This study focuses on modulation of lightning and convective vertical structure in the southern Amazon as a function of the South American monsoon V index (VI). Four wet seasons (December–March 1998–2001) of Tropical Rainfall Measuring Mission (TRMM) Lightning Imaging Sensor (LIS) and Precipitation Radar (PR) data are examined together with two wet seasons (2000–01) of ground-based Brazilian Lightning Detection Network (BLDN) data. These observations are composited by VI phase (northerly or southerly) for a region of the southern Amazon and discussed relative to VI-regime environmental characteristics such as thermodynamic instability and wind shear. Relative comparisons of VI-regime convective properties reveal 1) slightly larger (20%–25%) PR pixel-mean rainfall during periods of northerly VI due to increased stratiform precipitation, 2) a factor of 2 or more increase in lightning flash density and the lightning diurnal cycle amplitude during periods of southerly VI, 3) a factor of 1.5–2 increase in the conditional probability of any PR radar reflectivity pixel exceeding 30 dBZ above the −10°C level during periods of southerly VI, and 4) an associated factor of 2 or more increase in southerly VI pixel-mean ice water path, with the ice water path being highly correlated to trends in lightning activity. During periods of southerly VI, convection occurs in an environment of increased thermodynamic instability, weak southeasterly low-level, and deep upper-tropospheric easterly wind shear. During periods of northerly VI, low-level westerly shear opposes stronger deep tropospheric easterly shear in a relatively moist environment of weaker thermodynamic instability, consistent with the occurrence of more widespread stratiform precipitation. The composite results of this study point to 1) regime differences in convective forcing that alter the prevalence of ice processes and, by inference, the vertical profile of latent heating and 2) the utility of lightning observations in delineating convective regime changes.

The global climate models for the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) predict very different changes of rainfall over the Amazon under the SRES A1B scenario for global climate change. Five of the eleven models predict an increase of annual rainfall, three models predict a decrease of rainfall, and the other three models predict no significant changes in the Amazon rainfall. We have further examined two models. The UKMO-HadCM3 model predicts an El Nin˜o-like sea surface temperature (SST) change and warming in the northern tropical Atlantic which appear to enhance atmospheric subsidence and consequently reduce clouds over the Amazon. The resultant increase of surface solar absorption causes a stronger surface sensible heat flux and thus reduces relative humidity of the surface air. These changes decrease the rate and length of wet season rainfall and surface latent heat flux. This decreased wet season rainfall leads to drier soil during the subsequent dry season, which in turn can delay the transition from the dry to wet season. GISS-ER predicts a weaker SST warming in the western Pacific and the southern tropical Atlantic which increases moisture transport and hence rainfall in the Amazon. In the southern Amazon and Nordeste where the strongest rainfall increase occurs, the resultant higher soil moisture supports a higher surface latent heat flux during the dry and transition season and leads to an earlier wet season onset.

During boreal summer, much of the water vapor and CO entering the global tropical stratosphere is transported over the Asian monsoon/Tibetan Plateau (TP) region. Studies have suggested that most of this transport is carried out either by tropical convection over the South Asian monsoon region or by extratropical convection over southern China. By using measurements from the newly available National Aeronautics and Space Administration Aura Microwave Limb Sounder, along with observations from the Aqua and Tropical Rainfall-Measuring Mission satellites, we establish that the TP provides the main pathway for cross-tropopause transport in this region. Tropospheric moist convection driven by elevated surface heating over the TP is deeper and detrains more water vapor, CO, and ice at the tropopause than over the monsoon area. Warmer tropopause temperatures and slower-falling, smaller cirrus cloud particles in less saturated ambient air at the tropopause also allow more water vapor to travel into the lower stratosphere over the TP, effectively short-circuiting the slower ascent of water vapor across the cold tropical tropopause over the monsoon area. Air that is high in water vapor and CO over the Asian monsoon/TP region enters the lower stratosphere primarily over the TP, and it is then transported toward the Asian monsoon area and disperses into the large-scale upward motion of the global stratospheric circulation. Thus, hydration of the global stratosphere could be especially sensitive to changes of convection over the TP.

Some of the results from the climate component of the Large Scale Biosphere-Atmosphere Experiment in Amazonia (LBA), which are presented in this Special Issue are summarised. Recent advances in Amazonian climate modelling are also discussed. There is a range of papers which fall into three groups: surface fluxes and boundary layer growth; convection, clouds and rainfall; and climate modelling. The new insight given by this work is discussed and an argument is made for future research to employ a wider approach to Amazonian climate modelling

By analyzing the 15-yr (1979-93) reanalysis data of the European Centre for Medium-Range Weather Forecasts (ECMWF), it has been found that the seasonal and synoptic time-scale variations of the South American low-level jets (LLJs) are largely controlled by an upper-level trough and associated low-level zonal flow, rather than by horizontal temperature gradients along the slope of the Andes. The northerly LLJs are maintained by strong zonal pressure gradients caused by the upstream trough and westerly flow crossing the Andes through lee cyclogenesis. The process involves both baroclinic development of the upper-level trough and mechanical deflection of the westerly flow by the Andes. When an anticyclonic circulation replaces the trough and westerly flow over the eastern South Pacific, the northerly LLJs tend to diminish or reverse into southerly LLJs. The dependence of the LLJs upon the upstream wind pattern helps to explain how the seasonal variation of the South American LLJs is related to the seasonal changes of the large-scale circulation pattern over the eastern South Pacific. On synoptic time scales, the relation between LLJs and cross-Andes zonal flow is strong in austral winter, spring, and fall. This relation weakens somewhat in summer, when Amazon convection is strongest. The analysis also demonstrated strong connections of the LLJs with South American precipitation, intensity of the South Atlantic convergence zone (SACZ), and low-level cross-equatorial flow. A method for up to 5-day forecasts of the LLJs based on 700-hPa zonal winds over the subtropical eastern South Pacific was also introduced. A cross validation indicates a certain degree of predictability for South American LLJs. The results further suggest that the upstream flow pattern over the South Pacific should be closely monitored to determine the variability of the South American LLJs.

Analysis of the fifteen years of European Centre for Medium Range Weather Forecasts (ECMWF) reanalysis suggests that the transition from dry to wet season in Southern Amazonia is initially driven by increases of surface latent heat flux. These fluxes rapidly reduce Convective Inhibition Energy (CINE) and increase Convective Available Potential Energy (CAPE), consequently providing favourable conditions for increased rainfall even before the large-scale circulation has changed. The increase of rainfall presumably initiates the reversal of the cross-equatorial flow, leading to large-scale net moisture convergence over Southern Amazonia. An analysis of early and late wet season onsets on an interannual scale shows that a longer dry season with lower rainfall reduces surface latent heat flux in the dry and earlier transition periods compared to that of a normal wet season onset. These conditions result in a higher CINE and a lower CAPE, causing a delay in the increase of local rainfall in the initiating phase of the transition and consequently in the wet season onset. Conversely, a wetter dry season leads to a higher surface latent heat flux and weaker CINE, providing a necessary condition for an earlier increase of local rainfall and an earlier wet season onset. Our results imply that if land use change in Amazonia reduces rainfall during dry and transition seasons, it could significantly delay the wet season onset and prolong the dry season.

Using 15-yr instantaneous European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA) data, the authors have examined the large-scale atmospheric conditions and the local surface fluxes through the transition periods from the dry to wet seasons over the southern Amazon region (5–15S, 45–75W). The composite results suggest that the transition can be divided into three phases: initiating, developing, and mature. The initiating phase is dominated by the local buildup of the available potential energy. This begins about 90 days prior to the onset of the wet season by the increase of local land surface fluxes, especially latent heat flux, which increases the available potential energy of the lower troposphere. The cross-equatorial flow and upper- tropospheric circulation remain unchanged from those of the dry season. The developing phase is dominated by the seasonal transition of the large-scale circulation, which accelerates by dynamic feedbacks to an increase of locally thermal-driven rainfall, starting about 45 days before the onset of the wet season. During this stage, the reversal of the low-level, cross-equatorial flow in the western Amazon increases moisture transport from the tropical Atlantic Ocean and leads to net moisture convergence in the southern Amazon region. In the upper troposphere, the divergent kinetic energy begins to be converted into rotational kinetic energy, and geopotential height increases rapidly. These processes lead to the onset of the wet season and the increase of anticyclonic vorticity at the upper troposphere. After onset, the lower-tropospheric potential energy reaches equilibrium, but the conversion from divergent to rotational kinetic energy continues to spin up the upper-tropospheric anticyclonic circulation associated with the Bolivian high until it reaches its full strength. This analysis suggests that a weaker (stronger) increase of land surface latent (sensible) heat flux during the dry season and the initiating phase tends to delay the large-scale circulation transition over the Amazon. The influence of land surface heat fluxes becomes secondary during the developing and mature phases after the transition of the large-scale circulation begins. A later northerly reversal and/or weaker cross-equatorial flow, a later southerly withdrawal of the upper-tropospheric westerly wind, and a stronger subsidence could delay and prolong the developing phase of the transition and consequently delay the onset of the Amazon wet season.

The relationship between South American precipitation and cross-equatorial flow over the western Amazon is examined using the 15-yr (1979–93) European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis dataset. A meridional wind index, the V index, is constructed to represent the variability of the cross- equatorial flow, based on area-averaged (5S–5N, 65–75W) daily 925-hPa meridional winds. The V index displays large submonthly, seasonal, and interannual variabilities, and correlates well with precipitation over South America. Two circulation regimes are identified, that is, a southerly regime and a northerly regime. Linear regression shows that when the V index is southerly, precipitation is mainly located to the north of the equator. When the V index is northerly, precipitation shifts toward the Amazon basin and subtropical South America. The V index is predominately southerly in austral winter and northerly in summer. The onset (demise) of the Amazon rainy season is led by an increase in the frequency of the northerly (southerly) V index. The relation between the V index and upper-level circulation is consistent with the seasonal cycle of the South American monsoon circulation. Hence, the V index is a good indicator for precipitation change over tropical and subtropical South America. The singular value decomposition (SVD) analysis suggests that the V-index-related variation represents 92% of the total covariance between the low-level meridional wind and precipitation over South America. It also represents 37% of the seasonal variance of precipitation. On the seasonal timescale, the V index appears to correlate well with the meridional migration of the Hadley cell globally. On submonthly timescales, the change of V index is not correlated with the meridional wind over the adjacent oceans except in the South Atlantic convergence zone, suggesting a control by more localized and higher-frequency dynamic processes. The SVD analysis also suggests that during spring and fall precipitation changes over the equatorial eastern Amazon are associated with the seasonal variations of sea surface temperature in the Pacific and the Atlantic Oceans.

Although the correlation between precipitation over tropical South America and sea surface temperatures (SSTs) over the Pacific and Atlantic has been documented since the early twentieth century, the impact of each ocean on the timing and intensity of the wet season over tropical South America and the underlying mechanisms have remained unclear. Numerical experiments have been conducted using the National Center for Atmospheric Research Community Climate Model Version 3 to explore these impacts. The results suggest the following. 1) Seasonality of SSTs in the tropical Pacific and Atlantic has an important influence on precipitation in the eastern Amazon during the equinox seasons. The eastern side of the Amazon is influenced both by the direct thermal circulation of the Atlantic intertropical convergence zone (ITCZ) and by Rossby waves. These pro- cesses are enhanced by the seasonal cycles of SSTs in the tropical Atlantic and Pacific. SSTs affect Amazon precipitation much less during the solstice seasons and in the western Amazon. 2) The seasonality of SSTs in the Atlantic more strongly affects Amazon rainfall than does that of the Pacific. Without the former, austral spring in the eastern equatorial Amazon would be a wet season, rather than the observed dry season. As a consequence of the lag at that time of the southward seasonal migration of the Atlantic SSTs behind that of the insolation, the Atlantic ITCZ centers itself near 10N, instead of at the equator, imposing subsidence and low-level anticyclonic flow over the eastern equatorial Amazon, thus drying the air above the planetary boundary layer and reducing the low-level moisture convergence. Consequently, convection in the eastern Amazon is suppressed despite strong surface heating. 3) Seasonality of the SSTs in the tropical Pacific also tends to reduce precipitation in the eastern Amazon during both spring and fall. In spring, subsidence is enhanced not only through a zonal direct circulation, but also through Rossby waves propagating from the extratropical South Pacific to subtropical South America. This teleconnection strengthens the South Atlantic convergence zone (SACZ) and the Nordeste low, in both cases reducing precipitation in the eastern Amazon. A direct thermal response to the Pacific SSTs enhances lower- level divergence and reduces precipitation from the northern tropical Atlantic to the northeastern Amazon

This study examines the interannual variability of winter upper-troposphere water vapor over the Northern Hemisphere using the National Aeronautics and Space Administration Water Vapor Project, the International Satellite Cloud Climatology Project data, and the European Centre for Medium-Range Weather Forecasting reanalysis. The El Nino-Southern Oscillation related tropical sea surface temperature (SST) anomalies dominate the upper-troposphere water vapor anomalies south of the climatological jet. The anomalies of baroclinic instability in the storm track regions, which relate to the Pacific-North American and the North Atlantic oscillation patterns, dominate those north of the climatological jet. The upper-troposphere water vapor increases in the eastern tropical Pacific, the Gulf of Mexico, and some areas of the North Atlantic with warmer tropical SST. It decreases in the subtropical and extratropical northeastern Pacific. Deep convection and vertical moisture fluxes dominate these changes. To the north of the climatological jet, stronger upper-level cyclonic flow dries the upper troposphere when the baroclinicity of the storm tracks is enhanced. Both vertical and meridional moisture transport contribute to these water Vapor anomalies in the midlatitudes. High clouds, as a possible source/sink of water vapor, respond to the tropical SST anomalies and extratropical circulation in a pattern similar to the upper-troposphere water vapor, and they consequently positively correlate to the latter. In the Tropics and extratropics where high clouds are relatively abundant, water vapor concentration increases with temperature. Thus, the increase of evaporation or sublimation of high clouds probably contributes to the observed moistening of the upper troposphere, in addition to enhanced vapor transport. Conversely: in the subtropics where high clouds appear infrequently, water vapor concentration decreases with temperature, suggesting that the downward advection of drier air associated with subsidence dominates the drying of the upper troposphere.

The characteristics of winter monthly mean extratropical circulation associated with Fl Nino, including precipitation and surface temperature over the United States, are examined for nine Fl Nino events during 1950-94. Precipitation and surface temperature over the United States, also the 500-mb geopotential height and sea level pressure over the North Pacific and North America, are significantly different between early winter (November and December) and late winter (January to March). The typical Fl Nino-related U.S. precipitation and surface temperatures identified in many previous studies, as well as the Pacific-North American (PNA) circulation pattern, emerge in January and persist through February and March. The PNA patterns during these late winter months are coupled both with the tropical Fl Nino sea surface temperature (SST) variation and with the North Pacific SST variation. In contrast, the PNA patterns in the early winter months correlate only with the North Pacific SST. The tendency for the PNA pattern to occur during Fl Nino years is much less in early winter months than in lace winter months. An ensemble analysis of 12 45-yr (1950-94) integrations of the National Center for Atmospheric Research Community Climate Model forced by the observed time-varying SST shows that the model 500-mb heights display a PNA-like pattern in both early and rate winters of Fl Nino. The ensemble model response to the Fl Nino SST is thus unable to reproduce the observed differences in the extratropical atmospheric circulation between early and late winter months.

Comparison of the upper tropospheric relative humidity (UTH) of CCM2 forced by observed sea surface temperatures (SSTs) with data from TOVS, MSU, and the GEOS-1 assimilation shows that the model reproduces the observed large-scale pattern of interannual changes of UTH in the tropics, but gives too much drying during the 1987 EI Nino. How UTH responds to the changes of precipitation is well-simulated in the areas of anomalous subsidence and drying in the subtropical Pacific. However, the exaggerated. increase of precipitation in the model in the equatorial central Pacific leads to excessive amplitudes of vertical motion, which dries the subtropical upper troposphere. Since the uplift does not result in excessive moistening in the equatorial Pacific, the effect of the too vigorous hydrological cycle is an unrealistically large reduction of UTH in the tropics during an El Nine in CCM2.

Connections between the large-scale interannual variations of clouds, deep convection, atmospheric winds. vertical thermodynamic structure, and SSTs over global tropical oceans are examined over the period July 1983 - December 1990. The SST warming associated with El Nino had a significant impact on the global tropical cloud field, although the warming itself was confined to the equatorial central and eastern Pacific. Extensive variations of the total cloud field occurred in the northeastern Indian, western and central Pacific, and western Atlantic Oceans. The changes of high and middle clouds dominated the total cloud variation in these regions. Total cloud variation was relatively weak in the eastern Pacific and the Atlantic because of the cancellation between the changes of high and low clouds. The variation of low clouds dominated the total cloud change in those areas. The destabilization of the lapse rate between 900 and 750 mb was more important for enhancing convective instability than was the change of local SSTs in the equatorial central Pacific during the 1997 El Nino. This destabilization is associated with anomalous rising motion in that region. As a result. convection and high and middle clouds increased in the equatorial central Pacific, In the subtropical Pacific, both the change of lapse rate between 900 and 750 mb associated A,ith anomalous subsidence and the decrease of boundary-layer buoyancy due to a decrease of temperature and moisture played an important role in enhancing convective stability. Consequently, convection, as well its high and middle clouds, decreased in these areas. The change ot'low clouds in the equatorial and southeastern Atlantic was correlated to both local SSTs and the SST changes in the equatorial eastern Pacific. In this area. the increase of low clouds was consistent with the sharper inversion during the 1987 El Nino, The strengthening of the inversion was not caused by a local SST change. although the local SST change appeared to he correlated to the change of low clouds. The coherence between clouds and SST tendency shows that SST tendency leads cloud variation in the equatorial Pacific. Thus, the change of clouds does not dominate the sign of SST tendency even though the cloud change was maximum during the 1987 El Nino. In some ideas of the Indian, subtropical Pacific, and North Atlantic Oceans, cloud change leads SST tendency. Cloud change might affect SST tendency in these regions.

This paper combines satellite measurements of the upwelling 6.7-m radiance from \TOVS\ with cloud-property information from \ISCCP\ and outgoing longwave radiative fluxes from \ERBE\ to analyze the climatological interactions between deep convection, upper-tropospheric humidity, and atmospheric greenhouse trapping. The satellite instruments provide unmatched spatial and temporal coverage, enabling detailed examination of regional, seasonal, and interannual variations between these quantities. The present analysis demonstrates that enhanced tropical convection is associated with increased upper-tropospheric relative humidity. The positive relationship between deep convection and upper-tropospheric humidity is observed for both regional and temporal variations, and is also demonstrated to occur over a wide range of space and time scales. Analysis of \ERBE\ outgoing longwave radiation measurements indicates that regions or periods of increased upper-tropospheric moisture are strongly correlated with an enhanced greenhouse trapping, although the effects of lower-tropospheric moisture and temperature lapse rate are also observed to be important. The combined results for the Tropics provide a picture consistent with a positive interrelationship between deep convection, upper-tropospheric humidity, and the greenhouse effect. In extratropical regions, temporal variations in upper-tropospheric humidity exhibit little relationship to variations in deep convection, suggesting the importance of other dynamical processes in determining changes in upper-tropospheric moisture for this region. Comparison of the observed relationships between convection, upper-tropospheric moisture, and greenhouse trapping with climate model simulations indicates that the Geophysical Fluid Dynamics Laboratory (\GFDL)\ \GCM\ is qualitatively successful in capturing the observed relationship between these quantities. This evidence supports the ability of the \GFDL\ \GCM\ to predict upper-tropospheric water vapor feedback, despite the model's relatively simplified treatment of moist convective processes.

The characteristic features, the diurnal cycle, and the spatial distribution of deep convection over the equatorial Pacific and the relationship of deep convection to SST and surface-wind convergence were examined using a combined visible-IR (VS-IR) threshold method and an IR-only threshold method for diagnosing deep convection clouds (DCCs). Results suggest that deep convection is latitudinally confined to a much smaller spatial scale than that suggested by maps of outgoing long-wave radiation. The results suggested that there are two types of relationships between deep convection, SST, and surface-wind convergence: the west Pacific type and the east Pacific type. The latter relationship is observed in the east Pacific only when SST is not abnormally warm.