Low-frequency Climate Variability Research Articles

Extracting long‐period climate fluctuations from tree‐ring chronologies over timescales of centuries to millennia

AbstractFor a long time, tree‐rings have been thought of as containing almost no variation at timescales of centuries and millennia, i.e. at low frequencies. Here, we show that this might be an issue of data analysis rather than an actual lack of variability. A data set of subfossil and living Scots pines from northern Fennoscandia was examined by means of their ring‐width time series. The premise was that the growth trends of individual time series could be quantitatively determined and decomposed into their different elements. It was shown that not all the components of growth trends were invariant over long periods of time, and that consequently the use of a single‐curve standardization (i.e. Regional Curve Standardization, RCS) may result in temporally inflated and deflated indices of ring‐widths. Observed non‐climatic bias in tree‐ring indices was probably due to gradually changing conditions in the pine population of the forest‐limit ecotone. Changes in population density seem to have hampered the previous attempts at palaeoclimate reconstruction by masking the actual low‐frequency climate variability. A new approach, expected to yield unbiased tree‐ring indices, was proposed. The new chronology constructed by this approach showed consistency with multi‐centennial variations that are based on independent palaeoclimate evidence. Copyright © 2005 Royal Meteorological Society.

OCEAN WARMING EFFECTS ON GROWTH, REPRODUCTION, AND SURVIVORSHIP OF SOUTHERN CALIFORNIA ABALONE

Traditional fisheries management in southern California has failed, in part because it is based on an assumption of an unvarying environment and is focused on size limits rather than insuring the persistence of aggregations of large fecund individuals. The combined effect of low frequency climatic variability and anthropogenic perturbations can have dramatic consequences for abalone in southern California. Abalone species are tightly linked to kelp forest ecosystems that, besides furnishing habitat, also provide the main food source for abalone. In southern California, kelp canopies are very sensitive to oceanographic climate because the kelp depend upon high nutrients in the water column. Oceanic warming, in turn, results in decreased nutrients in the surface water, and this is correlated with marked reductions in giant kelp biomass.Here we address the additive effects of ocean warming on two species of California abalone (the red abalone, Haliotis rufescens; and the green abalone, H. fulgens) by subjecting them to varied environmental conditions similar to cool, normal, and warm phases of the California current in the southern California Bight. Our experimental design simultaneously tested the synergistic effects of temperature and food quantity and quality on survivorship, growth, and reproduction. For red abalone, warm temperatures increased the onset of withering syndrome, a fatal abalone disease, and halted growth and reproduction. In contrast, green abalone survivorship, growth, and reproduction were relatively robust irrespective of temperature, while their growth and reproduction were most strongly influenced by food quantity. We found clear evidence suggesting that, combined with overfishing, California abalone populations are adversely affected by ecosystem responses to ocean warming: Cool‐water red abalone suffer stronger consequences in warm water than do green abalone. Conservation, restoration, and recovery plans of remnant California abalone populations must consider these relationships when taking any action.

Linkages between Regional Trends in Monthly Maximum Flows and Selected Climatic Variables

The potential impact of climate change on the hydrologic regime is a crucial question for water resources management. This study explores regional trends in monthly maximum flows and their possible linkages to trends in selected climatic variables in a hydroclimatologically homogeneous region. Trends are identified using the Mann-Kendall nonparametric test, with a modification for autocorrelated data. The regional significance of trends identified at the local scale is evaluated by means of a regional bootstrap algorithm. A trend significance index that accounts for both local and regional significance levels is proposed as a convenient tool for quantification and visual comparison of different trend results. The index is also used for identifying potential linkages between trends in hydroclimatic records. The plausibility of identified linkages is explored by means of cross-correlation analysis applied on residuals that are obtained from the original records after subtracting all serially dependent components. An uncertainty in regional trend analysis resulting from different observation periods is presented and quantified by calculating trend significance indices for several scenarios of different locations and lengths of a common observation period shifted on a timescale. The results show significant changes in the intraannual flood regime in the case study area of southern British Columbia. A regionally, strongly significant increase in the spring air temperature shifts the timing of the snowmelt process, resulting in a significant increase in early spring maximum flows and a significant decrease in late spring maximum flows. An autumn decrease in flows is related to increasing air temperature in the preceding summer months, which tends to dry out catchments more intensively, and is also related to precipitation activity in the previous months. The regional trend results are highly sensitive to the location and length of a given regional observation period on a timescale. Possible sources of the uncertainty in a low frequency climatic variability such as the Pacific Decadal Oscillation are discussed.

Associations between low frequency variability modes and winter temperature extremes in Canada

Relationships between patterns of low frequency climate variability including the El Niño‐Southern Oscillation (ENSO), the Arctic Oscillation (AO) and the Quasi‐Biennial Oscillation (QBO), and the occurrence of Canadian winter temperature extremes are examined for the 1950–98 period. Composite analyses reveal that ENSO plays a dominant role in the frequency and duration of both cold and warm spells. In particular, the warm phase of ENSO is associated with a significant increase (decrease) in the occurrence of warm (cold) spells and the number of extreme warm (cold) days across most of Canada. The findings are generally the opposite for the cold ENSO phase. The singular value decomposition (SVD) technique further shows that ENSO‐like interdecadal sea surface temperature variability plays a significant role in the variability of winter cold and warm spells over Canada. Differences between the high and low index phases of the AO show a significantly higher frequency and duration of cold spells over eastern Canada during positive AO winters, while the frequency of winter warm spells increases over the Canadian Prairies. These results are also confirmed by the SVD analysis using northern hemisphere 1000‐hPa circulation. The QBO, as determined by the phase of the stratospheric winds, has an effect on the spatial pattern of Canadian extreme temperatures that is similar to that of the AO. The most significant result involves increases in the frequency of warm spells and the number of extreme warm days over the southern Prairies during the westerly phase of the QBO. The diagnostic results from this study improve the understanding of the imapact of the low frequency variability modes on temperature extremes over Canada. They may also assist in the attribution of past trends and variability in extremes, and potentially, provide insight into future changes to extreme temperature events.

On the decadal and interdecadal variability in the Pacific Ocean

The Pacific decadal and interdecadal oscillation (PDO) has been extensively explored in recent decades because of its profound impact on global climate systems. It is a long-lived ENSO-like pattern of Pacific climate variability with a period of 10–30 years. The general picture is that the anomalously warm (cool) SSTs in the central North Pacific are always accompanied by the anomalously cool (warm) SSTs along the west coast of America and in the central east tropical Pacific with comparable amplitude. In general, there are two classes of opinions on the origin of this low-frequency climate variability, one thinking that it results from deterministically coupled modes of the Pacific ocean-atmosphere system, and the other, from stochastic atmospheric forcing. The deterministic origin emphasizes that the internal physical processes in an air-sea system can provide a positive feedback mechanism to amplify an initial perturbation, and a negative feedback mechanism to reverse the phase of oscillation. The dynamic evolution of ocean circulation determines the timescale of the oscillation. The stochastic origin, however, emphasizes that because the atmospheric activities can be thought as having no preferred timescale and are associated with an essentially white noise spectrum, the ocean response can manifest a red peak in a certain low frequency range with a decadal to interdecadal timescale. In this paper, the authors try to systematically understand the state of the art of observational, theoretical and numerical studies on the PDO and hope to provide a useful background reference for current research.

From proxy data to paleoclimate interpretation: the mid-Holocene paradox of the Atacama Desert, northern Chile

The question whether the mid-Holocene climate (between ca. 9 and 4 cal kyr B.P.) in the Atacama Desert and the Central Andes in general was humid or dry has wide implications with regard to the understanding of long-term climate variability in South America. Paleosols, regional groundwater tables, abiotic proxy data and pollen of aquatic plants in lake sediments show a marked and rapid shift from very humid late-glacial/early Holocene climatic conditions (between ca. 14 and 9.5 kyr B.P.) to extremely dry mid-Holocene conditions (more arid than today between ca. 9 and 4 kyr B.P.). An exception during this hyperarid period is a century-scale more humid interval around ca. 5.5–6 kyr B.P. that appears systematically in lake sediment archives. In contrast, pollen for most terrestrial plants preserved in lake sediments do not show major changes during the Holocene, whereas more humid mid-Holocene conditions (compared with late Holocene conditions) were inferred from plant macrofossils in rodent middens. Is the reason for this disagreement to be attributed to misinterpretation of the paleoenvironments or of the proxy records themselves, or to incomplete paleoclimatic interpretation of the paleoenvironments? We argue that these different paleoclimate archives record different aspects and facets of ‘climate’. While paleosols and groundwater in the Atacama Desert record low-frequency climate variability at century to millennium scales, lake sediments on the Altiplano record decade- to century-scale variability. Terrestrial vegetation responds to shorter high-frequency climate variability at seasonal to inter-annual scales and preferably to humid years. Vegetation remains in ‘hibernation’ or does not germinate during arid years. Thus information from these three types of archives is not a priori comparable and requires careful site-specific, archive-specific and time-scale-specific evaluation. What is natural in modern climatology is also true for paleoclimatology: a comprehensive assessment must account for the complex daily and seasonal cycles, for the range of climate variability and trends at different scales in space and time, for impacts of short-term extreme events, and for specific, often non-linear responses of individual bio-geo-physical archives to any of the numerous aspects of ‘climate’.

Collaborative research on the pelagic ecosystem of the southeastern Bering Sea shelf

Two Bering Sea marine research programs collaborated during the final years of the 1990s to forge advances in understanding the southeastern Bering Sea pelagic ecosystem. Southeast Bering Sea Carrying Capacity, sponsored by NOAA Coastal Ocean Program, investigated processes on the middle and outer shelf and the continental slope. The Inner Front Program, sponsored by NSF, investigated processes of the inner domain and the front between the inner and middle domains. The purposes of these programs were to (1) increase understanding of the southeastern Bering Sea ecosystem, including the roles of juvenile walleye pollock, (2) investigate the hypothesis that elevated primary production at the inner front provides a summer-long energy source for the food web, and (3) develop and test annual indices of pre-recruit pollock abundance. The observations occurred during a period of unusually large variability in the marine climate, including a possible regime shift. Sea-ice cover ranged from near zero to one of the heaviest ice years in recent decades. Sea-surface temperatures reached record highs during summer 1997, whereas 1999 was noted for its low Bering Sea temperatures. Moreover, the first recorded observations of coccolithophore blooms on the shelf were realized in 1997, and these blooms now appear to be persistent. The programs' results include an archive of physical and biological time series that emphasize large year-to-year regional variability, and an Oscillating Control Hypothesis that relates marine productivity to climate forcing. Further investigations are needed of the confluences of interannual and even intra-seasonal variability with low-frequency climate variability as potential producers of major, abrupt changes in the southeastern Bering Sea ecosystem.

Oceanic Influence on North Atlantic Climate Variability

Abstract The role of ocean dynamics in low-frequency climate variability in the North Atlantic is investigated using the Hadley Centre coupled climate model (HadCM3). An ensemble of three 100-yr experiments, in which low-frequency damping is applied below 500 m in the North Atlantic Ocean, is compared with the HadCM3 control integration that shows realistic variability in both SST and the North Atlantic Oscillation (NAO). All three damped runs have shown significantly reduced SST and NAO variability on decadal timescales, while interannual variability remains unchanged or even increased. A significance test of the ensemble mean low-frequency variance, against any three random 100-yr segments from the full HadCM3 control run, gives a confidence level above 95%. It is therefore concluded that ocean dynamics does play an active role in low-frequency variability of both the SST and the NAO at decadal and interdecadal timescales. By comparing mechanisms in the control and damped experiments it is also possible...

Modes of low-frequency climate variability and their relationships with land precipitation and surface temperature: application to the Northern Hemisphere winter climate

Variations in the Earth's climate have had considerable impact on society sectors such as energy, agriculture, fisheries, water resources, and environmental quality. This natural climate variability must be documented and understood in order to assess its potential impacts, its predictability and relationships with human-induced changes. Understanding the mechanisms responsible for natural variability proceeds through a strategy based on the use of a hierarchy of climate models and careful data analysis. In this paper, we examine primarily climate fluctuations on interannual-to-decadal time scales and their climate signature in terms of precipitation and temperature. First, space and time characteristics of two of the major variability modes, the Southern Oscillation (and its associated teleconnection patterns) and the North Atlantic Oscillation, are documented with a focus onto the midlatitudes of the Northern Hemisphere. Then, the current hypothesis regarding the nature of these modes (forced, coupled or internal) are reviewed based on both simulation results and statistical data analyses. Next, we address the potential predictability of seasonal surface temperature and land precipitation using an ensemble of atmospheric model simulations forced by observed sea surface temperatures. Finally, we review the relationships between the atmospheric variability modes and the recent low-frequency trends and suggest a possible influence of anthropogenic effects on these low-frequency variations.

A recurrent neural networks approach using indices of low-frequency climatic variability to forecast regional annual runoff

This paper evaluates the potential of using low-frequency climatic mode indices to forecast regional annual runoff in northern Quebec and the Labrador region. The impact of climatic trends in the forecast accuracy is investigated using a recurrent neural networks (RNN) approach, time-series of inflow to eight large hydropower systems in Quebec and Labrador, and indices of selected modes of climatic variability: El Nino-Southern Oscillation (ENSO), North Atlantic Oscillation (NAO), Pacific-North American (PNA), Baffin Island-West Atlantic (BWA) and sea-level pressure (SLP) at Iceland. A wavelet analysis is used to show that the selected climatic patterns are related to annual runoff from 1950 to 1996 in northern Quebec. The forecast results indicate that the use of BWA, PNA and ENSO indices results in better forecast skill than the use of SLP or NAO. Overall, the use of the BWA index is found to provide the best forecast improvement (38% on average), whereas the use of PNA provides 28% of improvement on average. Using the SLP index improves the forecast accuracy by 4%, and the use of an ENSO indicator leads to an improvement of 6%. The NAO index used here is found to provide only a modest improvement.

Multidecadal climate variability in the subtropics/mid-latitudes of the Southern Hemisphere oceans

Observations of multidecadal variability in sea surface temperature (SST), surface air temperatureand winds over the Southern Hemisphere are presented and an ocean general circulationmodel applied towards investigating links between the SST variability and that of the overlyingatmosphere. The results suggest that the dynamical effect of the wind stress anomalies is significantmainly in the neighbourhood of the western boundary currents and their outflows acrossthe mid-latitudes of each Southern Hemisphere basin (more so in the South Indian and SouthAtlantic than in the South Pacific Ocean) and in the equatorial upwelling zones. Over most ofthe subtropics to mid-latitudes of the Southern Hemisphere oceans, changes in net surface heatflux (particularly in latent heat) appear to be more important for the SST variability thandynamical effects. Implications of these results for modelling and understanding low frequencyclimate variability in the Southern Hemisphere as well as possible links with mechanisms ofdecadal/interdecadal variability in the Northern Hemisphere are discussed.

Reply Climate variability and crop yields in Europe

Hulme et al. reply — Porter and Semenov point out the importance of changes in climate variability on different timescales when assessing the significance of climate-change impacts. They are correct to assert that changes in inter-daily, inter-annual and multi-decadal climate variability are important, but our study1 was intended to draw attention to natural multi-decadal climate variability and to indicate how it might obscure the identification of significant effects of anthropogenic climate change. It was therefore primarily a study of signal-to-noise ratios in impacts indicators, something that has not previously been attempted using low-frequency climate variability. The effects on crop yields of changes in inter-annual and inter-daily climate variability have been considered previously2,3,4.

Interannual variability in western US precipitation

Low-frequency (interannual or longer period) climatic variability is of interest, because of its significance for the understanding and prediction of protracted climatic anomalies. Since precipitation is one of the key variables driving various hydrologic processes, it is useful to examine precipitation records to better understand long-term climate dynamics. Here, we use the multi-taper method of spectral analysis to analyze the monthly precipitation time series (both occurrence and amount) at a few stations along a meridional transect from Priest River, ID to Tucson, AZ. We also examine spectral coherence between monthly precipitation and widely used atmospheric indices, such as the central Northern Pacific (CNP) and southern oscillation index (SOI). This analysis reveals statistically significant ‘signals’ in the time series in the 5–7 and 2–3 year bands. These interannual signals are consistent with those related to El-Niño southern oscillation (ENSO) and quasi-biennial variability identified by others.

Air–sea interaction mechanisms and low-frequency climate variability in the South Indian Ocean region

Long-term observations indicate that the Indian Ocean displays significant low-frequency variability in mean sea-level pressure, near-surface wind, cloud and sea-surface temperature (SST). A general circulation model is used to study the response of the atmosphere to an idealized SST anomaly pattern (warm in southern mid-latitudes, cool in southern tropics) that captures the essence of observed multidecadal SST variability as well as that associated with ENSO in the South Indian Ocean. The major objectives are to investigate air–sea interaction mechanisms potentially associated with the variability and whether the atmospheric response to the SST is likely to lead to maintenance or damping of the original SST anomaly pattern, and on what time scale. Two types of experiment are performed to tackle these objectives. An ensemble of roughly 1-year-long integrations suggests that the seasonal-scale response of the atmosphere to the imposed SST anomaly includes reduced genesis and density of cyclones in the mid- to higher latitudes, and an indication of a shift in their tracks relative to climatology. It is argued that these changes together with those to the near-surface winds could be expected to lead to variations in surface fluxes that would tend to reinforce the original SST anomaly pattern on seasonal scales. A 21 year integration of the model with the SST anomaly pattern imposed throughout indicates that a low is generated near, and downstream of, the warm mid-latitude anomaly. On decadal/multidecadal scales, the associated changes to the surface winds are argued as being likely to lead to changes in surface fluxes and in the strength of the South Indian subtropical gyre that would oppose the original anomaly. The current and previous model results together with the observations then support the idea that the observed multidecadal variability in atmospheric circulation and SST of the South Indian Ocean during the past century may have arisen through a combination of basin scale atmosphere–ocean interaction and a remotely forced component. © 1998 Royal Meteorological Society

Causes of Decadal Climate Variability over the North Pacific and North America

The cause of decadal climate variability over the North Pacific Ocean and North America is investigated by the analysis of data from a multidecadal integration with a state-of-the-art coupled ocean-atmosphere model and observations. About one-third of the low-frequency climate variability in the region of interest can be attributed to a cycle involving unstable air-sea interactions between the subtropical gyre circulation in the North Pacific and the Aleutian low-pressure system. The existence of this cycle provides a basis for long-range climate forecasting over the western United States at decadal time scales.

Structure and Predictability of the El Niño/Southern Oscillation Phenomenon in a Coupled Ocean-Atmosphere General Circulation Model

The space-time structure and predictability of the El Niño/Southern Oscillation (ENSO) phenomenon was investigated. Two comprehensive datasets were analyzed by means of an advanced statistical method, one based on observational data and the other on data derived from an extended-range integration performed with a coupled ocean-atmosphere general circulation model. It is shown that a considerable portion of the ENSO-related low-frequency climate variability in both datasets is associated with a cycle involving slow propagation in the equatorial oceanic beat content and the surface wind field. The existence of this cycle implies the ability of climate predictions in the tropics up to lead times of about one year. This is shown by conducting an ensemble of predictions with our coupled general circulation model. For the first time a coupled model of this type was successfully applied to ENSO predictions.

Natural variability of the climate system and detection of the greenhouse effect

GLOBAL mean temperatures show considerable variability on all timescales. The causes of this variability are usually classified as external or internal1, and the variations themselves may be usefully subdivided into low-frequency variability (timescale ≳= 10 years) and high-frequency variability (≲=10 years). Virtually nothing is known about the nature or magnitude of internally generated, low-frequency variability. There is some evidence from models, however, that this variability may be quite large1,2, possibly causing fluctuations in global mean temperature of up to 0.4 °C over periods of thirty years or more (see ref. 2, Fig. 1). Here we show how the ocean may produce low-frequency climate variability by passive modulation of natural forcing, to produce substantial trends in global mean temperature on the century timescale. Simulations with a simple climate model are used to determine the main controls on internally generated low-frequency variability, and show that natural trends of up to 0.3 °C may occur over intervals of up to 100 years. Although the magnitude of such trends is unexpectedly large, it is insufficient to explain the observed global warming during the twentieth century.

The Impact of Climatology and Systematic Errors upon the Skill of DERF Forecasts

The average skill of 61 thirty-day forecasts of 500 mb geopotential height from the Dynamic Extended Range Forecast (DERF) experiment are investigated. These forecasts were made using the National Meteorological Center's Medium Range Forecast model and starting from initial analyses during the winter of 1986/87. The impact upon extended range forecast skill of the removal or retention of systematic errors and low-frequency climate variability is studied. If the systematic error is removed a posteriori at each forecast lead time, then the skill of forecasts of time averages may be improved considerably. The magnitude of this improvement is difficult to quantify with forecasts from a single season. Nearly all of the skill at extended range in the DERF experiments arises from the successful forecast of low-frequency fluctuations in the large-scale circulation.