The PDO is NOT a Simple Residual Like the AMO.
People understand the Atlantic Multidecadal Oscillation (AMO). It’s calculated very simply; subtract Global SST (sea surface temperature) anomalies from the North Atlantic SST anomalies. This simple process has been said to remove the global warming signal from the AMO. Many people believe the Pacific Decadal Oscillation (PDO) is calculated using the same basic equation, but it’s not. According to Nathan Mantua of JISAO, the details of how the PDO is calculated are found in this paper:
Calculating the PDO is a multistep process. It includes creating an SST anomaly time series for each 5 degree grid of the North Pacific (North of 20N), calculating the residual for each grid, and computing the EOFs ( empirical
orthogonal function) of these North Pacific residual SST anomaly fields. The PDO index is the leading PC (principal component) of that analysis. It’s far from a simple process.
The PDO has been found to be a function of ENSO. In “ENSO-Forced Variability of the Pacific Decadal Oscillation“, Newman et al state in the conclusions, “The PDO is dependent upon ENSO on all timescales.”
A few months ago, I discovered the instructions for retrieving Smith and Reynolds ERSST.v2 data (extended reconstructed SST) from the NOAA NOMADS system based on user selected dates and global coordinates:
One of the first data sets I downloaded was the time series of SST anomalies for the North Pacific, 20 to 65N, what I called the Mid-Latitude North Pacific SST Anomaly in the following graph. Note the 0.9 deg C drop then rebound in temperature from the late 19th to the mid-20th centuries. It’s tough to miss. It certainly appears to be related to Meridional Overturning Circulation (MOC), not ENSO.
Using the same simple process employed to calculate the AMO, that is, subtracting the Global SST Anomaly from the Mid-Latitude North Pacific Anomaly, provides a data set that I’ve dubbed the North Pacific Residual.
The North Pacific Residual bears no resemblance to the PDO. In fact, note that I had to scale the PDO to bring it back into line with the data set from which it is extracted. (The PDO data illustrated is from the ERSST.v2 data set, not the JISAO version. The curves of the two PDO data sets are similar, but the ERSST.v2 data extends further back in time.)
When compared to the AMO, the two Northern Hemisphere SST oscillations complement one another from the 1920s to present. Prior to that, they were out of synch, offsetting their individual impacts on global temperature. It is no coincidence that Northern Hemisphere and global temperatures follow the rises and falls of these two residual anomalies.
Past studies have estimated the contribution of the AMO to the rises and falls of Northern Hemisphere and Global temperatures over the 20th century. I would think that the North Pacific Residual would contribute similarly. Shouldn’t climatologists and climate change bloggers have another index of North Pacific temperature anomalies, one that could be used to determine the effect of the North Pacific SST oscillation on Northern Hemisphere and Global temperatures?
The links and graphs are from my series on Smith and Reynolds SST data:
Sea Surface Temperature Data is Smith and Reynolds Extended Reconstructed SST (ERSST.v2) available through the NOAA National Operational Model Archive & Distribution System (NOMADS):