Sergio H. Franchito

Interaction between Coastal Upwelling and Local Winds at Cabo Frio, Brazil: An Observational Study

Abstract The relationships between coastal upwelling and local winds at Cabo Frio (Brazil) are studied using SST and time series of surface wind for a 10-yr period (1971–80). The results show that the seasonal variations of SST and local winds are closely related. Sea-breeze circulation is intensified by the enhancement of the land–sea temperature gradient due to cold water upwelling near the coast; coastal upwelling, in turn, is associated with strong northeasterlies. This result confirms the conclusions of earlier modeling studies. Interannual variability is also apparent in the results. During the period from 1971 to 1980, the highest SST values occur during the years 1972–73 (strong El Nino event) and the lowest occur in 1977 (moderate El Nino event). This suggests some possible effects of atmospheric teleconnections on South Atlantic SSTs. However, a record longer than 10 yr is needed to confirm the connection with El Nino and La Nina events. Time–frequency analyses of the SST and zonal wind series f...

CLIMATIC CHANGE DUE TO LAND SURFACE ALTERATIONS

A primitive equations global zonally averaged climate model is developed. The model includes biofeedback mechanisms. For the Northern Hemisphere the parameterization of biofeedback mechanisms is similar to that used by Gutman et al. (1984). For the Southern Hemisphere new parameterizations are derived. The model simulates reasonably well the mean annual zonally averaged climate and geobotanic zones.Deforestation, desertification and irrigation experiments are performed. In the case of deforestation and desertification there is a reduction in the surface net radiation, evaporation and precipitation and an increase in the surface temperature. In the case of irrigation experiment opposite changes occurred. In all the cases considered the changes in evapotranspiration overcome the effect of surface albedo modification. In all the experiments changes are smaller in the Southern Hemisphere.

Seasonal Variations in the Southern Hemisphere Storm Tracks and Associated Wave Propagation

Abstract Seasonal variations in the Southern Hemisphere (SH) storm track characteristics and associated wave propagation are studied using 19 years of NCEP–NCAR gridded data. It is found that the SH storm track is strongest in the austral autumn season and weakest in spring. The characteristics of wave packets are studied by computing 1-point lag correlation maps with unfiltered meridional wind at 300 hPa. It is found that the eastward group velocities of waves are much higher than the phase velocities in the transition seasons. This shows that the downstream development occurs throughout the year and is a basic feature of upper-tropospheric waves in the midlatitudes of the SH. This extends previous studies made for winter and summer seasons. Based on the indices that show wave coherence and correlation maps, it is found that the pathways in the transition seasons split into two branches east of Australia, in a way similar to what happens in the zonal wind distribution.

Interannual variations of rainfall and corn yields in Northeast Brazil

Abstract Interannual variations of rainfall in Northeast (NE) Brazil are studied giving emphasis to the unusually heavy rainfall during 1984 and 1985. It is found that a more southward location of the Inter-tropical Convergence Zone is probably responsible for the higher rainfall in 1984 and 1985. In 6 of the 9 states, rainfall correlates significantly with com yields. In 7 of the 9 states, the Southern Oscillation (SO) index shows a strong positive correlation with annual corn yields. This suggests that the SO index can be used to predict annual corn yields in NE Brazil. The feasibility of predicting annual corn yields in Northeast Brazil from prior observations of the Southern Oscillation index is examined. It is found that good success can be obtained in some states.

A Diagnosis of Rainfall over South America during the 1997/98 El Niño Event. Part I: Validation of NCEP–NCAR Reanalysis Rainfall Data

Abstract A comparison between the National Centers for Environmental Predictions–National Center for Atmospheric Research (NCEP–NCAR) reanalysis rainfall data and the Agencia Nacional de Energia Eletrica (ANEEL) rain gauge data over Brazil is made. It is found that over northeast Brazil, NCEP–NCAR rainfall is overestimated. But over south and southeast Brazil, the correlation between the two datasets is highly significant showing the utility of NCEP–NCAR rainfall data. Over other parts of Brazil the validity of NCEP–NCAR rainfall data is questionable. A detailed comparison between NCEP–NCAR rainfall data over northwest South America and rain gauge data showed that NCEP–NCAR rainfall data are useful despite important differences between the characteristics in the two data sources. NCEP–NCAR reanalysis data seem to have difficulty in correctly reproducing the strength and orientation of the South Atlantic convergence zone.

A Coupled Biosphere–Atmosphere Climate Model Suitable for Studies of Climatic Change Due to Land Surface Alterations

Abstract A biosphere model based on BATS (Biosphere–Atmosphere Transfer Scheme) is coupled to a primitive equation global statistical–dynamical model in order to study the climatic impact due to land surface alterations. The fraction of the earth’s surface covered by each vegetation type according to BATS is obtained for each latitude belt. In the control experiment, the mean annual zonally averaged climate is well simulated when compared with observations. Deforestation and desertification experiments are performed. In the deforestation experiment, the evergreen broadleaf tree in the Amazonian region is substituted by short grass; in the desertification experiment the semidesert, and the tall grass and deciduous shrubs are substituted by desert and semidesert in the African continent, respectively. The results show that in both the experiments there is a reduction in evapotranspiration and precipitation in the perturbed region and an increase in the soil surface temperature, the temperature of the foliag...

Rainy-Season Duration Estimated from OLR versus Rain Gauge Data and the 2001 Drought in Southeast Brazil

Large precipitation deficits observed during the 2001 austral summer over the southeast region of Brazil contributed to the worsening of the energy crisis that was occurring in the country, with unprecedented social and economic consequences. Reliable information on the beginning of the rainy season was essential for the Brazilian government to manage the energy crisis. The purpose of this study is to determine the rainy season in this region and to point out the risk of using outgoing longwave radiation (OLR) data to estimate the beginning of it. The results show that when OLR data are used the beginning and the end dates of the rainy season are wrongly anticipated and delayed, respectively. The present study aims to provide useful information for the management of the impact of adverse climate conditions such as the one in 2001 by basing the analysis on rainfall data instead of on OLR.

Is the recent increasing trend of rainfall over northeast Brazil related to sub-Saharan drought?

Abstract In recent years, there has been an increase in rainfall over northeast Brazil (Nordeste), while over the sub-Saharan region there has been a drought. The correlation coefficients between the 11-yr running means of the rainfall series over the two regions are significant (at the 95% confidence level by a two-sided t test), suggesting that both trends are related. The rainfall variations over the two regions are connected to the position of the intertropical convergence zone (ITCZ) over the Atlantic. A more southward (northward) position of the ITCZ is favorable for higher than normal rainfall over Nordeste (sub-Sahara). The correlation coefficient between the position of the ITCZ over the Atlantic and the rainfall over Nordeste (sub-Sahara) is negative (positive) and highly significant, reaching values over 0.9. Thus, this study suggests that a more southward than normal location of the ITCZ in the Atlantic may be the cause for the recent increasing (decreasing) trend of rainfall over Nordeste (su...

A Diagnosis of Rainfall over South America during the 1997/98 El Niño Event. Part II: Roles of Water Vapor Transport and Stationary Waves

Abstract A diagnosis of rainfall over South America (SA) during the 1997/98 El Nino year is made examining the roles of water vapor transport and stationary waves. It is found that the low-level jet (LLJ) on the eastern side of the central Andes is stronger during the El Nino event and transports more moisture. This seems to be the source for higher rainfall over southeast SA noted during the El Nino years. A calculation of three-dimensional stationary wave activity (Fs) for 1997 and 1998 showed that in the summer of 1996/97 stationary waves propagate poleward and eastward from midlatitudes into the higher latitudes to the west of south SA and then propagate equatorward to the east of SA. During the autumn of 1997, the vertical component of Fs is consistent with a blocking high over the southeast Pacific. To the east of this high cold air advection from Antarctica occurs, and to the west warm air advection occurs. This is consistent with negative and positive centers seen in the vertical component of Fs t...

Tropical land savannization: impact of global warming

This study investigates the impact of global warming on the savannization of the tropical land region and also examines the relative roles of the impact of the increase of greenhouse gas concentration and future changes in land cover on the tropical climate. For this purpose, a mechanistic–statistical–dynamical climate model with a bidirectional interaction between vegetation and climate is used. The results showed that climate change due to deforestation is more than that due to greenhouse gases in the tropical region. The warming due to deforestation corresponds to around 60% of the warming in the tropical region when the increase of CO2 concentration is included together. However, the global warming due to deforestation is negligible. On the other hand, with the increase of CO2 concentration projected for 2100, there is a lower decrease of evapotranspiration, precipitation and net surface radiation in the tropical region compared with the case with only deforestation. Differently from the case with only deforestation, the effect of the changes in the net surface radiation overcomes that due to the evapotranspiration, so that the warming in the tropical land region is increased. The impact of the increase of CO2 concentration on a deforestation scenario is to increase the reduction of the areas covered by tropical forest (and a corresponding increase in the areas covered by savanna) which may reach 7.5% in future compared with the present climate. Compared with the case with only deforestation, drying may increase by 66.7%. This corroborates with the hypothesis that the process of savannization of the tropical forest can be accelerated in future due to global warming.