What is a Tropical Cyclone?
Tropical cyclones also known as typhoons or hurricanes, are some sort of organized system of clouds with intense circular storm that originates over warm tropical or subtropical oceans, characterized by low atmospheric pressure, high winds and heavy rain.
A tropical cyclone generates winds that exceed 119 km/h (74 miles). In extreme cases winds may exceed 240 km/h (150 miles), and gusts may surpass 320 km/h (200 miles). Drawing energy from the sea surface and maintaining its strength as long as it remains over warm water.
Accompanying these strong winds are torrential rains and a devastating phenomenon known as the storm surge (an elevation of the sea surface that can reach 6 meters [20 feet] above normal levels).
Such a combination of high winds and water makes cyclones a serious hazard for coastal areas in tropical and subtropical areas of the world.
Every year during the late summer months (July–September in the Northern Hemisphere and January–March in the Southern Hemisphere), cyclones strike regions as far apart as the Gulf Coast of North America, northwestern Australia, and eastern India and Bangladesh.
Formation of tropical cyclones
A cluster of thunderstorms can develop over warm tropical oceans. If that cluster persists in an area of low pressure, it can start rotating. If the conditions are just right, the cluster of thunderstorms can grow in size, sustaining itself then develop into a tropical cyclone.
Tropical Cyclones typically form when the sea-surface temperature is above 26.5°C. Cyclones can continue for many days, even weeks, and may follow quite erratic paths. They will dissipate once it moves over land or over cooler oceans.
Once developed, a tropical cyclone is like a giant, atmospheric heat engine. The moisture from the warm ocean acts as its fuel, generating huge amounts of energy as clouds form.
As long as the environmental conditions support this atmospheric heat engine, the tropical cyclone can maintain its structure and even intensify over several days.
Anatomy of a cyclone
Tropical cyclones are compact, circular storms, generally some 320 km (200 miles) in diameter, whose winds swirl around a central region of low atmospheric pressure.
The winds are driven by this low-pressure core and by the rotation of the Earth, which deflects the path of the wind through a phenomenon known as the Coriolis force.
As a result, tropical cyclones rotate in a counterclockwise direction in the Northern Hemisphere and in a clockwise direction in the Southern Hemisphere.
Effects of tropical cyclones
Tropical cyclones may be dangerous as they can produce extreme winds, heavy rainfall with flooding and damaging storm surge that can cause inundation of low-lying coastal areas.
Cyclones have gale force winds with wind gusts in excess of 90 km/h around their center. In the most severe cyclones, gusts can exceed 280 km/h. These winds can cause extensive property damage and turn airborne debris into potentially lethal missiles. It is important to remember when the eye of a cyclone passes over a location, there will be a temporary lull in the wind, but that will soon be replaced by destructive winds from another direction.
Heavy rainfall associated with the passage of a tropical cyclone can produce extensive flooding. This can cause further damage. The heavy rain can persist as the cyclone moves inland and weakens into a low pressure system, hence flooding due to an ex-tropical cyclone can occur a long way from where the cyclone made landfall.
Storm surges are powerful ocean movements caused by wind action and low pressure on the ocean’s surface. These types of events can swamp low-lying areas, sometimes for kilometers inland.
As well as extreme winds, a tropical cyclone can cause the sea to rise well above the highest tide levels of the year when it comes ashore. These storm surges are caused mainly by strong onshore winds and also reduced atmospheric pressure. Potentially, the storm surge is the most dangerous hazard associated with a tropical cyclone.
Tropical Cyclone Bulletin
This TC Potential Bulletin describes any current cyclone activity, along with details of possible cyclone development over the next five days in the Coral Sea and South Pacific area between Australia and 120W. It is normally issued during the TC season from 1 Nov to 30 Apr, but also outside this period if required for a potential or active cyclone.
Current status of cyclone activity
Tropical Cyclone Irene (Category 2) was located near 19.3S 168.1E at 7am NZ local time Thursday morning, or about 130km west of Tanna Island, Vanuatu and about 160 km south of Port Vila, Vanuatu.
Forecast to 12:00 20 Jan 2023 UTC (1am Sat, 21 Jan NZT)
Tropical Cyclone Irene is expected to maintain category 2 intensity as it moves east-southeast across southern Vanuatu today (Thursday), then it should take a more southeastwards track and weaken once it moves away from Vanuatu later today and Friday. The system is not expected to impact New Zealand.
Another tropical low, located in Coral Sea near 16.9S 149.7E at 7am Thursday morning, has a HIGH risk of developing into a tropical cyclone on Friday as it moves eastwards over open waters.
Outlook to 12:00 23 Jan 2023 UTC (1am Tue, 24 Jan NZT)
Tropical Cyclone Irene should be reclassified as a low by the time it reaches the Kermadec Islands on Saturday and is not expected to impact New Zealand.
The tropical low in the Coral Sea continues to have a HIGH risk of developing into a tropical cyclone on Saturday. It is expected to track east-southeast towards New Caledonia, then become slow moving just west of New Caledonia during the weekend.
Atlantic and Eastern Pacific Hurricane Season Normal Activity
The Atlantic hurricane season runs from June 1 to November 30. The Atlantic basin includes the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico.
Based on a 30-year climate period from 1991 to 2020, an average Atlantic hurricane season has 14 named storms, 7 hurricanes, and 3 major hurricanes (Category 3, 4, or 5 on the Saffir-Simpson Hurricane Wind Scale).
The first named storm typically forms in mid to late June, the first hurricane tends to form in early to mid-August, and the first major hurricane forms in late August or early September.
The eastern Pacific hurricane season runs from May 15 to November 30. The eastern Pacific basin extends from Mexico and Central America westward to 140°W. Based on a 30-year climate period from 1991 to 2020
An average eastern Pacific hurricane season has 15 named storms, 8 hurricanes, and 4 major hurricanes. The first named storm typically forms in early to mid-June, the first hurricane tends to form in late June, and the first major hurricane forms in mid-July.