It is well known that glacial periods were punctuated by abrupt climate changes, with large impacts on air temperature, precipitation, and ocean circulation across the globe. However, the long-held idea that freshwater forcing, caused by massive iceberg discharges, was the driving force behind these changes has been questioned in recent years. This throws into doubt the abundant literature on modelling abrupt climate change through “hosing” experiments, whereby the Atlantic Meridional Overturning Circulation (AMOC) is interrupted by an injection of freshwater to the North Atlantic: if some, or all, abrupt climate change was not driven by freshwater input, could its character have been very different than the typical hosed experiments? Here, we describe spontaneous, unhosed oscillations in AMOC strength that occur in a global coupled ocean–atmosphere model when integrated under a particular background climate state. We compare these unhosed oscillations to hosed oscillations under a range of background climate states in order to examine how the global imprint of AMOC variations depends on whether or not they result from external freshwater input. Our comparison includes surface air temperature, precipitation, dissolved oxygen concentrations in the intermediate-depth ocean, and marine export production. The results show that the background climate state has a significant impact on the character of the freshwater-forced AMOC interruptions in this model, with particularly marked variations in tropical precipitation and in the North Pacific circulation. Despite these differences, the first-order patterns of response to AMOC interruptions are quite consistent among all simulations, implying that the ocean–sea ice–atmosphere dynamics associated with an AMOC weakening dominate the global response, regardless of whether or not freshwater input is the cause. Nonetheless, freshwater addition leads to a more complete shutdown of the AMOC than occurs in the unhosed oscillations, with amplified global impacts, evocative of Heinrich stadials. In addition, freshwater inputs can directly impact the strength of other polar haloclines, particularly that of the Southern Ocean, to which freshwater can be transported relatively quickly after injection in the North Atlantic.
“Abrupt” climate changes were initially identified as decadal–centennial
temperature changes in Greenland ice deposited during the last ice age
However, other model simulations have shown that spontaneous changes in the
AMOC can occur in the absence of freshwater inputs
What is more, the arrival of ice-rafted detritus does not seem to precede
changes in the AMOC, where the two are recorded together. An analysis of
Heinrich event 2 in the NW Atlantic showed that weakening of the AMOC
preceded the widespread deposition of ice-rafted detritus by approximately 2 kyr
In order to explore these questions, we make use of a large number of long water-hosing simulations with CM2Mc, a state-of-the-art Earth system model, to show how the global response to hosing varies between a preindustrial and glacial background state, as well as under different orbital forcings. In addition, we take advantage of the fact that the same model exhibits previously undescribed spontaneous AMOC interruptions and resumptions, which appear very similar to stadial-interstadial variability, to reveal what aspects of the abrupt changes are a result of the hosing itself rather than consequences of the changing AMOC.
Time series of AMOC for the matrix of simulations with prescribed
atmospheric
The simulations shown here use the coupled ocean–atmosphere model CM2Mc, as
described in
The experiments shown here vary in terms of the prescribed atmospheric
CO
North Atlantic climate metrics for all simulations. The left column
shows four simulations under the glacial boundary conditions (LGM ice sheets
and bathymetry with closed Bering Strait; 180 ppm CO
Atlantic meridional overturning stream functions in strong and weak states. All plots show 100-year averages. In glacial and preindustrial hosing simulations, weak AMOC state is defined by averaging the last century of hosing (model years 1901–2000) and strong AMOC state is defined by averaging the same years from the corresponding control simulation (to correct for potential drift). In the unhosed simulation, the weak AMOC state is defined by averaging model years 7501–7600 and the strong AMOC state is defined by averaging model years 7901–8000. For the top two rows, the stream functions are averaged over the corresponding four sets of orbital configurations. The bottom row shows the stream functions for the unhosed simulation. Contours in sverdrups.
Freshwater forcing (“hosing”) was applied to four simulations run under
“preindustrial” conditions with the four possible combinations of obliquity
and precession, and a corresponding four under “glacial” conditions. For each
of these eight hosing simulations, the model was initialized from a previous
preindustrial or LGM state, run for 1000 years, a freshwater hosing was
applied for 1000 years, and then it was run for a further 1000 years with the
hosing off. Control simulations (without hosing) were also run for the same
length of time as the hosing to allow drift correction. During hosing,
freshwater was added in the North Atlantic by overriding the land-to-ocean
ice calving flux with a preindustrial annual mean plus an additional 0.2 Sv
evenly distributed in a rectangle bounded by 40 to 60
In addition, we show results from a simulation for which hosing was not
applied, but which exhibits spontaneous oscillations in the AMOC reminiscent
of D-O events. This “unhosed” simulation was conducted under glacial
CO
The final experiment is a modification of the unhosed simulation, in which the 0.2 Sv hosing perturbation is applied to the North Atlantic during a strong-AMOC interval. This final experiment provides an explicit test of the effect of a hosed vs. an unhosed AMOC weakening.
The eight hosed simulations show a number of common features in the North
Atlantic, which vary as a function of boundary conditions (Fig. 2). Within
the first two centuries of hosing, the Greenland temperature rapidly drops by
8–15
However, Fig. 2 also shows the same metrics of North Atlantic variability
for the unhosed simulation. Under the low CO
The spontaneous millennial AMOC oscillations that occur in ocean circulation
models can generally be described as “deep decoupling oscillations”
The changes in AMOC, sea ice, and surface salinity in the unhosed simulation
(Fig. 2) are consistent with this general scenario. It is remarkable that the
magnitude of the salinity change in the unforced simulations is of the order
of more than 1 PSU, nearly half as much as for the hosing simulation under
the same CO
Stadial winter mixed-layer depth anomaly. The top two panels show the difference in winter mixed-layer depth between weak and strong AMOC states (as defined in Fig. 3) under preindustrial and glacial boundary conditions, averaged between the four sets of orbital configurations (see Fig. S6 to view all eight hosing simulations individually). The bottom panel shows the winter mixed-layer depth between weak and strong AMOC states (as defined in Fig. 3) in the unhosed simulation. Black and red contours show the sea-ice edge for the weak AMOC and strong AMOC states, respectively. Sea-ice edge is defined as > 30 % of annually averaged ice concentration. Shading in m.
Thermohaline oscillations in the unhosed simulation.
Stadial surface air temperature anomaly. The top two panels show the
surface air temperature difference between weak and strong AMOC states (as
defined in Fig. 3) under preindustrial and glacial boundary conditions,
averaged between the four sets of orbital configurations (see Fig. S2 to view
all eight hosing simulations individually). Red contours show the standard
deviation between the four sets of orbital configurations at 1
Next, we show how the global consequences of changes in the AMOC depend on the background climate state, as well as the question of whether or not they are forced by freshwater addition. We do so by exploring a few key atmospheric and oceanic variables that have well-documented responses to abrupt climate change as recorded by palaeoclimate records.
Abrupt climate change was initially identified in ice core proxy records of
atmospheric temperature, and subsequently extended to temperature variations
recorded in multiple proxies from around the world
One notable difference in the spatial patterns is the temperature change in
the N Pacific, including the western margin of Canada and southern Alaska.
The temperature here is quite sensitive to the degree of northward transport
of warm ocean water from the subtropical gyre, which is quite variable
between simulations. All hosed simulations develop a Pacific Meridional
Overturning Circulation (PMOC) when the AMOC weakens, previously described in
models as an Atlantic–Pacific seesaw
The strength of the bipolar seesaw also varies significantly as a function of background climate state, due to differences in both the Southern Hemisphere and the North Atlantic. Stronger southern warming occurs under glacial conditions and with the combination of high obliquity and strong boreal seasons (Fig. S2). The temperature response to hosing in the North Atlantic is particularly dependant on obliquity in glacial simulations because of its influence on the initial sea-ice extent (Fig. 2, third row, compare initial sea-ice area under glacial and preindustrial).
In general, the temperature response in the unhosed simulation is very
similar to the ensemble means of the hosed simulations, with the exception
that it would appear to have a relatively weak southern warming. However, the
weak southern warming is partly due to the fact that southern warmings
develop slowly, and the time between the unhosed “stadial” and “interstadial”
is relatively short (
Changes in precipitation during abrupt climate changes have also been
well documented in speleothems and marine sediment records
In fact, many aspects of the precipitation changes vary more as a function of
background climate state, including orbital configuration, than they do
between hosed and unhosed (Fig. S4). Thus, both the
mean state of tropical precipitation
Stadial precipitation anomaly. The top two panels show the
precipitation difference between weak and strong AMOC states (as defined in
Fig. 3) under preindustrial and glacial boundary conditions, averaged between
the four sets of orbital configurations (see Fig. S4 to view all eight hosing
simulations individually). Red contours show the standard deviation between
the four sets of orbital configurations at
5
The observed footprint of abrupt climate change also extends to ocean
biogeochemistry, with pronounced and well-documented changes in both
dissolved oxygen concentrations and export production. Prior work has shown
that many aspects of the observed oxygenation changes
As shown by Fig. 8, our model simulations produce consistent changes in
intermediate-depth oxygen during AMOC weakening, hosed or unhosed, that agree
well with the bipolar seesaw-mode changes in oxygenation extracted from
sediment proxy records of the last deglaciation
Stadial intermediate-depth (400–1100 m) oxygen concentration
anomaly. The top two panels show the oxygen concentration difference between
weak and strong AMOC states (as defined in Fig. 3) under preindustrial and
glacial boundary conditions, averaged between the four sets of orbital
configurations (see Fig. S7 to view all eight hosing simulations individually).
Red contours show the standard deviation between the four sets of orbital
configurations at 1
The relative changes in export production, shown in Fig. 9, are locally
quite large (in excess of 100 %) but have weaker regional patterns that are
less consistent between simulations. The most consistent strong features are
a reduction of export in the northern North Atlantic, the western tropical
North Atlantic, and the southern margin of the Indo-Pacific subtropical gyre,
and an increase in export off of NW Africa, to the west of California, and in
the high-latitude Southern Ocean. Regions that do not always respond
consistently are the subarctic Pacific, which depends significantly on
whether or not a PMOC develops, and the northern Arabian Sea, which shows an
increase in export under preindustrial but not glacial hosing. It is
important to point out that, because the model does not resolve detailed
features of coastal upwellings, it is probably missing important changes. For
example, the fact that decreases in primary production are not simulated on
the Baja California margin during hosings, as reconstructed during stadials
The simulated changes in export production show an overall decrease globally,
consistent with prior results
(
The analyses above suggest that most of the large-scale responses of climate and ocean biogeochemistry to an AMOC disruption are similar regardless of whether the disruption is forced through hosing or as a spontaneous result of internal model dynamics. In order to further test this apparent insensitivity to the cause of AMOC weakening, we applied a freshwater forcing during a strong AMOC interval of the unhosed simulation, forcing the model into a weak AMOC state earlier than in the standard unhosed case. This experiment provides a direct comparison between an unforced and a freshwater-forced AMOC reduction. The freshwater forcing weakens the AMOC transport by an amount that exceeds the unhosed weakening (Fig. 2, black line in third column), and maintains this weakened state over the full 1000-year simulation, whereas the unhosed simulation returns to the strong-AMOC state after about 800 years. By comparing both the forced and unforced simulations after 800 years of stadial, we can estimate the differences caused by the freshwater itself.
Stadial export production anomaly. The top two panels show the ratio of organic matter export at 100 m between weak and strong AMOC states (as defined in Fig. 3) under preindustrial and glacial boundary conditions, averaged between the four sets of orbital configurations (see Fig. S8 to view all eight hosing simulations individually). Green contours show the standard deviation between the four sets of orbital configurations at 50 % intervals. The bottom panel shows the ratio of export between weak and strong AMOC states (as defined in Fig. 3) in the unhosed simulation. Shading and contours in %.
Stadial intermediate-depth (400–1100 m) ideal age anomaly. The top two panels show the age difference between weak and strong AMOC states (as defined in Fig. 3) under preindustrial and glacial boundary conditions, averaged between the four sets of orbital configurations (see Fig. S9 to view all eight hosing simulations individually). Red contours show the standard deviation between the four sets of orbital configurations at 30-year intervals. The bottom panel shows the age difference between weak and strong AMOC states (as defined in Fig. 3) in the unhosed simulation. Shading and contours in years.
The changes caused by the freshwater addition are shown in Fig. 11, compared with the unhosed stadial/interstadial variability. The change in surface air temperatures differs little as a result of the hosing, with significant contrasts only in the North Atlantic, North Pacific, and high-latitude Southern Ocean. Precipitation shows a much larger response under hosing, which is generally an amplification of the unhosed trends, though the impact over western Indonesia is a uniformly strong drying rather than the mixed response of the unhosed case. Dissolved oxygen also shows an amplification of the unhosed trends when hosed, while export production shows changes mainly in the N Atlantic and NE Pacific. The general amplification of changes can be understood by the fact that the freshwater-forced simulation has a weaker AMOC, and more extensive North Atlantic sea-ice coverage, leading to lower temperatures in the NE Atlantic and a correspondingly greater response in atmospheric circulation that amplifies most features of the weak-AMOC state. In addition, there is a significant strengthening of the PMOC in the freshwater-forced simulation, which explains most of the N Pacific changes. (We note that the Bering Strait is open in this simulation, allowing communication with the North Atlantic across the Arctic Basin.)
Impacts of hosing the unhosed. Left column: differences between weak
and strong AMOC states for the unhosed simulation as shown in Figs. 6, 7, 8,
and 9. Right column: differences between the last century (model
years 6151–6250) of the 800-year hosed–unhosed stadial and the corresponding
unhosed stadial. Scale and units are the same as in Figs. 6, 7, 8, and 9:
temperature in
The fact that precipitation responds most strongly suggests that, among the
metrics examined here, it has the greatest sensitivity to the intensification
of the unhosed stadial by hosing. The sensitivity of precipitation could
reflect a fairly direct link between the northern sea-ice edge and/or North
Atlantic cooling, latitudinal sea surface temperature gradients, and the
Hadley circulation
Apart from the amplification of the general trends, we note one distinct
additional feature: the Southern Hemisphere warming is decreased under
hosing, weakening the bipolar seesaw. This feature of the Antarctic response
appears to reflect the transport of freshwater from the North Atlantic to the
Southern Ocean, in addition to the small increase in calving flux from the
Antarctic margin (Fig. S5), so that the Southern Ocean surface
is freshened. The addition of freshwater strengthens the Southern Ocean
halocline, reducing ocean heat release and keeping a larger mantle of sea ice
around Antarctica. Thus, the cooler Southern Ocean does not reflect a
different response to the AMOC weakening itself but rather a secondary
effect of the freshwater addition through its direct influence on the
vertical density structure of the Southern Ocean. We note that
Our unhosed model simulation adds to a small but growing subset of complex
3-D ocean–atmosphere model simulations exhibiting unforced
oscillations similar to the abrupt climate changes in Dansgaard–Oeschger
cycles. Under a particular set of boundary conditions (low
CO
The unhosed simulation was integrated under perfectly stable boundary
conditions, with no variability in external factors such as solar output,
aerosols, or freshwater runoff. In nature, such additional variability might
have made it easier for spontaneous AMOC variations to occur, as long as the
AMOC was relatively weak due to the background climate state, leading to AMOC
oscillations under a wider range of background conditions. Thus, large
volcanic eruptions
As previously suggested, melting of floating ice shelves due to subsurface
ocean warming could provide an important feedback to an initial AMOC
weakening, accelerating ice sheet mass loss due to their buttressing effect
on upstream ice, and adding freshwater that would push the AMOC into a very
weak mode
Although they only occur in our model under an unrealistic combination of boundary conditions, the spontaneous nature of the unhosed oscillations allows a powerful comparison to be made with the more typical freshwater-hosed simulations of AMOC weakening. When hosed and unhosed simulations are compared, the general features of the atmospheric and oceanic responses are remarkably robust. Background climate state introduces as much variability into the the response as the contrast between spontaneous and forced AMOC weakening. These robust features are therefore likely to reflect consistent dynamical changes related to the AMOC interruption and its coupling with sea ice and atmospheric changes, independent of the ultimate cause of the AMOC interruption. Some aspects of the climate system show greater sensitivity to the magnitude of AMOC weakening than others. Of the variables examined here, tropical precipitation showed the strongest sensitivity to an intensification of AMOC interruption under additional hosing.
An important difference between hosed and unhosed simulations lay in the
direct impact of freshwater on ocean density structure. Polar haloclines are
very sensitive to freshwater input, which can stratify or destratify them
depending on the depth at which freshwater is injected. In our simulations,
an important consequence of freshwater addition is stratification of the
Southern Ocean, which raises an intriguing conflict with observational
evidence that Southern Ocean ventilation was enhanced when the AMOC was
weakened, at least during the last deglaciation
The CM2Mc model code is publicly available from GFDL, and the model output used here is available from Eric Galbraith (eric.d.galbraith@gmail.com) by request.
We thank the Canadian Foundation for Innovation (CFI) and an allocation to the Scinet supercomputing facility (University of Toronto), made through Compute Canada, for providing the computational resources. We are grateful to Olivier Arzel, the three anonymous reviewers, and editor Gerrit Lohmann for constructive comments that greatly improved the manuscript. Edited by: G. Lohmann Reviewed by: three anonymous referees