The Adequacy of Observing Systems in Monitoring the Atlantic Meridional Overturning Circulation and North Atlantic Climate

Abstract

The Atlantic meridional overturning circulation (AMOC) has an important influence on climate, and yet adequate observations of this circulation are lacking. Here, the authors assess the adequacy of past and current widely deployed routine observing systems for monitoring the AMOC and associated North Atlantic climate. To do so, this study draws on two independent simulations of the twentieth century using an Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) coupled climate model. One simulation is treated as ?truth? and is sampled according to the observing system being evaluated. The authors then assimilate these synthetic ?observations? into the second simulation within a fully coupled system that instantaneously exchanges information among all coupled components and produces a nearly balanced and coherent estimate for global climate states including the North Atlantic climate system. The degree to which the assimilation recovers the truth is an assessment of the adequacy of the observing system being evaluated. As the coupled system responds to the constraint of the atmosphere or ocean, the assessment of the recovery for climate quantities such as Labrador Sea Water (LSW) and the North Atlantic Oscillation increases the understanding of the factors that determine AMOC variability. For example, the low-frequency sea surface forcings provided by the atmospheric and sea surface temperature observations are found to excite a LSW variation that governs the long-time-scale variability of the AMOC. When the most complete modern observing system, consisting of atmospheric winds and temperature, is used along with Argo ocean temperature and salinity down to 2000 m, a skill estimate of AMOC reconstruction is 90% (out of 100% maximum). Similarly encouraging results hold for other quantities, such as the LSW. The past XBT observing system, in which deep-ocean temperature and salinity were not available, has a lesser ability to recover the truth AMOC (the skill is reduced to 52%). While these results raise concerns about the ability to properly characterize past variations of the AMOC, they also hold promise for future monitoring of the AMOC and for initializing prediction models.