The Seascapes

The Seascapes

Sunday, June 27, 2010

Ocean forecast for tomorrows sampling

Part of the reason for writing this is to try to understand the dynamics of the ocean before we get on the water to sample the biology living in it.  We won't know until tomorrow morning when somebody flips the coin (literally) whether we will be sampling off New Jersey or New York.  But by looking over recent ocean data, as well as some ocean forecasts we can develop strategies we might implement tomorrow.  Based on the SUNY MM-5 atmospheric weather model we are supposed to have weak south/southwesterly winds over the next 12 hours.  These winds are favorable to upwelling along the coast of New Jersey

                                                         The plot on the left shows the SUNY wind forecast for 8AM tomorrow morning.

Below is the ROMs ocean current forecast for tomorrow in the Mid Atlantic Bight .  It is not clearly stated on the webpage page whether the google earth kmz files are depth averaged currents or surface currents (or I can't find the statement). I am assuming they are surface currents.  The plot shows northward flows of water in the near shore along the coast of New Jersey and weak northeastward flows along Long Island.  Offshore are some really interesting eddies that could affect larval transport and dispersal but we won't be able to reach them on our boat.  The circulation pattern is complex.  It is much more complex than the mean southwestward flow of water through the mid-Atlantic Bight we all talk about.  As my oceanographer friend, Josh Kohut, says "the mean never happens".  From the point of view of a larval fish, whose larval life is ~30 days give or take a week or so, what matters is the "weather" in the ocean that  happens at time scales equal too and shorter than about 30 days.

                                                                                                                                                          Stevens Institute is also running a finer scale ocean model for the area that produces hourly forecasts of  currents, surface temperatures and salinities.  This "NYHOPS" model output (below) also shows northward surface flows of water along the coast near our New Jersey seascape and I believe weak flow toward the mouth of the estuary off New York.  (The current vectors are hard to see).  This is consistent with the stage of the tide at 8AM which will be nearly the time of slack before ebb (9:55AM).  As discussed earlier, we will be probably begin sampling around 9 with the water just beginning to ebb out of the estuary .

                                                              The last three plots show the NYHOPs forecasts of current speed and direction, surface salinity and surface temperature for ~8AM tomorrow.  They indicate that the warmer fresher water of the  Hudson-Raritan River estuarine plume will be offshore in the New Jersey seascape.  We would expect the estuarine water to be blown offshore by the southwesterly winds we have been seeing over the past week and forecast for tomorrow by the SUNY MM-5 model (see Bob Chants Bulging plume paper referenced June 23).  So in the New Jersey Seascape we should be sampling in upwelling conditions with cold salty water inshore and fresher warmer water on the surface offshore.  Based on the model we might expect to see strong front running through a point located at ~ 40°23'5.71"N,  73°56'10.92"W .  If the coin flip puts us in NJ we will try to target this front and put it in the middle of our 10 to 20 kilometer long sampling transect.  On the New York side the NYHOPs model shows a salt front at  40°31'35.45"N  73°45'34.51"W and a strong temperature front at about the same location. But the cold (but fresher) water also forms a bulge closer to the mouth of the estuary.  I don't understand this feature and need some help from a physical oceanographer.  Our usual strategy is to make the transects slice ocean fronts in the dimension perpendicular to the coast.  This is clearly the way to go off New Jersey, but I am less sure whether this is the best way to sample off Long Island where the ocean dynamics seem much more complex.  Early tomorrow we will download another forecast, flip a coin to choose the New York or New Jersey seascape, and then do a rapid survey with surface temperature and salinity sensors and acoustic instruments to find the position of the ocean front forecast by the models. Then we will lay our our transect and begin to sample fish larvae with our tucker trawl on the front and in the water masses on either side of it.  The fronts can change position by a few kilometers as the tides and winds change during our 5-6 hours of plankton sampling so we are constantly chasing a moving target.  But this is lagrangian sampling of larval fish that live in a lagrangian world.

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