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Wednesday, April 8, 2009

Individual Ocean SST Anomalies In Perspective

I’ve moved to WordPress.  This post can now be found at Individual Ocean SST Anomalies In Perspective
CORRECTION: Figure 2 and its discussion has been corrected to reflect the discussion in The Atlantic Multidecadal Oscillation - Correcting My Mistake. The North Atlantic Residual is not the AMO.


I first posted the ERSST.v3b version of global, hemispheric and individual ocean (north and south) SST anomalies in KNMI Added ERSST.v3b Data To Climate Explorer. This is follow-up post. In it, I compare the individual ocean SST anomalies to global SST anomalies, which should help put the variability of the individual ocean datasets into perspective. I also present the residuals (individual ocean SST anomalies minus global SST anomalies) and compare the residuals where appropriate.


The SST anomaly graphs are based on ERSST.v3b data, from January 1854 to February 2009. The ERSST.v3b data is available through the KNMI Climate Explorer website. The data have been smoothed with a 37-month running-average filter. The linear trends are calculated by EXCEL for the entire time span of the data. The coordinates used for the individual oceans are listed on the graphs.


Figure 1 is a comparative graph of North Atlantic and Global SST anomalies. The deviations of the North Atlantic SST anomalies from the global dataset are the results of Atlantic Meridional Overturning Circulation and the impact of ENSO events on North Atlantic SST anomalies. Refer to my post There Are Also El Nino-Induced Step Changes In The North Atlantic for further information. Note the exaggerated rise in the North Atlantic SST anomalies from the early 1990s to the early 2000s, but in looking back to the warming period in late 1920s-early 1930s, the recent rate of rise is not unusual. Also note that the linear trend for the North Atlantic SST anomalies (0.025 deg C/decade) is less than the global trend (0.031 deg C/decade).
Figure 1

Figure 2 shows the North Atlantic residual. (It is not the same as the Atlantic Multidecadal Oscillation (AMO), which is detrended North Atlantic SST anomaly.) The lack of a sizable multidecadal oscillation in the early years may result from sparseness of the data, or it is possible that the two variables, North Atlantic and Global SST anomalies, were in synch at the time. At least one longer-term reconstruction shows that North Atlantic SST anomalies do vary on a semi-periodic basis. Refer to Figures 6 and 7 in my post SST Reconstructions.
Figure 2


The South Atlantic and Global SST anomalies are compared in Figure 3. The drop in South Atlantic SST anomalies from the late 1880s to the about 1905 is severe and contributes to the higher linear trend of that dataset, as does the anomalous spike in the early 1970s. Note that the linear trend for the South Atlantic (0.052 deg C/decade) is more than double that of the North Atlantic (0.025 deg C/decade).
Figure 3

The late-19th/early-20th century dip and the unusual early 1970s rise are clearly visible in the South Atlantic residual data, Figure 4.
Figure 4


Considering the contribution of the Pacific Ocean to the global SST anomaly dataset, it makes sense that the North Pacific and Global SST anomalies would be similar. Refer to Figure 5. The two linear trends are fundamentally the same.
Figure 5

The North Pacific Residual is illustrated in Figure 6. It bears no likeness to the Pacific Decadal Oscillation (PDO). Refer to the post The Common Misunderstanding about the PDO for additional clarification on what the PDO is and what it is not.
Figure 6

Note: I’ll show an interesting correlation with the North Pacific Residual later in the post.


Like the North Pacific, the South Pacific SST anomalies, Figure 7, are very similar to the global dataset. The linear trend for the South Pacific (0.027 deg C/decade) is slightly less than the global linear trend (0.031 deg C/decade).
Figure 7

The South Pacific Residual is shown in Figure 8.
Figure 8


Where the SST anomaly linear trend for the South Pacific was slightly less than the global trend, the Indian Ocean linear trend, Figure 9, is slightly higher (0.036 deg C/decade). Note how the Indian Ocean SST anomalies are also very similar to the global SST anomalies, but that the Indian Ocean SST anomalies have been increasing faster that the global SST anomalies since 1970.
Figure 9

This is very visible as a shift in the Indian Ocean Residual data in 1970, Figure 10.
Figure 10

Note that the shift in the Indian Ocean Residual corresponds with the opposing shift in the North Pacific Residuals. See Figure 11. The two datasets oppose one another as far back as the 1870s. What causes the relationship? We’ll have to examine that further in a future post.
Figure 11

And how much did the rise (and fall) in the Southern Ocean SST anomalies that began in the mid-1960s influence the shift in the Indian Ocean Residuals? Refer to Figure 12, which is a comparative graph of Southern Ocean SST anomalies and Indian Ocean Residuals.
Figure 12


Figure 13 compares Arctic Ocean and Global SST anomalies. As you’ll note, the linear trend in the Arctic Ocean SST anomalies (0.014 deg C/decade) is significantly less than the Global linear trend (0.031 deg C/decade). It’s less than half, actually. This should be due in part to the low density of readings in early years. Part of it should also result from there being greater ice cover in earlier years. Visually note the rate of Arctic SST anomaly rise from the late 1960s to approximately 1998. In that period, about 30 years, Arctic Ocean SST anomalies rose ~0.17 deg C. Then from ~1998 to ~2006, after the 1997/98 El Nino, Arctic Ocean SST anomalies accelerated and rose 0.24 deg C in less than 10 years.
Figure 13

The Arctic Ocean Residuals, Figure 14, are misleading, because the Global SST anomalies are the dataset with the greater variability.
Figure 14


The Southern Ocean and Global SST anomalies are compared in Figure 15. Note that the Southern Ocean linear trend is negative. The two late 19th century peaks in Southern Ocean SST anomalies were higher than they were in the late 20th century. Note also that the Southern Ocean SST anomalies have been declining since the 1980s, or the early 1990s, depending on your perspective, while the global SST anomalies rose until recently. This is not unusual. Southern Ocean and Global SST anomalies were also out of synch from ~1910 to the early 1930s.
Figure 15

The Southern Ocean Residual, Figure 16, appears similar to an inverted Global SST anomaly curve. This makes sense, since the Southern Ocean SST anomalies are comparatively flat.
Figure 16


The ERSST.v3b Extended Reconstructed Sea Surface Temperature anomaly data is available through the KNMI Climate Explorer website:


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