Decadal-scale Climate Variability Research Articles

Thirty-eight years of CO<sub>2</sub> fertilization has outpaced growing aridity to drive greening of Australian woody ecosystems

Abstract. Climate change is projected to increase the imbalance between the supply (precipitation) and atmospheric demand for water (i.e., increased potential evapotranspiration), stressing plants in water-limited environments. Plants may be able to offset increasing aridity because rising CO2 increases water use efficiency. CO2 fertilization has also been cited as one of the drivers of the widespread “greening” phenomenon. However, attributing the size of this CO2 fertilization effect is complicated, due in part to a lack of long-term vegetation monitoring and interannual- to decadal-scale climate variability. In this study we asked the question of how much CO2 has contributed towards greening. We focused our analysis on a broad aridity gradient spanning eastern Australia's woody ecosystems. Next we analyzed 38 years of satellite remote sensing estimates of vegetation greenness (normalized difference vegetation index, NDVI) to examine the role of CO2 in ameliorating climate change impacts. Multiple statistical techniques were applied to separate the CO2-attributable effects on greening from the changes in water supply and atmospheric aridity. Widespread vegetation greening occurred despite a warming climate, increases in vapor pressure deficit, and repeated record-breaking droughts and heat waves. Between 1982–2019 we found that NDVI increased (median 11.3 %) across 90.5 % of the woody regions. After masking disturbance effects (e.g., fire), we statistically estimated an 11.7 % increase in NDVI attributable to CO2, broadly consistent with a hypothesized theoretical expectation of an 8.6 % increase in water use efficiency due to rising CO2. In contrast to reports of a weakening CO2 fertilization effect, we found no consistent temporal change in the CO2 effect. We conclude rising CO2 has mitigated the effects of increasing aridity, repeated record-breaking droughts, and record-breaking heat waves in eastern Australia. However, we were unable to determine whether trees or grasses were the primary beneficiary of the CO2-induced change in water use efficiency, which has implications for projecting future ecosystem resilience. A more complete understanding of how CO2-induced changes in water use efficiency affect trees and non-tree vegetation is needed.

Rebuilding in the face of climate change

Decadal-scale climate variability and change can cause trends in oceanographic conditions that impact demographic rates. Rebuilding scenarios, therefore, developed assuming constant demographic rates may not be realistic. Winter flounder (Pseudopleuronectes americanus) is an important commercial and recreational species that has declined in the southern portion of its range despite reduced exploitation. Laboratory and mesocosm studies suggest that stock productivity is reduced under warmer conditions and that rebuilding to historical levels may not be possible. Our goal was to examine the rebuilding potential of winter flounder in the face of regional warming. We integrated winter temperature into a population model to estimate environmentally driven stock–recruitment parameters and projected the stock into the future under different climate and fishing scenarios. The inclusion of winter temperature had minor impacts on the estimates of current abundance, but provided greater understanding of the drivers of recruitment. Projections that included the environment suggest that rebuilding the stock to historical levels is unlikely. The integration of both fishing and the environment has the potential to provide more realistic expectations of future stock status.

Modulation of extreme temperatures in Europe under extreme values of the North Atlantic Oscillation Index.

This paper reports on the influence that extreme values in the tails of the North Atlantic Oscillation (NAO) Index probability density function (PDF) can exert on temperatures in Europe. When the NAO Index enters into its lowest (10% quantile or less) and highest (90% quantile or higher) modes, European temperatures often exhibit large negative or positive departures from their mean values, respectively. Analyses of the joint quantiles of the Index and temperatures (i.e., the simultaneous exceedance of particular quantile thresholds by the two variables) show that temperatures enter into the upper or lower tails of their PDF when the NAO Index also enters into its extreme tails, more often that could be expected from random statistics. Studies of this nature help further our understanding of the manner by which mechanisms of decadal-scale climate variability can influence extremes of temperature-and thus perhaps improve the forecasting of extreme temperatures in weather and climate models.

Antarctic climate variability on regional and continental scales over the last 2000 years

Abstract. Climate trends in the Antarctic region remain poorly characterized, owing to the brevity and scarcity of direct climate observations and the large magnitude of interannual to decadal-scale climate variability. Here, within the framework of the PAGES Antarctica2k working group, we build an enlarged database of ice core water stable isotope records from Antarctica, consisting of 112 records. We produce both unweighted and weighted isotopic (δ18O) composites and temperature reconstructions since 0 CE, binned at 5- and 10-year resolution, for seven climatically distinct regions covering the Antarctic continent. Following earlier work of the Antarctica2k working group, we also produce composites and reconstructions for the broader regions of East Antarctica, West Antarctica and the whole continent. We use three methods for our temperature reconstructions: (i) a temperature scaling based on the δ18O–temperature relationship output from an ECHAM5-wiso model simulation nudged to ERA-Interim atmospheric reanalyses from 1979 to 2013, and adjusted for the West Antarctic Ice Sheet region to borehole temperature data, (ii) a temperature scaling of the isotopic normalized anomalies to the variance of the regional reanalysis temperature and (iii) a composite-plus-scaling approach used in a previous continent-scale reconstruction of Antarctic temperature since 1 CE but applied to the new Antarctic ice core database. Our new reconstructions confirm a significant cooling trend from 0 to 1900 CE across all Antarctic regions where records extend back into the 1st millennium, with the exception of the Wilkes Land coast and Weddell Sea coast regions. Within this long-term cooling trend from 0 to 1900 CE, we find that the warmest period occurs between 300 and 1000 CE, and the coldest interval occurs from 1200 to 1900 CE. Since 1900 CE, significant warming trends are identified for the West Antarctic Ice Sheet, the Dronning Maud Land coast and the Antarctic Peninsula regions, and these trends are robust across the distribution of records that contribute to the unweighted isotopic composites and also significant in the weighted temperature reconstructions. Only for the Antarctic Peninsula is this most recent century-scale trend unusual in the context of natural variability over the last 2000 years. However, projected warming of the Antarctic continent during the 21st century may soon see significant and unusual warming develop across other parts of the Antarctic continent. The extended Antarctica2k ice core isotope database developed by this working group opens up many avenues for developing a deeper understanding of the response of Antarctic climate to natural and anthropogenic climate forcings. The first long-term quantification of regional climate in Antarctica presented herein is a basis for data–model comparison and assessments of past, present and future driving factors of Antarctic climate.

Spatio-temporal temperature trends and extreme hydro-climatic events in southern Zimbabwe

Atmospheric warming and extreme weather events have increasingly become major contemporary issues of global concern, yet relatively few studies have investigated recent decadal-scale climate change and variability in Zimbabwe. This study investigates spatiotemporal temperature and extreme weather events in Mzingwane catchment of southern Zimbabwe for the period 1967–2015. This is achieved through using the Non-parametric Modified Mann-Kendal trend test, while magnitudes of trends are estimated using Sen’s slope estimator. Temporal and spatial trends for extreme dryness and wetness are established by using the Standard Precipitation and Evapotranspiration Index. The results for annual anomalies show a strong positive anomaly (.44 °C warmer) at all five stations for the summer months, while winter months recorded cool anomaly averaging −.28 °C. Trends in annual Tmax significantly increased (p < .05) at an average of .16 °C decade−1 at four stations, but excluding Matopos, which decreased significantly at .29 °C decade−1. The mean Tmax for the period 1897–1904 was .7 and 1.7 °C cooler than the reference average and 2000–2015 average respectively, while historic Tmin is .3 and 2.1 °C cooler. Results of extreme events indicate a significant increase (p < .05) in the occurrence of extreme dry periods since the 1980s. The findings are important for developing appropriate sustainable and adaptive strategies as climate changes.

Temperature variations since 1750 CE inferred from an alpine lake in the southeastern margin of the Tibetan Plateau

Abstract High-altitude regions are thought to be very sensitive to global climate change, especially under current global warming. However, due to the lack of paleoclimate archives such as ice core and tree-rings above the modern tree line, limited paleoclimate data have been extracted to extend instrumental data over the past few centuries, which will help understand the interannual-to decadal-scale climate variability in high-altitude regions. Here we present a high-resolution (1–6 yr) quantitative temperature record from a remote alpine lake at the southeastern margin of the Tibetan Plateau, by applying a novel proxy based on branched glycerol dialkyl glycerol tetraethers (brGDGTs). Contrary to the often-documented warming trend over the past few centuries, but consistent with temperature record from the northern Tibetan Plateau, our data show a gradual decreasing trend of 0.3 °C in mean annual air temperature from 1750 to 1970 CE. This result suggests a gradual cooling trend in some high altitude regions over this interval, which could provide a new explanation for the observed decreasing Asian summer monsoon. In addition, our data indicate an abruptly increased interannual-to decadal-scale temperature variations of 0.8–2.2 °C after 1970 CE, in terms of both magnitude and frequency, indicating that the climate system in high altitude regions would become more unstable under current global warming.

Using the Twentieth Century Reanalysis to assess climate variability for the European wind industry

We characterise the long-term variability of European near-surface wind speeds using 142 years of data from the Twentieth Century Reanalysis (20CR), and consider the potential of such long-baseline climate data sets for wind energy applications. The low resolution of the 20CR would severely restrict its use on its own for wind farm site-screening. We therefore perform a simple statistical calibration to link it to the higher-resolution ERA-Interim data set (ERAI), such that the adjusted 20CR data has the same wind speed distribution at each location as ERAI during their common period. Using this corrected 20CR data set, wind speeds and variability are characterised in terms of the long-term mean, standard deviation and corresponding trends. Many regions of interest show extremely weak trends on century timescales, but contain large multidecadal variability. Since reanalyses such as ERAI are often used to provide the background climatology for wind farm site assessments, but contain only a few decades of data, our results can be used as a way of incorporating decadal-scale wind climate variability into such studies, allowing investment risks for wind farms to be reduced.

Tracing the North Atlantic decadal-scale climate variability in a late Holocene pollen record from southern Siberia

Abstract This paper presents a new palynological record from a 146 cm long finely laminated sediment core obtained in 2009 from the deep-water meromictic Lake Shira (54°30′38″N, 90°12″09′E; ca. 353 m a.s.l.) situated in the Khakassian steppe region of southern Siberia between the rivers Ob' and Yenisei. The area is rich in lakes and represents an exceptionally well preserved sequence of Bronze and Iron Age archeological cultures. Little is known about the changes in vegetation and climate of the region during the Holocene. The palynological analysis of the core allows us to partly fill up this gap in current knowledge. The record of pollen and non-pollen palynomorphs presented here covers the past 2450 year interval with an average resolution of 22 years. The results obtained support the interpretation that the late Holocene vegetation changes around Lake Shira are mainly associated with large-scale atmospheric circulation processes controlling the regional water balance rather than with human activities. An attempt to trace human impact in the pollen assemblages provides no clear evidence for anthropogenic activity, except for the last few decades since ca. 1955, though the region has a long history of mobile pastoralists. For explanation of decadal-scale changes in the regional vegetation cover, the Artemisia/Chenopodiaceae (A/C) pollen ratio proved to be a reliable indicator of effective moisture availability. Using available fossil and published instrumental data our study suggests a link between the North Atlantic warmer/colder temperatures and higher/lower atmospheric precipitation (or moisture availability) in southern Siberia at multi-decadal to centennial scales.

Externally driven changes in the abundance of summer and winter flounder

Abstract Marine organisms that utilize nearshore environments for major components of their life histories are subject to both local-scale forcing such as water quality and estuarine degradation as well as large-scale forcing such as fishing and decadal-scale climate variability. Large-scale forcing has the potential to synchronize the dynamics of subpopulations, while local-level forcing can produce asynchronous subpopulation trends. Summer flounder (Paralichthys dentatus) and winter flounder (Pseudopleuronectes americanus) are important commercial and recreational flatfish along the east coast of North America which spend their first year of life in coastal habitats. We found that the two exhibited significant within-species coherence in commercial landings and fisheries-independent surveys across the northeast shelf of the United States, suggesting large-scale external drivers. In laboratory studies, temperature has been found to be an important factor regulating survival during the egg, larva and settlement phases of both species. We reconstructed a 40-year time-series of coastal water temperature for the major spawning and nursery areas to examine changes in the thermally available habitat. Estimates of winter flounder abundance were negatively correlated with the winter water temperature, but not with fishing mortality. Summer flounder abundance, by contrast, was negatively correlated with fishing mortality, but exhibited no link with temperature. In addition, time-varying stock–recruitment relationships indicated that stock productivity declined for winter flounder over time, while summer flounder productivity has varied without a trend. While both species declined in the 1980s and early 1990s due to heavy fishing pressure, the reduction in fishing over the last two decades has led to rebuilding of the summer flounder stock and an expansion of its age structure. Declining productivity due to warming estuarine conditions has kept the winter flounder stock at low levels despite low fishing pressure. The two stocks illustrate the importance of controlling fishing mortality in the management of natural marine resources while also accounting for changes in productivity due to climate variability and change.

The annual particle cycle in Lake Van (Turkey)

The varved sediments of Lake Van provide a high-quality continental archive of seasonal to decadal-scale climate variability. In order to read the natural record, modern varve formation was studied on the basis of (1) remotely-sensed total suspended-matter (TSMrs) concentrations; (2) time-series of particle flux and water temperatures; and (3) turbidity, temperature, and oxygen profiles. TSMrs, validated by contemporaneous water-column sampling, shows great temporal and lateral variations (whitings and turbidity plumes). From 2006 to 2009, sequential sediment traps recorded high particle fluxes during spring and fall, medium fluxes during summer, and almost zero flux during winter. The mean total mass flux of 403mgm−2day−1 comprised 33% (seasonally up to 67%) calcium carbonate, 7% aquatic organic matter, 6% biogenic opal, and 54% detrital minerals. The CaCO3 fluxes are controlled by river discharge (precipitation and snowmelt) during spring, by high productivity during summer, and by river discharge (precipitation before snowfall starts) and mixing during fall. In November 2007, an anomalously high CaCO3 flux occurred as a result of a warm water surface supersaturated with calcite coinciding with an anomalous runoff event. The results demonstrate that the couplets of light and dark laminae in the short sediment cores are true varves representing spring–summer–fall and winter conditions, respectively. Consequently, varve formation can be linked to the seasonal climate pattern, providing a calibration that can be used to interpret the partially varved paleo-record of Lake Van and related environmental processes.

Statistical properties of hurricane surge along a coast

[1] The validity and accuracy of approaches used to determine hurricane surge hazard risk received much attention following the hurricane seasons in mid- to late-2000, which caused record surge-related damage along the Gulf of Mexico coastline. Following Hurricane Katrina in 2005, research showed that most extreme-value statistics approaches underestimated the risk associated with this surge event. In this paper, two of the most popular methods for determining hurricane surge extreme-value statistics are reviewed: the historical surge population approach and the joint probability method. Here, it is demonstrated that both limited historical record length and random along-coast variability in hurricane landfall location can introduce significant errors into surge estimates. For example, the historical surge population approach gives errors of 9% to 17% for return periods between 50 and 1000 years when a surge record of 100 years is considered. In contrast, it is shown that the joint probability method yields significantly more reliable surge estimates, with errors of 2% to 3% for return periods between 50 and 1000 years when a storm record of 100 years is considered. Finally, we show that both methods remain robust when decadal-scale climate variability in the storm rate of occurrence is considered, so long as the hurricane history is long enough to capture the full decadal cycle. When used in conjunction with continuous surge response information, it can be concluded that the joint probability method is a practical and reliable approach for determining extreme-value hurricane surge statistics.

The amplifying effects of humans on fire regimes in temperate rainforests in western Patagonia

During European colonization and settlement of southern hemisphere temperate ecosystems, historical fire regimes were often dramatically altered by either burning vast areas to create farmland, or reducing fire frequencies by suppressing fires or by eliminating aboriginal populations that formerly set fires. To determine the historical range of variability of wildfire and the potential human influences on wildfire activity in temperate rainforests of western Patagonia, we used tree rings to reconstruct fire history over the past ca. 400 years. Over a 6° latitudinal range, we examined spatiotemporal changes in fire history and compared it to ethnohistorical evidence of human activities. Time series of fire years were developed from fire-scars at 27 sites for comparison with a priori defined periods of land use in each of six areas of homogeneous land-use history. We also examined the influence of climate variability to discriminate the relative roles of human ignitions and decadal-scale climate variability on fire activity. Fires were relatively common in the forest-bog habitats sampled in our study, at least two centuries prior to any likely impact from Euro-Chilean settlers, implying that fires set by the indigenous peoples in this rainforest climate were much more common (and sometimes even widespread) than previously known. Our results also show that coincident with Euro-Chilean settlement, fire regimes shifted into long-lasting regimes of substantially higher fire frequency. However, decadal-scale climate variability, also clearly was a driver of shifts in fire regimes, and the second half of the 20th century has been a time of increased temperatures and drought throughout the region. Overall, our findings indicate that although in both pre-historic and modern times climate variability is the dominant control on years of widespread fires, aboriginals and Euro-Chilean settlers have amplified fire activity (particularly during the 20th–21st centuries) and shifted the region's fire regimes to new behaviors.

On North American Decadal Climate for 2011–20

Abstract The predictability of North American climate is diagnosed by taking into account both forced climate change and natural decadal-scale climate variability over the next decade. In particular, the “signal” in North American surface air temperature and precipitation over 2011–20 associated with the expected change in boundary conditions related to future anthropogenic greenhouse gas (GHG) forcing, as well as the “noise” around that signal due to internally generated ocean–atmosphere variability, is estimated. The structural uncertainty in the estimate of decadal predictability is diagnosed by examining the sensitivity to plausible scenarios for the GHG-induced change in boundary forcing, the model dependency of the forced signals, and the dependency on methods for estimating internal decadal noise. The signal-to-noise analysis by the authors is thus different from other published decadal prediction studies, in that this study does not follow a trajectory from a particular initial state but rather considers the statistics of internal variability in comparison with the GHG signal. The 2011–20 decadal signal is characterized by surface warming over the entire North American continent, precipitation decreases over the contiguous United States, and precipitation increases over Canada relative to 1971–2000 climatological conditions. The signs of these forced responses are robust across different sea surface temperature (SST) scenarios and the different models employed, though the amplitude of the response differs. The North American decadal noise is considerably smaller than the signal associated with boundary forcing, implying a potential for high forecast skill for 2011–20 North American climate even for prediction methods that do not attempt to initialize climate models. However, the results do suggest that initialized decadal predictions, which seek to forecast externally forced signals and also constrain the internal variability, could potentially improve upon uninitialized methods in regions where the external signal is small relative to internal variability.

Tropical ocean-atmosphere controls on inter-annual climate variability in the Cretaceous Arctic

[1] The first annually resolved sedimentary record from the Cretaceous is used to develop time series of inter-annual and decadal scale climate variability from the Arctic Ocean. Analysis of records spanning 1000 years reveals strong periodicities in the quasi-biennial oscillation and El Nino - Southern Oscillation (ENSO) band as well as a 14 year period, which all closely match periodicities typical of modern high latitude climate variability. This supports the view that an Arctic Ocean free of permanent sea ice would be driven by similar forcing to the present state, implicating tropical ocean atmosphere interaction and demonstrating that stratosphere-troposphere coupling likely played a prominent role in the transmission of Cretaceous equatorial climate forcing to polar latitudes as has recently been established for the modern earth system. On the other hand, the prominent ENSO periodicities in our records argue against the hypothesized link between past warm climates and “permanent El Nino” states.

A molluscan perspective on hydrological cycle dynamics in northwestern Europe

AbstractShell aragonite δ180 values of unionid freshwater mussels are applied as a proxy for past river discharges in the rivers Rhine and Meuse, using a set of nine shells from selected climatic intervals during the late Holocene. A single Meuse shell derives from the Subboreal and its δ180 values are similar to modern values. The Rhine specimens represent the Subboreal, the Roman Warm Period and the Medieval Warm Period (MWP). These shells also show averages and ranges of aragonite δ180 values similar to modern specimens. This indicates that environmental conditions such as Rhine river dynamics, Alpine meltwater input and drought severity during these intervals were similar to the 20th century. These shells do not record subtle centennial to millennial climatic variation due to their relatively short lifespan and the large inter-annual and intra-seasonal variation in environmental conditions. However, they are very suitable for studying seasonal to decadal scale climate variability. The two shells with the longest lifespan appear to show decadal scale variability in reconstructed water δ180 values during the MWP, possibly forced by the North Atlantic Oscillation (NAO), which is the dominant mode of variability influencing precipitation regimes over Europe.

The response of a jack pine forest to late-Holocene climate variability in northwestern Wisconsin

Terrestrial plant communities have the potential to respond to climate change rapidly, if dominant species are killed by a series of extreme events, or slowly, if the cumulative effects of shorter-term climate fluctuations result in long-term compositional change. We used pollen and charcoal records from a lake and a testate amoebae-derived history of water-table depth in a nearby peatland to assess the response of the jack pine-dominated forests of northwestern Wisconsin to the climate variability of the last ~2000 years. The hydrology record and the charcoal record indicate that the climate near Warner Lake over the last ~2000 years was characterized by multidecadal variation in moisture availability with no apparent multicentennial-long trends in moisture balance or fire frequency. However, the pollen record suggests that there were multicentennial-scale changes in the vegetation composition around Warner Lake. Direct comparison of the three proxy records is challenging, because of their differing temporal resolutions and the complexity of potential ecological responses to climate variability. Therefore, we developed an interpretive model to compare multiple simulated proxies under two scenarios of environmental variability in order to determine under what conditions apparently contradictory records are likely to be found. The interpretive model reveals that a record of multicentennial-long change in vegetation is possible if multidecadal climate variability interacts with ecological processes influencing the direction and magnitude of succession. Compositional changes in the Warner Lake pollen record could reflect long-term variation in temperature, seasonality or other climate factors independent of moisture balance; however it is also possible that multidecadal moisture variability interacted with ecological processes affecting recruitment and mortality of species following fires of varying size and severity. Decadal-scale climatic variability can lead to altered successional pathways and to changes in forest composition that last for centuries.

A quantitative high-resolution summer temperature reconstruction based on sedimentary pigments from Laguna Aculeo, central Chile, back to AD 850

We present a pigment-based quantitative high-resolution (five years) austral summer DJF (December to February) temperature reconstruction for Central Chile back to AD 850. We used non-destructive in situ multichannel reflection spectrometry data from a short sediment core of Laguna Aculeo (33°50′S/70°54′W, 355 m a.s.l., central Chile). Calibration-in-time (period AD 1901—2000, cross-validated with split periods) revealed robust correlations between local DJF temperatures and total sedimentary chlorin (relative absorption band depth (RABD) centred in 660—670 nm RABD660;670: r=0.79, P&lt;0.01; five-years triangular filtered) and the degree of pigment diagenesis ( R660nm/670 nm: r=0.82, P&lt;0.01; five-years triangular filter). Root Mean Squared Error values are small (between 0.24 and 0.34°C) suggesting that most of the reconstructed decadal-scale climate variability is significant. Our data provide quantitative evidence for the presence of a Medieval Climate Anomaly (in this case, warm summers between AD 1150 and 1350; Δ T = +0.27 to +0.37°C with respect to (wrt) twentieth century) and a very cool period synchronous to the ‘Little Ice Age’ starting with a sharp drop between AD 1350 and AD 1400 (−0.3°C/10 yr, decadal trend) followed by constantly cool (Δ T = −0.70 to −0.90°C wrt twentieth century) summers until AD 1750. The structure of variability is consistent in great detail with annually resolved tree-ring based warm-season temperature and river discharge reconstructions from northern Patagonia (42°S) for the past 400 years, with qualitative climate reconstructions from Andean glacier fluctuations, and with hydrological changes in Patagonian lake sediment records.

The importance of decadal-scale climate variability to wind-driven modulation of hypoxia in Chesapeake Bay

AbstractMillions of dollars are spent annually to reduce nutrient loading to Chesapeake Bay, with a fundamental goal of reducing the extent and severity of low dissolved oxygen (hypoxia) during the summertime months^1^. Yet despite recent reductions in nutrient loading, large volumes of the Bay continue to be impacted by hypoxia and anoxia during the summer months^2-3^. One obstacle to assessing efforts to improve water quality in the Bay and other estuarine systems is a complete understanding of the physical processes that modulate dissolved oxygen and the long-term variability of these processes. Here I analyze a 58-year data set of estimated hypoxic volume in the Bay^2^ and demonstrate the importance that wind direction plays in controlling the extent and severity of summertime hypoxia. This analysis indicates that wind direction explains a greater percentage of the observed inter-annual variation in hypoxic volume than estimates of nutrient loading. The implication is that physical processes play a dominant role in modulating hypoxia and that much of the increased hypoxia observed since the early 1980s can be attributed to changes in wind forcing that are the result of decadal-scale climate variability. These findings emphasize the importance of understanding the physical processes that modulate dissolved oxygen in coastal and estuarine systems and highlight the potential impact that climate change may have on water quality in Chesapeake Bay and other estuarine systems.

ENSO related decadal scale climate variability from the Indo-Pacific Warm Pool

The El Nino-Southern Oscillation (ENSO) is a climatic phenomenon that affects socio-economical welfare in vast areas in the world. A continuous record of Holocene ENSO related climate variability of the Indo-Pacific Warm pool (IPWP) is constructed on the basis of stable oxygen isotopes in shells of planktic foraminifera from a sediment core in the western Pacific Ocean. At the centennial scale, variations in the stable oxygen isotope signal (delta O-18) are thought be a representation of ENSO variability, although an imprint of local conditions cannot be entirely excluded. The record for the early Holocene (10.3-6 ka BP) shows, in comparison with the mid- to late Holocene, small amplitude variations in the delta O-18 record of up to 0.3 parts per thousand indicating relatively stable and warm sea surface conditions. The mid- to late Holocene (6-2 ka BP) exemplified higher variability in delta O-18 and thus in oceanic IPWP conditions. Climatically, we interpret this change (5.5-4.2 ka BP) as a phenomenon induced by variability in frequency and/or intensity changes of El Nino. In the period 4.2-2 ka BP we identified several periods, centred on 1.9, 2.1, 2.7, 3.3, 3.7 and 4.2 ka BP, with in general heavy 6180 values. During these periods, the IPWP was relocated to a more eastward position, enhancing the susceptibility for El Nino-like conditions at the core site. Over the last 2000 yr precipitation increased in the area as a response to an increase in Asian monsoon strength, resulting in a freshening of the surface waters. This study corroborates previous findings that the present-day ENSO activity started around 5.5 ka BP. (c) 2006 Elsevier B.V. All rights reserved.

Effects of Temperature and Precipitation Variability on Snowpack Trends in the Western United States*

Abstract Recent studies have shown substantial declines in snow water equivalent (SWE) over much of the western United States in the last half century, as well as trends toward earlier spring snowmelt and peak spring streamflows. These trends are influenced both by interannual and decadal-scale climate variability, and also by temperature trends at longer time scales that are generally consistent with observations of global warming over the twentieth century. In this study, the linear trends in 1 April SWE over the western United States are examined, as simulated by the Variable Infiltration Capacity hydrologic model implemented at 1/8° latitude–longitude spatial resolution, and driven by a carefully quality controlled gridded daily precipitation and temperature dataset for the period 1915–2003. The long simulations of snowpack are used as surrogates for observations and are the basis for an analysis of regional trends in snowpack over the western United States and southern British Columbia, Canada. By isolating the trends due to temperature and precipitation in separate simulations, the influence of temperature and precipitation variability on the overall trends in SWE is evaluated. Downward trends in 1 April SWE over the western United States from 1916 to 2003 and 1947 to 2003, and for a time series constructed using two warm Pacific decadal oscillation (PDO) epochs concatenated together, are shown to be primarily due to widespread warming. These temperature-related trends are not well explained by decadal climate variability associated with the PDO. Trends in SWE associated with precipitation trends, however, are very different in different time periods and are apparently largely controlled by decadal variability rather than longer-term trends in climate.