Adiós a las noches frías/Goodbye to cold nights

Debido al impacto que los extremos climáticos ocasionan en la agricultura y la salud en España, investigadores de la Universidad de Salamanca (USAL) han analizado las dos variables más representativas de estos extremos térmicos de 1950 a 2006: los días cálidos y las noches frías. Los resultados para la Península Ibérica indican un aumento de los días cálidos superior al del resto del planeta, y una disminución de las noches frías.
Son pocos los estudios que se han centrado en los extremos climáticos y en los cambios que se están produciendo en las temperaturas máximas y mínimas o en las variables de días cálidos y noches frías. Hasta ahora, la mayoría de las investigaciones habían analizado los cambios de temperatura promedio a escala global. Estos resultados indicaban un aumento provocado “lo más probable” por factores antropogénicos.
El nuevo estudio, publicado en la revista Climatic Change, ha permitido analizar desde el punto de vista físico las causas de las variaciones de los extremos climáticos, es decir, “qué cambios se están produciendo en las masas de aire que llegan a la Península Ibérica, así como en la temperatura del mar”, asegura Concepción Rodríguez, autora principal del trabajo e investigadora en el Departamento de Física General y Atmósfera de la USAL.
“Los resultados indican una tendencia creciente en la frecuencia de días cálidos y una disminución en la frecuencia de las noches frías. La tendencia de disminución de noches frías se corresponde con la obtenida a escala global según el IV Informe del Panel Intergubernamental sobre Cambio Climático (IPCC). Sin embargo, el crecimiento de días cálidos en la Península Ibérica es superior que el obtenido globalmente para todo el planeta”, señala la científica.
Para explicar estas diferencias, el equipo científico vinculó el aumento de días cálidos con índices de teleconexión climática, que representan la variabilidad de las características de la atmósfera y de los océanos. “Los días cálidos están relacionados con los patrones de teleconexión atmosféricos, mientras que las noches frías dependen, principalmente, de la temperatura del mar (del Atlántico norte)”, explica la investigadora.
El tiempo .que trae la masa de aire desde el norte de África es la principal causa del aumento de días cálidos “El tipo de tiempo que provoca más noches frías es la depresión sobre del golfo de Génova, que aporta aire seco y frío del centro de Europa a España", argumenta Rodríguez, quien afirma que los cambios en el número de días cálidos y noches frías son más pronunciados en el suroeste y el noreste de la Península Ibérica.. “Una de las causas probables de estos cambios es la variación de la temperatura superficial del mar en el Atlántico oriental”, puntualiza

Given the impact of climatic extremes on agriculture and health in Spain, researchers at the University of Salamanca (USAL) have analysed the two factors most representative of these thermal extremes between 1950 and 2006 - warm days and cold nights. The results for mainland Spain show an increase in the number of warm days greater than that for the rest of the planet and a reduction in the number of cold nights.
Few studies to date have focused on climatic extremes and the changes occurring in maximum and minimum temperatures and in warm day and cold night variables. Until now, most research studies had analysed average temperature changes on a global scale. These results indicated an increase "most probably" caused by human factors.
The new study, published in the journal Climatic Change, has made it possible to analyse the causes of the variations in climatic extremes from a physical point of view, in other words "which changes are taking place in the air masses reaching the Iberian Peninsula, as well as sea temperature", as Concepción Rodríguez, lead author of the study and a researcher at the General and Atmospheric Physics Department at the USAL, tells SINC.
"The results indicate an increasing trend in the frequency of warm days and a reduction in the frequency of cold nights. The trend towards the reduction of cold nights correlates with that obtained at global level, according to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). However, the increase in warm days in mainland Spain is higher than the number obtained globally for the planet as a whole", the scientist explains.
In order to explain these differences, the scientific team linked the increase in warm days with climate teleconnection indices, which show the variability of atmospheric and oceanic characteristics. "Warm days are related to atmospheric teleconnection patterns, while cold nights are caused, principally, by the temperature of the sea (in the North Atlantic)", the researcher adds.
Weather that draws air masses up from the north of Africa is the leading cause of warm days. "The type of weather that causes more cold nights is the depression over the Gulf of Genoa, which brings cold and dry air from central Europe to Spain", explains Rodríguez, who says that the change in the number of warm days and cold nights is much more pronounced in the south west and north east of the Iberian Peninsula. "One of the most probable causes of these changes is the variation in the surface temperature of the sea in the eastern Atlantic", she points out.

Tomado de Plataforma SINC (Español)/AlphaGalileo (English)

Abstract of the paper
Trends in warm days and cold nights over the Iberian Peninsula: relationships to large-scale variables
Concepción Rodríguez-Puebla, Ascensión H. Encinas, Luis Alberto García-Casado and Susana Nieto
Climatic Change, Volume 100, Numbers 3-4, 667-684, DOI: 10.1007/s10584-009-9721-0

Abstract
The aims of this study are to identify the trend of warm days and cold nights over the Iberian Peninsula and to connect the variations with large-scale variables. The reasons for performing this analysis are the effects that extremes events have on different ecosystems. Here, we present the results on spatial and temporal variability of warm days (TX90), or those exceeding the 90th percentile of maximum temperature, and cold nights (TN10), or those falling below the 10th percentile of minimum temperature. The extreme indices were derived from daily observations at stations and gridded data over land area for the period 1950 to 2006. Significant trends of more warm days and fewer cold nights were found. The trend to fewer cold nights is within the interval of global results given by the IPCC AR4 report; however, the trend to warm days is greater than the corresponding global trend. The influence of large-scale variables on these extreme indices was examined by means of the Empirical Orthogonal Function, correlation, composite maps and multiple regression analyses. Changes in TX90 are connected with the Scandinavian teleconnection index and a preferred mode of geopotential height at 500 hPa over the North Atlantic. Changes in TN10 are connected with the East Atlantic teleconnection index and the leading mode of Sea Surface Temperature (SST) variability over the North Atlantic area. Based on the links between the extreme indices and the large-scale variables we derived statistical models to describe the response of TX90 and TN10 to atmospheric circulation and SST variations. The models characterized the observed variations of TX90 and TN10 reasonably well. The results of this study encourage us to analyze, in further work, how temperature extremes might change over the Iberian Peninsula under warmer climate conditions.