Friday, 2 August 2013

The effect of ENSO on sea-level in the South-western Pacific and Australia

In a previous post, I explored the connection between ENSO ((El Niño/La Niña Southern Oscillation) and sea-levels at Darwin and Fremantle on the west coast of Australia, in particular, the remarkably close correlation between smoothed monthly average levels and a smoothed, trended multiple of the SOI (Southern Oscillation Index).

It had already occurred to me that if the correlation was so convincing, that perhaps it might be possible to adjust monthly average data by the multiple of SOI to remove its effect and reveal the underlying pattern and trend of sea-level change at such strongly-affected locations. First attempts showed that the tenfold multiple I'd used to show correlation was too high, and that a multiple of 7 was "just right" for ENSO adjustment. Lower multipliers didn't reduce the variability ("lumpiness") sufficiently, and too high a value increased variability in the opposite direction. Lucky 7 turned out to be the "Goldilocks" factor. Here's my revised chart for Darwin:

... and with 7 times SOI subtracted from monthly values

The revised chart for Fremantle for the same period

... and with 7 times SOI subtracted from monthly values

Those two removals are quite convincing, I'd say - "extreme ironing" indeed. Note that the trends for both removals are slightly higher than the originals, despite the post-1990s upticks having been removed. It's because the earlier trend from the mid-1970s was down, which effectively pulled the trend-lines down.

High rates of rise in the western Pacific (as shown by satellite sea-level maps) have been a thorn in the side of some sceptics for some time. They conveniently ignore the fact that sea-levels along the Pacific coasts of the Americas show low or negative rates on the maps, supported by tide-gauge data, and that the high rates in the west are also supported by tide-gauge data, when exactly the same time-spans as the satellite maps are compared. They also ignore the reasoned, researched and informed voices which explain that both phenomena are effects of ENSO.

The island of Pohnpei (Federated States of Micronesia) is in that western "hotspot", and using PSMSL data for the two tide-gauges covering the period I've been able to recreate the record from 1974-2012.

The "ENSO profile" being clear, I went ahead and adjusted the monthly data as before.

Majuro atoll is in the Marshall Islands group, and I've extended my previous reconstruction to end 2012, and added the SOI plot. Note that sea-level is lagging SOI on the extreme right.

Kwajelein is also in the Marshall Islands, not that far from Majuro. Sea level is clearly leading SOI on the right.

The uptick is the subject of a couple of recent posts on, ENSO not being very high on the list of possible reasons under discussion. Nils-Axel Mörner thinks it's due to subsidence because of recent building, but then he would, he doesn't understand ENSO and the magnitude of its effects. Here's the latest data to June 2013 for Majuro from the South Pacific Sea Level and Climate Monitoring Project.

As you can see, the uptick has now reversed, following the SOI back to zero.

Pago Pago, American Samoa, shows a less-satisfactory correlation overall, but it's still reasonably convincing. It also shows  the sharp recent uptick.

I've had a look at correlation on the eastern side in California, but it's less clear. California is well north of the equator, and ENSO is the Southern Oscillation after all. I'll see if I can find an SOI widget (or make one) for my sidebar. I'll add captions with source data links very soon.

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