Monitoring the Atlantic meridional overturning circulation

Abstract

A recent hydrographic section at 24.5°N in the Atlantic and 6 months of observations from a moored array show that Antarctic Bottom Water (AABW), the densest and deepest water mass in the world oceans, has been warming. While Johnson et al. (2008) showed that northward AABW transport at 24.5°N has been declining from 1981 to 2004, suggesting that the lower cell of the overturning circulation could halt in the near future, estimates from the latest hydrographic section in 2010 indicate a partial recovery of northward AABW transport. From 6 months of temperature and salinity observations at a deep moored array at 24–26°N, we find that short-term variability between April and November 2009 is of the same magnitude as the changes observed from hydrographic sections between 1981 and 2004. These observations highlight the possibility that transport changes estimated from hydrographic sections may be aliased by short-term variability. The observed AABW transport variability affects present estimates of the upper meridional overturning circulation by ±0.4 Sv (1 Sv = 106 m3s−1).

Figure 2. Schematic of the MOC monitoring array at 261N. The MOC is decomposed into three components: (1) Gulf Stream transport TGS through the Florida Straits (red arrow), (2) the near-surface wind driven Ekman transport TEK (green arrow) arising from the zonal wind stress and (3) geostrophic (thermal wind) contribu- tion TINT (light blue arrows) calculated between adjacent pairs of ‘‘moorings’’ (vertical lines). Yellow arrows indicate a spatially constant velocity correction that ensures mass balance across the section. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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.