M. C. Gallego

Trends in Block-Seasonal Extreme Rainfall over the Iberian Peninsula in the Second Half of the Twentieth Century

Abstract In this study trends in extreme rainfall over the Iberian Peninsula at a daily scale in the second half of the twentieth century have been detected and analyzed. For this goal 35 stations evenly distributed over the region of study covering the period 1958–97 have been studied. Two different approaches have been used. The first one consists of the nonparametric Mann–Kendall test and the Sen method. The second approach is based on the statistical theory of extreme values, involving time-dependent parameters in order to be able to reflect possible temporal changes in the frequency distribution. Results from both methods agree, confirming the reliability of the analysis. Negative trends are found for the west and southwest of the Iberian Peninsula in spring and winter. In autumn a spatial dipolar pattern appears, but trends are not so evident.

Revisited Sunspot Data: A New Scenario for the Onset of the Maunder Minimum

The Maunder minimum forms an archetype for the Grand minima, and detailed knowledge of its temporal development has important consequences for the solar dynamo theory dealing with long-term solar activity evolution. Here, we reconsider the current paradigm of the Grand minimum general scenario by using newly recovered sunspot observations by G. Marcgraf and revising some earlier uncertain data for the period 1636-1642, i.e., one solar cycle before the beginning of the Maunder minimum. The new and revised data dramatically change the magnitude of the sunspot cycle just before the Maunder minimum, from 60-70 down to about 20, implying a possibly gradual onset of the minimum with reduced activity started two cycles before it. This revised scenario of the Maunder minimum changes, through the paradigm for Grand solar/stellar activity minima, the observational constraint on the solar/stellar dynamo theories focused on long-term studies and occurrence of Grand minima.

Peaks-over-Threshold Study of Trends in Extreme Rainfall over the Iberian Peninsula

A peaks-over-threshold (POT) approach is used to study trends in extreme rainfall over the Iberian Peninsula (IP) at a daily scale. Records from 52 observatories regularly distributed over Iberia with no missing data were available for the common period from 1958 to 2004. The POT approach was used because it is particularly effective at extracting information concerning true extreme events. A generalized Pareto distribution fit was made to the data involving time-dependent parameters to account for possible temporal changes in the frequency distribution. These parameters were analyzed for trends in the return-level period, of importance for engineering purposes. A time-varying threshold was defined and an automatic declustering scheme was used to select independent extreme events exceeding the threshold. The results indicate a high variability of extreme events over the coastline of the IP, greater over the Mediterranean coast than over the Atlantic coast. The calculation of the trends for the 2-yr return level yielded a large proportion of negative trends for all three seasons considered: 58% for winter, 63% for spring, and 69% for autumn. The parametric approach also revealed an increase in the area with a positive trend of the 20-yr return level relative to the 2-yr return period, especially in autumn in the east of the IP.

Comparing historic records of storm frequency and the North Atlantic Oscillation (NAO) chronology for the Azores region

The storminess of the Azores region was investigated using newspaper records from AD 1836 onwards. The information obtained was rank-ordered for intensity and the time series of storm frequency analysed for interannual- to century-scale variability. The documentary data set was validated by comparison with objective cyclones intensity for the period AD 1958—2000. Results indicate that four periods of contrasting storm frequency are present (AD 1836—1870, 1870—1920, 1920—1940 and 1940—1998). The average storm lasts for 2.3 days and the average secular storm frequency is 3.1 storms/yr. Low intensity events occur four times every five years whereas an extreme storm occurs on average once every seven years. The documentary index of storminess is highly variable at different timescales, which is consistent with other studies of storminess in the North Atlantic. Nevertheless, an objective comparison between late nineteenth- and late twentieth-century storm frequency does not reveal a significant difference. Between AD 1865 and the late twentieth century the winter NAO and storminess indices show a statistically significant anti-correlation pattern at the monthly and seasonal scales. In the late nineteenth century and between AD 1950 and 1970 the NAO index was low and the storminess index high, whilst the opposite occurred from the early twentieth century until the middle 1950s; since AD 1970 both indexes reveal positive trends and are predominantly positive. The NAO mode of circulation is partially responsible for the storminess spatial pattern and temporal distribution over the Azores region since AD 1865 and for about a century, however this relation appears to have weakened since the 1960s.

A Revised Collection of Sunspot Group Numbers

We describe a revised collection of the number of sunspot groups from 1610 to the present. This new collection is based on the work of Hoyt and Schatten (Solar Phys. 179, 189, 1998). The main changes are the elimination of a considerable number of observations during the Maunder Minimum (hereafter, MM) and the inclusion of several long series of observations. Numerous minor changes are also described. Moreover, we have calculated the active-day percentage during the MM from this new collection as a reliable index of the solar activity. Thus, the level of solar activity obtained in this work is greater than the level obtained using the original Hoyt and Schatten data, although it remains compatible with a grand minimum of solar activity. The new collection is available in digital format.

Level and length of cyclic solar activity during the Maunder minimum as deduced from the active-day statistics

Aims. The Maunder minimum (MM) of greatly reduced solar activity took place in 1645‐1715, but the exact level of sunspot activity is uncertain as based, to a large extent, on historical gener ic statements of the absence of spots on the Sun. Here we aim, using a conservative approach, to assess the level and length of sol ar cycle during the Maunder minimum, on the basis of direct historical records by astronomers of that time. Methods. A database of the active and inactive days (days with and without recorded sunspots on the solar disc respectively) is constructed for three models of different levels of conservatism (loose ML, optimum MO and strict MS models) regarding generic no-spot records. We have used the active day fraction to estimate the group sunspot number during the MM. Results. A clear cyclic variability is found throughout the MM with peaks at around 1655‐1657, 1675, 1684 and 1705, and possibly 1666, with the active day fraction not exceeding 0.2, 0.3 or 0.4 during the core MM, for the three models. Estimated sunspot numbers are found very low in accordance with a grand minimum of solar activity. Conclusions. We have found, for the core MM (1650-1700), that: (1) A large fraction of no-spot records, corresponding to the solar meridian observations, may be unreliable in the conventional database. (2) The active day fraction remained low (below 0.3‐0.4) throughout the MM, indicating the low level of sunspot activity. (3) The solar cycle appears clearly during the core MM. (4) The length of the solar cycle during the core MM appears 9± 1 years, but there is an uncertainty in that. (5) The magnitude of the sunspot cycle during MM is assessed to be below 5‐10 in sunspot numbers; A hypothesis of the high solar cycles during the MM is not confirmed.

Auroras observed in the Iberian Peninsula (1700-1855) from Rico Sinobas' catalogue

Abstract A catalogue of auroras was compiled by the Spanish physicist Rico Sinobas in 1855. It presents the auroras observed in the Iberian Peninsula in the period 1700–1855, and is of special interest because observations were at low-latitude sites. We analyse the reliability of the data, and find that this compilation confirms for low latitudes the absence of auroras during the Dalton minimum.

Sunspot Numbers and Areas from the Madrid Astronomical Observatory (1876 – 1986)

The solar program of the Astronomical Observatory of Madrid started in 1876. Observations were made in this institution to determine sunspot numbers and areas for ten solar cycles. The program was completed in 1986 and the resulting data have been published in various Spanish scientific publications. Four periods of this program (with different observers and instruments) were identified with the aid of the interesting metadata that has been made available. In the present work, the published data were retrieved and digitized. Their subsequent analysis showed that most of these data could be considered reliable given their very high correlation with reference indices (international sunspot number, group sunspot number, and sunspot area). An abrupt change emerged in the sunspots/groups ratio in 1946, which lasted until 1972.

On the Reliability of the de la rue Sunspot Area Measurements

In the 19th century De la Rue, Stewart, and Loewy carried out a compilation of drawings and photographs of the solar sunspots in the period 1832–1868. From these drawings and photographs, they determined fortnightly values of the sunspot areas. In this work, monthly values of the sunspot areas for the period 1832–1868 are calculated and the reliability of these data in terms of the solar activity indices is discussed.

Evidence for a sunspot in A.D. 939 in an Arabian Source

It is the purpose of this paper to present evidence concerning the appearance of a sunspot in A.D. 939 observed by Arabs from the Iberian Peninsula. The sunspot record appears in the Arab source known as al-Muqtabis V.