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H. M. van Aken

Abstract

The density stratification of interleaving frontal intrusions is investigated with a statistical method. For this purpose a descriptive model of temperature inversions is formulated in which the thickness of the inversion is proportional to the temperature increase in the inversion. With this model an unbiased estimate can be made of the density deficit or density excess in intrusions with respect to an undisturbed background density stratification. Analysis of temperature inversion data from the North Rockall Trough obtained during the JASIN experiment gave the following results:

  1. The thickness of the temperature inversions was proportional to the inverse of the background Brunt-Väisälä frequency N −1. This result disagrees with excisting theoretical models and laboratory measurements but agrees with earlier results indicating that the thickness of intrusions in the North Rockall Trough was proportional to N−1.
  2. The salty intrusions had a density deficit, whereas cold, fresh intrusions had a density excess. This result is supported by models in which the dynamics of the intrusions are governed by horizontal density gradients caused by the vertical mass flux at the salt-finger interface of the intrusions. Therefore the cross-frontal interleaving in the Rockall Trough arms to be driven by salt-finger convection.
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H. M. van Aken

Abstract

The variability of the lowest 400 dbar in the Deep Iceland Basin has been studied for the years 1988–1991. It appears that hydrographic parameters of the Iceland–Scotland Overflow Water in the lowest 100 dbar mainly vary on longer timescales (interannual). Centered around 140 dbar from the bottom, at a more or less constant density level, salinity, dissolved silica, oxygen, and potential vorticity show a maximum in variability. This is attributed to the isopycnal advection of lenses of water with distinct, coherent hydrographic properties. These lenses are assumed to form a mechanism for lateral isopycnal advection of deep water, enriched with either Lower Deep Water or Subpolar Mode Water. The vertical extent of the lenses is determined by a density window between Labrador Sea Water and Iceland–Scotland Overflow Water, centered around σ2.5 = 39.27 kg m−3. The lateral, isopycnal advection, followed by subsequent diapycnal mixing, is proposed as the mechanism to incorporate Lower Deep Water into the Deep Northern Boundary Current and to explain the reported downstream modification of water mass characteristics of Iceland–Scotland Overflow Water in the Deep Northern Boundary current.

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M. F. de Jong, S. S. Drijfhout, W. Hazeleger, H. M. van Aken, and C. A. Severijns

Abstract

The performance of coupled climate models (CCMs) in simulating the hydrographic structure and variability of the northwestern North Atlantic Ocean, in particular the Labrador and Irminger Seas, has been assessed. This area plays an important role in the meridional overturning circulation. Hydrographic properties of the preindustrial run of eight CCMs used in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) are compared with observations from the World Ocean Circulation Experiment Repeat section 7 (WOCE AR7). The mean and standard deviation of 20 yr of simulated data are compared in three layers, representing the surface waters, intermediate waters, and deep waters. Two models simulate an extremely cold, fresh surface layer with model biases down to −1.7 psu and −4.0°C, much larger than the observed ranges of variability. The intermediate and deep layers are generally too warm and saline, with biases up to 0.7 psu and 2.8°C. An analysis of the maximum mixed layer depth shows that the low surface salinity is related to a convective regime restricted to the upper 500 dbar. Thus, intermediate water formed by convection is partly replaced by warmer water from the south. Model biases seem to be caused by the coupling to the atmospheric component of the CCM. Model drift during long spinup periods allows the initially small biases in water mass characteristics to become significant. Biases that develop in the control run are carried over to the twentieth-century runs, which are initialized from the control runs.

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