Experimental set-up

Data is taken from a five-member ensemble of simulations performed with the coupled climate model ECHAM5/MPI-OM32,33. The model ensemble uses historical forcing of greenhouse gases and aerosols between years 1950 and 2000. After year 200 it follows the SRES A1B scenario, with each member starting in 1950 with slightly different initial conditions. On January 1, 2000, an AMOC collapse is enforced by adding permanently a freshwater anomaly of 1 Sv (Sverdrup = 106 m3 s−1) between 50°N and 70°N in the Atlantic. All simulations end in 2100, hundred years after the onset of the AMOC collapse. Also, a control ensemble without freshwater forcing following the same SRES A1B scenario has been carried out and this ensemble has been used for comparison.

Heat uptake and surface temperature response

During the first fifteen years the greenhouse gas emissions from the SRES A1B scenario only slightly modify the response to the AMOC collapse, which dominates temperature and radiation changes. This response, without modification due to increased greenhouse gases, has been discussed before in other models26,27,34,35 and also using the present set of model runs analysed here22,23,24,36.

In the fifty years before the AMOC collapse (1950–2000), a TOA radiation imbalance develops with outgoing radiation being smaller than incoming radiation. The TOA radiation imbalance goes hand in hand with net ocean heat uptake6. In response to an AMOC collapse, net ocean heat uptake, that was already keeping the planet’s surface cooler than its equilibrium value associated with the increasing radiative forcing, is further enhanced (Fig. 1a).

Figure 1 The heat uptake response. (a) Time series of global mean heat uptake anomaly relative to the 1950–1960 average, for the A1B scenario run without (A1B) and with additional freshwater hosing (A1B+hosing) and the heat uptake anomaly associated with the shutdown of the AMOC (hosing), all smoothed with an 11-year Welch filter. (b) Same, but for the Northern Hemispheric mean. This figure has been created with the free Ferret software package developed by the National Oceanic and Atmospheric Administration (NOAA) and available from www.ferret.noaa.gov. Full size image

In response to the AMOC collapse heat transport across the equator decreases with 0.6 1015 W within 15 years. For the NH this decrease in heat transport is equivalent to a drop in energy transport of about 0.15 Wm−2 per year, implying a much stronger trend than that associated with the anthropogenic greenhouse forcing, where a doubling of CO 2 concentrations is equivalent to a radiative forcing of 3.7 Wm−2, implying a trend of at most 0.04 Wm−2 per year, even less in the first half of the 21st century. Although ocean heat convergence and radiative forcing are not completely comparable, these numbers give a correct indication of magnitudes and timescales involved. The drop in surface forcing of 2 Wm−2 in 15 years, associated with the decreased northward heat transport from the SH to the NH translates into a fast increase in ocean heat uptake of 2 Wm−2 when the NH is considered in isolation (Fig. 1b). The globally averaged increase in heat uptake is 0.7 Wm−2, indicating ocean heat loss in the SH. These changes are about 3 (global) to 10 (NH) times larger than the increase in radiatively forced increase in heat uptake over the same period.

The increase in greenhouse gases is too slow to compete with the response of an abrupt shutdown of the AMOC, which is reflected in the temperature response of the planet as a whole, although reduced ocean heat transport does not impose a net forcing on the globe. Figure 2a shows the evolution of GMT under an SRES A1B scenario, with and without the forced abrupt collapse of the AMOC. In the latter case, GMT immediately starts to decrease, reaching a minimum after 11 years that is 0.7 °C cooler compared to the year when the AMOC collapse started. When short fluctuations are filtered out, the minimum occurs somewhat later, after 15 years and mounts to 0.6 °C cooling. Without the forced AMOC collapse GMT increases by 0.3 °C after 15 years, indicating that the negative trend in radiative forcing associated with an AMOC collapse is roughly three times stronger than the upward trend due to increased greenhouse gases. After 15 years the negative temperature trend halts and after 20 years it reverses and becomes comparable with the trend observed in the SRES A1B without an AMOC shutdown, but with an almost constant offset.

Figure 2 Temperature response to hosing and radiative forcing. (a) Annual mean time series of global mean temperature in °C in the A1B climate change scenario with and without an AMOC collapse. (b) Temperature anomaly after 15 years since the onset of the AMOC collapse, smoothed with an 11-year Welch filter. (c) Same as (b) but for the temperature difference between years 95 and 15, showing the footprint of the response to radiative forcing. This figure has been created with the free Ferret software package developed by NOAA. Full size image

The length of the modelled hiatus period is governed by the time for increased greenhouse forcing to overcome a negative forcing of 0.7 Wm−2. For the SRES A1B scenario this period is roughly 40 years. If the AMOC collapse would develop slower (in the order of 40–50 years), GMT would have been characterised with a period of 40 years of zero temperature rise, after which global warming had resumed. If the AMOC collapse would have developed even slower, the increase in heat uptake would not have been able to compensate the increased radiative forcing and GMT would have been characterised with a positive trend, albeit weaker than in case of no AMOC shutdown. It should be noted that other ocean circulation changes may also affect net ocean heat uptake17,37,38.

Footprints in temperature. a) AMOC collapse

Both an AMOC collapse and radiative forcing are associated with net ocean heat uptake, but the first features global cooling, while the second features global warming. As a result, their spatial temperature patterns are very different (Fig. 2b,c). Apart from the sign difference, the temperature footprint linked to an AMOC collapse shows much more small-scale structure and is asymmetric across the equator (Fig. 2b). A very similar pattern (0–10 year average after the AMOC collapse) was found in Fig. 4 of ref. 31, although due to differences in experimental set-up the responses are not fully comparable.

Figure 4 Heat uptake response to radiative forcing. (a) Heat uptake anomaly in W m−2 between 95 and 15 years after the onset of the AMOC collapse, smoothed with an 11-year Welch filter, illustrating the imprint of radiative forcing versus Fig. 3 where the imprint of the AMOC collapse is shown. (b) Same as (a), but for the turbulent heat fluxes. (c) Zonally averaged turbulent heat flux anomaly relative to year 2015 as a function of time. (d) Same as (c) for longwave radiation. This figure has been created with the free Ferret software package developed by NOAA. Full size image

In response to reduced convective mixing in and over the Labrador Sea, SST and surface air temperature (SAT) anomalies develop which are immediately coupled to each other via air-sea heat exchange. These anomalies are spread eastward by prevailing winds and southward by ocean currents that circulate anti-clockwise in the subpolar gyre. After about 5 years a new minimum develops over the Greenland Sea, north of Iceland, where a second region of deep-water formation is affected. The declining inflow of the North Atlantic Drift into the Arctic induces a tongue of cold SST and SAT spreading northward east of Svalbard. After about 10 years ice-albedo feedbacks cause a temperature minimum east of Svalbard that will persist for a century. Also, a tongue of cold air progressing south-westwards from the North African coast can be seen. Through a wind-evaporation-SST (WES)-feedback39 enhanced trade winds cool the surface of the ocean, which on its turn enhances the trades. During this process the Intertropical Convergence Zone (ITCZ) shifts southward21,22,33,40 with in its wake a tongue of colder and drier air. A similar scenario may develop spontaneously, i.e. without freshwater hosing, when ocean-sea ice-atmospheric blocking feedbacks are strong enough to collapse Labrador Sea convection41.

b) Radiative forcing

The temperature footprint associated with the radiative response (Fig. 2c) is of larger scale and does not deviate significantly from the pattern associated with global warming without an induced AMOC collapse. Interestingly a small warming hole42 in the North Atlantic subpolar gyre is still present, suggesting that a slower adjustment of the North Atlantic circulation takes place after the AMOC abruptly collapsed.

The AMOC temperature-footprint implies that the hemispheric temperature difference reverses sign, with the SH becoming 1 °C warmer than the NH, instead of being more than 1 °C colder, supporting a previous conclusion that this temperature difference is almost entirely due to the AMOC43. Immediately after the collapse has been completed, however, the NH increases its temperature faster than the SH in response to anthropogenic warming, as evidenced by the radiative forcing-footprint and 40 years later the NH already has become warmer again than the SH. This response underscores that land-sea contrasts play an important role too in establishing the hemispheric temperature difference and that the equilibrium hemispheric temperature difference cannot be assessed from transient runs where the climate is not in equilibrium with the forcing, as it involves ocean heat uptake and radiative adjustment on a millennium time scale.

Footprints in heat uptake. a) AMOC collapse

The spatial patterns of heat uptake and SAT anomaly after 15 years from the onset of the AMOC shutdown are very similar (Figs 2b and 3a), almost each others mirror image. The temperature response is associated with the collapse of Labrador Sea and Greenland Sea convection. The added freshwater reduces the density of surface waters, preventing convective instability. Increasingly smaller amounts of relatively warm water are mixed upward by deep ocean convection and the associated convective heat loss to the overlying atmosphere wanes. This process is illustrated by comparing the heat uptake anomaly and climatology (Fig. 3a,b), clearly showing that north of 25°N increased ocean heat uptake occurs over regions of annual mean heat loss to the atmosphere. This implies that net ocean heat uptake due to an AMOC reduction is predominantly reduced ocean heat loss to the atmosphere. The temperature different between upper and deep ocean is maintained by a balance between downward heat transport by diffusive mixing and upward heat transport associated with convective overturning. It is the latter process that dominates the change in net ocean heat uptake, also in climate change simulations in which the AMOC does not collapse13.

Figure 3 The heat uptake response after an AMOC collapse. (a) Heat uptake anomaly in W m−2 after 15 years since the onset of the AMOC collapse, smoothed with an 11-year Welch filter. (b) Heat uptake climatology for years 1950–2000. (c) Same as a) for the anomalous turbulent heat flux. Positive values are into the ocean. (d) Global mean heat uptake anomaly and its separate contributions relative to year 2000 for the A1B scenario with additional freshwater hosing, smoothed with an 11-year Welch filter. This figure has been created with the free Ferret software package developed by NOAA. Full size image

Despite the strong similarities between heat uptake and surface temperature, changes in longwave radiation hardly affect the pattern of heat uptake associated with an AMOC collapse. Also changes in net shortwave radiation at the surface cannot compete with the changed turbulent heat fluxes (Fig. 3d). The total heat uptake pattern is completely dominated by reduced turbulent heat loss (Fig. 3c), mostly accomplished by the latent heat associated with reduced evaporation. Increased sea-ice cover implies colder and drier air with less evaporation. As a result, the deep ocean heats up and the atmosphere and surface ocean are cooled.

b) Radiative forcing

Global warming reinforces the heat uptake anomaly and the globally averaged heat uptake anomaly increases further after the AMOC collapse is completed (15 years). The small scales of this pattern are still dominated by reduced turbulent heat loss, especially over the North Atlantic subpolar gyre and associated with further ocean adjustment to the collapsing AMOC (Fig. 4a,b). Globally integrated, however, turbulent heat loss increases after 15 years due to global warming, counteracting the increase in ocean heat uptake. This occurs especially at low and polar latitudes, while in mid latitudes and subpolar regions the turbulent heat loss continues to decrease. When the fast feedbacks associated with the collapse of the AMOC no longer prevail over the slower response to enhanced greenhouse gas concentrations, changes in longwave radiation become the dominant factor in enhancing ocean heat uptake (Fig. 4c,d). As can be seen from Fig. 4d this especially hold for the tropics, where the oceans take up most of the increase in back radiation due to enhanced greenhouse gas concentrations.

The response at TOA. a) AMOC collapse

During the AMOC collapse a downward radiation anomaly develops, governed by reduced outgoing longwave radiation (OLR) and only partly cancelled by decreased absorbed solar radiation (ASR) (Fig. 5a,b). At the surface both net upward shortwave and longwave radiation increase, due to higher surface albedo and reduced downward longwave radiation associated with a weaker greenhouse effect, despite increasing CO 2 concentrations, because the atmosphere initially dries and contains less water vapour in response to the reduced evaporation (Fig. 5c). Although water vapour increases in the SH, it decreases much more in the NH.

Figure 5 The greenhouse effect. (a) Top of atmosphere global mean radiation anomalies in W m−2 relative to year 2000, smoothed with an 11-year Welch filter. (b) Same as a) but decomposed into surface radiation anomalies, anomalous radiation emitted by the troposphere and top of atmosphere radiation anomalies. (c) Water vapour content anomaly in kg m−2 after 15 years from the onset of an AMOC collapse, smoothed with an 11-year Welch filter. (d) Same as (c) but for the difference between years 95 and year 15. This figure has been created with the free Ferret software package developed by NOAA. Full size image

b) Radiative forcing

After 15 years, however, this scenario drastically changes: water vapour increases everywhere in response to the increasing CO 2 concentrations. Eventually the NH negative water vapour anomaly resulting from an AMOC collapse is obliterated (Fig. 5d). When the response to increased CO 2 overrules the fast atmospheric feedback to an AMOC collapse, enhanced longwave back radiation and decreasing surface albedo cause ASR and OLR to increase at TOA. They counteract the trends induced by an AMOC collapse, but in such a way that the net downward radiation anomaly keeps increasing (Fig. 5a). Most interesting is that the net downward TOA anomaly associated with ocean-induced surface cooling and the one associated with increased radiative forcing have such different characteristics. In case of cooling due to changed dynamics (which occurs by internal variability) the net downward TOA flux is accomplished by reduced OLR, partially counteracted by reduced absorbed shortwave radiation (ASR). In case of increased radiative forcing the net downward TOA flux is accomplished by larger ASR, partially counteracted by increased OLR44. This characteristic also features the CMIP5 ensemble-mean response to historical and RCP forcing as can be evidenced with the use of the KNMI climate explorer45.

Footprints in OLR and ASR

Figure 6 shows the spatial pattern of the net TOA downward radiation anomaly, OLR and ASR for the AMOC shutdown (Fig. 6a,c,e) and the radiative response (Fig. 6b,d,f). A comparison of the patterns of Fig. 6 with observations of recent TOA radiation anomalies7 does not lead to attribution of the recent climate hiatus to one single cause. Enhanced radiative forcing dominates the pattern of the downward TOA anomaly, partly associated with the recovery from the Pinatubo eruption. There are also signals, especially the dipole over the South Atlantic in ASR and OLR that are consistent with a reduced AMOC. Many aspects of the observed anomalies, however, cannot be related to each of the patterns shown in Fig. 6, indicating multiple drivers, possibly both in radiative forcing8,9 as well in ocean-atmosphere dynamics17,37,38,46,47. The globally averaged time series of TOA radiation48, however, indicates a role for unforced ocean heat-uptake changes. Firstly, the observed increased TOA downward flux, when being a response to radiative forcing only, would have been associated with a surge in global mean temperature instead of a hiatus. In addition there are two periods where the change in OLR clearly deviates from counteracting changes in ASR, namely 1997–2003 and from 2007 onwards (Fig. 4 from ref. 48).Changes in TOA radiative energy flux correlate with changes in ocean heat content, but are weaker and reflect a smoother and damped image of the implied ocean heat uptake48 and estimates from an ocean reanalysis10. The periods where OLR deviates from the expected response to radiative forcing indeed coincide with periods where ocean heat-content strongly increases48. For an unequivocal attribution, similar patterns as shown in Fig. 6 have to be calculated for different ocean/atmosphere processes that give rise to ocean heat uptake changes, such as changes in ENSO49. Also, the response patterns to volcanic eruptions and pattern changes in aerosol emissions would have to be assessed. In addition, for weaker internal variations in heat uptake the relation between TOA anomalies and anomalous upper ocean heat content change was found to consist of OLR and ASR anomalies of similar sign, with OLR dominating but lagging ASR14. This underscores the fact that the observed changes in TOA probably consist of a combination of processes with each its own footprint and that we need to establish many more of these footprints for attribution purposes when many processes are acting in concert.