Tropical Cyclones and Global Warming: A Reality Check

Recently there was widespread media reporting of Queensland Emergency Services Minister Craig Crawford’s release of “a plan designed to help first responders get ready for future weather extremes.”

In the ABC Online report, these quotes from Mr Crawford are emphasised:

“There are plenty of people out there who are climate change sceptics… but the consensus is our fire seasons are getting hotter and longer and our flood and cyclone seasons are certainly getting stronger and more frequent.”

“If we’re going to have cyclones happening in parts of Queensland that they don’t normally happen right now it means that we’re going to have to expand all the areas where we have response training, capability and everything like that,” Mr Crawford said.

Cyclone seasons getting stronger and more frequent?  Cyclones happening in parts of Queensland that they don’t normally happen right now?  Time for a reality check.

The Bureau of Meteorology has a useful resource in its Southern Hemisphere Tropical Cyclone Data Portal  which shows the tracks of all cyclones since the 1969-1970 season.  By clicking on each track you find details of each.   This is the 2017/18 season:

Fig. 1:  Cyclones of the 2017/18 season

I have used it to look at all cyclones that have crossed the coast of Australia (and I have included TC Nancy which came very close and whose impact was strongly felt without actually crossing the coast.)  I have counted the cyclones that crossed the coast in every month from October 1969 to now, allocating them to those parts of the northern coastline that they predominantly affected- the north-west, Northern Territory, Gulf of Carpentaria and northern Cape York, north-east Queensland, south-east Queensland (south of the Tropic of Capricorn), and New South Wales.

So here are some facts to annoy our Global Warming Enthusiast friends, and to demonstrate how ill-informed our Emergency Services Minister is.

Fig. 2: Total number of cyclones per season

There has been a decrease in the number of cyclones over the past 48 years, a rate of five less in 100 years.  There has been little change in Western Australian cyclones:

Fig. 3: Total number of cyclones per season hitting North-West Australia

Whereas there has been a very noticeable decrease on the east coast (Queensland and NSW):

Fig. 4: Total number of cyclones per season hitting the east coast

which is well illustrated by this plot of cyclones crossing the Queensland coast south of the Tropic of Capricorn:

Fig. 5: Total number of cyclones per season hitting south-east Queensland

And these images of cyclone tracks are instructive:

Fig. 6: Cyclones of south-east Queensland 1969-1992

Fig. 7: Cyclones of south-east Queensland 1992-2018

Oswald, Marcia and Debbie crossed the coast north of the Tropic of Capricorn and were rain depressions by the time they reached the south-east.

The difference is obvious.  No cyclone has crossed the coast south of Yeppoon since TC Fran in 1992.  26 years without a cyclone- people (and Mr Crawford) forget we had three in 1971.  If we do get another one no doubt it will be blamed on climate change.

So what connection is there between temperature and cyclones?

Fig. 8:  Australian tropical cyclones as a function of sea surface temperature

As temperatures go up, cyclones go down!

Fig. 9:  Australian tropical cyclones as a function of Southern Oscillation Index

The SOI is an indicator of El Nino, La Nina, or neutral conditions.  According to the BOM, consistently below -7 indicates El Nino, and above +7 indicates La Nina.  It is obvious that there have been very few cyclones in seasons with El Nino conditions, with the vast majority in neutral or La Nina conditions, and higher SOI indicates greater likelihood of cyclones crossing the coast.  This is not new, and the Bureau makes this clear.

Fig. 10:  Tropical cyclones in La Nina years

Fig. 11:  Tropical cyclones in El Nino years

Future trends:

The Bureau discusses future trends at length at http://www.bom.gov.au/cyclone/climatology/trends.shtml

but seems to base its conclusions entirely on climate models:

There remains uncertainty in the future change in tropical cyclone frequency (the number of tropical cyclones in a given period) projected by climate models, with a general tendency for models to project fewer tropical cyclones in the Australia region in the future climate and a greater proportion of the high intensity storms (stronger wind speeds and heavier rainfall).

This is the BOM plot of severe and non-severe cyclones, which includes all tropical cyclones from 90E to 160E south of the Equator, many of which remained well offshore.

Fig. 12: Severe and non-severe tropical cyclones

Is there any evidence for cyclones becoming stronger, if fewer?  According to the BOM’s history of cyclones, no.  This graph plots the number of cyclones rated as severe by the Bureau (<970 hPa central pressure at peak intensity- low pressure is a good predictor of wind speed).  Interestingly, Marcia and Debbie are not listed as severe, but are described as severe in their reports, and definitely were, so I have included them in the tally.

Fig. 13: Severe land-falling tropical cyclones

And showing how the proportion of severe tropical cyclones as a percentage of all land-falling cyclones has changed:

Fig. 14: Proportion of land-falling tropical cyclones rated as severe

Tropical cyclones in the past 48 years have decreased in number and intensity, and the proportion of severe tropical cyclones has also decreased, although it is entirely likely that this situation could reverse due to natural variability.

The Government’s Response

The Queensland Government is concerned cyclones may strike further south than they currently do.  They have records of cyclones going back 150 years.  Many, many of them have affected south-east Queensland and NSW.

The worst natural disaster in recorded Australian history was in March 1899 when TC Mahina (the Bathurst Bay cyclone) killed 307 people.

Here are some other significant tropical cyclones recorded by the Bureau:

February 1893 a cyclone crossed near Yeppoon.  This led to the Brisbane River floods.

January 1918. The Mackay cyclone, which caused many deaths.  There was a large storm surge and a barometric pressure reading of 932.6 hPa in a private barometer, and less than 944.8 hPa at the Post Office as the flange on the instrument prevented the needle from going lower.  Inland rainfall caused the highest recorded flood in the Fitzroy River.

March 1918. The Innisfail cyclone.  The pressure dropped to 926 hPa at Mourilyan Sugar Mill.  There was a large storm surge.  Almost all buildings in the town were destroyed or badly damaged.

March 1949.  A cyclone struck Rockhampton and Gladstone.

1967 TC Dinah affected southern Queensland and NSW.  The pressure dropped to 944.8 hPa at Sandy Cape.

In Queensland, counting only those cyclones that have actually crossed the coast, not just approached, here is a list of tropical cyclones since 1970 (see Figure 6) that have struck south of the Tropic of Capricorn (Rockhampton or Yeppoon.)

February 1971 TC Dora

February 1972 TC Daisy

March 1972 TC Emily

January 1974 TC Wanda

March 1974 TC Zoe

February 1976 TC Beth

March 1976 TC Dawn

February 1981 TC Cliff

March 1992 TC Fran

TC Nancy (January 1990) came close but did not actually cross the coast.

TC Marcia in February 2015 crossed the coast near Shoalwater Bay before moving south over Rockhampton.

There is also an impressive list of cyclones which have caused deaths and wind, wave, and flooding damage in NSW.   These include cyclones from 1892.  Included are:

March 1939, TC crossed the coast at Cape Byron.

January 1950   The Sydney cyclone of 1950, when the pressure dropped to 988 hPa in Sydney.

February 1954, TC crossed the coast at Tweed Heads, where the pressure dropped to 973 hPa.

February 1957 TC crossed the coast south of Port Macquarie.

January 1967 TC Dinah caused a large storm surge in the Tweed River.

February 1967 TC Barbara crossed the coast near Lismore.

March 1974 TC Zoe crossed the coast just north of the border and travelled through northern NSW.

January 1990  TC Nancy did not cross the coast but passed about 50km east of Cape Byron.

The Reality

Contrary to Minister Crawford’s claim, and the media’s breathless and uncritical reporting, tropical cyclones in the past 48 years have decreased in number and intensity, and the proportion of severe tropical cyclones has also decreased.  Predictions of future trends are purely speculative.  The current 26 year lull in tropical cyclones hitting the south of Queensland and northern NSW is unusual.  In the past it was normal for cyclones to strike much further south than they do now.  We should not become complacent.

TC Debbie

TC Debbie hit the Whitsunday coast and areas to the south and inland yesterday.  As I spent nearly half my life in places not far from Mackay and have many friends in the region, I was very interested to see what was happening.   I began checking online from 5 a.m. Tuesday morning.

Here is some initial analysis of TC Debbie.  Firstly, here is the table of cyclone intensities as found at http://www.bom.gov.au/cyclone/faq/index.shtml#definitions .

Fig. 1:  Cyclone Intensity

I began checking online from 5 a.m. Tuesday morning.

Fig. 2:  0500 forecast cyclone track map.

How accurate was the Bureau’s forecast?  Here is the forecast 22 hours later, at 0300 Wednesday morning.

Fig. 3:  Wednesday 0300 forecast cyclone track map.

The track forecast was pretty good.

The next images show Debbie’s progress across the Whitsunday Islands until the eyewall crossed the coast near Airlie Beach.

Fig. 4:  0720 Eyewall about to hit Hamilton Island

Fig. 5:  0910  Hamilton Island near the eyewall, Hayman Island in the eye

Fig. 6:  10.30  Hamilton Island near the eyewall, Hayman Island in the eye, and the eyewall about to pass over Airlie Beach

And four and a half hours later, the worst is over at Hamilton and Hayman Island and the eye is collapsing over Proserpine.

Fig. 7:  1510  Debbie weakening near Proserpine

Note the “gap” in the image in the northwest sector.  The Bowen radar failed and the Mackay radar was blocked by high mountains to the west.

What about forecasts of the cyclone’s intensity?

The next figures show plots of wind gusts, pressure, temperature, and rain at Hamilton Island, Proserpine, and Bowen, the closest stations to the cyclone’s track.

Fig. 8:  Wind gusts at Hamilton Island

The black line shows the period from just before 8.00 a.m. until about 2.30 p.m. during which Hamilton Island was close to the eyewall, the area of maximum wind strength.   For nine hours from before 6.00 a.m. until nearly 3.00 p.m. wind gusts were of Category 3 strength.  From 8.00 a.m. until 12.30 p.m. gusts approached or exceeded 225 km/hr, bordering on category 4, and between 10.35 and 10.30 reached 263 km/hr three times at least- and the Bureau had forecast winds up to 270 km/hr.  While the station at Hamilton Island is too high to be completely reliable, these data are indicative that winds at 10 metres were at cat 4 level for some time.

Fig. 9:  Air Pressure at Hamilton Island

The red line shows the period from just before 8.00 a.m. until about 2.30 p.m. during which Hamilton Island was near the eyewall, the area of maximum wind strength.    From 2.00 a.m. until 5.00 p.m.  pressure was below 985 hPa (Cat, 2) and from 10.00 a.m. until 1.30 p.m. was below 970 hPa (Cat.3) but did not reach 955 hPa (Cat. 4).  Remember however that Hamilton Island was some 50 km from the centre of the eye, so 955 hPa is quite possible for central pressure.

On the basis of wind gusts and pressure at Hamilton Island, I believe Debbie was a strong Category 3, weak Category 4 system.

Fig. 10:  Air temperature at Hamilton Island

Note the sudden jump in temperature from 8.12 a.m.- 3 degrees in 3 minutes- coinciding with a wind gust of 212 km/hr, and kept climbing to unbelievable values.  (Compare with Proserpine below.)  It is likely that the AWS probe malfunctioned, and failed altogether at 12.00 noon.

Fig. 11:  Rain at Hamilton Island

Rain measurement is unlikely to be accurate in such ferocious winds.  Note how rainfall levelled off from 11.00 a.m until 2.00 p.m., then increased after 3.00 p.m.

Fig. 12:  Wind gusts at Proserpine

Proserpine Airport is some 20 km inland, 41 km west of Hamilton Island and 56 km from Bowen.  As the cyclone arrived over land it began losing strength and the eye began to shrink.  From 10.00 a.m. until 2.00 p.m. gusts were at Category 2 strength and at 1.00 p.m. reached the magic 165 km/hr of Cat 3 strength.  They were very probably much stronger in the town itself 9.1 km north.

Fig. 13:  Pressure at Proserpine Airport

From 12.30 p.m. until 5.00 p.m. the pressure at the airport, some 20-30 km from the centre, was below the Category 3 value of 970 hPa.

Wind gust and pressure data indicate Debbie was very likely still Category 3 as it passed over Proserpine town.

Fig. 14:  Air temperature at Proserpine

Fairly stable temperature with only about 1.5C range all day.

Fig. 15:  Rain at Proserpine

Steady rain all day, fairly typical of cyclonic conditions.  At Strathdickie not far from Proserpine, 193mm fell in one hour that morning, and at Dalrymple Heights about 50km south 814mm fell in 24 hours.

Fig. 16:  Wind gusts at Bowen

For four and a half hours wind gusts reached Category 2 strength, and were above 100 km/hr from 9.00 a.m. to 8.00 p.m.

Fig. 17:  Pressure at Bowen

Pressure was at Category 2 levels from 9.00 a.m.

Fig. 18:  Air temperature at Bowen

Winds were west south west most of the day, but as Debbie passed and winds turned northwest (over the ocean), the temperature climbed.

Fig. 19:  Rain at Bowen

Steady rain all day: 12 inches in 12 hours.

While no stations were directly in the cyclone’s path, nearby station data indicate that Debbie was a large Category 3 to Category 4 tropical cyclone when it hit the coast and brought very strong winds, very heavy rainfall, and widespread destruction.  It is still lingering as a tropical low 300 km inland, bringing more strong winds and very heavy rain, and will head south over the next couple of days.  The clean up begins.  We await the report from James Cook University engineers who will provide their assessment of damage and wind loadings in a few weeks’ time.

Give credit where credit is due: the Bureau of Meteorology got this one pretty right.