Tag Archives: atmospheric and ocean circulation systems

The Green-House Gas Forcer vs. The Winter Gatekeeper Round 2: Climate Shifts – Are They for Real?

From Watts Up With That?

The Battle of the Climate Hypotheses: The Green-House Gas Forcer vs. The Winter Gatekeeper Round 2: Climate Shifts – Are They for Real?

Gabriel Oxenstierna

The climate system persistently tries to restore radiative imbalances through the meridional transport of heat via atmospheric and ocean circulation systems that control the poleward flux of latent and sensible heat, see the first article in this series, here.

The “Winter Gate-keeper hypothesis” [WGH] claims that the climate exhibits decades-long heat transport regimes separated by more or less abrupt shifts: “… climate regimes are distinct states of atmospheric circulation with different levels of poleward heat transport. Rather than changing gradually, these regimes can shift abruptly from one state to another.”[2, p. 337]

Climate regimes that are ended by climate shifts are a fundamental feature of the WGH, as they are an expression of the natural cycles that are essential to it.[2, ch. 32-33] These regimes manifest as trends in oceanic and atmospheric oscillations, in varying heat transport intensities, and in altering surface temperature trends.

Importantly, the WGH claims that changes in the flux resulting in climate shifts will be sufficient to change the radiative balance so that planet Earth either cools or warms as a result. The shifting transport regimes of heat and moisture are the fundamental explanation to climate change, says WGH.

The Green-house Gas forcer climate hypothesis doesn’t recognize the naturally occurring climate shifts on a global scale, as they don’t fit the hypothesis of a changing climate responding to gradually increasing GHG levels. Even if some climate shifts can be shown to exist, they are presumed not to be significant enough to change the global radiative balance. IPCC also doesn’t mention climate shifts or climate regimes as concepts per se.[3] Climate shifts are sorted under the category ‘internal variability’, whose effects are set to zero:

Figure 1. Assessed contributions to observed warming in 2010–2019 relative to 1850–1900. The figure is cropped from the latest  IPCC climate report, where they heroically claim that natural variations for the first time in history have ceased to influence the climate: all climate change is nowadays exclusively assumed to be caused by humans. Scale in degrees K. Source: [3, fig. 2b]

The only climate shift in the modern era that is somewhat accepted in the climate science occurred 1976. It has been studied extensively and is evident in many climate-related variables and had a lot of effects on e.g. marine ecology.[4][5][6] This is how the 1976 climate shift is described in the WGH literature:

“Recent global warming began in 1976 with a sudden climate shift in the Pacific Ocean that increased zonal atmospheric circulation and decreased poleward heat transport, affecting the global temperature trend. As a result, the multidecadal oceanic oscillations changed from a cold phase, which had led to the 1945-1975 cooling period, to a warm phase.
The abrupt climate shift of 1976 revealed the existence of multidecadal climate regimes separated by abrupt transitions. They result from changes in the global atmospheric circulation that establish distinct poleward heat transport regimes.”[2, p.342, my emph.]

The 1976 shift is thus the starting point for looking at presumptive climate shifts with opposite effects on the dominating wind patterns, the poleward heat transport and warming. One such shift happened around 1944/45 and a more recent one is claimed by the WGH to have happened around 1997.

What drives the climate shifts?

The basic concept of WGH is that polewards Meridional Transport (MT) of heat and moisture controls climate change. An increase in MT speeds up the energy transport to the polar regions, enhances radiation to space especially in the Arctic, and consequently acts as a negative feedback.

The first order driver of MT is the steep latitudinal temperature gradient (LTG; or “gradient” for short) between the tropics and the polar regions. “The LTG is a central property of Earth’s climatic system at all time scales. It drives the atmospheric-oceanic circulation and helps explain the propagation of orbital signatures through the climatic system, including the Monsoon, Arctic Oscillation, and ocean circulation.”[1, p.86][10]

The gradient arises from the differential radiative heating between tropical and polar latitudes and drives the poleward heat transport. It thereby acts as a thermodynamic engine of the planet’s climate:  “MT is the climate control knob, and it responds primarily to the LTG”. [1, p.542] Now, this gradient displays a multi-decadal variation:

Figure 2. The observed latitudinal temperature gradient between the Arctic and the tropics. It is  calculated by subtracting latitudinal temperature anomalies between 64–90N and 0–24N. Positive values reflect a weaker gradient relative to the base period 1951-1980. The yellow bars indicate the weak gradient around 1940/45 and more recently from 2000 and onwards, i.e. the periods when the temperature difference between the Arctic and the tropics was smallest. They are also the periods when polar and subpolar latitudes experience enhanced warming. The letters A-D show different climate regimes and the vertical orange lines have been added to show the claimed climate shifts. Source: [8, fig. 4]

A higher temperature gradient enhances the polewards heat transport, and vice versa. During the climate shift around 1945 global temperatures peaked as the gradient reached its weakest value and heat transport slowed, (period ‘A’/yellow bar in figure 2). From there the gradient started to increase, heat transport picked up steam which helped the climate to cool down in the following decades, ‘B’:

Figure 3. Multi-decadal temperature trends indicate multi-decadal climate regimes A-D with shifts around 1910, 1945 and 1976, but not in 1997.

In the 1960s and early 70s the gradient grew bigger, the poleward heat transport improved and global temperatures cooled somewhat. But again the climate shifted. In the decades after the climate shift 1976, the gradient has gotten ever smaller (‘C’ and ‘D’ in fig. 2). This has negatively influenced the heat transport since 1976 and contributed to a warming climate, especially in the high north.

Transporting heat and moisture

MT is a polewards transport of heat and moisture. Sensible heat is transported in all layers of the atmosphere up to ToA, whereas latent heat is transported as moisture (water vapor) in the lower atmosphere. Water vapor in the atmosphere acts as a means of storing heat that can be released later. (See the appendix for some further explanation.)

The total precipitable water (TPW) in the air column can be used as a proxy for the amount of moisture available for MT, and for the rate of atmospheric overturning. The climate regimes and shifts (A-D) line up nicely in the TPW developments:

Figure 4. Total precipitable water (TPW) as measured and reanalysed by the ECMWF ERA5 (0.5×0.5 deg) from 1940, and ERA20C from 1900-1940. The latter dataset has been level adjusted to fit with the higher level in ERA5. Climate regimes and shifts as in figures 2 and 3. Data source: ECMWF.

The TPW increases long-term as a result of global warming and there is almost 10 percent more water in the atmosphere today than 120 years ago. Warmer air holds more water, which enhances the convection and advection processes in the water cycle. It is well established that the intensity of the tropical Hadley cells have increased, and they have also significantly expanded polewards since 1997. Also the Ferrel cells show similar increases. As a result of these shifts, we have a positive trend in the polewards export of net energy from the tropics since around 2000, driving the MT, see previous post (fig. 4).

Was there a climate shift 1997?

The variations in the gradient (fig. 2) as well as in TPW corroborate the claims of climate shifts in 1945 and 1976. But what about the climate shift 1997 that has been proposed by WGH? [1, ch. 11.4] We had a peak in global temperatures during the strong El Niño 1997/98, followed by a temperature hiatus up until 2015. Still, the longer-term global warming trend has remained intact since 1976 with no signs of a climatologically relevant climate shift in global temperature data (fig. 3). The gradient in figure 2 also shows no signs of a shift after 1997. There are also no signs of a climate shift 1997 in the Earth energy imbalance (EEI) or Ocean heat content (OHC) data. However, there are other signs of a shift in 1997, e.g. in the water cycle (fig. 4) and in various climate indexes in the Arctic.

There is an on-going change in the global heat transport system, and it has resulted in an increase in the magnitude of poleward transport from 1997 in spite of the LTG getting continuously smaller. Apparently, some counter-balancing factors have been more important, such as the shift in the TPW. The Arctic has had a dramatic warming during this period, the ‘Arctic amplification’, caused by enhanced heat transport in the atmosphere and the oceans. This has reduced the temperature gradient, and apparently made other factors such as the TPW shifts more important.

Changes in the water cycle are consistent with all the claimed climate shifts, including 1997. This implies that changes in the water cycle are essential to MT, as seen in the significant developments of TPW in figure 4. We also have a lot of evidence of the 1997 shift in the Arctic, as well as in cumulative indexes of various multi-decadal climate oscillations such as AMO, ENSO and the PDO.[1, fig. 11.10]

Answering the headline question: Yes, it is clear from the data and the literature that climate regimes and climate shifts driven by transport of heat and moisture are for real. That is true not only in the modern era, but has characterized the climate throughout the Holocene, and more.[10] But it is also clear, that there are no regular interactions, or regular cycles: some fundamental global climate variables involved in the 1976 shift were not affected in 1997.

Finally, green-house gas forcing via CO2 has no role to play in the climate shifts during the modern era. First of all, CO2 is monotonically increasing throughout all the mentioned climate shifts, and secondly, it’s effect on heat transport has been deemed to be “negligible”.[10]

The next round will bring the battle to the Arctic, the home arena for the WGH.

Appendix: a note on sensible and latent heat

The water cycle involves around 10 times more vertical net heat transport as latent flux compared to sensible heat transport. But if we look at meridional heat transport beyond the tropics, less than half is horizontally advected as latent energy compared to sensible heat.

The meridional heat transport in the climate system can be thought of as being maintained by three components: the dry air heat transport AHTDSE, the ocean heat transport, and the latent heat ‘joint’ mode AHTLE.[7] In the polar latitudes the transport of dry air heat dominates over the transport of latent heat:

Figure 5. The mean Atmospheric Heat Transport, calculated directly from the velocity and temperature (AHTVT; y axis unit: PW). The total AHTVT (solid black) comprises the Dry Static Energy transport of sensible heat (AHTDSE, red), and the Latent Energy transport (AHTLE, solid blue). The latent heat transport (HTEMP) obtained from the ocean surface Evaporation Minus Precipitation (EMP) is plotted as the dashed blue line. For further explanations, see sections 4.5-4.7 and figures 9-12 in [7].

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 AR6 WG1, Summary for Policymakers (SPM), figure SPM.2, https://www.ipcc.ch/report/ar6/wg1/chapter/summary-for-policymakers/

[4] Recent observed interdecadal climate changes in the northern-hemisphere. Trenberth, AMS 1990, https://doi.org/doi:10.1175/1520-0477(1990)071<0988:ROICCI>2.0.CO;2

[5] From Anchovies to Sardines and Back: Multidecadal Change in the Pacific Ocean, Chavez and 3 co-authors, Science 2003, https://doi.org/10.1126/science.1075880

[6] Global Variations in Oceanic Evaporation (1958–2005): The Role of the Changing Wind Speed, Lisan Yu, J.of Cl. 2007, https://doi.org/10.1175/2007JCLI1714.1

[7] Decomposing the meridional heat transport in the climate system, Yang and 4 co-authors, Clim Dyn 2015, https://doi.org/10.1007/s00382-014-2380-5

[8] Ocean-atmosphere climate shift during the mid-to-late Holocene transition, Morley and 2 co-authors, 2014, https://doi.org/10.1016/j.epsl.2013.11.039

[9] A new dynamical mechanism for major climate shifts, Tsonis and 4 co-authors, https://doi.org/10.1029/2007GL030288

[10] Heat Transport Compensation in Atmosphere and Ocean over the Past 22 000 Years, Yang and 5 co-authors, Nature 2015, https://doi.org/10.1038/srep16661