Ensemble simulations of a regional climate model (RegCM3) forced by aerosol radiative forcing suggest that biomass burning aerosols can work against the seasonal monsoon circulation transition, thus re-enforce the dry season rainfall pattern for Southern Amazonia. Strongly absorbing smoke aerosols warm and stabilize the lower troposphere within the smoke center in southern Amazonia (where aerosol optical depth \textgreater0.3). These changes increase the surface pressure in the smoke center, weaken the southward surface pressure gradient between northern and southern Amazonia, and consequently induce an anomalous moisture divergence in the smoke center and an anomalous convergence in northwestern Amazonia (5°S-5°N, 60°W-70°W). The increased atmospheric thermodynamic stability, surface pressure, and divergent flow in Southern Amazonia may inhibit synoptic cyclonic activities propagated from extratropical South America, and re-enforce winter-like synoptic cyclonic activities and rainfall in southeastern Brazil, Paraguay and northeastern Argentina.

We have applied a scalable and extensible geo-fluid model (SEGMENT) that considers soil mechanics, vegetation transpiration and root mechanical reinforcement, and hydrological processes to simulate two dimensional maps of the landslides occurrence following the 2008 Wenchuan earthquake. Modeled locations and areas generally agree with observations. The model suggests that the potential energy of earth was lowered by 1.52 x 10(15) J by these landslides. With this, the vegetation destroyed transfer similar to 235 Tg C to the dead respiring pool and transforms 5.54 x 10(-2) Tg N into unavailable sediments pools and the atmosphere. The cumulative CO2 release to the atmosphere over the coming decades is comparable to that caused by hurricane Katrina 2005 (similar to 105 Tg) and equivalent to similar to 2% of current annual carbon emissions from global fossil fuel combustion. The nitrogen loss is twice as much as that released by the 2007 California Fire (similar to 2.5 x 10(-2) Tg). A significant proportion of the nitrogen loss (14%) is in the form of nitrous oxide, which can affect the atmospheric ozone layer. Citation: Ren, D., J. Wang, R. Fu, D. J. Karoly, Y. Hong, L. M. Leslie, C. Fu, and G. Huang (2009), Mudslide-caused ecosystem degradation following Wenchuan earthquake 2008, Geophys. Res. Lett., 36, L05401, doi: 10.1029/2008GL036702.

The factors that control the influence of deep convective detrainment on$\backslash$nwater vapor in the tropical upper troposphere are examined using$\backslash$nobservations from multiple satellites in conjunction with a trajectory$\backslash$nmodel. Deep convection is confirmed to act primarily as a moisture$\backslash$nsource to the upper troposphere, modulated by the ambient relative$\backslash$nhumidity (RH). Convective detrainment provides strong moistening at low$\backslash$nRH and offsets drying due to subsidence across a wide range of RH.$\backslash$nStrong day-to-day moistening and drying takes place most frequently in$\backslash$nrelatively dry transition zones, where between 0.01% and 0.1% of$\backslash$nTropical Rainfall Measuring Mission Precipitation Radar observations$\backslash$nindicate active convection. Many of these strong moistening events in$\backslash$nthe tropics can be directly attributed to detrainment from recent$\backslash$ntropical convection, while others in the subtropics appear to be related$\backslash$nto stratosphere-troposphere exchange. The temporal and spatial limits of$\backslash$nthe convective source are estimated to be about 36-48 h and 600-1500 km,$\backslash$nrespectively, consistent with the lifetimes of detrainment cirrus$\backslash$nclouds. Larger amounts of detrained ice are associated with enhanced$\backslash$nupper tropospheric moistening in both absolute and relative terms. In$\backslash$nparticular, an increase in ice water content of approximately 400%$\backslash$ncorresponds to a 10-90% increase in the likelihood of moistening and a$\backslash$n30-50% increase in the magnitude of moistening.

The U.S. Climate Variability and Predictability (CLIVAR) working group on drought recently initiated a series of global climate model simulations forced with idealized SST anomaly patterns, designed to address a number of uncertainties regarding the impact of SST forcing and the role of land–atmosphere feedbacks on regional drought. The runs were carried out with five different atmospheric general circulation models (AGCMs) and one coupled atmosphere–ocean model in which the model was continuously nudged to the imposed SST forcing. This paper provides an overview of the experiments and some initial results focusing on the responses to the leading patterns of annual mean SST variability consisting of a Pacific El Niño–Southern Oscillation (ENSO)-like pattern, a pattern that resembles the Atlantic multidecadal oscillation (AMO), and a global trend pattern. One of the key findings is that all of the AGCMs produce broadly similar (though different in detail) precipitation responses to the Pacific forcing pattern, with a cold Pacific leading to reduced precipitation and a warm Pacific leading to enhanced precipitation over most of the United States. While the response to the Atlantic pattern is less robust, there is general agreement among the models that the largest precipitation response over the United States tends to occur when the two oceans have anomalies of opposite signs. Further highlights of the response over the United States to the Pacific forcing include precipitation signal-to-noise ratios that peak in spring, and surface temperature signal-to-noise ratios that are both lower and show less agreement among the models than those found for the precipitation response. The response to the positive SST trend forcing pattern is an overall surface warming over the world's land areas, with substantial regional variations that are in part reproduced in runs forced with a globally uniform SST trend forcing. The precipitation response to the trend forcing is weak in all of the models. It is hoped that these early results, as well as those reported in the other contributions to this special issue on drought, will serve to stimulate further analysis of these simulations, as well as suggest new research on the physical mechanisms contributing to hydroclimatic variability and change throughout the world.

Satellite and in situ observations in the equatorial Atlantic Ocean during 2002-03 show dominant spectral peaks at 40-60 days and secondary peaks at 10-40 days in sea level and thermocline within the intraseasonal period band (10-80 days). A detailed investigation of the dynamics of the intraseasonal variations is carried out using an ocean general circulation model, namely, the Hybrid Coordinate Ocean Model (HYCOM). Two parallel experiments are performed in the tropical Atlantic Ocean basin for the period 2000-03: one is forced by daily scatterometer winds from the Quick Scatterometer (QuikSCAT) satellite together with other forcing fields, and the other is forced by the low-passed 80-day version of the above fields. To help in understanding the role played by the wind-driven equatorial waves, a linear continuously stratified ocean model is also used. Within 3 degrees S-3 degrees N of the equatorial region, the strong 40-60-day sea surface height anomaly (SSHA) and thermocline variability result mainly from the first and second baroclinic modes equatorial Kelvin waves that are forced by intraseasonal zonal winds, with the second baroclinic mode playing a more important role. Sharp 40-50-day peaks of zonal and meridional winds appear in both the QuikSCAT and Pilot Research Moored Array in the Tropical Atlantic (PIRATA) data for the period 2002-03, and they are especially strong in 2002. Zonal wind anomaly in the central-western equatorial basin for the period 2000-06 is significantly correlated with SSHA across the equatorial basin, with simultaneous/ lag correlation ranging from-0.62 to 0.74 above 95% significance. Away from the equator (3 degrees-5 degrees N), however, sea level and thermocline variations in the 40-60-day band are caused largely by tropical instability waves (TIWs). On 10-40-day time scales and west of 10 degrees W, the spectral power of sea level and thermocline appears to be dominated by TIWs within 5 degrees S-5 degrees N of the equatorial region. The wind-driven circulation, however, also provides a significant contribution. Interestingly, east of 10 W, SSHA and thermocline variations at 10 40- day periods result almost entirely from wind-driven equatorial waves. During the boreal spring of 2002 when TIWs are weak, Kelvin waves dominate the SSHA across the equatorial basin (2 degrees S-2 degrees N). The observed quasi-biweekly Yanai waves are excited mainly by the quasi-biweekly meridional winds, and they contribute significantly to the SSHA and thermocline variations in 1 degrees-5 degrees N and 1 degrees-5 degrees S regions.

Remote sensing of land surface temperature (LST) using infrared (IR) sensors, such as the Moderate Resolution Imaging Spectroradiometer (MODIS), is only capable of retrieval under clear-sky conditions. Such LST observations over tropical forests are very limited due to clouds and rainfall, particularly during the wet season and high atmospheric water-vapor content. In comparison, low-frequency microwave radiances are minimally influenced by meteorological conditions. Exploring this advantage, we have developed an algorithm to retrieve LST over the Amazonian forest. The algorithm uses multifrequency polarized microwave brightness temperatures from the Advanced Microwave Scanning Radiometer on NASA's Earth Observing System (AMSR-E). Relationships between polarization ratio and surface emissivity are established for forested and nonforested areas, such that LST can solely be calculated from microwave radiance. Results are presented over three time scales: at each orbit, daily, and monthly. Results are evaluated by comparing with available air-temperature records on daily and monthly intervals. Our findings indicate that the AMSR-E-derived LST agrees well with in situ measurements. Results during the wet season over the tropical forest suggest that the AMSR-E LST is robust under all-weather conditions and shows higher correlation to meteorological data (r = 0.70) than the IR-based LST approaches (r = 0.42).

Summer convective systems (CSs) initiated over the Tibetan Plateau$\backslash$nidentified by the International Satellite Cloud Climatology Project$\backslash$n(ISCCP) deep convection database and associated Tropical Rainfall$\backslash$nMeasuring Mission (TRMM) precipitation for 1998-2001 have been analyzed$\backslash$nfor their basic characteristics in terms of initiation, distribution,$\backslash$ntrajectory, development, life cycle, convective intensity, and precipitation.$\backslash$nSummer convective systems have a dominant center over the Hengduan$\backslash$nMountain and a secondary center over the Yaluzangbu River Valley.$\backslash$nPrecipitation associated with these CSs contributes more than 60%$\backslash$nof total precipitation over the central-eastern area of the Tibetan$\backslash$nPlateau and 30%-40% over the adjacent region to its southeast. The$\backslash$naverage CS life cycle is about 36 h; 85% of CSs disappear within$\backslash$n60 h of their initiation. About 50% of CSs do not move out of the$\backslash$nTibetan region, with the remainder split into eastward and southward-moving$\backslash$ncomponents. These CSs moving out the Tibetan Plateau are generally$\backslash$nlarger, have longer life spans, and produce more rainfall than those$\backslash$nstaying inside the region. Convective system occurrences and associated$\backslash$nrainfall present robust diurnal variations. The midafternoon maximum$\backslash$nof CS initiation and associated rainfall over the plateau is mainly$\backslash$ninduced by solar heating linked to the unique Tibetan geography.$\backslash$nThe delayed afternoon-late night peak of rainfall from CSs propagating$\backslash$nout of this region is a combined outcome of multiple mechanisms working$\backslash$ntogether. Results suggest that interactions of summer Tibetan CSs$\backslash$nwith the orientation of the unique Tibetan geography and the surrounding$\backslash$natmospheric circulations are important for the development, intensification,$\backslash$npropagation, and life span of these CSs.

The extent of evapotranspiration (E T) over the Brazilian Amazon rainforest remains uncertain because in situ measurement sites do not cover the entire domain, and the fetch of these sites is only of the order of 103m. In this investigation we developed an empirical method to estimate E T over the Brazilian Legal Amazon (BLA). The work was based on an improved physical understanding of what controls E T over the Amazonia rainforest resulting from analyses of recent in situ observations. Satellite data used in this study include the Enhanced Vegetation Index (EVI) from the Moderate Resolution Imaging Spectroradiometer (MODIS) and the surface radiation budget from the International Satellite Cloud Climatology Project (ISCCP). The empirical model was validated by measurements performed at four upland forest sites. The observed values and the calculated modelled values at these sites had the same mean and variance. On a seasonal scale, regional modelled E T peaks during the austral spring (September to November), as reported in the literature. In addition, the empirical model allows us to estimate the regional seasonal and interannual distributions of E T/precipitation rates.

The surface water budget of the Amazon River basin derived from the$\backslash$nERA40 reanalysis is evaluated by comparing it with observed precipitation$\backslash$n(P), streamflow/runoff (R), and evapotranspiration (ET) data sets$\backslash$nfor the period of 1980&\#8211;2002. The rainfall is averaged over$\backslash$n90% of the Amazon River basin, corresponding to the catchments of$\backslash$nthe �bidos and Altamira streamflow gauges. The annual rainfall and$\backslash$nthe interannual changes from ERA40 fall within the range of the two$\backslash$nprecipitation data sets. On the seasonal timescale, ERA40 reproduces$\backslash$nwell the rainfall during the dry and transition seasons, but it underestimates$\backslash$nthe wet season rainfall by 4&\#8211;11% when compared with the$\backslash$ntwo precipitation data sets. On the subbasin scale, the disparity$\backslash$nin precipitation between ERA40 and observations is as much as �40%.$\backslash$nThe annual runoff integrated over the two catchments is underestimated$\backslash$nin ERA40 by 25%. The rain-rates in ERA40, which affect both throughfall$\backslash$nand runoff, are comparable to those measured by the Tropical Rainfall$\backslash$nMeasurement Mission (TRMM 3B42V6), when these are rescaled to the$\backslash$nresolution of the 2.5� ERA40 data. However, even the native resolution$\backslash$nof ERA40 (&\#8764;1.125�) is greater than the scale of tropical$\backslash$nconvection. ET in ERA40 appears to be higher than observations by$\backslash$nabout 20%, although observed ET may have a 10% low bias. The difference$\backslash$nbetween precipitation and runoff, P-R, in ERA40 generally agrees$\backslash$nwith observations. However, annual ERA40 ET is greater than P-R,$\backslash$nbecause soil moisture nudging adds water to the soil. On the seasonal$\backslash$nscale, soil moisture nudging is largest during the dry season, because$\backslash$nERA40 provides only a 45 mm surplus of P-R relative to ET during$\backslash$nthe wet season, whereas the deficit in the dry season is almost four$\backslash$ntimes greater. This low bias in wet season soil moisture recharge$\backslash$nmay be caused by the underestimation of wet season rainfall in ERA40.$\backslash$nIt is possible that the model interception may have a high bias,$\backslash$nwhich contributes to the high ET in the rainy season and reduces$\backslash$nthe wet season storage.

Observations show that the standard precipitation index (SPI) over the southern Amazon region decreased in the period of 1970-1999 by 0.32 per decade, indicating an increase in dry conditions. Simulations of constant pre-industrial climate with recent climate models indicate a low probability (p=0%) that the trends are due to internal climate variability. When the 23 models are forced with either anthropogenic factors or both anthropogenic and external natural factors, approximately 13% of sampled 30-year SPI trends from the models are found to be within the range of the observed SPI trend at 95% confidence level. This suggests a possibility of anthropogenic and external forcing of climate change in the southern Amazon. On average, the models project no changes in the frequency of occurrence of low SPI values in the future; however, those models which produce more realistic SPI climatology, variability and trend over the period 1970-1999 show more of a tendency towards more negative values of SPI in the future. The analysis presented here suggests a potential anthropogenic influence on Amazon drying, which warrants future, more in-depth, study.

Knowledge on atmospheric precipitable water is necessary as input to hydrological, energetic and radiation models. Short historical review of parameterization of precipitable water is given. For Tallinn (Estonia), simple formulas are proposed to calculate precipitable water from observations of surface water vapor pressure. Seasonal changes of precipitable water in Tallinn are expressed by time series for 1990-2001 as well as by tabulation of monthly averages for this period. Parameterization of precipitable water, decadal time series, and tabulation of monthly averages are also given for three neighboring stations -St. Petersburg (Russia), Jokioinen and Sodankylä (Finland).

The applicability of NASA QuikSCAT ocean surface wind was tested for predicting South American low-level jets (SALLJs) with a statistical model. Our previous study (Wang, H. and Fu, R., 2004, Influence of cross-Andes flow on the South American low-level jet. Journal of Climate, 17, pp. 1247–1262) has examined the dynamic process associated with austral winter SALLJs using the ECMWF Reanalyses (ERA) and identified the mechanism that controls the seasonal and synoptic variations of the SALLJ. It was found that the SALLJ is maintained by strong zonal pressure gradients, with a maximum near 850 hPa caused by deflection of upstream zonal flow crossing the Andes and lee cyclogenesis. The robustness of this mechanism was further examined in this study using the NCEP–NCAR Reanalysis 1 (NCEP-R1) and NCEP–DOE Reanalysis 2 (NCEP-R2). The northerly LLJs to the east of the Andes are strongest in ERA, with wind speeds well above 15 m s-1. In NCEP-R1 and R2, typical wind speeds are about 12 and 10 m s-1, respectively. A statistical analysis of the three reanalysis datasets indicates that the SALLJ significantly correlates with the zonal wind of previous days over the South Pacific, particularly with the surface zonal wind. Based on this result, a statistical model introduced in Wang and Fu (2004) was employed in this study for forecasting the SALLJ using the QuikSCAT ocean surface wind as a predictor. The model was applied to June, July and August of 1999 to 2006 for up to 5 day forecasts of the SALLJ. Cross validations of the hindcasts indicate significant predictability of strong LLJ events with the QuikSCAT ocean surface wind data.

Using outgoing longwave radiation (OLR) and Tropical Rainfall Measuring Mission (TRMM) daily rain-rate data, systematic changes in intensity and location of the Atlantic intertropical convergence zone (ITCZ) were detected along the equator during boreal spring. It is found that the changes in convection over the tropical Atlantic may be induced by deep convection in equatorial South America. Lagged regression analyses demonstrate that the anomalies of convection developed over the land propagate eastward across the Atlantic and then into Africa. The eastward-propagating disturbances appear to be convectively coupled Kelvin waves with a period of 6-7.5 days and a phase speed of around 15 m s(-1). These waves modulate the intensity and location of the convection in the tropical Atlantic and result in a zonal variation of the Atlantic ITCZ on synoptic time scales. The convectively coupled Kelvin wave has substantial signals in both the lower and upper troposphere. Both a reanalysis dataset and the Quick Scatterometer (QuikSCAT) ocean surface wind are used to characterize the Kelvin wave. This study suggests that synoptic-scale variation of the Atlantic ITCZ may be linked to precipitation anomalies in South America through the convectively coupled Kelvin wave. The results imply that the changes of Amazon convection could contribute to the large variability of the tropical Atlantic ITCZ observed during boreal spring.

Rainfall in South America as simulated by a 24-ensemble member of the ECHAM 4.5 atmospheric general circulation model is compared and contrasted with observations (in areas in which data are available) for the period 19762001. Emphasis is placed on determining the onset and end of the rainy season, from which its length and rain rate are determined. It is shown that over large parts of the domain the onset and ending dates are well simulated by the model, with biases of less than 10 days. There is a tendency for model onset to occur early and ending to occur late, resulting in a simulated rainy season that is on average too long in many areas. The model wet season rain rate also tends to be larger than observed. To estimate the relative importance of errors in wet season length and rain rate in determining biases in the annual total, adjusted totals are computed by substituting both the observed climatological wet season length and rate for those of the model. Problems in the rain rate generally are more important than problems in the length. The wet season length and rain rate also contribute substantially to interannual variations in the annual total. These quantities are almost independent, and it is argued that they are each associated with different mechanisms. The observed onset dates almost always lie within the range of onset of the ensemble members, even in the areas with a large model onset bias. In some areas, though, the model does not perform well. In southern Brazil the model ensemble average onset always occurs in summer, whereas the observations show that winter is often the wettest period. Individual members, however, do occasionally show a winter rainfall peak. In southern Northeast Brazil the model has a more distinct rainy season than is observed. In the northwest Amazon the model annual cycle is shifted relative to that observed, resulting in a model bias. No interannual relationship between model and observed onset dates is expected unless onset in the model and observations has a mutual relationship with SST anomalies. In part of the near-equatorial Amazon, there does exist an interannual relationship between onset dates. Previous studies have shown that in this area there is a relationship between SST anomalies and variations in seasonal total rainfall.

The extent to which soil water storage can support an average dry season evapotranspiration (ET) is investigated using observations from the Rebio Jarú site for the period of 2000 to 2002. During the dry season, when total rainfall is less than 100 mm, the soil moisture storage available to root uptake in the top 3-m layer is sufficient to maintain the ET rate, which is equal to or higher than that in the wet season. With a normal or less-than-normal dry season rainfall, more than 75% of the ET is supplied by soil water below 1 m, whereas during a rainier dry season, about 50% of ET is provided by soil water from below 1 m. Soil moisture below 1-m depth is recharged by rainfall during the previous wet season: dry season rainfall rarely infiltrates to this depth. These results suggest that, even near the southern edge of the Amazon forest, seasonal and moderate interannual rainfall deficits can be mitigated by an increase in root uptake from deeper soil. How dry season ET varies geographically within the Amazon and what might control its geographic distribution are examined by comparing in situ observations from 10 sites from different areas of Amazonia reported during the last two decades. Results show that the average dry season ET varies less than 1 mm day1 or 30% from the driest to nearly the wettest parts of Amazonia and is largely correlated with the change of surface net radiation of 25% and 30%. Thus the geographic variation of the average dry season ET appears to be mainly determined by the surface radiation.

Li, Wenhong, Robert E. Dickinson, Rong Fu, Guo-Yue Niu, Zong-Liang Yang, and Joseph G. Canadell. 2007. Future precipitation changes and their implications for tropical peatlands. Geophys. Res. Lett. 34, L01403, doi:10.1029/2006GL028364

Aerosol and cloud data from the MODerate resolution Imaging Spectroradiometer (MODIS) onboard the Earth Observing System (EOS) Aqua are used to investigate interannual variability of smoke and warm cloud relationships during the dry-to-wet transition season (August-October) over the Amazon for two??years and its association with meteorological conditions. In one??year (2003), smoke aerosols are associated with an increase of cloud fraction and a decrease of cloud effective radius. These effects amplify the cooling at the surface and at the top of the atmosphere (TOA) caused by the aerosol extinction. However, in another year (2002) the cloud fraction decreases with increasing aerosol optical depth. Such a decrease of cloud fraction could offset the effect of increased reflection of solar radiation by the aerosols both at the surface and at TOA. The changes in radiative fluxes between these years would contribute to interannual changes of surface energy fluxes and radiative balance at the top of the atmosphere and influence variability of the wet season onset in the basin. In 2003, the atmosphere was more humid and less stable. These conditions may be relatively favorable for the activation of aerosol particles into cloud condensation nuclei and hence cloud droplets. In 2002, the clouds were less extensive and thinner in a relatively dry atmosphere and presumably dissipated more easily. This study suggests that the aerosol-cloud relation can be influenced by atmospheric structure and convective motions, in addition to changes in aerosols properties. An adequate characterization of aerosol-cloud relationship would require a longer time series of data that includes a variety of climate conditions. The caveat of this analysis is that differences in aerosol absorption and its vertical distribution may have contributed to the observed interannual change of smoke-cloud relationship but could not be determined due to lack of adequate measurements. ?? 2007 Elsevier Inc. All rights reserved.

Despite early speculation to the contrary, all tropical forests studied to date display seasonal variations in the presence of new leaves, flowers, and fruits. Past studies were focused on the timing of phenological events and their cues but not on the accompanying changes in leaf area that regulate vegetation-atmosphere exchanges of energy, momentum, and mass. Here we report, from analysis of 5 years of recent satellite data, seasonal swings in green leaf area of approximately 25% in a majority of the Amazon rainforests. This seasonal cycle is timed to the seasonality of solar radiation in a manner that is suggestive of anticipatory and opportunistic patterns of net leaf flushing during the early to mid part of the light-rich dry season and net leaf abscission during the cloudy wet season. These seasonal swings in leaf area may be critical to initiation of the transition from dry to wet season, seasonal carbon balance between photosynthetic gains and respiratory losses, and litterfall nutrient cycling in moist tropical forests.

Using 15-yr data from the European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA- 15), the authors found that rapid southeastward expansion of the rainy area from the western Amazon to southeastern Brazil is a result of midlatitude cold air intrusions. During austral spring, as the large-scale thermodynamic structure over Amazonia becomes destabilized, the incursions of extratropical cold air can trigger intense rainfall along the leading edge of northwest–southeast-oriented cold fronts east of the Andes. As these fronts penetrate into Amazonia, the northerly or northwesterly wind transports warm, moist air from the western Amazon to southeast Brazil. Moisture convergence consequently intensifies, resulting in northwest–southeast-elongated rainy areas. The latter contribute to the observed rapid, southeastward expansion of rainy areas shown in rainfall climatology during austral spring. The authors' analysis suggests that cold air intrusions during austral spring collectively assist the transformation of large-scale thermodynamic and dynamic environments to those favorable for the wet season onsets. Each time the cold fronts pass by, they tend to increase the atmospheric humidity and the buoyancy of the lower troposphere, which destabilizes the atmosphere. In the upper troposphere, the cold air intrusions supply kinetic energy for the development of anticyclonic flow. Cold air intrusions in the transitional season are not different from those occurring immediately before the wet season onsets except that the latter occurs under a more humid and unstable atmospheric condition. Thus, cold air intrusions can trigger the wet season onsets only when atmospheric and land surface conditions are “ready” for the onset. Comparisons among early, normal, and late onsets on an interannual scale further suggest that more frequent and stronger cold air intrusions trigger the early onsets of wet seasons given suitable large-scale thermodynamic conditions. Likewise, less frequent and weaker cold air intrusions could delay the wet season onset even though the large-scale thermodynamic conditions appear to be favorable. Occasionally, strong unstable atmospheric thermodynamic conditions and northerly reversal of cross-equatorial flow can lead to wet season onsets without cold air intrusions. In such cases, enhanced precipitation is centered over central and eastern Amazon, and rainfall increases more gradually compared to the onset with cold air intrusions.