Trade Winds and Australian Sea levels

In my post Is Climate Change Threatening the Solomon Islands? I showed that sea levels at Honiara are predominantly caused by variations and strengthening of the south-east trade winds blowing across the Pacific.

Trade wind strength is also an indicator of sea levels all around Australia- as far south as Tasmania.

I use scaled trade wind index data from NOAA, and mean sea level data from the BOM’s Australian Baseline Sea Level Monitoring Project.  Sea level is in metres and all data are monthly anomalies.

Here’s a map showing the location of the ABSLMP stations.

Figure 1:  Sea level stations

I did not use those stations with large gaps (e.g. Thevenard) or very short records (Thursday Island).

Figure 2 shows sea level and the trade wind index (scaled down by a factor of 60).

Figure 2:  Trade Winds and East Coast Sea Levels

Sea levels appear to loosely match trade winds (a symptom of the El Nino- Southern Oscillation-ENSO).  Sea levels are averaged in Figure 3.

Figure 3:  Trade Winds and Averaged East Coast Sea Levels

Across the north of Australia, the match is close and strong.

Figure 4:  Trade Winds and North Australian Sea Levels

Figure 5 shows the average of the tide gauges, including Cocos Island, far out in the Indian Ocean.

Figure 5:  Trade Winds and Average North Australian Sea Levels

Figure 6:  Trade Winds and Average North Australian Sea Levels Excluding Cocos Island

The surprise is that the same effect is seen across southern Australian ports, with the TWI scaled down by 30.

Figure 7:  Trade Winds and Southern Australian Sea Levels

Figure 8:  Trade Winds and Average Southern Australian Sea Levels

When the trades are weak, sea level is lower, and vice versa, with a delay of one or two months.  The trade winds have become stronger over the last 40 years, and sea levels have increased.

Across southern Australia the intensity of high pressure systems has also increased:

Figure 9:  Strength of southern high pressure systems

The strength of high pressure systems in the sub-tropical ridge has increased.  On the southern side blow the Roaring Forties, and on the northern side the South-East Trades.  Stronger winds in the Pacific roughly match stronger winds in the Southern Ocean, pushing the sea up against the coastlines in the north and south.

It could be that stronger circulation is a symptom of global warming (which you may remember I don’t doubt, just the amount and cause).   However water finds its own level.   Sea level rise at Australian ports and some Pacific islands that has been caused by wind-driven water movement has to be matched by sea level fall across broad areas elsewhere.  That’s why coastal tide gauges are not good at measuring global sea level.

There’s more to sea level than you might think.

Is Climate Change Threatening the Solomon Islands?

Since the first talk of an agreement between China and the Solomon Islands to establish a Chinese presence there, accusations have flown thick and fast between the Australian government and their opponents.

One of the points of contention is whether Australia’s supposed lack of urgency in addressing climate change has led to distrust of Australia by Pacific island nations, thus encouraging them to seek help from China.  Considering China’s record and plans for emissions, that is hardly likely.  However, The Guardian thinks so, saying two days ago:

There might not be a direct link between Australia’s climate policy and the security deal – Morrison certainly thinks there isn’t, dismissing such a connection as “nonsense” today – but it is without doubt that Australia’s climate policy has contributed to the dimming of Australia’s reputation in the region, especially given Australia claims to be family.

So is climate change – specifically sea level rise- threatening the Solomons?

Time for a reality check.  Here is a map courtesy of Google, showing where the tide gauge in the Solomons is in relation to Australia.

Figure 1:  Solomons tide gauge location

Not that far away.

Over the last 28 years since the BOM began monitoring sea level at Honiara, sea level has definitely risen.  Figure 2 shows monthly anomalies of mean tidal data.

Figure 2:  Monthly mean sea level, Honiara

Oh no!  Climate change!

Figure 3 shows inverted mean barometric pressure anomalies plotted with mean sea level.

Figure 3:  Monthly sea level and barometric pressure (inverted)

Hmm.  As air pressure falls, sea level rises, and vice versa.  Figure 4 shows 12 month means (from July to June, which covers most ENSO events):

Figure 4:  12 month means of monthly sea level and inverted barometric pressure

Still not a close match, but let’s include the effect of the trade winds (data from NOAA).

12 month means of trade wind anomalies, scaled down by a factor of 10 show a much better match:

Figure 5:  12 month means of monthly sea level and scaled trade winds index

Now we see the connection, and cause of the apparent trend in sea level- the combination of air pressure and trade winds.  Barometric pressure has been decreasing, and trade wind strength has increased.  These are symptoms of the El Nino Southern Oscillation (ENSO).  When atmospheric pressure is unusually high (as in very big El Ninos), sea levels are lower, mainly because the normal trade winds slacken and less water than normal is pushed westwards across the Pacific.  As trade winds strengthen, more water is pushed westwards and sea level rises.  (This also affects the eastern coast of Australia, and strengthens the East Australian current as well.) 

When we get the next big El Nino (cue droughts, bushfires, and wailing and gnashing of teeth) it is likely that the sea level trend will mysteriously flatten.

Sorry, guys, unless climate change predicts fewer and weaker El Ninos, climate change is not to blame: and certainly not the Australian government.

It’s all about the money.