Tonga Meteorological Services
(27th March 2020)
Summary
The tropical cyclone (TC) activity expected for the last month of the official 2019/2020 TC Season (April) to affect Tonga is likely to be very low for this last month of this TC Season.
Historical records, dynamical and statistical analysis indicate that Tonga would be expecting a continued reduced chance of 25% for at most 1 or more tropical cyclone systems for the last month (April) of this season, with very little chances for the occurrence of any severe Tropical Cyclone occurring (category 3 or more).
Any tropical cyclones possibly passing close to the country, associated active cloud and rain bands may occasionally affect Tonga with marked rainfall and possible flooding, including sea flooding of low‐lying coastal areas.
Given the event of a possible Tropical cyclone to affect Tonga for the remainder of this 2019/2020 season it should expect to be mostly coming from the NW (Fiji Area – 42%), West (29%), NE (Samoa Area – 16%), North (Wallis/Futuna – 13%).
Current dynamics indicate that the next period with chances of any tropical cyclone activity formation in the South West Pacific is expected to be from Mid to Late April 2020, but with lower chances for any tropical cyclone activity as the Madden-Julian Oscillation (MJO) signal is expected to be very weak.
Tropical Depressions and Tropical Cyclone occurrence for this 2019/20 Season
Let’s now look at the most recent tropical systems that has affected Tonga and in the South West Pacific during this tropical cyclone season since November 2019 up to today.
There have been 11 Tropical Depressions (TD) in total that has occurred during this 2019/2020 TC Season. From these 11 Tropical Depressions, a total of 6 Tropical Cyclones had managed to develop from Tropical Depression status. These 5 Tropical Cyclones were: TC Rita (Category 1), TC Sarai (Category 2), Severe TC Tino (Category 3), TC Vicky (Category 1), TC Wasi (Category 1 and TC Gretel (Category 2).
TC Sarai (Category 1) was the first cyclone to affect the Tonga group, during this season from the 27th December to 01st January 2020. Severe TC Tino (Category 3) as the second cyclone to affect the group, mainly the Niuas, Vava’u and Ha’apai from the 16th to 19th January 2020. TD05F also affected the group but with very minor impacts between 17th to 20th January 2020.
Tropical Depression and Cyclones, that developed near our area of responsibility but had no immediate threats for Tonga, especially the Niuas was TC Vicky (Category 1), TC Wasi (Category 1), TD07F and TD08F. These systems occurred during the first to second weeks of February 2020 which affected Samoa, with very little threat for the Niuas.
Accuracy of the Tropical Cyclone Outlook for the TC Season 2019/2020
The first TC outlook released from the Tonga MET Office was on the 17th October 2019. According to our TC outlook, it was predicted that on average, Tonga could be affected by at least 1 to 2 Tropical Cyclones for this season with 85% chances of 1 Severe (category 3 or more) Tropical Cyclone to occur.
TC Sarai (Category 1) and Severe TC Tino (Category 3) are the two cyclones on record that has affected Tonga during this TC Season, with one reaching Severe TC strength as the justifications to the outlook that was predicted. So far, the actual occurrence of these cyclone events provides verification that the tropical cyclone outlook provided has been accurate since the start of the season up to now.
Furthermore, it was also predicted in the TC outlook that about 5 to 8 named Tropical Cyclones were expected for the South West Pacific (Fiji Area of Responsibility) in 2019/2020 season with 2 to 4 reaching severe intensity (Category 3 to 5). Six (6) Tropical Cyclone systems has been named to date, with only two (2) more systems to be expected for this last part of the season as per our predictions but with at a lower likelihood as of the current dynamics and analysis.
Analysis and Outlook for Tonga for March – April 2020.
The official 2019/20 TC Season had begun on 01st November 2019 and will end on 30th April 2020. It should be noted that tropical cyclones have occasionally formed outside the official TC season period. For example, Tropical Cyclone “Keli” which affected Northern Tonga in June 1997. Historically, the peak time for the occurrence of tropical cyclones in Tonga is from January to March with most events occurring in February.
Tonga would be expecting a continued reduced chance of 25% for at most 1 or more tropical cyclone systems for the last month (April) of this season, with very little chances for the occurrence of any severe Tropical Cyclone occurring (category 3 or more).
International forecast guidance in the last few months indicate the El Niño–Southern Oscillation (ENSO) remains neutral—neither El Niño nor La Niña for the whole Pacific Ocean basin. The tropical Pacific Ocean is expected to remain in an ENSO-neutral phase for the remainder of the 2019/2020 Cyclone Season.
Cyclones that have affected Tonga in the past, where current and forecast conditions are similar to the 2019/20 Cyclone Season are indicated in Table 1.
Statistical Analysis
Dynamical Analysis
Likely Sector Impacts during the 2019/20 Season
1. Agriculture
There is an elevated risk that the agriculture sector will be affected. Particularly crops prone to high winds e.g. Banana, Papaya, Breadfruit and lower than normal summer rainfall e.g. Kava, Taro and Vegetables. Seek advice from MAFFF.
2. Water
Water shortages during the could be experienced in areas dependent on rainwater harvesting but risk of extensive meteorological drought occurring is considered low. Smaller islands that rely on rainwater should use water wisely.
3. Health
Higher risk of water contamination related diseases occurring due to the higher risk of a severe cyclone occurring.
4. Infrastructure
High risk of damage to poorly constructed homes due to increased likelihood of a Severe Cyclone.
5. Tourism
Moderate to high risk for Tourism activities due to higher chance of a severe cyclone happening.
6. Fisheries
Tuna catches should be normal due to neutral ENSO conditions. Expect low to moderate coral bleaching or algae blooms associated with changes in sea surface temperature which could expose inshore fisher folk.
7. Utilities
Moderate to High risk to Communications, Water and Power utilities expected.
8. Transport
Moderate risk of above than normal cancellations to ferry and aircraft operations due to tropical cyclone activity.
Note of caution
It should be noted that the information provided is only to be used as guidance and the given range of tropical cyclone numbers is indicative only. It is expected that the total number of tropical cyclones could be in the vicinity of the listed values, and not necessarily within the given range. The values are the most likely number of tropical cyclones based on statistical and scientific evidence, including the influences by regional and global weather and climate variability drivers and indices.
All communities should remain alert and prepared throughout the 2019/20 Tropical Cyclone Season and take heed of tropical cyclone alerts, warnings and advisories seriously whenever it is issued to reduce the effects on life and property. All communities are urged to be vigilant and follow forecast information provided by the Meteorology Department of the Ministry of MEIDECC throughout the season. An update to this Tropical Cyclone outlook will be issued towards the end of April 2020.
ENDS
Issued by the: Ministry of Meteorology, Energy, Information, Disaster Management, Environment, Climate Change & Communications.