Arctic Air Masses Research Articles

Postglacial climate in the St. Lawrence lowlands, southern Québec: pollen and lake-level evidence

Pollen and lake-level data are used to reconstruct past climate changes in the St. Lawrence lowlands, southern Québec. Past lake-level changes are assessed from sedimentological, pollen and macrofossil records from a single shallow-water core from Lac Hertel, which lies in the central part of the studied area. Three low lake-level phases are recognised: prior to 8000, 7600–6600 and 4800–3400 cal. BP. The modern analogue method is applied to pollen data from seven well-dated sites from the St. Lawrence lowlands and adjacent mountain areas, constrained and unconstrained by lake-level changes. The reconstructed climate changes are congruent with the pattern of climate changes known from eastern North America: a dry and cold late-glacial episode due to the presence of pro-glacial lakes and seas; a rapid warming between 12 500 and 11 000 cal. BP possibly caused by increasing summer insolation; a dry period from 10 000 to 6500 cal. BP; a brief cooling between 9000 and 8000 cal. BP, possibly related to a summer cooling of Arctic airmasses; a temperature maximum around 8000 cal. BP; and finally, a progressive decrease in summer temperature and an increase in (winter?) precipitation over the 4500 last years. These results show that it is possible to reveal seasonal patterns in climate by combining pollen and lake-level data.

Carbonyl Sulfide Concentration in the Arctic Lowermost Stratosphere and Stratosphere-troposphere Transport

Concentrations of carbonyl sulfide (COS) were measured in whole air samples collected between East Asia (Japan) and the Arctic (Spitsbergen) up to an altitude of 12 km during the Arctic Airborne Measurement Program 98 (AAMP98). Continuously measured O3 and CO2 concentrations were used to interpret the distribution of COS concentrations. A latitudinal gradient to lower concentrations of COS was observed poleward of 70°N; concentrations of COS were 421±27 pptv poleward of 70°N and 461±30 pptv at 60-70°N. The relationship between COS concentration and the O3/CO2 ratio indicates that the concentrations of COS near the ozonopause were almost equal to those in the troposphere, and then decreased as the O3/CO2 ratio increased. Tropospheric influence was observed up to potential temperatures of 350 K in this study. According to the AAMP98 observations, on the basis of the trajectory analysis and the O3/O2 ratio, distribution of COS in the Arctic lowermost stratosphere is certainly effected by the downward transport of an arctic stratospheric air mass and by the horizontal transport of a tropospheric air mass across the tropopause over midlatitudes. The sulfate aerosol produced by COS oxidation is estimated to be very small at the observed altitudes between East Asia and the Arctic in late winter.

Analysis of the Algerian severe weather event in November 2001 and its impact on ozone and nitrogen dioxide distributions

An analysis of the severe weather event in November 2001 over the western Mediterranean is presented focusing on satellite-based trace gas measurements from the Global Ozone Monitoring Experiment (GOME) on board the European Remote Sensing Satellite (ERS-2). This study is supplemented by a synoptic analysis and simulations of the three-dimensional stratospheric chemical transport model ROSE. Arctic air masses moved rapidly from Scandinavia to the Iberian peninsula and were mixed with subtropical air over the still warm Mediterranean Sea. This caused severe thunderstorms and extreme rainfall along the coasts of Morocco and Algeria and later on the Balearic Islands. Associated with the meridional transport an intrusion of stratospheric air below 3 km above sea level was observed. The maximum potential vorticity (PV) derived from UK Meteorological Office analysis data was about 9.3 potential vorticity units (pvu) at 330Kat the equatorward position of 35°N. These very high values went along with remarkably enhanced total ozone levels obtained from GOME backscatter measurements of collocated GOME/ERS-2 overpasses. Further investigation of GOME data showed unusually high levels of nitrogen dioxide (NO2) above the western Mediterranean.We present a new method to derive the tropospheric content of nitrogen dioxide (NO2) from a combination of satellite measurements and results of a chemical transport model.We show that about two-third of the total atmospheric content of nitrogen dioxide in the observed plume is found in the troposphere, due to lightning activity, advection and vertical transport in the thunderstorms from the planetary boundary layer (PBL) to atmospheric levels above clouds.

A proposed mechanism for the regulation of minimum midtropospheric temperatures in the Arctic

This paper documents an observed lower extreme of midtropospheric (500 mbar) temperatures in the Arctic of approximately −45°C during the winter season in several data sets. Each data set shows that the coldest air masses in the Arctic reach −45°C during the fall months but seldom get much colder even into late winter despite a continued net radiative loss. We demonstrate that midtropospheric temperatures are significantly skewed toward warmer temperatures, indicating a regulatory mechanism at work. We further provide evidence that minimum Arctic midtropospheric temperatures are regulated by moist convective processes and that minimum 500 mbar temperatures are controlled to a large extent by high‐latitude sea surface temperatures. The temperature −45°C is the expected 500 mbar temperature in an atmosphere regulated by moist adiabatic ascent from a surface temperature of 1°–2° below 0°C, the approximate freezing point of seawater. This implies that Arctic air masses are regularly in contact with unfrozen seawater to the south, an easily verified observation. Climate model simulations of the effects of increased greenhouse gasses hypothesize that high, northern latitude regions should warm at a faster rate than the globe as a whole, a hypothesis which does not appear to have strong observational support. We discuss the implications of this result for the accelerated Arctic warming hypothesis.

Small-particle concentration fluctuations at a coastal site

Aerosol size spectra ( d=10 nm–10 μm) were measured with an electrical aerosol spectrometer (EAS) at Mace Head on the west coast of Ireland. Several small aerosol particle (diameter 10–32 nm) concentration bursts were observed during the measurement period. Relationships between the events, air mass trajectories, tide height, and meteorological parameters are examined. Series of bursts were observed when a spectral transformation due to subsequent particle growth from 10 to 56–100 nm can be identified in an Eulerian experiment. Particle growth rates of between 1 and 3 nm/h were determined. These bursts appear in cold and comparatively clean arctic or polar air masses with temperature and relative humidity fluctuations, and do not correlate with low tide in some cases. These episodes, similar to those frequently found in the continental boundary layer, are thought to occur over a wide area and, for clear detection, require stable airflow for a few days. Elevated small-particle concentration events are more common during low tide or shortly after, and are typically associated with low wind speeds. Here, the increased shore exposure during low tide is thought to influence the nucleation and the subsequent growth of these aerosol particles. The occurrences of the bursts are found to depend on local wind direction. The highest d=10–32 nm particle concentrations appeared for wind sectors furthest from the tidal regions when the wind direction was 150–160°(south-easterly). Most of the events occurred during daytime when solar irradiation is most intense.

Synoptic tracer gradients in the upper troposphere over central Canada during the Stratosphere‐Troposphere Experiments by Aircraft Measurements 1998 summer campaign

During the July 1998 Stratosphere‐Troposphere Experiments by Aircraft Measurements (STREAM) intensive campaign, eight measurement flights were conducted from Timmins airport (Ontario, Canada, 48.2°N, 79.3°W). In situ measurements of ozone, carbon monoxide, carbon dioxide, and nonmethane hydrocarbons, as well as three‐dimensional back trajectories based on European Centre for Medium‐Range Weather Forecasts wind field analyses, are used to characterize upper tropospheric air masses with respect to their origin. In the upper troposphere between 8 km and the local tropospause, about 40% of the air masses originated from the subtropics/tropics south of 30°N, about 50% originated from midlatitudes (between 30° and 50°N), and 12% originated from the polar region north of 50°N. In general, highest trace gas levels were observed in air masses originating from midlatitudes (e.g., CO, 107 ± 25 ppbv; ethane, 983 ± 385 pptv; acetylene, 119 ± 43 pptv), with the exception of CO, which had highest mixing ratios (121 ± 11 ppbv) in air masses of polar origin. The lowest concentrations were generally observed in air masses of (sub)tropical origin (e.g., CO, 64 ± 2 ppbv; ethane, 424 ± 86 pptv; acetylene, 39 ± 14 pptv). For CO and most of the alkanes a significant positive latitudinal gradient was observed, while CO2 and CH3Cl exhibited an inverse gradient with lowest mixing ratios at high northern latitudes. This reflects the different source and sink distributions of the investigated species. Ozone mixing ratios significantly increased in the upper troposphere above 8 km altitude, at least partially because of stratosphere‐troposphere exchange. Approximately 10% of the upper tropospheric air was directly influenced by recent cross‐tropopause exchange. A case study indicates that the convergence of air masses of different origin along a cold front creates sharp gradients not only in temperature and humidity but also in trace gas concentrations. It is shown that these mesoscale gradients result from the organized flows of tropical, arctic, and stratospheric air masses.

Impact of Arctic treeline on synoptic climate

The position of the Arctic treeline has been associated with the preferred location of Arctic fronts that demarcate the boundary between Arctic and polar air masses. It has been argued that differences in summer energy balance, particularly in sensible heating, across treeline between forest and tundra may help determine the location of these frontal zones. Our observations of the energy balance at treeline show that the daily averaged summer heating contrast between treeline forest and tundra is an order of magnitude less than that previously proposed and that the influence on the atmosphere is confined to a shallow layer. In addition, maximum heating contrasts occurred in spring rather than in summer when the Arctic frontal zone is best expressed. Hence energy‐balance contrasts at treeline are unlikely to be the major determinant of the position of the Arctic front.

Effects of continental boundary layer evolution, convection, turbulence and entrainment, on aerosol formation

Aerosol nucleation events occurring in the continental boundary layer over the boreal forest region in Finland, during the BIOFOR experiment, have been examined to elucidate the rôle of micrometeorology in promoting such events. Invariably, during the spring campaign of 1999, nucleation events occurred in Arctic and polar air masses during cold air outbreaks. Under clear-sky conditions, typical of these synoptic meteorological patterns, the boundary layer evolution was characterized by the rapid growth of a mixed layer, convection and strong entrainment, first from the residual later and later from the free troposphere. It was found that the freshly nucleated particles were detected within two hours from the onset of strong turbulent kinetic energy, independent of how fast the boundary layer evolved. When considering the growth time from cluster size of = 1 nm to detectable sizes of 3 nm, the nucleation and onset of strong turbulence coincided almost exactly. The most likely site for nucleation to take place was the mixed layer or the entrainment zone, while the forest canopy and the free troposphere could be excluded as the nucleation region. There are several possible explanations for the correlation between the onset of turbulence and nucleation: (1) new aerosols or clusters may have been entrained from the residual layer into the mixed layer where they then (in the case of clusters) underwent growth to detectable sizes; (2) two or more precursor gases may have been mixed with each other over the entrainment zone; (3) the adiabatic cooling in the rising convective plumes and the turbulent fluctuation in temperature and vapors by the entrainment flux may have enhanced aerosol formation; (4) a sudden decrease in preexisting aerosol due to dilution of the mixed layer aerosol by entrained air may have reduced the vapor sink enough to initiate nucleation. However, the lack of vertical profile measurements of nucleation mode aerosols, precursor vapors and turbulent fluctuations throughout and above the mixed-layer results in it remaining an open question as to which one of these processes dominates.

Holocene climatic change in Swedish Lapland inferred from an oxygen-isotope record of lacustrine biogenic silica

Holocene climatic variability was studied in a 9500-year lake-sediment sequence from the Abisko region in Swedish Lapland, using the oxygen-isotope ratio in diatom biogenic silica (d18Osi). Oxygen-and hydrogen-isotope ratios of waters from the Abisko area suggest that in this region the evaporative flux is small and the isotopic composition of most lakes reflects that of the local precipitation. The hydrological setting of the region and sensitivity analysis of isotopic response to changing climatic parameters such as humidity, inflow and evaporation show that the downcore diatom d18Osi record is primarily controlled by changes in the summer isotopic composition of the lake water. The overall 3.5‰ depletion in d18Osi since the early Holocene is interpreted as an increase in the influence of the Arctic polar continental air mass that carries depleted precipitation. We estimate that this change is associated with a 2.5–4°C cooling that has occurred since the early Holocene. In general, the diatom d18Osi record resembles the average annual air temperature reconstructed for the Greenland ice core GISP2, especially during the past 4000 years, with a pronounced cooling starting at 2000 years BP.

Response of interior North America to abrupt climate oscillations in the North Atlantic region during the last deglaciation

Several broad-scale climatic oscillations during the last deglaciation are well documented in regions around the North Atlantic Ocean. This paper reviews empirical evidence for these deglacial climatic oscillations from non-coastal North America and discusses implications for testing climatic simulations and for understanding the cause and transmittal mechanisms. Paleoclimatic interpretation of oxygen-isotope records from several small sites in the eastern Great Lakes region indicates a classic deglacial climatic sequence that is comparable with records from Europe and Greenland. The climatic events as interpreted from Crawford Lake oxygen isotopes include the Bølling–Allerød (BOA) warming at ∼12,700 14C BP, a warm BOA at ∼12,500–10,920 14C BP, an intra-Allerød cold period shortly before 11,000 14C BP, a cold Younger Dryas (YD) climate reversal at 10,920–10,000 14C BP, the Holocene warming at 10,000 14C BP, a brief Preboreal Oscillation (PB) at 9650 14C BP, and an early-Holocene cold event at 7500 14C BP (8200 cal BP). Some of these events were also evident from changes in upland and aquatic vegetation and sediment lithology. The pronounced YD climatic reversal has been documented from pollen records along the ecotones at this time and from glacier readvances in the Great Lakes region. Along the Rocky Mountains from Alberta to Colorado, the YD event is indicated by alpine glacier advance and/or shift in timberline vegetation. In Minnesota and upper New York, early-Holocene climatic instability is also suggested by oxygen-isotope records and/or varve thickness. The regional variations in evidence for the YD and other events in North America suggest that climatic oscillations may have different expressions in paleo-records, depending on geographic location and characteristics of a particular site. The extent and magnitude of these climatic oscillations across North America suggest that these oscillations are an expression of climatic change that was probably widespread rather than locally induced by a nearby glacier. The location of these sites implies that climatic signals were likely carried over the Northern Hemisphere through the atmosphere, as indicated by general circulation models. We hypothesize that the lack of evidence for a cold YD in interior North America west of the Great Lakes region and east of the Rocky Mountains was caused by the trapping of cold arctic air mass north of the Laurentide ice sheet and by uninterrupted northward expansion of warm Caribbean air; this strongly contrasted climate also produced non-analogous biological assemblages.

Effects of air masses and synoptic weather on aerosol formation in the continental boundary layer

Nucleation of near nm sized aerosol particles and subsequent growth to ~100 nm in 1–2 days has in recent years been frequently observed in the continental boundary layer at several European locations. In 1998–99, this was the focus of the BIOFOR experiment in Hyytiälä in the boreal Finnish forest. Nucleation occurred in arctic and to some extent in polar air masses, with a preference for maritime air in transition to continental air masses, and never in sub-tropical air. The air masses originated north of the BIOFOR experiment by paths from the southwest to northeast sector. The nucleation was also associated with cold air advection behind cold fronts, never warm air advection. This may relate to low pre-existing aerosol concentration, low cloudiness and large diurnal amplitudes in the continental boundary layer associated with cold air advection and clear skies. Arctic and polar air together with cold air advection did not always lead to nucleation. The most important limiting meteorological factors were cold front passages and high cloudiness, probably through reduced photochemistry and wet scavenging of precursor gases and new aerosol particles. The preference for nucleation to occur in arctic air masses, which seldom form in the summer, suggests a meteorological explanation for the annual cycle of nucleation, which has a minimum in summer. The connection to cold-air outbreaks suggests that the maximum in nucleation events during spring and autumn may be explained by the larger latitudinal temperature gradients and higher cyclone activity at that time of the year. Nucleation was observed on the same days over large parts (1000-km distance) of the same air mass. This suggests that the aerosol nucleation spans from the microphysical scale to the synoptic scale, perhaps connected through boundary layer and mesoscale processes.

Impact of landscape features on spring phenological phases of maple and bird cherry in Estonia

We analysed the spatial distribution of the pollinationof maple (Acer platanoides L.) and bird cherry (Prunus padus L.) in Estonia on the basis of phenological data from the time series for 1948–1995, in 27 observation points, and from the special field observation programme 1996–1999, in 46 observation points. Phenological maps show that these springtime phenophases spread in the landscape at a rate of 3–6 days per 100 km, with different rates in early and late springs. The maple has a steeper gradient on northeastern islands and the bird cherry on the western islands; the values of standard deviation have a similar spatial pattern. The distribution of phenological phases in Estonia is influenced by differences between the temperature regimes of the Baltic Sea and inland areas, different climatic conditions in north-eastern Estonia, local altitude-impact of uplands with an absolute height of 150–300 m asl, and the effects of bigger lakes and wetland areas. On the basis of spring phenology, three seasonally different landscape types can be determined in Estonia: (1) Relatively continental South-East Estonian Plain and uplands – have the earliest spring with the smallest deviations and stable intervals between phases, (2) Central, western and northern Estonian plains – with temperate influence of the temperature regime of the Baltic Sea, and big variations from year to year. Large variability is caused by the presence and duration of ice cover on the sea in cold springs, and direct access by warm air masses in early springs. (3) North-east Estonia – has the most boreal climate with longer snow cover and very late spring, influenced by arctic air masses, local influence of the Baltic Sea, uplands, and large wetlands.

Effects of continental boundary layer evolution, convection, turbulence and entrainment, on aerosol formation

Aerosol nucleation events occurring in the continental boundary layer over the boreal forest region in Finland, during the BIOFOR experiment, have been examined to elucidate the rôle of micrometeorology in promoting such events. Invariably, during the spring campaign of 1999, nucleation events occurred in Arctic and polar air masses during cold air outbreaks. Under clear-sky conditions, typical of these synoptic meteorological patterns, the boundary layer evolution was characterized by the rapid growth of a mixed layer, convection and strong entrainment, first from the residual later and later from the free troposphere. It was found that the freshly nucleated particles were detected within two hours from the onset of strong turbulent kinetic energy, independent of how fast the boundary layer evolved. When considering the growth time from cluster size of = 1 nm to detectable sizes of 3 nm, the nucleation and onset of strong turbulence coincided almost exactly. The most likely site for nucleation to take place was the mixed layer or the entrainment zone, while the forest canopy and the free troposphere could be excluded as the nucleation region. There are several possible explanations for the correlation between the onset of turbulence and nucleation: (1) new aerosols or clusters may have been entrained from the residual layer into the mixed layer where they then (in the case of clusters) underwent growth to detectable sizes; (2) two or more precursor gases may have been mixed with each other over the entrainment zone; (3) the adiabatic cooling in the rising convective plumes and the turbulent fluctuation in temperature and vapors by the entrainment flux may have enhanced aerosol formation; (4) a sudden decrease in preexisting aerosol due to dilution of the mixed layer aerosol by entrained air may have reduced the vapor sink enough to initiate nucleation. However, the lack of vertical profile measurements of nucleation mode aerosols, precursor vapors and turbulent fluctuations throughout and above the mixed-layer results in it remaining an open question as to which one of these processes dominates.

Holocene changes in seasonal precipitation highlighted by fire incidence in eastern Canada

Postglacial fire history has been reconstructed for eastern Canada from charcoal-influx anomalies from 30 sites taken from a lacustrine charcoal database. The reconstruction exhibits coherent patterns of fire occurrence in space and time. The early Holocene is characterised by high fire incidence. There is a major change to much lower occurrence slightly after 8 ka BP. A return to more fire appears after 3 ka BP. This sequence does not fit with the hydro-climatic reconstruction deduced from lake level reconstructions for northeastern North America, which indicates a dry early and mid-Holocene, and a wet late-Holocene. Fire occurrence however closely matches summer relative humidity inferred from δ18O. The differences between fire frequency and lake level history, are due to changes in the seasonality of precipitation and drought frequency. Lake levels are essentially controlled by winter precipitation while summer precipitation controls fire occurrence. The early Holocene before 8–7.5 ka BP experienced dry summers due to higher solar radiation and dry adiabatic winds from the residual Laurentide Ice Sheet. The middle Holocene was dominated by wet summers due to stability of the Atlantic air mass over eastern Canada. After 2.5 ka BP, summers became drier, albeit not as fire-conducive as during the early Holocene. Late-Holocene summers conducive to fire are explained by more frequent incursions of dry Cool Pacific or Cold Arctic air masses over eastern Canada.

The Relative Importance of Warm Rain and Melting Processes in Freezing Precipitation Events

The importance of warm rain and melting processes in freezing precipitation events is investigated by analyzing 972 rawinsonde soundings taken during freezing precipitation. The soundings cover regions of the United States east of the Rocky Mountain states for the period 1970–94. The warm rain process was found to be unambiguously responsible for freezing precipitation in 47% of the soundings. In these soundings, the clouds had temperatures entirely below freezing, or had top temperatures that were above freezing. Another 28% of the soundings had cloud top temperatures between 0° and −10°C. Clouds with top temperatures >−10°C also can support an active warm rain process. Considered together, the warm rain process was potentially important in about 75% of the freezing precipitation soundings. This estimate is significantly higher than the estimate of 30% in a previous study by Huffman and Norman. The temperature, moisture, and wind profiles of the soundings, their geographic distribution, and the common occurrence of freezing drizzle at the sounding sites suggest that most of these events were associated with shallow cloud decks forming over arctic cold air masses. The “classic” freezing rain sounding, with a deep moist layer and a midlevel warm (>0°C) layer, was observed in only 25% of the sample. In these soundings, the depth of the cloud layer implies that melting processes were important to precipitation production. From the geographic distribution, the common occurrence of freezing rain, and the sounding profile, these cases appear to be associated primarily with cold air damming and overrunning along the U.S. East Coast, and with warm-frontal overrunning in the midwestern United States.

Wet Deposition of Radon-222 Progeny in Northern Finland Measured with an Automatic Precipitation Gamma Analyser

An automatic wet deposition gamma analyser has been constructed. Precipitation is collected with a 1 m 2 funnel and its 214 Pb and 214 Bi contents measured in a Marinelli beaker with an NaI(Tl) detector and a multichannel analyser. The amount of water in the beaker is measured with an ultrasonic sensor. All the sensor and spectrum data are gathered, and the magnetic valves controlled, with a data logger and a microcomputer. The instrument was tested during a five-month field measurement campaign in northern Finland. A value of 750 Bq.l -1 for the geometric mean 222 Rn progeny concentration in precipitation, and a geometric mean washout ratio of 9.1 x 10 5 was obtained in this study. Their variations, however, were large: four orders of magnitude. The Rn progeny concentrations and washout ratios are inversely correlated with low precipitation intensities. With higher precipitation intensities this behaviour levels out. Higher washout ratios were observed during rainfall compared to snowfall. Low Rn progeny concentrations in precipitation are usually connected with cold or occluded fronts and surges of arctic air masses. On the other hand, high Rn progeny concentrations are usually related to warm fronts or sectors. They are also observed between two successive cyclones, when continental air masses from southern Russia move northwest. The results obtained could also be applied to wet-depositing chemical substances like heavy metals, which can be difficult to measure using on-line methods.

The Interaction of a Pacific Cold Front with Shallow Air Masses East of the Rocky Mountains

Abstract The study presented here describes the interactions that occurred between an advancing Pacific cold front and shallow Gulf of Mexico and Arctic air masses situated east of the Rocky Mountains during the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX) field campaign on 17–18 April over Oklahoma and adjacent states. These interactions were driven largely by the complex topography of the region. Four air masses of distinctly different origin (i.e., Pacific polar, high-altitude continental, Gulf of Mexico, and Arctic), and the boundaries that separated them (i.e., Pacific cold front, dryline, and Arctic front), were observed within the experimental domain. This event produced more than $1 million worth of damage in the experimental domain due to severe weather. A dense network of ground-based in situ and remote observing systems and two research aircraft equipped with in situ sensors and Doppler radars gathered data that allowed the authors to document the passage of a vigoro...

Abrupt changes in North American climate during early Holocene times

Recent studies of the Greenland ice cores have offered many insights into Holocene climatic dynamics at decadal to century timescales1,2,3. Despite the abundance of continental records of Holocene climate, few have sufficient chronological control and sampling resolution to compare with the Greenland findings4. Butannually laminated sediments (varves) from lakes can provide high-resolution continental palaeoclimate data with secure chronologies. Here we present analyses of varved sediments from Deep Lake in Minnesota, USA. Trends in the stable oxygen-isotope composition of the sedimentary carbonate indicate a pronounced climate cooling from 8.9 to 8.3 kyr before present, probably characterized by increased outbreaks of polar air, decreased precipitation temperatures, and a higher fraction of the annual precipitation falling as snow. The abrupt onset of this climate reversal, over several decades, was probably caused by a reorganization of atmospheric circulation and cooling of the Arctic airmass in summer that resulted from the final collapse of the Laurentide ice near Hudson Bay and the discharge of icebergs from the Quebec and Keewatin centres into the Tyrell Sea. The timing and duration of this climate reversal suggest that it is distinct from the prominent widespread cold snap that occurred 8,200 years ago in Greenland and other regions1,5,6. No shifts in the oxygen-isotope composition of sediment carbonate occurred at 8.2 kyr before present at Deep Lake, but varve thickness increased dramatically, probably as a result of increased deposition of aeolian dust. Taken together, our data suggest that two separate regional-scale climate reversals occurred between 9,000 and 8,000 years ago, and that they were driven by different mechanisms.

Intercomparison of ATMOS, SAGE II, and ER‐2 Observations in Arctic Vortex and Extra‐Vortex Air Masses during Spring 1993

We have compared measured abundances of NOy, O3, H2O, CH4, HCl, and N2O from ATMOS and aerosol from SAGE II from inside and outside the Arctic vortex with in situ observations from the SPADE aircraft campaign in April/May 1993. When the distinction between vortex and extra‐vortex air is taken into account, SPADE measurements of NOy, O3, H2O, CH4, HCl, and aerosol abundances, correlated with [N2O], demonstrate excellent agreement with ATMOS and SAGE II data. Observations of low [HCl] and high [ClNO3] inside the vortex by ATMOS and low [HCl] in vortex fragments by ALIAS suggest that vortex‐influenced air masses maintained unusually high levels of [ClNO3] throughout spring 1993.

Status of and Outlook for Large-Scale Modeling of Atmosphere–Ice–Ocean Interactions in the Arctic

Arctic air masses have direct impacts on the weather and climatic extremes of midlatitude areas such as central North America. Arctic physical processes pose special and very important problems for global atmospheric models used for climate simulation and numerical weather prediction. At present, the observational database is inadequate to support research aimed at overcoming these problems. Three interdependent Arctic field programs now being planned will help to remedy this situation: SHEBA, which will operate an ice camp in the Arctic for a year; ARM, which will supply instruments for use at the SHEBA ice camp and which will also conduct longer-term measurements near Barrow, Alaska; and FIRE, which will conduct one or more aircraft campaigns, in conjunction with remote-sensing investigations focused on the SHEBA ice camp. This paper provides an introductory overview of the physics of the Arctic from the perspective of large-scale modelers, outlines some of the modeling problems that arise in attempting to simulate these processes, and explains how the data to be provided by the three field programs can be used to test and improve large-scale models.