HYDROGRAPHIC VARIABILITY IN THE BOTTOM LAYER OF THE ICELAND BASIN

The variability of the lowest 400 dbar in the Deep Iceland Basin has b een studied for the years 1988-1991. It appears that hydrographic para meters of the Iceland-Scotland Overflow Water in the lowest 100 dbar m ainly vary on longer timescales (interannual). Centered around 140 dba r from the bottom, at a more or less constant density level, salinity, dissolved silica, oxygen, and potential vorticity show a maximum in v ariability. This is attributed to the isopycnal advection of lenses of water with distinct, coherent hydrographic properties. These lenses a re 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 be tween Labrador Sea Water and Iceland-Scotland Overflow Water, centered around sigma(2.5) = 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 Curre nt and to explain the reported downstream modification of water mass c haracteristics of Iceland-Scotland Overflow Water in the Deep Northern Boundary Current.