Links to the previous rounds: 1, 2 and 3.

Gabriel Oxenstierna

The battle of climate hypotheses, Round 4:
The Green-house Gas Forcer vs. The Winter Gatekeeper

For the first time, the IPCC’s doctrine of CO₂ as a ‘control knob’ in our climate faces a serious challenger in the form of a comprehensive hypothesis about what drives climate and its shifts. – This article is the fourth in a series evaluating this new hypothesis of natural climate variability, the so-called Winter Gate-keeper Hypothesis [WGH]. The basic concept of WGH is that natural variations in the polewards meridional transport of heat and moisture controls climate change.[1][2]

Alarmists, including the IPCC, love to speculate about tipping points. One of their favourites is that the Atlantic meridional overturning circulation (AMOC), will shut down due to our emissions of greenhouse gases.[3][4]

The AMOC is an essential part of the global heat transport system. This is how bitterly cold the climate could get in parts of the North if it collapsed:[5]

Figure 1: Simulated outcome of an AMOC shutdown. Everything warms up significantly, except the North Atlantic and Northern Europe. Source: [7]

A collapsed AMOC would also lead to a host of knock-on effects due to ‘tele-connections’.We are warned that a weakening of AMOC would have severe impacts and increase the risk of “cascading problems” for other major Earth systems, “such as the Antarctic ice sheet, tropical monsoon systems and Amazon rainforest”.[6] Climate crise effects would occur in many other areas as well. Stormier weather, more floods, collapsed plankton production, and widespread oxygen death in the oceans (anoxia) are forecast, says the IPCC.[3] The issue of AMOC’s whereabouts is therefore of great interest.

The cold blob

The AMOC is like a giant conveyor belt of energy. It is primarily driven by the Denmark Strait overflow: warm and salty water from the northward extension of the Gulf Stream meets the cold ocean currents off south-east Greenland. There they sink to the bottom as a huge ‘waterfall’, with a drop of more than 3 000 meters (at the threshold of the Denmark Strait). The heavy, cold bottom water then flows south to the Southern Ocean, before turning north again.

A key part of the AMOC is the convection zone south of Iceland and Greenland, where we find the subpolar Atlantic gyre. The ocean current rotates counterclockwise around the so-called cold blob:

Figure 2. The thermohaline circulation in the North Atlantic together with the surface water temperature trend from 1993 to 2021. Surrounding the cold spot with deep convection, the AMOC flows counterclockwise.

The cold blob is not only present at the surface but goes way down to the depths:[8]

Figure 3. Change in ocean heat for the top 2000 meters, from 1958-2023, [4] adapted from [8, fig 7]

The hypothesis that the AMOC is weakening is based on the existence of the cold blob as a long-term climate phenomenon in the North Atlantic, as the AMOC strength is strongly positively correlated with the cold blob temperature. A colder cold blob corresponds to a slower AMOC.[9][10]

The cold blob disappears!

However, it now appears that the feared slowdown of the AMOC isn’t happening at all. Deep-sea flow data show stable flows over the past 24 years. This was described in an article on WUWT, here.

In addition, the cold blob has more or less disappeared in recent years:
https://www.youtube.com/watch?v=rfKXLPJxDog

Figure 4: Animation of North Atlantic surface temperatures, annual anomalies for 2013-2023 compared to the average for the period 1979-2010. Monthly data show that it is mainly during summer and fall that the cold blob has disappeared measured as an anomaly, while it is still present during spring. Data: ECMWF ERA5

However, if we look at the long-term development of the sea surface temperature in the area, we find no significant trend, but that the development shown in the animation is part of an oscillation:

Figure 5. Sea surface temperature anomaly in the North Atlantic waters, (50N-65N; 50W-10W). Blue thick line is a Loess smooth. Climate shifts are marked with yellow, see a discussion of those here. Data source: NOAA ERSST V5

The cold blob comes and goes in multidecadal cycles. The 30 years from 1965-1995 were particularly cold. And this is not only true for the surface water, but equally so at depth: even the heat content at depth oscillates with the change in surface water temperature:

Figure 6. Heat content of deep water down to 1000 meters depth in the North Atlantic. The 1995-2005 climate shift is highlighted in yellow. Graph from the Met Office.

AMOC is known as a thermohaline circulation, where the word -haline means salt. AMOC is therefore not only about the transport of heat, but also about the transport of salt. The key to the circulation is the density of the water: the colder and saltier the water, the heavier it is. The saltiness comes from the AMOC passing through the tropics where evaporation is high, thus increasing the salinity in the Gulf Stream.

As the Gulf Stream (and its extension) flows northwards, cooling occurs by evaporation of the warm surface water and by heat being released to the colder surroundings. This increases salinity and thus the density of seawater in the area. Salinity determines the strength of the circulation: saltier water sinks faster to the bottom, thus driving it.

One could say that the AMOC circulates because the seawater in the North Atlantic is salty – and the water is salty precisely because the northward volume transport of the AMOC is so large. But there is also a negative feedback lurking here: when we periodically get increased freshwater inflows into the North Atlantic, the circulation of the AMOC is negatively affected. Fresh water is supplied as meltwater from Greenland and also from the Arctic as ice export through the Fram Strait.

Also the salinity shows oscillations. Observations show large-scale freshening events in the area of the cold blob:

Figure 7. Changes in salinity in the North Atlantic deep water measured as a freshwater volume anomaly (LFC). The 1995-2005 climate shift is highlighted in yellow. Source: [11]

The liquid freshwater content (‘LFC’ in figure 7) estimates demonstrate decade-long freshening events starting around 1965, 1980 and 2010. These events have been called ‘Great Salinity Anomalies’. They appear to be a natural mode of Arctic/Atlantic Ocean variability that have occurred for at least the last century.[11]

Increasing the influx of fresh water (which is less dense than saltier water) lowers the salinity of the upper layers, leading to a cold, fresh, light upper layer once cooled by the atmosphere, i.e. the cold blob. As a result, the AMOC is weakened.

However, there is a time lag in this pattern. The rate of warm to cold transformation of water masses at high latitudes is found to lead the observed AMOC at 45∘N by 5–6 years and to drive its strong, ongoing recovery as now seen in the vanishing cold blob.[12]

The climate system is characterized by oscillations

Oscillations in the Atlantic part of the Northern hemisphere are not limited to the cold blob, or the AMOC. Corresponding variations have been observed in the Arctic sea-ice cover, in the air up to the stratosphere, and in various climate indexes. The long-term variations in the Atlantic Multidecadal Oscillation (AMO) and the North Atlantic Oscillation (NAO), or its close relative the Arctic oscillation, show similar turning points in their cumulatives. The oscillations of the cold blob and the NAO are e.g. closely linked on a multi-decadal scale.[9]

One example of these inter-dependent multi-decadal oscillations in the vicinity of the cold blob is the Arctic oscillation’s correspondence with the Arctic sea ice extent and solar cycle variations:

Figure 8. The Arctic Oscillation as a cumulative, detrended index over time is shown in blue (left scale, inverted). The sea ice extent at its minimum level in September in the Arctic is shown in brown (right scale, Mkm²). The minima of the 44-year solar cycle (2*22 years) are marked in yellow.[14] Ice extent is from satellite data from 1979, before that it is from the reconstruction in [15].

According to the WGH, these oscillations are linked to solar variations, and how they influence on the strength of the polar vortex, as well as the meridional transport (solar cycle variations are indicated with the yellow spots showing the solar cycle minima in figure 8). This is but one example. A whole range of similar oscillatory relationships are further described in figure 9 in the Appendix.

Summing up

According to the novel Winter Gate-keeper climate hypothesis, the main characteristic of the climate system is that it oscillates on different time scales. These oscillations control climate change via heat transport. The AMOC heat conveyor is a major part of the heat transport system that helps our climate to regulate itself.

The AMOC varies in irregular cycles. The cold blob is also a cyclical phenomenon in itself. This applies both to surface water temperature and salinity (Figures 5 and 7).

Advocates of climate tipping point events speculate that the climate functions as a quasi-linear system that reacts to radiative forcing coming from CO₂ and other sources. The IPCC gives anthropogenic greenhouse gas emissions the primary role in its speculations about AMOC developments, including suggested future tipping points.[3] The issue of natural variability is pertinent to all discussions on the AMOC, but remains unresolved.

Appendix: Multidecadal climate variability and meridional transport

Figure 9. Illustration of various multi-decadal oscillations and their connections with climate regimes and shifts. MT=Meridional transport. From figure 11.10 in [1].
a) Black line: Aleutian Low– Icelandic Low seesaw 25-year moving correlation as a proxy for polar vortex strength. Grey line: cumulative winter Arctic Oscillation index.
b) Black line: 4.5-year average of the AMO index. Grey line: cumulative 1870– 2020 detrended cold season (Nov– Apr average) NAO index.
c) Cumulative Pacific Decadal Oscillation, PDO. 1870– 2018 detrended annual average cumulative PDO index. Black dots mark the years 1925, 1946, 1976 and 1997 when PDO regime shifts took place.
d) Black line: zonal atmospheric circulation index, cumulative anomaly.
Grey line: 1900– 2020 inverted detrended annual ∆ Length Of Day.
e) Annual global surface average temperature, detrended. 10-year average.
f) Dashed line: 8.2– 16.6 years band-pass of the monthly mean total sunspot number. Grey line, 6.6– 11 years band-pass of the monthly AMO index. Black line: inverted 20-year running correlation of the band-pass sunspot and AMO data. Black dots as in c, showing their position with respect to solar minima.

References

[1] Vinós, Javier, Climate of the Past, Present and Future: A scientific debate, 2nd ed., Critical Science Press, 2022.

[2] Vinós, Javier. Solving the Climate Puzzle: The Sun’s Surprising Role, Critical Science Press, 2023.

[3] IPCC SROCC “Extremes, Abrupt Changes and Managing Risks”, Chapter 6.7, https://www.ipcc.ch/site/assets/uploads/sites/3/2022/03/08_SROCC_Ch06_FINAL.pdf

[4] Is the Atlantic Overturning Circulation Approaching a Tipping Point?, Stefan Rahmstorf, Oceanography 2024, https://doi.org/10.5670/oceanog.2024.501

[5] Warning of a forthcoming collapse of the Atlantic meridional overturning circulation, Ditlevsen, P., Ditlevsen, S., Nature 2023, https://doi.org/10.1038/s41467-023-39810-w

[6] Overlooked possibility of a collapsed Atlantic Meridional Overturning Circulation in warming climate, Liu and 3 co-authors, Science 2017, https://doi.org/10.1126/sciadv.1601666

[7] Exceeding 1.5°C global warming could trigger multiple climate tipping points, Armstrong McKay and 5 co-authors, Science 2022, https://doi.org/10.1126/science.abn7950

[8] Improved Quantification of the Rate of Ocean Warming, Cheng and 3 co-authors, AMS 2022, https://doi.org/10.1175/JCLI-D-21-0895.1

[9] North Atlantic Oscillation contributes to the subpolar North Atlantic cooling in the past century, Fan and 3 co-authors, Clim Dyn 2023, https://doi.org/10.1007/s00382-023-06847-y

[10] Observed fingerprint of a weakening Atlantic Ocean overturning circulation, Caesar and 4 co-authors, Nature 2018, https://doi.org/10.1038/s41586-018-0006-5

[11] Arctic freshwater impact on the Atlantic Meridional Overturning Circulation: status and prospects, Thomas W. N. Haine, 2023
https://doi.org/10.1098/rsta.2022.0185

[12] Surface predictor of overturning circulation and heat content change in the subpolar North Atlantic, Desbruyères and 3 co-authors, EGU Ocean Science 2019, https://doi.org/10.5194/os-15-809-2019

[13] Coupled stratosphere-troposphere-Atlantic multidecadal oscillation and its importance for near-future climate projection, Omrani and 6 co-authors, Nature 2022, https://doi.org/10.1038/s41612-022-00275-1

[14] Periodicities observed in the solar and geomagnetic indexes and in SABER thermospheric infrared power measurements, Nowak and 4 co-authors, Science 2024, https://doi.org/10.1016/j.jastp.2024.106234

[15] On assessment of the relationship between changes of sea ice extent and climate in the Arctic, Alekseev and 2 co-authors, 2015, https://doi.org/10.1002/joc.4550

Links to the previous rounds: 1, 2 and 3.