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1. The Role of the Greenland-Scotland Ridge in Step-wise Cooling of the Nordic Seas from the Middle to Late Miocene

   O'Mara, N. A. ; Polissar, P. J. ; Uno, K. T. ; Phelps, S. R. ; Buzeta, R. K. ; deMenocal, P. B. ( December 2018 , American Geophysical Union, Fall Meeting 2018, abstract #PP13E-1364)

   Neogene ocean temperatures are characterized by sustained warmth during the mid-Miocene Climatic Optimum followed by gradual cooling through the late Miocene culminating in Northern Hemisphere glaciation in the early Pleistocene. While the magnitude of sea surface temperature (SST) cooling is enhanced at higher latitudes, existing records suggest that the timing is nearly synchronous across the world's oceans. However, the Nordic Seas, north of the Greenland-Scotland Ridge (GSR), experienced rapid cooling steps (14.5-14 Ma, 12.5-12 Ma, 8-6 Ma) that are out of sync with the global SST cooling trend. Here we present a new alkenone paleo-SST record from Ocean Drilling Program (ODP) site 985 in the western Norwegian Sea (66°56' N, 6°27' W) and investigate the relationships between rapid SST change, depth of the GSR, ocean circulation, and deep-water formation using proxy and model data. We find significant (p < 0.01) inverse relationships between the depth of the GSR and SSTs at ODP sites north of the ridge (985 and 907), positive relationships between GSR depth and the SST gradient across the ridge, and inverse relationships between deep water production and SST at ODP sites 985 and 907. In sum, these observations suggest that during global Miocene cooling, intervals of GSRmore »deepening allowed for increased sea water exchange and an invigoration of deep-water production in the North Atlantic. We posit that enhanced surficial cyclonic flow in the Nordic Seas and a strengthened East Greenland Current caused rapid cooling in the western Nordic Seas. This cooling is consistent with Pliocene coupled climate model runs with altered tectonic boundary conditions simulating a deeper GSR, implying that this SST response to changes to GSR depth may be an important mechanism in high latitude Neogene climate. Furthermore, a strong linear relationship (r2 = 0.84) between ODP 985 SST and global deep ocean δ13C suggests that ocean circulation responses to tectonically forced variability in the GSR may have had an important impact on the Neogene carbon cycle.« less
2. Miocene Evolution of North Atlantic Sea Surface Temperature

   https://doi.org/10.1029/2019PA003748  

   Super, James R. ; Thomas, Ellen ; Pagani, Mark ; Huber, Matthew ; O'Brien, Charlotte L. ; Hull, Pincelli M. ( May 2020 , Paleoceanography and Paleoclimatology)

   We reconstruct sea surface temperatures (SSTs) at Deep Sea Drilling Project Site 608 (42.836°N, 23.087°), north of the Azores Front, and Ocean Drilling Program Site 982 (57.516°N, 15.866°), under the North Atlantic Current, in order to track Miocene (23.1–5.3 Ma) development of North Atlantic surface waters. Mean annual SSTs from TEX₈₆and U^K′₃₇proxy estimates at both sites were 10–15 °C higher than modern through the Miocene Climatic Optimum (17–14.5 Ma). During the global cooling of the Middle Miocene Climate Transition (~14.5–12.5 Ma), SSTs at midlatitude Site 608 cooled by ~6 °C, whereas high‐latitude Site 982 cooled by only ~2 °C, resulting in an ~4 Myr collapse of the SST gradient between the two sites. This regional pattern is inconsistent with an increased latitudinal surface temperature gradient, as generally associated with global cooling episodes linked to decreasingpCO₂levels. Instead, the pattern is best explained by enhanced ocean heat transport into the high‐latitude North Atlantic superimposed on the global cooling trend, probably due to enhanced Atlantic meridional overturning circulation and/or a stronger North Atlantic Current. During global late Miocene cooling (~8–7 Ma), surface waters cooled by ~6 °C at Site 982 while minimal change occurred at Site 608, reestablishing the North Atlantic SST gradient. The collapse and reemergence of the SSTmore »gradient between the middle‐ and high‐latitude North Atlantic suggests that interaction between changes in regional ocean circulation and the global response to changes in greenhouse gas concentration was important in Miocene climate evolution.

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3. Greenhouse-gas forced changes in the Atlantic meridional overturning circulation and related worldwide sea-level change

   https://doi.org/10.1007/s00382-022-06386-y  

   Couldrey, Matthew P. ; Gregory, Jonathan M. ; Dong, Xiao ; Garuba, Oluwayemi ; Haak, Helmuth ; Hu, Aixue ; Hurlin, William J. ; Jin, Jiangbo ; Jungclaus, Johann ; Köhl, Armin ; et al ( August 2022 , Climate Dynamics)

   The effect of anthropogenic climate change in the ocean is challenging to project because atmosphere-ocean general circulation models (AOGCMs) respond differently to forcing. This study focuses on changes in the Atlantic Meridional Overturning Circulation (AMOC), ocean heat content ($$\Delta$$$\Delta$OHC), and the spatial pattern of ocean dynamic sea level ($$\Delta \zeta$$$\Delta \zeta$). We analyse experiments following the FAFMIP protocol, in which AOGCMs are forced at the ocean surface with standardised heat, freshwater and momentum flux perturbations, typical of those produced by doubling$$\hbox {CO}_{{2}}$$${\text{CO}}_2$. Using two new heat-flux-forced experiments, we find that the AMOC weakening is mainly caused by and linearly related to the North Atlantic heat flux perturbation, and further weakened by a positive coupled heat flux feedback. The quantitative relationships are model-dependent, but few models show significant AMOC change due to freshwater or momentum forcing, or to heat flux forcing outside the North Atlantic. AMOC decline causes warming at the South Atlantic-Southern Ocean interface. It does not strongly affect the global-mean vertical distribution of$$\Delta$$$\Delta$OHC, which is dominated by the Southern Ocean. AMOC decline strongly affects$$\Delta \zeta$$$\Delta \zeta$in the North Atlantic, with smaller effects in the Southern Ocean and North Pacific. The ensemble-mean$$\Delta \zeta$$$\Delta \zeta$and$$\Delta$$$\Delta$OHC patterns are mostly attributable to the heat added bymore »the flux perturbation, with smaller effects from ocean heat and salinity redistribution. The ensemble spread, on the other hand, is largely due to redistribution, with pronounced disagreement among the AOGCMs.

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4. Transient Overturning Compensation between Atlantic and Indo-Pacific Basins

   https://doi.org/10.1175/JPO-D-20-0060.1  

   Sun, Shantong ; Thompson, Andrew F. ; Eisenman, Ian ( August 2020 , Journal of Physical Oceanography)

   Abstract Climate models consistently project (i) a decline in the formation of North Atlantic Deep Water (NADW) and (ii) a strengthening of the Southern Hemisphere westerly winds in response to anthropogenic greenhouse gas forcing. These two processes suggest potentially conflicting tendencies of the Atlantic meridional overturning circulation (AMOC): a weakening AMOC due to changes in the North Atlantic but a strengthening AMOC due to changes in the Southern Ocean. Here we focus on the transient evolution of the global ocean overturning circulation in response to a perturbation to the NADW formation rate. We propose that the adjustment of the Indo-Pacific overturning circulation is a critical component in mediating AMOC changes. Using a hierarchy of ocean and climate models, we show that the Indo-Pacific overturning circulation provides the first response to AMOC changes through wave processes, whereas the Southern Ocean overturning circulation responds on longer (centennial to millennial) time scales that are determined by eddy diffusion processes. Changes in the Indo-Pacific overturning circulation compensate AMOC changes, which allows the Southern Ocean overturning circulation to evolve independently of the AMOC, at least over time scales up to many decades. In a warming climate, the Indo-Pacific develops an overturning circulation anomaly associated withmore »the weakening AMOC that is characterized by a northward transport close to the surface and a southward transport in the deep ocean, which could effectively redistribute heat between the basins. Our results highlight the importance of interbasin exchange in the response of the global ocean overturning circulation to a changing climate.« less
5. Combining shell-based isotope records with numerical model simulations to investigate the time of emergence of widespread warming of the western North Atlantic shelf

   Whitney, N.M. ; Wanamaker, A.D. ; Ummenhofer, C.C. ; Johnson, B. ; Cresswell-Clay, N. ; and Kreutz, K.J. ( October 2021 , American Geophysical Union Fall Meeting, December 13-17, 2021, New Orleans, LA)

   Abstract The Gulf of Maine and surrounding western North Atlantic shelf are some of the fastest warming regions of the worlds oceans. The lack of long-term observational records from this area inhibits the ability to assess the timing and initial causes of this warming and consequently accurately predict future changes to this ecologically and economically important region. Here we present oxygen, nitrogen, and radiocarbon isotope data measured in Arctica islandica shells collected in the western North Atlantic to better understand the past temperature and ocean circulation variability of the region over the last 300 years. We combine these results with output from the Community Earth System Model Last Millennium Ensemble simulations to assess the temporal and spatial context of these isotope records. We find that the isotope records capture the end and reversal of a millennium-scale cooling trend in the Gulf of Maine. Last Millennium Ensemble single-forcing simulations indicate that this cooling trend appears to be largely driven by volcanic forcing. The nitrogen and radiocarbon records indicate that ocean circulation is in part driving the reconstructed hydrographic changes, pointing to a potential role of the Atlantic Meridional Overturning Circulation in regulating Gulf of Maine temperatures as suggested by the Lastmore »Millennium Ensemble simulations. Both isotope and model results suggest that the Gulf of Maine began to warm in the late 19th century, ultimately driven by increased greenhouse gas forcing. Plain-language Summary The Gulf of Maine, located off of the Eastern Coast of the United States, has experienced significant temperature increases recently. Because the instrumental record only began in 1905, we do not have a good idea of when this warming began and what may have initially caused the warming. Here, we analyze the chemistry of clam shells, which have grown in the Gulf of Maine for hundreds of years, to infer past changes in ocean temperatures and water properties. We combine these results with output from a climate model to reveal that the temperatures reconstructed from the clams shells agree well with the model during the period of overlap. Both the chemical records and the model suggest the Gulf of Maine started warming in the late 1800s as a result of increased atmospheric greenhouse gas concentrations. Before this warming began, the Gulf of Maine region appears to have been cooling. The model suggests that this cooling trend is likely due to the influence of volcanic eruptions. The chemical records from the clam shells also suggest that part of this cooling is likely related to changing ocean circulation patterns.« less