Renfrew Ian A.

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Ian A.

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Now showing 1 - 13 of 13
  • Article
    Offshore transport of dense water from the East Greenland Shelf
    (American Meteorological Society, 2014-01) Harden, Benjamin E. ; Pickart, Robert S. ; Renfrew, Ian A.
    Data from a mooring deployed at the edge of the East Greenland shelf south of Denmark Strait from September 2007 to October 2008 are analyzed to investigate the processes by which dense water is transferred off the shelf. It is found that water denser than 27.7 kg m−3—as dense as water previously attributed to the adjacent East Greenland Spill Jet—resides near the bottom of the shelf for most of the year with no discernible seasonality. The mean velocity in the central part of the water column is directed along the isobaths, while the deep flow is bottom intensified and veers offshore. Two mechanisms for driving dense spilling events are investigated, one due to offshore forcing and the other associated with wind forcing. Denmark Strait cyclones propagating southward along the continental slope are shown to drive off-shelf flow at their leading edges and are responsible for much of the triggering of individual spilling events. Northerly barrier winds also force spilling. Local winds generate an Ekman downwelling cell. Nonlocal winds also excite spilling, which is hypothesized to be the result of southward-propagating coastally trapped waves, although definitive confirmation is still required. The combined effect of the eddies and barrier winds results in the strongest spilling events, while in the absence of winds a train of eddies causes enhanced spilling.
  • Article
    Multidecadal mobility of the North Atlantic Oscillation
    (American Meteorological Society, 2013-04-15) Moore, G. W. K. ; Renfrew, Ian A. ; Pickart, Robert S.
    The North Atlantic Oscillation (NAO) is one of the most important modes of variability in the global climate system and is characterized by a meridional dipole in the sea level pressure field, with centers of action near Iceland and the Azores. It has a profound influence on the weather, climate, ecosystems, and economies of Europe, Greenland, eastern North America, and North Africa. It has been proposed that around 1980, there was an eastward secular shift in the NAO’s northern center of action that impacted sea ice export through Fram Strait. Independently, it has also been suggested that the location of its southern center of action is tied to the phase of the NAO. Both of these attributes of the NAO have been linked to anthropogenic climate change. Here the authors use both the one-point correlation map technique as well as empirical orthogonal function (EOF) analysis to show that the meridional dipole that is often seen in the sea level pressure field over the North Atlantic is not purely the result of the NAO (as traditionally defined) but rather arises through an interplay among the NAO and two other leading modes of variability in the North Atlantic region: the East Atlantic (EA) and the Scandinavian (SCA) patterns. This interplay has resulted in multidecadal mobility in the two centers of action of the meridional dipole since the late nineteenth century. In particular, an eastward movement of the dipole has occurred during the 1930s to 1950s as well as more recently. This mobility is not seen in the leading EOF of the sea level pressure field in the region.
  • Article
    The annual salinity cycle of the Denmark Strait Overflow
    (American Geophysical Union, 2022-03-22) Opher, Jacob G. ; Brearley, J. Alexander ; Dye, Stephen R. ; Pickart, Robert S. ; Renfrew, Ian A. ; Harden, Benjamin E. ; Meredith, Michael P.
    The Denmark Strait Overflow (DSO) is an important source of dense water input to the deep limb of the Atlantic Meridional Overturning Circulation (AMOC). It is fed by separate currents from the north that advect dense water masses formed in the Nordic Seas and Arctic Ocean which then converge at Denmark Strait. Here we identify an annual salinity cycle of the DSO, characterized by freshening in winter and spring. The freshening is linked to freshening of the Shelfbreak East Greenland Current in the Blosseville Basin north of the Denmark Strait. We demonstrate that the East Greenland Current advects fresh pycnocline water above the recirculating Atlantic Water, which forms a low salinity lid for the overflow in Denmark Strait and in the Irminger Basin. This concept is supported by intensified freshening of the DSO in lighter density classes on the Greenland side of the overflow. The salinity of the DSO in the Irminger Basin is significantly correlated with northerly/northeasterly winds in the Blosseville Basin at a lag of 3–4 months, consistent with estimated transit times. This suggests that wind driven variability of DSO source water exerts an important influence on the salinity variability of the downstream DSO, and hence the composition of the deep limb of the AMOC.
  • Article
    The Greenland Flow Distortion Experiment
    (American Meteorological Society, 2008-09) Renfrew, Ian A. ; Petersen, Guðrún N. ; Outten, S. ; Sproson, D. ; Moore, G. W. K. ; Hay, C. ; Ohigashi, T. ; Zhang, S. ; Kristjansson, J. E. ; Fore, I. ; Olafsson, H. ; Gray, S. L. ; Irvine, E. A. ; Bovis, K. ; Brown, P. R. A. ; Swinbank, R. ; Haine, Thomas W. N. ; Lawrence, A. ; Pickart, Robert S. ; Shapiro, M. ; Woolley, A.
    Greenland has a major influence on the atmospheric circulation of the North Atlantic–western European region, dictating the location and strength of mesoscale weather systems around the coastal seas of Greenland and directly influencing synoptic-scale weather systems both locally and downstream over Europe. High winds associated with the local weather systems can induce large air–sea fluxes of heat, moisture, and momentum in a region that is critical to the overturning of the thermohaline circulation, and thus play a key role in controlling the coupled atmosphere–ocean climate system. The Greenland Flow Distortion Experiment (GFDex) is investigating the role of Greenland in defining the structure and predictability of both local and downstream weather systems through a program of aircraft-based observation and numerical modeling. The GFDex observational program is centered upon an aircraft-based field campaign in February and March 2007, at the dawn of the International Polar Year. Twelve missions were flown with the Facility for Airborne Atmospheric Measurements' BAe-146, based out of the Keflavik, Iceland. These included the first aircraft-based observations of a reverse tip jet event, the first aircraft-based observations of barrier winds off of southeast Greenland, two polar mesoscale cyclones, a dramatic case of lee cyclogenesis, and several targeted observation missions into areas where additional observations were predicted to improve forecasts. In this overview of GFDex the background, aims and objectives, and facilities and logistics are described. A summary of the campaign is provided, along with some of the highlights of the experiment.
  • Article
    Buoy observations from the windiest location in the world ocean, Cape Farewell, Greenland
    (American Geophysical Union, 2008-09-17) Moore, G. W. K. ; Pickart, Robert S. ; Renfrew, Ian A.
    Cape Farewell, Greenland's southernmost point, is a region of significant interest in the meteorological and oceanographic communities in that atmospheric flow distortion associated with the high topography of the region leads to a number of high wind speed jets. The resulting large air-sea fluxes of momentum and buoyancy have a dramatic impact on the region's weather and ocean circulation. Here the first in-situ observations of the surface meteorology in the region, collected from an instrumented buoy, are presented. The buoy wind speeds are compared to 10 m wind speeds from the QuikSCAT satellite and the North American Regional Reanalysis (NARR). We show that the QuikSCAT retrievals have a high wind speed bias that is absent from the NARR winds. The spatial characteristics of the high wind speed events are also presented.
  • Article
    Meteorological buoy observations from the central Iceland Sea
    (John Wiley & Sons, 2015-04-24) Harden, Benjamin E. ; Renfrew, Ian A. ; Petersen, Guðrún N.
    We present the first continuous in situ atmospheric observations from the central Iceland Sea collected from a meteorological buoy deployed for a 2 year period between 23 November 2007 and 21 August 2009. We use these observations to evaluate the ERA-Interim reanalysis product and demonstrate that it represented low-level meteorological fields and surface turbulent fluxes in this region very well. The buoy observations showed that moderate to strong winds were common from any direction, while wind speeds below 5 ms−1 were relatively rare. The observed low-level air temperature and surface heat fluxes were related to the wind direction with cold-air outbreaks most common from the northwest. Mean wintertime turbulent heat fluxes were modest (<60 Wm−2), but the range was substantial. High heat flux events, greater than 200 Wm−2, typically occurred every 1–2 weeks in the winter, with each event lasting on average 2.5 days with an average total turbulent heat flux of ∼200 Wm−2 out of the ocean. The most pronounced high heat flux events over the central Iceland Sea were associated with cold-air outbreaks from the north and west forced by a deep Lofoten Low over the Norwegian Sea.
  • Article
    Spatial distribution of air-sea heat fluxes over the sub-polar North Atlantic Ocean
    (American Geophysical Union, 2012-09-27) Moore, G. W. K. ; Renfrew, Ian A. ; Pickart, Robert S.
    On a variety of spatial and temporal scales, the energy transferred by air-sea heat and moisture fluxes plays an important role in both atmospheric and oceanic circulations. This is particularly true in the sub-polar North Atlantic Ocean, where these fluxes drive water-mass transformations that are an integral component of the Atlantic Meridional Overturning Circulation (AMOC). Here we use the ECMWF Interim Reanalysis to provide a high-resolution view of the spatial structure of the air-sea turbulent heat fluxes over the sub-polar North Atlantic Ocean. As has been previously recognized, the Labrador and Greenland Seas are areas where these fluxes are large during the winter months. Our particular focus is on the Iceland Sea region where, despite the fact that water-mass transformation occurs, the winter-time air-sea heat fluxes are smaller than anywhere else in the sub-polar domain. We attribute this minimum to a saddle point in the sea-level pressure field, that results in a reduction in mean surface wind speed, as well as colder sea surface temperatures associated with the regional ocean circulation. The magnitude of the heat fluxes in this region are modulated by the relative strength of the Icelandic and Lofoten Lows, and this leads to periods of ocean cooling and even ocean warming when, intriguingly, the sensible and latent heat fluxes are of opposite sign. This suggests that the air-sea forcing in this area has large-scale impacts for climate, and that even modest shifts in the atmospheric circulation could potentially impact the AMOC.
  • Preprint
    Open-ocean convection becoming less intense in the Greenland and Iceland Seas
    ( 2015-05-18) Moore, G. W. K. ; Våge, Kjetil ; Pickart, Robert S. ; Renfrew, Ian A.
    The air-sea transfer of heat and freshwater plays a critical role in the global climate system. This is particularly true for the Greenland and Iceland Seas, where these fluxes drive ocean convection that contributes to Denmark Strait Overflow Water, the densest component of the lower limb of the Atlantic Meridional Overturning Circulation (AMOC). Here we show that the wintertime retreat of sea ice in the region, combined with different rates of warming for the atmosphere and sea surface of the Greenland and Iceland Seas, has resulted in statistically significant reductions of approximately 20% in the magnitude of the winter air-sea heat fluxes since 1979. We also show that modes of climate variability other than the North Atlantic Oscillation (NAO) are required to fully characterize the regional air-sea interaction. Mixed-layer model simulations imply that further decreases in atmospheric forcing will exceed a threshold for the Greenland Sea whereby convection will become depth limited, reducing the ventilation of mid-depth waters in the Nordic Seas. In the Iceland Sea, further reductions have the potential to decrease the supply of the densest overflow waters to the AMOC.
  • Article
    The Iceland Greenland Seas Project
    (American Meteorological Society, 2019-09-27) Renfrew, Ian A. ; Pickart, Robert S. ; Vage, Kjetil ; Moore, G. W. K. ; Bracegirde, Thomas J. ; Elvidge, Andrew D. ; Jeansson, Emil ; Lachlan-Cope, Thomas ; McRaven, Leah T. ; Papritz, Lukas ; Reuder, Joachim ; Sodemann, Harald ; Terpstra, Annick ; Waterman, Stephanie N. ; Valdimarsson, Héðinn ; Weiss, Albert ; Almansi, Mattia ; Bahr, Frank B. ; Brakstad, Ailin ; Barrell, Christopher ; Brooke, Jennifer K. ; Brooks, Barbara J. ; Brooks, Ian M. ; Brooks, Malcolm E. ; Bruvik, Erik Magnus ; Duscha, Christiane ; Fer, Ilker ; Golid, H. M. ; Hallerstig, M. ; Hessevik, Idar ; Huang, Jie ; Houghton, Leah A. ; Jonsson, Steingrimur ; Jonassen, Marius ; Jackson, K. ; Kvalsund, K. ; Kolstad, Erik W. ; Konstali, K. ; Kristiansen, Jorn ; Ladkin, Russell ; Lin, Peigen ; Macrander, Andreas ; Mitchell, Alexandra ; Olafsson, H. ; Pacini, Astrid ; Payne, Chris ; Palmason, Bolli ; Perez-Hernandez, M. Dolores ; Peterson, Algot K. ; Petersen, Guðrún N. ; Pisareva, Maria N. ; Pope, James O. ; Seidl, Andrew D. ; Semper, Stefanie ; Sergeev, Denis ; Skjelsvik, Silje ; Søiland, Henrik ; Smith, D. ; Spall, Michael A. ; Spengler, Thomas ; Touzeau, Alexandra ; Tupper, George H. ; Weng, Y. ; Williams, Keith D. ; Yang, Xiaohau ; Zhou, Shenjie
    The Iceland Greenland Seas Project (IGP) is a coordinated atmosphere–ocean research program investigating climate processes in the source region of the densest waters of the Atlantic meridional overturning circulation. During February and March 2018, a field campaign was executed over the Iceland and southern Greenland Seas that utilized a range of observing platforms to investigate critical processes in the region, including a research vessel, a research aircraft, moorings, sea gliders, floats, and a meteorological buoy. A remarkable feature of the field campaign was the highly coordinated deployment of the observing platforms, whereby the research vessel and aircraft tracks were planned in concert to allow simultaneous sampling of the atmosphere, the ocean, and their interactions. This joint planning was supported by tailor-made convection-permitting weather forecasts and novel diagnostics from an ensemble prediction system. The scientific aims of the IGP are to characterize the atmospheric forcing and the ocean response of coupled processes; in particular, cold-air outbreaks in the vicinity of the marginal ice zone and their triggering of oceanic heat loss, and the role of freshwater in the generation of dense water masses. The campaign observed the life cycle of a long-lasting cold-air outbreak over the Iceland Sea and the development of a cold-air outbreak over the Greenland Sea. Repeated profiling revealed the immediate impact on the ocean, while a comprehensive hydrographic survey provided a rare picture of these subpolar seas in winter. A joint atmosphere–ocean approach is also being used in the analysis phase, with coupled observational analysis and coordinated numerical modeling activities underway.
  • Article
    What causes the location of the air-sea turbulent heat flux maximum over the Labrador Sea?
    (John Wiley & Sons, 2014-05-19) Moore, G. W. K. ; Pickart, Robert S. ; Renfrew, Ian A. ; Våge, Kjetil
    The Labrador Sea is a region of climatic importance as a result of the occurrence of oceanic wintertime convection, a process that is integral to the Atlantic Meridional Overturning Circulation. This process requires large air-sea heat fluxes that result in a loss of surface buoyancy, triggering convective overturning of the water column. The Labrador Sea wintertime turbulent heat flux maximum is situated downstream of the ice edge, a location previously thought to be causal. Here we show that there is considerable similarity in the characteristics of the regional mean atmospheric circulation and high heat flux events over the Labrador Sea during early winter, when the ice is situated to the north, and midwinter, when it is near the region of maximum heat loss. This suggests that other factors, including the topography of the nearby upstream and downstream landmasses, contribute to the location of the heat flux maximum.
  • Article
    Coupled atmosphere–ocean observations of a cold‐air outbreak and its impact on the Iceland Sea
    (Royal Meteorological Society, 2022-12-24) Renfrew, Ian A. ; Huang, Jie ; Semper, Stefanie ; Barrell, Christopher ; Terpstra, Annick ; Pickart, Robert S. ; Våge, Kjetil ; Elvidge, Andrew D. ; Spengler, Thomas ; Strehl, Anna‐Marie ; Weiss, Alexandra
    Marine cold‐air outbreaks (CAOs) are vigorous equatorward excursions of cold air over the ocean, responsible for the majority of wintertime oceanic heat loss from the subpolar seas of the North Atlantic. However, the impact of individual CAO events on the ocean is poorly understood. Here we present the first coupled observations of the atmosphere and ocean during a wintertime CAO event, between 28 February and 13 March 2018, in the subpolar North Atlantic region. Comprehensive observations are presented from five aircraft flights, a research vessel, a meteorological buoy, a subsurface mooring, an ocean glider, and an Argo float. The CAO event starts abruptly with substantial changes in temperature, humidity and wind throughout the atmospheric boundary layer. The CAO is well mixed vertically and, away from the sea‐ice edge, relatively homogeneous spatially. During the CAO peak, higher sensible heat fluxes occupy at least the lowest 200 m of the atmospheric boundary layer, while higher latent heat fluxes are confined to the surface layer. The response of the ocean to the CAO is spatially dependent. In the interior of the Iceland Sea the mixed layer cools, while in the boundary current region it warms. In both locations, the mixed layer deepens and becomes more saline. Combining our observations with one‐dimensional mixed‐layer modelling, we show that in the interior of the Iceland Sea, atmospheric forcing dominates the ocean response. In contrast, in the boundary current region lateral advection and mixing counteract the short‐term impact of the atmospheric forcing. Time series observations of the late‐winter period illustrate a highly variable ocean mixed layer, with lateral advection and mixing often masking the ocean's general cooling and deepening response to individual CAO events.Simultaneous observations of the atmosphere and ocean during a cold‐air outbreak over the Iceland Sea. The top panel shows near‐surface potential temperature from two research flights and a research vessel at the onset of the event; the distribution is relatively homogeneous, although affected by the sea‐ice off the East Greenland coast. The bottom panel shows observed ocean mixed‐layer depth during the first part of the event; the response of the ocean is spatially dependent due to counteracting vertical and lateral processes.
  • Article
    An evaluation of surface meteorology and fluxes over the Iceland and Greenland Seas in ERA5 reanalysis: the impact of sea ice distribution
    (Royal Meteorological Society, 2020-10-29) Renfrew, Ian A. ; Barrell, Christopher ; Elvidge, Andrew D. ; Brooke, Jennifer K. ; Duscha, Christiane ; King, John C. ; Kristiansen, Jorn ; Lachlan-Cope, Thomas ; Moore, G. W. K. ; Pickart, Robert S. ; Reuder, Joachim ; Sandu, Irina ; Sergeev, Denis ; Terpstra, Annick ; Våge, Kjetil ; Weiss, Alexandra
    The Iceland and Greenland Seas are a crucial region for the climate system, being the headwaters of the lower limb of the Atlantic Meridional Overturning Circulation. Investigating the atmosphere–ocean–ice processes in this region often necessitates the use of meteorological reanalyses—a representation of the atmospheric state based on the assimilation of observations into a numerical weather prediction system. Knowing the quality of reanalysis products is vital for their proper use. Here we evaluate the surface‐layer meteorology and surface turbulent fluxes in winter and spring for the latest reanalysis from the European Centre for Medium‐Range Weather Forecasts, i.e., ERA5. In situ observations from a meteorological buoy, a research vessel, and a research aircraft during the Iceland–Greenland Seas Project provide unparalleled coverage of this climatically important region. The observations are independent of ERA5. They allow a comprehensive evaluation of the surface meteorology and fluxes of these subpolar seas and, for the first time, a specific focus on the marginal ice zone. Over the ice‐free ocean, ERA5 generally compares well to the observations of surface‐layer meteorology and turbulent fluxes. However, over the marginal ice zone, the correspondence is noticeably less accurate: for example, the root‐mean‐square errors are significantly higher for surface temperature, wind speed, and surface sensible heat flux. The primary reason for the difference in reanalysis quality is an overly smooth sea‐ice distribution in the surface boundary conditions used in ERA5. Particularly over the marginal ice zone, unrepresented variability and uncertainties in how to parameterize surface exchange compromise the quality of the reanalyses. A parallel evaluation of higher‐resolution forecast fields from the Met Office's Unified Model corroborates these findings.
  • Article
    The impact of resolution on the representation of southeast Greenland barrier winds and katabatic flows
    (John Wiley & Sons, 2015-04-19) Moore, G. W. K. ; Renfrew, Ian A. ; Harden, Benjamin E. ; Mernild, Sebastian H.
    Southern Greenland is characterized by a number of low-level high wind speed weather systems that are the result of topographic flow distortion. These systems include barrier winds and katabatic flow that occur along its southeast coast. Global atmospheric reanalyses have proven to be important tools in furthering our understanding of these orographic winds and their role in the climate system. However, there is evidence that the mesoscale characteristics of these systems may be missed in these global products. Here we show that the Arctic System Reanalysis, a higher-resolution regional reanalysis, is able to capture mesoscale features of barrier winds and katabatic flow that are missed or underrepresented in ERA-I, a leading modern global reanalysis. This suggests that our understanding of the impact of these wind systems on the coupled-climate system can be enhanced through the use of higher-resolution regional reanalyses or model data.