Trick or Treat? Both if you are a geek!

Three gliders patrol the inshore. They are flying in formation, like a graceful,,,,, but slowwww,,, pod of dolphins despite strong currents. We have three gliders in the nearshore formation (RU05, RU21, RU23). The decision in the beginning of the week will be on how to split the formation. The goal will be to provide optimal coverage of the shelf.

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The major goal of the experiment is to test the planning and prosecution software of the OOI; however we have chosen this time as any data in the fall will be scientifically valuable for understanding the dynamics of the Mid-Atlantic Bight. The waters on the MAB exhibit considerable seasonal and inter-annual variability in temperature and salinity. In late spring and early summer, a strong thermocline develops at about the 20 m depth across the entire shelf, isolating a continuous mid-shelf “cold pool” (formed in winter months) that extends from Nantucket to Cape Hatteras. This stratification forms one of the most extreme coastal thermoclines on Earth with temperatures ranging from 30º to 8ºC with the majority temperature change occurring within five meters. The cold pool persists throughout the summer until fall when the water column overturns and mixes in the fall, which provides a source of nutrients to the MAB shelf. Thermal stratification re-develops in spring as the frequency of winter storms decrease and surface heat flux increases. While it has long been appreciated that seasonal phytoplankton blooms are important in shelf and slope waters of the MAB many studies have found that the maximum chlorophyll concentration appeared during fall-winter in middle and outer shelf waters and that slope waters possess a secondary spring peak in addition to the a fall-winter bloom. The fall winter bloom is the largest and most recurrent event on the shelf. Despite the importance of this biological event for the MAB ecosystem, we have very little data. This experiment will collect data at a critical time!

So what is the status of the MAB? The three nearshore gliders show a well mixed with no real stratification of the shallow waters. Despite this lack of stratification there is enhanced chlorophyll and optical backscatter in the bottom waters. The depth average currents are strong and flowing to the north. There is good agreement between all three gliders.

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In contrast, RU15 is offshore and the conditions show some differences from the nearshore glider fleet. The offshore glider shows southward flowing currents. The offshore shelf waters are still stratified. The salinity shows the inner shelf contains more saline waters. The optical backscatter and chlorophyll are enhanced in the surface waters above the thermocline. The Colored Dissolved Organic Matter fluorescence show distinct differences above and below the thermocline. There are also strong nearshore and offshore gradients.

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ru15_091028t0706_091030t1556_muri_chla_xsec1

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Current thinking is that the winter bloom proper should begin once the thermocline is eroded and nutrients are replenished in the surface waters. The second factor that is key is that winds should die for period of time allow the water column to stabilize. This allows the cells to overcome chronic light limitation in fully mixed water column. Therefore with a little luck we can watch in the comes days, erosion of shelf stratification, water column stabilization and the beginning of the MAB winter bloom.

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