James D. Irish, and William J. WilliamsAs Edouard moved up the US Coast, on 2 September 1996 it passed about 10 nm to the North of the GLOBEC Long-Term Moored Program's Southern Flank mooring. The mooring remained in position and recorded good data until scheduled recovery four days later. This data set gives us a unique record of the response of Georges Bank to a hurricane.
The wind record from the Southern Flank Buoy at 3 m elevation showed a 22 m/s peak velocity on 2 September 1996 around 1400 UTC. The high winds occurred over a 72 hour period starting on 1 September 1996 and ending on 4 September 1996. The waves from NDBC buoy 44011 on Georges Bank peaked at a 9.7 meter significant wave height with a 9 second period.
The downward looking 300 kHz Workhorse ADCP showed the water velocity response to the hurricane. The response was depth dependent, with the strongest currents in upper 20 meters. A hodograph of the surface water velocity shows the non-tidal residual currents with the down-shelf mean of about 20 cm/sec at the start and end of the record. When the hurricane effects were starting to be felt, this down-shelf circulation was shut off. The currents then ran up onto the bank and back to zero, then up- shelf to the East and then a strong off-bank flow (about 1 m/s) for several hours. The flow then resumed the normal tidal with down-shelf flow. The duration of the velocity perturbation extended for about 4 tidal cycles. The top 20 meters moved the fastest and there is indication of a reversal in flow near the bottom which is confirmed by the water property time series.
The temperature and salinity response to the hurricane was largely due to advection in the lower waters, and mixing in the upper waters. As the wind picked up, the water in the upper 30 to 35 meters of the water column became well mixed. When the wind dropped, there was some restratification in the surface waters. The deeper waters showed greater fluctuations during and after the hurricane (due to the advection of a stronger horizontal gradient by the tides), with colder and saltier water brought in by the bottom counter flow. The stratification (sigma-theta difference from the top to the bottom of the water column) was only slightly reduced by the hurricane. The stratification in the upper half of the water column was reduced by wind mixing, but was compensated by the advection of more dense water at the bottom which kept the overall stratification about the same.