V. Brahmananda Rao

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...

Zonal variations of peak ionization rates in upper atmosphere of Mars at high latitude using Mars Global Surveyor accelerometer data

[1] The Accelerometer Experiment onboard Mars Global Surveyor (MGS) measured many density profiles in the upper atmosphere of Mars during aerobraking at many latitudes, longitudes, altitudes, local solar time (LST), and seasons. Here, in this paper, we use the accelerometer data of 57 orbits (P0588-P0648) from 30 September 1998 to 24 October 1998 between latitude ranges (50°-70°N) at LST 1600 hours, under spring equinox and medium solar activity conditions (average F 10.7 ∼ 120). Using these densities, the neutral densities of different gases are derived from their mixing ratio. From these neutral densities the longitudinal distribution of peak photoionization rates, peak photoelectron impact ionization rates, and the total peak ionization rates of CO + 2 , N + 2 and O + are obtained for solar zenith angle 78° at wavelength range 10-1025.7 A due to solar EUV radiation using analytical yield spectrum approach (AYS). These calculations are made at different altitudes and longitudes starting from 115 to 220 km and 0° to 360°E using intervals of 0.1 km and 5°, respectively. These conditions are appropriate for MGS phase 2 aerobraking period from which the accelerometer data are used. The Fourier analysis of the various peak ionization rates of CO + 2 , N + 2 , and O + indicates the presence of two dominant harmonic regions at high latitude in the upper atmosphere of Mars. The first is a class of long planetary-scale waves that may be associated with the fixed topography of Martian surface. The second is a class of rapidly moving transient disturbances that may be associated with baroclinic instability processes.

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.

Amazonian Deforestation: Impact of Global Warming on the Energy Balance and Climate

AbstractA coupled biosphere–atmosphere statistical–dynamical model is used to study the relative roles of the impact of the land change caused by tropical deforestation and global warming on energy balance and climate. Three experiments were made: 1) deforestation, 2) deforestation + 2 × CO2, and 3) deforestation + CO2, CH4, N2O, and O3 for 2100. In experiment 1, the climatic impact of the Amazonian deforestation is studied. In experiment 2, the effect of doubling CO2 is included. In experiment 3, the concentrations of the greenhouse gases (GHGs) correspond to the A1FI scenario from the Intergovernmental Panel on Climate Change Special Report on Emissions Scenarios. The results showed that the percentage of the warming caused by deforestation relative to the warming when the increase in GHG concentrations is included is higher than 60% in the tropical region. On the other hand, with the increase in GHG concentrations, a reduction in the decrease of evapotranspiration and precipitation in the tropical regi...

Genesis of the Northeast Brazil Upper-Tropospheric Cyclonic Vortex: A Primitive Equation Barotropic Instability Study

Abstract The primitive equation barotropic unstable linear normal modes are computed using an eigenvalue approach for daily latitudinal profiles of zonal flow in the upper-tropospheric layer of 100–350 hPa before and after formation of cyclonic vortices during January 1993 and November 2001 off the coast of northeast Brazil. The wave kinetic energy equation for u- and υ-motion is presented. Equations are derived to isolate the contribution of divergence and other dynamical processes in the movement and growth of unstable modes. Numerical accuracy and physical nature of unstable modes are tested. In a short span of 2–3 days, prior to formation of vortices, a progressive and a sharp intensification of the basic flow shear zone and its barotropic instability are seen with time. The horizontal structure, momentum transport, and zonal and meridional scales of the most unstable normalized wave are obtained and compared with the vortex extracted from the 200-hPa observed winds using a bandpass smoother. A close ...

Impact of high potential vorticity intrusions into the tropical upper troposphere in South Atlantic on precipitation over northeast Brazil

[1] Interannual variations of high potential vorticity intrusions at 10°S in the upper troposphere during NDJFM are shown to be negatively correlated with rainfall over northern northeast Brazil (NEB). That is, higher intrusions associated with stronger equatorial westerlies may provoke droughts. Higher intrusions lead to the formation of an anomalous cyclonic vortex over NEB, which causes convergence in the upper troposphere and sinking motion at lower levels. The number of intrusions in NDJFM has highest correlation, significant at 99% confidence level, with the rainfall of the principal rainy season in FMAM.

Comments on “Blocking over the South Pacific and Rossby Wave Propagation”

In a recent study Renwick and Revell (1999, hereafter RR99) investigated the atmospheric blocking over the South Pacific. They found that the blocking events occur more frequently over the southeast Pacific during El Niño events in austral spring. Their analysis showed that blocking events are associated with large-scale wave trains lying across the South Pacific from Australia to southern South America. RR99 performed numerical experiments with a linearized barotropic model and showed that the tropical convective heating associated with OLR anomalies, presumably generated during El Niño events, can generate similar wave trains. The purpose of the present comment is to provide observational evidence to show that the Rossby wave propagation similar to the one noted by RR99 is in fact stronger and better organized in austral spring than in other seasons. Several recent studies (Marques and Rao 1999, 2000; Renwick 1998; Sinclair 1996; Rutllant and Fuenzalida 1991) have found a new region of blocking in the South Pacific. Blocking in this region has important implications for weather over South America (Marques and Rao 1999). Rutllant and Fuenzalida (1991) noted the connection between the El Niño–Southern Oscillation (ENSO) and blocking over the southeast Pacific. Renwick (1998) found that the frequency of blocking over the southeast Pacific is related to ENSO. Using 25 years of National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis data Marques and Rao (2000) found that the blocking frequency over the southeast Pacific has a negative correlation with the Southern Oscillation index during the austral spring, indicating a higher incidence of blocking during El Niño events. We used monthly mean values of geopotential height for the period 1950–98 to obtain anomalies (observed minus mean) for the El Niño and La Niña events. Geopotential height data at several pressure levels were obtained from NCEP–NCAR reanalysis (Kalnay et al.