Timing and structure of early-Holocene climate anomalies inferred from north Chinese stalagmite records

In this paper, a new decadal resolution stalagmite δ18O record covering 10.4–6.5 ka BP from Kulishu cave in Beijing, north China is presented in combination with the published stalagmite δ18O record covering 10.4–14.0 ka BP in the same cave. Five significant monsoon collapses were identified around 11.5, 11.0, 10.0, 9.4, and 8.2 ka BP as well as three smaller ones around 10.3, 9.0, and 8.6 ka BP. The weak monsoon episodes around 8.6 and 8.2 ka BP form the two-step structure of the 8.2 ka event. All monsoon collapses, coeval with the cooling in northern high-latitude records, are correlated with Lakes Agassiz-Ojibway outbursts. Thus, our data support the idea of freshwater forcing of abrupt climate anomalies during the early Holocene. Nevertheless, the decreased irradiance together with freshwater outburst may account for the 9.2/9.3 ka event, which is expressed more significantly in low-latitude records.

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.

Timing of iceberg scours and massive ice-rafting events in the subtropical North Atlantic

High resolution seafloor mapping shows extraordinary evidence that massive (>300 m thick) icebergs once drifted >5,000 km south along the eastern United States, with >700 iceberg scours now identified south of Cape Hatteras. Here we report on sediment cores collected from several buried scours that show multiple plow marks align with Heinrich Event 3 (H3), ~31,000 years ago. Numerical glacial iceberg simulations indicate that the transport of icebergs to these sites occurs during massive, but short-lived, periods of elevated meltwater discharge. Transport of icebergs to the subtropics, away from deep water formation sites, may explain why H3 was associated with only a modest increase in ice-rafting across the subpolar North Atlantic, and implies a complex relationship between freshwater forcing and climate change. Stratigraphy from subbottom data across the scour marks shows there are additional features that are both older and younger, and may align with other periods of elevated meltwater discharge.

Internal salt content: a useful framework for understanding the oceanic branch of the water cycle

The global water cycle is dominated by an atmospheric branch which transfers fresh water away from subtropical regions and an oceanic branch which returns that fresh water from subpolar and tropical regions. Salt content is commonly used to understand the oceanic branch because surface freshwater fluxes leave an imprint on ocean salinity. However, freshwater fluxes do not actually change the amount of salt in the ocean and – in the mean – no salt is transported meridionally by ocean circulation. To study the processes which determine ocean salinity we introduce a new variable: “internal salt” and its counterpart “internal fresh water”. Precise budgets for internal salt in salinity coordinates relate meridional and diahaline transport to surface freshwater forcing, ocean circulation and mixing, and reveal the pathway of fresh water in the ocean. We apply this framework to a 1° global ocean model. We find that in order for fresh water to be exported from the ocean’s tropical and subpolar regions to the subtropics, salt must be mixed across the salinity surfaces that bound those regions. In the tropics, this mixing is achieved by parameterized vertical mixing, along-isopycnal mixing, and numerical mixing associated with truncation errors in the model’s advection scheme, while along-isopycnal mixing dominates at high latitudes. We analyze the internal freshwater budgets of the Indo-Pacific and Atlantic Ocean basins and identify the transport pathways between them which redistribute fresh water added through precipitation, balancing asymmetries in freshwater forcing between the basins.

River Freshwater Contribution in Operational Ocean Models along the European Atlantic Façade: Impact of a New River Discharge Forcing Data on the CMEMS IBI Regional Model Solution

River freshwater contribution in the European Atlantic margin and its influence on the sea salinity field are analyzed. The impacts of using a new river discharge database as part of the freshwater forcing in a regional ocean model are assessed. Ocean model scenarios, based on the CMEMS (Copernicus Marine Environment Monitoring Service) operational IBI-MFC (Iberia Biscay Ireland Monitoring Forecasting Centre) model set-up, are run to test different (observed, modeled and climatological) river and coastal freshwater forcing configurations throughout 2018. The modelled salinity fields are validated, using as a reference all known available in-situ observational data sources. The IBI model application is proven to adequately simulate the regional salinity, and the scenarios showcase the effects of varying imposed river outflows. Some model improvement is achieved using the new forcing (i.e., better capture of salinity variability and more realistic simulation of baroclinic frontal structures linked to coastal and river freshwater buoyancy plumes). Major impacts are identified in areas with bigger river discharges (i.e., the French shelf or the northwestern Iberian coast). Instead, the Portuguese shelf or the Gulf of Cadiz are less impacted by changes in the imposed river inflows, and other dynamical factors in these areas play a major role in the configuration of the regional salinity.

Feedback processes modulating the sensitivity of Atlantic thermohaline circulation to freshwater forcing timescales

Paleo proxy records indicate that abrupt changes in thermohaline circulation (THC) were induced by rapid meltwater discharge from retreating ice sheets. Such abrupt changes in the THC have been understood as a hysteresis behavior of nonlinear system. Previous studies, however, primarily focused on a near-static hysteresis under fixed or slowly varying freshwater forcing (FWF), reflecting the equilibrated response of the THC. This study aims to improve the current understanding of transient THC responses under rapidly varying forcing and its dependency on forcing timescales. The results simulated by an Earth system model suggest that the bifurcation is delayed as the forcing timescale is shorter, causing the Atlantic meridional overturning circulation collapse (recovery) to occur at higher (lower) FWF values. The delayed shutdown/recovery occurs because bifurcation is determined not by the FWF value at the time but by the total amount of freshwater remaining over the THC convection region. The remaining freshwater amount is primarily determined by the forcing accumulation (i.e., time-integrated FWF), which is modulated by the freshwater/salt advection by ocean circulations and freshwater flux by the atmospheric hydrological cycle. In general, the latter is overwhelmed by the former. When the forced freshwater amount is the same, the modulation effect is stronger under slowly varying forcing because more time is provided for the feedback processes.

Magnitude of the 8.2 ka event freshwater forcing based on stable isotope modelling and comparison to future Greenland melting

The northern hemisphere experienced an abrupt cold event ~ 8200 years ago (the 8.2 ka event) that was triggered by the release of meltwater into the Labrador Sea, and resulting in a weakening of the poleward oceanic heat transport. Although this event has been considered a possible analogue for future ocean circulation changes due to the projected Greenland Ice Sheet (GIS) melting, large uncertainties in the amount and rate of freshwater released during the 8.2 ka event make such a comparison difficult. In this study, we compare sea surface temperatures and oxygen isotope ratios from 28 isotope-enabled model simulations with 35 paleoproxy records to constrain the meltwater released during the 8.2 ka event. Our results suggest that a combination of 5.3 m of meltwater in sea level rise equivalent (SLR) released over a thousand years, with a short intensification over ~ 130 years (an additional 2.2 m of equivalent SLR) due to routing of the Canadian river discharge, best reproduces the proxy anomalies. Our estimate is of the same order of magnitude as projected future GIS melting rates under the high emission scenario RCP8.5.

Pathways and timescales of connectivity around the Antarctic continental shelf

<p><span xml:lang="EN-US" data-contrast="auto"><span>The Antarctic margin is surrounded by two westward flowing currents: the Antarctic Slope Current and the Antarctic Coastal Current. The former influences key processes near the Antarctic margin by regulating the flow of heat and nutrients onto and off the continental shelf, while together they </span></span><span xml:lang="EN-US" data-contrast="auto"><span>advect</span></span><span xml:lang="EN-US" data-contrast="auto"><span> nutrients, biological organisms, and temperature and salinity anomalies around the coastline, providing a connective link between different shelf regions. However, the extent to which these currents transport water from one sector of the continental shelf to another, and the timescales over which this occurs, remain poorly understood. Concern that crucial water formation sites around the Antarctic coastline could respond to non-local freshwater forcing </span></span><span><span xml:lang="EN-US" data-contrast="auto"><span>from ice shel</span></span></span><span><span xml:lang="EN-US" data-contrast="auto"><span>f meltwater</span></span></span> <span xml:lang="EN-US" data-contrast="auto"><span>motivates a more thorough understanding of zonal connectivity around Antarctica. In this study, we use daily velocity fields from a global high-resolution ocean-sea ice model, combined with the <span>Lagrangian</span> tracking software Parcels, to investigate the pathways and timescales connecting different regions of the Antarctic continental shelf<span> with a view to understanding</span><span> the timescales of meltwater transport around the continent</span>. Virtual particles are released over the continental shelf, poleward of the 1000 <span>metre</span> isobath, and are tracked for 20 years. Our results show a strong seasonal cycle connecting different sectors of the Antarctic continent, with more particles arriving further downstream during winter than during summer months. Strong advective links exist between West Antarctica and the Ross Sea while shelf geometry in some other regions acts as barriers to transport. We also highlight the varying importance of the Antarctic Slope Current and Antarctic Coastal Current in connecting different sectors of the coastline. Our results help to improve our understanding of circum-Antarctic connectivity <span>and the timescales </span><span>of meltwater transport from source regions to downstream </span><span>shelf locations. </span><span>Further</span><span>more, t</span><span>he timescales and pathways we </span><span>present </span><span>p</span>rovide a baseline from which to assess long-term changes in Antarctic coastal circulation due to local and remote forcing.<br></span></span></p>

Antarctic Peninsula warming triggers enhanced basal melt rates throughout West Antarctica

<p>The acceleration of ice-shelf basal melt rates throughout West Antarctica, as well as their potential to destabilize the ice sheets they buttress, is well documented.  Yet, the mechanisms that determine both trends and variability of these melt rates remain uncertain.  Explanations for the intensification of melting have largely focused on local processes in seas surrounding the ice shelves, including variations in wind stress over the continental slope and shelf.  Here, we show that non-local freshwater forcing, propagated between shelf seas by the Antarctic Coastal Current (AACC), can have a significant impact on ice-shelf melt rates.  </p><p>We present results from a suite of high-resolution (~3-km) numerical simulations of the ocean circulation in West Antarctica that includes a dynamic sea-ice field, ice-shelf cavities and forcing from ice shelf-ocean interactions.  Motivated by persistent warming at the northern Antarctic Peninsula since the 1950’s, freshwater perturbations are applied to the West Antarctic Peninsula.  This leads to a strengthening of the AACC and a westward propagation of the freshwater signal.  Critically, basal melt rates increase throughout the WAP, Bellingshausen and Amundsen Seas in response to this perturbation.  The freshwater anomalies stratify the ocean surface near the coast, enhancing lateral heat fluxes that lead to greater ice-shelf melt rates.  A suite of sensitivity studies show that changes in meltrates are linearly proportional to the magnitude of the freshwater anomaly, changing by as much as 30% for realistic perturbations, but are relatively insensitive to the distribution of the perturbation across the WAP shelf.  These results indicate that glacial run-off on the Antarctic Peninsula, one of the first signatures of a warming climate in Antarctica, could be a key trigger for increased melt rates in the Amundsen and Bellingshausen Seas.</p>