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  • 1
    Publication Date: 2021-05-06
    Description: While archaeological records indicate an intensive Mesolithic occupation of dune areas situated along river valleys, relatively little knowledge exists about environmental interactions in the form of land-use strategies and their possible local impacts. The combination of geoarchaeological, chronological, geochemical and palaeoecological research methods and their application both on a Mesoltihic site situated on top of a dune and the adjacent palaeochannel sediments allows for a detailed reconstruction of the local environmental development around the Soven site in the Jeetzel valley (Northern Germany) since ~10.5 ka cal BP. Based on the results, we identified four phases that may be related to local human impact twice during the Mesolithic, the Neolithic and the Iron Ages and are discussed on the backdrop of the regional settlement history. Although nearby Mesolithic occupation is evident on archaeological grounds, the identification of synchronous impacts on the vegetation in the local environmental records remains tentative even in respect of the broad methodical spectrum applied. Vice versa, human impact is strongly indicated by palaeoecological and geochemical proxies during the Neolithic period, but cannot be connected to archaeological records in the area so far. A younger phase of human impact – probably consisting of seasonal livestock farming in the wetlands – is ascribed to the Iron Age economy and comprises local soil erosion, raised concentrations of phosphates and urease, and the facilitation of grazing related taxa.
    Description: research
    Keywords: 551.7 ; aeolian sand ; pollen ; mesolithic ; iron age ; charcoal ; human impact ; OSL ; Neolithisation
    Language: English
    Type: article , Verlagsversion
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  • 2
    Publication Date: 2021-05-06
    Description: Remarkable polygenetical structures were observed at a refinery rehabilitation site in Wedel/Holstein. The polygonal shaped, channel-like structures are incised in mid-Saalian clayey, chalk-rich till. They are symmetrically semicircular shaped and filled with calcareous, silty sands that can be interpreted as sandy reworked till with aeolian components. The width reaches from approx. 0.3 to 1.5 metres, the depth up to approx. 0.8 metres. Horizontal cracks up to more than 10 meters in lengths, occurring as narrow belts of sand with reddish colour, are often centrally aligned in the channel-like structures. The present-day red colour is not natural but related to pollution. These belts reach down to the bottom of the channel like structures. At the lower site of the channel-like structures glacitectonic fissures with a width up to a few centimetres were to be traced into a depth of several metres to the basis of the excavat ion. The genesis of the channel-like structures is discussed. e. g. a possible relation with the pre-existing glacitectonic joints resp. the expected periglacial paleohydrogeological setting.
    Description: research
    Keywords: 551.7 ; periglacial channels ; Weichselian periglacial ; frost wedges ; glacitectonism ; paleohydrogeology ; talik ; Elbe spillway
    Language: German
    Type: article , Verlagsversion
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  • 3
    Publication Date: 2021-05-06
    Description: Hochterrassen (High or Higher Terraces) are a prominent geomorphological feature of the Northern Alpine Foreland and have traditionally been attributed to the Rissian glaciation. However, distinct morphological sublevels observed for this feature have often raised the question of their age. This issue is exemplarily investigated here on the Langweider and Rainer Hochterrassen in the lower Lech valley using different relative and numerical dating techniques. The lowest sublevel, the Übergangsterrasse is only preserved in small patches at the western rim of the Rainer Hochterrasse and is most probably of early Würmian age. The sublevel of the Jüngere Hochterrasse is older than the Last Interglacial, as indicated by luminescence ages of overlying loess/palaeosol sequence and the development of a luvisol on top of the terrace gravel. This terrace is composed of stacked gravel units that represent at least two accumulation phases correlating with Marine Isotope Stage (MIS) 6 for the top gravel and MIS 7 to MIS 10 (or older) for the basal gravel. It is not yet clear, if the deposition of the basal gravel unit corresponds to one or more aggradation phases during the Middle Pleistocene. The highest sublevel, the Ältere Hochterrasse also shows a compositon of two stacked gravel units but so far, no numerical ages have been achieved for these units.
    Description: research
    Keywords: 551.7 ; northern alpine foreland ; middle pleistocene ; luminescence dating ; Hochterrassen ; fluvial terraces
    Language: English
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  • 4
    Publication Date: 2021-05-06
    Description: Crocuta crocuta spelaea (Goldfuss 1823) cranial and postcranial remains of the Pößneck region in the Zechstein Karst region of the Thuringian Mountains (Central Germany) were excavated historically in the Wüste Scheuer Cavity at Döbritz. Nearby, at the Krölpa gypsum karst open air site, additionally a woolly rhinoceros, partially scavenged by Ice Age spotted hyenas, was found. The amount at Wüste Scheuer Cavity includes chew damaged Coelodonta antiquitatis remains and is classified herein as communal den type. At both den/scavenging sites, only a small amount of prey material of Late Pleistocene megafauna of rare M. primigenius, mainly C. antiquitatis, E. c. przewalskii, and fewer B. priscus and R. tarandus was accumulated. The dominance of woolly rhinoceros, bison and Przewalski horse bones are typical for hyena bone assemblages in European low mountain regions, where mammoth was nearly absent as a result of topography. In the Thuringian Karst Mountains nine Late Pleistocene Ice Age spotted hyena den sites are identified. Solely hyena dens are present in Zechstein open air gypsum and limestone karstic regions of Bad Köstritz, Krölpa and Fuchsluken Cavities near Saalfeld. In the Wüste Scheuer their remains overlap with Middle Palaeolithic Neanderthal human camp sites, similar as in the Ilsen Cave at Ranis and Lindenthal Cave in Gera, which demonstrates competition for prey and shelter cavities. At such cave sites, bone remains were historically misinterpreted as „solely of Neanderthal human kitchen rubbish” or even as “bone tools” (e.g. “bone scrapers” = woolly rhinoceros tibia bones chewed by hyenas).
    Description: research
    Keywords: 551.7 ; late pleistocene ; Ice Age spotted hyenas ; den types ; Thuringian Mountains ; Central Germany
    Language: English
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  • 5
    Publication Date: 2021-03-29
    Description: Eine durch einen Bodenschurf aufgeschlossene Boden-Sediment-Folge einer alten Seeterrasse im Einzugsgebiet des Priesterbäker Sees zeigt deutliche Fazieswechsel im Uferbereich des Gewässers. Die Abfolge von liegenden humosen Sanden, welche einen ehemaligen Verlandungssaum nachzeichnen, limnischen Sanden mit Bodenbildung und hangenden Flugsanden belegt Änderungen in der Sedimentation, welche mit säkularen Grundwasser- bzw. Seespiegelschwankungen zu erklären sind. Mit Hilfe von geophysikalischen Datierungen lassen sich gegenüber dem heutigen Niveau höhere Wasserstände für den Priesterbäker See bzw. die so genannten „Specker Seen“ östlich der Müritz während des 7.–9. und 12.–14. Jahrhunderts ableiten.
    Description: research
    Keywords: 551 ; 554.3 ; seespiegelschwankungen ; mecklenburg ; Seeterrasse ; Priesterbäker See ; Slawenzeit
    Language: German
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  • 6
    Publication Date: 2021-05-06
    Description: C/O-stable isotope composition (VPDB) of speleothems from the Makkaronihalle of the Hüttenbläserschacht Cave in Iserlohn shows a clear separation between glacial and interglacial calcites. In contrast to normal speleothems (stalagmites, excentriques, crystals in cave ponds, draperies; δ18O: –4.0 to –6.1 ‰, δ13C: –4.9 to –10.9 ‰), rhombohedral crystal sinter and spherulitic speleoparticles are characterised by lower δ18O (–8.9 to –17.9 ‰) and higher δ13C values (+0.7 to –6.1 ‰). This suggests that these speleogenetic particles were formed in slowly freezing waterpools on ice during the transition from a stadial to an interstadial phase. Precise 230Th/U-dating shows younger Weichselian ages of 28.6 to 33.0 ka for these speleogenetic particles from north-west Germany. These formation periods indicate freezing conditions overlain by 34 m of hostrock and provide the minimum depth of permafrost penetration for the younger Weichselian in the area of the northern Sauerland (north-west Germany).
    Description: research
    Keywords: 551.7 ; permafrost ; Cryogenic calcites ; C/O isotopes ; Upper Weichselian ; ice caves ; north-western Germany
    Language: German
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  • 7
    Publication Date: 2021-05-06
    Description: On the basis of the work done by Strahl et al. (1994), the mollusc specimens in the Late Saalian glacial and Eemian interglacial sediments from the cliff outcrop of Klein Klütz Höved (NW Mecklenburg) were re-examined and, in the process, a few vertebrates (fishes, micromammals) were also found. The mollusc specimens from the Late Saalian sand and mud alone comprise approximately 40 species among which terrestrial varieties, e.g. Succinella oblonga, Pupilla loessica, P. pratensis, Vallonia tenuilabris or Vertigo genesii, are clearly predominant as far as the number of individual specimens is concerned. Among the limnic varieties were also found the glacial index species Pisidium obtusale f. lapponicum and P. stewarti. It was also possible to prove the northern-most presence for Germany of the steppe lemming Lagurus lagurus in the Late Saalian. In the Eemian mud, the molluscs found were almost exclusively limnic. This about 20 species comprising fauna is characte-ristic of a river system with calmer areas, oxbow lakes and in-coming springs. These fluvial conditions were confirmed by the freshwater bivalves Pisidium amnicum, P. supinum, P. moitessierianum, P. ponderosum and Unio tumidus. For the first time in Mecklenburg-Western Pomerania, evidence was found of the spring snail Belgrandia germanica as an interglacial index species. The limnic-fluvial section of the Eemian is topped by a cryoturbate structures. Its sandy, gravelly pockets contain marine molluscs. The index species for the marine Eemian found in the southern Baltic region are Polititapes senescens and Bittium reticulatum.
    Description: research
    Keywords: 551.7 ; molluscs ; Saalian deposits ; limnic and marine Eemian facies ; Belgrandia germanica ; micromammals ; Lagurus lagurus ; Mecklenburg-Western Pomerania
    Language: German
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  • 8
    Publication Date: 2021-03-29
    Description: Tendenziell fallende Grundwasserstände können vielfältige Ursachen haben, z.B. die Änderung der klimatischen Bedingungen, Modifikationen der Landnutzung oder die Entnahme von Grundwasser. Am Beispiel Südwest-Usedoms wird dargestellt, wie mögliche Einflussfaktoren analysiert und quantifiziert werden können. Für das Untersuchungsgebiet zeigte sich, dass die über Jahrzehnte gesunkenen Grundwasserstände überwiegend aus der Kahlschlagbewirtschaftung des Waldes resultieren und damit weniger einen Trend als eine überjährliche Schwankung ausweisen.
    Description: research
    Keywords: 551 ; 554.3 ; forest ; Sickerwasser ; Grundwasser ; Grundwasserneubildung ; Wald ; Landschaftswasserhaushalt ; Trend ; Grundwasserentnahme ; geohydraulisches Modell ; Landscape water regime ; Groundwater ; Groundwater recharge ; percolate water ; Groundwater extraction ; Groundwater modelling ; Geohydraulic model ; Grundwasserströmung ; Modellierung
    Language: German
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  • 9
    Publication Date: 2021-05-06
    Description: A soil-sediment sequence in NE Germany has provided information about the duration and intensity of formation of a Bw horizon in Holocene slope deposits. With a combination of optically stimulated luminescence (OSL), 14C- and archaeological dating methods, colluvial layers taken from a former castle wall trench constructed during the Bronze Age were dated. With this chronology, the relative age of the postsedimentary Bw horizon formation within the colluvial sediments was derived, resulting in the first valid pedochronological data (maximum and minimum age) for a Holocene Bw horizon in NE Germany. The horizon was formed within 2400 years. Weathering and brunification have altered the Holocene parent material. However, the geochemical characteristics of the Holocene soil formation are weak compared to Bw horizons from the Late Glacial and the Late Glacial to Holocene. The results presented here enhance our understanding of soil formation processes in northern Germany, while highlighting the role of colluvial layers as sedimentological tracers of Holocene soil formation processes.
    Description: research
    Keywords: 551.7 ; holocene ; ne germany ; dating ; Bw horizon ; Slope deposits ; soil formation
    Language: English
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  • 10
    Publication Date: 2021-05-06
    Description: A radiocarbon-dated peat profile from Rond Pertuis supérieure mire in the uplands of the Northern Vosges is studied using palynological methods. The profile dates from the middle Atlantic period (4500–3100 B.C.) to recent times. During the middle of the Atlantic period an oak forest rich in pine covered the Northern Vosges. Fir and beech immigrated at the end of the Atlantic leading to the decline of oak and pine in the forest. This also marked the onset of decisive human influence on the development of the terrestrial vegetation. Five land use phases were detected, the first one at the end of the Neolithic period (~4000–2200 B.C.). Subsequently, continuous land use is evident from the Bronze Age (2200–800 B.C.) up to now. In the late Middle Ages (A.D. 900–1500) and the early Modern Era (since A.D. 1500), the woodlands were completely altered by human activities. During the 19th century modern forestry introduced spruce into the investigated area in the Northern Vosges. Spruce afforestation then accelerated in the middle of the 20th century.
    Description: research
    Keywords: 551.7 ; holocene ; pollen analysis ; france ; human impact ; Northern Vosges
    Language: English
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  • 11
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    AMS (American Meteorological Society)
    In:  Journal of the Atmospheric Sciences, 72 (7). pp. 2786-2805.
    Publication Date: 2019-01-08
    Description: In Ammassalik, in southeast Greenland, downslope winds can reach hurricane intensity and represent a hazard for the local population and environment. They advect cold air down the ice sheet and over the Irminger Sea, where they drive large ocean–atmosphere heat fluxes over an important ocean convection region. Earlier studies have found them to be associated with a strong katabatic acceleration over the steep coastal slopes, flow convergence inside the valley of Ammassalik, and—in one instance—mountain wave breaking. Yet, for the general occurrence of strong downslope wind events, the importance of mesoscale processes is largely unknown. Here, two wind events—one weak and one strong—are simulated with the atmospheric Weather Research and Forecasting (WRF) Model with different model and topography resolutions, ranging from 1.67 to 60 km. For both events, but especially for the strong one, it is found that lower resolutions underestimate the wind speed because they misrepresent the steepness of the topography and do not account for the underlying wave dynamics. If a 5-km model instead of a 60-km model resolution in Ammassalik is used, the flow associated with the strong wind event is faster by up to 20 m s−1. The effects extend far downstream over the Irminger Sea, resulting in a diverging spatial distribution and temporal evolution of the heat fluxes. Local differences in the heat fluxes amount to 20%, with potential implications for ocean convection.
    Type: Article , PeerReviewed
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  • 12
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    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 45 . pp. 1709-1734.
    Publication Date: 2021-05-18
    Description: We perform eddy-resolving and high-vertical-resolution numerical simulations of the circulation in an idealized equatorial Atlantic Ocean in order to explore the formation of the deep equatorial circulation (DEC) in this basin. Unlike in previous studies, the deep equatorial intraseasonal variability (DEIV) that is believed to be the source of the DEC is generated internally by instabilities of the upper ocean currents. Two main simulations are discussed: Solution 1, configured with a rectangular basin and with wind forcing that is zonally and temporally uniform; and Solution 2, with realistic coastlines and with an annual cycle of wind forcing varying zonally. Somewhat surprisingly, Solution 1 produces the more realistic DEC: The large-vertical-scale currents (Equatorial Intermediate Currents or EICs) are found over a large zonal portion of the basin, and the small-vertical-scale equatorial currents (Equatorial Deep Jets or EDJs) form low-frequency, quasi-resonant, baroclinic equatorial basin modes with phase propagating mostly downward, consistent with observations. We demonstrate that both types of currents arise from the rectification of DEIV, consistent with previous theories. We also find that the EDJs contribute to maintaining the EICs, suggesting that the nonlinear energy transfer is more complex than previously thought. In Solution 2, the DEC is unrealistically weak and less spatially coherent than in the first simulation probably because of its weaker DEIV. Using intermediate solutions, we find that the main reason for this weaker DEIV is the use of realistic coastlines in Solution 2. It remains to be determined, what needs to be modified or included to obtain a realistic DEC in the more realistic configuration.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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  • 13
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    AMS (American Meteorological Society)
    In:  Journal of Atmospheric and Oceanic Technology, 32 (8). pp. 1536-1543.
    Publication Date: 2020-08-04
    Description: A yet unexplained drift of (some) oxygen optodes during storage/transport and thus significant deviations from factory/laboratory calibrations have been a major handicap for autonomous oxygen observations. Optode drift appears to be systematic and is predominantly a slope effect due to reduced oxygen sensitivity. A small contribution comes from a reduced luminophore lifetime, which causes a small positive offset. A reliable in situ reference is essential to correct such a drift. Traditionally, this called for a ship-based reference cast, which poses some challenges for opportunistic float deployments. This study presents an easily implemented alternative using near-surface/in-air measurements of an Aanderaa optode on a 10-cm stalk and compares it to the more traditional approaches (factory, laboratory, and in situ deployment calibration). In-air samples show a systematic bias depending on the water saturation, which is likely caused by occasional submersions of the standard-height stalk optode. Linear regression of measured in-air supersaturation against in-water supersaturation (using ancillary meteorological data to define the saturation level) robustly removes this bias and thus provides a precise (0.2%) and accurate (1%) in situ correction that is available throughout the entire instrument’s lifetime.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
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  • 14
    Publication Date: 2017-10-24
    Description: Predictability on seasonal time scales over the North Atlantic–Europe region is assessed using a seasonal prediction system based on an initialized version of the Max Planck Institute Earth System Model (MPI-ESM). For this region, two of the dominant predictors on seasonal time scales are El Niño–Southern Oscillation (ENSO) and sudden stratospheric warming (SSW) events. Multiple studies have shown a potential for improved North Atlantic predictability for either predictor. Their respective influences are however difficult to disentangle, since the stratosphere is itself impacted by ENSO. Both El Niño and SSW events correspond to a negative signature of the North Atlantic Oscillation (NAO), which has a major influence on European weather. This study explores the impact on Europe by separating the stratospheric pathway of the El Niño teleconnection. In the seasonal prediction system, the evolution of El Niño events is well captured for lead times of up to 6 months, and stratospheric variability is reproduced with a realistic frequency of SSW events. The model reproduces the El Niño teleconnection through the stratosphere, involving a deepened Aleutian low connected to a warm anomaly in the northern winter stratosphere. The stratospheric anomaly signal then propagates downward into the troposphere through the winter season. Predictability of 500-hPa geopotential height over Europe at lead times of up to 4 months is shown to be increased only for El Niño events that exhibit SSW events, and it is shown that the characteristic negative NAO signal is only obtained for winters also containing major SSW events for both the model and the reanalysis data.
    Type: Article , PeerReviewed
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  • 15
    Publication Date: 2020-10-21
    Description: Classical theory concerning theEliassen–Palmrelation is extended in this study to allowfor a unified treatment of midlatitude inertia–gravity waves (MIGWs), midlatitude Rossby waves (MRWs), and equatorial waves (EQWs). A conservation equation for what the authors call the impulse-bolus (IB) pseudomomentum is useful, because it is applicable to ageostrophic waves, and the associated three-dimensional flux is parallel to the direction of the group velocity of MRWs. The equation has previously been derived in an isentropic coordinate system or a shallow-water model. The authors make an explicit comparison of prognostic equations for the IB pseudomomentum vector and the classical energy-based (CE) pseudomomentum vector, assuming inviscid linear waves in a sufficiently weak mean flow, to provide a basis for the former quantity to be used in an Eulerian time-mean (EM) framework. The authors investigate what makes the three-dimensional fluxes in the IB and CE pseudomomentum equations look in different directions. It is found that the two fluxes are linked by a gauge transformation, previously unmentioned, associated with a divergence-form wave-induced pressure L. The quantity L vanishes for MIGWs and becomes nonzero for MRWs and EQWs, and it may be estimated using the virial theorem. Concerning the effect of waves on the mean flow, L represents an additional effect in the pressure gradient term of both (the three-dimensional versions of) the transformed EM momentum equations and the merged form of the EMmomentumequations, the latter of which is associated with the nonacceleration theorem.
    Type: Article , PeerReviewed
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  • 16
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    AMS (American Meteorological Society)
    In:  Bulletin of the American Meteorological Society, 96, Special supplement (7). S157-S160.
    Publication Date: 2018-06-20
    Description: [in “State of the Climate in 2014” : Special Supplement to the Bulletin of the American Meteorological Society Vol. 96, No. 7, July 2015]
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  • 17
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    AMS (American Meteorological Society)
    In:  Journal of the Atmospheric Sciences, 72 . pp. 4029-4045.
    Publication Date: 2018-04-16
    Description: Atmospheric deposition contributes potentially significant amounts of the nutrients iron, nitrogen and phosphorus (via mineral dust and anthropogenic aerosols) to the oligotrophic tropical North Atlantic Ocean. Transport pathways, deposition processes and source strengths contributing to this atmospheric flux are all highly variable in space and time. Atmospheric sampling was conducted during 28 research cruises through the Eastern Tropical North Atlantic (ETNA) over a 12 year period and a substantial dataset of measured concentrations of nutrients and trace metals in aerosol and rainfall over the region was acquired. This database was used to quantify (on a spatial- and seasonal-basis) the atmospheric input of ammonium, nitrate, soluble phosphorus and soluble and total iron, aluminium and manganese to the ETNA. The magnitude of atmospheric input varies strongly across the region, with high rainfall rates associated with the Inter-tropical Convergence Zone contributing to high wet deposition fluxes in the south, particularly for soluble species. Dry deposition fluxes of species associated with mineral dust exhibited strong seasonality, with highest fluxes associated with winter-time low-level transport of Saharan dust. Overall (wet plus dry) atmospheric inputs of soluble and total trace metals were used to estimate their soluble fractions. These also varied with season and were generally lower in the dry north than in the wet south. The ratio of ammonium plus nitrate to soluble iron in deposition to the ETNA was lower than the N:Fe requirement for algal growth in all cases, indicating the importance of the atmosphere as a source of excess iron.
    Type: Article , PeerReviewed
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  • 18
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    AMS (American Meteorological Society)
    In:  Bulletin of the American Meteorological Society, 96 (9). pp. 1561-1564.
    Publication Date: 2015-11-11
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  • 19
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    AMS (American Meteorological Society)
    In:  Journal of Atmospheric and Oceanic Technology, 32 . pp. 2305-2317.
    Publication Date: 2020-08-04
    Description: We investigated the effect of hydrostatic pressure of up to 6000 dbar on Aanderaa and Sea-Bird oxygen optodes both in the laboratory and in the field. The overall pressure response is a reduction in the O2 reading by 3 – 4 % per 1000 dbar which is closely linear with pressure and increases with temperature. Closer inspection reveals two superimposed processes with opposite effect: an O2-independent pressure response on the luminophore which increases optode O2 readings and an O2-dependent change in luminescence quenching which decreases optode O2 readings. The latter process dominates and is mainly due to a shift in the equilibrium between sensing membrane and sea water under elevated pressures. If only the dominant O2-dependent process is considered, Aanderaa and Sea-Bird optodes differ in their pressure response. Compensation of the O2-independent process, however, yields a uniform O2 dependence for Aanderaa optodes with standard foil and fast-response foil as well as Sea-Bird optodes. A new scheme to calculate optode O2 from raw data is proposed to account for the two processes. The overall uncertainty of the optode pressure correction amounts to 0.3 % per 1000 dbar, mainly due to variability between sensors.
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  • 20
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    AMS (American Meteorological Society)
    In:  Journal of Climate, 28 (24). pp. 9697-9706.
    Publication Date: 2020-08-04
    Description: The upper tropical Atlantic Ocean has markedly warmed since the 1960s. It has been shown that this warming was not due to local heat fluxes, and that the trade winds that drive the coastal and equatorial upwelling have intensified rather than weakened. Remote forcing might thus have played an important role. Here model experiments are used to investigate the contribution from an increased inflow of warm Indian Ocean water through Agulhas leakage. A high-resolution hindcast experiment with interannually varying forcing for the time period 1948 to 2007, in which Agulhas leakage increases by about 45% from the 1960s to the early 2000s, reproduces the observed warming trend. To tease out the role of Agulhas leakage, a sensitivity experiment designed to only increase Agulhas leakage is used. Compared to a control simulation it shows a pronounced warming in the upper tropical Atlantic Ocean. A Lagrangian trajectory analysis confirms that a significant portion of Agulhas leakage water reaches the upper 300m of the tropical Atlantic Ocean within two decades, and that the tropical Atlantic warming in the sensitivity experiment is mainly due to water of Agulhas origin. Therefore, it is suggested that the increased trade winds since the 1960s favor upwelling of warmer subsurface waters, which in parts originate from the Agulhas, leading to higher SSTs in the tropics
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  • 21
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    AMS (American Meteorological Society)
    In:  Journal of Climate, 28 (1). pp. 168-185.
    Publication Date: 2020-08-04
    Description: Variations in the global tropospheric zonal mean zonal wind ([U]) during boreal winter are investigated using Rotated Empirical Orthogonal Functions applied to monthly means. The first two modes correspond to the Northern and Southern Annular Mode and modes 3 and 4 represent variability in the tropics. One is related to El Niño Southern Oscillation and the other has variability that is highly correlated with the time series of [U] at 150 hPa between 5°N and 5°S ([U150]E) and is related to activity of the Madden-Julian Oscillation. The extratropical response to [U150]E is investigated using linear regressions of 500 hPa geopotential height onto the [U150]E time series. We make use of reanalysis data and of the ensemble mean output from a relaxation experiment using the European Center for Medium Range Weather Forecasts model in which the tropical atmosphere is relaxed towards reanalysis data. The regression analysis reveals that a shift of the Aleutian low and a wave train across the North Atlantic are associated with [U150]E. We find that the subtropical waveguides and the link between the North Pacific and North Atlantic are stronger during the easterly phase of [U150]E. The wave train over the North Atlantic is associated with Rossby wave sources over the subtropical North Pacific and North America. Finally, we show that a linear combination of both [U150]E and the Quasi Biennial Oscillation in the lower stratosphere can explain the circulation anomalies of the anomalously cold European winter of 1962/63 when both were in an extreme easterly phase.
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  • 22
    Publication Date: 2020-08-04
    Description: To investigate the influence of atmospheric model resolution on the representation of daily precipitation extremes, ensemble simulations with the atmospheric general circulation model ECHAM5 at different horizontal (T213 to T31) and vertical (L31 to L19) resolutions and forced with observed sea surface temperatures and sea ice concentrations have been carried out for 01/1982 - 09/2010. All results have been compared with the highest resolution, which has been validated against observations. Resolution affects both the representation of physical processes and the averaging of precipitation across grid boxes. The latter, in particular, smoothes out localized extreme events. These effects have been disentangled by averaging precipitation simulated at the highest resolution to the corresponding coarser grid. Extremes are represented by seasonal maxima, modeled by the generalized extreme value distribution. Effects of averaging and representation of physical processes vary with region and season. In the tropical summer hemisphere, extreme precipitation is reduced by up to 30% due to the averaging effect, and a further 65% owing to a coarser representation of physical processes. Towards mid- to high latitudes, the latter effect reduces to 20%; in the winter hemisphere it vanishes towards the poles. A strong drop is found between T106 and T63 in the convection dominated tropics. At the lowest resolution, northern hemisphere winter precipitation extremes, mainly caused by large scale weather systems, are in general represented reasonably well. Coarser vertical resolution causes an equatorward shift of maximum extreme precipitation in the tropics. The impact of vertical resolution on mean precipitation is less pronounced; for horizontal resolution it is negligible.
    Type: Article , PeerReviewed
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  • 23
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    AMS (American Meteorological Society)
    In:  Bulletin of the American Meteorological Society, 96 (7). ES1-ES32.
    Publication Date: 2019-09-23
    Type: Article , PeerReviewed
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  • 24
    Publication Date: 2022-03-09
    Description: Many aspects of the coupling between the ocean and atmosphere at the mesoscale (on the order of 20–100 km) remain unknown. While recent observations from the Southern Ocean revealed that circular fronts associated with oceanic mesoscale eddies leave a distinct imprint on the overlying wind, cloud coverage, and rain, the mechanisms responsible for explaining these atmospheric changes are not well established. Here the atmospheric response above mesoscale ocean eddies is investigated utilizing a newly developed coupled atmosphere–ocean regional model [Consortium for Small-Scale Modeling–Regional Ocean Modelling System (COSMO-ROMS)] configured at a horizontal resolution of ~10 km for the South Atlantic and run for a 3-month period during austral winter of 2004. The model-simulated changes in surface wind, cloud fraction, and rain above the oceanic eddies are very consistent with the relationships inferred from satellite observations for the same region and time. From diagnosing the model’s momentum balance, it is shown that the atmospheric imprint of the oceanic eddies are driven by the modification of vertical mixing in the atmospheric boundary layer, rather than secondary flows driven by horizontal pressure gradients. This is largely due to the very limited ability of the atmosphere to adjust its temperature over the time scale it takes for an air parcel to pass over these mesoscale oceanic features. This results in locally enhanced vertical gradients between the ocean surface and the overlying air and thus a rapid change in turbulent mixing in the atmospheric boundary layer and an associated change in the vertical momentum flux.
    Type: Article , PeerReviewed
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  • 25
    Publication Date: 2022-03-09
    Description: This study examines the role of processes transporting tracers across the Polar Front (PF) in the depth interval between the surface and major topographic sills, which this study refers to as the “PF core.” A preindustrial control simulation of an eddying climate model coupled to a biogeochemical model [GFDL Climate Model, version 2.6 (CM2.6)– simplified version of the Biogeochemistry with Light Iron Nutrients and Gas (miniBLING) 0.1° ocean model] is used to investigate the transport of heat, carbon, oxygen, and phosphate across the PF core, with a particular focus on the role of mesoscale eddies. The authors find that the total transport across the PF core results from a ubiquitous Ekman transport that drives the upwelled tracers to the north and a localized opposing eddy transport that induces tracer leakages to the south at major topographic obstacles. In the Ekman layer, the southward eddy transport only partially compensates the northward Ekman transport, while below the Ekman layer, the southward eddy transport dominates the total transport but remains much smaller in magnitude than the near-surface northward transport. Most of the southward branch of the total transport is achieved below the PF core, mainly through geostrophic currents. This study finds that the eddy-diffusive transport reinforces the southward eddy-advective transport for carbon and heat, and opposes it for oxygen and phosphate. Eddy-advective transport is likely to be the leading-order component of eddy-induced transport for all four tracers. However, eddy-diffusive transport may provide a significant contribution to the southward eddy heat transport due to strong along-isopycnal temperature gradients.
    Type: Article , PeerReviewed
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