by Bill Johns
At sea again!
No, not an OSNAP cruise this time, but in the balmy subtropics at 26°N.
I am leading a group from the University of Miami and NOAA’s Atlantic Oceanographic and Meteorological Laboratory (AOML) on this 18 day cruise, where we will recover and replace several deep moorings and collect hydrographic profiles near the ocean’s western boundary off the Bahamas as part of the RAPID/MOCHA program (http://www.rapid.ac.uk/rapidmoc/). One of our key goals is to monitor the strength of the Deep Western Boundary Current (DWBC) that carries deep waters formed in the subpolar region southward toward the equator, in the lower branch of the Atlantic Merdional Overturning Circulation.
Unlike our OSNAP cruises, we are wearing t-shirts on deck and scanning for the elusive “green flash” on clear days at sunset – a somewhat more comfortable existence to be sure, but as on all cruises the work is nonstop. We’ve experienced a short November gale on this cruise that shut down our sampling for awhile, but we are mindful of the fact that we’d much rather be here now than up in the high North Atlantic!
The last CTD recovery before shutting down our overboarding operations.
Even though the subpolar gyre seems far, far away, the data we are collecting on this cruise is a constant reminder of the connection between what is happening here in the subtropics and in the subpolar region. We can see clear evidence in the DWBC water mass properties of changes in the intensity of deep water mass formation in the North Atlantic over the past few decades. Although the RAPID program only started in 2004, the AOML group has been making measurements of the water mass composition of the DWBC here since the mid-80’s as part of their Western Boundary Time Series (WBTS) program. The biggest change occurred in 1995 when a new pulse of much colder and fresher (less salty) water originating from the Labrador Sea arrived at 26°N. This pulse followed a period of very strong cooling in the Labrador Sea starting about 9 years earlier that resulted in the deepest and densest formation of Labrador Sea Water in more than 60 years. The 9-year transit time for that pulse to arrive off the Bahamas means it couldn’t have all come in a fast-track pathway within the DWBC itself, but very likely followed one or more pathways through the ocean interior, for which there is other independent evidence. The peak of that event occurred in about 2003 off the
Bahamas, again just about 9 years after the peak of deep convection in the Labrador Sea in 1994. Since that time the waters in the DWBC off Abaco have gradually warmed, while deep convection in the Labrador Sea has generally decreased.
All that changed in 2014 with the onset of very strong cooling again across the subpolar gyre and extensive deep convection in the Labrador Sea – coincidentally (but auspiciously) timed with the start of OSNAP. If this turns out to be a sustained multi-year event, which it seems to have the makings of, then the next several years will be very interesting. Will this be a playback of the mid-90’s event, or will something different happen? We know the ocean is taking up a great deal of the excess carbon dioxide we are putting into the atmosphere, and that the deep water mass formation in the North Atlantic is a key element of that uptake. What we know much less about are the pathways and processes by which carbon is transported and stored in the ocean and he time scales of those deep ocean transport processes. Obviously we’ll have to wait a while to find out what happens, but the difference this time around is that we will have the OSNAP, RAPID, and other deployed AMOC arrays, as well as the fully-deployed Argo array, to help us out. THAT is progress.