Estimating the Deep Overturning Transport Variability at 26°N Using Bottom Pressure Recorders

E L Worthington,G D Mccarthy,E Frajka‐Williams

doi:10.1029/2018jc014221

Abstract

AbstractThe RAPID mooring array at 26°N in the Atlantic has been observing the Atlantic meridional overturning circulation (AMOC) since 2004, with estimates of AMOC strength suggesting that it has declined over the 2004–2016 period. When AMOC transport is estimated, an external transport is added to the observed Ekman, Florida Straits, and baroclinic geostrophic transports to ensure zero net mass transport across the section. This approach was validated using the first year of RAPID data by estimating the external component directly from in situ bottom pressure data. Since bottom pressure recorders commonly show low‐frequency instrument drift, bottom pressure data had to be dedrifted prior to calculating the external component. Here we calculate the external component from 10 years of in situ bottom pressure data and evaluate two choices for dedrifting the records: traditional and adjusted using a Gravity Recovery and Climate Experiment (GRACE) bottom pressure solution. We show that external transport estimated from GRACE‐adjusted, in situ bottom pressure data correlates better with the RAPID compensation transport (r=0.65,p<0.05) than using individually dedrifted bottom pressure recorders, particularly at low frequencies on timescales shorter than 10 years, demonstrating that the low‐frequency variability added from GRACE is consistent with the transport variability at RAPID. We further use the bottom pressure‐derived external transport to evaluate the zonal distribution of the barotropic transport variability and find that the transport variability is concentrated west of the Mid‐Atlantic Ridge rather than uniformly distributed across the basin, as assumed in the RAPID calculation.

Highlights

The Atlantic meridional overturning circulation (AMOC) is a key component of the climate system, comprising northward transport of warm upper waters and southward transport of deep cold water
We show that external transport estimated from Gravity Recovery and Climate Experiment (GRACE)-adjusted, in situ bottom pressure data correlates better with the RAPID compensation transport (r = 0.65, p < 0.05) than using individually dedrifted bottom pressure recorders, at low frequencies on timescales shorter than 10 years, demonstrating that the low-frequency variability added from GRACE is consistent with the transport variability at RAPID
External Transports Compared to RAPID Compensation Term bottom pressure (BP) variability derived from the recorder data at WB2 shows a strong correlation with the GRACE ocean BP data for the closest mascon (r = 0.67, Figure 5b) and is well within the uncertainty (Figure 5a), whereas the BP variability at EBH1 shows almost no correlation with the corresponding GRACE data (r = 0.02 and not significant, Figure 5h) and frequently exceeds the GRACE BP uncertainty (Figure 5g)

Summary

Introduction

The Atlantic meridional overturning circulation (AMOC) is a key component of the climate system, comprising northward transport of warm upper waters and southward transport of deep cold water. Since most of the heat carried by the upper limb of the AMOC is lost to the atmosphere as it moves poleward, it plays a vital role in maintaining the milder climate of northwest Europe (Rhines et al, 2008). Global climate models simulating an AMOC shutdown show widespread cooling in northwest Europe and the North Atlantic, relative to rising global temperatures, together with changes to precipitation patterns and more winter storms (Jackson et al, 2015). Based on its role in the Earth's energy budget and the potential for it to slow down, the AMOC has been measured at 26◦N in the Atlantic since 2004 by the RAPID Climate Change measurement program, a joint UK/U.S effort, which we will refer to as RAPID

Methods

Results

Conclusion