Guide to hurricane seasons

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AlphaToOmega
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Guide to hurricane seasons

#1 Postby AlphaToOmega » Tue Aug 31, 2021 7:29 am

This is a simple guide for people who are new to tracking hurricanes and who are unsure about terminology and how certain indicators impact hurricane seasons. This is not a guide for hurricane season safety.



Overview

Tropical cyclones are non-frontal areas of low pressure with closed circulation (i.e. winds around the low pressure forming a circular band) and deep convection. They derive their energy from thermal energy from the ocean. Subtropical cyclones are like tropical cyclones, but they derive their energy from baroclinicity (i.e. an atmospheric pressure gradient that is not parallel to the ground).

A tropical or subtropical cyclone with wind speeds below 34 knots is a tropical or subtropical depression; a tropical or subtropical cyclone with wind speeds between 34 and 63 knots is a tropical or subtropical storm; a tropical or subtropical cyclone with wind speeds above 63 knots is a hurricane and assumed to be fully tropical.

Hurricane seasons are sets of storms that form within a certain year in a certain basin. Each year, there are three hurricane seasons: one for the North Atlantic, one for the Northeast Pacific, and one for the North Central Pacific (e.g. the 2005 North Atlantic Hurricane Season is the set of all storms that form in the North Atlantic during 2005).

Each basin can be split into multiple bodies of water. The North Atlantic can be split into the North Atlantic Main Development Region (the Tropical Atlantic extending from the West Coast of Africa to the Lesser Antilles), Caribbean Sea, Gulf of Mexico, and North Atlantic Subtropics. The Northeast and Central Pacific can each be split into the Northeast and Central Pacific Main Development Region (the Tropical Northeast and Central Pacific) and the Northeast and Central Pacific Subtropics.

Tropical and subtropical cyclones

Tropical cyclones typically develop from tropical waves (broad low pressure areas that develop in the tropics during hurricane seasons) or extratropical cyclones (areas of low pressure embedded within fronts). Should these low pressure areas detach from fronts and/or find conducive conditions for development, they might become tropical cyclones.

Tropical and subtropical cyclones require warm waters, low vertical wind (difference in wind speeds between different layers of the atmosphere) shear, and humid air to form and maintain themselves. They require warm waters because they derive energy from thermal energy in the ocean. They require low vertical wind shear because high vertical wind shear can shear apart storms. They require humid air because dry air disrupts convection. Tropical and subtropical cyclones typically require vertical wind shear values <10 knots to form; tropical cyclones typically require sea surface temperatures >26° C to form, but subtropical cyclones typically require sea surface temperatures >22° C to form.

Tropical and subtropical cyclones are categorized by the Beaufort and Saffir-Simpson Scales. Any tropical or subtropical cyclone with sustained wind speeds lower than 8 (<34 knots) on the Beaufort Scale is a depression; Any tropical or subtropical cyclone with sustained wind speeds between 8 and 11 (34-63 knots) on the Beaufort Scale is a storm; any tropical or subtropical cyclone with sustained wind speeds above 11 (>63 knots) on the Beaufort Scale is a hurricane. Any tropical cyclone with hurricane-force winds is classified using the Saffir-Simpson Scale. A Category 1 hurricane has sustained wind speeds between 64 and 82 knots; a Category 2 hurricane has sustained wind speeds between 83 and 95 knots; a Category 3 hurricane has sustained wind speeds between 96 and 112 knots; a Category 4 hurricane has sustained wind speeds between 113 and 136 knots; a Category 5 hurricane has sustained wind speeds above 136 knots. Hurricanes that are Category 3 or higher are major hurricanes.

The accumulated cyclone energy of a tropical or subtropical cyclone is the sum of the squares of the maximum sustained wind speeds during six-hour intervals (excluding intervals during which the cyclone has tropical depression wind speeds) divided by 10000. In general, longer-lasting, stronger tropical or subtropical cyclones have more accumulated cyclone energy than shorter-lived, weaker tropical cyclones.

Tropical and subtropical cyclones are steered by ridges and troughs. Ridges are areas of high pressure, and troughs are areas of low pressure. In the Northern Hemisphere, ridges create clockwise trade winds, and troughs create counterclockwise trade winds. Tropical and subtropical cyclones and steered clockwise around ridges and counterclockwise around troughs.

When tropical or subtropical cyclones become storms, they receive names. For the North Atlantic and the Northeast Pacific, each year has a different naming list, and they are arranged in alphabetical order; the first storm always receives the name that starts with "A". For the North Central Pacific, storms receive names from a rotating list.

Hurricane seasons

In the North Atlantic and North Central Pacific, hurricane season starts on June 1 and ends on November 30. In the Northeast Pacific, hurricane season starts on May 15 and ends on November 30. However, tropical cyclones can form any time of the year. Typically, the most active part of the North Atlantic hurricane season is from August to October; for the Northeast and North Central Pacific hurricane seasons, it is typically from July to September. The average North Atlantic hurricane season has 14 storms, 7 of which are hurricanes, 3 of which are major hurricanes. The average North Pacific (Northeast and North Central combined) hurricane season has 15 storms, 8 of which are hurricanes, 4 of which are major hurricanes. North Atlantic hurricane seasons are typically less active than North Pacific hurricane seasons because dry Saharan dust does inhibit tropical cyclone formation in the North Atlantic during the early months of the hurricane season.

During a hurricane season, subseasonal variation is driven primarily by the Madden-Julian Oscillation. The Madden-Julian Oscillation is a dipole that moves around the globe that creates rising air in some parts of the tropics and sinking air in other parts of the tropics. The Madden-Julian Oscillation causes active and inactive periods of activity for every tropical cyclone basin during spring and early summer. In general, favorable subseasonal conditions for the Northeast and North Central Pacific mean unfavorable subseasonal conditions for the North Atlantic and vice-versa.

Seasonal activity of hurricane seasons is driven by sea surface temperatures. Sea surface temperature anomalies dictate how much warmer than average sea surface temperatures are for a given area. Positive sea surface temperature anomalies in the Tropical North Atlantic (Tropical North Atlantic) in the the North Atlantic in general (Atlantic Multidecadal Oscillation) favor active North Atlantic hurricane seasons. Positive sea surface temperatures in the Equatorial Pacific (Oceanic Niño Index) and the Northeast Pacific (Pacific Decadal Oscillation) favor active North Pacific hurricane seasons and inactive North Atlantic hurricane seasons; they create rising air over the North Pacific and sinking air over the North Atlantic. The opposite is true for negative sea surface temperature anomalies.

Seasonal activity of hurricane seasons is also driven by rising air patterns. Negative velocity potential anomalies indicate rising air, which indicates low wind shear, humid air, and positive sea surface temperature anomalies. Positive velocity potential anomalies indicate sinking air, which indicates high wind shear, dry air, and negative sea surface temperature anomalies. Negative velocity potential anomalies over Africa and the Indian Ocean, and positive velocity potential anomalies over the Pacific and the Americas favor active Atlantic hurricane seasons and inactive Pacific hurricane seasons. Positive velocity potential anomalies over Africa and the Indian Ocean, and negative velocity potential anomalies over the Pacific and the Americas favor active Pacific hurricane seasons and inactive Atlantic hurricane seasons.

Tropical cyclone landfalls are dictated by the North Atlantic Oscillation. The North Atlantic Oscillation measures the strength of the Azores-Bermuda High, a ridge in the Subtropical North Atlantic. A positive North Atlantic Oscillation means a strong Azores-Bermuda High, leading to more landfalling storms and fewer out-to-sea storms. A negative North Atlantic Oscillation means a weak Azores-Bermuda High, leading to fewer landfalling storms and fewer out-to-sea storms.

Forecasting

Models are used to determine positions and strengths of tropical and subtropical cyclones, and they are also used to determine general climate patterns. For specific storms, global models (such as the GFS (American), ECMWF (European), CMC (Canadian), ICON (German), or UKMET (British)), hurricane-specific models such as the HWRF, or ensemble models (such as the GEFS (American) or ECENS (European)) are used. For seasons, climate models (such as the CFS (American), CanSIPS (Canadian), or JMA (Japanese)) are used. Models can be found on websites such as tropicaltidbits.com, pivotalweather.com, or weatheronline.co.uk.

The National Hurricane Center and Central Pacific Hurricane Center use models to forecast positions and strengths of tropical and subtropical cyclones, and they use models to forecast rainfall from tropical and subtropical cyclones. Moreover, they use models to mark areas of potential tropical cyclone development. In general, forecasting strength is much more difficult than forecasting position. The National Oceanic and Atmospheric Administration uses climate models for hurricane season forecasts.

Summary

Tropical and subtropical cyclones require warm waters, humid air, and low vertical wind shear to form. Their intensities are rated on the Beaufort and Saffir-Simpson Scales. A set of tropical and subtropical cyclones that form in a certain basin during a certain year is a hurricane season. The activity of hurricane seasons is based on sea surface temperature anomaly and rising air patterns. Subseasonal variation is driven by the Madden-Julian Oscillation. Rising air over Africa and the Indian Ocean promote active North Atlantic hurricane seasons; rising air over the Pacific Ocean and the Americas promote active North Pacific hurricane seasons. Positive sea surface temperature anomalies in the North Atlantic promote active North Atlantic hurricane seasons; positive sea surface temperature anomalies in the North Pacific promote active North Pacific hurricane seasons. In general, favorable patterns for the North Atlantic mean unfavorable patterns for the North Pacific and vice-versa. Global, hurricane, and ensemble models are used for forecasting positions and strengths of cyclones, and climate models are used to forecast hurricane season activity.
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