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Saturday, August 9, 2008

Indian Ocean and South Atlantic Ocean


While rummaging through the JISAO Climate Data Archive, I found a webpage that contained a Surface Air Temperature data set for the Indian Ocean. That reminded me that I hadn’t posted on the Indian Ocean or the South Atlantic.

JISAO Indian Ocean SAT Anomalies:
JISAO Climate Data Archive:

Two things struck me about the Indian Ocean SAT data. First, the study extended back to 1800. This is unusual since most SST studies don’t begin until 1850. If, however, you elect to visit the link, you’ll note the minimum number of readings and the wild swings in data from 1800 to 1850, which surely indicates how unreliable the data was during that early period and explains the use of dampening where data is scarce. The second thing that struck me was the limited area included in the data set. Refer to Figure 1. The east border of the Indian Ocean is normally represented to include the Malaysian Peninsula, Sumatra, the Lesser Sunda Islands, on to Australia, where it follows the coast to the Indian Ocean eastern longitudinal extent at Tasmania. Refer to Figure 2. This representation of the Indian Ocean also includes a portion of the Southern Ocean.

Figure 1
Figure 2

To assure the abridged area wasn’t masking anything or providing a much amplified view of Indian Ocean anomalies, I’ve compared the SSTs of the two areas. Figure 3 illustrates the areas included in the comparison of the abridged Indian Ocean and the extended Indian Ocean data sets. I’ve included data as far south as 60S. The east border of the abridged set is 100E, and for the extended set, it is 145E. Since NOMADS is a coordinate-based system, there is no way for me to eliminate the included West Pacific (the Pacific Warm Pool plus).
Figure 3

As shown in Figure 4, there is a minor difference between the abridged and extended Indian Ocean data sets. The smaller area has a slightly greater variation, so it will be used for the rest of this post.
Figure 4

Figure 5 illustrates the SST anomalies for the Indian Ocean (abridged) for January 1854 to May 2008, smoothed with a 37-month filter. The first eye-catching feature is the massive rise and fall centered around 1942. The second item of note is the scale. Indian Ocean SSTs rose approximately 0.9 deg C from 1910 to 2004.
Figure 5

In Figure 6, I’ve provided a comparison of Indian Ocean and Global SSTs. It’s easy to see the influence the abrupt changes in Indian Ocean SSTs have on Global SSTs.
Figure 6

Looking for the cause of the additional warming in the Indian Ocean, I subtracted Global SST from Indian Ocean SST and plotted it in Figure 7. Having had my head stuck in the ERSST.v2 data set for so long, I see a relative flat curve in the residual data until the late 1960s, where it takes an upward climb, reminiscent of the Southern Ocean data set.
Figure 7

In Figures 8 and 9, the Indian Ocean Residual is compared to Southern Ocean SST. The Southern Ocean influence becomes apparent. Figure 8 is the raw data. In Figure 9, I’ve shifted the Southern Ocean data down by 0.12 deg C.

Figure 8

Figure 9


The South Atlantic SST anomalies, Figure 10, also rise 0.9 deg C, but the South Atlantic does it between 1895 and 1974. Curious. Like the Indian Ocean, the South Atlantic data includes the hump at 1940, but it also includes a second anomalous rise and fall centered on 1974. Very curious.
Figure 10

In Figure 11, Global SST anomalies are compared to South Atlantic SST anomalies, illustrating the influence the South Atlantic has on the Global SST.
Figure 11

The South Atlantic Residual (Global SST anomalies subtracted from South Atlantic SST anomalies) is illustrated in Figure 12.
Figure 12

Is the South Ocean the cause of the unusual rise in the early 1970s? Refer to Figure 13. Could currents have shifted in 1974, changing the influence of the Southern Ocean on the Southern Atlantic?
Figure 13

Or are Southern Ocean SSTs being influenced by the South Atlantic?


Sea Surface Temperature Data is Smith and Reynolds Extended Reconstructed SST (ERSST.v2) available through the NOAA National Operational Model Archive & Distribution System (NOMADS).

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