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A tropical cyclone is a rapidly-rotating storm system characterized by a low-pressure center, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain. Tropical cyclones typically form over large bodies of relatively warm water. They derive their energy from the evaporation of water from the ocean surface, which ultimately recondenses into clouds and rain when moist air rises and cools to saturation.
This energy source differs from that of mid-latitude cyclonic storms, such as nor'easters and European windstorms, which are fueled primarily by horizontal temperature contrasts. The strong rotating winds of a tropical cyclone are a result of the (partial) conservation of angular momentum imparted by the Earth's rotation as air flows inwards toward the axis of rotation. As a result, they rarely form within 5° of the equator. Tropical cyclones are typically between 100 and 4,000 km (62 and 2,500 mi) in diameter.
The term "tropical" refers to the geographical origin of these systems, which usually form over the tropical oceans. The term "cyclone" refers to their cyclonic nature, with wind blowing counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The opposite direction of circulation is due to the Coriolis force.
Depending on its location and strength, a tropical cyclone is referred to by names such as hurricane (/ˈhʌrɨkeɪn/ or /ˈhʌrɨkən/), typhoon /taɪˈfuːn/, tropical storm, cyclonic storm, tropical depression, and simply cyclone.
Recent historically low global tropical cyclone activity
Tropical cyclone accumulated cyclone energy (ACE) has exhibited strikingly large global interannual variability during the past 40‐years. In the pentad since 2006, Northern Hemisphere and global tropical cyclone ACE has decreased dramatically to the lowest levels since the late 1970s. Additionally, the global frequency of tropical cyclones has reached a historical low.
Here evidence is presented demonstrating that considerable variability in tropical cyclone ACE is associated with the evolution of the character of observed large‐scale climate mechanisms including the El Niño Southern Oscillation and Pacific Decadal Oscillation. In contrast to record quiet North Pacific tropical cyclone activity in 2010, the North Atlantic basin remained very active by contributing almost one‐third of the overall calendar year global ACE.
During the past four decades, integrated and secular frequency measures of global tropical cyclone (TC) activity have undergone considerable interannual variability. Since 2006, global and Northern Hemisphere (NH) TC accumulated cyclone energy (ACE) has decreased dramatically, nearly cut in half from the previous peak in 2005 to the lowest levels since the 1970s.
While the annual global frequency of tropical storm TCs has not exhibited a long‐term trend (∼87 per year), the calendar year total of 69 observed in 2010 represented a decades’ low. Indeed while the veryactive North Atlantic (NATL) basin generated 19 named storms in 2010, the rest of the global tropics produced only 50 TCs including the fewest Western North Pacific (WNP) typhoons and Northeast Pacific (NEP) hurricanes counted in the reliable historical record.
There is no systematic upward or downward trend in global tropical storm frequency and intensity, the media hype on the other hand ...
NOAA Tropical Cyclone Climatology
NOAA Hurricane Center
(Wiki not up to date, the ACE is now a global-scale index including all storms/all ocean basins)
Global Tropical Cyclone Activity Update
follow up post in next few days
reply to post by surrealist
I tried to imagine what you could mean by 'unusual weather events'. Could you expand on that?