Terms

A glossary vocabulary

A
AO	Arctic Oscillation - is technically defined as the first empirical orthogonal function (EOF) of sea level pressure. EOF is a statistical technique to identify common modes or patterns in a given dataset. The AO is the most common mode or pattern of variability in the Northern Hemisphere sea level pressure field. The AO is found in all months of the calendar year but is most active/strongest in the winter months. EOF is linear statistical technique so the identified pattern exists in both the negative or positive phase. The positive AO phase is characterized by a low-pressure center anomaly over the entire Arctic basin or polar cap (the region poleward of 60°N) and high-pressure center anomalies in each ocean basin in the mid-latitudes. The positive AO is often associated with cold weather in the Arctic but mild weather across the mid-latitudes including the Eastern United States, Europe and East Asia. The negative AO phase is characterized by a high-pressure center anomaly over the entire Arctic basin or polar cap and low-pressure center anomalies in each ocean basin in the mid-latitudes. The negative AO is often associated with milder weather in the Arctic but severe winter weather, including cold-air or Arctic outbreaks and snowstorms across the mid-latitudes including the Eastern United States, Europe and East Asia. The AO can be considered an atmospheric mixing index. During the positive AO phase there is little mixing of atmospheric masses between the Arctic and lower latitudes (mid-latitudes and subtropics) so the cold air stays bottled up in the Arctic and mild air dominates the mid-latitudes. During the negative AO phase there is increased mixing of atmospheric masses between the Arctic and lower latitudes (mid-latitudes and subtropics) so the cold air is release from the Arctic and mild air in the mid-latitudes and subtropics flows more easily into the Arctic region.

Arctic Oscillation - is technically defined as the first empirical orthogonal function (EOF) of sea level pressure. EOF is a statistical technique to identify common modes or patterns in a given dataset. The AO is the most common mode or pattern of variability in the Northern Hemisphere sea level pressure field. The AO is found in all months of the calendar year but is most active/strongest in the winter months. EOF is linear statistical technique so the identified pattern exists in both the negative or positive phase. The positive AO phase is characterized by a low-pressure center anomaly over the entire Arctic basin or polar cap (the region poleward of 60°N) and high-pressure center anomalies in each ocean basin in the mid-latitudes. The positive AO is often associated with cold weather in the Arctic but mild weather across the mid-latitudes including the Eastern United States, Europe and East Asia. The negative AO phase is characterized by a high-pressure center anomaly over the entire Arctic basin or polar cap and low-pressure center anomalies in each ocean basin in the mid-latitudes. The negative AO is often associated with milder weather in the Arctic but severe winter weather, including cold-air or Arctic outbreaks and snowstorms across the mid-latitudes including the Eastern United States, Europe and East Asia. The AO can be considered an atmospheric mixing index. During the positive AO phase there is little mixing of atmospheric masses between the Arctic and lower latitudes (mid-latitudes and subtropics) so the cold air stays bottled up in the Arctic and mild air dominates the mid-latitudes. During the negative AO phase there is increased mixing of atmospheric masses between the Arctic and lower latitudes (mid-latitudes and subtropics) so the cold air is release from the Arctic and mild air in the mid-latitudes and subtropics flows more easily into the Arctic region.

G
GFS	Global Forecast System - the American weather model that provides forecasts out to 15 days. Created and run operationally by NOAA.

M
MMW	Major Mid-winter Warming (MMW) - The predominant wind direction around North Pole circulating around the stratospheric polar vortex is from west to east. However periodically due to increased poleward heat flux the winds slacken around the polar vortex and may even reverse direction and flow instead from east to west. This is refereed to as a major warming and ti typically occurs in mid-winter

P
PCH	Polar Cap Height (PCH) - Is the area averaged geopotential height anomaly averaged over every grid point from 60°N to the North Pole and from the surface through the stratosphere (typically 10 hPa). It often is an excellent proxy for the Arctic Oscillation, since the AO is characterized by a one sign anomaly around the polar cap.
PV	Polar Vortex (PV) – is a deep low-pressure center in the upper atmosphere that usually sits over the Pole during the winter months and is encircled or surrounded by a fast flowing ribbon or river of air. Air flows counterclockwise around low pressure in the Northern Hemisphere or in a west to east direction. The polar vortex resides in the stratosphere generally above 50 hPa. Also see our webpage on the polar vortex: https://www.aer.com/science-research/climate-weather/climate-dynamics/seasonal-weather-forecasts/polar-vortex-impact-winter-weather/

PCH

Polar Cap Height (PCH) - Is the area averaged geopotential height anomaly averaged over every grid point from 60°N to the North Pole and from the surface through the stratosphere (typically 10 hPa). It often is an excellent proxy for the Arctic Oscillation, since the AO is characterized by a one sign anomaly around the polar cap.

Polar Vortex (PV) – is a deep low-pressure center in the upper atmosphere that usually sits over the Pole during the winter months and is encircled or surrounded by a fast flowing ribbon or river of air. Air flows counterclockwise around low pressure in the Northern Hemisphere or in a west to east direction. The polar vortex resides in the stratosphere generally above 50 hPa. Also see our webpage on the polar vortex: https://www.aer.com/science-research/climate-weather/climate-dynamics/seasonal-weather-forecasts/polar-vortex-impact-winter-weather/

S
SSW	Sudden Stratospheric Warming (SSW) - when temperatures rise dramatically in the polar stratosphere in a mater of days.

V
VNIR	visible and near-infrared

W
WAFz	Wave Activity Flux in the vertical direction (or z coordinate) - Is the vertical transfer of energy from waves in the atmosphere. Only the largest or Rossby waves (wavenumbers 1-2) produce energy strong enough to escape the troposphere into the stratosphere. When the vertical energy is absorbed in the polar stratosphere it leads to warming of the polar stratosphere or a weakening of the polar vortex. If it is of sufficient amplitude it will result in a sudden stratospheric warming (SSW). The vertical transfer of energy is directly proportional to the poleward transport or advection of heat. When the WAFz is zonally averaged it is equal to the vertical component of the Eliassen Palm (EP) Flux.

WAFz

Wave Activity Flux in the vertical direction (or z coordinate) - Is the vertical transfer of energy from waves in the atmosphere. Only the largest or Rossby waves (wavenumbers 1-2) produce energy strong enough to escape the troposphere into the stratosphere. When the vertical energy is absorbed in the polar stratosphere it leads to warming of the polar stratosphere or a weakening of the polar vortex. If it is of sufficient amplitude it will result in a sudden stratospheric warming (SSW). The vertical transfer of energy is directly proportional to the poleward transport or advection of heat. When the WAFz is zonally averaged it is equal to the vertical component of the Eliassen Palm (EP) Flux.