Influence of ocean cycles on the current warm phase in Germany

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From KlimaNachrichten Redakteur

By Dr. Ludger Laurenz

To take away:

  • The high temperature level of the last 20 years in Germany can primarily be attributed to the current warm phase of the Atlantic Multidecadal Oscillation AMO and the spread of the “Western Hemisphere Warm Pool” WHWP.
  • Over the next 10 years, the temperature of surface water in the Atlantic, Caribbean and eastern tropical Pacific is expected to decrease due to the cycle. As a result, temperature levels and sunshine duration will most likely decrease again in Germany.
  • The spring drought that has occurred since 2008 is based on an approximately 15-year low-precipitation cycle phase, which is repeated regularly during the approximately 65-year AMO cycle. Over the next few years, spring precipitation activity is expected to gradually normalize.
  • It is surprising that the German Weather Service does not say a word about the influence of ocean cycles on weather and climate in Germany in its climatological review of 2022, justifies the high temperature level of recent decades with anthropogenic influence and calls for political measures against climate change.

For five years I have been searching for natural cycles in historical climate data. My motivation is based on the mistrust of the claim that the current warm phase is solely or primarily anthropogenic caused, as in Climatological Review of 2022.

The results of my research so far are documented in a publication (Solar influence on rain in Europe, 2019) and several articles in The publication would never have been produced without considerable support from Sebastian Lüning and Horst Lüdecke.

Again and again I have come across the variation of solar activity as the main cause in the search for natural influence on weather and climate, both in the search with the help of google scholar and in the search in historical climate data series, which are abundantly available on the Internet.

Method for visualizing solar influence: stacking the same cycle phases on top of each other with cycle lengths of approx. 11, 22 and 65 years

In the search for solar influence in historical climate data, the publication by S. C. Chapman et al. 2021 helped me a lot.

For the first time, he has defined and named the starting years of the approximately 22-year Hale cycles of the Sun on the basis of solar physical measurements. With the help of these starting years, it is easy to detect solar influence in historical weather data series. An example is the precipitation sum of the 2nd half of the year in the Netherlands in the following figure. In this illustration, five identical approximately 22-year cycle phases from the period since 1926 are stacked on top of each other.

Figure 1: Precipitation total in the second half of the year in the Netherlands in five Hale cycles of the Sun

The decrease in the precipitation sum from cycle year 7 to 8 cannot be a coincidence, but can only be explained by the influence of solar activity on the Earth’s atmosphere, which is repeated approximately every 22 years. The solar influence is similar in the other Benelux countries and Germany. Numerous proofs of solar influence, supported by literature references, are documented in klimanachrichten.deMs. S. Veretenenko 2022 provides a plausible explanation for the influence of varying solar activity on the Earth’s atmosphere and precipitation.

New project: How the Caribbean and Atlantic periodically heat us up and bring us beautiful summers

Is the current warm phase in Germany with 1.7 degrees higher temperature compared to 1881 anthropogenic or the result of natural cycles, or both? In order to present the answer to this question as vividly as possible, the first step is to show the temperature trend of the ocean surface, which warms the air before it is carried to Europe by southwesterly winds. For this purpose, two data sets provided by the American weather agency NOAA are used, the Atlantic Multidecadal Oscillation AMO and the Oscillation of the extent of the “Western Hemisphere Warm Pool” WHWP, Monthly Atmospheric and Ocean Time Series . The areas from which the data originate can be seen in the following figure.

Figure 2: Area reference of AMO and WHWP

The AMO influences the climate throughout the northern hemisphere from the Arctic to the Sahel or India. See Google Scholar. It has a period of over 60 years. Currently, the AMO is at the end of a warm phase. The AMO is a natural temperature fluctuation that has been detectable for several centuries. The fluctuation range of the AMO with about 0.5 degrees is also currently within the framework of natural variability.

The Western Hemisphere Warm Pool (WHWP), where water is warmer than 28.5°C in summer, extends from the eastern tropical North Pacific to the Gulf of Mexico and the Caribbean and overlaps at its tip with the tropical North Atlantic. I included the WHWP to check the cyclical influence of the second warmest “bathtub” on earth with a summer temperature level of over 28.5 °C on the European temperature level. The variability of the WHWP influences the hurricane activity and the temperature level of large areas of the northern hemisphere (literature).

In order to be able to illustrate the influence of AMO and WHWP on the temperature trend as clearly as possible, periods of the same cycle phases are first stacked on top of each other, as in the example in Figure 1. The cycle length for the oscillations mentioned is the period of three Hale cycles of the Sun, corresponding to about 65 years. From recent publications, there is evidence of a link between the approximately 60-year cycle, which can be detected on the Sun, and the approximately 60-year cycles, such as in the AMO, WHWP or PDO. (Scafetta 2020Veretenko 2021Olilla 2022). The period of about 65 years results if three Hale cycles are ranked consecutively with the starting years mentioned by Chapman, see Table 1.

Table 1: Hale cycles of the Sun and about 65-year AMO cycles since 1837, in brackets duration of cycles in years. Determination of the first year of the Hale cycles based on S. C. Chapman et al. 2021

The trend of AMO is stable in the last three cycles

With the help of the phase length determined in this way, approximately 65-year periods of the AMO can be defined as in the following figure, in which the cycle repeats.

Figure 3: Trend of the AMO since 1870 with the starting years of the approximately 65-year AMO cycles

If the three approximately 65-year periods of the AMO are stacked on top of each other, it becomes visible how the course of the AMO repeats itself from cycle to cycle.

Figure 4: Trend of the AMO in the approximately 65-year AMO cycle since 1858

The three cycles follow a common trend. Currently (grey column, 2022), the AMO is at the end of the last warm phase and still at a very high level compared to the two preceding cycles (blue and green). In the next few years, a more or less sharp decline in the AMO index and a cooling of the northern Atlantic are to be expected.

Area extent of the WHWP varies cyclically and in step with the AMO

At the same time and congruent as the AMO, the area extent of the WHWP fluctuates, see the following figure. Here, the AMO trend is transferred from the previous figure with a blue dashed line.

Figure 5: Trend of WHWP since 1950 in the approximately 65-year AMO cycle

AMO and WHWP appear to be synchronized by the same external drive. Whether the periodically fluctuating solar activity is actually the driving force will certainly be clarified in the next few years.

While the trend of water temperature at the AMO is approximately the same in the three cycles (see Fig. 4), the area extent of the WHWP differs between the current (red) and previous cycle (green) in the middle of the 3rd Hale cycle (yellow rectangle) by about 4 million km², more than ten times the area of Germany. This difference may explain at least part of the exceptionally high temperature level of the last 20 years in Germany.

The WHWP is also expected to enter a longer cooling phase. This is currently confirmed in the Caribbean in the trend of anomaly of the water temperature of recent months, source.

Figure 6: Trend of Caribbean surface water temperature anomaly in recent months

Temperature trend in Iceland closely linked to AMO and oscillation of WHWP

The extent to which the temperature trend from the Atlantic incl. the Caribbean and the eastern tropical Pacific is transferred to Europe is shown in the following figure with the cyclical temperature trend of Iceland. Due to its insularity in the Atlantic, temperature transmission from the ocean surface is still most likely to be expected in Iceland.

Figure 7a: Trend of temperature in the summer half year in Iceland since 1903 in the approximately 65-year AMO cycle

Also in this representation the temperature trend of the AMO is inserted. The temperature trend in Iceland runs parallel to the AMO trend. Thus, the temperature increase of the last 30 years in Iceland of a good one degree can be explained solely by the trend of AMO and oscillation of the WHWP. If the temperature trend is transferred to Greenland, it is understandable that the current AMO warm phase may have contributed significantly, if not dominantly, to the ice loss of recent decades.

Current warm phase in Germany influenced by the trend of AMO and oscillation of WHWP

Also in the area average of Germany, the AMO and the oscillation of the WHWP leave deep traces in the temperature trend of the summer half-year, see next figure.

Figure 7b: Trend of temperature in the summer half year in the area average of Germany since 1881 in the approximately 65-year AMO cycle

In the current cycle (red), the curves for the trend of the mean temperature and the AMO (blue) run parallel. The total temperature increase in Germany over the last 30 years can be attributed to the influence of the AMO and the oscillation of the WHWP.

But why does the temperature trend of the current cycle (red) from the middle of the 65-year AMO cycle around the year 2000 decouple from the previous older cycles and rise about 1 degree higher? The cause could lie in the oscillation of the WHWP. For the WHWP in Figure 5, the area with > 28°C in the period 2016 to 2021 (red line in the yellow field) is about 4 million km² larger than 65 years earlier from 1950 to 1955 (green). This difference could have had an effect on the temperature level in Germany.

Significant influence of AMO and WHWP on sunshine duration in Potsdam

The sunshine duration in Germany is also influenced by AMO and WHWP, see next figure. Here the data from Potsdam are used because they go back to 1893, while the data for the area average of Germany are only available since 1951.

Figure 8: Trend of annual sunshine duration in Potsdam since 1893 in the approximately 65-year AMO cycle

The trend of sunshine duration increases in the two cycles in similar orbits until the last third of the cycle by a remarkable 200 hours. The trend in the current cycle (red) runs parallel to the previous cycle (green) until 2017. Until 2017, the increase in sunshine duration over the past 30 years can be attributed solely to the trend of AMO and oscillation of the WHWP. Further evidence for the link between Atlantic temperature and sunshine duration in Europe is provided by Andrzej A. Marsz et al. 2021.

The regular increase in sunshine duration during the AMO cycle acts as a temperature booster of the AMO cycle. The air in Europe is preheated in warm phases by the Atlantic and Caribbean surface and finally further heated by prolonged sunshine. (Laurenz 2021)

The cause of extremely high sunshine duration in 2018, 2019, 2020 and 2022 due to a long-term trend and interpolating into the future cannot be deduced from the presentation. Rather, the current solar activity is likely to be responsible for these extreme years. (Laurenz 2021)

Spring drought of the last 15 years in Germany correlates with approx. 65-year AMO cycle

The 65-year period can also be used to explain other climate and weather phenomena. In Germany, the spring has been rather too dry since 2008, with below-average precipitation in April. April is considered the front month of summer. Drought and parched soils at the end of April reduce cloud and precipitation formation in the following period. Thus, an April drought can develop into an early summer and summer drought. The next figure shows that this trend is likely to reverse soon:

Figure 9: Trend of the April precipitation sum in the area average of Germany since 1881 in the approximately 65-year AMO cycle

The precipitation trend follows a common trend in all three cycles. In the first half of the third Hale cycle, the precipitation total in the three cycles (black, green, red) is almost 20 mm or a third lower than in the cycle time before or after. In the next five years, the level of April precipitation should return to normal, and the risk of early summer drought in Germany should decrease again.

Why does the German Weather Service overlook the influence of ocean cycles on the current warm phase in Europe?

The link between AMO and WHWP cycles with the temperature trend in Germany and Europe is extensively described in the literature and cannot be overlooked in the illustrations in this article. Against this background, I am surprised that the German Weather Service does not say a word about ocean cycles in its last annual report, justifying warming practically solely with anthropogenic influence (climatological review of 2022). Here is a small excerpt of the summary:

“Since 1881, we have now had an increase in the annual mean temperature in Germany of 1.7 degrees. This can only be explained by man-made climate change. Since the early seventies, this warming trend has accelerated significantly and there is no reason to assume that it will slow down in the next few years. We are now experiencing heat waves and intensities that we would have expected from the climate models in a few decades. All this must be a strong motivation for us to significantly promote climate protection in Germany and globally, because adaptation is much more expensive and even here only possible to a limited extent.”

Law on the German Weather Service (DWD-Gesetz) § 4 Tasks, point 6: Analysis and forecasting of meteorological and climatological processes as well as the analysis and projection of climate change and its impact.