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Wednesday, March 31, 2010

Which Has The Greater Impact On Global SST Anomalies, The Rise In The SST Anomalies Of The Arctic Ocean Or The Decrease In The Southern Ocean?

This post illustrates an observation I’ve made a number of times while investigating other subjects, so I thought I’d post it. It shows the relative strengths of the polar SST anomalies.

Using the NCDC’s Optimum Interpolation (OI.v2) SST Anomaly dataset, which begins in November 1981, the Arctic Ocean shows a significant positive linear trend. Refer to Figure 1. Over the same period, the Southern Ocean shows a negative linear trend that’s much less significant. The latitudes I’ve used for the Arctic Ocean are 65N-90N, while the Southern Ocean latitudes are 90S-60S.
Figure 1

According to Wikipedia, the area of the Arctic Ocean is 14,056,000 sq. km. The Southern Ocean, on the other hand, has an area of 20,327,000 sq. km or about 45% larger than the Arctic Ocean. Sea ice also varies in the polar oceans, as is well known.

Taking those factors into consideration, which plays a greater role in global SST anomaly trends, the rise in the Arctic Ocean SST anomalies, or the drop in the Southern Ocean SST anomalies?

The easiest way to illustrate it is to compare global SST anomalies with and without the polar oceans. (Or someone other than me could go through all of the calculations to factor in area, monthly sea ice extent, etc., using polar ocean data.) If the rise in the SST anomalies of the Arctic Ocean is having the greater impact on global SST anomalies than the drop in the Southern Ocean data, then the Global SST anomaly dataset including the polar oceans would have a higher trend than the a global dataset without the polar oceans. Refer to Figure 2. The opposite takes place.
Figure 2

While the two linear trends are close, the drop in the Southern Ocean SST anomalies has the greater effect. That is, adding the polar oceans to the data caused the global SST anomaly trend to decrease.

Someone will note that the latitudes of the Arctic and Southern Ocean datasets are not the same. The Southern Ocean is generally defined as the ocean south of 60S. But the Arctic Ocean borders aren’t defined by one latitude. Does this skew the results? Since the NOAA NOMADS system requires latitudes and longitudes as inputs, I can’t customize the Arctic Ocean borders. But I can extend the latitudinal range to 60N-90N. Same results. The linear trend of the dataset without the polar data has the higher linear trend, though the gap is closing.
Figure 3

SST anomaly data is available through the NOAA NOMADS website:http://nomad1.ncep.noaa.gov/cgi-bin/pdisp_sst.sh


John said...

Hi Bob -

You have likely seen this, but I thought it appropriate given the topic of this post.


Bob Tisdale said...

Thanks, John.

Andrew said...

Bob-From what I understand the Arctic Ocean SST data is highly suspect due to the sea ice retreat allowing new gridcells to come in that had zero data before. They have nothing but short strings of positive anomalies with nothing but fixed values in the base period.

Bob Tisdale said...

Andrew: The Arctic presents a unique problem: how to address the simple fact that some years the ice melt is greater than others, exposing more sea surface. There is no base period for anomalies, since it’s brand spanking new data. I don’t know that there’s a correct way to address it. Each of the SST anomaly datasets appears to take a different approach.

(Note the unusual 12-month shift, 1996, in the ERSST.v3b data. I’ll have to call that to their attention. Looks like a glitch.)

Anonymous said...

Highly OT

Mr Tisdale,

Would you be kind and have a look into those images/maps/plots on PWP (Pacific Warm Pool):


It seems to me the PWP is losing energy (big magenta area). At the same time red-white area is deminishing (and not bulging I think).

Meantime, correlated or not, there is higher upper atmosphere temperature:


And lower than average global surface temperatures (see huge blue areas):


Would you be kind to elaborate on the issue if it is a "real issue", of course.

The question is (for me) - where would the energy from PWP dissipate (if it realy happened)?

Best regards
Przemysław Pawełczyk

Bob Tisdale said...

Przemysław Pawełczyk: You asked, "...where would the energy from PWP dissipate (if it realy happened)?"

As you know, much of the decline in Sea Level anomaly in the PWP is caused by the shifting of the warm water east during the El Nino. Some of it is caused by the change (reversal/decline) in trade winds. During the transfer from El Nino to La Nina over the next six to nine months, the PWP will regain much of decline as the warm water sloshes back to the west.

The east and west equatorial Pacific Warm Water Volume data from the TAO project website illustrates this pretty well:

They run a little out of synch at times and with the latest data, the eastern equatorial Pacific appears to have peaked while western equatorial Pacific has not. The west looks like it's still declining.

Here's a link to the WWV graphs page from the TAO project:


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