Hi Dr. Towe, thank you for sharing your thought. Today, I've read a news article emphasizing the role of sea surface temperature and monsoonal wind.

Yes. By the way, what are your thoughts on the average intensity of the tropical storms on our earth? Some people in my country worry about possibility of so-called super typhoons. Are they going to be more frequent than now under global warming environment?

To the leading order, sea surface temperature.

A Tropical Storms are also known as Hurricanes. Hurricanes happen with the change in temperature, when the oceans have been warmed during summer months. The spin of the Hurricane is due to the rotation of the Earth

The number of tropical storms on earth is usually determined as a result of the differences in sea and surface temperature which result in high sinds, hurricanes, tropical storms and cyclones.

How hot is the water in a particular part of the sea. If it gets very hot, the result would be many hurricanes, if not, only a few, if any.

Tropical cyclones with maximum sustained winds greater than 74 miles per hour are described as hurricanes or typhoons depending on where the storm originated. The conditions of their development are relatively well known; the most common causes are high surface water temperature, low level convergence, lack of wind shear and unstable vertical stratification. In the case of hurricanes, we also know that the important element favoring the development of powerful storms is the absence of a Saharan hot air layer with a high concentration of dust particles.

The frequency of occurrence of all these factors depends on the dynamics of large-scale atmospheric circulation and is not always easy to determine precisely because of the inherent limitations of predictability. Despite this, we are still trying to determine how current changes in the atmospheric system can affect the frequency distribution of tropical storms. This difficult question is not new, I would like to quote the text from the time before the dawn of computer models:

“What kind of secular changes may have existed in the frequency and intensity of the hurricane vortices of the Earth? And what changes may be expected in the future? We know nothing about these things, but I hope [to] have shown that even quite a small change in the different factors controlling the life history of a hurricane may produce, or may have produced, great changes in the paths of hurricanes and in their frequency and intensity. A minor alteration of the surface temperature of the sun, in the general composition of the earth's atmosphere, or in the rotation of the earth, might be able to change considerably the energy balance and the balance of forces within such a delicate mechanism as the tropical hurricane.

- Tor Bergeron, 1954

According to the empirical study, the warming trend is characterized by a change in the vertical structure of baroclinicity (increase at the higher levels and reduction at the surface).

https://www.atmos-chem-phys.net/9/9143/2009/acp-9-9143-2009.pdf

Considering that baroclinicity is the major factor influencing the development of tropical cyclones, we have a link to explore.

Monsoon wind, Sea surface and also sub sea surface temperature as per my opinion.

I am not confident about my answer.

Thank you, all. In particular, Dr. Pudykiewicz and Dr. Towe remind us of not only our current level of understanding on tropical cyclones but also their complex, nonlinear, and unpredictable nature.

If we regard the number of tropical cyclones as a way of reducing energy gaps between polar regions (very high latitude) and tropical regions (low latitude), then what can be practical and easy methods to measure or represent such gaps?

Tropical cyclones are characterized by several parameters, including intensity (maximum wind), size (extent of the vortex) and strength (average wind speed of the vortex). How these parameters depend on the extensive list of parameters characterizing large-scale circulation is still not completely explained. However, it is generally accepted that the Atlantic and Pacific systems are controlled by different large scale forcing. Tropical cyclones in the Pacific are also larger than their Atlantic counterparts. Given the lack of a common classification system, it is difficult to provide a simple answer to your second question. Some useful facts are itemized, however, in the interesting study: A comparison of large and small scale tropical cyclones by Merrill and Gray (available on the web).

Considering the fact that global warming is associated with a change of the vertical structure of baroclinicity (increase at upper levels and reduction at lower levels), I would like to suggest exploring the link between these changes and the number and strength of tropical cyclones. The paper mentioned in my first comment provides additionally some explanation of the distribution of energy between vertical modes, this information can also shed some light on your question.

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Ken ... I agree with your suggestion. The problem of classifying tropical cyclones and their dependence on external forcing is very difficult because of the lack of reliable databases and the absence of a commonly accepted theory. One of the interesting approaches starts from the concept of critical phenomena. See for example: Ossó et al. (2009): Scaling of Tropical-Cyclone Dissipation

https://arxiv.org/abs/0910.0054

The results from the above are for the time interval between 1900 and 2007 and they should address your concerns.

Ken... The reference to old papers is indeed very useful, the examples discussed show that powerful weather systems can change the course of history. Accidentally, they can also serve as "switches" modifying the phase trajectory of an atmospheric system in the range of sub-climatic variability. This fact has been demonstrated using computer simulation with high resolution atmospheric models. For this reason, calculating the mean is not always the best method to describe the effects of tropical cyclones.

With regard to the frequency of storms, I agree with your observation. It corresponds to information available from all major meteorological centers and confirms the data shown in Fig. 1a of Ossó et al. (2009).

Thank you for the interesting papers.

Ocean heat content, humidity at different levels in the atmosphere and wind shear are probably the main factors controling the number of tropical cyclones on earth. If we look at the historical data, the number of events is very constant at a global basis, but it does changes year to year depending on the basin. For instance, if it is an intense ENSO year, we have very high heat content in the eastern Pacific and low wind shear, resulting in a lot of tropical cyclones in that basin, on the contrary, the Atlantic basin experiences very high wind shear, inhibiting the formation of tropical cyclones in that basin during ENSO years. Thus, the lack of events in the Atlantic is compensated by the high amount of events in the eastern Pacific. The opposite occurs during La Niña years. Yearly forecast of the number of events and then based on such estimations, although they sometimes fail as a year that is starting as ENSO, may change and be a normal year when we get to the hurricane season.

If we consider climate change, well, global warming, then we know the ocean heat content will increase as well as humidity, so we would expect more tropical cyclones, right? Wrong! Since wind shear also increases, thus the formation of tropical cyclones is not really more likely. Nevertheless, if the tropical cyclones forms, its chances of undergoing rapid intensification and become a very intense events is more likely as there is more energy readibily available in the form of ocean heat.

The answear is ver complex and interesting, beause the number of tropical cyclones is related for many factors, for example the ENOS event (change the possititon of subtropical jet stream in North Atlantic), wind shear (in my opinion ver important for develompement) ocean heat content (temperature of sea over 27C up to 60-100 m).

Several factors are there like SST, Atmosphere temp, ocean heat content etc.

Very simple answer: the temperature of the ocean. Remember storms and hurricanes transfer the heat out of the equatorial area. If very hot, many storms, if not so hot, less storms.

The estimate of kinetic energy production by tropical cyclones presented by Emanuel in the article “Environmental Factors Affecting Tropical Cyclones Energy Dissipation” (J. of Climate, Vol 20, 15 November 2007) indicates that the strong correlation with sea surface temperature is observed at time scales of several years to several decades in the tropical Atlantic. In the North West Pacific, this correlation is much less pronounced. We need to include other factors such as potential intensity, low level vorticity, and vertical wind shear.

Tropical storms move the heat from the equator towards the poles. When the ocean gets warm, it evaporates. The warmer the water, the more evaporation and the more precipitation. Right now the oceans are still warmer than normal, therefore, the number of storms is greater and the ferocity of storms also greater. The power of the storms also has one source: the latent heat of condensation every time water vapour turns into water.