Before returning to the series on the tropics, I felt it would be good idea to learn more about the Indian Ocean, more than what I’d reported in a prior post.
The Indian Ocean is the third largest ocean, representing approximately 20% of global surface waters. It’s obviously impacted by neighboring oceans: the Atlantic, Pacific, and Southern Oceans. It shares the Indo-Pacific Warm Pool (aka Pacific Warm Pool) with the Pacific, and, due to its proximity with the Pacific, it is strongly impacted by ENSO events. The Indian Ocean is unlike the Atlantic and Pacific Oceans in one major detail; the Northern Hemisphere portion of the Indian Ocean is very much limited in size, while the Atlantic and Pacific Oceans extend northward to interact with the Arctic Ocean. There are a good many phenomena taking place in the Indian Ocean: the Indian Ocean Dipole (IOD) for one. In this post, I have no intention of delving deeper than an introductory look at the IOD.
Figure 1 illustrates monthly Indian Ocean SST anomaly data from 1854 to 2008. (The data has been smoothed with a 37-month filter.)
The Indian Ocean SST anomalies exhibit an overall trend that’s slightly different than Global SST anomalies, and the periodic variations due to ENSO events and other perturbations are amplified. Refer to Figure 2. The clearly visible difference is the magnitude of the rise up to and the spike centered around 1940, which coincides with a series of significant El Nino events.
Subtracting the Global SST anomalies from the Indian Ocean SST anomalies, Figure 3, illustrates the influence of the Southern Ocean as shown in Figure 4.
INDIAN OCEAN CURRENTS AND THE INDIAN OCEAN GRIDS USED IN THIS POST
Figure 5 is the most detailed, easily readable illustration of Indian Ocean sea surface currents I could readily find. (Figure 5 is a large file; it will expand greatly when downloaded.)
While the South Equatorial Current appears to be the dominant current in the Indian Ocean, there are a multitude of others: equatorial counter currents, coastal currents, monsoon drifts. The southernmost current identified as the West Wind Drift is better known as the Antarctic Circumpolar Current or ACC. I’ve elected to continue to identify it as the West Wind Drift to maintain a common name between illustrations. I did, however, note it as the Antarctic Circumpolar Current parenthetically in Figure 13.
Using the currents as a guide, but without segmenting the data too far, I first divided the Indian Ocean into latitude bands. Refer to Figure 6. The first division isolated the Arabian Sea and the Bay of Bengal. Then I separated the areas just north and south of the equator. Further south, I divided the Indian Ocean Gyre (for lack of a better identifier) into north and south components. That left the West Wind Drift (ACC). Each of the latitude bands I subdivided into west, central, and east areas, with hope of identifying significant differences, if any existed. That left the area west and southwest of Madagascar, which I have identified as the Mozambique Channel (Plus).
SEGMENTED INDIAN OCEAN SST ANOMALIES
Initial note: The exaggerated spike in temperatures around 1940 does decrease in amplitude as the data sets proceed farther south. Keep an eye out for it, because it does resurrect itself in the western portion of the South Gyre data set.
Figure 7 shows the SST anomalies for the Arabian Sea and the Bay of Bengal. Keying off the 1940 spike, the Arabian Sea has greater periodic variations. Its overall trends are slightly larger too. Yet, it is larger in area than the Bay of Bengal, indicating the larger variations in signal are not size related. Are the SST anomalies amplified as we travel farther west?
It will be tough to determine from the next group, since the westernmost North Equatorial data set is the smallest in area. Refer to Figure 8, which illustrates the SST anomalies for the west, central, and east portions of the North Equatorial section of the Indian Ocean. Note, though, that the perturbations in the east section are significantly less than the central section, implying that some of the exaggeration of the west area may result from its location.
Let’s drop another tier to the South. The SST anomalies for the west, central, and east sections of the South Equatorial Indian Ocean are shown in Figure 9. Again, the westernmost data set has the greater periodic variations even though its area is not significantly smaller than the others. Also note that the western data set also flattens somewhat, while the central and eastern SSTs continue to rise. Is this an indication of a Southern Ocean influence? Referring again to the currents in Figure 5, it would be possible. Is it likely?
A BRIEF OVERVIEW OF THE INDIAN OCEAN DIPOLE
Figure 10 is an illustration of the positive mode of the Indian Ocean Dipole (IOD). During a positive IOD, anomalously warm SSTs form in the western equatorial Indian Ocean, while anomalously cool SSTs form in the southeastern equatorial Indian Ocean. The reverse occurs during a negative IOD. As a result, anomalous sea level pressures and convection occur, which alter rainfall patterns in eastern Africa and Indonesia.
For a further overview of the IOD, refer to the source of Figure 10:
There are also many studies of the IOD that can be found online.
RETURNING TO SEGMENTED INDIAN OCEAN SST ANOMALIES
Figure 11 shows SST anomalies (west, central, and east) for the northern portion of the Indian Ocean Gyre. The greatest swings at this tier occur in the eastern portion, which is impacted by a number of major currents. The Indonesian Throughflow brings water into the East Indian Ocean from the western Pacific, passing through the Indonesian Archipelago. This is shown but not identified in Figure 5. The West Australian Current carries water north into the area, while the Leeuwin Current off the Australian coast (not illustrated in Figure 5) transports water southward. The western Australian coast is also an upwelling area.
Note in Figure 11 how the west and central SST anomalies in the northern portion of the Indian Ocean Gyre periodically appear out of synch with the east SST anomalies. Is this representative of the IOD?
In Figure 12, we’ve dropped south another tier to the southern portion of the Indian Ocean Gyre and again illustrate SST anomalies for the west, central, and east portions. What catches the eye in this graph are the significant anomalous drops in SST in the early 1960s in the west and central data sets.
That drop in the 1960s can also be seen in the central section SST data of the West Wind Drift (Antarctic Circumpolar Current) illustration. Refer to Figure 13. Also, what seems counterintuitive, the SSTs in the West Wind Drift areas upstream and downstream (west and east sections) rise in opposition to the decrease in SSTs in the central section. Most illustrations of the global Thermohaline Circulation circuit show an upwelling in the Indian Ocean. Is the anomalous decrease in SST in the central West Wind Drift (Figure 13) and in the west and central sections of the South Gyre (Figure 12) a component of THC?
Note how the curves of the data no longer bear a resemblance to the curve of the Indian Ocean, that they are taking on more charateristics of South Ocean, Figure 4.
The SST anomalies for the area identified as the Mozambique Channel are illustrated in Figure 14. The curve appears very similar to the western data in the South Equatorial section of the Indian Ocean, Figure 9. This is logical since the Mozambique Current transports waters southward along the East African coast.
The final illustration, Figure 15, is my favorite of the entire post. It shows SST anomalies for the area south of South Africa, bordered by the coordinates 30-60S and 20-30E. It brought to mind Igor’s line, “What hump?” in “Young Frankenstein” by Mel Brooks.
That graph has prompted me to add the Antarctic Circumpolar Current to my list of future posts. Considering the opposing anomalies in the 1960s in areas that are adjacent to one another, a post on the ACC should be interesting.
While this post introduced more questions than it answered, it does give me enough of a background to return to the series on the tropics.
Sea Surface Temperature Data is Smith and Reynolds Extended Reconstructed SST (ERSST.v2) available through the NOAA National Operational Model Archive & Distribution System (NOMADS).
Figure 5, which shows Indian Ocean currents, is from Wikipedia and is originally titled “Indian Ocean Gyre”. Refer to: