CL4.11

The height of the snow-rain transition during infrequent but high impact precipitation events, closely related to the 0⁰C-isotherm, is a crucial variable for snow cover extent, high discharge flows and flash floods in semi-arid northern Chile. Estimations of the snow-rain transition zone and its past and future changes are therefore fundamental for adaptation strategies and might eventually serve to develop early warning systems in this region. However, there are important challenges that hinder the assessment of the snow-rain transition zone in semi-arid environments and little is known about past and future changes under different global warming scenarios. For example, there are few radiosonde observations along the Andes and most weather stations are located in valley bottoms, influenced by local conditions and the assumption of free-air temperature lapse rates contributes to the uncertainty. We combine different data sets to estimate the past snow-rain transition zone of our study site, the semi-arid Elqui river catchment. Pictures of the snow line after precipitation events - available from social networks - are used to visually estimate the snow line elevation. These values are in high agreement with vertically extrapolated temperature from meteorological stations. Furthermore, we identified considerable biases between the extrapolated 0⁰C-isotherm from meteorological stations and ERA5 reanalysis data. These large biases are probably due to the lowering of the freezing level over complex terrain and need further analysis. Our results contribute to an improved understanding of the snow-rain transition in this region, but also serve to derive a climatology of this key variable along the Andes mountain range, needed for future projections.

How to cite: Schauwecker, S., Palma, G., MacDonell, S., and Goubanova, K.: Estimating the snow-rain transition zone in a semi-arid Andean catchment, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2477, https://doi.org/10.5194/egusphere-egu21-2477, 2021.

Tropical cyclones are one of the most important causes of disasters in Central America. Using historical (1970–2010) tracks of cyclones in the Caribbean and Pacific basin, we identify critical path locations where these low-pressure systems cause the highest number of floods in a set of 88 precipitation stations in the region. Results show that tropical cyclones from the Caribbean and Pacific basin produce a large number of indirect impacts on the Pacific slope of the Central American isthmus. Although the direct impact of a tropical cyclone usually results in devastation in the affected region, the indirect effects are more common and sometimes equally severe. In fact, the storm does not need to be an intense hurricane to cause considerable impacts and damage. The location of even a lower intensity storm in critical positions of the oceanic basin can result in destructive indirect impacts in Central America. The identification of critical positions can be used for emergency agencies in the region to issue alerts of possible flooding and catastrophic events.

How to cite: Hidalgo, H. G., Alfaro, E. J., Hernández-Castro, F., and Pérez-Briceño, P. M.: Identification of Tropical Cyclones’ Critical Positions Associated with Extreme Precipitation Events in Central America, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6316, https://doi.org/10.5194/egusphere-egu21-6316, 2021.

We classified events of extreme Orinoco Low-Level Jet (OLLJ) activity using the ERA5 time series of daily winds at 925 hPa averaged over the 6°S–8°N/67°W–69°W area for the period 1981-2019. This area exhibits an overall mean of 3.7 m/s easterly wind speed and an overall standard deviation of 3.5 m/s. Then, during December-January-February (June-July-August), the season of strong (weak) OLLJ activity, we defined the events below (above) one standard deviation from the overall mean. Hence, days with easterly wind speeds higher than 7.2 m/s are considered events with strong activity during DJF. In contrast, days with westerly wind speed higher than 0.2 m/s are the events with weak activity during JJA. A composite analysis of precipitation from CHIRPS dataset during the days classified as strong or weak OLLJ activity showed that during the most active period (DJF), daily precipitation values are close to 0 mm/day; except for increased precipitation in the border between Colombia, Ecuador, Peru, and Brazil. In contrast, precipitation composites during the period of non-activity of the OLLJ (JJA), showed that precipitation increases in the range 5–10 mm/day along the OLLJ corridor. A detailed analysis of the precipitation time series used for composite analysis indicates that the probability of precipitation during DJF (JJA) is less (more) than 20% (80%) over Venezuela and the Guianas. In terms of advective water transport (qV) during the most active events of the OLLJ water is transported from the Tropical Atlantic towards northern South America through the OLLJ corridor, whilst during the less active events water transport along the OLLJ corridor comes from the north Amazon basin towards northern South America. In conclusion, during DJF the OLLJ is associated with the northerly cross-equatorial flow and dry season, whereas during JJA the southerly cross-equatorial flow from the Amazon river basin predominates, which contributes to the rainy season over the Orinoco region.

How to cite: Builes-Jaramillo, A., Yepes, J., and Salas, H. D.: Strong activity of the Orinoco Low-Level Jet and its association with moisture transport in northern South America, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9935, https://doi.org/10.5194/egusphere-egu21-9935, 2021.

Mesoscale Convective Systems (MCSs) are associated with an important fraction of total precipitation in the vicinity of the Tropical Andes, and are related to high impact weather events and extreme rainfall.  Important ingredients include input of moisture and synoptic conditions particular of each location, depending on the regional scale circulation and the local topography.  Convection-Permitting (CP) simulations can help to better describe events with MCSs, including details of surface processes, low-level moisture transport and mountain-related circulations. Here we present a description of two MCSs in the vicinity of the Tropical Andes based on gridded observation-based data (ERA5 and GPM), in situ measurements and CP simulations with the Weather Research and Forecasting (WRF) model.  One of the events took place near the Andes-Amazon transition region (Mocoa-Colombia), with, reportedly, more than 100mm of precipitation accumulated in 3 hours in one location, accompanied with strong low-level transport of moisture by the (nocturnal) Orinoco Low-Level Jet (OLLJ) and strong mid-tropospheric easterly winds towards the Andes, favorable for orographic enhancenment of precipitation.  The other event took place over the low-lands of the Magdalena-Cauca basin (Cordoba-Colombia), with an approximate size of 71304 km² , according to its cloud top temperature pattern.  In this region a sea-breeze provides moisture from oceanic origin, and the nearby Andes might help to enhance low-level convergence via orographic blocking and other mountain-related effects.  Based on kilometer-scale CP simulations we describe details of the initiation and life cycle of these two MCSs as simulated by WRF, including a description of the low-level input of moisture provided by the sea-breeze and the nocturnal jet during the initiation and mature stages, the corresponding mesoscale circulations in the vicinity of the Andes, and the intensity of the simulated precipitation.  Preliminary 3-km simulations of the Mocoa event show the low-level flow blocking by the Andes, the enhanced orographic precipitation, and an underestimation of the maximum intensity of rainfall. This study might help on understanding the skill and limitations of CP simulations for representing weather systems associated to extreme rainfall in the Tropical Andes. 

How to cite: Martinez, J. A., Camacho, J. C., Vasquez, D., Espinosa, D., and Arias, P. A.: Simulation of Mesoscale Convective Systems near the Tropical Andes: Insights from Convection-Permitting Simulations of Two Events, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10264, https://doi.org/10.5194/egusphere-egu21-10264, 2021.

The impact of the El Niño Southern Oscillation (ENSO) on rivers are well known, but most existing studies involving streamflow data are severely limited by data coverage. Time series of gauging stations fade in and out over time, which makes hydrological large scale and long time analysis or studies of rarely occurring extreme events challenging. Here, we use a machine learning approach to infer missing streamflow data based on temporal correlations of stations with missing values to others with data. By using 346 stations, from the “Global Streamflow Indices and Metadata archive” (GSIM), that initially cover the 40 year timespan in conjunction with Gaussian processes we were able to extend our data by estimating missing data for an additional 646 stations, allowing us to include a total of 992 stations. We then investigate the impact of the 6 strongest El Niño (EN) events on rivers in South America between 1960 and 2000. Our analysis shows a strong correlation between ENSO events and extreme river dynamics in the southeast of Brazil, Carribean South America and parts of the Amazon basin. Furthermore we see a peak in the number of stations showing maximum river discharge all over Brazil during the EN of 1982/83 which has been linked to severe floods in the east of Brazil, parts of Uruguay and Paraguay. However EN events in other years with similar intensity did not evoke floods with such magnitude and therefore the additional drivers of the 1982/83  floods need further investigation. By using machine learning methods to infer data for gauging stations with missing data we were able to extend our data by almost three-fold, revealing a possible heavier and spatially larger impact of the 1982/83 EN on South America's hydrology than indicated in literature.

How to cite: Deppner, M. and Goswami, B.: Impact of strong El Niño events on river discharge in South America, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10383, https://doi.org/10.5194/egusphere-egu21-10383, 2021.

The Central Andes are characterized by a steep climatic and environmental gradient with large spatial and temporal variations of associated hydrological parameters. There are two main atmospheric processes that influence climate conditions in our study area in northwestern Argentina: the South American Monsoon System that transports moisture via the low-level jet and the orographic barrier of the Eastern Cordillera that forces focused rainfall at the windward facing slopes.
As part of the International Research Training Group-StRATEGy project, our research aims at monitoring integrated water vapour (IWV) in the south-central Andes, in order to track moisture propagation. In accordance with the needs of the research, we processed data from two new Global Navigation Satellite System (GNSS) ground stations that were installed in spring 2019 along with - already calculated - solutions that were derived from an existing network. We used 10 year-long time-series from 31 stations spanning an altitude range from 198 to 5141m asl and stretching from the mountain front to the interior of the mountain range. This enhanced network helped us to examine spatial correlations, as well as differences in behaviour of the IWV across the climatic gradient. Moreover, we retrieved the gradients of the IWV at single positions, in order to study seasonal correlations between wind and gradient direction.

How to cite: Antonoglou, N., Balidakis, K., Bookhagen, B., Dick, G., Zus, F., Wickert, J., and de la Torre, A.: Atmospheric monitoring with a new GNSS network in the south-central Andes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12018, https://doi.org/10.5194/egusphere-egu21-12018, 2021.

The relationship between multiple hydroclimatic variables and vegetation conditions in the upper Madeira Basin (southwestern Amazon) has been analyzed. Vegetative dynamics are characterized using NDVI dataset as an indicator of the photosynthetic capacities of vegetation. Hydroclimatic variability is analyzed using satellite-based precipitation datasets, observed river discharge, and satellite measurements of terrestrial water storage (TWS). Our results show that the vegetation in the Basin varies from energy- to water-limited. During the peak of the wet season (January-February), rainfall, discharge, and TWS are negatively correlated with NDVI (r=-0.48 to -0.65), suggesting that during this period the vegetation is mainly energy-dependent. Outside this period, these correlations are positive (r=0.55 to 0.9), suggesting that vegetation depends mainly on water availability. This higher water dependence is more noticeable during the vegetation dry season (VDS; June-October). Considering the predominant land cover types, differences in the hydroclimate-NDVI relationship are observed. Evergreen forests remain energy-limited during the beginning of the VDS, but they become water-dependent almost at the end. Savannas show a different behavior, where water dependence occurs months before the onset of the VDS. On the other hand, unlike the other variables, the TWS better explains the NDVI in evergreen forests during the VDS (r=0.7 to 0.85). This is probably because evergreen forests are more dependent on deep soil water. A spatial analysis between hydroclimatic variables and the NDVI shows the predominance of positive correlations in most of the basin. However, specific areas do not show significant correlations. The weak relationship in these areas is explained by two factors i) very wet conditions during most of the year in the "rainfall hotspot" regions, where the vegetation is not water-limited, and ii) recent land-use changes (deforestation) that break the natural response in the hydroclimate-vegetation system. These findings provide new evidence on the impacts of the land cover changes on the natural relationship between vegetation and hydroclimatic variability, which is particularly relevant given the increasing rates of deforestation in this region during recent years.

How to cite: Gutierrez-Cori, O., Espinoza, J. C., Z X Li, L., Wongchuig-Correa, S., Arias, P. A., Ronchail, J., and Segura, H.: The hydroclimate-vegetation relationship in the Amazon basin during the last 20 years: an analysis focused on the southwestern region, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13445, https://doi.org/10.5194/egusphere-egu21-13445, 2021.

The relationship between precipitation over the southern tropical Andes (STA; 20S-12S) and the Bolivian High has been revisited in a recent study (Segura et al., 2020). Western Amazon convection during the austral summer (DJF), which is located on the western side of the regional Hadley cell associated with the mature phase of the South America Monsoon System (SAMS), has been proposed as a new mechanism controlling interannual precipitation in this Andean region. This change in the controlling mechanisms is associated with the recent intensification of this regional Hadley cell, in particular, convection over the western Amazon, which has decreased the atmospheric stability in most of western tropical South America, including the southern tropical Andes. In this study, we explore the relationship of precipitation over the STA and these two atmospheric mechanisms by using the WRF model on a global scale to simulate 38 December-February seasons (1980-2017). First, we performed a series of experiments by changing the scheme of parametrization to select the one reproducing two characteristics of the SAMS: the regional Hadley Cell and the Bolivian High. On the other hand, the best set of parameterization schemes, even if reproducing these two climatic features, presented bias in precipitation and atmospheric circulation over South America. Additionally, WRF could not reproduce the long-term variability of precipitation over the STA. Aside from these expected biases, precipitation over the STA is also related to both identified mechanisms (the Bolivian High and the regional Hadley cell) in WRF simulation. Using the moist static energy approach, we explore the reasons for the relationship between the precipitation over the STA and western Amazon convection. In the following experiment, we explore the influence of western Amazon convection on the regional circulation over South America by no permitting the development of deep convection in this Amazonian region.

How to cite: Segura, H., Junquas, C., Espinoza, J. C., and Lebel, T.: Understanding the atmospheric connection between the western Amazon and the Altiplano: A modelling approach, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15928, https://doi.org/10.5194/egusphere-egu21-15928, 2021.