Subtropical Air Masses Research Articles

Holocene insolation and sea surface temperature influences on the polar front jet stream and precipitation in the midcontinental United States

Variability in the source and seasonality of precipitation in the midcontinental United States during the Holocene was investigated using isotopic and sedimentological data from Martin Lake, northeastern Indiana, USA. Between 7100 and 4000 years before present (yr BP; present = 1950 CE), high δ18Ocal and δ13Ccal values with low variability indicate that moisture was predominantly derived from subtropical, southerly sources and delivered primarily during the warm season. Mean state shifts toward lower δ18Ocal and δ13Ccal occurred at ca. 4000 and 2550 yr BP, respectively, indicating an increase in northerly-sourced cold-season precipitation during the Late Holocene (i.e., the past 4200 years) and a subsequent reduction in warm season duration after 2550 yr BP. Record low %lithics from ca. 5000 to 4000 yr BP indicates major reductions in warm-season rain storms, consistent with regional evidence of drought at this time. An increase in the amplitude of centennial-scale variability in δ18Ocal, δ13Ccal, and %lithics after 1900 yr BP indicates greater precipitation source variability during the Common Era. During this interval, precipitation fluctuated between southerly-sourced, convective rainstorms when the Northern Hemisphere (NH) was warm (e.g., during the Medieval Climate Anomaly; 700–1000 yr BP) and northerly-sourced rain and snow when the NH was cool (e.g., during the Little Ice Age; 150–550 yr BP). These trends, especially the change at ca. 4000 yr BP, are consistent with other North American paleoclimate records that collectively suggest a continental-scale shift in precipitation seasonality during the Middle to Late Holocene transition as the tropical Pacific Ocean transitioned from La Niña-like conditions to a more El Niño-like mean state. Concurrent NH cooling and persistent El Niño-like conditions during the Late Holocene would have favored a southerly polar front jet stream with enhanced ridge and trough atmospheric circulation over North America – conditions resembling the positive mode of the Pacific-North American teleconnection (PNA). This would have increased interactions between high-latitude and subtropical airmasses over the midcontinent, increasing the proportion of northerly precipitation with low δ18O delivered during the cold season (i.e., snowfall) and during extended periods with +PNA-like atmospheric circulation (e.g., the Little Ice Age).

Tracking rapid ice recession in a major Southern Alps valley during the last glacial termination

The last glacial termination featured a major reconfiguration of Earth's climate and cryosphere, whose underlying cause remains unresolved. To investigate this problem, we combine 10Be surface-exposure dating of moraines with former equilibrium line altitudes to determine the magnitude and timing of atmospheric temperature warming that ended the Last Glaciation in the Takapō/Tekapo valley in the central Southern Alps of New Zealand. We show mountain-valley glacier recession from a nearly full-glacial configuration to a near-interglacial configuration early in the termination between ∼18,000 and ∼17,000 yrs ago, commensurate with a net atmospheric warming of ∼3.8 °C (from −6.25 °C to −2.5 °C cooler than present). Similar recession also affected mid-latitude mountain glaciers in South America. We suggest trans-South Pacific glacier withdrawal early in the termination resulted from a decisive poleward shift of the austral westerlies that increased the proportion of warm subtropical air masses flowing over southern mid-latitude mountains, markedly raising glacier ablation rates. Farther south, the poleward-shifted westerlies drove increased ocean upwelling and surface warming, outgassing of carbon dioxide, and progressive ocean destratification, together raising atmospheric temperature over the Antarctic Ice Sheet, but at a rate slower than over mid-latitude mountain glaciers. Overall, we consider that Southern Hemisphere middle-latitude glacier recession was linked to Antarctic warming by a poleward displacement in latitude and an increase in strength of the Southern Hemisphere westerlies.

Examining Atmospheric River Life Cycles in East Antarctica

AbstractDuring atmospheric river (AR) landfalls on the Antarctic ice sheet, the high waviness of the circumpolar polar jet stream allows for subtropical air masses to be advected toward the Antarctic coastline. These rare but high‐impact AR events are highly consequential for the Antarctic mass balance; yet little is known about the various atmospheric dynamical components determining their life cycle. By using an AR detection algorithm to retrieve AR landfalls at Dumont d'Urville and non‐AR analogs based on 700 hPa geopotential height, we examined what makes AR landfalls unique and studied the complete life cycle of ARs reaching Dumont d'Urville. ARs form in the mid‐latitudes/subtropics in areas of high surface evaporation, likely in response to tropical deep convection anomalies. These convection anomalies likely lead to Rossby wave trains that help amplify the upper‐tropospheric flow pattern. As the AR approaches Antarctica, condensation of isentropically lifted moisture causes latent heat release that—in conjunction with poleward warm air advection—induces geopotential height rises and anticyclonic upper‐level potential vorticity tendencies downstream. As evidenced by a blocking index, these tendencies lead to enhanced ridging/blocking that persist beyond the AR landfall time, sustaining warm air advection onto the ice sheet. Finally, we demonstrate a connection between tropopause polar vortices and mid‐latitude cyclogenesis in an AR case study. Overall, the non‐AR analogs reveal that the amplified jet pattern observed during AR landfalls is a result of enhanced poleward moisture transport and associated diabatic heating which is likely impossible to replicate without strong moisture transport.

몽골의 기후지형학적 특징과 토양발달

Mongolia's high latitude/altitude and strong continentality make its climate and vegetation, topography and soil development different from those of a typical periglacial or arid zone. The wide range of daily/annual temperatures, glaciers and permafrost means that freeze-thaw processes are very active in almost all areas, and forest/steppe landscapes develop. It is an arid environment with low precipitation, but subtropical air masses in summer can cause seasonal flooding and inundation. These features make Mongolia a region with a very different character from region to region. Zuunkharaa is characterised by relatively low temperatures/evaporation and permafrost, leading to the development of periglacial features such as ice lenses, ice wedges and hummocks, while Lun, a tributary lowland formed by summer flooding, dries up and takes on the characteristics of a deflation lake. In the Karakorum, at the junction of mountains and plains, extensive alluvium is deposited in the form of alluvial fans. In Bayanzag, strong evaporation causes solonchacks to form, but active freeze-thaw processes and winds produce a hardened surface similar to desert pavements. Mongolia's landforms and soils are the result of a combination of strong freeze-thaw processes/evaporation, surface water and wind erosion, seasonal precipitation and flooding, and the influence of permafrost.

Magnitude, Scale, and Dynamics of the 2020 Mei-yu Rains and Floods over China

Large parts of East and South Asia were affected by heavy precipitation and flooding during early summer 2020. This study provides both a statistical and dynamical characterization of rains and floods affecting the Yangtze River Basin (YRB). By aggregating daily and monthly precipitation over river basins across Asia, it is shown that the YRB is one of the areas that was particularly affected. June and July 2020 rainfall was higher than in the previous 20 years, and the YRB experienced anomalously high rainfall across most of its sub-basins. YRB discharge also attained levels not seen since 1998/1999. An automated method detecting the daily position of the East Asian Summer Monsoon Front (EASMF) is applied to show that the anomalously high YRB precipitation was associated with a halted northward progression of the EASMF and prolonged mei-yu conditions over the YRB lasting more than one month. Two 5-day heavy-precipitation episodes (12−16 June and 4−8 July 2020) are selected from this period for dynamical characterization, including Lagrangian trajectory analysis. Particular attention is devoted to the dynamics of the airstreams converging at the EASMF. Both episodes display heavy precipitation and convergence of monsoonal and subtropical air masses. However, clear differences are identified in the upper-level flow pattern, substantially affecting the balance of airmass advection towards the EASMF. This study contextualizes heavy precipitation in Asia in summer 2020 and showcases several analysis tools developed by the authors for the study of such events.

The influence of tropical and subtropical air masses on the chemical composition of the tropospheric aerosol of Western Siberia based on the results of systematic aircraft sounding in 1997-2017

Almost monthly sensing of the troposphere from 500 to 7000 m above Karakansky bor (Novosibirsk Province) are conducted since 1997 up to now by using of the aircraft-laboratory "Optik" based on Antonov-30 or Tupolev-134. Aerosol sampling on to Petryanov filters AFA-CP-20 for subsequent laboratory quantitative analysis of the content of inorganic ions and the next elements in the aerosol composition: Al, Ba, Ca, Cu, Fe, Mg, Mn, Mo, Ni, Pb, Sn, Ti, V, Be, Cd, Si, Co, Cr, Sr, Zn, Zr was performed for each flight. For the region of the study of tropospheric aerosol, the chemical composition of the aerosol was classified for the main air masses during the period of airborne sounding from September 1997 to June 2017 - continental arctic, continental moderate, and combined into one “southern” group of the continental tropical and subtropical air masses. Explicit "southern" origin, along with silicon, have chromium, manganese, silver and bicarbonate-anion. To the same group, the ions of chlorine and sodium are attracted, which probably were transported together with salt particles originating from Central Asian deserts.

Analysis of the Exceptionally Warm December 2015 in France Using Flow Analogues

Capsule December 2015 in France was an extreme of circulation and temperature. Both circulation and climate change partly explain the 4°C anomaly. We found no link between climate change and circulation. The event The December 2015 average temperature broke a record in France, with an anomaly of +4.1°C (Fig. 1a) with respect to the 1949-2015 climatology. The linear trend of average December temperature (in red in Fig. 1a) is not significant (p-value > 0.05), as regional temperature variability is high in winter. Such a positive temperature anomaly has impacts on the vegetation cycle (the French press covered this topic in the daily newspaper Le Monde 1). It also affects local economies, e.g. tourism in ski resorts. The temperature anomaly was concomitant with a zonal atmospheric circulation over Western Europe (Fig. 1b), directing mild subtropical air masses towards France. We found that the mean monthly SLP (sea level pressure) anomaly over the black box of Fig.1b is also a record high for the NCEP reanalysis. Such a circulation type generally leads to warm temperatures overs France (Yiou and Nogaj, 2004). In this paper we seek to address three questions: How much does the circulation anomaly explain the temperature anomaly during December 2015 in France? What is the influence of climate change on the occurrence of the circulation anomaly? How does the distribution of temperature conditional to the atmospheric circulation evolve with climate change?

Continuous measurements of stable isotopes of carbon dioxide and water vapour in an urban atmosphere: isotopic variations associated with meteorological conditions

ABSTRACTIsotope ratios of carbon dioxide and water vapour in the near-surface air were continuously measured for one month in an urban area of the city of Nagoya in central Japan in September 2010 using laser spectroscopic techniques. During the passages of a typhoon and a stationary front in the observation period, remarkable changes in the isotope ratios of CO2 and water vapour were observed. The isotope ratios of both CO2 and water vapour decreased during the typhoon passage. The decreases can be attributed to the air coming from an industrial area and the rainout effects of the typhoon, respectively. During the passage of the stationary front, δ13C–CO2 and δ18O–CO2 increased, while δ2H–H2Ov and δ18O–H2Ov decreased. These changes can be attributed to the air coming from rural areas and the air surrounding the observational site changing from a subtropical air mass to a subpolar air mass during the passage of the stationary front. A clear relationship was observed between the isotopic CO2 and water vapour and the meteorological phenomena. Therefore, isotopic information of CO2 and H2Ov could be used as a tracer of meteorological information.

Multilevel cloud structures over Svalbard.

The presented picture of the month is a superposition of space-borne lidar observations and high-resolution temperature fields of the ECMWF integrated forecast system (IFS). It displays complex tropospheric and stratospheric clouds in the Arctic winter 2015/16. Near the end of December 2015, the unusual northeastward propagation of warm and humid subtropical air masses as far north as 80°N lifted the tropopause by more than 3 km in 24 h and cooled the stratosphere on a large scale. A widespread formation of thick cirrus clouds near the tropopause and of synoptic-scale polar stratospheric clouds (PSCs) occurred as the temperature dropped below the thresholds for the existence of cloud particles. Additionally, mountain waves were excited by the strong flow at the western edge of the ridge across Svalbard, leading to the formation of mesoscale ice PSCs. The most recent IFS cycle using a horizontal resolution of 8 km globally reproduces the large-scale and mesoscale flow features and leads to a remarkable agreement with the wave structure revealed by the space-borne observations.

Greenland ice sheet melt from MODIS and associated atmospheric variability.

Daily June-July melt fraction variations over the Greenland ice sheet (GIS) derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) (2000–2013) are associated with atmospheric blocking forming an omega-shape ridge over the GIS at 500 hPa height. Blocking activity with a range of time scales, from synoptic waves breaking poleward (<5 days) to full-fledged blocks (≥5 days), brings warm subtropical air masses over the GIS controlling daily surface temperatures and melt. The temperature anomaly of these subtropical air mass intrusions is also important for melting. Based on the years with the greatest melt (2002 and 2012) during the MODIS era, the area-average temperature anomaly of 2 standard deviations above the 14 year June-July mean results in a melt fraction of 40% or more. Though the summer of 2007 had the most blocking days, atmospheric temperature anomalies were too small to instigate extreme melting.Key PointsShort-term atmospheric blocking over Greenland contributes to melt episodesAssociated temperature anomalies are equally important for the meltDuration and strength of blocking events contribute to surface melt intensity

Urban climate research in Warsaw: the results of microclimatic network measurements

The paper presents some aspects of Warsaw’s climate, in particular the urban heat island. UHI changes in different seasons and in different air mass types were analysed over the years 2011-2012. Average UHI in Warsaw is of a diamond shape which reflects the distribution of the densest built area and exceeds 2.0°C in the city centre compared to the airport station. In subtropical air mass, the intensity of UHI on the left side of the Vistula River reached 7.7°C. The basis for the analysis is the microclimatic measurement network of 28 permanent points in Warsaw and its surroundings, operated by IGSO PAS and completed by data from 7 other stations. This dense network became the IGSO PAS’ input into an UHI project titled ‘Development and application of mitigation and adaptation strategies and measures for counteracting the global Urban Heat Islands phenomenon (UHI)’ implemented through the Central Europe Program and co-financed by the ERDF.

Multiyear variations in aerosol condensation activity in Tomsk

We consider the results of 13 years (1998–2010) of studies of the aerosol condensation activity in Tomsk. The long-term behavior of the condensation activity parameter exhibits, in addition to the annual cycle and its second harmonic, a tendency toward slower, ∼6–7-year oscillations. The values were maximum in 1998–1999 and 2005–2007. The annual average values were minimum in 2003 and 2008, i.e., in years when intrusions of subtropical air masses from arid regions of Kazakhstan and Central Asia were the most frequent.

NO&amp;lt;sub&amp;gt;x&amp;lt;/sub&amp;gt; production by lightning in Hector: first airborne measurements during SCOUT-O3/ACTIVE

Abstract. During the SCOUT-O3/ACTIVE field phase in November–December 2005, airborne in situ measurements were performed inside and in the vicinity of thunderstorms over northern Australia with several research aircraft (German Falcon, Russian M55 Geophysica, and British Dornier-228. Here a case study from 19 November is presented in detail on the basis of airborne trace gas measurements (NO, NOy, CO, O3) and stroke measurements from the German LIghtning Location NETwork (LINET), set up in the vicinity of Darwin during the field campaign. The anvil outflow from three different types of thunderstorms was probed by the Falcon aircraft: (1) a continental thunderstorm developing in a tropical airmass near Darwin, (2) a mesoscale convective system (MCS), known as Hector, developing within the tropical maritime continent (Tiwi Islands), and (3) a continental thunderstorm developing in a subtropical airmass ~200 km south of Darwin. For the first time detailed measurements of NO were performed in the Hector outflow. The highest NO mixing ratios were observed in Hector with peaks up to 7 nmol mol−1 in the main anvil outflow at ~11.5–12.5 km altitude. The mean NOx (=NO+NO2) mixing ratios during these penetrations (~100 km width) varied between 2.2 and 2.5 nmol mol−1. The NOx contribution from the boundary layer (BL), transported upward with the convection, to total anvil-NOx was found to be minor (&lt;10%). On the basis of Falcon measurements, the mass flux of lightning-produced NOx (LNOx) in the well-developed Hector system was estimated to 0.6–0.7 kg(N) s−1. The highest average stroke rate of the probed thunderstorms was observed in the Hector system with 0.2 strokes s−1 (here only strokes with peak currents ≥10 kA contributing to LNOx were considered). The LNOx mass flux and the stroke rate were combined to estimate the LNOx production rate in the different thunderstorm types. For a better comparison with other studies, LINET strokes were scaled with Lightning Imaging Sensor (LIS) flashes. The LNOx production rate per LIS flash was estimated to 4.1–4.8 kg(N) for the well-developed Hector system, and to 5.4 and 1.7 kg(N) for the continental thunderstorms developing in subtropical and tropical airmasses, respectively. If we assume, that these different types of thunderstorms are typical thunderstorms globally (LIS flash rate ~44 s−1), the annual global LNOx production rate based on Hector would be ~5.7–6.6 Tg(N) a−1 and based on the continental thunderstorms developing in subtropical and tropical airmasses ~7.6 and ~2.4 Tg(N) a−1, respectively. The latter thunderstorm type produced much less LNOx per flash compared to the subtropical and Hector thunderstorms, which may be caused by the shorter mean flash component length observed in this storm. It is suggested that the vertical wind shear influences the horizontal extension of the charged layers, which seems to play an important role for the flash lengths that may originate. In addition, the horizontal dimension of the anvil outflow and the cell organisation within the thunderstorm system are probably important parameters influencing flash length and hence LNOx production per flash.

Ozone loss driven by nitrogen oxides and triggered by stratospheric warmings can outweigh the effect of halogens

Ozone loss in the lower and middle stratosphere in spring and summer, in particular over polar regions, is driven mainly by halogens and nitrogen oxides (NOx). Whereas the stratospheric chlorine levels are expected to decrease in the future, the role of NOx for the O3 budget in a changing climate is not well quantified. Here we combine satellite measurements and model simulations to diagnose the accumulated O3 loss during winter and spring 2002–2003 in the Arctic polar stratosphere. We show that in a winter stratosphere strongly disturbed by warmings, O3 loss processes driven by halogens and NOx can significantly overlap within the polar column and become comparable in magnitude even if a significant, halogen‐induced O3 loss has occurred. Whereas, until the beginning of March 2003, polar column O3 loss was mainly caused by the halogen chemistry within the vortex at an altitude around 18 km, the chemical O3 destruction in March and April was dominated by the NOx chemistry in O3‐rich air masses transported from the subtropics and mixed with the polar air above the region affected by the halogens. This NOx‐related O3 loss started around mid‐December 2002 in subtropical air masses above 30 km that moved poleward after the major warming in January, descended to 22 km with an increasing magnitude of O3 loss and reached surprisingly high values of up to 50% local loss around the end of April. To some extent, the NOx‐driven O3 loss was enhanced by mesospheric air trapped in the vortex at the beginning of the winter as a layer of few km in the vertical and transported downward within the vortex. The effect of NOx transported from the subtropics dominated the O3 loss processes in the polar stratosphere in spring 2003, both relative to the effect of the halogens and relative to the contribution of the mesospheric NOx sources. A comparison with the 1999/2000 Arctic winter and with the Antarctic vortex split event in 2002 shows that wave events triggered by stratospheric warmings may significantly enhance O3 loss driven by NOx when O3‐ and NOx‐rich air masses from the subtropics are transported poleward and are mixed with the vortex air.

Spatial and Temporal Variations of Atmospheric 85Kr Observed During 1995-2001 in Japan: Estimation of Atmospheric 85Kr Inventory in the Northern Hemisphere

Atmospheric 85Kr concentrations have been continuously monitored since 1995 at the Meteorological Research Institute (MRI) in Tsukuba, Japan. They have also been observed once a year at several stations over the Japanese islands since 1995. The annual growth rate of the background atmospheric 85Kr concentrations in Tsukuba was 0.03 Bq x m(-3) x yr(-1) during 1996-2001. The atmospheric 85Kr concentrations at several stations over Japan were within the range of the annual variations in Tsukuba. However, higher and lower 85Kr concentrations in early winter, compared with those in Tsukuba (36.1 degrees N, 140.1 degrees E), occurred in Sapporo (43.1 degrees N, 141.3 degrees E) and Ishigaki (24.3 degrees N, 124.2 degrees E), respectively. The reason for this is that Sapporo is covered by a continental air mass, some from European sources, whereas Ishigaki is still covered by a subtropical air mass. The Northern Hemispheric background 85Kr concentrations from 1994 to 2001 was calculated from the 85Kr inventory and the release rate of 85Kr from the nuclear fuel reprocessing plants in Europe. Calculated 85Kr concentrations in surface air were in good agreement with annual average observed values at the MRI, Tsukuba. The global atmospheric inventory of 85Kr in December 2001 was also estimated to be approximately 5 EBq by using observed data in Tsukuba.

Winter‐spring evolution and variability of HOx reservoir species, hydrogen peroxide, and methyl hydroperoxide, in the northern middle to high latitudes

The Tropospheric Ozone Production about the Spring Equinox (TOPSE) experiment examined the evolution of tropospheric chemical compositions from February to May 2000 over North America, 40 to 85 N. Hydrogen peroxide (H2O2) and methyl hydroperoxide (CH3OOH) were investigated using instrumental observations aboard the NCAR C‐130 research aircraft. Primary TOPSE results indicate both photochemistry and atmospheric dynamics are critical factors controlling the variability of peroxides in this region. From February to May, H2O2 and CH3OOH mixing ratios increased with the greatest relative changes at mid‐altitudes. H2O2 ranged from below the detection limit (BDL = 25 pptv) to 380 pptv in winter and from BDL to 1330 pptv during spring. Winter measurements of CH3OOH were from BDL (35 pptv) to 740 pptv with higher levels of BDL to 1400 pptv measured during spring. Peroxides also decreased with latitude at all altitudes. These findings are consistent with those expected from photochemical theory. Evidence also supports a source of CH3OOH to the Arctic from the transport of subtropical air masses. Air mass back trajectories and GOES‐derived specific humidity products indicate transport of moist tropical air to the study region coincides with elevated levels of CH3OOH up to 940 pptv. The concurrence of this transport regime with episodic elevated CH3OOH events suggests a source of HOx to the Arctic. However, evidence from this study shows CH3OOH does not greatly contribute to total HOx production which is dominated primarily by reactions of O(1D) and H2O at low latitudes and CH2O at high latitudes.

On the relative role of convection, chemistry, and transport over the South Pacific Convergence Zone during PEM-Tropics B: A case study

A mesoscale 3D model (Meso-NH) is used to assess the relative importance of convection (transport and scavenging), chemistry, and advection in the vertical redistribution of HO X and their precursors in the upper tropical troposphere. The study is focused on marine deep convection over the South Pacific Convergence Zone (SPCZ) during the PEM-Tropics B Flight 10 aircraft mission. The model reproduces well the HO X mixing ratios. Vertical variations and the contrast between north and south of the SPCZ for O 3 are captured. Convection uplifted 0 3 -poor air at higher altitude, creating a minimum in the 9-12 km region, in both modeled and observed profiles. The model captured 60% of the observed HCHO variance but fails to reproduce a peak of HCHO mixing ratio at 300 hPa sampled during the northern spirals. Simulated HCHO mixing ratios underestimate observations in the marine boundary layer. In the model, convection is not an efficient process to increase upper tropospheric HCHO, and HCHO is unlikely to serve as a primary source of HO x . Convection plays an important role in the vertical distribution of CH 3 OOH with efficient vertical transport from the boundary layer to the 10-15 km region where it can act as a primary source of HO X . The SPCZ region acts as a barrier to mixing of tropical and subtropical air at the surface and at high altitudes (above 250 hPa). The 400-270 hPa region over the convergence zone was more permeable, allowing subtropical air masses from the Southern Hemisphere to mix with tropical air from NE of the SPCZ and to be entrained in the SPCZ-related convection. In this altitude range, exchange of subtropical and tropical air also occurs via airflow, bypassing the convective region SW and proceeding toward the north of the SPCZ.

High resolution Holocene palynological record from the Scotian Shelf

Because of their location at the confluence of polar and subtropical airmasses and near a transition zone between the cold Labrador Current and the Gulf Stream, the Atlantic Provinces experience some of the most dynamic climate conditions in Canada. Major climate changes occurred during the Holocene, as shown by pollen records from lakes, but previous paleoceanographic studies, based on low-resolution proxy-data records do not show major changes during the past 8000 years. Therefore, the Holocene history of Canada's Atlantic region was examined using a high-resolution palynological record from the Scotian Shelf (La Have Basin). Sea surface conditions were reconstructed using proxy-data from dinoflagellate cysts and paleobioclimatic transfer functions. Ocean–atmosphere interactions are determined by onshore–offshore correlation of marine and pollen records from Nova Scotia. Results show a succession of major paleoceanographic events. Sea surface temperatures (SST) (February and August) up to 5°C higher than today's average and slightly higher salinity are reconstructed between 10.5 and 8.5 ka. The last pulse of meltwater from the residual ice sheet affected the shelf waters between 8.5 and 6.5 ka by lowering the SST (in February) and the salinity. Most previous studies failed to record this event. Since 6.5 ka, August temperature generally remained around today's value, while February temperature was generally 2°C above it, except for recurring colder and lower salinity intervals. These cold intervals have a recurrence of about 1000 years. A slight cooling of summer SST is recorded in the last 500 years. Comparison with climatic reconstructions from Nova Scotia pollen records shows a difference in timing between ocean and atmosphere. The onset of the climatic optimum (hypsithermal) in Nova Scotia lags by about 2000 years (until 8 ka) relative to the ocean but it lasted longer. The Neoglacial cooling in Atlantic Canada however, started earlier (2 ka) than the late Holocene ocean cooling. La Have Basin's Holocene paleoceanography presents some differences from most other studies from the region: (1) the hypsithermal started earlier; (2) the last pulse of meltwater is recorded; (3) the last 6500 years are punctuated by colder intervals. Increased productivity and blooms of toxic algae in the early Holocene are probably due to a combination of factors: increased nutrients and a greater stability of the water column because of meltwaters, higher SST and increased upwelling or storm activity.

Characteristics of episodes with extremely low ozone values in the northern middle latitudes 1957−2000

Abstract. A number of episodes are observed when the total ozone for 2 to 3 days has fallen below 220 matm-cm in the northern mid- and polar latitudes in autumn. The occurrences of such episodes represent ozone deviations of about one-third from the pre-1976 Oct-Nov-Dec monthly mean! By using primarily quality checked Dobson data, a clear identification was made of more than three dozen short spells with extremely low ozone in the 1957–1978 period. In the following twenty-two years (1979–2000), using mainly TOMS data, one can identify ~ 46 cases with ozone values falling below 220 matm-cm for longer than 1 day, with each time over an area greater than 500,000 km2 . The Ozone Mass Deficiency (O3MD) from the pre-1976 average ozone values over the affected area was ~2.8 Mt per day, i.e. four to seven times greater than it would be, assuming only a long-term trend in the Oct-Nov-Dec period. The Extremely Low Ozone (ELO3) events on the day of their appearance over the N. Atlantic/European region contribute to the O3MD by representing 16% of the deficiency due to the Oct-Nov trend in the entire 40–65° N latitudinal belt. The O3MD of the greater pool with low ozone (here taken as &lt;260 matm-cm) surrounding the area of the lowest events could contribute on the day of their appearance in Oct-Nov up to 60% and in December, ~30% to the deficiency due to the trend over the entire 40–65° N belt. Analysis of synoptic charts, supported by a backward trajectory on the isentropic surfaces 350 and 380 K, shows that in most of the events, subtropical air masses with low ozone content were transported from the Atlantic toward the UK, Scandinavia, and in many cases, further to the western sub-polar regions of Russia. This transport was sometimes combined with upward motions above a tropospheric anticyclone which lifted low ozone mixing ratios to higher altitudes. The ELO3 events cause a significant deficiency above the tropopause where, in general, the subtropical air is injected. In fact, the overall amount of ozone is not depleted, but redistributed on the hemispheric scale. Review of low ozone events, defined as days with negative deviations from the pre-1976 averages greater than 25% show, in general, similar origin. The seasonally averaged area with ELO3 and the associated O3MD, as well as for the cases with deviations &gt; –25%, has increased during the 1990s, which could be an indication of stronger and/or more frequent subtropical air intrusions. Their occurrences could contribute noticeably to the ozone deficiency of the middle latitude ozone during the days of ELO3 appearances; however, their contribution to the long-term trend of the ozone seasonal decline is of the order of ~10%.Key words. Atmospheric composition and structure (middle atmosphere - composition and chemistry) Meteorology and atmospheric dynamics (middle atmosphere dynamics)

Dissipation and mixing of a small‐scale stratospheric intrusion in the upper troposphere

Ground‐based lidar measurements of ozone mixing ratio and aerosol backscatter profiles above Fritz Peak Observatory near Boulder, Colorado (39.9° N, 105.3°W), are used in conjunction with nearby radiosonde profiles and GOES‐9 satellite water vapor imagery to describe the mixing of a dry, ozone‐rich streamer of stratospheric air into the upper troposphere over the western United States on June 30 and July 1, 1997. The satellite images show that the streamer extended over more than 2000 km from the eastern Pacific Ocean to the central United States beneath the subtropical jet with a width of ∼200 km over Colorado. The streamer persisted with little change for more than 6 hours of lidar observations and then abruptly disappeared to be replaced by a region of unusually low ozone. The disappearance of the streamer is attributed to localized convective activity induced by the incursion of a moist, ozone‐poor subtropical air mass between the streamer and the convective boundary layer. This activity disrupted the streamer and, together with shear‐induced mixing beneath the descending subtropical jet, merged ∼6.5 × 1031 molecules of ozone into the free troposphere over Colorado between 8 and 10 km. These results illustrate the effectiveness of small‐scale processes for the irreversible transfer of stratospheric air into the upper troposphere.