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By Frank Bosse and Fritz Vahrenholt

[Translated, edited by P Gosselin]

Our primary “fusion reactor” remains in a weak phase in its current solar cycle, number 24 since systematic observations began in the year 1749. In May sunspot activity was below normal. The observed sunspot number (SSN) was 58.8. The mean of all previous cycles for the current 78th month into the cycle is SSN=79. Thus May saw 75% of the usual activity.

Figure 1: The current cycle 24 (started in December 2008) is shown in red and is compared to the mean cycle (blue) and to cycle no. 5 (black).

A pronounced lull

Figure 1 shows that current solar cycle 24 has never exceeded the mean (blue) at any time since it began. In the 78 months since the it began, SC 24 has always been below normal. This has never been observed for any previous cycle. The low solar activity since December 2008 is unique when it comes to its consistency when compared to the other cycles since observations began!

Even when activity reached a maximum in October 2011 in the sun’s northern hemisphere, and in February 2014 for the southern hemisphere, it remained just below the mean value. Together with the delayed start of the cycle we now have a record 10 years of quiet solar activity.

Figure 2: The accumulated sunspot anomaly of all cycle up to the 78th solar cycle month.

Figure 2 depicts a comparison of all the cycles with respect to solar activity. So far the current cycle is in 4th place in terms of low activity. But 3rd place is very reachable because SC 7 saw high sunspot values in its last third of the cycle, and so the chances are good that the total activity of SC 24 will be quieter than the last cycles of the Dalton Minimum.

Atlantic waves…

…are really high when it’s stormy. In early May off the coast of Portugal one of the co-authors of this article came to realize this in a 14-meter long sail boat. But the Atlantic also created other types of waves in the past month. A team of scientists led by Gerard D. McCarthy of the University of Southampton went on the search for internal North Atlantic variability, see www.nature.com/nature/journal.html. They determined that the Atlantic Multidecadal Oscillation (AMO) not only has ups and downs in sea surface temperature (SST) in the extratropic Atlantic region, but that these temperature variations lead to changes in sea level (SSH) along the east coast of the USA. The pattern appears as follows:

Figure 3: The “circulation series” shown in blue. In the paper the SSH variation is determined by comparing the sea level south of and north of Cape Hatteras. The AMO is black. Source: Figure 3 of the cited McCarthy publication.

The relatively long time series of tide measurements at the East Coast is thus a proxy for the ocean heat content (OHC) of the North Atlantic. Its direct measurement since the 1950 entails large uncertainty. But beginning in 2004 it has been much more precise thanks to the submerged ARGO measurement buoys and the RAPID network.

What implications does this study have? First of all, the existence of natural Atlantic Multidecadal Oscillations is confirmed, and not only as a variation in sea surface temperature (SST) as it was previously defined. It is now sure that the AMO is a large-scale North Atlantic water mass circulation pattern. It is an independent internal natural variability of our climate system, and not just one involving global temperature.

Already in January 2013 we pointed to falling North Atlantic ocean heat content (OHC) since 2007. What follows is the data plot:

Figure 4: The ocean heat content (OHC) of the extratropical North Atlantic since 1979. Source: Climate4you .

In the paper and its accompanying press release it is explained that the current decline in the OHC means it is announcing that the probability of the North Atlantic cooling more than 10 years is very high. The AMO’s impact on temperatures in the northern hemisphere was major in the past, as the following plot shows:

Figure 5: The AMO (green) compared to temperature changes of the Northern Hemisphere (red).

If the AMO exists as an internal variability, as the McCarthy paper tells us, then that could imply that 0.5°C warming seen in the northern hemisphere since 1975 was due to the AMO and that the remaining 0.5°C of warming was due to impacts from greenhouse gases and other factors, such as varying solar activity.

For estimating climate sensitivity from greenhouse gases, this has far-reaching implications: Up to now we were not able to completely exclude the impact of aerosols on the cooling of temperatures between 1945-1975, but now it is appearing as increasingly improbable. Indeed it is becoming more evident that the cooling was due to the weakening AMO during that time period (see Figure 3).

If indeed aerosols have a lesser cooling effect than previously assumed, then the climate sensitivity with respect to greenhouse gases must be less. Since 1975 for the northern hemisphere it was not 0.26 °C / decade increase, but rather only 0.13. This is close to being identical to the southern hemisphere. We’ve often discussed this 50:50 order here …and once again we are confirmed.