Thirty Years of GOSHIP and WOCE Data: Atlantic Overturning of Mass, Heat, and Freshwater Transport

Abstract

The Atlantic Meridional Overturning Circulation (AMOC) transports relatively warm surface, thermocline, and intermediate waters northwards. This warm upper limb of the overturning circulation releases heat to the atmosphere on its way northward, loses buoyancy, and eventually sinks in the subpolar North Atlantic (SPNA) and Nordic Seas, returning south as North Atlantic Deep Water (NADW; Srokosz et al., 2012). Recent publications have addressed the long-term millennial evolution of the AMOC and suggest that it has been in a relative weak state in recent decades (

Anthropogenic Carbon Transport Variability in the Atlantic Ocean Over Three Decades

“…The meridional circulation in the Atlantic follows two‐counter rotating cells (Caínzos et al., 2022; Kersalé et al., 2020). The balance in these cells involves northward transports of upper and abyssal layers, compensated by southward‐flowing North Atlantic Deep Water (NADW) in between.…”

“…This transport is opposed by a southward normalized transport for the upper mid‐ocean, computed as the upper transport discounting the Florida Straits transport (UMO; −0.333 ± 0.015, −0.376 ± 0.019, and −0.309 ± 0.016 PgC yr −1 , for 1990–1999, 2000–2009, and 2010–2019, respectively) between 26.14 and 27.84 kg m −3 (Figure 3). In the SPNA (at 47°N and 55°N) the core of the NAC (North Atlantic Current) gets ∼4% denser on its way northward (Caínzos et al., 2022), with stronger C anth transports between 27.23 and 27.58 kg m −3 . Deep water formation in the Nordic and Labrador Seas offers a return southward flow of C anth transport with stronger values in the upper limb of the NADW, between 27.84 and 28.04 kg m −3 , a pattern that is maintained as it is advected southward, despite decreasing its strength.…”

Energetic overturning flows, dynamic interocean exchanges, and ocean warming observed in the South Atlantic

“…Another well-known disagreement between observations and models is the direction of the freshwater transport induced by the overturning circulation (M ov ) near the southern boundary of the Atlantic 16,90,91,120,166,167 . Contrary to the positive values of the M ov from numerical models 35,50,[240][241][242] , most recent observationbased estimates of the M ov in the South Atlantic 16,120,166,167,184 gave negative values ranging from −0.28 to −0.05 Sv (with the exception of the recent snapshot estimate from a trans-basin cruise at 34.5°S 56 ), suggesting that changes in the AMOC would amplify the freshwater anomalies in the convection sites. As a result, contrary to the monostable state of the AMOC from models, observations suggest that the AMOC is bistable and might collapse with a large enough freshwater perturbation.…”

“…Combined with historical hydrographic observations, XBT transects are used to examine the seasonal-to-interannual variability of volume and heat transports 16,53,233,248 . A few repeated full-depth conductivity-temperature-depth (CTD) transects have provided estimates of volume, heat, and freshwater transports that can be compared over the decades 56,166,167 and combined with inverse models to produce globally-balanced mean transport estimates 61,184,249,250 . Ship-based transport estimates are considered "instantaneous snapshots", but in reality, each trans-basin section can take weeks to months to complete.…”

“…Estimates of the meridional freshwater transport by the overturning circulation, computed either from the sea surface to the seafloor or from the sea surface to the base of the upper AMOC cell (nominally 3000 dbar), have been limited to full-depth CTD sections 56,[166][167][168] , XBT transects 16 , and inverse models 61,184,185 . Most trans-basin observations collected to date have shown that the freshwater transport associated with the overturning circulation (M ov ) between 11°S and 34.5°S is southward 16,166,167,184 , and may have increased in strength over the past 30 years associated with increasing salinity trends in the upper ocean 184 (Fig. 3c).…”