The variety of storms that form over the ocean often leads to confusion, particularly when distinguishing between tropical and subtropical cyclones. A subtropical storm is a unique weather system, representing a hybrid between familiar tropical storms and extratropical cyclones common in middle latitudes. This classification recognizes that the storm possesses characteristics of both major storm types, utilizing different mechanisms to sustain its circulation. Understanding this hybrid nature provides clarity on its structure and how it differs from its counterparts.
Defining Characteristics and Hybrid Nature
A subtropical storm is a non-frontal low-pressure system that develops over tropical or subtropical waters and exhibits an organized circulation with features from both tropical and extratropical cyclones. Unlike a purely tropical system, which is highly symmetrical, a subtropical storm often displays an asymmetrical structure on satellite imagery. The core of the system is neither fully cold nor fully warm; it exhibits a cold core aloft that transitions to a warmer core nearer the surface, reflecting its hybrid nature.
The wind field of a subtropical storm is broader than that of a tropical cyclone. Its strongest sustained winds are located relatively far from the center of circulation, often exceeding a radius of 60 nautical miles. This contrasts with tropical systems, where the maximum winds are concentrated tightly around the center, near the eye wall. To be classified as a subtropical storm, the maximum sustained surface wind speed must fall within the range of 39 to 73 miles per hour (63 to 118 km/h).
The distribution of thunderstorm activity reflects the storm’s intermediate status. Convection tends to be displaced farther from the center in a subtropical storm, sometimes appearing in a band well removed from the core. As a result, the center of circulation may be large and relatively cloud-free, distinguishing it from the dense cloud cover typically surrounding a tropical storm’s center. This broader, less centralized structure indicates its partial reliance on non-tropical energy sources.
Formation and Energy Sources
The environmental conditions required for a subtropical storm to develop place it between the tropical and extratropical zones. These storms typically form at higher latitudes than their tropical counterparts, often developing over the western portions of ocean basins. While tropical cyclones require sea surface temperatures (SSTs) of at least 79°F (26°C) for formation, subtropical storms can develop over somewhat cooler water, generally needing SSTs around 73°F (23°C).
The difference in power source is significant. Tropical cyclones are fueled solely by the release of latent heat, which occurs when water vapor evaporated from warm ocean surfaces condenses into clouds and rain. This process establishes a warm core that drives the tropical system.
Subtropical storms, however, draw energy from both latent heat release and baroclinic processes. Baroclinic energy is derived from horizontal temperature gradients, or the contrast between cold and warm air masses, which primarily drives extratropical cyclones. This dual energy source allows subtropical systems to form in environments where the SSTs are not warm enough to support a purely latent-heat-driven storm.
The development process for a subtropical storm begins with a non-tropical low-pressure system that stalls over sufficiently warm water. As the system remains over the warm ocean, convection builds near the center, and the release of latent heat begins to warm the storm’s core in the lower atmosphere. This transition from a cold-core system to one with a partial warm core, while retaining mid-level cold air and frontal remnants, marks its classification as a subtropical cyclone.
Comparison to Tropical and Extratropical Cyclones
The three main types of oceanic cyclones—tropical, subtropical, and extratropical—are best understood by comparing their characteristics. The primary distinction is their thermal structure. Tropical cyclones are purely warm-core systems throughout the troposphere, sustained by the latent heat released from condensation. Extratropical cyclones are cold-core systems driven by baroclinic instability, involving the clash of air masses. Subtropical storms bridge this gap, beginning with a cold core aloft but developing a shallow warm core near the surface due to convection.
The energy source reflects the core temperature difference. Tropical systems rely entirely on warm ocean water and low wind shear to provide moisture and release latent heat. Extratropical systems are driven by large-scale atmospheric dynamics, such as the jet stream, which enhances the temperature contrast needed for baroclinic processes. Subtropical storms, utilizing both latent heat and baroclinic processes, are more robust against environmental factors like moderate wind shear, which can quickly dissipate a purely tropical cyclone.
The symmetry of the wind field is another difference. Tropical cyclones are characterized by a symmetrical structure, with the strongest winds and rainfall concentrated in a tight ring around the center. Extratropical cyclones are defined by their asymmetry, with distinct warm and cold fronts extending from the center of circulation. Subtropical storms typically exhibit the broad, asymmetrical wind field of their extratropical parent, where the strongest winds are displaced farther out, but they lack the distinct fronts that define a purely extratropical system.
The location of formation is also a distinguishing factor. Tropical cyclones must form in low-latitude areas where the Coriolis force is sufficient to initiate rotation and sea surface temperatures are highest. Extratropical cyclones form in the mid- to high-latitudes, often following the path of the jet stream. Subtropical storms are found in the transitional zone between these regions, forming in the higher tropical or lower mid-latitudes where the water is warm enough to initiate convection, but the atmospheric environment retains some cold-core characteristics.