Fristedt Tim

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Fristedt
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  • Article
    Supplement to Cirene : air-sea interactions in the Seychelles-Chagos thermocline ridge region
    (American Meteorological Society, 2009-01) Vialard, Jérôme ; Duvel, J. P. ; McPhaden, Michael J. ; Bouruet-Aubertot, Pascale ; Ward, Brian ; Key, E. ; Bourras, Denis ; Weller, Robert A. ; Minnett, Peter ; Weill, A. ; Cassou, Christophe ; Eymard, L. ; Fristedt, Tim ; Basdevant, C. ; Dandonneau, Y. ; Duteil, O. ; Izumo, T. ; de Boyer Montegut, C. ; Masson, S. ; Marsac, F. ; Menkes, C. ; Kennan, S.
    The Vasco—Cirene field experiment, in January—February 2007, targeted the Seychelles—Chagos thermocline ridge (SCTR) region, with the main purpose of investigating Madden—Julian Oscillation (MJO)-related SST events. The Validation of the Aeroclipper System under Convective Occurrences (Vasco) experiment (Duvel et al. 2009) and Cirene cruise were designed to provide complementary views of air—sea interaction in the SCTR region. While meteorological balloons were deployed from the Seychelles as a part of Vasco, the Research Vessel (R/V) Suroît was cruising the SCTR region as a part of Cirene.
  • Article
    Kinematic structure and dynamics of the Denmark Strait Overflow from ship-based observations
    (American Meteorological Society, 2020-11-01) Lin, Peigen ; Pickart, Robert S. ; Jochumsen, Kerstin ; Moore, G. W. K. ; Valdimarsson, Héðinn ; Fristedt, Tim ; Pratt, Lawrence J.
    The dense outflow through Denmark Strait is the largest contributor to the lower limb of the Atlantic meridional overturning circulation, yet a description of the full velocity field across the strait remains incomplete. Here we analyze a set of 22 shipboard hydrographic–velocity sections occupied along the Látrabjarg transect at the Denmark Strait sill, obtained over the time period 1993–2018. The sections provide the first complete view of the kinematic components at the sill: the shelfbreak East Greenland Current (EGC), the combined flow of the separated EGC, and the North Icelandic Jet (NIJ), and the northward-flowing North Icelandic Irminger Current (NIIC). The total mean transport of overflow water is 3.54 ± 0.29 Sv (1 Sv ≡ 106 m3 s−1), comparable to previous estimates. The dense overflow is partitioned in terms of water mass constituents and flow components. The mean transports of the two types of overflow water—Atlantic-origin Overflow Water and Arctic-origin Overflow Water—are comparable in Denmark Strait, while the merged NIJ–separated EGC transports 55% more water than the shelfbreak EGC. A significant degree of water mass exchange takes place between the branches as they converge in Denmark Strait. There are two dominant time-varying configurations of the flow that are characterized as a cyclonic state and a noncyclonic state. These appear to be wind-driven. A potential vorticity analysis indicates that the flow through Denmark Strait is subject to symmetric instability. This occurs at the top of the overflow layer, implying that the mixing/entrainment process that modifies the overflow water begins at the sill.
  • Article
    Cirene : air-sea iInteractions in the Seychelles-Chagos thermocline ridge region
    (American Meteorological Society, 2009-01) Vialard, Jérôme ; Duvel, J. P. ; McPhaden, Michael J. ; Bouruet-Aubertot, Pascale ; Ward, Brian ; Key, E. ; Bourras, Denis ; Weller, Robert A. ; Minnett, Peter ; Weill, A. ; Cassou, Christophe ; Eymard, L. ; Fristedt, Tim ; Basdevant, C. ; Dandonneau, Y. ; Duteil, O. ; Izumo, T. ; de Boyer Montegut, C. ; Masson, S. ; Marsac, F. ; Menkes, C. ; Kennan, S.
    The Vasco—Cirene program ex-plores how strong air—sea inter-actions promoted by the shallow thermocline and high sea surface temperature in the Seychelles—Chagos thermocline ridge results in marked variability at synoptic, intraseasonal, and interannual time scales. The Cirene oceano-graphic cruise collected oceanic, atmospheric, and air—sea flux observations in this region in Jan-uary—February 2007. The contem-poraneous Vasco field experiment complemented these measure-ments with balloon deployments from the Seychelles. Cirene also contributed to the development of the Indian Ocean observing system via deployment of a moor-ing and 12 Argo profilers. Unusual conditions prevailed in the Indian Ocean during Janu-ary and February 2007, following the Indian Ocean dipole climate anomaly of late 2006. Cirene measurements show that the Seychelles—Chagos thermocline ridge had higher-than-usual heat content with subsurface anomalies up to 7°C. The ocean surface was warmer and fresher than average, and unusual eastward currents prevailed down to 800 m. These anomalous conditions had a major impact on tuna fishing in early 2007. Our dataset also sampled the genesis and maturation of Tropical Cyclone Dora, including high surface temperatures and a strong diurnal cycle before the cyclone, followed by a 1.5°C cool-ing over 10 days. Balloonborne instruments sampled the surface and boundary layer dynamics of Dora. We observed small-scale structures like dry-air layers in the atmosphere and diurnal warm layers in the near-surface ocean. The Cirene data will quantify the impact of these finescale features on the upper-ocean heat budget and atmospheric deep convection.