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. Lateral dispersion of EDW is assessed using CLIMODE temperature and velocity data from 40 acoustically-tracked, bobbing profiling floats.

EDW destruction occurs primarily at the top of the EDW layer. 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. Within the framework of the EDW potential vorticity budget, EDW destruction is dominated by 1-D vertical diffusion, while along-isopycnal stirring comprises approximately 1/3 of the total EDW destruction. Dispersion via lateral processes is more prevalent near the Gulf Stream than in the southern part of the EDW region, due to higher PV gradients and enhanced mesoscale activity.

Observations of dissolved oxygen from CLIMODE profiling floats reveal the annual cycle of EDW oxygen and are used together with observations of EDW destruction to infer EDW oxygen utilization rates.