In the tropical Andes, the identification of the present synoptic mechanisms associated with the diurnal cycle of precipitation and its interaction with orography is a key step to understand how the atmospheric circulation influences the patterns of precipitation variability on longer time-scales. In particular we aim to better understand the combination of the local and regional mechanisms controlling the diurnal cycle of summertime (DJF) precipitation in the Northern Central Andes (NCA) region of Southern Peru. A climatology of the diurnal cycle is obtained from 15 wet seasons (2000–2014) of 3-hourly TRMM-3B42 data (0.25° × 0.25°) and swath data from the TRMM-2A25 precipitation radar product (5 km × 5 km). The main findings are: (1) in the NCA region, the diurnal cycle shows a maximum precipitation occurring during the day (night) in the western (eastern) side of the Andes highlands, (2) in the valleys of the Cuzco region and in the Amazon slope of the Andes the maximum (minimum) precipitation occurs during the night (day). The WRF (Weather Research and Forecasting) regional atmospheric model is used to simulate the mean diurnal cycle in the NCA region for the same period at 27 km and 9 km horizontal grid spacing and 3-hourly output, and at 3 km only for the month of January 2010 in the Cuzco valleys. Sensitivity experiments were also performed to investigate the effect of the topography on the observed rainfall patterns. The model reproduces the main diurnal precipitation features. The main atmospheric processes identified are: (1) the presence of a regional-scale cyclonic circulation strengthening during the afternoon, (2) diurnal thermally driven circulations at local scale, including upslope (downslope) wind and moisture transport during the day (night), (3) channelization of the upslope moisture transport from the Amazon along the Apurimac valleys toward the western part of the cordillera.

Understanding the influence of orography on the precipitation diurnal cycle and the associated atmospheric processes in the central Andes

The Andes-Amazon transition, along the eastern Peruvian Andes, features “hotspots” with strong precipitation Using 15 years of TRMM PR data we established a robust relation between terrain elevation and mean surface precipitation, with the latter peaking around 1000 masl, coinciding with the moisture flux peak of the South American Low Level Jet (SALLJ) There is strong diurnal variability, with afternoon (13-18 LT) convection in the Amazon plains, while on the eastern slopes (1000-2000 masl), after the forcing associated with the thermal heating of the Andes subsides, convection grows during the night and surface precipitation peaks around 01-06 LT, and organizes into mesoscale convective systems (MCSs) These then displace downslope to an terrain elevation of 700 masl with stratiform regions spreading upslope and downslope, and then decay during the remainder of the morning The large MCSs contribute with at least 50% of daily rainfall (40 % of the 01-06 LT rainfall) On synoptic scales, the large MCSs are more common in stronger SALLJ conditions, although subtropical cold surges are responsible for 16% of the cases

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2016JD026282

Orographic rainfall hot spots in the Andes‐Amazon transition according to the TRMM precipitation radar and in situ data

In this study, we analyze the atmospheric mechanisms associated with the main rainfall patterns in the tropical Andes (
 $$20^{\circ }\hbox {S}$$
 –
 $$1^{\circ }\hbox {N}$$
 ) on seasonal and interannual time scales. Using a homogeneous and high spatial resolution precipitation data set (
 $$0.05^{\circ }\times 0.05^{\circ }$$
 ) at monthly time step (CHIRPS; 1981–2016), in-situ precipitation from 206 rain-gauge stations, power spectrum and EOF analysis, we identify three Andean regions characterized by specific seasonal and interannual rainfall modes: the equatorial Andes (EA, $$5^{\circ }\hbox {S}$$
 –
 $$1^{\circ }\hbox {N}$$
 ), the transition zone (TZ, $$8^{\circ }\hbox {S}$$
 –
 $$5^{\circ }\hbox {S}$$
 ) and the southern tropical Andes (STA, $$20^{\circ }\hbox {S}$$
 –
 $$8^{\circ }\hbox {S}$$
 ). On seasonal time scales, the main mode of precipitation in the EA and STA are characterized by a unimodal regime, while the TZ is represented by a bimodal regime. The EA and the TZ share the same wet season in the February–April period, which is associated with a weakened Walker Cell, the southerly position of the Intertropical Convergence Zone (ITCZ) and a strong westward humidity transport from the equatorial Amazon. This latter mechanism and a reduced elevation of the Andes are associated with the October–November wet season in the TZ. The presence of the Bolivian High and the northward extension of the Low Level Jet are associated with the precipitation over Andean regions between 20
 $$^{\circ }$$
 S and 8
 $$^{\circ }$$
 S in the December–March period. On interannual time scales, extreme monthly wet events (EMWE) in the STA (TZ) are related to convection over the western (equatorial) Amazon during the December–March (February–April) period, showing an atmospheric relationship between the Amazon and the Andes. Extreme monthly dry events (EMDE) in the TZ and in the EA during the February–April period are related to a strengthened Walker Cell, especially in the eastern Pacific. In addition, EMWE (EMDE) in the EA are associated with an anomalous southward (northward) displaced eastern Pacific ITCZ. Moreover, we find a relationship between precipitation at higher elevations in the Andes north of $$10^{\circ }\hbox {S}$$
 and easterly winds at 200 hPa during February–April EMWE. Finally, extreme monthly events in the EA (STA) are related to sea surface temperature anomalies in the western (central) equatorial Pacific.

New insights into the rainfall variability in the tropical Andes on seasonal and interannual time scales

Remarkably heavy and devastating rainfalls affected large parts of Peru during the austral summer 2016–2017. These rainfalls favoured widespread land sliding and extensive flooding and generated one of the most severe disasters of Peru since the 1997–1998 El Nino event. The amount of rainfall recorded between January and March 2017 only compares to the biggest El Nino events of the last 40 years (i.e. 1982–1983 and 1997–1998) and exceeded the 90th percentile of available records (1981–2017) over much of the northern and central coasts of Peru, the Andean region and Amazonia. The occurrence of these heavy rainfalls was highly anomalous as it occurred during the first austral summer following the development and decay of a very strong El Nino in 2015–2016. Here, we propose that the likely cause of the anomalous rainfalls is linked to the combination of an especially intense wet spell over the Central Andes related to a deep, long-lasting anticyclone located adjacent to the Chilean coast, and to the unusual development of warm water off the coast of Peru in the nominal El Nino 1 + 2 region. This warming has been related to an anomalous weakening of the mid-upper level subtropical westerly flow, which in turn led to a weakening of the southeasterly trades off the coast, thus hindering the upwelling near the Peruvian coast and favoring the eastern Pacific warming. This development is counter to the usual evolution of sea surface temperature in the eastern equatorial Pacific following very strong El Nino events, such as those occurred in 1982–1983, 1997–1998, and 2015–2016. This paper explores the unusual nature of this event in the observational record and illustrates its consequences.

The anomalous 2017 coastal El Niño event in Peru

In June 2021 a new extreme flood was reported in the Amazon Basin, the largest hydrological system on Earth. During this event water level was above 29 m (the emergency threshold) for 91 days at Manaus station (Brazil), surpassing even the previous historical flood of 2012. Since the late 1990s, 9 extreme floods occurred, while only 8 events were reported from 1903 to 1998. Here we report that the 2021 flood is associated with an intensification of the atmospheric upward motion in the northern Amazonia (5°S-5°N), which is related to an intensification of the Walker circulations. This atmospheric feature is associated with an enhanced of deep convective clouds and intense rainfall over the northern Amazonia that produce positive anomalies of terrestrial water storage over northern Amazonia in the 2021 austral summer. The intensification of Walker circulation is associated with La Niña conditions that characterize the major floods observed in Amazonia during the 21st century (2009, 2012 and 2021). However, during the 2021 an intensification of the continental Hadley circulation is also observed. This feature produces simultaneous dry conditions over southern and southeastern Amazonia, where negative rainfall anomalies, low frequency of deep convective clouds and negative anomalies of terrestrial water storage are observed. 

The new historical flood of 2021 in the Amazon River compared to major floods of the 21st century: Atmospheric features in the context of the intensification of floods

This study investigates the spatial and temporal characteristics of cold surges that propagates northward along the eastern flank of the Andes from subtropical to tropical South America analysing wintertime in situ daily minimum temperature observations from Argentina, Bolivia and Peru and ERA-40 reanalysis over the 1975–2001 period. Cold surges usually last 2 or 3 days but are generally less persistent in the southern La Plata basin compared to tropical regions. On average, three to four cold surges are reported each year. Our analysis reveals that 52 % of cold episodes registered in the south of La Plata basin propagate northward to the northern Peruvian Amazon at a speed of around 20 m s−1. In comparison to cold surges that do not reach the tropical region, we demonstrate that these cold surges are characterized, before they reach the tropical region, by a higher occurrence of a specific circulation pattern associated to southern low-level winds progression toward low latitudes combined with subsidence and dry condition in the middle and low troposphere that reinforce the cold episode through a radiative effect. Finally, the relationship between cold surges and atmosphere dynamics is illustrated for the two most severe cold intrusions that reached the Peruvian and Bolivian Amazon in the last 20 years.

Revisiting wintertime cold air intrusions at the east of the Andes: propagating features from subtropical Argentina to Peruvian Amazon and relationship with large-scale circulation patterns

Automobile emissions in urban cities, such as Peru, are significant; however, there are no published studies of the effects of these emissions on PM2.5 (fine particulate matter) formation. This study aims to analyze the contributions of vehicle aerosol emissions to the surface mass concentration of PM2.5 in the Metropolitan Area of Lima and Callao (MALC), one of the most polluted cities in Latin America and the Caribbean (LAC) known to have high concentrations of PM2.5. In February 2018, we performed two numerical simulations (control and sensitivity) using the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem). We considered both trace gasses and aerosol emissions from on-road traffic for the baseline simulation (hereinafter referred to as “control”); gasses without particulate emissions from vehicles were considered for the sensitivity simulation (hereinafter referred to as WithoutAerosol). For control, the model’s performance was evaluated using in situ on-ground PM2.5 observations. The results of the predicted PM2.5 concentration, temperature, and relative humidity at 2 m, with wind velocity at 10 m, indicated the accuracy of the model for the control scenario. The results for the WithoutAerosol scenario indicated that the contributions of vehicular trace gasses to secondary aerosols PM2.5 concentrations was 12.7%; aerosol emissions from road traffic contributed to the direct emissions of fine aerosol (31.7 ± 22.6 µg/m3).

/pdf/evaluating-the-impact-of-vehicular-aerosol-emissions-on-2g6fro8e.pdf

Evaluating the Impact of Vehicular Aerosol Emissions on Particulate Matter (PM2.5) Formation Using Modeling Study

Benefits of the coupling in the downscaling the South American climate

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Revisiting wintertime cold air intrusions at the east of the Andes: propagating features from subtropical Argentina to Peruvian Amazon and relationship with large-scale circulation patterns

Evaluating the Impact of Vehicular Aerosol Emissions on Particulate Matter (PM2.5) Formation Using Modeling Study

Benefits of the coupling in the downscaling the South American climate