>I recently posted An Introduction To ENSO, AMO, and PDO — Part 3, which provided a discussion of the Pacific Decadal Oscillation (PDO) for those new to climate and climate change. On the thread, TallBloke left a comment about my description of one of the figures. Unfortunately, the comment is lost in the ether. TallBloke took exception this part of the post, “Comparing the North Pacific Residual to the PDO, Figure 13, the two datasets have no relationship with one another. This means that the contribution of the North Pacific (north of 20N) to Global SST anomalies is independent of the PDO.” I’ve reproduced Figure 13 here as Figure 1. He noted that the two curves appeared to be negatively correlated.
Note: The North Pacific SST residuals in this post are for the coordinates of 20N-65N, 100W-100E.
Smoothing both the North Pacific Residuals (North Pacific SST anomalies minus Global SST anomalies) and the scaled PDO with 121-month filters, Figure 2, helps to illustrate this. They do appear to be inversely related on decadal timescales.
And if we invert the PDO data by using a negative scaling factor (-0.2), Figure 3, the two curves definitely show similar variations over similar time periods.
Why would the North Pacific warm faster than Global SST anomalies during periods when the PDO is negative? (This discussion of course relates to the multidecadal variations in both signals as illustrated in Figure 2 and 3. It may not be visible in the yearly variations.) First, for the PDO to be negative over decadal periods, the frequency and magnitude of La Niña events have to exceed the frequency and magnitude of El Niño events, and this is because the PDO represents the ENSO-like pattern of the SST anomalies in the North Pacific, north of 20N. (See note below.) During La Niña events, Pacific trade winds strengthen, which reduces cloud cover over the tropical Pacific. This increases the amount of Downward Shortwave Radiation (visible light) reaching the ocean surface and, in turn, warms the tropical Pacific. The warmer water is pushed to the west by the trade winds and is carried northward by the western boundary current, the Kuroshio Current. Then the warm water is carried eastward by the western boundary current extension, the Kuroshio Extension. This is why there is the area of warm SST anomalies east of Japan during La Niña events. During El Niño events, the trade winds decrease or reverse and less warm water than normal is carried from the tropics up to the Kuroshio Extension.
Note: The PDO also appears to be impacted by changes in sea level pressure. Refer to Is The Difference Between NINO3.4 SST Anomalies And The PDO A Function Of Sea Level Pressure? Would sea level pressures also impact the “gyre spin up” of warm waters from the tropics to the Kuroshio Extension? One would think this could impact the duration of the PDO.
There is also another phenomenon that allows SST anomalies in the Kuroshio Extension to persist for periods longer than ENSO, and it’s called the reemergence. Refer to The Reemergence Mechanism. I’ll also have to add a short sentence about it in the post An Introduction To ENSO, AMO, and PDO — Part 3
The HADISST data used in this post is available through the KNMI Climate Explorer:http://climexp.knmi.nl/selectfield_obs.cgi?someone@somewhere
The PDO data from JISAO is available through the KNMI Climate Explorer “Climate Indices” webpage, but I used the data directly from the JISAO website for this post:http://jisao.washington.edu/pdo/PDO.latest