On the seasonal cycles and variability of Florida Straits, Ekman and Sverdrup transports at 26° N in the Atlantic Ocean 1National Oceanography Centre, European Way, Southampton, SO14 3ZH, UK
2Ozeanzirkulation und Klimadynamik, Leibniz-Institut für Meereswissenschaften an der Universität Kiel, Düsternbrooker Weg 20, 24105 Kiel, Germany
Received: 20 Apr 2010 – Published in Ocean Sci. Discuss.: 29 Apr 2010Abstract. Since April 2004 the RAPID array has made continuous measurements of the
Atlantic Meridional Overturning Circulation (AMOC) at 26° N. Two key
components of this system are Ekman transport zonally integrated across
26° N and western boundary current transport in the Florida Straits.
Whilst measurements of the AMOC as a whole are somewhat in their infancy,
this study investigates what useful information can be extracted on the
variability of the Ekman and Florida Straits transports using the decadal
timeseries already available. Analysis is also presented for Sverdrup
transports zonally integrated across 26° N.
Revised: 10 Aug 2010 – Accepted: 17 Sep 2010 – Published: 01 Oct 2010
The seasonal cycles of Florida Straits, Ekman and Sverdrup transports are
quantified at 26° N using harmonic analysis of annual and semi-annual
constituents. Whilst Sverdrup transport shows clear semi-annual periodicity,
calculations of seasonal Florida Straits and Ekman transports show
substantial interannual variability due to contamination by variability at
non-seasonal frequencies; the mean seasonal cycle for these transports only
emerges from decadal length observations. The Florida Straits and Ekman mean
seasonal cycles project on the AMOC with a combined peak-to-peak seasonal
range of 3.5 Sv. The combined seasonal range for heat transport is 0.40 PW.
The Florida Straits seasonal cycle possesses a smooth annual periodicity in
contrast with previous studies suggesting a more asymmetric structure. No
clear evidence is found to support significant changes in the Florida Straits
seasonal cycle at sub-decadal periods. Whilst evidence of wind driven Florida
Straits transport variability is seen at sub-seasonal and annual periods, a
model run from the 1/4° eddy-permitting ocean model NEMO is used to
identify an important contribution from internal oceanic variability at
sub-annual and interannual periods. The Ekman transport seasonal cycle
possesses less symmetric structure, due in part to different seasonal
transport regimes east and west of 50 to 60° W. Around 60% of
non-seasonal Ekman transport variability occurs in phase section-wide at
26° N and is related to the NAO, whilst Sverdrup transport
variability is more difficult to decompose.
Citation: Atkinson, C. P., Bryden, H. L., Hirschi, J. J-M., and Kanzow, T.: On the seasonal cycles and variability of Florida Straits, Ekman and Sverdrup transports at 26° N in the Atlantic Ocean, Ocean Sci., 6, 837-859, doi:10.5194/os-6-837-2010, 2010.