Observed Interannual Variability of the Atlantic MOC at 26.5°N

Abstract

The Atlantic meridional overturning circulation (MOC) plays a critical role in the climate system and is responsible for much of the heat transported by the ocean. A mooring array, nominally at 26°N between the Bahamas and the Canary Islands, deployed in Apr 2004 provides continuous measurements of the strength and variability of this circulation. With seven full years of measurements, we now examine the interannual variability of the MOC. While earlier results highlighted substantial seasonal and shorter timescale variability, there had not been significant interannual variability. The mean MOC from 1 Apr 2004 to the 31 March 2009 was 18.5 Sv with the annual means having a standard deviation of only 1.0 Sv. From 1 April 2009 to 31 March 2010, the annually averaged MOC strength was just 12.8 Sv, representing a 30% decline. This downturn persisted from early 2009 to mid-2010. We show that the cause of the decline was not only an anomalous wind-driven event from Dec 2009–Mar 2010 but also a strengthening of the geostrophic flow. In particular, the southward flow in the top 1100 m intensified, while the deep southward return transport—particularly in the deepest layer from 3000–5000 m—weakened. This rebalancing of the transport from the deep overturning to the upper gyre has implications for the heat transported by the Atlantic.

(top) Transport anomaly time series after removing the average seasonal cycle (calculated over the full timeseries) and smoothing with a 180-day low-pass Tukey filter. Components include Ekman (black), upper mid-ocean with fixed Ekman (magenta) and overturning with fixed Ekman (red). Transports in units of Sverdrups. (bottom) Transport time series with the average seasonal cycle removed of the upper NADW (1100–3000 m, cyan) and lower NADW (3000–5000 m, dark blue), calculated with fixed Ekman. These are the main water masses of the deep mid-ocean transports

Eleanor Frajka-Williams

Professor of Ocean Dynamics in a Changing Climate

I am a physical oceanographer who uses ocean observations to investigate ocean dynamics and circulation in a changing climate. I have a particular interest in problems spanning scales (from micro- to large-scale) or spheres (biogeosphere, cryosphere, atmosphere), and in methods that leverage traditional observations with new platforms and satellite data.