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Sunday, February 22, 2009

The Impact of the North Atlantic and Volcanic Aerosols on Short-Term Global SST Trends

I’ve moved to WordPress.  This post can now be found at The Impact of the North Atlantic and Volcanic Aerosols on Short-Term Global SST Trends

I took this post from the prior one “A Secondary (Repeated) ENSO Signal?” and added to the narrative because it is worthy of its own post. I’ve also added long-term trend comparisons at the end to illustrate a point.

I prepared a post on SST trends for the globe, Figure 1, and for individual ocean subsets. (That post has now been put to the side.)
Figure 1

The disparity between the North Atlantic SST anomaly trend, Figure 2, and the rest of the subsets was striking. The North Atlantic SST anomaly linear trend for the period of November 1981 (the start of the OI.v2 SST dataset) and January 2009 is ~0.264 deg C/decade, while the global linear trend is ~0.0948 deg C/decade. The North Atlantic linear trend is approximately 2.8 times the global linear trend, driven by Atlantic Meridional Overturning Circulation and El Ninos, (yes, El Ninos).
Figure 2

NOTE: El Nino-induced step changes in the North Atlantic were illustrated in the post There Are Also El Nino-Induced Step Changes In The North Atlantic. Recall, also, that the Atlantic Meridional Overturning Circulation appears to be impacted by ENSO events as well. Refer to the post titled Atlantic Meridional Overturning Circulation Data.


So I decided to remove the North Atlantic SST anomaly data from the global. There is no simple way to do this with a coordinate-based system such as NOMADS or KNMI Climate Explorer, so I made an assumption. The Atlantic Ocean surface area is approximately 30% of the global ocean surface area, and I assumed the North Atlantic represented 50% of that. I then scaled the North Atlantic SST anomaly data by 0.15 and subtracted it from the global SST anomalies. The resulting dataset, noted as “Global SST Anomalies Without North Atlantic,” is illustrated in Figure 3. There isn’t a significant difference between the peaks and troughs of this adjusted dataset and those of the global SST anomalies. Visually, the curve appears as though it's been rotated. What stands out, however, is the decrease in trend to 0.0546 deg C/decade.
Figure 3


Volcanic aerosols lower the SST anomalies at the time of the eruption and for a few years afterwards, so SSTs would have dropped as a result of the 1982 El Chichon and 1991 Mount Pinatubo eruptions. This increased the trend over the term of the dataset. Figure 4 is a comparative graph of the “Global SST Anomalies Without North Atlantic” and inverted Sato Index of Stratospheric mean optical thickness. I’ll use the Sato Index data to remove the impacts of the volcanic eruptions.
Figure 4

If we consider that the peak of the decrease in global land plus sea surface temperature caused by the Mount Pinatubo eruption is considered to be between 0.2 to 0.5 deg C, the 0.15 deg C illustrated at 1991 would be toward the conservative side for SST. Eyeballing it, it appears to be in line, so there doesn’t seem to be any need to scale the Sato Index data. I then added the Sato Index to the “Global SST Anomalies Without North Atlantic” data. The resulting changes in the “Global SST Anomalies Without North Atlantic” data is shown in Figure 5. Note how the trend has decreased to 0.036 deg C/decade.
Figure 5


Figures 6 and 7 are long-term comparisons of global SST anomaly data and the global SST anomaly data with the North Atlantic and the effects of volcanic aerosols removed. Figure 6 uses ERSST.v2 SST anomaly data and Figure 7 uses HADISST SST anomaly data. I used the same scaling factors that I used in the short-term data. All datasets have been smoothed with 37-month filters (but the trends were determined from the unsmoothed data, which is why the trend lines extend beyond the smoothed curves). For ERSST.v2 Global SST anomaly data, the linear trend from January 1880 to January 2009 is approximately 0.040 deg C/decade, but with the North Atlantic and volcanic aerosols removed, the linear trend drops to approximately 0.033 deg C/decade. A similar drop results with the HADISST SST anomaly data, with the Global SST anomaly linear trend dropping from 0.041 deg C/decade to 0.035 deg C/decade when the North Atlantic and volcanic aerosols are removed. Note that over the period of 1880 to 2008 there is no significant change in the trends caused by volcanic aerosols. The Mount Pinatubo and Krakatau eruptions were approximately the same magnitudes, both had a DVI of 1,000, so they would tend to “balance” one another at opposite ends of the period.
Figure 6
Figure 7


The Global SST anomaly trend from November 1981 to January 2009 is approximately 0.0948 deg C/ decade or 0.95 deg C/century. Without the natural variations in North Atlantic SST anomalies and without the impacts of two significant volcanic eruptions, the SST trend would project out to a rise of only 0.36 Deg C over the next 100 years, which is more in line with the long-term trends.

Figure 8 is the upper half of “FAQ 3.1, Figure 1” from the IPCC’s “Frequently Asked Question 3.1 - How are Temperatures on Earth Changing?”

The caption reads, “Note that for shorter recent periods, the slope is greater, indicating accel­erated warming. The blue curve is a smoothed depiction to capture the decadal variations. To give an idea of whether the fluctuations are meaningful, decadal 5% to 95% (light grey) error ranges about that line are given (accordingly, annual values do exceed those limits). Results from climate models driven by estimated radiative forcings for the 20th century (Chapter 9) suggest that there was little change prior to about 1915, and that a substantial fraction of the early 20th-century change was contributed by naturally oc­curring influences including solar radiation changes, volcanism and natural variability. From about 1940 to 1970 the increasing industrialisation following World War II increased pollution in the Northern Hemisphere, contributing to cooling, and increases in carbon dioxide and other greenhouse gases dominate the observed warming after the mid-1970s.”
Figure 8

Are the IPCC taking advantage of the natural cycles of Atlantic Meridional Overturning Circulation and of periodic volcanic eruptions to illustrate “ACCELERATED WARMING” in more recent decades?

Of course they are!


OI.v2 SST data can be accessed through the NOAA NOMADS system:

ERSST.v2 data and HADISST data can be accessed through the KNMI Climate Explorer website:

The Sato Index Data is available from GISS at:

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