Saturday, 21 January 2012

The Footprint of El Niño - South Pacific Sea level

I've mentioned the intermittent effect of El Niño on South Pacific sea level in several posts. How and why does this part of the ENSO cycle affect sea level?
The primary cause of anomalous ocean conditions is the El Niño/Southern Oscillation (ENSO) in the equatorial Pacific Ocean. Westward winds normally maintain slightly higher water levels in the western Pacific relative to the eastern Pacific. Every three to five years, in a non-periodic pattern, the winds weaken and water levels in the western Pacific drop below normal. The southern equatorial current is weakened and water temperatures in the eastern Pacific rise. This condition is known as El Niño. The opposite condition, known as La Niña, occurs when westward equatorial winds are unusually strong and water levels in the western Pacific become anomalously high. The south equatorial current strengthens, accompanied by below normal water temperatures in the eastern Pacific.
Boiled down, in terms of the effect on sea level, is that levels in the western and south-western Pacific tend to drop, often significantly, during El Niño years, while those in the east tend to rise.

The island of Majuro, in the Marshall Islands reflects this trend particularly well. The opposite La Niña effect, though present, is not so well illustrated. Using data from an earlier gauge maintained by the University of Hawaii, and the later SEAFRAME gauge installed in 1993 by the National Tidal Centre based in Adelaide, Australia, I've been able to reconstruct a history of sea level at Majuro Atoll from 1978 to 2011.

For charts for Majuro and other Pacific Islands, including Tuvalu and Kiribati, see my reference page "South Pacific Sea Level 2011".

Here's a table of El Niño years. Those from 1969 to 2010 are clearly represented on the Majuro chart. The 1982-3 and 1997-8 El Niños were particularly strong events, and I've highlighted them in red

El Niño Years
1902-1903 1905-1906 1911-1912 1914-1915
1918-1919 1923-1924 1925-1926 1930-1931
1932-1933 1939-1940 1941-1942 1951-1952
1953-1954 1957-1958 1965-1966 1969-1970
1972-1973 1976-1977 1982-1983 1986-1987
1991-1992 1994-1995 1997-1998 2002-2003
2006-2007 2009-2010

On the other (eastern) side of the Pacific, continuous long-term records are difficult to find, but that for Monterey, California illustrates the opposite effects quite well, although earlier El Niño years aren't quite so clearly defined.

Not much sign of the 1972-1973 and 1976-1977 El Niños, but the later events are all represented, though to varying degrees. Most of the west and south-west Pacific islands show the "El Niño" effect to differing degrees, but Tuvalu has become the "poster child" of the Global Warming scenario, so it's worthwhile reproducing my reconstruction for that island, covering 1977-2011.

The linear trend shows a value of 0.28 mm/month, which translates to 3.36 mm/year. However, the effect of the El Niños is to pull the trend line down progressively from right to left; it's always below the 13-month running mean (in red), more so on the left. A simple way to eliminate the negative effect of the El Niño "spikes" on the trend is to remove them from the data. However, this is not as easy as it sounds; there's a clear (approximately) annual cycle, illustrated by the sharp upward spikes.  These "king tides" usually occur in late February or early March, occasionally as late as April, and their timing is due to the "lining up" of the tidal effects of both moon and sun. The all-time peak was in February 2006 (very clear on the graph), and despite dire predictions, hasn't yet been surpassed.

An Australian newspaper dispatched a team to Tuvalu in February 2011 to document what was expected to an all-time record "king tide". However, nature being as quirky as ever, organised things so that they'd already "missed the boat" (a metaphor becomes a bad pun!) as the highest tide had already occurred the previous month; indeed the February peak was lower than both January and March peaks. That and the fact that the January peak was no record, produced a "non-event" and no follow-up report was published.

However, I digress - removing the El Niño "spikes" must be done with care if an opposite upward bias is to be avoided. One way round this is to use annual data so the monthly cycle is not a problem, though it effectively removes more data. I'll be following up with a detailed analysis of late-20th century sea level rise at Tuvalu.

Data Sources

Marshall Islands (Majuro): Permanent Service for Mean Sea Level and South Pacific Sea Level and Climate Monitoring Project

Monterey: Permanent Service for Mean Sea Level

Tuvalu (Funafuti): Permanent Service for Mean Sea Level and South Pacific Sea Level and Climate Monitoring Project

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