P09 The North Atlantic and Climate Change

IAPSO (Physical Oceanography)

28-Jun-2015, 08:30 - 10:00

Abstract content:

Seasonal cycle and annual destruction of Eighteen Degree Water

Eighteen Degree Water (EDW), the subtropical mode water of the western North Atlantic, is a voluminous, weakly-stratified upper ocean water mass. This thick layer acts as a subsurface reservoir of heat, nutrients, and CO2 as it persists throughout the year, but nearly half of its volume is dispersed or mixed away from the time of its formation until it is renewed the following winter, diffusing its properties into the thermocline. The relevant processes responsible for this large annual cycle of EDW destruction are being investigated as part of CLIMODE. CTD observations from Argo and CLIMODE profiling floats are used to observe the cycle of the seasonal pycnocline, which isolates EDW from the surface mixed layer during spring, summer, and fall, and to quantify EDW destruction rates. The highest EDW destruction rates occur during summer when the stratification of the seasonal pycnocline is increasing, while slower, steady EDW destruction is observed in early winter, as stratification weakens. Profiling float observations suggest that EDW is mixed away via three-dimensional processes near the Gulf Stream in the northern part of the subtropical gyre, whereas EDW destruction via 1-D vertical diffusion likely prevails in the southern part of the gyre. Lateral dispersion of EDW is assessed using CLIMODE temperature and velocity data from 40 acoustically-tracked, bobbing profiling floats. The relative contributions of vertical vs. lateral mixing are compared directly within the framework of the EDW potential vorticity budget.

S. Billheimer1, L. Talley2.
1University of California- San Diego, Scripps Institution of Oceanography, San Diego, USA.
2University of California- San Diego, Scripps Intstituion of Oceanography, San Diego, USA.


subtropical mode water     Eighteen Degree Water     Gulf Stream Extension