Sea-level in Australia and the Southern Oscillation Index

I've read a fair bit about the Southern Oscillation Index (SOI) over the years, about how much ENSO (El Niño/La Niña Southern Oscillation) affects sea-level in the Pacific. I've noted effects, particularly the El Niño "dip" in the western and central Pacific, and the corresponding "spike" in the east (especially the US Pacific shore). However, my impression was that it was just the extremes, El Niño and the less-well defined La Niña that had any real effect on Pacific sea-level. Before I continue, it's worthwhile quoting what Australia's Bureau of Meteorology has to say about the SOI; it's succinct and informative:

The Southern Oscillation Index, or SOI, gives an indication of the development and intensity of El Niño or La Niña events in the Pacific Ocean. The SOI is calculated using the pressure differences between Tahiti and Darwin.

Sustained negative values of the SOI below −8 often indicate El Niño episodes. These negative values are usually accompanied by sustained warming of the central and eastern tropical Pacific Ocean, a decrease in the strength of the Pacific Trade Winds, and a reduction in winter and spring rainfall over much of eastern Australia and the Top End. You can read more about historical El Niño events and their effect on Australia in the Detailed analysis of past El Niño events.

Sustainted [sic] positive values of the SOI above +8 are typical of a La Niña episode. They are associated with stronger Pacific trade winds and warmer sea temperatures to the north of Australia. Waters in the central and eastern tropical Pacific Ocean become cooler during this time. Together these give an increased probability that eastern and northern Australia will be wetter than normal. You can read more about historical La Niña events and their effect on Australia in the Detailed analysis of past La Niña events. 

The ENSO Wrap-Up includes the latest 30-day SOI value, as well as other information on indicators of El Niño and La Niña events.

 The graph below shows monthly values of the SOI in recent years.

  Source:BOM

On the following page they have links to data tables; Wanting to create a spreadsheet of monthly SOI values (from 1876!) I was dismayed to find the table to be structured as years down and months across. However, using Excel's Copy and Paste-Special/Transpose functions I was able to do it, laboriously year by year. Here's the result for 1959 to May 2013. The reason for the not-so-obvious start year will become obvious very soon.

SOI index 1959-2013    Data source: BOM

I've added a 25-month (2 year) centred moving average to smooth out the spikes without suppressing the signal. A 13-month MA would seem to be more appropriate, but gives too "lumpy" a trace; a 37-month MA smooths just too much. Like Goldilocks' porridge the 25-month MA is "just right".

Comparing the SOI plot with a sea-level plot is easy, but I wondered if I could add the SOI to a sea-level chart in some way. One problem is that the signal is relatively small, and the other is that it varies around a (flat, obviously) zero value. I hit on the wheeze of "magnifying" the SOI signal, and normalising the start of the SOI moving average with the start of the sea-level moving average, incrementing the magnified SOI by the monthly sea-level-trend increment. I tried a factor of 10 for magnification, on the basis that the SOI signal is based on air pressure at sea level, and one hPa change leads to a 10 mm sea-level change (in the opposite direction). My assumption may not have had much maths behind it, but the "porridge effect" operated and it was "just right". Here's the result for Darwin:

Darwin complete sea-level record 1959-2012       Data source: BOM/NTC

I don't now what you think, but I'd call that a rather good correlation, especially over the right-hand half of the chart. Here's one for Fremantle over the same period.

Fremantle sea-level 1959-2013  Data source: BOM/NTC

For both Darwin and Fremantle, note that the recent (since 1994) sharp upward trend has begun a downward reverse, more clearly predicted on the SOI chart which extends to last month (May 2013). The large and broad downward bulge around 1983 corresponds with the intense (some would say most intense on record) El Niño of 1982-3. It shows up well on the 25m MA sea-level plot for Darwin (and most Australian stations), but sea-level rose at Fremantle during that event. All the other El Niños show up on both charts; the broad low during the early 1990s (1991-2 and 1994-5 El Niños, with just 1993 between) and the intense but shorter 1997-8 El Niño. The 2010 El Niño was a more subdued affair.

I think it's clear that the SOI doesn't just affect variations in sea-level, it drives them, at least on the west coast of Australia. In a future post I'll look at other Oz stations for correlation, and where correlation is poor, explore possible reasons.

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