Ryan Svenja

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  • Article
    Late 20th century Indian Ocean heat content gain masked by wind forcing
    (American Geophysical Union, 2020-10-26) Ummenhofer, Caroline C. ; Ryan, Svenja ; England, Matthew H. ; Scheinert, Markus ; Wagner, Patrick ; Biastoch, Arne ; Böning, Claus W.
    Rapid increases in upper 700‐m Indian Ocean heat content (IOHC) since the 2000s have focused attention on its role during the recent global surface warming hiatus. Here, we use ocean model simulations to assess distinct multidecadal IOHC variations since the 1960s and explore the relative contributions from wind stress and buoyancy forcing regionally and with depth. Multidecadal wind forcing counteracted IOHC increases due to buoyancy forcing from the 1960s to the 1990s. Wind and buoyancy forcing contribute positively since the mid‐2000s, accounting for the drastic IOHC change. Distinct timing and structure of upper ocean temperature changes in the eastern and western Indian Ocean are linked to the pathway how multidecadal wind forcing associated with the Interdecadal Pacific Oscillation is transmitted and affects IOHC through local and remote winds. Progressive shoaling of the equatorial thermocline—of importance for low‐frequency variations in Indian Ocean Dipole occurrence—appears to be dominated by multidecadal variations in wind forcing.
  • Article
    Marine Heatwaves and their depth structures on the Northeast U.S. continental shelf
    (Frontiers Media, 2022-06-15) Großelindemann, Hendrik ; Ryan, Svenja ; Ummenhofer, Caroline C. ; Martin, Torge ; Biastoch, Arne
    Marine Heatwaves (MHWs) are ocean extreme events, characterized by anomalously high temperatures, which can have significant ecological impacts. The Northeast U.S. continental shelf is of great economical importance as it is home to a highly productive ecosystem. Local warming rates exceed the global average and the region experienced multiple MHWs in the last decade with severe consequences for regional fisheries. Due to the lack of subsurface observations, the depth-extent of MHWs is not well-known, which hampers the assessment of impacts on pelagic and benthic ecosystems. This study utilizes a global ocean circulation model with a high-resolution (1/20°) nest in the Atlantic to investigate the depth structure of MHWs and associated drivers on the Northeast U.S. continental shelf. It is shown that MHWs exhibit varying spatial extents, with some only occurring at depth. The highest intensities are found around 100 m depth with temperatures exceeding the climatological mean by up to 7°C, while surface intensities are typically smaller (around 3°C). Distinct vertical structures are associated with different spatial MHW patterns and drivers. Investigation of the co-variability of temperature and salinity reveals that over 80% of MHWs at depth (>50 m) coincide with extreme salinity anomalies. Two case studies provide insight into opposing MHW patterns at the surface and at depth, being forced by anomalous air-sea heat fluxes and Gulf Stream warm core ring interaction, respectively. The results highlight the importance of local ocean dynamics and the need to realistically represent them in climate models.
  • Article
    Depth structure of Ningaloo Niño/Niña events and associated drivers
    (American Meteorological Society, 2021-02-04) Ryan, Svenja ; Ummenhofer, Caroline C. ; Gawarkiewicz, Glen G. ; Wagner, Patrick ; Scheinert, Markus ; Biastoch, Arne ; Böning, Claus W.
    Marine heatwaves along the coast of Western Australia, referred to as Ningaloo Niño, have had dramatic impacts on the ecosystem in the recent decade. A number of local and remote forcing mechanisms have been put forward; however, little is known about the depth structure of such temperature extremes. Utilizing an eddy-active global ocean general circulation model, Ningaloo Niño and the corresponding cold Ningaloo Niña events are investigated between 1958 and 2016, with a focus on their depth structure. The relative roles of buoyancy and wind forcing are inferred from sensitivity experiments. Composites reveal a strong symmetry between cold and warm events in their vertical structure and associated large-scale spatial patterns. Temperature anomalies are largest at the surface, where buoyancy forcing is dominant, and extend down to 300-m depth (or deeper), with wind forcing being the main driver. Large-scale subsurface anomalies arise from a vertical modulation of the thermocline, extending from the western Pacific into the tropical eastern Indian Ocean. The strongest Ningaloo Niños in 2000 and 2011 are unprecedented compound events, where long-lasting high temperatures are accompanied by extreme freshening, which emerges in association with La Niñas, that is more common and persistent during the negative phase of the interdecadal Pacific oscillation. It is shown that Ningaloo Niños during La Niña phases have a distinctively deeper reach and are associated with a strengthening of the Leeuwin Current, while events during El Niño are limited to the surface layer temperatures, likely driven by local atmosphere–ocean feedbacks, without a clear imprint on salinity and velocity.
  • Article
    FRIS revisited in 2018: on the circulation and water masses at the Filchner and Ronne Ice Shelves in the Southern Weddell Sea
    (American Geophysical Union, 2021-05-18) Janout, Markus A. ; Hellmer, Hartmut H. ; Hattermann, Tore ; Huhn, Oliver ; Sultenfuß, Jurgen ; Østerhus, Svein ; Stulic, Lukrecia ; Ryan, Svenja ; Schröder, Michael ; Kanzow, Torsten
    The Filchner-Ronne Ice Shelf (FRIS) is characterized by moderate basal melt rates due to the near-freezing waters that dominate the wide southern Weddell Sea continental shelf. We revisited the region in austral summer 2018 with detailed hydrographic and noble gas surveys along FRIS. The FRIS front was characterized by High Salinity Shelf Water (HSSW) in Ronne Depression, Ice Shelf Water (ISW) on its eastern flank, and an inflow of modified Warm Deep Water (mWDW) entering through Central Trough. Filchner Trough was dominated by Ronne HSSW-sourced ISW, likely forced by a recently intensified circulation beneath FRIS due to enhanced sea ice production in the Ronne polynya since 2015. Glacial meltwater fractions and tracer-based water mass dating indicate two separate ISW outflow cores, one hugging the Berkner slope after a two-year travel time, and the other located in the central Filchner Trough following a ∼six year-long transit through the FRIS cavity. Historical measurements indicate the presence of two distinct modes, in which water masses in Filchner Trough were dominated by either Ronne HSSW-derived ISW (Ronne-mode) or more locally derived Berkner-HSSW (Berkner-mode). While the dominance of these modes has alternated on interannual time scales, ocean densities in Filchner Trough have remained remarkably stable since the first surveys in 1980. Indeed, geostrophic velocities indicated outflowing ISW-cores along the trough's western flank and onto Berkner Bank, which suggests that Ronne-ISW preconditions Berkner-HSSW production. The negligible density difference between Berkner- and Ronne-mode waters indicates that each contributes cold dense shelf waters to protect FRIS against inflowing mWDW.
  • Article
    Exceptionally warm and prolonged flow of warm deep water toward the Filchner-Ronne Ice Shelf in 2017
    (Wiley, 2020-06-09) Ryan, Svenja ; Hellmer, Hartmut H. ; Janout, Markus A. ; Darelius, Elin ; Vignes, Lucie ; Schröder, Michael
    The Filchner‐Ronne Ice Shelf, fringing the southern Weddell Sea, is Antarctica's second largest ice shelf. At present, basal melt rates are low due to active dense water formation; however, model projections suggest a drastic increase in the future due to enhanced inflow of open‐ocean warm water. Mooring observations from 2014 to 2016 along the eastern flank of the Filchner Trough (76°S) revealed a distinct seasonal cycle with inflow if Warm Deep Water during summer and autumn. Here we present extended time series showing an exceptionally warm and long inflow in 2017, with maximum temperatures exceeding 0.5°C. Warm temperatures persisted throughout winter, associated with a fresh anomaly, which lead to a change in stratification over the shelf, favoring an earlier inflow in the following summer. We suggest that the fresh anomaly developed upstream after anomalous summer sea ice melting and contributed to a shoaling of the shelf break thermocline.
  • Article
    Understanding physical drivers of the 2015/16 marine heatwaves in the Northwest Atlantic
    (Nature Research, 2021-09-02) Perez, Elena ; Ryan, Svenja ; Andres, Magdalena ; Gawarkiewicz, Glen G. ; Ummenhofer, Caroline C.
    The Northwest Atlantic, which has exhibited evidence of accelerated warming compared to the global ocean, also experienced several notable marine heatwaves (MHWs) over the last decade. We analyze spatiotemporal patterns of surface and subsurface temperature structure across the Northwest Atlantic continental shelf and slope to assess the influences of atmospheric and oceanic processes on ocean temperatures. Here we focus on MHWs from 2015/16 and examine their physical drivers using observational and reanalysis products. We find that a combination of jet stream latitudinal position and ocean advection, mainly due to warm core rings shed by the Gulf Stream, plays a role in MHW development. While both atmospheric and oceanic drivers can lead to MHWs they have different temperature signatures with each affecting the vertical structure differently and horizontal spatial patterns of a MHW. Northwest Atlantic MHWs have significant socio-economic impacts and affect commercially important species such as squid and lobster.
  • Article
    Autonomous tracking of salinity-intrusion fronts by a long-range autonomous underwater vehicle
    (Institute of Electrical and Electronics Engineers, 2022-04-18) Zhang, Yanwu ; Yoder, Noa ; Kieft, Brian ; Kukulya, Amy L. ; Hobson, Brett W. ; Ryan, Svenja ; Gawarkiewicz, Glen G.
    Shoreward intrusions of anomalously salty water along the continental shelf of the Middle Atlantic Bight are often observed in spring and summer. Exchange of heat, nutrients, and carbon across the salinity-intrusion front has a significant impact on the marine ecosystem and fisheries. In this article, we developed a method of using an autonomous underwater vehicle (AUV) to detect a salinity-intrusion front and track the front's movement. Autonomous front detection is based on the different vertical structures of salinity in the two distinct water types: the vertical difference of salinity is large in the intruding saltier water because of the salinity “tongue” at mid-depth, but is small in the nearshore fresher water due to absence of the salinity anomaly. Every time the AUV crosses and detects the front, the vehicle makes a turn at an oblique angle to cross the front, thus zigzagging through the front to map the frontal zone. The AUV's zigzags sweep back and forth to track the front as it moves over time. From June 25 to 30, 2021, a Tethys-class long-range AUV mapped and tracked a salinity-intrusion front on the southern New England shelf. The frontal tracking revealed the salinity intrusion's 3-D structure and temporal evolution with unprecedented detail.
  • Article
    Editorial: Advances in marine heatwave interactions
    (Frontiers Media, 2023-03-30) Sen Gupta, Alex ; Ryan, Svenja ; Hernaman, Vanessa
    With growing appreciation of the predominant role of climate extremes as the most impactful manifestation of anthropogenic warming, marine heatwave (MHW) research has exploded. Changes to the physical environment driven by MHWs can have devastating effects on ecosystems and reliant industries, with losses of tens of millions of US$ associated with many individual MHW events (Smith et al., 2021). This special edition explores new directions in MHW research around forecasting events across different timescales, understanding drivers, delving below the surface, examining the interaction between different types of extremes, and understanding the ecosystem and fisheries implications and how to manage these disruptive events.