Holocene

The gyral resonance into the band 576-1152 years

Solar activity in the band 576-1152 years is significant. A coupling occurs in the North Atlantic between the total solar irradiance and the global mean temperature into this band  characteristic of the 128×6=768 year period, mainly within the interval covering 9000 to 6000 years BP.

Comparison of forcing multiplied by its effectiveness and average global temperature, into the band 576-1152 years. A 100-year delay is applied to forcing - a) the North Atlantic (GISP2) - b) the southern hemisphere (EPICA) – c) effectiveness of forcing. The average global temperature results from the sea surface temperature anomalies of the subtropical gyres at mid-latitudes, proportional to the amplitude of gyral waves, and the change in the albedo at high latitudes.
Comparison of forcing multiplied by its effectiveness and average global temperature, into the band 576-1152 years. A 100-year delay is applied to forcing – a) the North Atlantic (GISP2) – b) the southern hemisphere (EPICA) – c) effectiveness of forcing. The average global temperature results from the sea surface temperature anomalies of the subtropical gyres at mid-latitudes, proportional to the amplitude of gyral waves, and the change in the albedo at high latitudes.

Between 9000 and 5000 years BP, there is a great similarity between the GISP2 and EPICA data. Between 5000 and 2500 years BP the amplitude of solar irradiance into the band 576-1152 years decreases and the gyral waves disassociate from the solar irradiance cycle, both the amplitude as period. During this period of decoupling the gyral waves do not weaken because of the remanence of geostrophic forces throughout the gyre, but their period lengthens, which indicates that the centroid of the gyres drifts poleward. The waves are coupled to the solar cycle again between 2500 years BP and present in the North Atlantic whereas the coupling occurs later in the Southern hemisphere, despite the upsurge of the solar cycle.

Thus the gyral resonance into the band 576-1152 years appears to be the main driver of climate during the Holocene. Effectiveness of forcing evolves considerably, equal to 3.0 °C(W/m2)-1 between 9000 and 6500 years BP, it is 1.2 °C(W/m2)-1 since 4800 years BP. For a same variation in solar irradiance the oscillation of the global mean temperature decreases by a factor of 2.5 during the Holocene. This reflects the progressive retreat of the polar front, and the effectiveness loss of forcing that is accompanied by a reduction in the amplitude of the gyral wave, including the velocity of radial and polar currents. The fall in efficiency is compounded by changing the albedo at high latitudes, the impact of which is much larger at 65 °N than at 80 °N.

The Bond events (little ice ages)

Proxies of global mean temperature: gross data and data filtered into the band 576-1152 years: a) EPICA, b) GISP2.
Proxies of global mean temperature: gross data and data filtered into the band 576-1152 years: a) EPICA, b) GISP2.

More sudden climate changes around 8,200 years ago appear superimposed on the cooling cycle. A question arises: is there a link between the cycle of 768 year average period and the occurrence of Bond events? With regard to the first half of the Holocene, the answer is clear: these events always occur during the cooling phase of the cycle whether the GISP2 or the EPICA data although these events are manifested in very different ways. Indeed, cooling occurs very rapidly in the North Atlantic but more slowly in the southern hemisphere.

Bond events occur during the cooling phase of every cycle of large magnitude. In the North Atlantic they appear at times that can be identified accurately given their brevity. The main events happen in 9400, 8200, 7500, 5400, 4700 and 1100 years BP. The most striking event occurs 8200 years BP, i.e. during the minimum of the cooling phase of a cycle of large amplitude as benefiting both the advance of the polar front for the early Holocene, and a large magnitude solar cycle into the band 576-1152 years. The estimate of the average period of Bond events does not make much sense; only the period of successive cooling cycles can be determined accurately, but the events may be poorly differentiated when the amplitude of the gyral wave is too low. In the Southern Hemisphere the intervals during which the main events occur are 8500-8000, 7500-7100, 5500-5400, 2400-2300 years BP.

Although the duration of events is very different from one hemisphere to the other, the cause of their formation is certainly the same, being related to the advance of the polar front during the successive cooling cycles.  But what physical phenomena occur precisely while the density of sea water increases significantly along the drift current at high latitudes (the circumpolar current in the southern hemisphere), due to both an increase in salinity and a decrease in temperature? This has the effect of changing the buoyancy of the current, sinking at lower latitudes while reducing the exchanges with the atmosphere and the transfer of heat from the tropics. The current resurfaces while renewing with sea water warmer and less salted after few decades. In this way, the importance of the North Atlantic Drift on Europe’s climate could explain the little ice ages by the southward offset of the interglacial deep water overturning. Such a shift of the convection zone had been hypothesized by Ganopolski and Rahmstorf, [2001] to explain the spatial pattern and time evolution of D-O events.