PAGASA (2024)

The following are the necessary requirements established for tropical cyclone formation:

Sufficiently large ocean areas with a surface temperature of more than 26°C or 27°C that air lifted from the lowest atmospheric layers and expanded moist adiabatically remain considerably warmer than the surrounding undisturbed atmosphere at least up to a level of about 40,000 feet.

Initial disturbances from which storms later developed may be detected within 5° of latitude of the equator, but these disturbances do not intensify into typhoons or hurricanes until they are more than 5° of latitude from the equator (since the value of the coriolis parameter should be larger than a certain minimum value.

Weak vertical wind shear in the basic current so in those areas of small mean zonal-wind shear are also areas of active storm formation.

A pre-existing low level disturbance over a warm ocean area and a region of upper-level divergence or outflow above the surface disturbance (though not all these areas of organized convective activity develop into tropical cyclones or greater intensity).

Several theories have been formulated on the formation of tropical cyclones. These are the "convective theory" and the "frontal" or "counter-current" theory.

According to the convective theory , a large mass of air becomes convectively unstable and moist compared with its surroundings, which results in an upward motion of air. The air from the surroundings tend toward the low pressure area formed, so that, a cyclonic circulation is formed. The combined effects of the earth's rotation and the centrifugal force, retards the movements of air towards the center causing further pressure fall. The process continues until a vigorous cyclonic wind system is developed. Likewise, the outward flow of air from the center at high levels also makes the pressure lower.

The frontal theory indicates that many tropical cyclones form along the front between the trade winds and the equatorial air in the doldrums . Winds develop along this front and when conditions are favorable, forms into tropical cyclones. The convergence of the two air masses results in the upward motions which in addition to the deflective effect of the earth's rotation, centrifugal force , and divergence at the upper levels results in allow pressure area with a spiral circulation toward the center.

Likewise, as already listed above, tropical cyclones develop over sea surfaces having at least 26°C. Though these heat sources are not sufficient to start a hurricane going, the heat of condensation supports the process once started. Tropical cyclones are also generated in disturbances along the intertropical convergence zone, on traverse waves or under superimposed upper disturbances. But the upper divergence must exceed low-level convergence in order to cause surface pressures to decrease (which is called deepening ).

In general, therefore, development of a tropical cyclone takes place when there is proper combination of circulation, divergence and convergence which is maintained over a considerable period of time on a proper scale.

The following shows the regions of the world where tropical cyclones form:

Tropical North Atlantic Ocean

• East of the Lesser Antilles and the Caribbean, east of 70°W during the months of July to October

• North of the West Indies in June to October

• Western Caribbean during the months of June and late September to early November

• Gulf of Mexico during the months of June to November

Western North Pacific Ocean , including the Philippines , during the months of May to November, but storms sometimes occur in all months.

North Pacific off the West Coast of Central America during the months of June to October.

Bay of Bengal and Arabian Sea from May to June and October to November.

South Pacific Ocean, West of 140°W from December to April.

South Indian Ocean from December to April.

• Northwestern Coast of Australia during the months of November to April

• West of 90°E from November to May

Tropical cyclones form over oceans of the World except in the South Atlantic ocean and in the South eastern Pacific. During the Southern Hemisphere Summer, the intertropical front in these areas moves only a degree or so south of the equator which is not far enough for the coriolis force to become effective. Tropical cyclones are very rare within 5° latitude of the equator.

In the Pacific Ocean, the tropical cyclones that form normally move slowly towards the west or west northwest, threatening the Philippines. They usually move at an average speed of 19 kilometers per hour, often guided by the main airstream above them. Those that form in the South China Sea move generally northward or toward the northeast which also affects the Philippines.

Below are the frequencies of tropical cyclones per 10 years in the different areas where tropical cyclones are formed:

• North Atlantic Ocean - 73

• North Pacific, off West Coast of Mexico - 57

• North Pacific Ocean, west of 170°E - 211

• North Indian Ocean, Bay of Bengal - 60

• North Indian Ocean, Arabian Sea - 15

• South Indian Ocean, west of 90°E - 61

• South Indian Ocean, Northwestern Australia - 9

Tropical cyclones undergo constant metamorphosis from birth through maturity to decay. They last for about six days, in general, before they enter the land or reach sub-tropical latitudes. However, some can be detected only a few hours or perhaps a day or two, while others are observed as long as a fortnight.

The four stages of the life history of a cyclone are:

Formative Stage

The incipient stage when the tropical cyclone form in waves and in shear lines of pre-existing disturbances and winds usually remain below the typhoon force.

Immature Stage

The deepening stage of the cyclone during which it continues to deepen until the lowest central pressure and the maximum wind intensity are reached. However, intensification does not usually takes place since some have been known to die down even though the winds has attained typhoon force.

Mature Stage

The stage of maturity of the tropical cyclones where the areas of circulation expands while the surface pressure no longer falls and no increase in maximum winds speed can be observed which may last for a week.

Decaying Stage

The dissipating stage of the tropical cyclone where the surface pressure rises and the area affected by the cyclones diminishes in size as it recurves or dissipate due to friction and lack of moisture over continents or when colder and drier air enters through when they go poleward.

CLASSIFICATION OF TROPICAL CYCLONES

Tropical cyclones derive their energy from the latent heat of condensation which made them exist only over the oceans and die out rapidly on land. One of its distinguishing features is its having a central sea-level pressure of 900 mb or lower and surface winds often exceeding 100 knots. They reach their greatest intensity while located over warm tropical waters and they begin to weaken as they move inland. The intensity of tropical cyclones vary, thus , we can classify them based upon their degree of intensity.

The classification of tropical cyclones according to the strength of the associated windsas adopted by PAGASA as of 23 March 2022 are as follows:

TROPICAL DEPRESSION (TD) - a tropical cyclone with maximum sustained winds of up to 62 kilometers per hour (kph) or less than 34 nautical miles per hour (knots) .

TROPICAL STORM (TS) - a tropical cyclone with maximum wind speed of 62 to 88 kph or 34 - 47 knots.

SEVERE TROPICAL STORM (STS) , a tropical cyclone with maximum wind speed of 87 to 117 kph or 48 - 63 knots.

TYPHOON (TY) - a tropical cyclone with maximum wind speed of 118 to 184 kph or 64 - 99 knots.

SUPER TYPHOON (STY) - a tropical cyclone with maximum wind speed exceeding 185 kph or more than 100 knots.

The atmospheric pressure decrease from the periphery of the circulation towards the center of the eye and reaches its lowest value in the "eye" itself. On the other hand, as the wind blows inward, its speed increases and reaches its maximum value just outside of the "eye" near the surface of the ocean, the winds converge towards the center. The converging air is forced upward carrying with it moisture in the form of water vapor. As the air rises, the water vapor it contains cools by expansion and eventually condenses to form clouds. The condensation of the water vapor causes the release of the latent heat trapped within it. The latent heat released increases the buoyancy of the cloud and provides the energy for the sustenance of the tropical cyclone circulation. In view of the vigorous ascent of air, the clouds formed around the "eye" have large vertical extent with tops reaching beyond 12 kilometers above the surface. Such massive cloud formation produces heavy rains with large-sized raindrops.

At the top of the storm system, the rising warm air is transported outward and form an anvil-shaped cloud called "cumulonimbus". Further away from the center, at the tip the air becomes colder and dry and starts "sinking" downward. In this area, which is outside the storm system, the weather is abnormally good. This is the basis for the saying "lull before the storm" which many perceptive people notice before the arrival of the storm.

Tropical cyclones owe their existence to the release of latent heat in intense convection. This convection depends on eddy transfers of heat, moisture and momentum at the sea surface and radiative effects, as well as on the tropical-cyclone-scale circulation itself.

The relationship between the ocean and the atmosphere during tropical cyclone conditions is not a one-way interaction. The stress exerted by strong winds on the surface water and the negative pressure anomaly leads to a rise of mean sea level under the storm of about 1 cm per mb of pressure drop. This mound of water follows the storm and contributes to the storm surge when the hurricane makes landfall. The strong winds generate surface waves with amplitudes of 20 m or more. The curl of the stress generates divergence in the upper layer of the ocean, producing regions of upwelling and downwelling . Turbulence is also generated in the ocean by the wind stress and this turbulence mixes warm surface waters with deeper cooler water. The combination of upwelling and vertical mixing typically produces a decrease in the surface ocean temperature of 1-3°C and may occasionally produce a decrease as large as 5°C which affects the intensity of slow-moving or stationary storms by reducing evaporation into the atmosphere.

As we know, the ocean is divided into an upper layer of constant (in the vertical) temperature and a lower layer in which the temperature decreases with depth. The upper layer is termed the mixed layer because the constant temperature in the vertical is maintained by vertical mixing. Temperature across the interface (thermocline) between the mixed layer and the lower layer is depicted as discontinuous.

The response of the ocean to the approaching storm. As the storm approaches, the increasing winds produce stronger turbulence and a deepening and slight cooling of the mixed layer. Outside the radius of maximum wind, the anticyclonic relative vorticity is associated with a stress field with negative curl. Convergence is induced in the mixed layer and downwelling occurs, which also acts to deepen the mixed layer. As the radius of maximum winds passes, the vorticity becomes strongly positive, and a positive stress curl induces horizontal divergence of mixed-layer water and a strong upwelling. Behind the storm, the reverse sequence of events occurs. In addition, imbalances between the current velocities and pressure field in the ocean lead to eddies which between the current velocities and pressure field in the ocean lead to eddies which persists far behind the storm. Since the eddy circulations in the ocean which are induced by tropical cyclones and the sea-surface temperature decreases may persist for many days after a storm's passage, the behavior of subsequent storms which cross the modified ocean surface may be affected, although the small area of significant sea surface temperature decreases makes a large influence unlikely.

In addition to the cooling of the ocean by upwelling and mixing, there are four other processes that may also affect the oceanic temperature. These include the following:

• Radiation

• Cooling by Precipitation

• Sensible Heat Flux to the Atmosphere

• Latent Heat Flux to the Atmosphere

Radiative effects are negligible near the center because of the presence of thick, multilevel clouds which reflect most of the incoming short-wave radiation while blocking long wave radiation loss.

As tropical storms make landfall, the combined action of the pressure anomaly and the wind stress produces the most destructive aspect of the hurricane to coastal regions - the storm surge. Storm surge is the abnormal rise in sea level at the coast during the passage of an intense tropical cyclone (TC), usually land falling or touching land. It is best described as the highest water level rise as the peak of the storm surges usually coincides with the time of passage of typhoon across a coastline. The exact distribution and amplitude of the storm surge depend in a complicated way on the bottom topography as well as the size, intensity, direction and speed of movement of the tropical cyclone. In addition to the relatively simple barotropic and baroclinic responses that are produced over the open ocean, rapidly decreasing depths induce nonlinear responses as the perturbation depths become large compared to the mean depth. Peninsulas and islands provide walls to reflect, refract and channel waves. Flooding of low-lying areas expands the area of the ocean and reduces the surge height in the waters adjacent to the coast.

Storm swell is an indicator of an approaching tropical cyclone. The appearance of a swell of a particular type may give quite reliable indications of a tropical storm as much as 805 to 1610 kilometers or more distant. The height of the waves from which swell develop is determined by the fetch or water distance over which the wind has blown without significant deviation in direction. Based on researches, the maximum height of a wave as a function of fetch is H = 1/3 (F)1/2 ; where H is in meters, and F is in kilometers.

The magnitude of waves is dependent not only upon the fetch, but also upon the wind velocity. Over oceanic areas with 600 - 1000 miles or more of sea room, waves 35-40 feet high are developed in ordinary storms and in more intense storms may exceed 45 feet. Based on some studies, the quotient obtained by dividing the wind velocity (probably average for hour) in miles per hour by 2.05 represents the average height in feet of waves developed by the wind. This should be used with caution and only as an approximation. Since there are always other factors to be taken into consideration, and a wind, constant in speed and direction (in a hurricane at least), does not act on a wave for any great length of time. The breaking wave or swell is one of the most destructive elements of tropical cyclone, since a cubic yard of water weighs 1500 pounds and waves moving forward many feet per second may be very destructive to beaches and harbor facilities, especially when they contain debris such as tree trunks and heavy beams.

During the occurrence of a tropical cyclone it was observed that the wind energy is concentrated in the storm causing a system of swell waves to spread out of the storm area. The swell moves with a speed of three or four times greater than the speed of the storm center. Now the swell generated in the rear right quadrant will move forward in the direction of the movement of the storm. These waves will be under the influence of the strong winds for a long time, and we say that the fetch is large. To the left of the storm track, the waves are under the influence of the wind for a relatively short time, and we say that the fetch is small. The energy that goes into the swell increases with fetch, with result that the swell generated on the right of the storm becomes prominent. This swell travels a long way, it may be observed as far as 1000 miles away from the center of the storm, and this provides a warning. The direction from which the swell arrives points toward the place where the swell was generated. The warning is, however, not very precise, for it provides no information on the behavior of the storm since the swell left it. Nevertheless, the arrival of the swell is a useful early alert to the man on the bridge, the harbor master, and the beach dweller.

The main energy source of a tropical cyclone is water vapor which is abundant in the oceans and seas. When the sun heats up the earth surface, water vapor evaporates into the atmosphere and condenses into water droplets, a great amount of heat energy, which is locked up in the water vapor, is released. This process is known as condensation. It is the reverse process of evaporation, which requires considerable amount of heat to evaporate water.

The heat energy absorbed by water during the process of evaporation is locked in the water and is released only when the same amount of water condenses back into the liquid state.

Through this process, an average-sized typhoon will get an energy supply in one day equivalent to the energy release by 40,000 hydrogen bombs. By comparison, the energy released by one hydrogen is very small against the energy of a typhoon in one day.

Thus, the typhoon will dissipate once the supply of water vapor is cut-off. This is manifested when a typhoon from the ocean passes over land. While still in the water areas, the typhoon is strongest, but its strong winds will normally diminish when it is over land. When it moves over mountains, the effect of topography further retards the air strength.

In latter information, we can concede that tropical cyclone is closely related to ocean. There is a close link between the two, likewise with air and the ocean. There are conditions necessary for the development of tropical cyclone, which greatly depends on the ocean:

The instability of the lower atmospheric layers.

Simultaneous with the warming of water masses along their way from east to west in the individual oceans within west Pacific regions heating, and, owing to the strong evaporation, an increase in water vapor in the lower atmosphere occur, both of which contribute to the instability of the atmospheric layers.

The reduced air friction over the ocean.

This is important in as much as no tropical cyclone develop over land and in coastal vicinities.

The two other conditions were being mentioned in the theories of formation. The interaction between air and ocean happens at the sea surface, this interface between the two is not a rigid boundary between the fluid and gaseous envelopes of the earth. It is a transfer station of the exchange of matter and energy. When favorable conditions exist the interaction leads to the formation of the most destructive phenomenon, tropical cyclone formation. Before the occurrence of this phenomenon, storm swell appears as an indication that there is an approaching tropical disturbance. During the passage, storm surge is generated. So all that energy content of the tropical cyclone is from the ocean, and the occurrence of one is highly impossible without the existence of the other.