Research

PhD thesis: A reconstruction of the Atlantic meridional overturning circulation at 26°N

Abstract

A decline in Atlantic meridional overturning circulation (AMOC) strength has been observed between 2004 and 2012 by the RAPID-MOCHA-WBTS (RAPID –Meridional Overturning Circulation and Heatﬂux Array – Western Boundary Time Series, hereafter RAPID) array with this weakened state of the AMOC persisting until 2017. Climate model and paleo-oceanographic research suggests that the AMOC may have been declining for decades or even centuries before this; however direct observations are sparse prior to 2004, giving only “snapshots” of the overturning circulation. Proxy reconstructions extend the AMOC back in time, but tend to be either implicitly single-layer reconstructions of 20–30 years that do not capture all the observed variability, or longer reconstructions with high uncertainty and for the most part, not based on hydrography.

To bridge the gap between observations and proxy reconstructions, we developed a higher-ﬁdelity statistical model of AMOC variability based on RAPID data and associated physically with changes in thickness of the persistent upper, intermediate, and deep water masses at 26°N and associated transports. We applied historical hydrographic data to the empirical model to create an AMOC time series extending from 1981 to 2016, and constrained the associated uncertainty by evaluating the statistical model within a high resolution ocean simulation.

Increasing the resolution of the observed AMOC to approximately annual shows multi-annual variability in agreement with RAPID observations, AMOC indices based on sea-surface temperatures, and palaeoclimate reconstructions. It captures a recovery of the AMOC during the 1990s and shows that the downturn between 2008 and 2012 was the weakest AMOC since the mid–1980s. This work creates the longest continuous AMOC reconstruction based on both shallow and deep sub-surface hydrographic data, and has created an independent AMOC index. The length of our AMOC time series is greater than reconstructions based on modern observations, and approaches the length of detectable climate-change related decline, but shows no overall AMOC decline as indicated by high-resolution climate models. Our results reinforce that adequately capturing changes to the deep circulation is key to detecting any anthropogenic climate-change-related AMOC decline.

Key figure


UMO and LNADW transports estimated by empirical models using density anomalies from hydrographic CTD profiles, compared to estimates from RAPID, and from ship section hydrography (Bryden et al. (2005), Atkinson et al. (2010)). The 920 dbar eastern boundary density anomaly for the UMO model was replaced by RAPID monthly climatology. The monthly mean Florida Current and Ekman transports are also shown, and were added to the UMO model-estimated transports to give the estimated AMOC transport.

UMO and LNADW transports estimated by empirical models using density anomalies from hydrographic CTD profiles, compared to estimates from RAPID, and from ship section hydrography (Bryden et al. (2005), Atkinson et al. (2010)). The 920 dbar eastern boundary density anomaly for the UMO model was replaced by RAPID monthly climatology. The monthly mean Florida Current and Ekman transports are also shown, and were added to the UMO model-estimated transports to give the estimated AMOC transport.