The Atlantic and Pacific basins tend to see changes in temperature and circulation patterns on multi-decadal, century, and even multi-century time scales. These changes have large impacts on the climate on short and long-term time scales.
There is decadal variability in the Pacific Ocean, known as the PDO (Pacific Decadal Oscillation), which has been theorized to exist due to oceanic processes. The Atlantic Multidecadal Oscillation (AMO) is another decadal variation in the ocean, but in the Atlantic basin, which has been theorized to exist due to the power of the thermohaline circulation, also referred to as the “Great Ocean Conveyor Belt,” which we learned about in The Ocean section of this website.
THE PACIFIC DECADAL OSCILLATION (PDO)
The Pacific Decadal Oscillation (PDO) is a “switch,” as Dr. Roy Spencer puts it, between two different circulation patterns in the North Pacific, which change from one to the other on approximately 30-year time scales.
The PDO was first mentioned by University of Washington scientists (Mantua et al., 1997) in 1997, while they were explaining salmon fishery production in the Pacific, which was based off ocean temperatures referred to as the Pacific Decadal Oscillation. The trend that was noticed was that the sea surface temperatures would overlay with the Aleutian low air pressure, which remained in the positive mode for about 30 years, then would flip to the opposite for another 30. It has also been noted that while the PDO is in its warm or positive phase, that the Northern Hemisphere tends to become warmer, whereas during the negative or cool phase, the PDO would bring cooler temperatures to the Northern Hemisphere.
Prior to 1997, climatologists often noted about the Great Pacific Climate Shift in 1977, which was the reorganization of the ocean currents and the temperatures in the Pacific, which has likely been one of the biggest factors in recent warming trends. As you can see in the graph below, the PDO went from its negative phase, which started in 1947, to its positive phase in 1977, and it has been in its positive phase since then.
It has been theorized that the PDO is a vital factor in long-term climatic changes, and especially, a large factor in the current climate debate that has been going on since 1988. Even small changes in ocean-atmosphere circulation patterns can have a huge effect on global cloud cover. With an increase in cloud cover, more sunlight gets reflected from reaching Earth’s surface, which in turn cools the planet.
Furthermore, atmospheric pressure changes tend to correlate well with water temperatures. The Aleutian low correlates inversely with the Pacific Decadal Oscillation. When the PDO is in its positive phase, the Aleutian low becomes stronger (lower pressure) while it becomes weaker during the negative phases of the PDO (Figure 2).
With a stronger Aleutian low, southerly winds are brought to Alaska, which in turn bring warmer water to the coast. This is the main reason that Alaska has been warm in recent decades. During the Great Pacific Climate Shift of ’77, the temperatures rose in the first two years of the shift, which is why (in the graph below) (Figure 3), that we see a warmer Alaska than we did, say, 50 years ago. Since the shift, temperatures have been relatively steady with up and down trends.
In addition, aside from Alaska being warm in recent decades, much of the warmth in western North America can be attributed to the same thing; the transition into the positive phase of the PDO in 1977. This correlated with warm waters in the Niño regions, which meant that more El Niños would occur than La Niñas, which is exactly what we have seen in recent decades. Therefore, a negative PDO would mean the opposite, a cold Alaska and a warm western U.S., which once again, it what we observed.
Typically, El Niño events are associated with short-term global warming, while La Niña is generally associated with short-term global cooling. However, because the phase of the PDO determines which phase of ENSO will be dominant, the effects we see from that are long-term.
If we look at the global temperature data from University of Alabama, Huntsville (UAH) (Figure 5), we can easily see where El Niño and La Niña events occurred. Dr. Spencer’s and Dr. Christy’s measurements date back to 1979, which began right after the Great Pacific Climate Shift, in which we will notice that more El Niño events have typically occurred since 1979.
I also want you to take note of how cold the early and mid 1980s as well as the early and mid 1990s were. Despite La Niñas being dominant during those particular years, the main reason for the cooling, as you will learn in the next section, is due to volcanic eruptions. Mt. St. Helens and El Chichon erupted in 1980 and 1982, respectively, which was the dominant factor of global cooling during the 1980s, whereas the cooling during the 1990s was brought on by the eruptions of Pinatubo and Cerro Hudson, both in 1991 (Figure 6). It is also interesting to point out that the past 20 years have been relatively quiet in volcanic activity, which has likely contributed some to recent warming. This was also prominent during the 1930s and 1940s, which is likely what also contributed to warming during those decades. During the 1800s, 1960s, and 1980s, volcanic activity was very high, which likely contributed to the cooling seen during those decades.
Since the Great Climate Shift of ’77, the Arctic has been warming, which has resulted in some ice melt over the last 40 years. However, despite news reports claiming that this is due to your SUV, you need to realize that this is cyclical. In late 2007, the Northwest Passage opened, which allowed ships to go from the Atlantic to Pacific, through the Arctic. Thus, this event caused the media to flip out stating that this event was “unprecedented,” despite the Northwest Passing being open during the late 1930s and early 1940s, when the Arctic was just as warm as today (Figure 7).
My research has shown that the PDO has a 22% correlation with global temperature between 1900 and 2017 (Figure 8).
THE ATLANTIC MULTI-DECADAL OSCILLATION (AMO)
The Atlantic Multi-Decadal Oscillation (AMO) (Figure 9) is very similar to the PDO, in which it undergoes changes in ocean temperatures and that each phase lasts about 30 years, but the difference is that the AMO is obviously in the Atlantic and the PDO is in the Pacific basin.
The AMO has been in its positive phase since 1995, which means that it tends to favor a more active Atlantic in terms of tropical cyclone activity. It also tends to favor high latitude blocking during the winter months. However, for the Northern Hemisphere land masses and globally, a positive AMO favors warmer temperatures, similar to a positive PDO.
When the PDO and AMO are in the same phase (Figure 10), there tends to be even more global warming or global cooling taking place depending upon whether they are both in their positive or negative phases.
In the graph below, you can see how the phases of the PDO and the AMO correlate relatively well with the United States temperatures over the last century and a half (Figure 11).
My research has shown that the AMO has a correlation of 71% to global temperatures between 1900 and 2017, which tells me that the AMO is the primary inner climate driver (Figure 12). Also note that the AMO has seemingly had a stronger control of global temperatures in the period 1981 to 2017 than it did between 1900 and 1980. This can likely be the result of the PDO having a slightly downward trend since the mid 1990s as well as sunspot count going in opposite direction to temperature. And indeed, research has shown that the global temperature can be controlled by different factors to different extents and with those extents, they can increase or decrease, come, or go.
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Christy, John R., and Roy W. Spencer. “UAH LT Global Anomaly.” The University of Alabama Huntsville Global Temperatures, The University of Alabama Huntsville, http://www.nsstc.uah.edu/data/msu/v6.0/tlt/uahncdc_lt_6.0.txt.
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