Sub-Tropical Cyclone

Primary reference(s)

WMO, 2017. Regional Association IV - Hurricane Operational Plan for North America, Central America and the Caribbean. Tropical Cyclone Programme, Report No. TCP-30, World Meteorological Organization (WMO). Accessed 14 October 2020.

Additional scientific description

A sub-tropical cyclone is a low-pressure system, developing over tropical or subtropical waters which initially contains few tropical characteristics. With time the subtropical cyclone can become a tropical cyclone (WMO, 2018).

Sub-tropical cyclones have organised moderate to deep convection, but lack a central dense overcast. Unlike tropical cyclones, subtropical cyclones derive a significant proportion of their energy from baroclinic sources and are generally cold-core in the upper troposphere, often being associated with an upper-level low or trough. In comparison to tropical cyclones, these systems generally have a radius of maximum winds occurring relatively far from the centre (usually greater than 60 nautical miles), and generally have a less symmetric wind field and distribution of convection (WMO, 2017; NOAA, no date).

Metrics and numeric limits

Not applicable.

Examples of drivers, outcomes and risk management

Impacts from sub-tropical cyclones are from storm surges and significant rainfall which cause flooding. Strong wind gusts can also cause impacts.

Human health can be severely affected by wind-related hazards such as subtropical cyclones and other windstorms. Direct effects occur during the impact phase of a storm, causing death and injury due to the force of the wind. Becoming airborne, being struck by flying debris or falling trees and road traffic accidents are the main dangers. Indirect effects, occurring during the pre- and post-impact phases of the storm, include falls, lacerations and puncture wounds, and occur when preparing for, or cleaning up after a storm. Power outages are a key issue and can lead to electrocution, fires and burns and carbon monoxide poisoning from gasoline powered electrical generators. Worsening of chronic illnesses due to lack of access to medical care or medication is also an issue. Other health impacts include infections and insect bites (Goldman et al., 2014).

The effects of flooding on health are extensive and significant, ranging from mortality and injuries resulting from trauma and drowning, to infectious diseases and mental health problems (acute and long-term). While some of these outcomes are relatively easy to track, ascertaining the human impact of floods is still weak. For example, it has been reported that two-thirds of deaths associated with flooding are from drowning, with the other third from physical trauma, heart attacks, electrocution, carbon monoxide poisoning and fire. Often, only immediate traumatic deaths from flooding are recorded (WHO, 2013).

Morbidity associated with floods is usually due to injuries, infections, chemical hazards and mental health effects (acute as well as delayed) (WHO, 2013). Hypothermia may also be a problem, particularly in children, if trapped in floodwaters for lengthy periods (WHO, no date). There may also be an increased risk of respiratory tract infections due to exposure (loss of shelter, exposure to flood waters and rain). Power cuts related to floods may disrupt water treatment and supply plants thereby increasing the risk of water-borne diseases and may also affect the proper functioning of health facilities, including cold chain (WHO, no date). Floods can potentially increase the transmission of the following communicable diseases: water-borne diseases (such as typhoid fever, cholera, leptospirosis and hepatitis A) and vector-borne diseases (such as malaria, dengue and dengue haemorrhagic fever, yellow fever, and West Nile Fever) (WHO, no date).

The longer-term health effects associated with a flood are less easily identified. They include effects due to displacement, destruction of homes, delayed recovery and water shortages (WHO, 2013).