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 wattsupwiththat.com, 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.

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 South-west "Cold Spot" - the other side of the coin

There was a lot in the press and on blogs back in June about the so-called "North-east Hot Spot" or NEH, an invention of the US Geological Survey. The authors of the USGS paper know what the weaknesses of their argument are, I'm sure - if they don't know, then they should have left their paper languishing in the "pending" tray. I'm planning another post on that subject soon, but there are two sides to every story, and there are two sides to every continent. If there's a "North-east hot spot", is there perhaps a "South-west cold spot"? There is indeed - and along a coast which is also subsiding slowly (which tends to raise sea level relative to the land), from San Diego in the south to just north of San Francisco, sea level hasn't risen in the last three decades. There are reasons for this of course, just as there are reasons behind the claimed "acceleration" in the north-east, but unlike the USGS authors, I'll not skate over or omit them. In my post on the continuing moderate sea-level rise around the Hawaiian Islands, I included this satellite graphic.

Map 1 - Linear trend (1993-2010) of satellite altimeter sea surface height showing he region of high rates in the western Tropical Pacific (Source)

I accept the data summarised therein, but it's a bad graphic - there's hardly any difference in the colours used for small negative, and small positive trends. Whether that's deliberate of not, I can't say, but the same criticism can't be levelled at this graphic representing a  slightly shorter period (to 2008). Its authors chose a switch from bright green to bright yellow either site of the scale zero, and I applaud them for it.

Map 2   Source: Why sea level rise isn’t level at all (NASA blog)

What stands out is the swathe of green (negative change) from Alaska down along the US west coast to Mexico and beyond. Incidentally, some sceptics (who should know better) have scoffed at graphics such as these, claiming that the very high rates of increase shown in the western Pacific are "fiction", and therefore the entire maps aren't to be trusted. I tended to agree (mea culpa) , until I began actually checking the depicted rates against tide gauge data over the time period(s) of the map(s). To date I've not found any but minor discrepancies, and where it's possible to take account of changes in land (and therefore gauge) height any such differences are mostly of the order of a few tenths of a mm/year at the very most.

However, looking at some of the actual plots will illustrate the situation and confirm the veracity of the satellite data. I initially planned to start in the south and work northwards, but decided to show what I think is the most interesting plot, for Port San Luis, N of Santa Barbara.

Note that while there is, overall, a small rate of rise (0.65 mm/year), the level since 1996 is lower than the 5-year-average peak in 1959. The level since 1980 is falling:

Port San Luis, CA                                                                          source: PSMSL

I'm not stating that sea level will continue to fall at Port San Luis. I'm not even suggesting it. I'm merely pointing out what is happening now (to the end of the last complete year of published data), and what has happened in recent decades.

My question at this point is whether city and state planners, local businesses and citizens actually know that sea level along the Californian coast has been stable for decades, and has been falling at almost all gauge sites for the last decade or two? If not, why not? Authorities around San Francisco Bay are wetting themselves about possible huge future sea level rise "predictions", and many appear to have convinced themselves that effects such as beach erosion are due to recent sea level rise, when the records show nothing of the sort. For example, there's been a gauge at Alameda Naval Air Station (East across the Bay from San Francisco) since 1939.

Alameda (Naval Air Station), CA                                                 source: PSMSL

... where the current 5-year average is lower than it was in the late 1960s, and the current rate is clearly negative. Most assessment reports I've seen brush the inconvenient facts "under the carpet" and only mention global data and the scary predictions. Some (see Missing the Point in California) are so mesmerised by this misinformation they can't see what's under their noses. There's no doubt that sea levels are rising globally. There's no doubt that effects such as ENSO raise and lower sea levels across the Pacific. The peaks from the strong 1982-3 and 1997-8 El Niños stand out for the two previous examples, and for all west-coast sites from southern California to Alaska. The Pacific Decadal Oscillation has an impact - it was strongly positive from the late 1970s to around 2008, and in that phase tends to raise sea levels in the west, and lower them in the east, our area of study.

Is the story the same at all stations in California? Not quite - Humboldt Bay (about 120 km S of the Oregon state line) is an outlier, and shows up on map 2 above (click to get the original) as a pale orange blob at the apex of the "bulge" of the Californian coast.

Source: PSMSL

Note however, that while the calculated trend is 4.7 mm/year overall, the rolling profile of the 3-year moving average is similar to the previous examples, and this indicates that North Spit must be sinking at a greater rate than other sites to the south, and a greater rate than the sea level is falling, resulting in a small positive rate since the early 1990s.

Plots for San Francisco and Point Reyes can be seen here, and data for all the west coast stations are linked on the Permanent Service for Mean Sea Level (PSMSL) page here - scroll down three-quarters of the page on the scroll bar.

Things will change, there's no doubt, but precisely which way, and to what extent, no one knows. Now that's a fact. Don't believe what you read, not even what you read here. I do my best to be truthful and unbiased, but I still have an overall point of view, and it may colour my presentation of what I see as facts. Check my references, check other blogger and web author's references, and remember - the truth may only be a click away (or two clicks if you have Windows Vista).

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

South Pacific Sea Level to September 2011

Note: A permanent and (to be) regularly updated page has been created (see top of sidebar) located here. I've updated this page with data to December 2011.

The island sea level charts are drawn from monthly data from the South Pacific Sea Level and Climate Monitoring Project. I've included a chart for a second Fiji station, and also one at the end for a New Zealand station, Jackson Bay (South Island). Neither is part of the project, but data is provided on the website.

The high resolution SEAFRAME (Sea Level Fine Resolution Acoustic Measuring Equipment) monitoring stations comprise modern integrated housings which measure and record sea level, barometric pressure, water temperature and air temperature. Most of the stations were installed in 1992 and 1993, though a few were later. It's important to note that the effect of local land movement is eliminated from sea level data:

The Continuous Global Positioning System (CGPS) network monitors vertical movement in the earth's crust, such as subsidence or tectonic shifts, at the SEAFRAME tide gauges and adjacent land. Sea level data can then be adjusted to compensate for the earth's movement to within a millimeter, enabling the absolute sea level to be determined. 

I have voiced criticism of Australia's Bureau of Meteorology on various topics in the past, but as far as the presentation of sea level data is concerned, I rate their National Tidal Centre as the best. The page I linked to above has a table of the 12 stations in the project (and the two others I mentioned) which links to PDF plots and data tables for sea level, barometric pressure, water temperature and air temperature. The data tables in turn link to online graphical plots and text files for easy import to spreadsheets. A map links directly to the data tables. I liked that Idea so much I've pinched it for use here.

What should be evident from the plots is that any generalisation of the situation over this wide area is invalid. Apart from an almost universal downward spike during the 1997/98 ENSO event, the history and trends differ widely. It should also be clear that claims of "25 mm/year" or "no rise" since the early 1990s are also invalid.

Most of the charts are dominated by a downward "spike" in 1997/98. The level drop was due to unusually high barometric pressure during the 1997/98 ENSO ("El Niño/La Niña-Southern Oscillation") event. The correlation is well illustrated for the Marshall Islands (Majuro Atoll), so I've placed this first. Kiribati and Tuvalu have been given much attention in the news media and on the 'net recently, so they're listed next.

Readers are welcome to reproduce any of the plots - all I ask is that attribution be given, preferably with a link to this post. I haven't used thumbnails - right-click on the image and select "save image as" or whatever your browser prompts.

Level data has been converted from metres to millimetres to overcome loss of precision in Excel's trend data. Gaps in the plots indicate gaps in the original data. Note that the trend slope is monthly - multiply by 12 to get the annual value (e.g. y = 0.2269x gives 2.7228 mm/year).

Select a location from the map to view a graph of the monthly sea level statistics for that location. Click on the bottom of the blue area for the NZ station, Click on your browser's back button to return to the map.

Source: Bureau of Meteorology     

Marshall Islands

Island: Majura   Location: Uliga

Note the 1997/98 ENSO "spike" and the correspondence with the abnormally high atmospheric pressure from late 1997 to late 1998 shown in the barometric pressure plot below:

Kiribati

Island: Tarawa   Location: Betio

The trend line is pulled down by the ENSO dip from the end of 1997 to end 1998. To give an better view of the trend from 1992 to present, I replotted the chart with that data excluded:

The resulting trend is effectively zero.
A downward trend is evident from end 2001 to present:

Tuvalu

Atoll:Funafuti   Island: Fongafale

As with Kiribati, the deep ENSO dip in 1997/98 pulls the trend line down on the left; it's below the 1994/1997 average. A zero trend is evident from 1999 to present:

Papua New Guinea

Island: Manus   Location: Lombrum

Here's a clearer view of the 1999-2011 trend of 3.4 mm/year, slightly above the global average:

Solomon Islands

Island: Guadalcanal   Location: Honiara

Again the trend is pulled down on the left; the average level 1999 to present is around 750 mm, and the trend 2.1 mm/year.

Vanuatu

Island: Efate   Location: Port Vila

The big 1997/98 downward spike evident in most of the other plots is absent; instead there's a relatively steady upward trend.

Fiji

Island: Viti Levu   Location: Lautoka

Island: Viti Levu   Location: Suva

Tonga

Island: Tongatapu   Location: Nuku'alofa

Although there's a steep trend from 1993, current levels are not dissimilar to those between 2000 and 2003.

Cook Islands

Island: Rarotonga   Location: Avatiu

Levels seem to have stabilised from 2006.

Samoa

Island: Upolu   Location: Apia

The familiar ENSO dip pulls the trend down at the LHS, but there is a sharp upward "spike" from 2010.

Nauru

Island: Nauru   Location: Aiwo

Overall trend is effectively flat - levels at present similar to those in 1993. However, the trend from 2002 is downward.

Federated States of Micronesia

Island: Pohnpei   Location: Dekehtik

An overall upward trend, though little change from 2007 to present.

New Zealand

Island: South Island   Location: Jackson Bay

There seems to be little overall change from mid-1998 (after the ENSO dip) to present. A plot from 1999 confirms that:

Sea Level in Australia - the Inexorable Rise in Alarmism

Is the sea-level around the Australian coast rising at an alarming rate? The Australian Climate Commission would have citizens and policy-makers in Australia and elsewhere believe that it is. In their 2011 report "The Critical Decade - Climate science, risks and responses" the following map is shown (mm per year)

Figure 8. Local sea-level rise (mm/year) around Australia from the early 1990s to 2008

Looks scary, doesn't it? In the southwest, somewhere near Perth, 8.1 mm/year, implying a rise of 81 cm in a century. In the north, somewhere near Darwin, a slightly lower value. Are these figures correct? As quoted "from the early 1990s to 2008" they are, but they are also very misleading - at most of the (as yet) unidentified monitoring stations shown on that map, there has been no significant sea-level rise after 1998. I can also show that most, if not all of the rise around Australia in the second half of the 20th century occurred during 1997 and 1998. during what's been called "one of the most severe ENSO events in history". ENSO is short for "El Niño/La Niña-Southern Oscillation"  for those not familiar with the mnemonic.This is not what the Climate Commission, the CSIRO, nor the Australian government want known.

Here's my version, based on the same data, but starting from 1999, at the end of the late 90s ENSO "ramp" which grossly distorts the "early 1990s to 2008" rates shown above.

Local sea-level rise (mm/year) around Australia from 1999 to 2010

Looks rather less scary, doesn't it? Let's see where the CC got their data, and why there's such a massive difference between those two maps.