Yes, it's short, but fortunately it starts at about the peak of the last positive AMO ... so we should be able to track this whole
downward portion of the cycle. The last time this happened was 1960. According to the undetrended AMO values plotted on
www.climate4you.com/ the index value dropped by about 0.5 C between 1960 and 1965. There was a slight pause
and it dropped again for a total loss between 1960 and 1974 of about 0.75 C.
While the 11-year time frame may be limiting, the wealth of data from Argo does allow us to look at how temperature and
salinity changes between the lower and upper areas of the North Atlantic Basin have changed within relatively narrow time bands.
I selected two relatively equal, slightly staggered areas to represent the sub-tropical source area of the AMOC and the high-latitude,
receiving (business) end of that flow. I've called them: the southwest area (25-45 N, 30-80 W) and the northeast area (45-65 N, 10-60 W).
Forty-five north seemed to be a logical breaking line as it extends from Labrador to just north of Spain, and is north of the eastward
flowing leg of the sub-tropical gyre.
It appears that NOAA serves its Argo temperature and salinity data as gridded horizontal surfaces, where edited point data
from the buoys are modeled into continuous cell values using some type of typographic (or similar) model. These may be packaged
as predefined depth stacks between 0 and 2000 m or they may be retrieved by some type of dynamic depth assembly function within
the software. Either way, one can conduct custom statistical manipulations and summary functions both vertically and horizontally across
the data base, and can output either plan-view or vertically-sliced graphics either as maps or line graphs, with ascii text file output
capabilities. The internal apps are limited, but it appears to be a well-designed, user-friendly public-access data base. I applaud their efforts.
Currently, the Argo Atlas does not include any data north of Scotland and Iceland.
For this presentation, I extracted temperature and salinity data averaged for 0-700 m depth for the two basin subareas and loaded it into
a spreadsheet. Data are plotted as monthly deviations from a 12-year mean where monthly values are a 12-month running average. These
trends from 2004-2015 are displayed in the left-hand graph below.
OBSERVATIONS
Temperatures for both sub-basins and the basin-wide average declined relatively steadily by about 0.2 to 0.3 C from 2004 to mid-2013, at
which point the SW Area increased abruptly by about 0.25 C and the NE area declined dramatically by about 0.45 C. Within the time series
there appear to be alternating periods of heat accumulation in the NE area and heat loss in the SW area (2005-07 and 2010-11) followed by
the reverse (2008-09 and 2011-12. This almost seems like a pulse between the sub-basins, where heat builds up in the sub-tropical source
region while the destination area cools, then this heat is emptied northward and the source area cools. So what happened in 2014?? The
magnitude of this cycle (if it is one) has just increased dramatically! Heating in the source area has increased by about the same amount
as in the 2010-11 time period, but heat loss in the destination area has been twice as much as in any comparable time period.
Note also the trends in salinity deviations in each sub-basin seem to mirror temperature changes. Salinities in the upper 700 m appear
to have 'freshened' (if this is the correct term), largely since 2010 ... and there is a coupled large rise in the SW and large decline in the
NE that matches the bifurcation in the temperatures. Any comments???
For comparison, I am attaching plan-view maps showing the alternating heat pulses in the two sub-basins, two in this and three in the
following post. Note that in 2004, both sub-basins are warm. After that I'll follow up with temperature and velocity cross sections.
Argo data plotted using Argo Global Marine Atlas
Roemmich, D. and J. Gilson, 2009: The 2004-2008 mean and annual cycle of temperature, salinity, and steric
height in the global ocean from the Argo Program. Progress in Oceanography, 82, 81-100.