Pacific Decadal Oscillation - Effect on Climate.


It is only in recent years that scientists are starting to recognize the influence of atmospheric and oceanic cycles in influencing climate.

A 2008 study - "Oceanic Influences on Recent Continental Warming", by Compo, G.P., and P.D. Sardeshmukh, (Climate Diagnostics Center, Cooperative Institute for Research in Environmental Sciences, University of Colorado, and Physical Sciences Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration), Climate Dynamics, 2008) stated:

 "Evidence accrued over the last 50 years has indicated that the recent worldwide land warming occurred largely in response to a worldwide warming of the oceans rather than as a direct response to increasing greenhouse gases (GHGs) over land."

Several recent studies suggest that the observed Sea Surface Temperature (SST) variability may be misrepresented in the coupled models used in preparing the IPCC's Fourth Assessment Report, with substantial errors on annual and decadal scales. There is a hint of an underestimation of simulated decadal SST  variability even in the published IPCC Report.

The Pacific Decadal Oscillation (PDO) was defined by fisheries scientist Steven Hare in the mid-1990's, based on observations of Pacific fisheries cycles. The PDO index is calculated from sea surface temperatures and sea level pressures. 

The PDO goes through warm and cool phases of a repeating cycle with phases typically lasting about 30 years. The causes of the oscillation appear to be related to 60 year Solar Wind / Sunspot cycles that change the rotation of the Earth and cause major warm ocean currents to move more northerly (high sunspots) or southerly (low sunspots) directions, bringing associated weather extremes along with them.   

The recent phases of the PDO can be seen in the above figure, with a cool phase starting around 1945 and switching to a warm phase in 1977. This correlates to sunspot observations. It appears that 2008 may be the start of the next cool phase. 

The current shift of the PDO has been shown to have occurred at the same time as the recent 30 year rise in global temperatures (blamed on rise in C02). It is now widely accepted that a climatic regime shift transpired in the North Pacific Ocean in the winter of 1976-77.  This is on target with observations of global warming for the following 30 years.

This regime shift has had far reaching consequences for the large marine ecosystems of the North Pacific. Subsequent research has suggested that this event was not unique in the historical record but merely the latest in a succession of climatic regime shifts.

The following figure is from a NOAA study of the impact of the PDO variability on the California Current ecosystem and shows the approximately 60-year cycle of the PDO and the corresponding northern Pacific Ocean temperature regimes.

Does the PDO affect Arctic Ice ? - It is tempting to suggest that the PDO would extend its influence into the Arctic during high Sunspot phases when the warm currents push high into the northern hemisphere. The PDO definitely affects Alaskan temperatures and climate as well as ocean biosystems but as far as currents getting into the Arctic this is somewhat unlikely because of the narrowness of access to the Arctic ocean. This is shown clearly on the following map.

It can be clearly seen that the Arctic is much more an extension of the Atlantic system than it is of the Pacific System. Therefore it is probably wise to attribute the warming or cooling of the arctic to the movements of the Atlantic currents, predominantly the Arctic and Gulf Streams. These switch in the same manner as the PDO on a sixty to seventy year cycle - known as the Atlantic Multidecadal Oscillation (AMO).

Morner has shown the movements of these currents and suggested the mechanism is related to Sun cycles causing solar wind / earth interactions that change rotation speeds and shift current directions. 

What does the PDO affect then? 

Many studies have shown that the major climatic events in the US are linked to cycles in ocean temperatures. Here are some PDO examples:

1. ) The Jet Stream and Droughts - NASA Explains 'Dust Bowl' Drought (NASA/Goddard Space Flight Center (2004, March 19).)  

Cooler than normal tropical Pacific Ocean temperatures and warmer than normal tropical Atlantic Ocean temperatures contributed to a weakened low-level jet stream and changed its course. 

The Jet stream, a ribbon of fast moving air near the Earth's surface, normally flows westward over the Gulf of Mexico and then turns northward pulling up moisture and dumping rain onto the Great Plains.

 As the low level jet stream weakened, it traveled farther north than normal, missing the Gulf and not picking up the rain it would normally  carry to the more northerly climates. The Great Plains dried up and dust storms formed. 

Analysis of other major U.S. droughts of the 1900s suggests a cool tropical Pacific was a common factor.

Note: Gulf of Mexico app. 25 N to 30 N
Note: Gulf of Mexico app. 25 N to 30 N

Note: How to read these images. There is really only two actual jet streams - the polar and the subtropical that affect the northern hemisphere. These jet streams generally move from the Pacific Ocean to the Atlantic Ocean in an easterly flow pattern rotating in an anticlockwise fashion but causing the centre stream to move in a clockwise motion.  

These three streams are depicted below but remember this is the direction of a TUBE OF AIR that is also rotating in a clockwise (Ferrel Cell) or anticlockwise (Hedley and Polar Cells).

The diameter of these "screws" can be quite large as much as 30 degrees in latitude. Envision the mid-latitude jet stream (actually caused by the effect of the subtropical and the polar streams) would span the range from the Gulf of Mexico to the lower edge of Alaska by this reasoning. Shifts north or south caused by subtropical or polar  "pushing" would have a large effect as they carry southerly weather to the northern ranges or the opposite.

Image used for Perspective.
Image used for Perspective.

It is this clockwise motion of the Hedley cell that picks up Gulf moisture and moves it north to the states above. This is shown in the cross section above.

The net Jet stream is highly variable, being pushed up and down by each other. The image below shows how the average mid stream bends first north and then drops dramatically south changing the state of weather below them. 

2.) The Jet Stream and Midwest Rains - Temperature of Pacific Ocean Influences Midwest Rains (University Of Illinois At Urbana-Champaign (1997, September 11).)  

The Jet Stream, a ribbon of fast moving air near the Earth's surface, normally flows westward over the Gulf of Mexico and then turns northward pulling up moisture and dumping rain onto the Great Plains. Sea-surface temperatures in the Pacific Ocean affect both the position and the intensity of the jet stream over the central United States, which in turn modifies the Jet Stream circulation pattern coming off the gulf. 

Warmer sea-surface temperatures shifts the jet stream farther south with more exposure to the Gulf, leading to more storm activity, which pumps even more moisture up from the gulf.  

In contrast, cooler sea-surface temperatures shift the jet stream farther north away from the Gulf, resulting in reduced storm activity and drier conditions as the air is less moisture laden.

3.) Northern Hemisphere land temperatures - The Significance of the 1976 Pacific Climate Shift in the Climatology of Alaska (Geophysical Institute, University of Alaska, 2004)  

In 1976, the North Pacific region, including Alaska, underwent a dramatic shift to a climate regime that saw great increases in winter and spring temperatures .

The shift in the climate regime now is known to have coincided with a shift in the phase of the Pacific Decadal Oscillation (PDO). ...all of the regions in sub- Arctic Alaska experienced a net warming since 1977.

4.) Global air Temperatures - Tropical Pacific Decadal Variability and Global Warming (Amy J. Bratcher & Benjamin S. Giese, Department of Oceanography, Texas A&M University, Geophysical Research Letters, 29(19), 2002)  

An analysis of ocean surface temperature records show that low frequency changes of tropical Pacific temperature lead global surface air temperature changes by about 4 years. Anomalies of tropical Pacific surface temperature are in turn preceded by subsurface temperature anomalies in the southern tropical Pacific by approximately 7 years.

 The results suggest that much of the decade to decade variations in global air temperature may be attributed to tropical Pacific decadal variability. The results also suggest that subsurface temperature anomalies in the southern tropical Pacific can be used as a predictor for decadal variations of global surface air temperature. 

Since the southern tropical Pacific temperature shows a distinct cooling over the last 8 years, the possibility exists that the warming trend in global surface air temperature observed since the late 1970's may soon weaken.

5.) Herring / Sardines - The PDO shows a strong correlation with sardine catch. A report about the sardine fishery states: 

Fish scales found in sediments of the Santa Barbara Basin reveal that, over the past 1,700 years, West Coast sardine populations seem to follow a 60-year cycle of abundance and disappearance (Baumgartner, Soutar et al. 1992).  Sardines proliferate in warm Ocean waters and decrease when those waters cool. 

The following figure compares Pacific sardine catch with the PDO (see green line).

6.) Zooplankton - The following figure shows a strong correlation between the PDO and zooplankton (copepod) in the Pacific Northwest from a NOAA study of the impact of the PDO variability on the California Current ecosystem. Zooplankton grow well in warm ocean temperatures (sometimes too much and we get blooms) and slow growth during cool ocean temperature periods. 

7.) Salmon -  lag zooplankton and sardine / herring growth patterns (makes sense) tending to proliferate during cooler ocean periods after herring stocks have replenished and food is available. The following figure is from the same study showing a strong negative correlation between the PDO and California Chinook salmon abundance.

These are from a study of the impacts of the PDO on salmon production. It shows a positive correlation with the PDO for Alaskan salmon:  

A remarkable characteristic of Alaskan salmon abundance over the past half-century has been the large fluctuations at interdecadal time scales which resemble those of the PDO. 

Time series for Washington-Oregon-California (WOC) coho and Columbia River spring chinook landings tend to be out of phase with the PDO index. 

Recent work suggests that the marine ecological response to the PDO-related environmental changes starts with phytoplankton and zooplankton at the base of the food chain and works its way up to top level predators like salmon.


High Sun Magnetic Activity - High Sunspots - High Solar Wind - Decreased Cloud Cover - Decreased Earth rotation speed - Ocean Currents turn North - Strong Jet Stream flowing west over Gulf then north to Midwest - increased Northern Pacific Ocean temperatures ---  all  causing increased northern hemisphere temperatures / decreased droughts due to  increased gulf based midwest rain storms / increased herring / increased zooplankton / less salmon (lags zooplankton and sardine population increases). 

Low Sun Magnetic Activity - Low Sunspots - Decreased Solar Wind - Increased Cloud Cover - Increased Earth rotation speed - Ocean currents turn South - Weak Jet Stream drops southward below Gulf and doesn't flow north to Midwest -  decreased  Northern Pacific Ocean temperatures ---  all causing decreased northern hemisphere temperatures / increased midwest droughts due to  reduced midwest Gulf based rain / decreased herring /  decreased zooplankton / but improved salmon catch.

Note:  I've relied heavily on one source for my summary of events above.  I've deleted some of the references to materials listed originally in this source to make the blog "readable". If you wish to see this subject (and others) in much better detail as well as to obtain source papers for the sections above, please use the button below where you will find what you need.