Possible predictability in overflow from the Denmark Strait

The overflow and descent of cold dense water from the Denmark Strait sill-a submarine passage between Greenland and Iceland-is a principal means by wh ich the deep ocean is ventilated, and is an important element in the global thermohaline circulation. Previous investigations of its variability-in pa rticular, direct current measurements(1,2) in the overflow core since 1986- have shown surprisingly little evidence of long-term changes in now speed. Here we report significant changes in the overflow characteristics during t he winter of 1996-97, measured using two current-meter moorings and an inve rted echo sounder located at different depths in the fastest part of the no w. The overflow warmed to the highest monthly value yet recorded (2.4 degre es C), and showed a pronounced slowing and thinning at its lower margin. We believe that the extreme warmth of the overflow caused it to run higher on the continental slope off east Greenland, so that the lower current meters and the echo sounder were temporarily outside and deeper than the fast-flo wing core; model simulations appear to confirm this interpretation, We sugg est that the extreme warmth of the overflow is a lagged response to a warmi ng upstream in the Fram Strait three years earlier (caused by an exceptiona l amplification of the winter North Atlantic Oscillation). If this is so, o ver-now characteristics may be predictable.