The Seascapes

The Seascapes

Friday, July 2, 2010

Sampling off Long Island on the last day of our June cruise

A fine looking fishing vessel off the Rockaways, Long Island

Yesterday we completed the last day of our 4 day June sampling cruise. Earlier I remarked about how lucky we were to have winds shift from the south to north after we sampled each of the seascapes once. The plot on the left shows the northward and eastward components of the wind over the last few days at the “Ambrose” buoy off the mouth of the Hudson-Raritan Estuary. Our sampling periods are indicated by the hatched bars. Winds blew toward the east throughout the 4 cruises but shifted from the south to the north midway through the week. (The plots indicate the direction the wind is pushing water towards rather than the direction the wind is blowing from.  This is a convention of the physicists and all fish biologists suffer physics envy.  But why is ocean physics really interesting?  Because it matters to the fish.)

The effects of the wind shift on the oceans surface is evident in the 24 average surface currents measured with HF radar at 1300 each of the days we sampled (You can animate hourly 24 hour averaged surface currents at the RU COOL HF radar site). In these images you can see the wind drive strong offshore flow particularly along the southern flank of the Hudson Shelf valley early in the week. This flow weakened dramatically as the winds shifted southward. As a result of this change in winds the warm water we sampled relatively close to the coast of Long Island on Tuesday had moved well offshore into the mid-Atlantic bight on Thursday (7/30/10).   

The edge of the warm water is indicated by arrow in this 1030 GMT  satellite image of sea surface temperature.  On clear mornings we can count on getting one good early morning satellite image.  Once the land heats up and clouds form the images are not so useful.

This google earth map shows the NYHOPs model prediction of temperature and surface current flows for 1400 GMT which was about the time we began to sample. The beginning and ends of our transect as well the site of the front we selected from the NYHOPs model (mid_07012010) and the site of the  front from the satellite image above are marked. We towed our plankton net at 6 stations instead of usual 5 along this transect so we could sample locations of the front indicated by both the model and the satellite image.   

The acoustic image of particle backscatter along our transect to the left is from our acoustic doppler current profiler.  The rectangles show the 6 locations where we towed the plankton nets.  TT_3_1506 is the site of front in the NYHOPs model and TT_4_1558 the site of the front in the satellite image above with the arrow.  The 4 digit numbers at the end of the station designations are the Greenwich mean times (GMT) of the sampling. We use GMT time so that it is easy to match our data with data collected by satellites and other sensors available through the ocean observing system.

The image below shows in much more detail the scattering layers we towed our net through at station 2 (TT_2_1430).  We fished one net from the surface to a depth of 9 meters, and a second net from 9-23 meters.  We selected these depths based on this image and the CTD cast (not shown. But see earlier post).  There seems to be a lot going on down there.  In general the acoustic complexity of the water column on the transect to Long Island on thursday was greater than on the other 3 days of the week.  This CTD data also seemed show this but we haven't processed the data yet. We have a lot more data to processes including the plankton samples which we now need to sort.


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