More frequent droughts, compared to the historical period, have been recorded during the past 30-40 years in subtropical western South America (SWSA; Fig. 1). Drought has a more direct and immediate impact on humans and ecosystems than temperature changes, but modelling of future hydroclimatic changes is problematic and current climate models have large deficiencies in simulating such variability.
Subtropical climate is largely related to the downward branch of the Hadley Circulation (HC). The descending arm of the HC circulation can be considered the “edge of the tropics”. During recent decades, the HC has expanded towards the poles likely in response to anthropogenic global changes, shifting rainfall patterns and broadening the subtropical dry zones, especially in the Southern Hemisphere (SH). Over the last 40 years, this expansion has been estimated to be 1-3° latitude in each hemisphere. These trends in HC expansion are projected to continue through the 21st century. However, the underlying mechanisms of this phenomenon are not well known nor the magnitude of this long-term change. Indeed, several relevant issues, such as the fraction of HC expansion, the causes of the HC broadening (natural, anthropogenic or both), the variability of HC expansion-contraction fluctuations in the past, and the future expansion rates of the subtropical deserts, need to be investigated. The knowledge gaps are partially ascribed to the shortness of observational records documenting changes in large-scale tropical-extratropical atmospheric circulation.
In this context, the overarching aim of the THEMES project is to build a multi-century perspective of HC dynamics using tree-ring records along the America cordillera sector of the SH, extending the analysis of large-scale atmospheric teleconnections beyond the short record provided by instrumental data. The specific objectives are to: (1) Update and develop new tree-ring records along the Andes extending from the outer tropics in the Altiplano (17-23°S), via Central Chile (32-36°S), to northern Patagonia (37-45°S). In addition to traditional ring-width measurements, state-of-the-art techniques in dendroclimatology will be used to develop isotopic and Blue Intensity annual records; (2) Based on these data, provide robust regional reconstructions of hydroclimate and temperature, as well as the latitudinal position and intensity of the HC along SWSA; (3) Characterize the patterns and processes underlying the 20th century amplitude and recurrence of droughts in the subtropical Andes, quantifying the relative importance of the HC expansion upon these events, and (4) Combine instrumental records, reanalysis data, past reconstructions of HC, and ensembles of model simulations to assess the mechanisms underlying changes in hydroclimate induced by the HC for the years 1600 to 2100. By addressing these objectives, THEMES will provide future hydroclimate scenarios that contribute to future socio-economic planning and adaptation strategies in SWSA.           

Projet "THEMES" : THE Mystery of the Expanding TropicS porté par Valérie Daux
Partners’ institutions : LSCE, FR; LOCEAN, FR. IANIGLA-CONICET, AR; Univ. St Andrews, GB;  LDEO, USA; UACh, CL.  (2018-2020)

Site web : https://andreu-hayles.ldeo.columbia.edu/research-projects/mystery-expanding-tropics-past-future-themes