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Tracking a cyclone

Dr Sushil K Sharma
Tuesday, February 13, 2018

WEATHER and climate, as expected over our nation during this wet season, are going to scientific expectations. Fewer chances of tropical cyclones and above average rains with wet and cloudy conditions are expected until April. La Nina has the upper hand at the moment.

Do you recall the 2009 and 2012 Nadi or Western Division floods when Nadi, Ba and Rakiraki were under so much water boats were seen where cars, buses and trucks usually ran?

We have made this far without much adverse conditions for our nation as would be expected in a La Nina phase of the cycle but let us hope for the best in the next two months for our nation as we slowly end our wet season on April 30, 2018.

El Nino and La Nina come and affect us all in three-seven year cycles and they regulate our extreme dry years and extreme wet years during our November to April wet season. You may recall that for nearly four years we have not had major floods as in 2009 and 2012.

Cooling of the Pacific Ocean in the central and eastern equatorial Pacific region continues unabatedly covering about 80 per cent of the entire tropical Pacific Ocean as we progress into 2018. This cooling is of gigantic proportions and is expected to progress into March -May 2018 period.

Over 30 years of data (1961-1991) used to derive the average oceanic temperatures of this vast region found oceanic temperatures had dropped continually since August 2017 when it was the average value in each successive month and had reached very low thresholds of at least — 0.8C triggering the onset of a La Nina.

Both air temperatures and deeper waters to hundreds of metres have cooled leading to negative anomalies with the temperatures expected to cool further by more than 1.2 C (from the 30-year average temperature for the region). These anomalies are expected to continue to the middle of May 2018.

La Nina is the opposite of El Nino. One warms the region while the other cools it driving our Walker Circulation which modulates in an irregular cyclical manner. The areal and temporal variability of our weather and climate and the extremes will be dictated by these modulations.

Warmer waters to great depths in the region of northern Australia, Papua New Guinea and Indonesia is causing intense cloud activity in that part of the world with a number of cyclones forming in that part of the world moving westwards while Fiji and our region have remained fairly quiet, as we had expected.

Tropical Cyclone Gita happens to be the first major system of our region. An extremely rapid development west of Samoa with enormous number of passing squalls, thunderstorms, winds gusting to 60 knots with very heavy continuous rains, caught Samoans by surprise as it moved inland last Friday night.

This system, after dumping tonnes of water flooding Samoa and blocking all access to Apia City, kept moving southwards still a Category 1 system and of a relatively extremely small diameter system, only 250 km wide, at 1.30pm Sunday Fiji time.

Because of an extremely intense area of high pressure to the south and with the absence of north-westerly steering fields, the system stalled and then started moving westwards on Sunday morning.

TC Gita slowly began to intensify late on Sunday and by Monday morning had become a major hurricane — Category 4 system, three to four times larger than before in its axial diameter and remained very symmetric with a definite eye but slightly covered by the upper level cirrus clouds.

By midday Monday, the system was estimated to be almost at Category 4-5, hurricane strength. This is almost similar to Severe TC Winston posing a major threat to Tonga where the front peripheries of the axial clouds bands were just arriving with the approaching system with the axial centre only some 250 km behind. This means the axial diameter of the system, 500km, had become much wider from its original 250km width.

In my educated opinion, TC Gita is expected to curve more sharply soon towards the southwest from its initial westwards movement, but not before affecting southern Tonga very badly indeed where death, destruction and carnage is expected initially similar to that of Fiji during T.C Winston.

At 08.30 GMT (8.30 PM Fiji Times) Nukualofa was directly under the front leading edge of the eye wall of the Hurricane, with a small break to come with the eye passing few hours later, with the winds changing to the opposite directions. i.e., 10-minute sustained winds of over 100 knots with gusts to 150 — 190 knots, with extremely low pressure of 910 hPa, leading to a rise in sea levels to 10 meters or more with extensive storm surge damage.

Hurricane Gita will trending more on a south-westerly quadrant from this point. Small islands just south of Nukualofa, the capital of Tonga which sit right in the path of the Hurricane Gita will also experience the worst effects of the system, and people will need to hide in caves to withstand the fury of the Category 5 system.

The eye of the cyclone by yesterday afternoon and evening was very clear 3o-40 km wide in diameter, with extremely good axial cemetery of cloud circulation around the system. After passing far to the south of Ono-i-Lau Tropical cyclone will curve in a very large anti-clock wise arch, gravitating towards the large area of low pressure and trough between Fiji and New Zealand, passing east of Norfolk Island and then re-curving to the southeast away, eastwards of Auckland.

Fiji will be spared the wrath from this system, as it was re-curving south-west wards last night, with intensity and strength at Category 5 level. After 36-48 hours it will come under very strong wind shear from aloft, which will slowly weaken the system apart and move it to the southeast on its normal course away from Fiji.

Three dimensional ocean-atmosphere circulations — both the Walker and the Hadley, are more enhanced at the moment. As forecast, Fiji region continues to have enhanced activity in cloudiness, rains, thunderstorms, floods, with the South Pacific Convergence Zone (SPCZ) much more pronounced and positioned over us.

Tropical cyclone genesis region is to the far northwest of us, in the Coral Sea ensuring systems move westwards away from us. Low pressure systems and depressions are running down the trough lines in a northwest to southeast manner across Fiji enhancing rains and cloudiness.

While we have been hearing many doom and gloom stories for the region in relation to likely regional climate change scenarios, it is worthwhile to discuss some of the latest findings of an interesting research.

Wellington (AFP) reports Tuvalu, long seen as a prime candidate to disappear as climate change forces up sea levels, is actually growing.

A University of Auckland study, using aerial photographs and satellite imagery, examined changes in the geography of Tuvalu's nine atolls and 101 reef islands between 1971 and 2014.

It found eight of the atolls and almost three-quarters of the islands grew during the study period, lifting Tuvalu's total land area by 2.9 per cent even though sea levels in the country rose at twice the global average.

Co-author Paul Kench said the research, published this week in the journal Nature Communications, challenged the assumption that low-lying island nations would be swamped as the sea rose.

La Nina is a phenomenon which regulates our weather and climate's natural variability within our earth-atmosphere system and can be considered a "climate driver".

People continue to confuse climate variability from natural causes and treat natural variability cycles in our weather and climate system as climate change, which is grossly inaccurate.

Tropical cyclones are not getting stronger and it is not correct that they are becoming more frequent. This is grossly inaccurate and based on speculation and not science and people should stop this type of talk without scientific evidence.

* Dr Sushil K Sharma BA MA MEng (RMIT) PhD (Melbourne) is a WMO accredited Class 1 professional meteorologist and an associate professor of meteorology at the Fiji National University. The views expressed are his and not that of the FNU or this newspaper.








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