Two summers of São Paulo drought: Origins in the western tropical Pacific

Two years of drought in Southeast Brazil have led to water shortages in São Paulo, the country's most populous city. We examine the observed drought during austral summers of 2013/2014 and 2014/2015 and the related large-scale dynamics. The 2013–2014 precipitation deficits were more concentrated in the state of São Paulo, while in 2014–2015 moderate deficits were seen throughout the region. We find that a persistent warm sea surface temperature (SST) anomaly in the western tropical Pacific Ocean was an important driver of drought via atmospheric teleconnection in the two December–February seasons. The warm SST and associated convective heating initiated a wave train across the South Pacific. The resulting anticyclonic geopotential height anomaly over the southwest Atlantic expanded the westward margin of the South Atlantic high and prevented low-pressure systems from entering southeast Brazil from midlatitudes. This mechanism suggests a hemispheric symmetry to that proposed for the recent California drought

Domain choice in an experimental nested modeling prediction system for South America

The purposes of this paper are to evaluate the new version of the regional model, RegCM3, over South America for two test seasons, and to select a domain for use in an experimental nested prediction system, which incorporates RegCM3 and the European Community-Hamburg (ECHAM) general circulation model (GCM). To evaluate RegCM3, control experiments were completed with RegCM3 driven by both the NCEP/NCAR Reanalysis (NNRP) and ECHAM, using a small control domain (D-CTRL) and integration periods of January–March 1983 (El Niño) and January–March 1985 (La Niña). The new version of the regional model captures the primary circulation and rainfall differences between the two years over tropical and subtropical South America. Both the NNRP-driven and ECHAM-driven RegCM3 improve the simulation of the Atlantic intertropical convergence zone (ITCZ) compared to the GCM. However, there are some simulation errors. Irrespective of the driving fields, weak northeasterlies associated with reduced precipitation are observed over the Amazon. The simulation of the South Atlantic convergence zone is poor due to errors in the boundary condition forcing which appear to be amplified by the regional model. To select a domain for use in an experimental prediction system, sensitivity tests were performed for three domains, each of which includes important regional features and processes of the climate system. The domain sensitivity experiments were designed to determine how domain size and the location of the GCM boundary forcing affect the regional circulation, moisture transport, and rainfall in two years with different large scale conditions. First, the control domain was extended southward to include the exit region of the Andes low level jet (D-LLJ), then eastward to include the South Atlantic subtropical high (D-ATL), and finally westward to include the subsidence region of the South Pacific subtropical high and to permit the regional model more freedom to respond to the increased resolution of the Andes Mountains (D-PAC). In order to quantify differences between the domain experiments, measures of bias, root mean square error, and the spatial correlation pattern were calculated between the model results and the observed data for the seasonal average fields. The results show the GCM driving fields have remarkable control over the RegCM3 simulations. Although no single domain clearly outperforms the others in both seasons, the control domain, D-CTRL, compares most favorably with observations. Over the ITCZ region, the simulations were improved by including a large portion of the South Atlantic subtropical high (D-ATL). The methodology presented here provides a quantitative basis for evaluating domain choice in future studies

Local Difference Measures between Complex Networks for Dynamical System Model Evaluation

Acknowledgments We thank Reik V. Donner for inspiring suggestions that initialized the work presented herein. Jan H. Feldhoff is credited for providing us with the STARS simulation data and for his contributions to fruitful discussions. Comments by the anonymous reviewers are gratefully acknowledged as they led to substantial improvements of the manuscript.Peer reviewedPublisher PD

An evaluation of the ETA model during SALLJEX

The South American Low Level Jet Experiment took place during austral summer 2002-2003 and provided unique data sets to evaluate the veracity of models to represent atmospheric motions in the vicinity of the central Andes. The experiment focused on the wind maximum east of the Andes (SALLJ, South American Low Level Jet) and its role to transport moisture that feeds meso-scale convective systems over the La Plata basin. Such systems, as well as the SALLJ, exhibited a pronounced diurnal oscillation during SALLJEX, with maximum values at night. Special radiosonde observations were collected during this experiment at 06 and 18Z, when the diurnal cycle displays maximum amplitude. Such observations were not incorporated in the operational analyses and thus provide a unique data set to test the ability of models to reproduce the amplitude of diurnal oscillations in this region. In particular, the ETA model is yet lacking a comprehensive evaluation of its biases in this region of highly variable and steep orography. At the University of Maryland, Collini and Berbery integrated and archived model ETA model runs at 80 and 22km resolution, eight times a day, over two regions of different extent, in real time. These runs, when compared against radiosonde measurements of winds, moisture and temperature profiles, allow to identify model biases related to resolution, boundary conditions, and model description of the diurnal cycle. The presentation explores the ability of the model to regionally downscale large scale features and its potential use for regional climate assessments based on its performance over this particular season of enhanced observations. Preliminary results indicate that benefits of the higher resolution to describe the three-dimensional structure of the SALLJ are only apparent close to the Andes, with runs at both resolutions underestimating the amplitude of the diurnal cycle at low levels. Furthermore, the ETA runs are compared to identify dependence of representation of the diurnal cycle on resolution.Pages: 51-5

Low-Frequency variability of the SALLJ

Considerable attention has recently focused on low-frequency variability of precipitation over southeastern South America (Nogués-Paegle et al., 2002 and Barros et al., 2002). The temporal and spatial variability of the South American low-level jet east of the Andes (SALLJ) is still not well-understood, in part because the available limited upper-air observational network is not of adequate resolution to accurately describe it. Thus gridded reanalysis such as those from ECMWF and NCEP, have, by necessity, been used as an approximation to the true atmospheric state over South America. Recent work has shown that the frequency and intensity of the SALLJ events exhibit considerable variability on intraseasonal, interannual and even longer time scales (Marengo et al., 2004). A discussion of such variability is presented in this paper.Pages: on lin