg N2O-N m-2h-1. The farming season lasted 246 days, from April to December.

The relative angular momentum (RAM) from 500 hPa to 200 hPa and from 55º N to 90 ºN was computed for the 1958-1998 period. NCAR-NCEP reanalysis data were used in this study, whose main purpose was to find an annual measurement to compare with the Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO). Annual values of RAM exhibited a clear positive trend consistent with positive trends found in AO and NAO. The correlations between RAM and AO were higher than those between RAM and NAO and the spectral analysis showed a common significant band of oscillation ca. to eight years.

In this paper the urban heat island (UHI) of the city of Salamanca, Spain is examined. The temporal evolution of the intensity of the UHI for the three-year period between 1996 and 1998 was studied by comparing the temperatures in an urban area and those in a nearby rural area. The results pointed to the existence of a nocturnal UHI, whose highest mean value was recorded in autumn (3.2 ºC). Using the transect selection method, the spatial distribution of the UHI was analyzed. The lowest nocturnal temperatures were observed in the outskirts of the city, mainly in green areas close to the river. The daytime mean temperatures revealed a negative UHI, the lowest temperatures were recorded in the urban area, where the buildings, which afford shade, prevent the arrival of direct solar radiation. Under conditions of atmospheric stability, the UHI is more intense than under conditions of instability.  

The potential for damage from hurricanes landfalling in Mexico is assessed. During the 1951-2000 period, Pacific hurricane hits were more frequent on coastal areas of the northwest of the country (e.g., Sinaloa and the southern half of Baja California Peninsula) as well as in southern México (Michoacán). On the Atlantic side, the Yucatán Peninsula and the northern state of Tamaulipas were the most exposed to these storms. The hurricane season reaches maximum activity in September for both the Atlantic and Pacific coasts of the country. During the 50 year period, five intense hurricanes (category 5) made landfall on the Gulf/Caribbean coasts, while only one such intense hurricane made a land hit on the Pacific side. While hurricanes affecting Pacific coasts show a marked increase during the last decade, those of the Atlantic side exhibit a marked de-crease since the 1970s. However, when considering the frequency of landfalling tropical storms and hurricanes impacting on both littorals of the country, their numbers have considerably increased during the 1990s.

Air temperature, vapour pressure and relative humidity differences at fixed hours in urban, suburban and rural districts of Cairo area, Egypt, have been investigated using data for the 1995-2000 period. It has been found that, on the basis of the vapour pressure differences, the urban atmosphere is drier throughout the year except for the months of December, January, May and September. In the afternoon, the atmosphere in the urban area is more humid throughout the year if compared to the suburban area and during the months from October to January in addition to May if compared to the rural area. On the basis of relative humidity, the urban atmosphere is always drier than its surroundings throughout the year, except in the afternoon when the urban-rural differences fluctuated between positive and negative. The urban atmosphere is always warmer than its surroundings throughout the year, except in November when there is a cool island. Relationships between heat island intensity and both vapour pressure and relative humidity differences reveal that local effects can be significant.

A series of laboratory experiments with forced temperature and salinity were conducted using an apparatus (channel between pole and equator boxes) designed to duplicate the conceptual Stommel’s model of thermohaline circulation. The flow patterns are discussed depending on modelling parameters (aspect ratio and buoyancy), and specified boundary conditions. Three steady states are found in the control space of the modelling parameters: two-layer thermal mode (T2), two-layer saline mode (S2), and three-layer hybrid state (H3). The thermohaline circulation exhibits both, smooth and abrupt transitions, depending on the rate of mixing. Free convection and double diffusion experiments with limited mixing show smooth transitions. Abrupt transitions were only found in the complete mixing experiments when two boxes were stirred by mixers.

The ion and electron temperature anomalies induced by the sprites associated with active thunderstorms in the ionosphere F2 region have been studied. For this study the data was obtained by the Retarded Potential Analyzer (RPA) payload experiment flown aboard the Indian SROSS-C2 satellite for the solar minimum year 1995-96. The data on thunderstorm activity has been obtained from the India Meteorological Department (IMD). The data analysis and comparison with the normal day’s temperature demonstrate the consistent enhancement during the sprites associated with active thunderstorms. The physical region for this temperature enhancement may be related to the generation of UHF to gamma ray frequencies due to lightning sprite activity, which propagate still upward at the satellite altitude (Taranenko and Roussel-Dupre, 1996; Lehtimen et al., 1996) and heat the local plasma in the ionosphere.  

Previous research has demonstrated that Pacific Region SSTs and SST anomalies can be separated into seven general synoptic classifications (clusters) (A-G). Clusters B and G (C, D, and F) [A and E] were shown to be generally representative of La Niña (El Niño) [neutral] type SST distributions. Further, an analysis of the SST patterns in 1955-1993 demonstrated that clusters A-D were prominent in 1955-1977, while types E and F dominated the later period. Type G clusters were comparatively rare, but occurred during both periods. In retrospect, this shift during 1977 corresponds roughly with a change in phase of the Pacific Decadal Oscillation (PDO). After updating the analysis to include the 1994 to 2005 period, there was a corresponding change in the predominant SSTs associated with a change in phase of the PDO during 1999 and 2000. The results show that SST patterns did evolve from predominantly E and F-type anomalies in 1994 to A, B, D and G-type anomalies through 2005. Thus, these results suggest that A through D-type (C, E, and F-type) SST clusters are characteristic of the negative (positive) phase of the PDO. Also, using a modified technique for generating phase diagrams, it is shown that there are interannual and interdecadal variations in the mid-Mississippi region monthly mean surface temperature and precipitation records that can be associated with the ENSO and PDO. Additionally, an analysis was performed to see if there was any statistical association between temperature and precipitation anomalies in the mid-Mississippi region and prolonged SST regimes. B, D and G anomalies were associated with warmer-than-normal conditions, while C and E type anomalies tended to be associated with cooler-than-normal conditions across the region. C, D, F, and G anomalies were associated with drier than normal conditions.