North Atlantic low-frequency changes of volume and
heat transports in the ECCO state estimates

K. Lorbacher, A. Koehl and D. Stammer, SIO

From a solution of the global ECCO state estimation we
diagnose
the low-frequency (seasonal to interannual) variability of meridional
volume and heat transports between 20N and 60N in the North Atlantic
Ocean. The model transports are compared with estimates across the
nominal "48N"-section (WOCE/A2) -- reaching from the English Channel
to the Grand Banks -- where a coherent hydrographic data set was
obtained during seven cruises from 1993 to 2000.

The standard deviation of the monthly-mean anomalies of the simulated
overturning rate at "48N" amounts to 2 Sv and of the heat transport to
0.08 PW, respectively; the year-to-year variability is half of those
numbers. In contrast to the model simulations, the observed amplitudes
lie 50% above the model transport variations on interannual timescale.
The percentage of the different components contributing to the total
heat transport in the model is consistent with the estimates from the
observations: In the time-mean, the baroclinic component of the heat
transport contributes 80% to the total integral and the Ekman component
10%. However, the latter explains 90% of the diagnosed low-frequency
variability. In the observations the baroclinic component is mainly
responsible for interannual variations and thereby changes in the sign
of the horizontal component appears to be dynamically relevant. From
the model output we compute error bars on the observed transport
estimates to test their representativeness of low-frequency variability.