Departamento de Física Tesis Doctoral ANALYSIS OF THE RAINFALL VARIABILITY IN THE SUBTROPICAL NORTH ATLANTIC REGION: BERMUDA, CANARY ISLANDS, MADEIRA AND AZORES Irene Peñate de la Rosa Las Palmas de Gran Canaria Noviembre de 2015 UNIVERSIDAD DE LAS PALMAS DE GRAN CANARIA Programa de doctorado Física Fundamental y Aplicada Departamento de Física ANALYSIS OF THE RAINFALL VARIABILITY IN THE SUBTROPICAL NORTH ATLANTIC REGION: BERMUDA, CANARY ISLANDS, MADEIRA AND THE AZORES Tesis Doctoral presentada por D" Irene Peñate de la Rosa Dirigida por el Dr. D. Juan Manuel Martin González y Codirigida por el Dr. D. Germán Rodríguez Rodríguez El Director, El Codirector, La Doctoranda, (firma) (firma) (firma) \ Las Palmas de Gran Canaria, a 17 de noviembre de 2015 DEPARTAMENTO DE FÍSICA PROGRAMA DE DOCTORADO: FÍSICA FUNDAMENTAL Y APLICADA TESIS DOCTORAL ANALYSIS OF THE RAINFALL VARIABILITY IN THE SUBTROPICAL NORTH ATLANTIC REGION: BERMUDA, CANARY ISLANDS, MADEIRA AND AZORES PRESENTADA POR: IRENE PEÑATE DE LA ROSA DIRIGA POR EL DR. D. JUAN MANUEL MARTÍN GONZÁLEZ CODIRIGIDA POR EL DR. D. GERMÁN RODRÍGUEZ RODRÍGUEZ LAS PALMAS DE GRAN CANARIA, 2015 Para Pedro y Ángela (mis padres), Andrés, Alejandra y Jorge Irene ACKNOWLEDGEMENTS This thesis has been carried out within the framework of a research collaboration between the Spanish Agency of Meteorology (AEMET) and the Bermuda Weather Service (BWS), such cooperative efforts have been very successful in accomplishing my meteorological training and research objectives. I would like to acknowledge the support to both institutions, especially to Mark Guishard (BWS) for his passionate discussions and by way of his outstanding knowledge about contemporary scientific theories relevant to tropical cyclone forecasting, including case studies of local events. It was a real pleasure learning from him during my stay at Bermuda Weather Service having a great experience in training in the above mentioned field. I would like to acknowledge the Government of Bermuda Department of Airport Operations (DAO), BAS-Serco Ltd. and The Bermuda Institute of Ocean Sciences (BIOS) for making possible this cooperation. I also want to thank to the Bermuda Weather staff, particularly to James Dodgson for his helpful comments and Ian Currie for his support mainly with questions related to data inquiries. Furthermore, I would like to acknowledge the collaboration and support of Agencia Estatal de Meteorología (AEMET) in particular to Manolo Palomares for making possible this cooperation, Juana Arolo, Juan Manzano, Sonia Ripado and the Spanish Weather Service staff in the Canary Islands, especially to Orlando Pazos, Pino Domínguez and Ricardo Sanz for their support with data sources and climate reports and to Ignacio Egea Ureña (observer in Lanzarote airport). I must also acknowledge the support of the the IPMA (Instituto Portugues do Mar e Atmosfera) for the development of this thesis. In particular to Álvaro Pimpão Silva, Fátima Espírito Santo Fátima Coelho and Luís F. Nunes. The data used in this study has been kindly provided by the Bermuda Weather Service (BWS), Spanish Agency of Meteorology (AEMET) and Consejo Insular de Aguas de Gran Canaria and IPMA (Instituto Portugues do Mar e Atmosfera). I gratefully acknowledge the input and suggestions of my colleagues at ULPGC (University of Las Palmas of Gran Canaria) in the preparation of this work, especially I would like to express my sincere gratitude to my thesis director Dr. Juan Manuel Martín González and also to my co-director Dr. Germán Rodríguez Rodríguez and to M. C. Cabrera-Santana and A. Rodríguez-González (GEOVOL Group Research Department of Physics) for providing topographic maps. I also want to thank A. Mazzarella, Paul E. Roundy and another anonymous reviewers for their helpful comments. I cannot conclude without to extend my sincere thanks to my fellows Candy and Daniel Sandoval and J.L. Gonçalves Teixeira for reading preliminary versions of this dissertation. Finally, I would like to give thanks to my entire family for their unconditional support and especially to my husband, Andrés. Irene ABSTRACT This study presents an analysis of the rainfall in the subtropical North Atlantic region, proceeding as a reference the archipelagos of Bermuda, Canary Islands, Madeira and Azores. The spatial and seasonal variability and the annual cycle of the rainfall, on the basis of daily rainfall data records in the past decades with particular emphasis on the normal period 1981-2010, have been the main focus of this work. Particular importance has been given to the annual pattern, due to its crucial role in freshwater resources management. Geographical and topographical features, such as relief, orientation or proximity to the sea are some of the factors that influence rainfall regime which define the different areas considered in this study. In general, the rainfall in the Canary Islands and Madeira both follow a similar pattern, which is different from the one that characterizes the Bermuda and Azores archipelagos. Non-linear models have been a suitable framework for this characterization. It stresses the random nature of the rainfall distribution over Bermuda and Azores where the duration of dry periods follows an exponential adjustment close to a discrete Poisson model more accurately than a Power Law or scale-free behaviour. However, the rainfall in Madeira and more markedly in The Canary Islands has a more complex character, which can be treated through an analysis of scale or a fractal point of view. Even though the Azores anticyclone is the common synoptic situation dominating this region almost throughout the year, a marked spatial- temporal variability was found when analysing the different time series from the selected weather stations. The analysis of the wind direction and speed has been a helpful tool in describing the different seasonal rainfall patterns, and to differentiate between frontal and convective rainfall types. Investigation of the inter-annual rainfall variability in Bermuda indicates preliminary results as an increasing trend in winter rainfall days, a decreasing one in rainfall rate and a potential relationship with certain modes of atmospheric variability such as the North Atlantic Oscillation Index (NAO). The findings from this study have allowed a better knowledge on the climate within this biogeographical region, which is considered as of a great scientific interest. ABSTRACT IN SPANISH Este estudio presenta un análisis de la precipitación en la región subtropical del Atlántico Norte, tomando como referencia los archipiélagos de Bermudas, Canarias, Madeira y Azores. La variabilidad espacial y estacional, así como el ciclo anual de la lluvia en las pasadas décadas con particular énfasis en el periodo normal 1981-2010 han supuesto el centro de atención del mismo. Particular énfasis se ha dado también a la tendencia anual debido al papel crucial que tiene en la gestión de los recursos hídricos. Las características geográficas y topográficas como el relieve, la orientación o la proximidad al mar son algunos factores que influyen en el régimen pluviométrico que define las distintas zonas consideradas en este estudio. En general, la precipitación en las islas Canarias y Madeira sigue un patrón similar el cual es diferente al que caracteriza a los archipiélagos de Bermudas y Azores. Los modelos no lineales han supuesto un marco adecuado para esta caracterización. Destaca el carácter aleatorio de la lluvia en Bermudas y Azores donde la distribución de la duración de periodos secos sigue un ajuste exponencial propio de un modelo de Poisson más que un comportamiento de escala o Power Law. Por el contrario, dicha distribución en Madeira y más marcadamente en Canarias posee un carácter más complejo pudiendo ser caracterizada a través de un análisis de escala o desde un punto de vista fractal. A pesar de que el anticiclón de Azores domina esta región prácticamente todo el año, una marcada variabilidad espacio-temporal es encontrada al analizar las diferentes series temporales de precipitación seleccionadas. El análisis de la dirección y la intensidad del viento han sido útiles en la descripción de los distintos patrones estacionales de la variable de estudio y para diferenciar entre precipitación frontal y convectiva. El análisis de la variabilidad interanual en Bermudas indica, como resultado preliminar, una tendencia hacia el aumento del número de días lluviosos, una disminución en la tasa de precipitación y una relación potencial con modos de variabilidad atmosférica como la NAO (North Atlantic Oscillation Index). Los resultados derivados de este estudio han servido para profundizar en el conocimiento de la climatología en esta región biogeográfica la cual es considerada de gran interés científico. INDEX 1. Introduction ……………………………………….…..….… 2 1.1. Previous study in the research area …………………..…….…. 13 1.2. Objectives ………………………………………………..….……15 2. Area of study ………………………………………….….…19 2.1. Bermuda …………………………………………………............ 20 2.1.1. Geographic and topographic features ………………..….. 20 2.1.2. Climatic characterization …………………...……..……….22 2.1.2.1. Temperature …........................................................22 2.1.2.2. Humidity and cloudiness……………….….….…..22 2.1.2.3. Precipitation …………………………….…...........23 2.1.2.4. Surface winds…………………………….…..……24 2.1.2.5. Visibility……………………………………...….…24 2.1.2.6. Dynamic climatology……………………….…..…24 Typical synoptic situations: 2.1.3. Oceanographic conditions………………………..….….…28 2.2. The Macaronesia …………………………………………...……30