Early Detection of Changes in the North Atlantic Meridional Overturning Circulation: Implications for the Design of Ocean Observation Systems

Abstract

Many climate models predict that anthropogenic greenhouse gas emissions may cause a threshold response of the North Atlantic meridional overturning circulation (MOC). These model predictions are, however, uncertain. Reducing this uncertainty can have an economic value, because it would allow for the design of more efficient risk management strategies. Early information about the MOC sensitivity to anthropogenic forcing (i.e., information that arrives before the system is committed to a threshold response) could be especially valuable. Here the focus is on one particular kind of information: the detection of anthropogenic MOC changes. It is shown that an MOC observation system based on infrequent (decadal scale) hydrographic observations may well fail in the task of early MOC change detection. This is because this system observes too infrequently and the observation errors are too large. More frequent observations and reduced observation errors would result in earlier detection. It is also shown that the economic value of information associated with a confident and early prediction of an MOC threshold response could exceed the costs of typically implemented ocean observation systems by orders of magnitude. One open challenge is to identify a feasible observation system that would enable such a confident and early MOC prediction across the range of possible MOC responses.

Detecting potential changes in the meridional overturning circulation at 26˚N in the Atlantic

“…Similar to Keller et al (2006) we also analyze how less frequent hydrographic sections influence the detection time. Keller et al (2006) found that an observing system consisting of a hydrographic transect every 5 years and an observation error of about 3 Sv results in a median detection time of approximately 70 years.…”

“…Detection time is a random variable as it depends on random realizations of the observation errors and the internal variability (Keller et al 2006). To account for observation error, we add random observation error (identically, independently and normally distributed) of different magnitudes (e.g., standard deviation of 1 Sv) to the simulated observations, and estimate the linear trend of each of the resulting time series.…”

“…The results of the two detection approaches differ, because the simple detection approach considers the observations in isolation, whereas the quantitative approach analyses a trend in a sequence of observations. Previous analysis typically considered a single realization of a model run (e.g., Santer et al 1995;Vellinga and Wood 2004;Keller et al 2006). Analyzing a single realization neglects the uncertainty due to the initial conditions.…”

“…All these studies employed numerical models to investigate the mechanisms or general nature of detecting changes in the MOC in the Atlantic, but were not directed at realistic observing systems. In contrast, Keller et al (2006) considered the effects of observation error and infrequent observations, analyzing the required frequency of hydrographic transects in the North Atlantic to detect changes in the MOC. In the present study, we simulate a realistic observing system, which was deployed recently Schiermeier 2004).…”

Timely Detection of Changes in the Meridional Overturning Circulation at 26°N in the Atlantic

“…Keller et al (2007) found that an observing system based on hydrographic transects every five years [with an observation uncertainty of about 5 Sverdrups (Sv)] is likely to result in detection of changes in the maximum MOC before a point of no return is reached. Brennan et al (2007) found that in the Commonwealth Scientific and Industrial Research Organisation (CSIRO) model, MOC detection times can be reduced by 40%, if in addition to a (univariate) MOC time series apparent oxygen utilization is measured along 26°N.…”