Ernesto Rodríguez Camino
Senior State Meteorologist and president of Spanish Meteorological Association
The Azores anticyclone (AA) is a high-pressure centre well known to all those who follow the weather in the media. The AA is one of the quasi-permanent pressure systems that form in the atmosphere in response to the uneven heating of the Earth's surface caused by the different amount of radiation from the Sun hitting the Earth's surface and its rotation. These high-pressure systems that form in subtropical latitudes are characterised by downward atmospheric motions that inhibit precipitation and make it possible to describe the evolution of weather and climate in terms of their changes. Therefore, a description of the evolution of the AA allows us to explain the changes in rainfall in the different regions affected by its radius of action. In the Iberian Peninsula, which is particularly vulnerable to the effects of climate change, a greater expansion and intensity of the Oscillationary Oscillation translates, especially in winter, into a shift towards more northerly latitudes of the low pressure train from the west (and its associated precipitation fronts) which do not reach our territory, favouring rainfall in northern Europe.
There is a higher frequency of extreme episodes of intense Azores anticyclone and a clear expansion of its geographical coverage, which can be attributed to climate change caused by human activities
The recent paper published in Nature Geoscience by Caroline Ummenhofer and her collaborators from several US research centres describes the reconstruction - through indirect observations and model simulations - of the evolution of AA over the last 1,200 years, allowing us to deduce differences between its behaviour in pre-industrial (pre-1850) and industrial periods affected by climate change. The climate change in which we are immersed - contrary to other previous climate changes of natural origin - has its origin in human activity and is caused by massive emissions of greenhouse gases, a consequence of the widespread use of fossil fuels and, to a lesser extent, by deforestation. The article also explicitly mentions the impact that the evolution of the AA may have on climatic conditions in the Iberian Peninsula, which is particularly vulnerable to the impacts of climate change, and in particular on the effects expected by the end of the century on the olive and wine sectors.
Indirect observations (proxies) in palaeoclimatology are indicators of certain physical or biophysical characteristics of the past that make it possible to reconstruct climatic conditions prior to instrumental measurements, which only began to be widely used in the mid-19th century with the creation of meteorological services. These palaeo-indicators include tree rings, ice cores from Greenland and Antarctica, historical documentation, etc. In this case, the calcareous formations in the Buraca Gloriosa grotto in western Portugal, which is strongly affected by precipitation variability and which in turn depends on the variability of the AA, have been used as palaeoindicators. Ultimately, the analysis of stalactites and stalagmites has allowed us to deduce the variability of OA over the last 1200 years, allowing comparison with the well-studied variability of OA over the last decades.
To complement the indirect observations, the authors have made use of numerical simulations covering the same 1,200-year period. The climate models used are reliable tools that have been calibrated, compared and systematically evaluated over a recent instrumental period. These models, once calibrated and evaluated, allow us to simulate the evolution of past climate, as in this case, or to simulate future climate in order to estimate the response of the climate system to our increasing greenhouse gas emissions.
Combining data from limestone formations with climate model simulations, the authors of this paper have shown that in the last 100 years - compared to the total period of 1200 years - there is a higher frequency of extreme episodes of intense OA and a clear expansion of their geographical coverage, and this difference can be attributed to climate change caused by human activities. Previous studies had not reached a consensus on whether the origin of this different behaviour of the AA in recent decades could be due either to natural variability - caused by volcanic eruptions and changes in incident solar radiation - or to increasing greenhouse gas emissions. This study unequivocally attributes the expansion of AA to the increasing concentration of greenhouse gases in the atmosphere. The authors conclude that the expansion of the AA will continue during the 21st century as the concentration of greenhouse gases in the atmosphere continues to increase, leading to an increased risk of droughts in the Iberian Peninsula.
Studies such as the current one are particularly useful as they allow recent changes - whether expressed in terms of climate variables (such as temperature or precipitation) or in terms of patterns (such as AA) - to be contextualised with changes over longer periods unaffected by human intervention.