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  • AMS (American Meteorological Society)  (16)
  • Wiley  (9)
  • American Meteorological Society  (3)
  • Deutscher Wetterdienst  (1)
  • Frontiers  (1)
  • John Wiley & Sons  (1)
  • American Geophysical Union
  • IFM-GEOMAR
  • 2015-2019  (20)
  • 2000-2004  (2)
  • 1990-1994  (9)
Publikationsart
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Jahr
  • 1
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    Composition and variability of the Denmark Strait Overflow Water in a high-resolution numerical model hindcast simulation (2017)
    AGU (American Geophysical Union) | Wiley
    In:  Journal of Geophysical Research: Oceans, 122 (4). 2830-2846 .
    Details
    Publikationsdatum: 2020-02-06
    Beschreibung: The upstream sources and pathways of the Denmark Strait Overflow Water and their variability have been investigated using a high-resolution model hindcast. This global simulation covers the period from 1948 to 2009 and uses a fine model mesh (1/20°) to resolve mesoscale features and the complex current structure north of Iceland explicitly. The three sources of the Denmark Strait Overflow, the shelfbreak East Greenland Current (EGC), the separated EGC, and the North Icelandic Jet, have been analyzed using Eulerian and Lagrangian diagnostics. The shelfbreak EGC contributes the largest fraction in terms of volume and freshwater transport to the Denmark Strait Overflow and is the main driver of the overflow variability. The North Icelandic Jet contributes the densest water to the Denmark Strait Overflow and shows only small temporal transport variations. During summer, the net volume and freshwater transports to the south are reduced. On interannual time scales, these transports are highly correlated with the large-scale wind stress curl around Iceland and, to some extent, influenced by the North Atlantic Oscillation, with enhanced southward transports during positive phases. The Lagrangian trajectories support the existence of a hypothesized overturning loop along the shelfbreak north of Iceland, where water carried by the North Icelandic Irminger Current is transformed and feeds the North Icelandic Jet. Monitoring these two currents and the region north of the Iceland shelfbreak could provide the potential to track long-term changes in the Denmark Strait Overflow and thus also the AMOC.
    Materialart: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    Format: text
    DOI: 10.1002/2016JC012158
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  • 2
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    Influences of Pacific Climate Variability on Decadal Subsurface Ocean Heat Content Variations in the Indian Ocean (2018)
    AMS (American Meteorological Society)
    In:  Journal of Climate, 31 (10). pp. 4157-4174.
    Details
    Publikationsdatum: 2021-02-08
    Beschreibung: Decadal variabilities in Indian Ocean subsurface ocean heat content (OHC; 50–300 m) since the 1950s are examined using ocean reanalyses. This study elaborates on how Pacific variability modulates the Indian Ocean on decadal time scales through both oceanic and atmospheric pathways. High correlations between OHC and thermocline depth variations across the entire Indian Ocean Basin suggest that OHC variability is primarily driven by thermocline fluctuations. The spatial pattern of the leading mode of decadal Indian Ocean OHC variability closely matches the regression pattern of OHC on the interdecadal Pacific oscillation (IPO), emphasizing the role of the Pacific Ocean in determining Indian Ocean OHC decadal variability. Further analyses identify different mechanisms by which the Pacific influences the eastern and western Indian Ocean. IPO-related anomalies from the Pacific propagate mainly through oceanic pathways in the Maritime Continent to impact the eastern Indian Ocean. By contrast, in the western Indian Ocean, the IPO induces wind-driven Ekman pumping in the central Indian Ocean via the atmospheric bridge, which in turn modifies conditions in the southwestern Indian Ocean via westward-propagating Rossby waves. To confirm this, a linear Rossby wave model is forced with wind stresses and eastern boundary conditions based on reanalyses. This linear model skillfully reproduces observed sea surface height anomalies and highlights both the oceanic connection in the eastern Indian Ocean and the role of wind-driven Ekman pumping in the west. These findings are also reproduced by OGCM hindcast experiments forced by interannual atmospheric boundary conditions applied only over the Pacific and Indian Oceans, respectively.
    Materialart: Article , PeerReviewed
    Format: text
    DOI: 10.1175/JCLI-D-17-0654.1
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  • 3
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    Decadal variability of Eddy Kinetic Energy in the South Pacific Subtropical Countercurrent in an Ocean General Circulation Model (2018)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 48 (4). pp. 757-771.
    Details
    Publikationsdatum: 2021-02-08
    Beschreibung: 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.
    Materialart: Article , PeerReviewed
    Format: text
    DOI: 10.1175/JPO-D-17-0173.1
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  • 4
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    Seasonal Cycles of Oceanic Transports in the Eastern Subpolar North Atlantic (2018)
    AGU (American Geophysical Union) | Wiley
    In:  Journal of Geophysical Research: Oceans, 123 (2). pp. 1471-1484.
    Details
    Publikationsdatum: 2021-02-08
    Beschreibung: The variability of the Atlantic Meridional Overturning Circulation (AMOC) may play a role in sea surface temperature predictions on seasonal to decadal time scales. Therefore, AMOC seasonal cycles are a potential baseline for interpreting predictions. Here we present estimates for the seasonal cycle of transports of volume, temperature, and freshwater associated with the upper limb of the AMOC in the eastern subpolar North Atlantic on the Extended Ellett Line hydrographic section between Scotland and Iceland. Due to weather, ship‐based observations are primarily in summer. Recent glider observations during other seasons present an opportunity to investigate the seasonal variability in the upper layer of the AMOC. First, we document a new method to quality control and merge ship, float, and glider hydrographic observations. This method accounts for the different spatial sampling rates of the three platforms. The merged observations are used to compute seasonal cycles of volume, temperature, and freshwater transports in the Rockall Trough. These estimates are similar to the seasonal cycles in two eddy‐resolving ocean models. Volume transport appears to be the primary factor modulating other Rockall Trough transports. Finally, we show that the weakest transports occur in summer, consistent with seasonal changes in the regional‐scale wind stress curl. Although the seasonal cycle is weak compared to other variability in this region, the amplitude of the seasonal cycle in the Rockall Trough, roughly 0.5–1 Sv about a mean of 3.4 Sv, may account for up to 7–14% of the heat flux between Scotland and Greenland.
    Materialart: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1002/2017JC013350
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  • 5
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    Annual and semiannual cycle of equatorial Atlantic circulation associated with basin mode resonance (2016)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 46 (10). pp. 3011-3029.
    Details
    Publikationsdatum: 2020-08-04
    Beschreibung: Seasonal variability of the tropical Atlantic circulation is dominated by the annual cycle, but semi-annual variability is also pronounced, despite weak forcing at that period. Here we use multi-year, full-depth velocity measurements from the central equatorial Atlantic to analyze the vertical structure of annual and semi-annual variations of zonal velocity. A baroclinic modal decomposition finds that the annual cycle is dominated by the 4th mode and the semi-annual cycle by the 2nd mode. Similar local behavior is found in a high-resolution general circulation model. This simulation reveals that the annual and semi-annual cycles of the respective dominant baroclinic modes are associated with characteristic basin-wide structures. Using an idealized linear reduced-gravity model to simulate the dynamics of individual baroclinic modes, it is shown that the observed circulation variability can be explained by resonant equatorial basin modes. Corollary simulations of the reduced-gravity model with varying basin geometry (i.e. square basin versus realistic coastlines) or forcing (i.e. spatially uniform versus spatially variable wind) show a structural robustness of the simulated basin modes. A main focus of this study is the seasonal variability of the Equatorial Undercurrent (EUC) as identified in recent observational studies. Main characteristics of the observed EUC including seasonal variability of transport, core depth, and maximum core velocity can be explained by the linear superposition of the dominant equatorial basin modes as obtained from the reduced-gravity model.
    Materialart: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1175/JPO-D-15-0248.1
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  • 6
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    Variability and trends in Southern Ocean eddy activity in 1/12° ocean model simulations (2016)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 43 . pp. 4517-4523.
    Details
    Publikationsdatum: 2019-02-26
    Beschreibung: The response of eddy kinetic energy (EKE) to the strengthening of Southern Hemisphere winds occurring since the 1950s is investigated with a global ocean model having a resolution of 1/12° in the Antarctic Circumpolar Current domain. The simulations expose regional differences in the relative importance of stochastic and wind-related contributions to inter-annual EKE changes. In the Pacific and Indian sectors the model captures the EKE variability observed since 1993 and confirms previous hypotheses of a lagged response to regional wind stress anomalies. Here, the multi-decadal trend in wind stress is reflected in an increase in EKE typically exceeding 5 cm2 sec-2 decade-1. In the western Atlantic EKE variability is mostly stochastic, is weakly correlated with wind fluctuations, and its multi-decadal trends are close to zero. The non-uniform distribution of wind-related changes in the eddy activity could affect the regional patterns of ocean circulation and biogeochemical responses to future climate change.
    Materialart: Article , PeerReviewed
    Format: text
    Format: text
    Format: video
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    DOI: 10.1002/2016GL069026
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  • 7
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    Seasonal variability of eddy kinetic energy in a global high-resolution ocean model (2015)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 42 (21). pp. 9379-9386.
    Details
    Publikationsdatum: 2020-11-04
    Beschreibung: A global ocean model with 1/12∘ horizontal resolution is used to assess the seasonal cycle of surface Eddy Kinetic Energy (EKE). The model reproduces the salient features of the observed mean surface EKE, including amplitude and phase of its seasonal cycle in most parts of the ocean. In all subtropical gyres of the Pacific and Atlantic, EKE peaks in summer down to a depth of ∼350 m, below which the seasonal cycle is weak. Investigation of the possible driving mechanisms reveals the seasonal changes in the thermal interactions with the atmosphere to be the most likely cause of the summer maximum of EKE. The development of the seasonal thermocline in spring and summer is accompanied by stronger mesoscale variations in the horizontal temperature gradients near the surface which corresponds, by thermal wind balance, to an intensification of mesoscale velocity anomalies towards the surface.
    Materialart: Article , PeerReviewed
    Format: text
    Format: text
    DOI: 10.1002/2015GL066152
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  • 8
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    Multidecadal Indian Ocean variability linked to the Pacific and implications for pre-conditioning Indian Ocean Dipole events (2017)
    AMS (American Meteorological Society)
    In:  Journal of Climate, 30 (5). pp. 1739-1751.
    Details
    Publikationsdatum: 2020-02-06
    Beschreibung: 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.
    Materialart: Article , PeerReviewed
    Format: text
    Format: text
    DOI: 10.1175/JCLI-D-16-0200.1
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  • 9
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    Sources of Eddy Kinetic Energy in the Labrador Sea (2002)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 32 (12). pp. 3346-3363.
    Details
    Publikationsdatum: 2020-08-04
    Beschreibung: Experiments with a suite of North Atlantic general circulation models are used to examine the sources of eddy kinetic energy (EKE) in the Labrador Sea. A high-resolution model version (112°) quantitatively reproduces the observed signature. A particular feature of the EKE in the Labrador Sea is its pronounced seasonal cycle, with a maximum intensity in early winter, as already found in earlier studies based on altimeter data. In contrast to a previously advanced hypothesis, the seasonally varying eddy field is not related to a forcing by high-frequency wind variations but can be explained by a seasonally modulated instability of the West Greenland Current (WGC). The main source of EKE in the Labrador Sea is an energy transfer due to Reynolds interaction work (barotropic instability) in a confined region near Cape Desolation where the WGC adjusts to a change in the topographic slope: Geostrophic contours tend to converge upstream of Cape Desolation, such that the topographically guided WGC narrows as well and becomes barotropically unstable. The eddies spawned from the WGC instability area, dominating the EKE in the interior Labrador Sea, are predominantly anticyclonic with warm and saline cores in the upper kilometer of the water column, while the few cyclones originating as well from the instability area show a more depth-independent structure. Companion experiments with a ⅓° model exhibit the strength of the WGC, influenced by either changes in the wind stress or heat flux forcing, as a leading factor determining seasonal to interannual changes of EKE in the Labrador Sea
    Materialart: Article , PeerReviewed
    Format: text
    DOI: 10.1175/1520-0485(2002)032〈3346:SOEKEI〉2.0.CO;2
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  • 10
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    Seasonal transport variation in the western subtropical North Atlantic: Experiments with an eddy resolving model (1991)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 21 . pp. 1271-1289.
    Details
    Publikationsdatum: 2020-08-04
    Beschreibung: A high-resolution model of the wind-driven and thermohaline circulation in the North and equatorial Atlantic Ocean is used to study the structure and variability of the boundary current system at 26°N, including the Florida Current, the Antilles Current, and the Deep Western Boundary Current (DWBC). The model was developed by Bryan and Holland as a Community Modeling Effort of the World Ocean Circulation Experiment. Subsequent experiments have been performed at IfM Kiel, with different friction coefficients, and different climatologies of monthly mean wind stress: Hellerman–Rosenstein (HR) and Isemer–Hasse (IH). The southward volume transports in the upper 1000 m of the interior Atlantic, at 26°N, are 25.0 Sv (Sv ≡ 106m3s−1) for HR, and 34.9 Sv for IH forcing, in good agreement with the transport from the integrated Sverdrup balance at this latitude (23.9 Sv for HR, 35.6 Sv for IH). The return flow of this wind-driven transport, plus the southward transport of the DWBC (6–8 Sv), is partitioned between the Florida Current and Antilles Current. With HR forcing, the transport through the Straits of Florida is 23.2 Sv; this increases to 29.1 Sv when the wind stresses of IH are used. The annual variation of the simulated Florida Current is very similar to previous, coarse-resolution models when using the same wind-stress climatology (HR); the annual range (3.4 Sv) obtained with HR forcing is strongly enhanced (6.3 Sv) with IH forcing. The meridional heat transport at 26°N, zonally integrated across the basin, is in phase with the Florida Current; its annual range increases from 0.44 PW (HR) to 0.80 PW (IH). The annual signal east of the Bahamas is masked by strong transport fluctuations on a time scale of O(100 days), caused by an instability of the Antilles Current. By averaging over several model years, an annual cycle is extracted, which is in phase with the wind stress curl over the western part of the basin.
    Materialart: Article , PeerReviewed
    Format: text
    DOI: 10.1175/1520-0485(1991)021〈1271:STVITW〉2.0.CO;2
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