Hurricane Concepts and Information
​2. El Niño - Southern Oscillation - ENSO
​ 2a. Atlantic El Niño's - El little brother
​5. Madden-Julian Oscillation
​6. North Atlantic Oscillation
Tropical Cyclone = What's in a name?
Meteorologists world-wide use the term, "Tropical Cyclone" to describe the intense rotating systems with heavy rain and with winds in excess of 74 mph that originate over warm tropical waters.
However, tropical cyclones are also referred to by different names depending on the point of origin.​
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Hurricanes: North Atlantic and the eastern/central North Pacific oceans. It is extremely rare for one to form in the South Atlantic Ocean.
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Cyclones: western South Pacific and Indian oceans
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Typhoons: western North Pacific (around the Philippines, Japan, and China)
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Medicane: Storms with characteristics similar to tropical cyclones will occasionally form over the Mediterranean Sea.
Requirements for Tropical Cyclone Formation
There are six main requirements for tropical cyclone development.
1. Ocean temperature >26.5 °C (79.7 °F) to a depth
of 50 meters.
2. Atmospheric instability (warm saturated air
rising)
3. High humidity in the lower to middle levels of
the Troposphere.
4. Enough Coriolis force to sustain a low-pressure
center.
5. Preexisting low-level focus or disturbance.
(tropical wave or the tailing end of a cold front.
6. Low vertical wind shear.
While these conditions are necessary for tropical cyclone formation, they do not guarantee that a tropical cyclone will form.
Times of the Year when tropical cyclones can form in each hemishere.
In the northern hemisphere, tropical cyclone activity peaks in late summer when water temperatures are warmest. Each basin, however, has its own seasonal patterns. Generally, May is the least active month, while September is the most active month.
Southern Hemisphere activity peaks in mid-February to early March. Virtually all the Southern Hemisphere activity is seen from the southern African coast eastward, toward South America. Tropical cyclones are rare events across the south Atlantic Ocean and the far southeastern Pacific Ocean.
Credit: Hurricanes, Cyclones, Typhoons -- What's in a name (NOAA)
Credit: Tropical cyclogenesis Wikipedia
Tropical Cyclone Season lengths and averages:
​
Basin Active Dates Avg. # Tropical
Cyclones per year
North Atlantic June 1-November 30 14.4
Eastern Pacific May 15-November 30 16.6
Western Pacific January 1-December 31 26.0
North Indian January 1-December 31 12.0
South-West Indian July 1-June 30 9.3
Australian region November 1-April 30 11.0
Southern Pacific November 1-April 30 7.1
Total: 96.4
Credit: Tropical cyclogenesis Wikipedia
Hurricanes and the Coriolis Effect
The Coriolis Effect affects weather patterns, ocean currents and wind currents. In simple terms, the Coriolis Effect makes things (like planes, ocean and air currents) traveling long distances around the earth appear to move as a curve as opposed to a straight line.
It's a pretty weird phenomenon, but the cause is simple, different parts of the Earth move at different speeds.
Credit: Coriolis Effect (NOAA SciJinks)
El Niño–Southern Oscillation (ENSO)
Though ENSO is a single climate phenomenon, it has three states, or phases, it can be in. The two opposite phases, “El Niño” and “La Niña,” require certain changes in both the ocean and the atmosphere because ENSO is a coupled climate phenomenon. “Neutral” is in the middle of the continuum.
Credit: 2024 NOAA's Atlantic Oceanographic and Meteorological Laboratory
Representation of the air flow around a low-pressure area (in this case, Hurricane Isabel) in the Northern hemisphere. The pressure gradient force is represented by blue arrows, the Coriolis acceleration (always perpendicular to the velocity) by red arrows. Take note of the counter-clockwise rotation.
Frequency of hurricane strikes along the Eastern and Gulf coasts
The frequency a location will likely be affected by a hurricane in the number of years. Wind speeds over 74 mph.
Credit: Hurricane return period (NOAA)
Tropical cyclogenesis is the development and strengthening of a tropical cyclone in the atmosphere.[1] The mechanisms through which tropical cyclogenesis occur are distinctly different from those through which temperate cyclogenesis occurs. Tropical cyclogenesis involves the development of a warm-core cyclone, due to significant convection in a favorable atmospheric environment.
​ Tropical cyclones tend to develop during the summer, but have been noted in nearly every month in most basins. Climate cycles such as ENSO and the Madden–Julian oscillation modulate the timing and frequency of tropical cyclone development.[4][5] The maximum potential intensity is a limit on tropical cyclone intensity which is strongly related to the water temperatures along its path.[6]
An average of 86 tropical cyclones of tropical storm intensity form annually worldwide. Of those, 47 reach strength higher than 74 mph (119 km/h), and 20 become intense tropical cyclones (at least Category 3 intensity on the Saffir–Simpson scale)
The frequency a location will likely be affected by a major hurricane in the number of years. Wind speeds over 111 mph.
Credit: Hurricane return period (NOAA)
Credit: Titoxd, CC BY-SA 3.0
Tropical Cyclogenesis
Yes, El Niño does have a little brother who lives just across the South American continent in the Atlantic Ocean. His name is Atlantic Niño, and he has an uncanny resemblance to his big brother: Like El Niño, Atlantic Niño is characterized by warmer-than-average sea surface temperatures in the eastern equatorial basin and weaker-than-average trade winds throughout the east-central equatorial Atlantic.
Credit: Sang-Ki Lee, ENSO blog team
Atlantic El Niño
Credit: Tropical cyclogenesis Wikipedia
Madden-Julian Oscillation
The Madden–Julian oscillation is characterized by an eastward progression of large regions of both enhanced and suppressed tropical rainfall, observed mainly over the Indian and Pacific Ocean. The anomalous rainfall is usually first evident over the western Indian Ocean, and remains evident as it propagates over the very warm ocean waters of the western and central tropical Pacific. This pattern of tropical rainfall generally becomes nondescript as it moves over the primarily cooler ocean waters of the eastern Pacific, but reappears when passing over the warmer waters over the Pacific Coast of Central America. The pattern may also occasionally reappear at low amplitude over the tropical Atlantic and higher amplitude over the Indian Ocean. The wet phase of enhanced convection and precipitation is followed by a dry phase where thunderstorm activity is suppressed. Each cycle lasts approximately 30–60 days. Because of this pattern, the Madden–Julian oscillation is also known as the 30- to 60-day oscillation, 30- to 60-day wave, or intraseasonal oscillation.
North Atlantic Oscillation
The North Atlantic Oscillation (NAO) is a weather phenomenon over the North Atlantic Ocean of fluctuations in the difference of atmospheric pressure at sea level (SLP) between the Icelandic Low and the Azores High. Through fluctuations in the strength of the Icelandic Low and the Azores High, it controls the strength and direction of westerly winds and location of storm tracks across the North Atlantic.[1]
The NAO was discovered through several studies in the late 19th and early 20th centuries.[2] Unlike the El Niño–Southern Oscillation phenomenon in the Pacific Ocean, the NAO is a largely atmospheric mode. It is one of the most important manifestations of climate fluctuations in the North Atlantic and surrounding humid climates.[3]