Browsing by Author "Gallego, D."
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PublicationA shift in the wind regime of the southern end of the Canary upwelling system at the turn of the 20th century(Amer Geophysical Union, 2021-05) Gallego, D.; García Herrera, Ricardo; Losada Doval, Teresa; Mohino Harris, Elsa; Rodríguez Fonseca, María BelénIn this study, we make use of historical wind direction observations to assemble an instrumental upwelling index (DUI) at the southern end of the Canary Current Upwelling System. The DUI covers the period between 1825 and 2014 and, unlike other upwelling indices, it does not rely neither in wind speed nor in reanalyzed data. In this sense, the DUI can be regarded as an instrumental index. Additionally, it avoids the suspected bias toward increasing wind speed of historical wind observations documented in previous research. Our results indicate that the frequency of the alongshore winds at the west coast of Africa between 10°N and 20°N measured by the DUI is significantly related with the wind stress and therefore the upwelling intensity in this region. The DUI presents a significant variability both at interannual and decadal timescales. We have not found any significant trend for the 20th century. However, when the entire length of the series is considered, a large shift toward more frequent alongshore winds is evidenced as a result of several decade-long fluctuations which took place between the late 19th century and the beginning of the 20th century. This fact would imply that a significant change in the upwelling intensity at the southern end of the Canary Current Upwelling System should have occurred at the turn of the 20th century. PublicationAnalysis of the precipitation and cloudiness associated with COLs occurrence in the Iberian Peninsula(Springer Wien, 2007-04) Nieto, R.; Gimeno, Luis; Añel, J. A.; Torre, L. de la; Gallego, D.; Barriopedro Cepero, David; Gallego, M.; Gordillo, A.; Redaño, A.; Delgado, G.The Iberian Peninsula is one of the regions in the world with higher occurrence of cut-off low systems (COL). The aim of this paper is to analyse the weather events (rainfall and cloudiness layer) associated to COLs in the Iberian Peninsula with tools not previously used: (a) the use of the new multidecadal COLs database developed by Nieto et al (2005) that permit us to study a 41 years period (1958-1998), (b) the checking of the expected weather events (rainfall and cloudiness layer) associated with COLs in a conceptual model (Winkler et al, 2005) and (c) the extensive use of radiosoundings to analyse convective instability in areas inside and close to the COL. Two points of view are used to make the analysis: (1) a source oriented method, when a particular COL is followed and its associated precipitation and cloudiness is analysed over four quadrants in which Iberia was divided and (2) a receptor oriented method, when the precipitation associated to COLs is analysed in given areas, defined by patterns of precipitation. Results reveal that the precipitation and cloudiness patterns associated to COLs in the conceptual model reproduce quite well the main characteristics found over the Iberian Peninsula. The generalized idea that most of the COLs produce intense convective rainfall is show to be misleading. Convective phenomena are important usually when the centre of the COL is located on the Mediterranean region. Most of the rainfall associated with COLs comes from the baroclinic shield; specially in cases located over the west half of the Iberian Peninsula. It is shown that nearly 30% of COLs do not induce any rainfall; most of them located in the southern half of the Peninsula, and mainly during autumn. Only 30% of COLs produce generalized rainfall over the whole analysed territory, being most of them (about 90%) located over the western half of the Iberian Peninsula. PublicationDeriving wind force terms from nautical reports through content analysis. The Spanish and French cases(Springer, 2005-11) Calvo Fernández, Natalia; Prieto, M. R.; Gallego, D.; García Herrera, RicardoRecords of wind strength taken onboard Spanish and French ships during the 1750-1850 period have been digitized and examined using content analysis techniques to derive the equivalent wind strength in terms of the current Beaufort scale, this conversion being a key step in any attempt to compare ancient records with modern climatologies. During the analysis it was evident that Spanish and French officers used a great number of different terms to describe the wind force. However, when the records are analyzed and homogenized, a broadly common and essentially narrow vocabulary was identified, indicating that, at this period, a great effort had been made to regulate the maritime language. Using contemporary dictionaries and navigation manuals, an equivalence was established between the original wind force terms and the Beaufort scale. PublicationDetection of the secondary meridional circulation associated with the quasi-biennial oscillation(American Geophysical Union, 2004-09-23) Ribera, P.; Peña Ortiz, C.; García Herrera, Ricardo; Gallego, D.; Gimeno, L.; Hernández Martín, EmilianoThe quasi-biennial oscillation (QBO) signal in stratospheric zonal and meridional wind, temperature, and geopotential height fields is analyzed based on the use of the National Centers for Environmental Prediction (NCEP) reanalysis (1958-2001). The multitaper method-singular value decomposition (MTM-SVD), a multivariate frequency domain analysis method, is used to detect significant and spatially coherent narrowband oscillations. The QBO is found as the most intense signal in the stratospheric zonal wind. Then, the MTM-SVD method is used to determine the patterns induced by the QBO at every stratospheric level and data field. The secondary meridional circulation associated with the QBO is identified in the obtained patterns. This circulation can be characterized by negative (positive) temperature anomalies associated with adiabatic rising (sinking) motions over zones of easterly (westerly) wind shear and over the subtropics and midlatitudes, while meridional convergence and divergence levels are found separated by a level of maximum zonal wind shear. These vertical and meridional motions form quasi-symmetric circulation cells over both hemispheres, though less intense in the Southern Hemisphere. PublicationInterannual variability of the annual cycle of temperature over Northern Africa(Springer, 2005-01) Tesouro, M.; Nieto, R.; De la Torre, L.; Ribera, P.; Gallego, D.; García Herrera, Ricardo; Hernández, E.In this study, the imprints of two major atmospheric variability modes - ENSO and NAO - on the annual cycle of temperature over Northern Africa, a region sensitive to both modes, are investigated. Results from adjusting the annual cycle from daily data on a high resolution grid, indicate that both NAO and ENSO are able to influence significantly the amplitude and phase of the seasonal cycle and, consequently, that interannual trends found in amplitude and phase can be not exclusively due to greenhouse gases effects. PublicationNorth Atlantic oscillation influence and weather types associated with winter total and extreme precipitation events in Spain(Elsevier Science INC, 2009-12) Queralt, S.; Barriopedro Cepero, David; Hernández, E.; Gallego, D.; Ribera, P.; Casanova, C.An analysis of winter intensity and frequency of precipitation is presented, based on 102 daily precipitation stations over Spain and the Balearic Islands for the 1997-2006 decade. Precipitation stations have been merged in the eight different regions which compose the analyzed area by the use of an EOF analysis. NAO influence on the intensity and frequency of precipitation of each region is described in terms of mean precipitation, mean rain frequency, the number of extreme events, changes in the precipitation distribution and the prevalent synoptic configuration. Results indicate a non-stationary response; NAO signal being more evident in mid-late winter. Strong regional differences in the response to NAO are also found, which vary according to the specific character of the precipitation under analysis. Thus, NAO exerts a clear effect on the intensity of total and extreme precipitation rates in northern and westernmost Spanish regions, whereas the frequency of precipitation is clearly affected by NAO in central and southwestern areas. While the correlation between NAO and precipitation is negative for most of the analyzed area, two regions reveal positive responses to NAO in total precipitation occurrence and intensity for specific months. Further analyses reveal asymmetric responses to opposite phases of NAO in the precipitation distributions of some regions. The complex regional relationship between NAO and precipitation is also revealed through the modulation of the former in the preferred Circulation Weather Types associated to precipitation in each region. This spatially non-homogeneous NAO signal stresses the need of caution when employing Iberian precipitation as a proxy for NAO. PublicationOn the quality of climate proxies derived from newspaper reports - a case study(Copernicus Gesellschaft MBH, 2008) Gallego, D.; García Herrera, Ricardo; Prieto, R.; Peña Ortiz, C.One of the main problems in climate reconstruction from documentary sources is the evaluation of the quality of non instrumental meteorological records in absence of instrumental observations to perform a calibration. In these cases it is mandatory to envision different approaches to assess the climatic signal in a reconstruction. This work is aimed to test the consistency of a snow frequency reconstruction in the central Argentinean Andes by studying the synoptic patterns related to the occurrence of precipitation in this area. While the original reconstruction covers the period between 1885 and 1996, the insufficiency of overlapping instrumental data limited the calibration to a short 15-year interval. In this paper we evaluate the performance of the reconstructed series for the entire 45-year period between 1958 and 1996 by analyzing the displacement in the jet stream and the patterns of geopotential height related to anomalies in the reconstructed snow frequency series. Previous works have linked the precipitation in the central Andes to the ENSO through the Pacific South American mode. We also have found this connection between ENSO and the reconstructed precipitation. Finally, it is shown that the ENSO relationship is the cause of a significant link between the precipitation anomalies in the central Argentinean Andes and the ice extent around the Antarctic Peninsula. PublicationSeasonal mean pressure reconstruction for the North Atlantic (1750-1850) based on early marine data(Copernicus Gesellschaft MBH, 2005) Gallego, D.; García Herrera, Ricardo; Ribera, P.; Jones, P. D.Measurements of wind strength and direction abstracted from European ships' logbooks during the recently finished CLIWOC project have been used to produce the first gridded Sea Level Pressure ( SLP) reconstruction for the 1750 - 1850 period over the North Atlantic based solely on marine data. The reconstruction is based on a spatial regression analysis calibrated by using data taken from the ICOADS database. An objective methodology has been developed to select the optimal calibration period and spatial domain of the reconstruction by testing several thousands of possible models. The finally selected area, limited by the performance of the regression equations and by the availability of data, covers the region between 28° N and 52° N close to the European coast and between 28° N and 44° N in the open Ocean. The results provide a direct measure of the strength and extension of the Azores High during the 101 years of the study period. The comparison with the recent land-based SLP reconstruction by Luterbacher et al. ( 2002) indicates the presence of a common signal. The interannual variability of the CLIWOC reconstructions is rather high due to the current scarcity of abstracted wind data in the areas with best response in the regression. Guidelines are proposed to optimize the efficiency of future abstraction work. PublicationSecular Variability of the Upwelling at the Canaries Latitude: An Instrumental Approach(Amer Geophysical Union, 2022-03) Gallego, D.; García Herrera, Ricardo; Mohino Harris, Elsa; Losada Doval, Teresa; Rodríguez de Fonseca, María BelénIn this research we make use of historical wind direction observations to assemble an instrumental upwelling intensity index (the so-called Directional Upwelling Index [DUI]) for the coast of Northwest Africa between 26 degrees and 33 degrees N and from 1825 to 2014. The DUI is defined as the persistence of the alongshore winds at the coast and unlike other upwelling indices, it relies on observed wind direction solely, avoiding the suspected bias toward increasing wind speed of historical wind observations documented in previous research. We have found that between June and October, when the upwelling intensity in the area is at its seasonal maximum, the persistence of the north-easterlies measured by the DUI is significantly related to the alongshore wind stress and subsequently with Sea Surface Temperature anomalies at the coast of NW Africa. The analysis of the DUI record does not display a consistent long-term trend but an oscillatory behavior. At interannual time scales this variability can be linked to the changes in the strength and location of the subtropical north Atlantic high-pressure center and at multidecadal scales, the upwelling seems mainly driven by the Atlantic Multidecadal Variability through the modulation exerted by this climatic pattern on the intensity of the Saharan low. PublicationThe importance of ship log data: reconstructing North Atlantic, European and Mediterranean sea level pressure fields back to 1750(Springer, 2010-06) Küttel, M.; Xoplaki, E.; Gallego, D.; Ñuterbarcher, J.; García Herrera, Ricardo; Allan, R.; Barriendos, M.; Jones, P.D.; Wheeler, D.; Wanner, H.Local to regional climate anomalies are to a large extent determined by the state of the atmospheric circulation. The knowledge of large-scale sea level pressure (SLP) variations in former times is therefore crucial when addressing past climate changes across Europe and the Mediterranean. However, currently available SLP reconstructions lack data from the ocean, particularly in the pre1850 period. Here we present a new statistically-derived 5° x 5° resolved gridded seasonal SLP dataset covering the eastern North Atlantic, Europe and the Mediterranean area (40°W–50°E; 20°N–70°N) back to 1750 using terrestrial instrumental pressure series and marine wind information from ship logbooks. For the period 1750–1850, the new SLP reconstruction provides a more accurate representation of the strength of the winter westerlies as well as the location and variability of the Azores High than currently available multiproxy pressure field reconstructions. These findings strongly support the potential of ship logbooks as an important source to determine past circulation variations especially for the pre-1850 period. This new dataset can be further used for dynamical studies relating large-scale atmospheric circulation to temperature and precipitation variability over the Mediterranean and Eurasia, for the comparison with outputs from GCMs as well as for detection and attribution studies. PublicationThe use of equivalent temperature to analyse climate variability(Springer, 2004-04) Ribera, P.; Gallego, D.; Gimeno, L.; Pérez Campos, J. F.; García Herrera, Ricardo; Hernández, E.; Torre, L. de la; Nieto, R.; Calvo Fernández, NataliaEquivalent temperature based in the NCEP/NCAR reanalysis database has been used as a simultaneous measure of temperature and humidity. Its variations during the 1958-1998 added to the effect of the inclusion of satellite data during the late seventies have been analyzed. An increase of the globally averaged equivalent temperature has been detected, the trend has been considerably greater during the first half of the study period and significant differences can be found between continental and oceanic areas. The relation of the trend with four of the main modes of climate variability has been assessed. The North Atlantic Oscillation and the Artic Oscillations are closely related to the equivalent temperature over the North Atlantic basin, extending toward Northern Asia in the second case. El Nino/Southern Oscillation and the Antarctic Oscillation seem to have a more global effect. PublicationTwo approaches for determining extreme years of global atmospheric temperature(Springer, 2004-04) Calvo Fernández, Natalia; Ribera, P.; Gimeno, L.; Gallego, D.; García Herrera, Ricardo; Hernández, E.; Torre, L. de la; Nieto, R.Two different groups of indices have been defined to analyze the evolution of global temperature between 1958 and 1998. All the indices were evaluated at three different levels (850, 500 and 200 hPa), and averaged indices were calculated using the whole globe, continental areas and oceanic areas. The first group of indices analyzes the area of the world covered with higher and lower than normal temperatures, detecting a slight cooling in the 200 hPa level. The second group of indices studies the annual frequency of extreme events, and again, it is at 200 hPa where the most intense variation is detected. Finally, an analysis is performed to determine the regions most sensible to variations in the occurrence of extreme events. Tropical areas are mostly responsible of the variations detected in the second group of indices.