A plausible atmospheric trigger for the 2017 coastal El Niño

The far eastern tropical Pacific experienced a rapid, marked warming in early 2017, causing torrential rains along the west coast of South America with a significant societal toll in Peru and Ecuador. This strong coastal El Nino was largely unpredicted, even a few weeks before its onset, and it developed differently from either central or eastern events. Here we provide an overview of the event, its impacts and concomitant atmospheric circulation. It is proposed that a remotely forced, sustained weakening of the free tropospheric westerly flow impinging the subtropical Andes leads to a relaxation of the southeasterly (SE) trades off the coast, which in turn may have warmed the eastern Pacific throughout the weakening of upwelling in a near-coastal band and the lessening of the evaporative cooling farther offshore.FONDAP-Chile 1511000

The coastal winds off western subtropical South America in future climate scenarios

Artículo de publicación ISIThe west coast of subtropical South America is characterized by a semi-arid climate and very persistent southerly winds that often exhibit a low-level jet structure. The nearly alongshore flow forces coastal and offshore upwelling of cold, nutrient-rich waters, thus supporting one of the most productive marine ecosystems in the world and a wealth of fishery resources. Therefore, the evaluation of the changes in the coastal winds in future climate is a key step to predict the regional environmental impacts of global climate change linked to anthropogenic greenhouse gas (GHG) increases. In this work we document the wind changes between present-day conditions and those projected for the end of the 21st century under two Intergovernmental Panel on Climate Change (IPCC) scenarios (A2 and B2). We first estimate and interpret the changes of the wind field over the southeast Pacific from 15 coupled atmosphere–ocean Global Circulation Models (GCMs). Very consistent among the GCMs is the strengthening of the southerlies along the subtropical coast as a result of a marked increase in surface pressure farther south. We then examine the coastal wind changes in more detail using the Providing REgional Climate for Impact Studies (PRECIS) regional climate model (RCM) with 25 km horizontal resolution nested in the Hadley Centre Atmospheric global Model (HadAM3). PRECIS results indicate that the largest southerly wind increase occurs between 37–41 °S during spring and summer, expanding the upwelling-favourable regime in that region, at the same time that coastal jets at subtropical latitudes will become more frequent and last longer than current events. During fall and winter, the strengthening of the southerlies occurs at subtropical latitudes maintaining a mean jet year-round. Finally, we discuss the possibility that strengthening of the coastal southerlies might actually lead to a relative regional cooling even as the world as a whole continues to warm up. Copyright 2008 Royal Meteorological Societ

Spatial variability of near-surface temperature over the coastal mountains in southern Chile (38°S)

The spatial distribution of the near-surface air temperature over a coastal mountain range in southern Chile [Nahuelbuta Mountains (NM), 38°S, maximum height 1300-m ASL] is investigated using in situ measurements, satellite-derived land-surface temperature, and simulations during the austral winter of 2011. Based on a few selected but representative cases, we found that under rainy conditions—either at day or night—temperature decreases with height close to the moist adiabatic lapse rate (~6.5 °C/km). Likewise, the temperature tends to follow the dry adiabat (~9.8 °C/km) during daytime under dry- and clear-skies conditions. During clear-skies nights, the temperature also decreases with height over the southeastern side of NM, but it often increases (at about 8 °C/km) over the northwestern side of the mountains. This temperature inversion extends up to about 700-m ASL leading to an average temperature contrast of about 7 °C between the northwestern and southeastern sides of Nahuelbuta by the end of dry nights. These dawns also feature substantial temperature differences (>10 °C) among closely located stations at a same altitude. High-resolution numerical simulations suggest that upstream blocking of the prevailing SE flow, hydrostatic mountain waves, and strong downslope winds is responsible for such distinctive nocturnal temperature distribution

Orographic Precipitation in Coastal Southern Chile: Mean Distribution, Temporal Variability, and Linear Contribution

Artículo de publicación ISIThe Nahuelbuta Mountains (NM) are a semielliptical massif 1300 m high in coastal southern Chile (37 degrees-38 degrees S) facing frontal storms that move from the Pacific. Mean precipitation between 900 and 1200 mm yr(-1) is observed in the surrounding lowland, but river flow measurements suggest values >= 3000 mm yr(-1) atop the mountains. To verify and characterize such marked orographic enhancement, 15 rain gauges were deployed around and over the NM. The observations were supplemented by a high-resolution WRF simulation and linear theory (LT) modeling during the winter of 2011. The estimated mean precipitation increases gradually from offshore (~1000 mm yr(-1)) to the north-facing foothills (2000 mm yr(-1)). The precipitation rapidly increases in the upslope sector to reach ~4000 mm yr(-1) over the northern half of the NM elevated plateau, and decreases farther south to reach background values 20-30 km downstream of the mountains. The upstream (downstream) orographic enhancement (suppression) was relatively uniform among storms when considering event accumulations but varied substantially within each storm, with larger modifications during pre- and postfrontal stages and minor modifications during the brief but intense frontal passage. WRF results are in good agreement with observations in terms of seasonal and daily mean rainfall distributions, as well as temporal variability. Given its linear, steady-state formulation, the LT model cannot resolve rainfall variability at short (hourly) time scales, which in WRF is at least characterized by transient, mesoscale rainbands. Nonetheless, the rainbands are mobile so the accumulation field at monthly or longer time scales produced by the linear model is remarkably similar to its WRF counterpart.CR2/FONDAP-15110009; FONDECYT-1110169; FONDECYT-1140637; CHRIAM/CONICYT/FONDAP-1513001

Alternative approaches for estimating missing climate data: application to monthly precipitation records in South-Central Chile

Background: Over the last decades interest has grown on how climate change impacts forest resources. However, one of the main constraints is that meteorological stations are riddled with missing climatic data. This study compared five approaches for estimating monthly precipitation records: inverse distance weighting (IDW), a modification of IDW that includes elevation differences between target and neighboring stations (IDWm), correlation coefficient weighting (CCW), multiple linear regression (MLR) and artificial neural networks (ANN). Methods: A complete series of monthly precipitation records (1995–2012) from twenty meteorological stations located in central Chile were used. Two target stations were selected and their neighboring stations, located within a radius of 25 km (3 stations) and 50 km (9 stations), were identified. Cross-validation was used for evaluating the accuracy of the estimation approaches. The performance and predictive capability of the approaches were evaluated using the ratio of the root mean square error to the standard deviation of measured data (RSR), the percent bias (PBIAS), and the Nash- Sutcliffe efficiency (NSE). For testing the main and interactive effects of the radius of influence and estimation approaches, a two-level factorial design considering the target station as the blocking factor was used. Results: ANN and MLR showed the best statistics for all the stations and radius of influence. However, these approaches were not significantly different with IDWm. Inclusion of elevation differences into IDW significantly improved IDWm estimates. In terms of precision, similar estimates were obtained when applying ANN, MLR or IDWm, and the radius of influence had a significant influence on their estimates, we conclude that estimates based on nine neighboring stations located within a radius of 50 km are needed for completing missing monthly precipitation data in regions with complex topography. Conclusions: It is concluded that approaches based on ANN, MLR and IDWm had the best performance in two sectors located in south-central Chile with a complex topography. A radius of influence of 50 km (9 neighboring stations) is recommended for completing monthly precipitation data.National Fund for Scientific and Technological Development (FONDECYT) 1151050 Chile's Education Ministry through the program MECESUP2 UCO070

CFD simulations of turbulent buoyant atmospheric flows over complex geometry: Solver development in OpenFOAM

Artículo de publicación ISIThis paper, first of a two-part work, presents an overview of the development of a computational fluid dynamics (CFD) solver in OpenFOAM platform to simulate the internal ventilation regime within an open pit including the effects of developed turbulence, buoyancy and stratification. To incorporate the effect of stratification in the simulations we have chosen a formulation that includes density as a variable in the system of equations, thus facilitating further study of buoyant flows. Given the importance of turbulence in this type of large-scale flows we have used Large Eddy Simulation (LES) to incorporate it in the calculation, using a Detached Eddy Simulation (DES) approach to solve the flow near walls. Specific initial and boundary conditions were defined. The results presented in this paper, including several tests of the solver where we compared our results with experimental or numerical data, have demonstrated the validity of using OpenFOAM to study this type of complex multiphysics problems. Especially advantageous in this regard are the flexibility provided by the modular structure of the code, the possibility of defining specific boundary and initial conditions for each case, and the ability of generating detailed meshes of complex geometries. Also we probed the benefits of using a DES approach, allowing us to solve developed turbulence and the interaction of the flow with detailed geometry. A second paper associated to this work will expose the application of the solver to large open pit mines, simulating the particular case of Chuquicamata, one of the largest open pit mines in the world, located in northern Chile

Coastal Lows along the Subtropical West Coast of South America: Mean Structure and Evolution

Artículo de publicación ISIThe typical conditions of the eastern boundary of the subtropical anticyclone [e.g., well-defined marine boundary layer (MBL), equatorward low-level flow] that prevail along the mountainous west coast of subtropical South America are frequently disrupted by shallow, warm-core low pressure cells with alongshore and crossshore scales of 1000 and 500 km, respectively. These so-called coastal lows (CLs) occur up to five times per month in all seasons, although they are better defined from fall to spring. Marked weather changes along the coast and farther inland are associated with the transition from pressure drop to pressure rise. The mean structure and evolution of CLs is documented in this work, using a compositing analysis of 57 episodes selected from hourly pressure observations at a coastal station at 308S during the austral winters of 1991, 1993, and 1994, and concurrent measurements from a regional research network of nine automatic weather stations, NCEP–NCAR reanalysis fields and high-resolution visible satellite imagery. Coastal lows tend to develop as a migratory surface anticyclone approaches southern Chile at about 408S producing a poleward-oriented pressure gradient and geostrophically balanced offshore component in the low-level wind. At subtropical latitudes the transition from negative to positive geopotential anomalies occurs around 850 hPa. Enhanced mid- and lowlevel subsidence near the coast and downslope flow over the coastal range and Andes Mountains leads to the replacement of the cool, marine air by adiabatically warmed air, lowering the surface pressure at the coast and offshore. As the midlatitude ridge moves to the east of the Andes, the alongshore pressure gradient reverts back and the easterly wind ceases to act. The recovery of the surface pressure toward mean values occurs as the cool, cloud-topped MBL returns to the subtropical coast, although the pressure rise can be attenuated by midlatitude troughing. The return of the MBL resembles a Kelvin wave propagating along the coast from northern Chile (where the MBL eventually thickened) into subtropical latitudes in about a day

Impact of Warmer Eastern Tropical Pacific SST on the March 2015 Atacama Floods

Northern Chile hosts the driest place on Earth in the Atacama Desert. Nonetheless, an extreme precipitation event affected the region on 24-26 March 2015 with 1-day accumulated precipitation exceeding 40mm in several locations and hourly mean rainfall rates higher than 10mm h(-1), producing floods and resulting in casualties and significant damage. The event is analyzed using ERA-Interim, surface station data, sounding observations, and satellite-based radar. Two main conditions favorable for precipitation were present at the time of the event: (i) a cutoff low (COL) off the coast of northern Chile and (ii) positive sea surface temperature (SST) anomalies over the eastern tropical Pacific. The circulation driven by the COL was strong but not extraordinary. Regional Climate Model, version 4 (RegCM4), is used to test the sensitivity of precipitation to SST anomalies by removing the warm SST anomaly in the eastern tropical Pacific. The cooler simulation produced very similar COL dry dynamics to that simulated in a control run (with observed SST), but suppressed the precipitation by 60%-80% over northern Chile and 100% in parts of the Atacama Desert due to the decreased availability of precipitable water. The results indicate that the warm SST anomaly over the eastern Pacific, favored by the onset of El Nino 2015/16, was instrumental to the extreme precipitation event by providing an anomalous source of water vapor transported to Atacama by the circulation ahead of the COL.Fondo de Financiamiento de Centros de Investigacion en Areas Prioritarias (FONDAP)/Comision Nacional de Investigacion Cientifica y Tecnologica (CONICYT) Chile 15110009-CR2 Fondo Nacional de Desarrollo Cientifico y Tecnologico (FONDECYT)-Chile 3150036 National Laboratory for High Performance Computing (NLHPC) ECM-0