Why are Australian Sea Levels Rising?

The answer, my friend, is blowin’ in the wind…. literally.

In brief…

  • At Sydney, the long term sea level rise is about 1 mm per year, with short periods of rapid increase and a long plateau of very small or zero trend in the second half of last century.  As Australia is geologically stable, it is likely that a similar pattern occurred all around the coast.
  • This gradual sea level rise is consistent with oceanic warming since the Little Ice Age, with fluctuations resulting from El Nino-Southern Oscillation (ENSO) changes.
  • Tide gauge data since 1990 from different locations show rises varying from 2.4 mm to 7.2 mm per year.  A significant proportion of this is due to ENSO wind circulation changes.
  • There is no sign of any unusual acceleration in Australian tide gauge data.

The Bureau of Meteorology maintains the Australian Baseline Sea Level Monitoring Project, with a number of tide gauges around the coastline, shown here:

Fig. 1:  Australian Baseline Sea Level Monitoring Project

MSL map

These sites have monthly data only from 1990, mostly later, and two (Thursday Island and Port Stanvac) have very limited data and were not used in this study.   I have used data for Mean Sea Level for all sites on the Australian coastline to find the current situation with sea level rise, and use the much longer dataset from Fort Denison in Sydney Harbour as well for a longer term perspective.  Figure 2 is a plot of all monthly data from all sites.

Fig. 2: Australian Mean Sea Levels

MSL plot abs

Points to note:

  • The mean is a measure of central tendency: the full tidal range is at least twice the values shown for each site.  Broome’s range is well over 11 metres.  Portland has a very small range.
  • An Australian average of these means is meaningless.
  • Each site has a seasonal signal which is not regular.
  • It is difficult to make any meaningful comparison.

However if we look at sites individually, we can at least compare any trends.  Figures 3 and 4 show MSL at sites with the greatest and least trends.

Fig. 3:  MSL at Hillarys

MSL plot abs Hillarys

Fig. 4:  MSL at Stony Point

MSL plot abs StonyPt

According to this very short record, the rate of Australian sea level rise varies in different locations, from a low of 2.4 mm per year in Bass Strait to 7.2 mm per year at Hillarys in Western Australia.  Why is this?

Australia is very stable geologically, and these tide gauges are carefully checked with levelling connections between them and Global Navigation Satellite System (GNSS) sites maintained by State land and survey departments.  Therefore differing rates of land movement are unlikely to be responsible.

We need to compare all sites, and as well remove the seasonal signal.  To do this I calculate monthly anomalies for each site, then plot the results in Figure 5.

Fig. 5:  Monthly anomalies for all Australian sites:

MSL plot all anoms

With the seasonal signal removed, the data show some roughly similar patterns for all sites.  I now plot the mean of these anomalies, to find an “average” Australian sea level trend.

Fig. 6:  Average of all MSL anomalies

MSL anoms trend

All sites show marked dips in 1997-98 and 2015-16, clearly shown in the average.  The influence of El Nino perhaps?  Figure 7 shows the mean of all MSL anomalies with the scaled Southern Oscillation Index (SOI).

Fig. 7:  Average of all MSL anomalies and SOI/200

Aust MSL and soi

My first response was “Wow!”  Next, sea level plotted against SOI:

Fig. 8: MSL as a function of SOI

MSL scatterplot all v soi

For every one point increase in the SOI, Australian sea level rises an average of 3.2 mm, and SOI change can account for more than a third of sea level rise.  Now we check how the SOI has behaved over the last 27 years.

Fig. 9:  Trend in SOI, 1991-2018

SOI plot trend

In this short record, the SOI has increased by about 8 points.

From this, we can deduce that a portion of the perceived sea level rise since 1991 is due to the influence of the El Nino- Southern Oscillation (ENSO), of which SOI is a strong indicator.

What mechanism could there be for this?  The SOI is calculated from the difference in atmospheric pressure between Tahiti and Darwin.  Darwin’s sea level is compared with the SOI in Figure 10.

Fig. 10:  Darwin MSL anomalies and SOI/100

MSL plot Darwin SOI

The match is very close, as the plot of MSL vs SOI shows:

Fig. 11:  Darwin MSL as a function of SOI

MSL plot Darwin vs SOI

SOI has about twice the effect on MSL at Darwin as it has on the Australian average, and more than half sea level rise can be accounted for by change in SOI.  Here’s my explanation:

During La Nina, when SOI is high, the northwest monsoon is strengthened, the monsoon trough penetrates further into northern Australia in summer with lower atmospheric pressure and stronger northwest winds.  This combination pushes the sea up against the northwest coast, raising the sea level.  In winter, the monsoon disappears and winds are predominantly from the east.  During El Nino, the monsoon is weakened and may fail completely.  Thus northwest winds are weaker and the sea level is markedly lower.

That’s all very well for Darwin and other sites in northern Australia, but take a look at Figure 12, which compares seal level at Darwin with Spring Bay, in southern Tasmania, and about as far from Darwin as you can get without a passport.

Fig. 12: MSL at Darwin and Spring Bay

MSL plot Darwin Springbay all

Note that in some (but not all) El Ninos (marked) Spring Bay sea level is also strongly affected.  Note also that sea level at Spring Bay appears to start rising again several months before Darwin, in other words before the SOI starts rising.

The 2015-16 comparison of anomalies shows the Spring Bay sea level at its lowest in September 2015, rising strongly and four months before Darwin’s.

Fig. 13: MSL at Darwin and Spring Bay 2015-16

Darwin SpringBay anoms 20152016

To understand this we need to consider circulation patterns as they change through the year and with ENSO events, and their effect on local sea levels.  The following plots show the absolute 2015-2016 monthly mean sea levels and the long term average for each month.

Fig. 14: MSL at Darwin 2015-16 compared with average monthly levels

Darwin abs 20152016

Darwin’s long term average sea level is highest at the peak of the Wet season (February – March) and lowest in the Dry (July – August).  In 2015, the high was reached in January and the low in July- both one month earlier- and the 2016 high was in March- one month later.  Below normal sea levels lasted from April 2015 to April 2016.

In contrast, Spring Bay’s average sea level is highest in the southern wet season (Winter-July) and lowest in the summer dry season (November to February).  In 2015 the high was reached in May and the low in September, and the 2016 high in May.

Fig. 15: MSL at Spring Bay 2015-16 compared with average monthly levels

Spring Bay abs 20152016

This happens at other sites in the southeast of Australia (from Portland to Port Kembla including Tasmania).

Fig. 16:  Australian sea level at sites in the north and southeast.

MSL plot Nth SE

Note that the same pattern applies: sea level is lower in strong El Ninos and rises before the north (in 1997-98 and 2015-16 but not so clearly in 2006-07).

A possible explanation is that circulation changes associated with the ENSO are not restricted to the tropics, although that is where the effects are largest and most visible. In normal (non-El Nino) years, the sub-tropical ridge moves north over the continent in winter, and the winter storms around the lows to its south bring rain and winds from the south-west quarter to the southern coast, particularly South Australia, Victoria, and Tasmania.  These winds cause the sea to pile up (by a few centimetres) against the southern coast.  In summer, the sub-tropical ridge moves south, rain bearing storms mostly pass to the south of the Australian region, and blocking highs in the Tasman Sea bring strong north-west winds across the south-east of Australia.  This causes sea level to fall.

In a strong El Nino, these conditions occur earlier, with a rapid retreat south of the sub-tropical ridge so that winter storms with south-westerly winds are fewer and weaker and sea level is lower in winter and spring.  Summer sea levels (November to January) are close to normal.

Figure 17 tests the response of sea level to barometric pressure at Spring Bay.

Fig. 17:  Spring Bay MSL anomalies as a function of barometric pressure anomalies

SpringBay MSL vs Press.jpg

The result is clear.  More than half of sea level change is due to pressure variation, which causes winds to change.

The effect is much greater at Darwin.

Fig. 18:  Darwin MSL anomalies as a function of barometric pressure anomalies

Darwin MSL vs Press

By the way, how much does increase in sea temperature affect sea level?

Fig. 19:  Spring Bay MSL anomalies as a function of temperature anomalies

SpringBay MSL vs SST

At Spring Bay, very little.  An increase of one degree could raise sea level by 17 mm, but R-squared of 0.033 is tiny compared with 0.527 for air pressure.

Whatever causes El Nino also causes the southern seasonal weather cycle to occur earlier, and sea levels rebound several months before they do in the tropics.

What of the longer term?

The Australian Baseline Sea Level Monitoring Project data are limited to sea levels since 1990, so the record is too short to make assumptions about long term sea level rise, and certainly not about the future.  There are longer datasets available however.  Sydney Harbour (Fort Denison) has data from 1914.

Fig. 20: MSL anomalies at Fort Denison (Sydney)

Sydney 1914 to 2018

That’s a long term sea level rise of 1 mm per year, or 104 mm in 100 years- a bit over 4 inches.  Now there has been an apparent “acceleration” since 1991, matching the data at nearby Port Kembla:

Fig. 21: MSL anomalies at Fort Denison (Sydney) 1991-2018

Sydney 19912018

But once again note the correspondence with the SOI:

Fig. 22: MSL anomalies and scaled SOI Sydney 1991-2018

Sydney 19912018 soi

A significant portion of the recent sea level rise at Sydney can be attributed to a short term rise in the SOI.

So is this recent rapid rise unique?  By calculating the trend in sea level over 10 year periods, we can see periods when sea level rise has accelerated or slowed in the past:

Fig. 23:  10 year running trend in MSL at Sydney

10yr trends MSLSydney

The most recent rise in sea level of 7 to 8 mm per year over 10 years is less than that of the rise to 1953, when sea level rose by 10 mm per year.

If you think 10 year trends are too short, Figure 21 shows 30 year trends at Sydney:

Fig. 24:  30 year running trend in MSL at Sydney

30yr trends MSL Sydney

The current 30 year trend is exactly the same as the trend to 1965:  2.4 mm per year.  For the 30 year period to the mid-1990s the trend was zero.

Conclusion:

Across all tide gauges of the Australian Baseline Sea Level Monitoring Project, a significant proportion of sea level rise since 1990 is due to circulation changes associated with the El Nino- Southern Oscillation.  The effect is much greater in the north and west, where sea level rise is highest, but also is evident in the south-east.

Sydney’s long term record tells us that sea level has been rising at an average rate of about 1 mm per year.  There have been short periods of rapid increase and a long plateau of very small or zero trend in the second half of last century.  As Australia is geologically stable, it is likely that a similar pattern occurred all around the coast.

This gradual sea level rise is consistent with oceanic warming since the Little Ice Age, with fluctuations resulting from ENSO changes.

There is no sign of any unusual acceleration in Australian tide gauge data.  In 100 years from now sea level could be expected to be 100 mm to 200 mm higher.  A sea level rise of 5 to 10 times this amount is purely speculative and not based on empirical data, but instead is based on the worst case scenario of computer models.

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17 Responses to “Why are Australian Sea Levels Rising?”

  1. trevor prowse Says:

    Hillarys has had land falling by 3mm per year due to water extraction

  2. Geoff Sherrington Says:

    Ken, I’ve not thought this through, but Darwin has annual rainfall of 1730 mm, most falling in Dec-Jan-Feb, with 6 months of each year called “The Dry”. It might not take much of a difference in drainage rates back to the sea to cause measurable annual fluctuations in sea level, some of which can be imagined capable of multi-year change. Geoff.

    • kenskingdom Says:

      Hi Geoff
      I find it hard to imagine local run off from the Darwin area changing mean sea level in Darwin harbour- it can’t bank up but will find its own level. whereas strong winds do cause water to bank up against land e.g. Venice, Lake Eyre, any storm surge in a cyclone.

      • Geoff Sherrington Says:

        Good one, Ken. Of course I should have extended this to the whole northern monsoon weather system impacting the northern shoreline. In the Wet, if you can get there, there are large areas sheet flooded with a few cm of water.. There is so much change in the weight of water in the air, in flood waters, and then in the Dry, a water table dropping my several cm,so that it is hard to imagine there are no side effects on sea water levels. Though, when the reverse is happening in the other hemisphere, a lot of scope for cancelling out is there. Geoff

  3. siliggy Says:

    Hi Ken Just wondering sort of like Geoff’s comment if water may not find its own level due to differences in salinity after rainfall or differences in content after erosion has fertilised planckton etc. Then differences like this from the sea such as volcanic input of just a different natural temperature distribution.
    I wonder if you could watch a diagonal yurt being built by putting salt water or alcohol in one end of this below or just by leaving one half in the sun (Doubt i could be so mean).
    Lance Pidgeon

  4. siliggy Says:

    Also curious to know if cloudy weather will steer the cyclic tides via electro-static deflection of flow as modulated by the UTC timed Carnegie curve.

  5. kenskingdom Says:

    Lance you never cease to amaze me, you’re way ahead of me!

    • siliggy Says:

      Not ahead at at all Ken. You did all the hard work and show a real connection with air pressure. I just throw thoughts around. Thinking about it again any attraction or deflection between air and water will move the easier of the two more. Air is easier to move than water. So the electro-static effects could contribute to the change in air pressure more than moving the water.
      Electric currents that flow in the water from either tidal magnetic dynamo generation or solar wind changes meeting the earths tilt first at one pole to make a pole to pole Voltage, may do different things.

      Quote from link below. “When exposed to a high-voltage electric field, water in two beakers climbs out of the beakers and crosses empty space to meet, forming the water bridge. The liquid bridge, hovering in space, appears to the human eye to defy gravity.”
      http://phys.org/news/2007-09-bridge-exposed-high-voltage.html

      • siliggy Says:

        A video of that. Wonder what salt water will do? It would have a far higher conductivity.

      • siliggy Says:

        The magnetic effects in the oceans like all good settled science keeps dramatically changing and has lots of unknowns. “As the Moon pulls the salty seas through our planet’s global magnetic field, electric currents are generated.

        And these currents then induce their own magnetic signals, which have now been mapped in exquisite detail by a trio of Esa satellites known as Swarm.”
        “There were hopes before Swarm launched that it might even be possible to tease out the magnetism related to specific currents as well, such as the famous Gulf Stream that shifts warm waters across the North Atlantic.
        The desire is still there, but Prof Olsen is a little pessimistic. It is the well-known periodicity of the Moon’s orbit around the Earth that helps betray the tidal signature.”
        https://www.bbc.com/news/science-environment-43720024

  6. siliggy Says:

    Another possible reason for sea level rise is an old process that may not have stopped.
    “”The huge size of the reservoir throws new light on the origin of Earth’s water. Some geologists think water arrived in comets as they struck the planet, but the new discovery supports an alternative idea that the oceans gradually oozed out of the interior of the early Earth”
    http://www.newscientist.com/article/dn25723-massive-ocean-discovered-towards-earths-core/

  7. siliggy Says:

    Just had a look at the 2015/16 rainfall anomaly maps for Tas and N.T. near Spring bay and Darwin. Both seem to have lower than average rainfall when your charts show lower sea level and both had higher sea level with higher rainfall.
    Boats float higher in salt water. So it stands to reason that fresher water would sit higher.
    Got a giggle from this too. S i think you may.
    “This conclusion suggests that future sea level change assessments must consider the regional impacts of salinity-driven changes; this effect is too large to continue to ignore.”
    https://www.sciencedaily.com/releases/2014/11/141120133426.htm

  8. kenskingdom Says:

    Ocean basin wide…. A few mm maybe, but I think winds might be stronger

    • siliggy Says:

      No not basin wide, local where the very heavy salt water flows underneath the fresh water without mixing quickly.
      Seems counter intuitive but sea levels Not finding a level due to weighing different amounts until mixed. A bit like sea ice not finding a level in the water. It finds a balance but not a level. Mixing would be delayed by long rivers taking time to drain as well.
      The effect from wind would certainly be real too. Which is greater would vary like the wind.

  9. Andrew Says:

    Outstanding article. Thanks ken for all the detailed work you put in.

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