Globally, tropical cyclone development is most common across the following areas: North Atlantic, West Pacific, Indian Ocean (both North and South), and Southwest Pacific. It is rare, though not impossible for tropical cyclones to develop in the Mediterranean Sea or South Atlantic. There are several different agencies that oversee these regions, each with their own varied thresholds and naming methodology. The below figure displays the agencies that deploy advisories on tropical activity.

In Areas I-III, the National Hurricane Center (NHC) based in Miami and Honolulu are responsible for forecasting each developing system and providing updates. Another American agency, the Joint Typhoon Warning Center (JTWC), provides updates in the Northwest Pacific (west of 180E), Indian Ocean, and Southern Pacific (Areas IV-XIII). In addition to JTWC, Area IV is monitored by the Japan Meteorological Agency (JMA), Area V by the India Meteorological Department, and Area VI by Météo France. Across the Southern Indian Ocean and through the Southern Pacific Ocean (Areas VII to XIII), there are several smaller agencies that forecast for local waters surrounding the country they are based in. The Australian Bureau of Meteorology, Indonesian Agency for Meteorology, Papua New Guinea National Weather Center, Fiji Meteorological Service, and the Meteorological Service of New Zealand are all responsible for tropical cyclones in respective waters of these areas.

Each of these agencies utilizes different criteria in their naming of tropical systems, primarily related to a system’s strength. The different criteria and nomenclatures can make understanding the current intensity of tropical cyclones difficult, especially when two different agencies are publishing advisories for the same system. Even within the same organization, there can be differing criteria. The JTWC, for example, uses different scales depending on which basin the system is located (to help minimize confusion with the agencies that cover the same basin). All tropical cyclones, no matter their strength, are classified only as a Tropical Cyclone in the Indian Ocean and South Pacific. However, in the West Pacific, the JTWC uses similar terminology to the Japan agency. In both areas, the scale is simplified compared to the other agencies’ scales.

The NHC's Saffir-Simpson scale is the most widely known tropical cyclone scale in the Western Hemisphere, characterized by its use of several different “categories.” The Australian scale also has five categories; however, these are somewhat broader than the Saffir-Simpson Scale and the scale does not include several weaker designations such as Tropical Depression or Tropical Storm. Thus, a Category 4 Severe Tropical Cyclone on the Australian scale could have a range of different categories on the Saffir-Simpson Scale.

The India Department of Meteorology designates all storms of gale force winds or higher as minimum “Cyclonic Storm.” Météo France only classifies systems as a Tropical Cyclone with winds of Beaufort Force 12 or higher. 

To further complicate matters, Tropical Cyclones can cross between basins, and thus the agency providing forecasts/advisories may change. This may also change the scale used to classify the system. A common instance of this occurring is the Southern Indian Ocean. Here, systems can seem to “downgrade” even when maintaining the same strength, just by virtue of the differences between the Australian Bureau of Meteorology (region VII) and Météo France (region VI). If using forecasts from the Joint Typhoon Warning Center, the system would not change classification.

At WRI, we base our discussions and guidance on tropical advisories from the National Hurricane Center and the Joint Typhoon Warning Center. Not only do these organizations stay consistent in their classification of named systems across regions, but they also utilize intensity scales that try to minimize confusion and blend together the classifications of all other local agencies within a given region. Whenever either of these organizations have an advisory on an active tropical cyclone or an area of potential development, we issue warnings multiple times a day to cover the areas of concern. We also monitor regions where the potential for tropical development may be low, or not yet identified by these organizations, and issue statements regarding potential tropical disturbances.

Our team at WRI continues to diligently monitor all basins around the world and will keep our clients well advised and briefed on any areas of developed/potential tropical activity. WRI provides 24/7 surveillance, please reach out with any questions you may have.