The 1600 Huaynaputina Eruption as Possible Trigger for Persistent Cooling in the North Atlantic Region

Paleoclimate reconstructions identify a period of exceptional summer and winter cooling in the North Atlantic region following the eruption of the tropical volcano Huaynaputina (Peru) in 1600 CE. Numerical climate simulations indicate a possible eruption-induced mechanism for the persistent cooling in a slowdown of the North Atlantic subpolar gyre (SPG) and consequent ocean-atmosphere feedbacks. To examine the possibility of such an eruption-induced cooling mechanism, this study compares simulations with and without volcanic forcing and an SPG shift to reconstructions from annual proxies in natural archives and historical written records as well as contemporary historical observations of relevant climate and environmental conditions. These reconstructions and observations demonstrate patterns of cooling and sea ice expansion consistent with, but not necessarily indicative of, an eruption trigger for the proposed SPG slowdown mechanism. The results point to possible improvements in future model-data comparison studies utilizing historical written records. Moreover, we consider historical societal impacts and adaptations associated with the reconstructed climatic and environmental anomalies.

Circulation adjustment in the Arctic and Atlantic in response to Greenland and Antarctic mass loss

Following a high-end projection for mass loss from the Greenland and Antarctic ice-sheets, a freshwater forcing was applied to the ocean surface in the coupled climate model EC-Earthv2.2 to study the response to meltwater release assuming an RCP8.5 emission scenario. The meltwater forcing results in an overall freshening of the Atlantic that is dominated by advective changes, strongly enhancing the freshening due to dilution by Greenland meltwater release. The strongest circulation change occurs in the western North Atlantic subpolar gyre and in the gyre in the Nordic Seas, leaving the North Atlantic subpolar gyre the region where most advective salt export occurs. Associated with counteracting changes in both gyre systems, the response of the Atlantic Meridional Overturning Circulation is rather weak over the 190 years of the experiment; it reduces with only 1 Sv ($$= 10^6$$ = 10 6 m $$^3$$ 3 s $$^{-1}$$ - 1 ), compared to changes in the subpolar gyre of 5 Sv. This relative insensitivity of the AMOC to the forcing is attributed to enhanced convection in the Nordic Seas and stronger overflows that compensate reduced convection in the Labrador and Irminger Seas, and lead to higher sea surface temperatures (SSTs) in the former and lower SSTs in the latter region. The weakened subpolar gyre in the west also shifts the North Atlantic Current and the subpolar-subtropical gyre boundary; with the subtropical gyre expanding, and the western subpolar gyre contracting. The SST changes are associated with obduction of Atlantic waters in the Nordic Seas that would otherwise obduct in the western subpolar gyre. The anomalous SSTs also induce a coupled ocean-atmosphere feedback that further strengthens the Nordic Seas circulation and weakens the western subpolar gyre. This occurs because the anomalous SST-gradient enhances the westerlies, especially between 65$$^{\circ }$$ ∘ N and 70$$^{\circ }$$ ∘ N; the associated increase in windstress curl consequently enhances the gyre in the Nordic Seas. This feedback is driven by the Greenland mass loss; Antarctic meltwater discharge causes a weaker, opposite response and more particularly affects the South Atlantic salinity budget through northward advection of low-salinity waters from the Southern Ocean. This effect, however, becomes visible only hundred years after the onset of Antarctic mass loss. We conclude that the response to freshwater forcing from both ice caps can lead to a complex response in the Atlantic circulation systems with opposing effects in different subbasins. The relative strength of the response is time-dependent and largely governed by internal feedbacks; the forcing acts mainly as a trigger and is decoupled from the response.

Ocean carbonate system variability in the North Atlantic Subpolar surface water (1993–2017)

The North Atlantic is one of the major ocean sinks for natural and anthropogenic atmospheric CO2. Given the variability of the circulation, convective processes or warming–cooling recognized in the high latitudes in this region, a better understanding of the CO2 sink temporal variability and associated acidification needs a close inspection of seasonal, interannual to multidecadal observations. In this study, we investigate the evolution of CO2 uptake and ocean acidification in the North Atlantic Subpolar Gyre (50–64∘ N) using repeated observations collected over the last 3 decades in the framework of the long-term monitoring program SURATLANT (SURveillance de l'ATLANTique). Over the full period (1993–2017) pH decreases (−0.0017 yr−1) and fugacity of CO2 (fCO2) increases (+1.70 µatm yr−1). The trend of fCO2 in surface water is slightly less than the atmospheric rate (+1.96 µatm yr−1). This is mainly due to dissolved inorganic carbon (DIC) increase associated with the anthropogenic signal. However, over shorter periods (4–10 years) and depending on the season, we detect significant variability investigated in more detail in this study. Data obtained between 1993 and 1997 suggest a rapid increase in fCO2 in summer (up to +14 µatm yr−1) that was driven by a significant warming and an increase in DIC for a short period. Similar fCO2 trends are observed between 2001 and 2007 during both summer and winter, but, without significant warming detected, these trends are mainly explained by an increase in DIC and a decrease in alkalinity. This also leads to a pH decrease but with contrasting trends depending on the region and season (between −0.006 and −0.013 yr−1). Conversely, data obtained during the last decade (2008–2017) in summer show a cooling of surface waters and an increase in alkalinity, leading to a strong decrease in surface fCO2 (between −4.4 and −2.3 µatm yr−1; i.e., the ocean CO2 sink increases). Surprisingly, during summer, pH increases up to +0.0052 yr−1 in the southern subpolar gyre. Overall, our results show that, in addition to the accumulation of anthropogenic CO2, the temporal changes in the uptake of CO2 and ocean acidification in the North Atlantic Subpolar Gyre present significant multiannual variability, not clearly directly associated with the North Atlantic Oscillation (NAO). With such variability it is uncertain to predict the near-future evolution of air–sea CO2 fluxes and pH in this region. Thus, it is highly recommended to maintain long-term observations to monitor these properties in the next decade.

Barotropic vorticity balance of the North Atlantic subpolar gyre in an eddy-resolving model

The circulation in the North Atlantic subpolar gyre is complex and strongly influenced by the topography. The gyre dynamics are traditionally understood as the result of a topographic Sverdrup balance, which corresponds to a first-order balance between the planetary vorticity advection, the bottom pressure torque, and the wind stress curl. However, these dynamics have been studied mostly with non-eddy-resolving models and a crude representation of the bottom topography. Here we revisit the barotropic vorticity balance of the North Atlantic subpolar gyre using a new eddy-resolving simulation (with a grid space of ≈2 km) with topography-following vertical coordinates to better represent the mesoscale turbulence and flow–topography interactions. Our findings highlight that, locally, there is a first-order balance between the bottom pressure torque and the nonlinear terms, albeit with a high degree of cancellation between them. However, balances integrated over different regions of the gyre – shelf, slope, and interior – still highlight the important role played by nonlinearities and bottom drag curls. In particular, the Sverdrup balance cannot describe the dynamics in the interior of the gyre. The main sources of cyclonic vorticity are nonlinear terms due to eddies generated along eastern boundary currents and time-mean nonlinear terms in the northwest corner. Our results suggest that a good representation of the mesoscale activity and a good positioning of mean currents are two important conditions for a better representation of the circulation in the North Atlantic subpolar gyre.

Predicting the 2015 North Atlantic Cold Blob

<p>Leading up to and during the summer of 2015 sea surface temperatures (SSTs) in the eastern North Atlantic Subpolar Gyre reached anomalously low values while in the subtropical gyre just to the SSTs were anomalously warm. Recent observation and modelling studies have found evidence showing that these SST anomalies can be linked to the heat wave experienced over Europe that summer.  The latest observation based data still shows anomalously cold temperatures, as well as the anomalously fresh conditions that went along the 2015 cold blob in the upper layers of the eastern North Atlantic Subpolar gyre.  A second heat wave over Europe occurred in the summer of 2018 where the SSTs reached another minimum value.  Therefore, being able to predict the development, enhancement and persistence of such an anomaly is essential for good seasonal and longer predictions.  At present several modelling systems have had difficulties in simulating/maintaining the 2015 cold blob. In this work we apply a novel initialization technique using anomalous initialization from a forced ocean simulation to simulate the 2015 cold blob.  Initial results show that the model is able to maintain the cold blob as well as have a strengthening of the cold blob, however, it has difficulties capturing the timing of this strengthening.</p>

Reconciling the role of the Labrador Sea overturning circulation in OSNAP and climate models

<p>The AMOC (Atlantic Meridional Overturning Circulation) is a key driver of climate change and variability. Since continuous, direct measurements of the overturning strength in the North Atlantic subpolar gyre (SPG) have been unavailable until recently, the understanding, based largely on climate models, is that the Labrador Sea has an important role in shaping the evolution of the AMOC. However, a recent high profile observational campaign (Overturning in the Subpolar North Atlantic, OSNAP) has called into question the importance of the Labrador Sea, and hence of the credibility of the AMOC representation in climate models. Here, we reconcile these viewpoints by comparing the OSNAP data with a new, high-resolution coupled climate model: HadGEM3-GC3.1-MM. Unlike many previous models, we find our model compares well to the OSNAP overturning observations. Furthermore, overturning variability across the eastern OSNAP section (OSNAP-E), and not in the Labrador Sea region, appears linked to AMOC variability further south. Labrador Sea densities are shown to be an important indicator of downstream AMOC variability, but these densities are driven by upstream variability across OSNAP-E rather than local processes in the Labrador Sea.</p>

Dynamical constraints on the choice of the North Atlantic subpolar gyre index

<p>The North Atlantic Subpolar Gyre (SPG) has been widely implicated as the source of large-scale changes in the subpolar marine environment. However, inconsistencies between different indices of SPG strength based on Sea Surface Height (SSH) observations have raised questions about the active role SPG strength and size play in determining water properties in the eastern subpolar North Atlantic (ENA). Here, by analyzing SSH-based and various other SPG-strength indices derived from observations and a global coupled model, we show that the interpretation of SPG strength-salinity relationship is dictated by the choice of the SPG index. Our results emphasize that SPG indices should be interpreted cautiously because they represent variability in different regions of the subpolar North Atlantic. Idealized Lagrangian trajectory experiments illustrate that zonal shifts of main current pathways in the ENA and meridional shifts of the North Atlantic Current (NAC) in the western intergyre region during strong and weak SPG circulation regimes are manifestations of variability in the size and strength of the SPG. Such shifts in advective pathways modulate the proportions of subpolar and subtropical water reaching the ENA, and thus impact salinity. Inconsistency among SPG indices arises due to the inability of some indices to capture the meridional shifts of the NAC in the western intergyre region. Overall, our results imply that salinity variability in the ENA is not exclusively sourced from the subtropics, instead the establishment of a dominant subpolar pathway also points to redistribution within the SPG.</p>