Changes In Circulation Patterns Research Articles

Mechanisms and reliability of future projected changes in daily precipitation

We isolate the contribution of warming, other large-scale changes and soil moisture decline and feedbacks in driving future projected changes in daily precipitation across Europe. Our confidence in each of these mechanisms differs, so this analysis then allows us to determine an overall confidence (or reliability) in the projected changes. In winter, increases in extreme precipitation over Europe as a whole are judged to be reliable, dominated by increased atmospheric moisture with warming. At scales less than about 2,000 km changing circulation patterns could enhance or offset this increase. Additionally, over the Scandinavian mountains warming-induced circulation changes do offset the effect of increased moisture and the overall change is unreliable. In summer, increases in extreme precipitation over northern Scandinavia and decreases over the Mediterranean are reliable in the absence of considerable circulation change. Over central Europe, an increase in the proportion of summer rainfall falling as extreme events is reliable.

Climate change, coral reef ecosystems, and management options for marine protected areas.

Marine protected areas (MPAs) provide place-based management of marine ecosystems through various degrees and types of protective actions. Habitats such as coral reefs are especially susceptible to degradation resulting from climate change, as evidenced by mass bleaching events over the past two decades. Marine ecosystems are being altered by direct effects of climate change including ocean warming, ocean acidification, rising sea level, changing circulation patterns, increasing severity of storms, and changing freshwater influxes. As impacts of climate change strengthen they may exacerbate effects of existing stressors and require new or modified management approaches; MPA networks are generally accepted as an improvement over individual MPAs to address multiple threats to the marine environment. While MPA networks are considered a potentially effective management approach for conserving marine biodiversity, they should be established in conjunction with other management strategies, such as fisheries regulations and reductions of nutrients and other forms of land-based pollution. Information about interactions between climate change and more “traditional” stressors is limited. MPA managers are faced with high levels of uncertainty about likely outcomes of management actions because climate change impacts have strong interactions with existing stressors, such as land-based sources of pollution, overfishing and destructive fishing practices, invasive species, and diseases. Management options include ameliorating existing stressors, protecting potentially resilient areas, developing networks of MPAs, and integrating climate change into MPA planning, management, and evaluation.

Reply to comment by W. M. Connolley on “Was the 2003 European summer heat wave unusual in a global context?”

[1] We agree with Connolley [2008] that the 2003 heat wave as measured by the near surface temperature does indeed appear to be much more unusual than the 1000–500 mb depth-averaged anomaly. We chose, however, the averaged temperature to characterize the heat wave for several reasons. First, a layer average permits a more comprehensive assessment of large scale temperature anomalies than do single point, near-surface temperature anomalies which are more affected by local conditions [e.g., see, Pielke et al., 2007]. Secondly, unlike the depth averaged temperatures which are strongly observationally based, the near-surface, NCAR-NCEP reanalysis temperature is not directly constrained by observations and is therefore, in large part, a model calculation [Kalnay et al., 1996]. Errors in such quantities as soil moisture distribution in the reanalysis (a weakness of most models), for example, would make the calculation of near-surface temperature extremes less precise, especially in a situation where soil moisture seems to have played a strong role. For example, a too dry soil would spuriously amplify a warm anomaly. Finally, the effects of increasing anthropogenic CO2 on atmospheric temperature are expected to extend vertically through the troposphere, at least climatologically [Trenberth et al., 2007; Climate Change Science Program, 2006]. Assuming the near-surface temperature measurements are spatially representative, the conclusion that the heat wave was a shallow phenomenon in terms of its unusualness argues against the point of view that it was a direct manifestation of the effects of increased atmospheric CO2 (note the use of “SD thresholds exceeded” normalizes variability with altitude making all levels directly comparable). [2] Therefore we also conclude that land surface conditions (low soil moisture) are the likely direct cause for such an “unusual” event near the surface. This is supported by Fischer et al. [2007] who reported the lack of rainfall the preceding spring as an important contributing factor in enhancing the 2003 heat wave. Many processes could be responsible for the lack of previous season rainfall including regional processes and natural or anthropogenic changes in circulation patterns. [3] Figure 1 updates Chase et al. [2006] through 2006 for 2.0 and 3.0 SD levels and adds to our original conclusion that 2003 was not very unusual in terms of the spatial coverage of extreme depth-averaged temperatures. For example, the summer of 2004 also exceeded the 2003 warm anomaly extent. This was due to a strong Alaskan heat wave which produced a record breaking wildfire season documented in National Climatic Data Center's [2004] Annual Climate Report (http://www.ncdc.noaa.gov/oa/climate/research/2004/ann/events.html), but otherwise received much less public attention than the European heat wave. However, the addition of three additional summers (2004–2006) to the time series, all of which appear to be relatively warm, now indicates the possible emergence of an upward trend as suggested in previous work [Stott et al., 2004]. For example 2.0 SD warm anomalies now appear to have an upward trend (p = 0.05) though this trend should be viewed with caution because of the small sample size and the dominant effect of data points at the end of the series. The rise in 3.0 SD anomalies comparable to the 2003 heat wave is, however, still insignificant (p = 0.16) and so the increased probability of such extremes with time suggested by Stott et al. [2004] is not yet apparent.

Dynamics of the Venus upper atmosphere: Outstanding problems and new constraints expected from Venus Express

A consistent picture of the dynamics of the Venus upper atmosphere from ∼ 90 to 200 km has begun to emerge [e.g., Bougher, S.W., Alexander, M.J., Mayr, H.G., 1997. Upper Atmosphere Dynamics: Global Circulation and Gravity Waves. Venus II, CH. 2.4. University of Arizona Press, Tucson, pp. 259–292; Lellouch, E., Clancy, T., Crisp, D., Kliore, A., Titov, D., Bougher, S.W., 1997. Monitoring of Mesospheric Structure and Dynamics. Venus II, CH. 3.1. University of Arizona Press, Tucson, pp. 295–324]. The large-scale circulation of the Venus upper atmosphere (upper mesosphere and thermosphere) can be decomposed into two distinct flow patterns: (1) a relatively stable subsolar-to-antisolar (SS–AS) circulation cell driven by solar heating, and (2) a highly variable retrograde superrotating zonal (RSZ) flow. Wave-like perturbations have also been observed. However, the processes responsible for maintaining (and driving variations in) these SS–AS and RSZ winds are not well understood. Variations in winds are thought to result from gravity wave breaking and subsequent momentum and energy deposition in the upper atmosphere [Alexander, M.J., 1992. A mechanism for the Venus thermospheric superrotation. Geophys. Res. Lett. 19, 2207–2210; Zhang, S., Bougher, S.W., Alexander, M.J., 1996. The impact of gravity waves on the Venus thermosphere and O 2 IR nightglow. J. Geophys. Res. 101, 23195–23205]. However, existing data sets are limited in their spatial and temporal coverage, thereby restricting our understanding of these changing circulation patterns. One of the major goals of the Venus Express (VEX) mission is focused upon increasing our understanding of the circulation and dynamical processes of the Venus atmosphere up to the exobase [Titov, D.V., Lellouch, E., Taylor, F.W., 2001. Venus Express: Response to ESA's call for ideas for the re-use of the Mars Express platform. Proposal to European Space Agency, 1–74]. Several VEX instruments are slated to obtain remote measurements (2006–2008) that will complement those obtained earlier by the Pioneer Venus Orbiter (PVO) between 1978 and 1992. These VEX measurements will provide a more comprehensive investigation of the Venus upper atmosphere (90–200 km) structure and dynamics over another period in the solar cycle and for variable lower atmosphere conditions. An expanded climatology of Venus upper atmosphere structure and wind components will be developed. In addition, gravity wave parameters above the cloud tops will be measured (or inferred), and used to constrain gravity wave breaking models. In this manner, the gravity wave breaking mechanism (thought to regulate highly variable RSZ winds) can be tested using Venus general circulation models (GCMs).

Changes in the Northwest Atlantic circulation for the 1992–95 high NAO period from a numerical model

Present research suggests that the North Atlantic Oscillation (NAO) has a significant impact on the circulation in the Northwest Atlantic. An ocean general circulation model was used to quantify the changes of circulation patterns and transport on this region during a high NAO period (between 1992 and 1995) when compared to climatological mean forcing. Upstream of the study region, the maximum barotropic transport in the Labrador Sea was decreased by 8 Sv (1 Sv=10 6 m 3s −1) during the high NAO conditions, leading to a reduction in transport of the Labrador Current by ∼1.6 Sv east of Newfoundland and by ∼0.4 Sv south of Newfoundland. In addition, a Lagrangian analysis was performed to examine the pathways of transport that advect water properties across the Grand Banks and how those pathways are modified owing to the high-NAO forcing. The potential implications of these changes for biological production processes in the adjacent downstream shelf regions such as the Scotian Shelf, Gulf of Maine, and Georges Bank are discussed.

Local Controls on Carbon Cycling in the Ordovician Midcontinent Region of North America, with Implications for Carbon Isotope Secular Curves

Abstract The carbon isotope record from ancient epicontinental seas may contain much more of a local-scale carbon cycling signal than is generally appreciated. A unique opportunity exists to examine this issue in the case of the Late Ordovician Mohawkian Sea of eastern Laurentia, where the Millbrig K-bentonite stratigraphic framework has been used to delineate a time slice at 454 Ma, extending over ~ 1,500,000 km2 of the eastern United States. Across the time slice, carbonate and organic carbon δ13C vary by 4.5‰ and 7.5‰ respectively, a spatial variation that is as large as temporal (secular) changes in epeiric-sea δ13C that have been reported in the past. These new data are considered in the context of geographic variations in lithological, biological, and other geochemical sediment characteristics. Collectively, these sediment properties distinguish regions of the Mohawkian Sea which likely differed in terms of the nature and relative importance of carbon cycling processes. Water-column depth and structure, and barriers to free exchange of water across the Mohawkian Sea, may have been overarching factors in the development of these regions, raising the possibility that changes in circulation patterns, such as those caused by sea-level change, played a role in driving secular carbon isotope excursions by changing the rate of exchange of dissolved inorganic carbon between water masses. If the observed effects of local carbon cycling on the distribution of Mohawkian Sea δ13C were commonplace in ancient epicontinental marine environments, it would imply that local-scale carbon cycling may have left a nontrivial imprint on epeiric-sea records of secular variations in δ13C, in addition to the imprint left by changes in the global carbon cycle. This may have contributed to the broad scatter in δ13C values observed in the Paleozoic portion of the global carbon isotope secular curve.

Boundary Intensification of Vertical Velocity in a β-Plane Basin

Abstract The buoyancy-driven circulation of simple two-layer models on the β plane is studied in order to examine the role of beta in determining the magnitude and structure of the vertical motions forced in response to surface heating and cooling. Both analytical and numerical approaches are used to describe the change in circulation pattern and strength as a consequence of the planetary vorticity gradient. The physics is quasigeostrophic at lowest order but is sensitive to small nonquasigeostrophic mass fluxes across the boundary of the basin. The height of the interface between the two layers serves as an analog of temperature, and the vertical velocity at the interface consists of a cross-isopycnal velocity, modeled in terms of a relaxation to a prescribed interface height, as well as an adiabatic representation of eddy thickness fluxes parameterized as lateral diffusion of interface displacement. In the numerical model the lateral eddy diffusion of heat is explicitly represented by a resolved eddy field. In the plausibly more realistic case, when the lateral diffusion of buoyancy dominates the diffusion of momentum, the major vertical velocities occur at the boundary of the basin as in earlier f-plane studies. The effect of the planetary vorticity gradient is to intensify the sinking at the western wall and to enhance the magnitude of that sinking with respect to the f-plane models. The vertical mass flux in the Sverdrup interior exactly balances the vertical flux in the region of the strong horizontal transport of the western boundary current, leaving the net flux to occur in a very narrow region near the western boundary tucked well within the western boundary current. On the other hand, if the lateral diffusion of heat is arbitrarily and unrealistically eliminated, the vertical mass flux is forced to occur in the interior. The circulation pattern is extremely sensitive to small net inflows or outflows across the basin perimeter. The cross-basin flux determines the interface height on the basin’s eastern boundary and affects the circulation pattern across the entire basin.

Investigation of variability of the North Atlantic Subtropical Mode Water using profiling float data and numerical model output

The basic characteristics of the North Atlantic Subtropical Mode Waters (STMW) are well documented in the literature from one-time hydrographic sections or from long-term measurements at one location. From July 1997 to March 1998, 71 vertical profiling floats were deployed in the Western Subtropical North Atlantic region to provide a broader spatial and temporal coverage of STMW. In this study, the STMW properties are estimated from a total of 5352 float temperature profiles in an area covering 30°W–80°W and 20°N–45°N, from January 1998 to December 2000. These STMW properties are compared to those derived from numerical simulation output using the Miami Isopycnic Coordinate Ocean Model (MICOM). The model is configured to the North Atlantic Ocean with a 1-deg horizontal resolution and 20 vertical layers. The model temperature profiles are collocated with the observed float profiles for a direct comparison. The values of the mean STMW temperature derived from both the float data and model output are roughly equal (18.1 °C for floats and 18.0 °C for the model). The model is found to capture well the observed annual cycle of the STMW volume. Given good agreement between float and model profiles, we use a longer MICOM simulation to investigate decadal variability of the STMW renewal rate in terms of the annual subduction rate and its relationship to the large-scale atmospheric circulation pattern changes measured by the North Atlantic Oscillation index. Good correlation is found between the model annual subduction rate anomaly at the Panulirus Station and the NAO winter index anomaly on decadal time scales with NAO leading by 2–3 years.

A Holocene lacustrine record in the central North Atlantic: proxies for volcanic activity, short-term NAO mode variability, and long-term precipitation changes

Lake and peat corings on three Azores islands in the central North Atlantic, resulted in the discovery of a 6000 year long lacustrine sequence in a small crater lake, Lake Caveiro, on the island of Pico. This island is dominated by Pico mountain (2351 m), Portugal's highest mountain, and the lake site is situated at 903 m asl. Two sediment profiles, one central and one littoral, were sampled. Due to large facial shifts and disconformities in the littoral cores the analyses were concentrated on the central core; only the earliest 1000 years of the littoral core were studied to complement the central profile. We used sedimentology, geochemistry, diatom analyses, magnetic properties, and multivariate statistics, together with 14C and 210Pb dating techniques, to analyse the environmental history of the lake. Volcanic activity seems to have had a dominating impact on sediment changes and partly also on the diatom assemblages; a large number of tephras are found and seem to be connected with large (diatom) inferred pH variations. However, by a combination of methods, including multivariate techniques, we infer that precipitation changes can be detected through the volcanic noise. In the youngest part of the record (AD 1600–2000), with its decadal resolution, these humidity variations seem partly related to shifts in dominating NAO mode. The more long-term precipitation changes further back in time (350–5100 cal yr BP) roughly correspond to the well-known North Atlantic drift-ice variations as well as other North Atlantic records; low precipitation during drift-ice periods. We think these alterations were driven by changes in the thermohaline circulation as large-scale equivalences to the Great Salt Anomaly; low sea surface temperatures and changes in circulation patterns of the central North Atlantic decreased the regional precipitation. Cooler/drier periods occurred 400–800, 1300–1800, 2600–3000, 3300–3400 and possibly also 4400–4600 cal yr BP, while 300–400, 900–1000, 2000–2400, 3100–3200, 3800–4000 and 4700–5000 cal yr BP seem to have been more humid phases on the Azores.

Optical properties of colored dissolved organic matter in the Northern Gulf of Mexico

Variations in the concentration and inherent optical properties of colored dissolved organic matter (CDOM) in river-dominated margins provide information on the cycling of carbon and its chemical composition. Large-scale temporal and spatial variability in CDOM concentration and optical properties were observed in the Northern Gulf of Mexico in June of 2000 and April of 2001. Terrestrial CDOM from the Mississippi and Atchafalaya Rivers dominates the region. Although the primary factor controlling CDOM in this region is the quantity of fresh water runoff, strong regional variability in fresh water sources also plays a major role. Physical complexity due to changes in circulation patterns, volume of river flow and multiple river sources makes observation of biological and photochemical effects challenging in this region. We have used two approaches to distinguish between source and transformation effects: mixing models, which include the concentration and inherent optical properties of CDOM from discrete samples, and high-resolution three-dimensional mapping of multi-spectral fluorescence.

Late Quaternary climates of the Australian arid zone: a review

Aridity in Australia has generally been associated with glacial intervals of the last few glacial cycles. The causes and nature of aridity varied from north to south over the continent. In northern and central Australia, reduced monsoon rain in global cold stages caused lakes and rivers to dry, vegetation to become more sparse, sand dunes to become active, and dust advection to increase. While the monsoon is correlated with warmer global temperatures and high sea levels overall, the exact timing of the greatest development of the monsoon in Stage 5 remains uncertain. In southeastern Australia, the proxy records give a less clear picture. While vegetation seems to have been more sparse in cold stages and sand dune activity and dust flux consequently greater, records of runoff in rivers and lakes have mixed or opposite behaviour to northern or central Australia. The nature of aridity in southeastern Australia is not analogous to conditions anywhere on the continent today. Greatly reduced temperatures over the central and southern interior, greater influence of snow and periglaciation on runoff, high groundwater tables, lower rainfall and reduced atmospheric carbon dioxide levels may all have contributed to and interacted to produce the reconstructed picture of a cold, sparsely vegetated arid landscape with large rivers and perennial lakes. The largest changes in circulation patterns over the glacial cycle probably occurred in the location and/or intensity of summer tropical convergence in northern Australia. Over southern Australia, changes to the temperature and humidity of the westerly circulation have been more significant than the small fluctuations in latitude of the sub-tropical high pressure ridge.

Dependency of larval fish survival on retention/dispersion in food limited environments: the Baltic Sea as a case study

AbstractA three‐dimensional hydrodynamic model has been used to analyse temporally and spatially resolved circulation patterns in the Baltic Sea with special emphasis on drifting particles representing larval fish. The main purpose of this study was (i) to investigate potential drift patterns of larval fish, (ii) to identify its intra‐ and inter‐annual variability for time periods based on the timing of spawning and (iii) to analyse its seasonal and spatial variability in dependence of the atmospheric forcing conditions. For the time period 1979–1998 temporally and spatially resolved simulated flow fields were used to describe the potential drift from the centre of main reproductive effort of Baltic cod (Gadus morhua). The results of the model runs demonstrate a general change in circulation pattern from retention during a first decade (1979–1988) to dispersion in the following decade (1989–1998). This increase in dispersion was related to an increase in the variability of the local wind forcing conditions over the Baltic. The more frequent occurrence of dispersion in spring of the recent decade was accompanied by a strong decay in biomass of one of the main larval fish feeding component in the central basin, the calanoid copepod Pseudocalanus elongatus. Larger dispersion of this prey organism may have affected the spatial overlap and thus the contact rates between predator and prey. Hence, this may have resulted in a food limitation for early life stages of Baltic cod and potentially contributed to the pronounced shift in cod peak spawning time from spring to late summer. Early life stages of cod originating from late spawning fish, benefited from a stronger dispersion in late summer and autumn, into shallow coastal areas with higher calanoid abundance.

Fire impacts on surface heat, moisture and carbon fluxes from a tropical savanna in northern Australia

Savannas form a large fraction of the total tropical vegetation and are extremely fire prone. We measured radiative, energy and carbon exchanges over unburned and burned (both before and after low and moderate intensity fires) open forest savanna at Howard Springs, Darwin, Australia. Fire affected the radiative balance immediately following fire through the consumption of the grass-dominated understorey and blackening of the surface. Albedo was halved following fire of both intensities (from 0.12 to 0.07 and from 0.11 to 0.06 for the moderate and low intensity sites, respectively), but the recovery of albedo was dependent on the initial fire intensity. The low intensity fire caused little canopy damage with little impact on the surface energy balance and only a slight increase in Bowen ratio. However the moderate fire resulted in a comprehensive canopy scorch and almost complete leaf drop in the weeks following fire. The shutdown of most leaves within the canopy reduced transpiration and altered energy partitioning. Leaf death and shedding also resulted in a cessation of ecosystem carbon uptake and the savanna turned from a sink to a source of carbon to the atmosphere because of the continued ecosystem respiration. Post-fire, the Bowen ratio increased greatly due to large increases in sensible heat fluxes. These changes in surface energy exchange following fire, when applied at the landscape scale, may have impacts on climate through local changes in circulation patterns and changes in regional heating, precipitation and monsoon circulation.

Weather Observations Across the Southern Andes at 53°S

Regional variations of weather pattern were analyzed along a west-to-east profile across the Southern Andes (53°S), one of the most pronounced climate-divides in the world. For the first time we present a meteorological record from an array of three automatic weather stations (AWS), operated by the authors, for the central part of the climate divide which, together with previously existing Chilean weather stations, complete the transect. These data cover a time period of 3 yr. from October 1999 until September 2002. Air temperatures along the profile are highly correlated. Annual precipitation drops from between 6000 mm and 7000 mm at sea level along the main divide of the mountains to only about 1000 mm at the eastern slopes of the Andes and to as little as 430 mm at Punta Arenas. The variations of rainfall with wind direction and synoptic weather types are markedly different between the central part of the Andes and Punta Arenas. At the center of the climate divide precipitation correlates positively with wind speed from the west, whereas at Punta Arenas, east of the Andes, higher rainfall rates occur with easterly air flow. It is assumed that this reflects the barrier effect of the mountain range of the Andes. The results indicate that in order to make references about present or past climatic variations in Patagonia, it is essential to consider the effect of changes in circulation patterns.

Sensitivity of modern and Holocene floods to climate change

Alluvial records of paleofloods show that natural floods resulting from excessive rainfall, snowmelt, or from combined rainfall and snowmelt are highly sensitive to even modest changes of climate equivalent or smaller than changes expected from potential future global warming in the 21st century. The high sensitivity results from effects of hemispheric or global-scale changes in circulation patterns of the ocean and atmosphere to influence the pathways and locations of air masses and storm tracks. Holocene paleoflood chronologies from the Upper Mississippi Valley in the Midwest United States and from the Colorado River drainage of the Southwest United States show that recurrence frequencies of large floods have been subject to abrupt changes over time. These flood chronologies and flood chronologies observed for other middle-latitude regions suggest that recurrence frequencies of large floods are increased when there is an increase in the number of waves and their amplitudes in the middle and upper tropospheric circum-polar westerly circulation. However, some middle-latitude regions on the western margins of continents experience increased frequencies of flooding during strong onshore zonal westerly circulation. Flood chronologies from several regions suggest that times of rapid climate change have a tendency to be associated with more frequent occurrences of large and extreme floods. The unusual high frequencies of large floods that have been observed in many regions since the early 1950s are often attributed to land use change, but the rapid climate forcing from the effects of increased atmospheric greenhouse gases may also be a contributing factor. Paleoflood records provide information that is useful for better interpretation and calibration of modern short-term instrumental records, and they provide unique event-scale information that is useful for calibrating and testing geophysical models of past and anticipated future climate conditions.

Molecular indicators of redox and marine photoautotroph composition in the late Middle Ordovician of Iowa, U.S.A.

Saturated and aromatic hydrocarbons were used to evaluate depositional redox conditions and marine photoautotroph contributions to Middle Caradocian strata of the central United States (IA). At the base of the Spechts Ferry Member of the Decorah Formation, 13C-enriched aryl isoprenoids, derivatives of green sulfur bacteria, become abundant, indicating the development of photic-zone anoxia. This is coincident with the disappearance of the organic-walled microfossil Gloeocapsomorpha prisca and a marked decrease in the relative abundances of cyanobacterial biomarkers. The development of dysoxic to anoxic conditions and/or associated changes in basin circulation potentially affected the distributions and abundances of these organisms. In the overlying Guttenberg Member, G. prisca-derived organic matter becomes dominant, but relative cyanobacteria abundances remain low. In addition, the percentage total organic carbon is greater than 20%, even though selected biomarker ratios (pristane/phytane ratios greater than 3, and homohopane indices less than 0.5) and the presence of bioturbation indicate that bottom waters were oxygenated. It is suggested that deposition of G. prisca affected both organic matter preservation and depositional redox conditions. Observed variations in redox indicators and marine photoautotroph contributions are associated with changes in siliciclastic deposition, reported macrofauna turnover and with evidence for oceanic cooling and a change in circulation patterns documented in the eastern United States.

Climate signals in Palaeozoic land plants

The Palaeozoic is regarded as a period in which it is difficult to recognize climate signals in land plants because they have few or no close extant relatives. In addition early, predominantly axial, representatives lack the features, e.g. leaf laminae, secondary growth, used later as qualitative and quantitive measures of past climates. Exceptions are stomata, and the preliminary results of a case study of a single taxon present throughout the Devonian, and analysis of stomatal complex anatomy attempt to disentangle evolutionary, taxonomic, habitat and atmospheric effects on stomatal frequencies. Ordovician–Silurian vegetation is represented mainly by spores whose widespread global distribution on palaeocontinental reconstructions with inferred climates suggest that the producers were independent of major climate variables, probably employing the physiology and behavioural strategies of extant bryophytes, further characterized by small size. Growth–ring studies, first possible on Mid–Devonian plants, have proved most informative in elucidating the climate at high palaeolatitudes in Late Permian Gondwana. Changes in the composition of Carboniferous–Permian low–latitude wetland vegetation are discussed in relation to tectonic activity and glaciation, with most confidence placed on the conclusion that major extinctions at the Westphalian–Stephanian boundary in Euramerica resulted from increased seasonality created by changes in circulation patterns at low latitudes imposed by the decrease of glaciations in most parts of Gondwana.

Variations in western Arctic temperatures in response to cloud radiative and synoptic‐scale influences

The analysis focuses on Barrow, Alaska, a site that is sensitive to changing conditions because it is located near cryospheric boundaries and is influenced by both extratropical and Arctic synoptic activity. Surface and upper air meteorological data for a 31‐year period (1965–1995) are used to evaluate temperature variations as they relate to dynamical and radiative processes. Both annual and monthly analyses indicate a tendency toward warming overall. However, the annual warming is not monotonic over time and varies seasonally. Comparisons of temperature time series from four sites along the Siberian‐Alaskan coastline show that Barrow is a representative site to evaluate climate change in the western Arctic coastal zone. Regionally, the warming is dominated by significant temperature increases during winter and spring, but cooling is indicated for autumn. These results are not entirely consistent with model predictions of a more uniform high‐latitude warming during the cold season in response to increasing concentrations of greenhouse gases in the atmosphere. Rather, the observed changes are attributed to well‐known natural processes that affect regional cloud distributions in response to changing circulation patterns. Coincident daily and hourly meteorological and radiation data are also used to demonstrate empirically how clouds modulate Arctic temperatures.

Joint Spatiotemporal Modes of Surface Temperature and Sea Level Pressure Variability in the Northern Hemisphere during the Last Century

Coherent spatiotemporal modes of climatic variability are isolated based on a multivariate frequency domain singular value decomposition (SVD) of nearly a century of monthly Northern Hemisphere sea level pressure (SLP) and surface temperature data. Insight into the underlying physical processes associated with potential climatic signals is obtained by examining the relationship between surface temperature and inferred atmospheric circulation patterns as they evolve over the a typical cycle, taking potential seasonal distinctions into account. Our analysis provides evidence for two significant independent secular variations describing a secular warming trend (and accompanying changes in circulation patterns) and a century timescale “oscillation” marked by high-amplitude variations in temperature and SLP in the North Atlantic that are similar to those observed in recent model simulations. Quasi-oscillatory interdecadal (16–18 yr timescale) variability also displays a pattern similar to those predicted in recent model experiments, with an apparent origin in the North Pacific. Weaker quasi-decadal (10–11-yr timescale), largely cold-season oscillatory behavior is more closely tied to the North Atlantic and may involve analogous mechanisms. Interannual variability is examined with an “evolutive” generalization of our procedure to captures the time-evolving frequency and amplitude characteristics of the associated climate signal. Variability exhibiting the characteristic climatic patterns of the global El Niño-Southern Oscillation (ENSO) phenomenon is described by two largely distinct frequency bands within the broader 3–7-yr ENSO band. The drifting central frequencies of these two dominant bands is suggestive of nonstationary behavior in ENSO. A quasibiennial signal exhibits a gradual trend toward increasing frequency. Prospects for improved long-range climate forecasting are discussed.

The Paleocene-Eocene benthic foraminiferal extinction and stable isotope anomalies

Abstract In the late Paleocene to early Eocene, deep sea benthic foraminifera suffered their only global extinction of the last 75 million years and diversity decreased worldwide by 30–50% in a few thousand years. At Maud Rise (Weddell Sea, Antarctica; Sites 689 and 690, palaeodepths 1100 m and 1900 m) and Walvis Ridge (Southeastern Atlantic, Sites 525 and 527, palaeodepths 1600 m and 3400 m) post-extinction faunas were low-diversity and high-dominance, but the dominant species differed by geographical location. At Maud Rise, post-extinction faunas were dominated by small, biserial and triserial species, while the large, thick-walled, long-lived deep sea species Nuttalides truempyi was absent. At Walvis Ridge, by contrast, they were dominated by long-lived species such as N. truempyi , with common to abundant small abyssaminid species. The faunal dominance patterns at the two locations thus suggest different post-extinction seafloor environments: increased flux of organic matter and possibly decreased oxygen levels at Maud Rise, decreased flux at Walvis Ridge. The species-richness remained very low for about 50 000 years, then gradually increased. The extinction was synchronous with a large, negative, short-term excursion of carbon and oxygen isotopes in planktonic and benthic foraminifera and bulk carbonate. The isotope excursions reached peak negative values in a few thousand years and values returned to preexcursion levels in about 50 000 years. The carbon isotope excursion was about −2‰ for benthic foraminifera at Walvis Ridge and Maud Rise, and about −4‰ for planktonic foraminifera at Maud Rise. At the latter sites vertical gradients thus decreased, possibly at least partially as a result of upwelling. The oxygen isotope excursion was about −1.5‰ for benthic foraminifera at Walvis Ridge and Maud Rise, −1‰ for planktonic foraminifera at Maud Rise. The rapid oxygen isotope excursion at a time when polar ice-sheets were absent or insignificant can be explained by an increase in temperature by 4–6°C of high latitude surface waters and deep waters world wide. The deep ocean temperature increase could have been caused by warming of surface waters at high latitudes and continued formation of the deep waters at these locations, or by a switch from dominant formation of deep waters at high latitudes to formation at lower latitudes. Benthic foraminiferal post-extinction biogeographical patterns favour the latter explanation. The short-term carbon isotope excursion occurred in deep and surface waters, and in soil concretions and mammal teeth in the continental record. It is associated with increased CaCO 3 -dissolution over a wide depth range in the oceans, suggesting that a rapid transfer of isotopically light carbon from lithosphere or biosphere into the ocean-atmosphere system may have been involved. The rapidity of the initiation of the excursion (a few thousand years) and its short duration (50 000 years) suggest that such a transfer was probably not caused by changes in the ratio of organic carbon to carbonate deposition or erosion. Transfer of carbon from the terrestrial biosphere was probably not the cause, because it would require a much larger biosphere destruction than at the end of the Cretaceous, in conflict with the fossil record. It is difficult to explain the large shift by rapid emission into the atmosphere of volcanogenic CO 2 , although huge subaerial plateau basalt eruptions occurred at the time in the northern Atlantic. Probably a complex combination of processes and feedback was involved, including volcanogenic emission of CO 2 , changing circulation patterns, changing productivity in the oceans and possibly on land, and changes in the relative size of the oceanic and atmospheric carbon reservoirs.