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Cristina Peña-Ortiz, David Barriopedro, and Ricardo García-Herrera

Abstract

This study analyzes the multidecadal variability of the European summer timing and length. The dates of the summer onset and end are computed through an objective algorithm based on locally defined temperature thresholds applied to the European daily high-resolution gridded dataset (E-OBS) during the period 1950–2012. The results reveal a European mean summer lengthening of 2.4 days decade−1 for the period 1950–2012. However, this trend is confined to the post-1979 period, when lengthening rates range between 5 and 12 days decade−1 over western Europe and the Mediterranean region. In contrast, a widespread summer shortening occurred for the 1950–78 period. The reported changes in the summer length are in agreement with temperature trends during June and September, which affect the summer onset and end dates.

It is shown that the shortening and lengthening with a turning point around 1979 is a leading mode of the summer length multidecadal variability. The trends in the summer length can be explained by the superposition of an Atlantic multidecadal oscillation signal and a long-term trend toward more persistent summers in Europe associated with global warming.

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David Barriopedro, Ricardo García-Herrera, Anthony R. Lupo, and Emiliano Hernández

Abstract

In this paper a 55-yr (1948–2002) Northern Hemisphere blocking climatology is presented. Traditional blocking indices and methodologies are revised and a new blocking detection method is designed. This algorithm detects blocked flows and provides for a better characterization of blocking events with additional information on blocking parameters such as the location of the blocking center, the intensity, and extension. Additionally, a new tracking procedure has been incorporated following simultaneously the individual evolution of blocked flows and identifying coherently persistent blocked patterns.

Using this method, the longest known Northern Hemisphere blocking climatology is obtained and compared with previous studies. A new regional classification into four independent blocking sectors has been obtained based on the seasonally preferred regions of blocking formation: Atlantic (ATL), European (EUR), West Pacific (WPA), and East Pacific (EPA). Global and regional blocking characteristics have been described, examining their variability from the seasonal to interdecadal scales.

The global long-term blocking series in the North Hemisphere showed a significant trend toward weaker and less persistent events, as well as regional increases (decreases) in blocking frequency over the WPA (ATL and EUR) sector. The influence of teleconnection patterns (TCPs) on blocking parameters is also explored, being confined essentially to wintertime, except in the WPA sector. Additionally, regional blocking parameters, especially frequency and duration, are sensitive to regional TCPs, supporting the regional classification obtained in this paper. The ENSO-related blocking variability is evident in blocking intensities and preferred locations but not in frequency. Finally, the dynamical connection between blocking occurrence and regional TCPs is examined through the conceptual model proposed by Charney and DeVore. Observational evidence of a dynamical link between the asymmetrical temperature distributions induced by TCPs and blocking variability is provided with a distinctive contrast “warm ocean/cold land” pattern favoring the blocking occurrence in winter. However, the conceptual model is not coherent in the WPA sector, suggesting different blocking mechanisms operating in this sector.

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Froila M. Palmeiro, David Barriopedro, Ricardo García-Herrera, and Natalia Calvo

Abstract

Sudden stratospheric warmings (SSWs) are characterized by a pronounced increase of the stratospheric polar temperature during the winter season. Different definitions have been used in the literature to diagnose the occurrence of SSWs, yielding discrepancies in the detected events. The aim of this paper is to compare the SSW climatologies obtained by different methods using reanalysis data. The occurrences of Northern Hemisphere SSWs during the extended-winter season and the 1958–2014 period have been identified for a suite of eight representative definitions and three different reanalyses. Overall, and despite the differences in the number and exact dates of occurrence of SSWs, the main climatological signatures of SSWs are not sensitive to the considered reanalysis.

The mean frequency of SSWs is 6.7 events decade−1, but it ranges from 4 to 10 events, depending on the method. The seasonal cycle of events is statistically indistinguishable across definitions, with a common peak in January. However, the multidecadal variability is method dependent, with only two definitions displaying minimum frequencies in the 1990s. An analysis of the mean signatures of SSWs in the stratosphere revealed negligible differences among methods compared to the large case-to-case variability within a given definition.

The stronger and more coherent tropospheric signals before and after SSWs are associated with major events, which are detected by most methods. The tropospheric signals of minor SSWs are less robust, representing the largest source of discrepancy across definitions. Therefore, to obtain robust results, future studies on stratosphere–troposphere coupling should aim to minimize the detection of minor warmings.

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Daniel Paredes, Ricardo M. Trigo, Ricardo Garcia-Herrera, and Isabel Franco Trigo

Abstract

March monthly accumulated precipitation in the central and western regions of the Iberian Peninsula presents a clear continuous decline of 50% during the 1960–97 period. A finer analysis using daily data reveals that this trend is exactly confined to the month of March. However, this is merely the most visible aspect of a larger phenomenon over the North Atlantic/European sector. The European precipitation trends in March for the period 1960–2000 show a clear distribution of increasing precipitation in the northern regions (the British Isles and parts of Scandinavia) together with decreasing trends throughout the western Mediterranean Basin.

Relevant circulation changes over the North Atlantic and European sectors explain these precipitation trends. First, a regional Eulerian approach by means of a weather-type (WT) classification shows that the major rainfall contributors in March display significantly decreasing frequencies for the Iberian Peninsula, in contrast to the corresponding “wet” weather types for the U.K./Ireland sector, which display increasing frequencies. Within a larger context, a Lagrangian approach, based on the analysis of storm tracks over Europe and the North Atlantic region, reveals dramatic changes in the location of cyclones in the last four decades that coincide with the corresponding precipitation trends in Europe. The North Atlantic Oscillation is suggested to be the most important large-scale factor controlling both the circulation changes and the precipitation trends over the Euro–Atlantic area in March. Finally, the potential impact of reduced precipitation for rivers and water resources in the Iberian Peninsula is considered.

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Javier Mellado-Cano, David Barriopedro, Ricardo García-Herrera, Ricardo M. Trigo, and Armand Hernández

Abstract

Recent studies have stressed the key role of the east Atlantic (EA) pattern and its interactions with the North Atlantic Oscillation (NAO) in Euro-Atlantic climate variability. However, instrumental records of these leading patterns of variability are short, hampering a proper characterization of the atmospheric circulation beyond the mid-nineteenth century. In this work, we present the longest (1685–2014) observational-based records of winter NAO and EA indices as well as estimates of the North Atlantic eddy-driven jet stream speed and latitude for the same period. The time series display large variability from interannual to multidecadal time scales, with, for example, positive (negative) EA (NAO) phases dominating before 1750 (during much of the nineteenth century). By identifying winters with different combinations of NAO/EA phases in the twentieth century, our results highlight the additional role of EA in shaping the North Atlantic action centers and the European climate responses to NAO. The EA interference with the NAO signal is stronger in precipitation than in temperature and affects areas with strong responses to NAO such as Greenland and the western Mediterranean, which prevents simplistic relationships of natural proxies with NAO. The last three centuries uncover multidecadal periods dominated by specific NAO/EA states and substantial interannual-to-centennial variability in the North Atlantic jet stream, thus providing new evidence of the dynamics behind some outstanding periods. Transitions in the NAO/EA phase space have been recurrent and pin down long-lasting anomalies, such as the displacement of the North Atlantic action centers in the late twentieth century, besides some disagreements between NAO indices.

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Javier Mellado-Cano, David Barriopedro, Ricardo García-Herrera, Ricardo M. Trigo, and Mari Carmen Álvarez-Castro

Abstract

This paper presents observational evidence of the atmospheric circulation during the Late Maunder Minimum (LMM, 1685–1715) based on daily wind direction observations from ships in the English Channel. Four wind directional indices and 8-point wind roses are derived at monthly scales to characterize the LMM. The results indicate that the LMM was characterized by a pronounced meridional circulation and a marked reduction in the frequency of westerly days all year round, as compared to the present (1981–2010). The winter circulation contributed the most to the cold conditions. Nevertheless, findings indicate that the LMM in Europe was more heterogeneous than previously thought, displaying contrasting spatial patterns in both circulation and temperature, as well as large decadal variability. In particular, there was an increase of northerly winds favoring colder winters in the first half of the LMM, but enhanced southerlies contributing to milder conditions in the second half of the LMM. The analysis of the atmospheric circulation yields a new and complete classification of LMM winters. The temperature inferred from the atmospheric circulation confirms the majority of extremely cold winters well documented in the literature, while uncovering other less documented cold and mild winters. The results also suggest a nonstationarity of the North Atlantic Oscillation (NAO) pattern within the LMM, with extremely cold winters being driven by negative phases of a “high zonal” NAO pattern and “low zonal” NAO patterns dominating during moderately cold winters.

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Ricardo García Herrera, Rolando R. García, M. Rosario Prieto, Emiliano Hernández, Luis Gimeno, and Henry F. Díaz

Spanish historical archives contain a vast store of information about Spain and its former colonies in America and Asia. Some searches for climate-related information within these archives have been undertaken recently, but they have been by no means exhaustive. This paper discusses the principal archives and shows, by means of several examples, that they exhibit a high potential for inferring past climate over a wide range of timescales and geographical areas. Extraction of such information is often time consuming, and requires a combination of archival, historical, and climatological expertise, and the development of individualized methodologies to fit each situation and type of data. In spite of these difficulties, the archives can be particularly useful in many cases where there are no alternative sources of climate data. Thus, the complexities of the multidisciplinary effort required should not discourage other researchers from undertaking similar studies.

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M. Cruz Gallego, Fernando Domínguez-Castro, José M. Vaquero, and Ricardo García-Herrera

Abstract

Some of the first systematic meteorological observations in Africa were made by two women in Equatorial Guinea in 1875. Sisters Isabel and Juliana Urquiola, together with Manuel Iradier, Isabel's husband, built a meteorological observatory on Little Elobey Island (0°59′46″N, 9°32′14″E), off the tiny nation's western coast. From 1 June to 31 December 1875, the sisters took subdaily readings (6, 12, 15, and 18 h past midnight local time) of humidity, temperature, precipitation, and wind direction and speed. To evaluate the quality of these historical observations, the authors have compared them with equivalent modern meteorological data from Cocobeach (the modern meteorological station nearest to Little Elobey). The monthly-mean distributions of maximum and minimum temperature are similar to those of Cocobeach, but minimum temperatures are 2.4°C higher than Cocobeach values. Despite this difference, the observations of the Urquiola sisters were found to be far better and more consistent than other observations of the time. Sadly, the duo never enjoyed an appropriate acknowledgment of their detailed weather measurements, some taken eight times per day, which were some of the first measurements in Equatorial Guinea. Their unappreciated task unfortunately also had a high cost on their health for the rest of their lives, made all the worse for Isabel Urquiola with the loss of a baby.

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Paulina Ordoñez, David Gallego, Pedro Ribera, Cristina Peña-Ortiz, and Ricardo García-Herrera

Abstract

The Indian summer monsoon onset is one of the most expected meteorological events of the world, affecting the lives of hundreds of millions of people. The India Meteorological Department has dated the monsoon onset since 1901, but its original methodology was considered subjective and it was updated in 2006. Unfortunately, the new method relies on OLR measurements, which impedes the construction of an objective onset series before the 1970s. An alternative approach is the use of the wind field, but the development of such an index is limited to the period covered by reanalysis products. In this paper historical wind records taken on board ships are used to develop a new onset series using only wind direction measurements, providing an objective record of the onset since the late nineteenth century. The new series captures the rapid precipitation increase associated with the onset, correlates well with previous approaches, and is robust against anomalous (bogus) onsets. A tendency for later-than-average onsets during the 1900–25 and 1970–90 periods and earlier-than-average onsets between 1940 and 1965 was found. A relatively stable relationship between ENSO and Indian monsoon onset dates was found; however, this link tends to be weaker during decades characterized by prevalent La Niña conditions. Furthermore, it was found that the link between the Pacific decadal oscillation (PDO) and the onset date is limited to the phases characterized by a shift from negative to positive PDO phases.

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Cary J. Mock, Michael Chenoweth, Isabel Altamirano, Matthew D. Rodgers, and Ricardo García-Herrera

Major hurricanes are prominent meteorological hazards of the U.S. Atlantic and Gulf coasts. However, the official modern record of Atlantic basin tropical cyclones starts at 1851, and it does not provide a comprehensive measure of the frequency and magnitude of major hurricanes. Vast amounts of documentary weather data extend back several centuries, but many of these have not yet been fully utilized for hurricane reconstruction. These sources include weather diaries, ship logbooks, ship protests, and newspapers from American, British, and Spanish archives. A coordinated effort, utilizing these historical sources, has reconstructed a major hurricane in August 1812, which is the closest to ever pass by New Orleans, Louisiana, including Hurricane Katrina. The storm became a tropical depression in the Caribbean Sea, passed south of Jamaica as a tropical storm, and then strengthened to hurricane strength in the Gulf of Mexico. It made landfall about 65 km southeast of New Orleans and passed just to the west of the city. Historical storm surge and damage reports indicate it as a major hurricane at landfall. Given that conditions during 1812 include having lower sea level, higher land elevation prior to human-induced subsidence, and more extensive wetlands, a recurrence of such a major hurricane would likely have a greater detrimental societal impact than that of Hurricane Katrina. The 1812 hurricane study provides an example of how historical data can be utilized to reconstruct past hurricanes in a manner that renders them directly comparable with those within our modern record.

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