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Tuesday, September 14, 2010

An Inverse Relationship Between The PDO And North Pacific SST Anomaly Residuals

I’ve moved to WordPress.  This post can now be found at An Inverse Relationship Between The PDO And North Pacific SST Anomaly Residuals
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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.

http://i56.tinypic.com/ipcodt.jpg
Figure 1

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.
http://i52.tinypic.com/ipaxjr.jpg
Figure 2

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.
http://i52.tinypic.com/15oz3eo.jpg
Figure 3

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

Thanks, TallBloke.

SOURCE

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

10 comments:

tallbloke said...

Bob, thank you, it's one thing to spot a relationship, quite another to be able to explain the reason for it. I think this is one of the reasons caution needs to be exercised when looking for vague wiggle matches between disparate datasets; co2 and temperature, the first derivative of solar cycle length and a particular ocean basin's temperature variation etc.

Your in-depth knowledge of the circulations of the Pacific ocean seem to have this one nailed anyway. Well done!

Gus said...

I am curious what you think.

In general I suspect the gyre produced by the PDO generates a general wind pattern the blowing from the northeast would help pile waters in the warm pool and thus fewer El Ninos, and alternates with a gyre motion that pushes winds more from the northwest and to some extent counteracting the trade winds thus allowing for more El Ninos as there is less force to keep piling up the warm water in the warm pool.

I saw a vertical cross section of the warm pool in a presentation by Mehta and there was a build up of warm temperature anomalies deep in the warm pool until 1976 that disappeared when the PDO shifted.

Bob Tisdale said...

Gus: Do you have a link to the Mehta study or presentation? The reason I ask is, you wrote, "there was a build up of warm temperature anomalies deep in the warm pool until 1976 that disappeared when the PDO shifted." And that seems to be contradicted by the NODC (Levitus et al) OHC data:
http://i36.tinypic.com/eqwdvl.png
That graph is from:
http://bobtisdale.blogspot.com/2009/09/enso-dominates-nodc-ocean-heat-content.html

Based on the OHC data, the additional warm water then fueled the additional El Nino events, which, in turn, caused the shift in the PDO.

Also, the PDO lags ENSO by a few months, so it would be difficult for it to drive the El Nino events. Refer to Figures 19 and 20:
http://bobtisdale.blogspot.com/2010/09/introduction-to-enso-amo-and-pdo-part-3.html

Gus said...

The profile is from a CRCES presentation titled

Natural decadal-multidecadal variability of the Indo-Pacific Warm Pool and its impacts on global climate

http://www.crces.org/presentations/dmv_ipwp/

The graph is right below the section titles:

"Vertical temperature structure and upper-ocean heat content in WPWP and equatorial eastern Pacific: SODA, 1950-2001"

The warm anomaly starts around 1965 and peaks 1976 between 80 and 200 meters

Bob Tisdale said...

Gus: Thanks for the link. I assume this is the plot you’re referring to:
http://www.crces.org/presentations/dmv_ipwp/images/figure9.gif

The mid-1970s hotspot coincides with the 1973/74/75/76 La Niña, which makes sense. La Nina yields an increase in trade winds, which yields less cloud cover, which yields more DSR, which yields the increase in OHC. And the cold spot in 1991 should be caused by Mount Pinatubo.

What’s odd about the graph of tropical Pacific OHC graph that I linked in my earlier reply is that it’s hard to find the volcanic eruptions.

Gus said...

Why would you conclude that the cooling at 100 m depth is due to Pinatubo? Due to 1000X heat capacity differences between atmosphere and oceans, if eruptions cooled the atmosphere I would expect a small near surface change but not a cool anomaly deeper.

I would think it is more likely the general long term wind pattern of the PDO in the 60's and 70' which encouraged great La Ninas and thus greater warming of the oceans, also piled up more warm water in the western Pacific. But since SST seldom exceed 28C due to rapid increase in evaporation, that evaporation resulted in denser warmer water sinking deeper.

I would also suspect a degree of positive feedback maintaining the warm anomalies in the eastern Pacific. Thus the low frequency of PDO

I would see along with the PDO regime shift of '76, as shifting wind patterns, slackening the positive feedback and encouraging more frequent El Nino's. Now with warm water more frequently covering the eastern Pacific there is less solar heating of the ocean and more "venting of the warm pool. Thus I see the change in that warm pool vertical profile as reflecting an interaction between PDO and ENSO. Mehta shows a interdecadal oscillation in the warm pool and I think that interdecadal oscillation is directly related to PDO. Together they modulate the degree and frequency of El NInos.

Bob Tisdale said...

Gus wrote, “Why would you conclude that the cooling at 100 m depth is due to Pinatubo? Due to 1000X heat capacity differences between atmosphere and oceans, if eruptions cooled the atmosphere I would expect a small near surface change but not a cool anomaly deeper.”

It’s not the atmosphere that cools the ocean. It’s the drop in downward shortwave radiation caused by the volcanic aerosols. Also keep in mind that you’re looking at anomalies in that plot, not absolute temperatures.

You wrote, “I would think it is more likely the general long term wind pattern of the PDO in the 60's and 70' which encouraged great La Ninas and thus greater warming of the oceans,..”

The PDO lags ENSO. The PDO is an aftereffect of ENSO.
http://bobtisdale.blogspot.com/2010/09/introduction-to-enso-amo-and-pdo-part-3.html

You wrote, “I would see along with the PDO regime shift of '76…”

It’s not only the PDO that shifted in 1976. The frequency and magnitude of ENSO switched.
Also in 1976, the SST anomaly for the South Pacific shifted upwards, and the SST anomaly for the eastern Pacific shifted upwards.

Regards

Gus said...

Bob you mentioned the oddity of not finding the Pinatubo in the Pacific OHC graph. Nor is it visible in the Nino 3.4. Pinatubo is assumed by so many to account for the .6 degree global temp drop but still it does not show up in the tropical ocean temperatures. You assumed that the 150 M deep cold spot in CRCES warm pool profile in the early 90's is due to Pinatubo but the surface is not more than a .3 negative anomaly while in the eastern Pacific the temperature is .a positive .6 degree anomaly.

SInce it is the exportation and redistribution of warm equatorial waters that largely determines the global temperatures, I would expect to see a larger drop in the equatorial OHC during Pinatubo, but it is just not there. To me that suggests that Pinatubo and aerosols are overrated.

Do you have a link to how the effect of Pinatubo was calculated?

Regards PDO lagging ENSO, my use of PDO needs clarification. I am referring to it as a low frequency oscillation affecting general patterns over a 40 year span. ENSO is a higher frequency oscillation, with many cycles within the larger PDO. So in that sense I fear we are mis-communicating.

Bob Tisdale said...

Gus: You wrote, "SInce it is the exportation and redistribution of warm equatorial waters that largely determines the global temperatures, I would expect to see a larger drop in the equatorial OHC during Pinatubo, but it is just not there."

I waver back and forth on it from time to time. It's tough to tell. Comparing the Tropical East and West OHC anomalies, the drop in the west from 1991 to 1995 seems excessive for the strength of the El Ninos at the time.
http://i51.tinypic.com/x55sna.jpg

You wrote, “I am referring to it as a low frequency oscillation affecting general patterns over a 40 year span. ENSO is a higher frequency oscillation, with many cycles within the larger PDO.”

Actually, you really need to plot the raw PDO data to see that it’s just as noisy as NINO3.4 SST anomalies.
http://i51.tinypic.com/2s85ij8.jpg

Or comparing the two…
http://i52.tinypic.com/11h6k3b.jpg

And there’s also a decadal and multidecadal low-frequency component of ENSO:
http://i54.tinypic.com/33di5qr.jpg

tallbloke said...

I agree with Gus that Pinatubo's effect on climate is overrated. It wasn't just trhat volcanic aerosols diominished insolation, there was a big drop in TSI anyway at that time.
See my graphs near the bottom of this discussion on my blog:
http://tallbloke.wordpress.com/2010/08/05/volcanos-dont-cause-global-cooling

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