Instrumental Time Series Research Articles

The global historical climate database HCLIM

There is a growing need for past weather and climate data to support science and decision-making. This paper describes the compilation and construction of a global multivariable (air temperature, pressure, precipitation sum, number of precipitation days) monthly instrumental climate database that encompasses a substantial body of the known early instrumental time series. The dataset contains series compiled from existing databases that start before 1890 (though continuing to the present) as well as a large amount of newly rescued data. All series underwent a quality control procedure and subdaily series were processed to monthly mean values. An inventory was compiled, and the collection was deduplicated based on coordinates and mutual correlations. The data are provided in a common format accompanied by the inventory. The collection totals 12452 meteorological records in 118 countries. The data can be used for climate reconstructions and analyses. It is the most comprehensive global monthly climate dataset for the preindustrial period so far.

Evaluation of climate variability on sustainability for transboundary water supply in Chihuahua, Mexico

Study regionThis is study of the Rio Conchos Watershed, Chihuahua, Mexico. Study focusThis study provides assessment of climate variability from analysis of instrumental climate and reservoir operations data, plus nearby tree-ring indices in Chihuahua, Mexico. Water demand include international treaty deliveries, growing agricultural land use, and intensified crop production. Analysis of climate cycles includes frequency analysis of local multidecadal instrumental hydrologic time series and multi-century tree-ring indices. In addition, tree-ring indices were compared with 14C content in tree rings to assess the presence of the millennial solar cycle. New hydrological insights for the regionMost of the climate variability within inter-annual to interdecadal periods are aligned with Pacific Decadal Oscillation (PDO)-like climate cycles. New multi-century climate cycles were discovered from tree-ring indices that have a profound impact on sustainability of food and water security. Historical droughts of 8-year average duration within PDO-like cycles that span longer (12–43 year) periods were newly identified which are longer than the 5-year delivery cycles used in the 1944 International Treaty of the Rivers. Mega-droughts were discovered to occur within multi-century climate cycles. Multi-century cycles identified in tree-ring indices were likely driven by solar luminosity changes from the millennial solar cycle and not just recent anthropogenic climate change. Climate variability and temperature increases will further amplify cycles of supply and demand and exacerbate sustainability, mitigation, and management strategies. Conjunctive use combined with reservoir operations will broaden sustainability portfolio of options, reducing demand effects as well as enhance supply options and replenishment effects.

Incorporation of Na and S in bamboo coral skeletons

Ocean environmental conditions can be inferred from the chemical composition of bamboo coral skeletons. The high magnesium calcite internodes of these long-living octocorals may therefore represent a potential archive for seawater properties such as salinity or temperature where instrumental time series are absent. To extend these time series into the past using a natural archive the principles of temperature and salinity signal incorporation into cold-water coral skeletal material need to be investigated. Since skeletal Na and S concentrations have been proposed as environmental proxies, we mapped the spatial distribution and concentration of these elements in two Atlantic specimens of Keratoisis grayi (family Isididae). These measurements were conducted with an electron microprobe applying a spatial resolution of 4 μm. The mean apparent distribution coefficient of Na/Ca for the two samples was within 2.5 and 2.8*10−4, while that of S shows a similar depletion relative to seawater with 3.8 and 3.6*10−3. The two elements show an inverse correlation in bamboo coral skeletons. The mean apparent distribution coefficient of Na is similar to that of abiotic calcites. This similarity can be interpreted as the absence of significant vital effects for skeletal Na/Ca. Hence it corroborates the idea that the average skeletal composition of bamboo corals holds the potential to record past seawater conditions. In contrast, it appears unlikely that the spatial variations of the element distribution of seemingly simultaneously precipitated material along growth rings are exclusively controlled by environmental factors. We further exclude Rayleigh fractionation, ion-specific pumping, and Ca/proton exchange as the driver of Na and S distribution in bamboo corals. Instead, we adapt a calcification model originally proposed for scleractinians to bamboo corals. This model can explain the observed distribution of Na and S in the skeleton by a combination of Ca/proton pumping, bicarbonate active transport, and the formation of an organic skeletal matrix. The adapted model can further be used to predict the theoretical behaviour of other elements and disentangle vital effects from external factors influencing compositional features. It is therefore a useful tool for future studies on the potential of bamboo corals as environmental archives.

On one approach to the detection of infrasonic signals of irregular natural phenomena in the instrumental observations time series at the student interdisciplinary testing ground

The work is devoted to the analysis of time series and the problem of processing signals obtained as a result of the design approach implementation during the organization of instrumental observations of irregular natural phenomena at the student interdisciplinary testing ground. The objective of the work is to study the methods of processing noisy signals obtained as a result of monitoring the infrasonic environment, which make it possible to automate the search for fragments of the time series generated by irregular natural phenomena. At the beginning of the work, a brief explanation of the essence of the measuring scientific experiment carried out within the framework of the project approach used in the additional education of students and schoolchildren shall be given. The following is a review of publications describing various approaches to the analysis of nonstationary time series obtained in the process of instrumental observations. As the main method of time series analysis, it is proposed to use the algorithm for calculating the fractal dimension of the time series, proposed by T. Higuchi [1]. During studying of the time series of infrasonic signals, a number of regularities were discovered that contribute to the development of an original procedure for processing and transforming the signal under study, which makes it possible to determine the time intervals of fragments of the time series corresponding to the signals of the desired natural phenomena. The essence of the proposed approach lies in the preliminary preparation of the time series by processing the data with a simple normalized difference filter, previously smoothed by performing the coenvolution (convolution) operation with a Gaussian kernel; determining the step of segmenting the normalized time series, calculating fractal dimensions and averaged amplitudes for each of the segments of the time series and obtaining on their basis vectors of changes in dimensions and amplitudes with their subsequent element-wise multiplication. It is shown that the maximum values of the components of the resulting vector are indicators of timestamps for the location of the desired signals.

200\xa0years of equilibrium-line altitude variability across the European Alps (1901\u22122100)

Mountain glaciers are key indicators of climate change. Their response is revealed by the environmental equilibrium-line altitude (ELA), i.e. the regional altitude of zero mass balance averaged over a long period of time. We introduce a simple approach for distributed modelling of the environmental ELA over the entire European Alps based on the parameterization of ELA in terms of summer temperature and annual precipitation at a glacier. We use 200 years of climate records and forecasts to model environmental ELA from 1901 to 2100 at 5 arcmin grid cell resolution. Historical environmental ELAs are reconstructed based on precipitation from the Long-term Alpine Precipitation reconstruction (LAPrec) dataset and temperature from the Historical Instrumental climatological Surface Time series of the greater Alpine region (HISTALP). The simulations of future environmental ELAs are forced with high-resolution EURO-CORDEX regional climate model projections for the European domain using three different greenhouse gas emissions scenarios (Representative Concentration Pathways, RCP). Our reconstructions yielded an environmental ELA across the European Alps of 2980 m above sea level for the period 1901−1930, with a rise of 114 m in the period 1971−2000. The environmental ELA is projected to exceed the maximum elevation of 69%, 81% and 92% of the glaciers in the European Alps by 2071−2100 under mitigation (RCP2.6), stabilization (RCP4.5) and high greenhouse gas emission (RCP8.5) scenarios, respectively.

Holocene glacier change in the Silvretta Massif (Austrian Alps) constrained by a new 10Be chronology, historical records and modern observations

Mountain glaciers are important water resources in Alpine regions and are sensitive to climate change. Reconstructing glacier oscillations improves our understanding of the amplitude and the frequency of climate variability and resolves time periods when the climate system was in transition – from glacial to interglacial conditions at the beginning of the Holocene, and from a naturally controlled system to an anthropogenically influenced system in the course of industrialization.With this study, we contribute a new Holocene mountain glacier chronology from the Eastern European Alps. The study area, the Ochsental in the Silvretta Massif, features pronounced Holocene moraine ridges and is an important catchment for hydropower production. We present 18 new 10Be exposure ages of bedrock outcrops (n = 2) and boulders (n = 16). We complement the 10Be glacier chronology with historical records and instrumental time series and correlate it with pre-existing climate proxy records for capturing ice margin positions at different times during the Holocene. The Ochsental chronology is compared to cosmogenic nuclide moraine records across the European Alps to provide an Alpine-wide perspective on the transition from the Younger Dryas (YD; c. 12.9 to 11.7 ka) to the Holocene (c. 11.7 ka to present).Results show that glaciers in the Ochsental stabilized at the position of a preserved Holocene moraine c. 9.9 ± 0.7 ka after retreating from their Late Glacial position. This Holocene moraine formation interval is concurrent with a cold spell detected in some climate proxy records in the Swiss and Austrian Alps, the Central European Cold Phase 1 (CE-1). Glaciers were presumably much smaller during the Mid-Holocene and readvanced to a position close to the preserved Early Holocene moraine during the Little Ice Age (LIA; c. 1250 to 1850 CE). LIA 10Be ages range from 390 ± 20 yrs to 135 ± 5 yrs and point to multiple advances within this time period with most robust evidence for a culmination during the 18th century. The 10Be record and the historical glacier records overlap and are remarkably consistent, which demonstrates that 10Be surface exposure dating produces reliable ages even for young glacial deposits. Within the last c. 170 years, Ochsentaler glacier has retreated c. 2.3 km, which highlights the impact of recent warming on Alpine mountain glaciers.

Lake Sediments Reveal Large Variations in Flood Frequency Over the Last 6,500 Years in South-Western Norway

Lake sediments can retain imprints of past floods, enabling reconstructions that span well beyond instrumental time series. Time series covering thousands of years can document the natural range of flood variability, which is critical for understanding the potential causality between changing flood patterns and climate. Here, we analyzed sediments from Lake Sandvinvatnet in southwest Norway. Detailed environmental magnetic analyses of an 830cm-long sediment core covering the last 6,500 years captured decadal scale trends in local flood frequency. Magnetic susceptibility (MS) assessments were carried out both on split cores and individual samples to track variability in sedimentary influx; the ratios of MS measured at 77 K and 293 K (MS ratios) provided information on potential changes in source regions. The results suggested that sediments from the Buerdalen valley dominate the signal in the core, and the amount of ferromagnetic (high MS) carriers increases during flood events. These carriers were assumed to be transported from slope deposits in Buerdalen during rainstorm-triggered flood events. The reconstructed flood frequency, based on sediment layers with ferromagnetic carriers, showed high variability over the past 6,500 years, and the finding was validated by overlapping with known historical floods in the area. We observed periods with a high frequency of extreme floods (4100-3140 cal. yr BP) compared with intervals with a few or no extreme floods (6050-4100 cal. yr BP). Floods in this region are commonly a result of intense rain events during fall and snow and glacial melt during late spring and summer. The systematic frequency changes during the past 6,500 years suggest a certain persistency in the processes that cause floods, where mean trends in summer temperature and precipitation may have played a role.

Reconstructing 800 Years of Carbonate Ion Concentration in the Cariaco Basin Using the Area Density of Planktonic Foraminifera Shells

AbstractAnthropogenically mediated ocean acidification (OA) has negative impacts on many marine organisms, especially calcifiers. However, systematic measurements of OA have only been made over the past four decades. In order to improve future predictions and understand how ongoing OA compares to natural variability on longer timescales, it is critical to extend records beyond observational time series. In the Cariaco Basin, located in the tropical Atlantic, near‐surface dissolved inorganic carbon reflects atmospheric carbon dioxide concentrations (CO2) since the Industrial Revolution, making it an ideal site for examining longer‐term variability. We extend the record of Cariaco Basin near‐surface [CO32−] back to 1240 CE, using the area density (shell weight (μg)/shell area (μm2)) of the planktonic foraminifer Globigerinoides ruber (pink). Multidecadal variability is observed throughout the record. Since the Industrial Revolution (1760–2007 CE), [CO32−] has declined by 0.22 μmol kg−1 year−1, in agreement with the magnitude and direction of change captured in the shorter instrumental time series. During the Little Ice Age (1500–1760 CE), a period marked by regional drought, substantial variability but no long‐term trend is observed, while a decrease in [CO32−] of 0.11 μmol kg−1 y−1 occurs at the end of the Medieval Climate Anomaly (MCA) (1240 – 1500 CE). Both the MCA and Little Ice Age contain substantial natural variability in near surface [CO32−] that we attribute to changes in regional upwelling and atmospheric CO2. However, the decline in [CO32−] occurring in the Post‐Industrial Period is anomalous against a backdrop of 800 years of natural variability, reflecting OA associated with anthropogenic increases in atmospheric CO2.

An oceanic perspective on Greenland\u2019s recent freshwater discharge since 1850

Instrumental data evidence an accelerating freshwater release from Arctic sea ice export and the Greenland Ice Sheet over the past three decades causing cooling and freshening in the subpolar North Atlantic region. However, evaluating the observed acceleration on a historical oceanic and climatic perspective remains challenging given the short available instrumental time series. Here we provide a marine perspective on the freshwater releases to the ocean since 1850 as reflected in the northern limb of the Subpolar Gyre. Our reconstructions suggest that the recent acceleration tracks back to the 1940s/50s and is unprecedented since 1850. The melting, initiated by the 1920s natural rise in solar irradiance, accelerated in response to a combined effect of natural and anthropogenic forcing factors. We find that Greenland’s freshwater discharge has contributed to a nutrient-driven fertilization of the upper ocean and consequently increased the marine primary productivity since the 1940s/50s.

Homogenization of instrumental time series of air temperature in Central Italy (1930-2015)

Homogenization of instrumental time series of air temperature in Central Italy (1930-2015)

BINCOR: An R package for Estimating the Correlation between Two Unevenly Spaced Time Series

This paper presents a computational program named BINCOR (BINned CORrelation) for estimating the correlation between two unevenly spaced time series. This program is also applicable to the situation of two evenly spaced time series not on the same time grid. BINCOR is based on a novel estimation approach proposed by Mudelsee (2010) for estimating the correlation between two climate time series with different timescales. The idea is that autocorrelation (e.g. an AR1 process) means that memory enables values obtained on different time points to be correlated. Binned correlation is performed by resampling the time series under study into time bins on a regular grid, assigning the mean values of the variable under scrutiny within those bins. We present two examples of our BINCOR package with real data: instrumental and paleoclimatic time series. In both applications BINCOR works properly in detecting well-established relationships between the climate records compared.

The influence of skeletal micro-structures on potential proxy records in a bamboo coral

Assessing the physicochemical variability of the deeper ocean is currently hampered by limited instrumental time series and proxy records. Bamboo corals (Isididae) form a cosmopolitan family of calcitic deep sea corals that could fill this information gap via geochemical information recorded in their skeletons. Here we evaluate the suitability of high-resolution chemical imaging of bamboo coral skeletons for temperature and nutrient reconstruction. The applied elemental mapping techniques allow to verify the suitability of the chosen transect on the sample section for paleo-reconstructions and enhance the statistical precision of the reconstruction. We measured Mg/Ca via electron microprobe at 1 µm resolution and Ba/Ca via laser ablation ICP-MS at 35 µm resolution in a historic specimen of Keratoisis grayi from the Blake Plateau off Eastern Florida. Long-term growth temperatures of 7.1 ± 3.4 °C (± 2 SD) that are in agreement with recent ambient temperature range can be reconstructed from Mg/Ca ratios provided that anomalously Mg-enriched structural features around the central axis and isolated features related to tissue attachment are avoided for reconstruction. Skeletal Ba/Ca measurements reflect mean seawater barium [Ba]SW concentrations ([Ba]SW = 51 ± 24 nmol kg-1 (± 2 SD)), in agreement with instrumental data (47 nmol kg-1). We show for the first time that Ba/Ca forms concentric structures in a bamboo coral skeleton section. Our investigations suggest that, while bamboo coral skeletons do record environmental parameters in their mean chemical composition, the magnitude of environmental variability reconstructed from high-resolution chemical maps exceeds that expected from instrumental time series. This necessitates additional investigation of the factors driving bamboo coral skeletal composition.

Impact of the Major Baltic Inflow in 2014 on Manganese Cycling in the Gotland Deep (Baltic Sea)

The deep basins of the Baltic Sea, including the Gotland and Landsort Deeps, are well known for the exceptional occurrence of sedimentary Mn carbonate. Although the details of the mechanisms of Mn carbonate formation are still under debate, a close relationship with episodic major Baltic inflows (MBIs) is generally assumed, at least for the Gotland Basin. However, the few studies on Mn cycling during MBIs suffer from a limited temporal resolution. Here we report on Mn dynamics in the water column and sediments of the Gotland Deep following an MBI that entered the Baltic Sea in December 2014. Water column profiles of dissolved Mn were obtained at a monthly to bi-monthly resolution between February 2015 and March 2017 and revealed an impact of the MBI on the Gotland Deep bottom waters beginning in March 2015. Water column profiles and budget estimates provided evidence for remarkable losses of dissolved Mn associated with the enhanced deposition of Mn oxide particles, as documented in sediment trap samples and surface sediments. In July 2015, subsequent to the nearly full oxygenation of the water column, clear signals of the re-establishment of bottom water anoxia appeared, interrupted by a second inflow pulse around February 2016. However, dissolved Mn concentrations of up to 40 µM in the bottom waters in June 2016 again indicated a pronounced reduction of Mn oxide and the escape of dissolved Mn back into the open water column. The absence of substantial amounts of Mn carbonate in the surface sediments at the end of the observation period suggested that the duration of bottom water oxygenation plays an important role in the formation of this mineral. Data from both an instrumental time series and a dated sediment core from the Gotland Deep supported this conclusion. Enhanced Mn carbonate formation occurred especially between the 1960s and mid-1970s, when several MBIs caused a long-lasting oxygenation of the water column. By contrast, Mn carbonate layers were much less pronounced or even missing after single MBIs in 1993, 2003, and 2014, each of which provided a comparatively short-term supply of O2 to the deeper water column.

Ion microprobe δ18O analyses to calibrate slow growth rate speleothem records with regional δ18O records of precipitation

Paleoclimate reconstructions based on speleothems require a robust interpretation of their proxies. Detailed transfer functions of external signals to the speleothem can be obtained using models supported by monitoring data. However, the transferred signal may not be stationary due to complexity of karst processes. Therefore, robust interpretations require the calibration of speleothem records with instrumental time series lasting no less than a decade. We present the calibration of a speleothem δ18O record from Postojna Cave (Slovenia) with the regional record of δ18O composition of precipitation during the last decades. Using local meteorological data and a regional δ18O record of precipitation, we developed a model that reproduces the cave drip water δ18O signal measured during a two-year period. The model suggests that the average water mixing and transit time in the studied aquifer is 11 months. Additionally, we used an ion microprobe to study the δ18O record of the top 500 μm of a speleothem from the studied cave gallery. According to U–Th dates and 14C analyses, the uppermost section of the speleothem was formed during the last decades. The δ18O record of the top 500 μm of the speleothem has a significant correlation (r2=0.64; p-value <0.001) with the modelled δ18O record of cave drip water. Therefore, we confirm that the top 500 μm of the speleothem grew between the years 1984 and 2003 and that the speleothem accurately recorded the variability of the δ18O values of regional precipitation filtered by the aquifer. We show that the recorded speleothem δ18O signal is not seasonally biased and that the hydrological dynamics described during monitoring period were stationary during recent decades. This research demonstrates that speleothems with growth rates <50 μm/yr can also be used for calibration studies. Additionally, we show that the fit of measured and modelled proxy data can be used to achieve annually resolved chronologies in speleothems that were not actively growing at the time of collection and/or that do not record annual laminae.

Winter climate signal in boreal clastic-biogenic varves: a comprehensive analysis of three varved records from 1890 to 1990 AD with meteorological and hydrological data from Eastern Finland

Clastic-biogenic varves are widely used for reconstructing past climate: in boreal environments, the accumulation of minerogenic clasts on the lake floor is generally considered a proxy for past variations in spring floods reflecting previous winter conditions. However, the physical mechanisms behind this winter climate sensitivity and the influence of catchment type on the varve formation are not fully investigated. Here, we present a winter climate record inferred from the clastic laminae of three lakes located on the region of fine-grained tills in Eastern Finland spanning from AD 1890 to 1990. The minerogenic varve data are compared with instrumental meteorological and hydrological time series in order to investigate the physical link between winter and spring climate and minerogenic matter accumulation. Our analyses reveal that the climate-catchment mechanisms operating in the region of fine-grained tills differ crucially from previously described climate catchment interactions on sand moraine-dominated catchments in Finland, where the maximum river discharge in spring controls the clastic lamina formation. However, in contrast to earlier boreal varve records from Central Finland, the clastic lamina formation in the studied region correlates negatively with spring temperatures and winter precipitation. This could be an artefact of varying catchment dynamics but also related to the regional climate. The lakes surrounded by catchments characterized by fine-grained tills are more sensitive to cold and dry winters. The differences in the sensitivity of varve characteristics to climate, highlights the importance of understanding the catchment dynamics in detail in order to better understand climatic forcing.

Gridded climate data products are an alternative to instrumental measurements as inputs to rainfall–runoff models

AbstractRainfall–runoff models are widely used to predict flows using observed (instrumental) time series of air temperature and precipitation as inputs. Poor model performance is often associated with difficulties in estimating catchment‐scale meteorological variables from point observations. Readily available gridded climate products are an underutilized source of temperature and precipitation time series for rainfall–runoff modelling, which may overcome some of the performance issues associated with poor‐quality instrumental data in small headwater monitoring catchments. Here we compare the performance of instrumental measured and E‐OBS gridded temperature and precipitation time series as inputs in the rainfall–runoff models “PERSiST” and “HBV” for flow prediction in six small Swedish catchments. For both models and most catchments, the gridded data produced statistically better simulations than did those obtained using instrumental measurements. Despite the high correspondence between instrumental and gridded temperature, both temperature and precipitation were responsible for the difference. We conclude that (a) gridded climate products such as the E‐OBS dataset could be more widely used as alternative input to rainfall–runoff models, even when instrumental measurements are available, and (b) the processing applied to gridded climate products appears to provide a more realistic approximation of small catchment‐scale temperature and precipitation patterns needed for flow simulations. Further research on this issue is needed and encouraged.

Norwegian fjord sediments reveal NAO related winter temperature and precipitation changes of the past 2800 years

The North Atlantic Oscillation (NAO) is the leading mode of atmospheric circulation variability in the North Atlantic region. Associated shifts of storm tracks, precipitation and temperature patterns affect energy supply and demand, fisheries and agricultural, as well as marine and terrestrial ecological dynamics. Long-term NAO records are crucial to better understand its response to climate forcing factors, and assess predictability and shifts associated with ongoing climate change. A recent study of instrumental time series revealed NAO as main factor for a strong relation between winter temperature, precipitation and river discharge in central Norway over the past 50 years. Here we compare geochemical measurements with instrumental data and show that primary productivity recorded in central Norwegian fjord sediments is sensitive to NAO variability. This observation is used to calibrate paleoproductivity changes to a 500-year reconstruction of winter NAO (Luterbacher et al., 2001). Conditioned on a stationary relation between our climate proxy and the NAO we establish a first high resolution NAO proxy record (NAOTFJ) from marine sediments covering the past 2800 years. The NAOTFJ shows distinct co-variability with climate changes over Greenland, solar activity and Northern Hemisphere glacier dynamics as well as climatically associated paleo-demographic trends. The here presented climate record shows that fjord sediments provide crucial information for an improved understanding of the linkages between atmospheric circulation, solar and oceanic forcing factors.

Sunshine duration variability and trends in Italy from homogenized instrumental time series (1936–2013)

AbstractA data set of quality checked daily sunshine duration measurements was collected from 104 Italian sites over the 1936 to 2013 period. Monthly mean values were homogenized, projected onto a grid, and subjected to principle component analysis, which identified two significantly different regions: North and South. Sunshine duration temporal evolution is presented, and possible reasons for differences between the two regions are discussed in the light of a comparison with the trends found in observations of total cloud cover and with results from two neighboring regions: the Alps and Spain. In addition, trends for irradiance records, estimated from sunshine duration records using the Ångström‐Prescott formula, are presented too. The major feature of the trends, an increase in sunshine duration from the mid‐1980s, was common to both northern and southern Italy; the decrease in the preceding 30 year period was not, as northern Italy had a lower rate of decrease than southern Italy. The few records available during the earliest period of the data set indicate that sunshine duration in Italy increased from the mid‐1930s to the mid‐1950s. The further steps needed to identify and quantify the mechanisms giving rise to the observed trends and to the reported regional differences in dimming and brightening are outlined.

Long-term oscillations in rainfall extremes in a 268 year daily time series

We analyze long-term fluctuations of rainfall extremes in 268 years of daily observations (Padova, Italy, 1725–2006), to our knowledge, the longest existing instrumental time series of its kind. We identify multidecadal oscillations in extremes estimated by fitting the GEV distribution, with approximate periodicities of about 17–21, 30–38, 49–68, 85–94, and 145–172 years. The amplitudes of these oscillations exceed the changes associated with the observed trend in intensity. This finding implies that even if climatic trends are absent or negligible, rainfall and its extremes exhibit an apparent nonstationarity if analyzed over time intervals shorter than the longest periodicity in the data (about 170 years for the case analyzed here). These results suggest that because long-term periodicities may likely be present elsewhere, in the absence of observational time series with length comparable to such periodicities (possibly exceeding one century), past observations cannot be considered to be representative of future extremes. We also find that observed fluctuations in extreme events in Padova are linked to the North Atlantic Oscillation: increases in the NAO Index are on average associated with an intensification of daily extreme rainfall events. This link with the NAO global pattern is highly suggestive of implications of general relevance: long-term fluctuations in rainfall extremes connected with large-scale oscillating atmospheric patterns are likely to be widely present and undermine the very basic idea of using a single stationary distribution to infer future extremes from past observations.

Sea Surface Temperature Variability at the Scripps Institution of Oceanography Pier*

Abstract Sea surface temperature (SST) has been measured from near the end of the Scripps Institution of Oceanography (SIO) pier daily since 1916. It is one of the world’s longest instrumental time series of SST. It is widely used in studies of climate and marine ecosystems and in fisheries management. The authors hypothesized that a discontinuity exists in 1988, when the old pier was replaced with the present pier. A regression of annual-mean SST at SIO (SSTSIO) on the Pacific decadal oscillation index for 1916–87 was used to predict annual-mean SST (SSTSIO, PDO) for 1916–present. The residual (ResSSTSIO = SSTSIO − SSTSIO,PDO) time series shows a positive discontinuity in 1988, when the present SIO pier was first used to measure SSTSIO. No discontinuity in 1988 was observed for ResSST at 12 other shore stations or in nearby waters. Use of the first principal component of other shore station time series of annual-mean SST as the predictor yields similar results. SSTSIO measured over 3 days shows a diel cycle and short-term variability consistent with rip current transport of warm surf-zone water to the end of the SIO pier. This study hypothesizes that rip current transport increased with the change from the old to the present pier and contributed to the observed discontinuity in SIO pier SST. The authors estimate an artifact of about +0.45°C due to both rapid (1988 pier change) and gradual processes. Adjusting the SIO pier SST time series for this artifact reduces the long-term trend from +1.1° to +0.6°C century−1, consistent with the global rate of change of SST over the past century.