By Andy May

In my last post I discussed the evolution from the ICOADS raw sea surface temperature (SST) data to the final ERSST and HadSST SST anomalies. These anomalies are compared in figure 1. The ICOADS anomalies are generated by subtracting the 1961-1990 mean value from all the final raw simple yearly means. This was done last after the simple global mean SST had been computed for all years.

The ERSST and HadSST values start out as anomalies. That is, they are created grid cell by grid cell before processing starts. Obviously, the measurements in each cell are normally from different vessels in different years or months, but the measurements are turned into anomalies by subtracting the 1961-1990 mean for each cell from each measurement captured in the same cell for any specific month. This is before any processing or corrections are performed. Since the cells may be as large as 12,300 sq. km this is of dubious value, but that is the way it is done.

On land with fixed measurement weather stations anomalies make more sense, the elevation of each weather station is different, and the individual weather station will often have been in the same location through the entire period from 1961-1990 and often with the same or similar equipment. Thus, making an anomaly at the beginning of the processing by subtracting the mean weather station value for each month from its 1961-1990 monthly mean is logical. On the ocean where every measurement in any given month is at about the same elevation, but from a different buoy or ship, with different equipment, it makes less sense.

Figure 1. A comparison of the ICOADS 3 simple mean sea surface temperature (SST) in green, converted into an anomaly, with the heavily processed “final” SST anomalies from ERSST 5 (NOAA, orange) and HadSST 4.1 (Hadley Centre, heavy black line).

This post is about the difference in the ICOADS data anomaly during the World War II era from 1939 to 1946 as marked in figure 1. It is very noticeable in the raw ICOADS data but disappears in the two final reconstructions shown. There are a lot of known problems that occurred during the war. Shipping lanes changed due to the presence of submarine “wolf packs,” SSTs were beginning to be measured inside engine water intakes rather than with buckets dipped in the ocean, and for those ships still using buckets the type of bucket often changed. These problems are apparent in the ICOADS data, but do they account for the entire radical correction shown in figure 1? The peak raw data anomaly is in 1944 (+2.14°C), yet the 1944 ERSST value is 0.091°C, thus the correction in 1944 is over 2°C. Is this realistic? The total warming since 1900 is estimated to be only about one degree, how is a two-degree correction for an entire year justified? ERSST and HadSST corrected the data, but how much confidence can we have in the corrections? What else was going on at the time?

The Climatic Conditions during World War II

Climatically there was a lot going on during the war. We are fortunate that Stefan Brönnimann of the University of Bern has dug out and digitized a very large database of meteorological data from Germany, the German occupied areas, Sweden, the United States, the Soviet Union, and the UK, and tried to come to some conclusions as to the climate of the Northern Hemisphere during the war. What is presented in this post is mostly from papers he published from 2003 to 2005 (Brönnimann S. , 2003), (Brönnimann, Luterbacher, & Staehelin, 2004), (Brönnimann & Luterbacher, 2004b), and (Brönnimann S. , 2005).

Using the data they collected, Brönnimann and colleagues built monthly maps of surface conditions, upper atmospheric temperature, and geopotential height. Brönnimann and Luterbacher reconstructed the upper atmosphere for the period 1939 to 1944. They found evidence of a weak and disturbed Northern Hemisphere winter polar vortex. This resulted in anomalously high winter temperatures in the upper atmosphere and the surface in Alaska, Canada, and Greenland and very cold winter temperatures in Europe. An example map is shown in figure 2 for January 1942 at an altitude of 500 hPa (around 18,000 feet or 5,500 meters).

Figure 2. Example map of the January 1942 temperature anomaly at 500 hPa (5,500 meters). It is a polar projection of the Northern Hemisphere, and the center is the North Pole. Source: after (Brönnimann & Luterbacher, Reconstructing Northern Hemisphere upper-level fields during World War II, 2004b).

Brönnimann and colleagues believe that the unusual weather during World War II was related to the very strong and persistent El Niño that occurred at the time. In figure 3 we show the Niño 3.4 index, the AMO and the PDO with World War II marked as “WWII.”

While the world as a whole was unusually warm during the war years on average, Europe, northern Siberia, and the central North Pacific suffered through three bitterly cold winters from late 1939 to 1942. The Southern Hemisphere was not spared, sea surface temperatures in the southern mid-latitudes were unusually cool and Australia suffered from a very prolonged drought from 1937 to 1945 (Hegerl, Brönnimann, Schurer, & Cowan, 2018).

Figure 3. The AMO, the PDO and the Niño 3.4 indices with World War II marked. Data source: NOAA.

In figure 3 the prominent World War II El Niño is very clear, and we can see that the AMO and PDO are positive. The North Atlantic Oscillation is not shown, but it is strongly negative during this period (see figure 4 for a plot of the similar Iceland to Aleutian sea level pressure anomaly). There is also evidence of high column ozone in both the Arctic and mid-latitudes, a weak winter polar vortex, and frequent stratigraphic warmings. As Brönnimann and colleagues report, “At the Earth’s surface and in the troposphere, the period 1940–42 represents an extreme climatic anomaly of hemispheric to global extent.”

Figure 4 compares the ENSO 3.4 temperature to central, northern, and eastern European temperature, the Iceland to Aleutian sea level pressure difference, the 100-mbar geopotential height difference between the poles and the mid-latitudes, and the total ozone measurement at Arosa, Switzerland. All these measurements show a distinct anomaly during World War II.

Figure 4. A comparison of the ENSO 3.4 temperature anomaly to European surface temperature, the Iceland to Aleutian sea-level pressure anomaly, the 100-mbar geopotential height, and total ozone in Switzerland. The yellow shading covers roughly 1937 to1945. Source: after (Brönnimann S. , 2005).

Figure 4 shows that analyzed in the context of the 20^th century, 1940-1942 stands out as a climatic anomaly that is unique. Thus, it seems unusual that the final ERSST and HadSST records shown in figure 1 just carry on through 1940-1942 on the same trend as before as if nothing unusual were happening. They do show a bit of the 1946 cliff in the raw ICOADS data, but it is very subdued compared to the ICOADS data.

I do not question the problems in the raw data, they are firmly documented. I do question the corrections applied by the Hadley Centre and NOAA, however. The corrected data appears to be too consistent with the time before the giant World War II El Niño event and the time after. I would expect some of the anomaly seen in the raw data to survive the correction process.

Works Cited

Brönnimann, S. (2003). A historical upper air-data set for the 1939–44 period. International Journal of Climatology, 23(7), 769-791. doi:10.1002/joc.914

Brönnimann, S. (2005). The Global Climate Anomaly in 1940-1942. RMetS Weather, 60(12).

Brönnimann, S., & Luterbacher, J. (2004b). Reconstructing Northern Hemisphere upper-level fields during World War II. Climate Dynamics, 22, 499-510. doi:10.1007/s00382-004-0391-3

Brönnimann, S., Luterbacher, J., & Staehelin, J. (2004). Extreme climate of the global troposphere and stratosphere in 1940–42 related to El Niño. Nature, 431, 971–974. doi:10.1038/nature02982

Hegerl, G. C., Brönnimann, S., Schurer, A., & Cowan, T. (2018). The early 20th century warming: Anomalies, causes, and consequences. WIREs Climate Change, 9(4). doi:10.1002/wcc.522