ANATOMY OF A STORM
Glossary of Weather Terms
Understand some of the frequent terms used in forecasting and weather phenomenon
PNA: THE PACIFIC-NORTH AMERICA
A climatological term for a large-scale weather pattern with two modes, denoted positive and negative, and which relates the atmospheric circulation pattern over the North Pacific Ocean with the one over the North American continent.
AO: ARCTIC OSCILLATION
Is a weather phenomenon at the Arctic poles north of 20 degrees latitude. It is an important mode of climate variability for the Northern Hemisphere. The southern hemisphere analogue is called the Antarctic oscillation or Southern Annular Mode (SAM).
ENSO: EL NINO-SOUTHERN OSCILLATION
ENSO is one of the most important climate phenomena on Earth due to its ability to change the global atmospheric circulation, which in turn, influences temperature and precipitation across the globe. We also focus on ENSO because we can often predict its arrival many seasons in advance of its strongest impacts on weather and climate.
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.
NAO: 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
MJO: MADDEN JULIAN OSCILLATION
The largest element of the intraseasonal (30- to 90-day) variability in the tropical atmosphere. It is a large-scale coupling between atmospheric circulation and tropical deep atmospheric convection. Unlike a standing pattern like the El Niño–Southern Oscillation (ENSO), the Madden–Julian oscillation is a traveling pattern that propagates eastward, at approximately 4 to 8 m/s (14 to 29 km/h, 9 to 18 mph), through the atmosphere above the warm parts of the Indian and Pacific oceans. This overall circulation pattern manifests itself most clearly as anomalous rainfall.
PDO: PACIFIC DECADAL OSCILLATION
A robust, recurring pattern of ocean-atmosphere climate variability centered over the mid-latitude Pacific basin. The PDO is detected as warm or cool surface waters in the Pacific Ocean, north of 20°N. Over the past century, the amplitude of this climate pattern has varied irregularly at interannual-to-interdecadal time scales (meaning time periods of a few years to as much as time periods of multiple decades).
EPO: EASTERN PACIFIC OSCILLATION
A dipole pattern similar to the NAO in the Atlantic, but located in the eastern Pacific. A negative EPO regime corresponds to ridging over the northeastern Pacific, and a positive EPO regime corresponds with a trough in the same location.
SOI: SOUTHERN OSCILLATION INDEX
Represents the difference in average air pressure measured at Tahiti and Darwin, Australia. More specifically, the SOI is calculated as the difference in monthly averages of standardized mean sea level pressure at each station.
CAPE: CONVECTIVE AVAILABLE POTENTIAL ENERGY
The integrated amount of work that the upward (positive) buoyancy force would perform on a given mass of air (called an air parcel) if it rose vertically through the entire atmosphere. Positive CAPE will cause the air parcel to rise, while negative CAPE will cause the air parcel to sink. Nonzero CAPE is an indicator of atmospheric instability in any given atmospheric sounding, a necessary condition for the development of cumulus and cumulonimbus clouds with attendant severe weather
CAP (ALSO CALLED LID)
A layer of relatively warm air aloft, usually several thousand feet above the ground, which suppresses or delays the development of thunderstorms. Air parcels rising into this layer become cooler than the surrounding air, which inhibits their ability to rise further and produce thunderstorms.