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  • Other Sources  (74)
  • Articles (OceanRep)  (74)
  • AMS (American Meteorological Society)  (64)
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  • Articles (OceanRep)  (74)
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  • 1
    Publication Date: 2020-06-24
    Description: Nitrogen fixation — the reduction of dinitrogen (N2) gas to biologically available nitrogen (N) — is an important source of N for terrestrial and aquatic ecosystems. In terrestrial environments, N2-fixing symbioses involve multicellular plants, but in the marine environment these symbioses occur with unicellular planktonic algae. An unusual symbiosis between an uncultivated unicellular cyanobacterium (UCYN-A) and a haptophyte picoplankton alga was recently discovered in oligotrophic oceans. UCYN-A has a highly reduced genome, and exchanges fixed N for fixed carbon with its host. This symbiosis bears some resemblance to symbioses found in freshwater ecosystems. UCYN-A shares many core genes with the 'spheroid bodies' of Epithemia turgida and the endosymbionts of the amoeba Paulinella chromatophora. UCYN-A is widely distributed, and has diversified into a number of sublineages that could be ecotypes. Many questions remain regarding the physical and genetic mechanisms of the association, but UCYN-A is an intriguing model for contemplating the evolution of N2-fixing organelles.
    Type: Article , PeerReviewed
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  • 2
    Publication Date: 2020-02-06
    Description: Arctic sea ice area (SIA) during late summer and early fall decreased substantially over the last four decades, and its decline accelerated beginning in the early 2000s. Statistical analyses of observations show that enhanced poleward moisture transport from the North Pacific to the Arctic Ocean contributed to the accelerated SIA decrease during the most recent period. As a consequence, specific humidity in the Arctic Pacific sector significantly increased along with an increase of downward longwave radiation beginning in 2002, which led to a significant acceleration in the decline of SIA in the Arctic Pacific sector. The resulting sea ice loss led to increased evaporation in the Arctic Ocean, resulting in a further increase of the specific humidity in mid-to-late fall, thus acting as a positive feedback to the sea ice loss. The overall set of processes is also found in a long control simulation of a coupled climate model.
    Type: Article , PeerReviewed
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  • 3
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    AMS (American Meteorological Society)
    In:  Journal of Climate, 30 (22). pp. 9321-9337.
    Publication Date: 2020-08-04
    Description: In the present study, the influence of some major tropical modes of variability on northern hemisphere regional blocking frequency variability during boreal winter is investigated. Reanalysis data and an ensemble experiment with the ECMWF model using relaxation towards the ERA-Interim reanalysis data inside the tropics are used. The tropical modes under investigation are El Niño Southern Oscillation (ENSO), the Madden-Julian Oscillation (MJO) and the upper tropospheric equatorial zonal-mean zonal wind . An early (late) MJO phase refers to the part of the MJO cycle when enhanced (suppressed) precipitation occurs over the western Indian Ocean and suppressed (enhanced) precipitation occurs over the Maritime Continent and the western tropical Pacific. Over the North Pacific sector, it is found that enhanced (suppressed) high latitude blocking occurs in association with El Niño (La Niña) events, late (early) MJO phases and westerly (easterly) . Over central to southern Europe and the east Atlantic, it is found that late MJO phases, as well as a suppressed MJO are leading to enhanced blocking frequency. Furthermore, early (late) MJO phases are followed by blocking anomalies over the western North Atlantic region, similar to those associated with a positive (negative) North Atlantic Oscillation. Over northern Europe, the easterly (westerly) phase of is associated with enhanced (suppressed) blocking. These results are largely confirmed by both the reanalysis and the model experiment.
    Type: Article , PeerReviewed
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  • 4
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    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 48 (4). pp. 757-771.
    Publication Date: 2021-02-08
    Description: The Eddy Kinetic Energy (EKE) associated with the Subtropical Countercurrent (STCC) in the western subtropical South Pacific is known to exhibit substantial seasonal and decadal variability. Using an eddy-permitting ocean general circulation model, which is able to reproduce the observed, salient features of the seasonal cycles of shear, stratification, baroclinic production and the associated EKE, we investigate the decadal changes of EKE. We show that the STCC region exhibits, uniquely among the subtropical gyres of the world’s oceans, significant, atmospherically forced, decadal EKE variability. The decadal variations are driven by changing vertical shear between the STCC in the upper 300 m and the South Equatorial Current below, predominantly caused by variations in STCC strength associated with a changing meridional density gradient. In the 1970s, an increased meridional density gradient results in EKE twice as large as in later decades in the model. Utilizing sensitivity experiments, decadal variations in the wind field are shown to be the essential driver. Local wind stress curl anomalies associated with the Interdecadal Pacific Oscillation (IPO) lead to up- and downwelling of the thermocline, inducing strengthening or weakening of the STCC and the associated EKE. Additionally, remote wind stress curl anomalies in the eastern subtropical South Pacific, which are not related to the IPO, generate density anomalies that propagate westward as Rossby waves and can account for up to 30–40 % of the density anomalies in the investigated region.
    Type: Article , PeerReviewed
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  • 5
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    AMS (American Meteorological Society)
    In:  Bulletin of the American Meteorological Society, 98 (8) (8). AMS (American Meteorological Society), Si-S280, 277 pp.
    Publication Date: 2020-10-21
    Type: Report , NonPeerReviewed
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  • 6
    Publication Date: 2020-02-06
    Description: The Indian Ocean has sustained robust surface warming in recent decades, but the role of multi-decadal variability remains unclear. Using ocean model hindcasts, characteristics of low-frequency Indian Ocean temperature variations are explored. Simulated upper-ocean temperature changes across the Indian Ocean in the hindcast are consistent with those recorded in observational products and ocean reanalyses. Indian Ocean temperatures exhibit strong warming trends since the 1950s limited to the surface and south of 30°S, while extensive subsurface cooling occurs over much of the tropical Indian Ocean. Previous work focused on diagnosing causes of these long-term trends in the Indian Ocean over the second half of the 20th Century. Instead, the temporal evolution of Indian Ocean subsurface heat content is shown here to reveal distinct multi-decadal variations associated with the Pacific Decadal Oscillation and the long-term trends are thus interpreted to result from aliasing of the low-frequency variability. Transmission of the multi-decadal signal occurs via an oceanic pathway through the Indonesian Throughflow and is manifest across the Indian Ocean centered along 12°S as westward propagating Rossby waves modulating thermocline and subsurface heat content variations. Resulting low-frequency changes in the eastern Indian Ocean thermocline depth are associated with decadal variations in the frequency of Indian Ocean Dipole (IOD) events, with positive IOD events unusually common in the 1960s and 1990s with a relatively shallow thermocline. In contrast, the deeper thermocline depth in the 1970s and 1980s is associated with frequent negative IOD and rare positive IOD events. Changes in Pacific wind forcing in recent decades and associated rapid increases in Indian Ocean subsurface heat content can thus affect the basin’s leading mode of variability, with implications for regional climate and vulnerable societies in surrounding countries.
    Type: Article , PeerReviewed
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  • 7
    Publication Date: 2021-02-08
    Description: The North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO) describe the dominant part of the variability in the Northern Hemisphere extratropical troposphere. Due to the strong connection of these patterns with surface climate, recent years have shown an increased interest and an increasing skill in forecasting them. However, it is unclear what the intrinsic limits of short-term predictability for the NAO and AO patterns are. This study compares the variability and predictability of both patterns, using a range of data and index computation methods for the daily NAO/AO indices. Small deviations from Gaussianity are found and characteristic decorrelation time scales of around one week. In the analysis of the Lyapunov spectrum it is found that predictability is not significantly different between the AO and NAO or between reanalysis products. Differences exist however between the indices based on EOF analysis, which exhibit predictability time scales around 12 - 16 days, and the station-based indices, exhibiting a longer predictability of 18 - 20 days. Both of these time scales indicate predictability beyond that currently obtained in ensemble prediction models for short-term predictability. Additional longer-term predictability for these patterns may be gained through local feedbacks and remote forcing mechanisms for particular atmospheric conditions.
    Type: Article , PeerReviewed
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  • 8
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    AMS (American Meteorological Society)
    In:  Bulletin of the American Meteorological Society, 98 (6). ES139-ES142.
    Publication Date: 2020-07-16
    Description: The 13th European Polar Low Workshop was organized by the European Polar Low Working Group (www.uni-trier.de/index.php?id=20308)and gathered scientists from nine countries focusing on polar mesocyclones in both hemispheres and other mesoscale weather phenomena such as katabatic winds, tip jets, boundary layer fronts, cold air outbreaks, and weather extremes in polar regions. Topics included experimental, climatological, theoretical, modeling, and remote sensing studies. The aim was to bring together scientists and forecasters to present their latest work and recent findings on these topics and to encourage discussions on improving forecasting and understanding of these phenomena.
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  • 9
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    AMS (American Meteorological Society)
    In:  Journal of Climate, 30 (22). pp. 8913-8927.
    Publication Date: 2020-02-06
    Description: The regional climate model COSMOin Climate Limited-AreaMode (COSMO-CLM or CCLM) is used with a high resolution of 15km for the entire Arctic for all winters 2002/03–2014/15. The simulations show a high spatial and temporal variability of the recent 2-m air temperature increase in the Arctic. The maximum warming occurs north of Novaya Zemlya in the Kara Sea and Barents Sea between March 2003 and 2012 and is responsible for up to a 208C increase. Land-based observations confirm the increase but do not cover the maximum regions that are located over the ocean and sea ice.Also, the 30-km version of theArctic SystemReanalysis (ASR) is used to verify the CCLM for the overlapping time period 2002/03–2011/12. The differences between CCLM and ASR 2-m air temperatures vary slightly within 18C for the ocean and sea ice area. Thus,ASR captures the extreme warming as well. The monthly 2-m air temperatures of observations and ERA-Interim data show a large variability for the winters 1979–2016. Nevertheless, the air temperature rise since the beginning of the twenty-first century is up to 8 times higher than in the decades before. The sea ice decrease is identified as the likely reason for the warming. The vertical temperature profiles show that the warming has a maximum near the surface, but a 0.58Cyr21 increase is found up to 2 km. CCLM, ASR, and also the coarser resolved ERA-Interim data show that February and March are the months with the highest 2-m air temperature increases, averaged over the ocean and sea ice area north of 708N; for CCLM the warming amounts to an average of almost 58C for 2002/03–2011/12.
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  • 10
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    AMS (American Meteorological Society)
    In:  Journal of the Atmospheric Sciences, 75 (8). pp. 2815-2826.
    Publication Date: 2021-02-08
    Description: The formation of secondary ice in clouds, i.e. ice particles that are created at temperatures above the limit for homogeneous freezing without the direct involvement of a heterogeneous ice nucleus is one of the longest standing puzzles in cloud physics. Here we present comprehensive laboratory investigations on the formation of small ice particles upon the freezing of drizzle-sized cloud droplets levitated in an electrodynamic balance. Four different categories of secondary ice formation (bubble bursting, jetting, cracking, breakup) could be detected and their respective frequencies of occurrence as a function of temperature and droplet size are given. We find that bubble bursting occurs more often than droplet splitting. While we do not observe the shattering of droplets into many large fragments, we find that the average number of small secondary ice particles released during freezing is strongly droplet-size dependent and may well exceed unity for droplets larger than 300 μm in diameter. This leaves droplet fragmentation an important secondary ice process effective at temperatures around -10 °C in clouds where large drizzle droplets are present.
    Type: Article , PeerReviewed
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