Common Terms
El Niño Southern Oscillation/ENSO: A see-saw pattern in surface pressures and ocean temperatures that occurs in the tropical Pacific Ocean. The warm oceanic phase is known as El Nino, and cold phase is called La Nina and includes high air pressure and colder ocean temperatures in the Eastern Pacific. ENSO affects the location of the Jet Stream in the Pacific and North America and, in turn, storm tracks. The state of ENSO affects global precipitation and temperature patterns and understanding the state of ENSO aids in snowfall, severe weather, and tropical cyclone forecasting. By convention we use the NINO 3.4 departure from normal SSTs as the measure for ENSO with departure from normal>0.5C for three consecutive months as threshold. Other measures include MEI and SOI.
La Nada: A condition where the central and eastern tropical Pacific Ocean temperatures remain neutral, or normal.
Subtropical Jet-stream: The jet-stream typically found between 20 and 30 degrees latitude and at altitudes of 12 to 14 kilometers. It carries a lot of moisture.
Polar Jet-stream: The jet-stream that is associated with the polar front in middle and high latitudes. It is usually located at altitudes between 9 and 12 kilometers.
Alberta Clipper: An air mass in which arctic air is carried usually from Canada to the northeast and mid-atlantic states. They are relatively dry, but may produce a small amount of snow.
Phasing: A term used when two pieces of energy, sometimes from different jet-streams combine to form a larger, stronger, and more complex system.
Stalling: A term used to indicate when an area of low pressure slows or stops, often providing extra moisture to the area of impact.
North Atlantic Oscillation/NAO: One of the most commonly looked at teleconnection patterns for weather forecasting. The value of the NAO is determined by the differences in pressures over the high latitudes and central latitudes of the North Atlantic. The positive phase of the NAO is characterized by a low difference in pressure between the two regions, a below average pressure over the high latitudes, and an above average pressure over the central latitudes. The opposite occurs during the negative phase. The NAO is significant in forecasting because its state affects the intensity and location of the jet stream and storm track in the North Atlantic. As a result, significant changes in temperature and precipitation occur in Europe and North America based on the NAO.
Southern Oscillation Index/SOI: The SOI is computed from normalized surface level pressure anomalies at Tahiti and Darwin Australia. Negative values of the SOI indicate a relatively small difference in pressure between Tahiti and Darwin, and is indicative of El Nino conditions. During a La Nina episodes the SOI is usually positive. During weaker ENSO events the SOI can oscillate positive and negative in response to the passage of the MJO waves.
Arctic Oscillation/AO: The AO refers to opposing pressure patterns in the middle and high latitudes of the Northern Hemisphere. The negative phase is represented by high pressure over the polar region and lower pressure at the mid-latitudes. The opposite occurs during the positive phase. The AO plays a role in weather patterns as it affects the track of storms, and the location of cold air. When the AO index is negative, there tends to be high pressure at the pole and weaker zonal winds, so more cold air is able to drop down into the mid-latitudes.
Madden-Jullian Oscillation/MJO: An eastward tracking wave extending to high levels of the atmosphere which can enhance and then suppress vertical motion in the atmosphere, and which thus results in enhanced and then suppressed cloudiness and rainfall. The MJO cycles typically last 30 to 60 days. The MJO index has 8 different stages, and the stage of the MJO can affect temperature patterns in the United States and affect the frequency can be used to forecast enhanced periods of tropical activity in both the Pacific and Atlantic.
Pacific Decadal Oscillation/PDO: A pattern of variability of climate that occurs in the Northern Hemisphere in the Pacific. Each stage generally lasts around 20 to 30 years. The cold phase features cooler ocean temperatures in the Eastern Pacific and warmer ocean temperatures in the western part of the Pacific. The opposite occurs during the warm phase. The PDO is often referenced in discussions of climate change since its state can significantly affect the global temperature and the occurrence of some types of weather events. It affects the favored ENSO state - the cold PDO favoring more frequent, stronger and longer lasting: La Ninas and the warm phase more frequent, stronger and longer lasting El Niños.
Pacific North American Pattern/PNA: The PNA is one of the most prominent modes of low-frequency variability in the Northern Hemisphere extra-tropics. The PNA pattern reflects a quadripole pattern of 500 millibar height anomalies, with anomalies of similar sign located south of the Aleutian Islands and over the southeastern United States. Anomalies with sign opposite to the Aleutian center are located in the vicinity of Hawaii, and over the intermountain region of North America (central Canada) during the winter and fall.
La Nada: A condition where the central and eastern tropical Pacific Ocean temperatures remain neutral, or normal.
Subtropical Jet-stream: The jet-stream typically found between 20 and 30 degrees latitude and at altitudes of 12 to 14 kilometers. It carries a lot of moisture.
Polar Jet-stream: The jet-stream that is associated with the polar front in middle and high latitudes. It is usually located at altitudes between 9 and 12 kilometers.
Alberta Clipper: An air mass in which arctic air is carried usually from Canada to the northeast and mid-atlantic states. They are relatively dry, but may produce a small amount of snow.
Phasing: A term used when two pieces of energy, sometimes from different jet-streams combine to form a larger, stronger, and more complex system.
Stalling: A term used to indicate when an area of low pressure slows or stops, often providing extra moisture to the area of impact.
North Atlantic Oscillation/NAO: One of the most commonly looked at teleconnection patterns for weather forecasting. The value of the NAO is determined by the differences in pressures over the high latitudes and central latitudes of the North Atlantic. The positive phase of the NAO is characterized by a low difference in pressure between the two regions, a below average pressure over the high latitudes, and an above average pressure over the central latitudes. The opposite occurs during the negative phase. The NAO is significant in forecasting because its state affects the intensity and location of the jet stream and storm track in the North Atlantic. As a result, significant changes in temperature and precipitation occur in Europe and North America based on the NAO.
Southern Oscillation Index/SOI: The SOI is computed from normalized surface level pressure anomalies at Tahiti and Darwin Australia. Negative values of the SOI indicate a relatively small difference in pressure between Tahiti and Darwin, and is indicative of El Nino conditions. During a La Nina episodes the SOI is usually positive. During weaker ENSO events the SOI can oscillate positive and negative in response to the passage of the MJO waves.
Arctic Oscillation/AO: The AO refers to opposing pressure patterns in the middle and high latitudes of the Northern Hemisphere. The negative phase is represented by high pressure over the polar region and lower pressure at the mid-latitudes. The opposite occurs during the positive phase. The AO plays a role in weather patterns as it affects the track of storms, and the location of cold air. When the AO index is negative, there tends to be high pressure at the pole and weaker zonal winds, so more cold air is able to drop down into the mid-latitudes.
Madden-Jullian Oscillation/MJO: An eastward tracking wave extending to high levels of the atmosphere which can enhance and then suppress vertical motion in the atmosphere, and which thus results in enhanced and then suppressed cloudiness and rainfall. The MJO cycles typically last 30 to 60 days. The MJO index has 8 different stages, and the stage of the MJO can affect temperature patterns in the United States and affect the frequency can be used to forecast enhanced periods of tropical activity in both the Pacific and Atlantic.
Pacific Decadal Oscillation/PDO: A pattern of variability of climate that occurs in the Northern Hemisphere in the Pacific. Each stage generally lasts around 20 to 30 years. The cold phase features cooler ocean temperatures in the Eastern Pacific and warmer ocean temperatures in the western part of the Pacific. The opposite occurs during the warm phase. The PDO is often referenced in discussions of climate change since its state can significantly affect the global temperature and the occurrence of some types of weather events. It affects the favored ENSO state - the cold PDO favoring more frequent, stronger and longer lasting: La Ninas and the warm phase more frequent, stronger and longer lasting El Niños.
Pacific North American Pattern/PNA: The PNA is one of the most prominent modes of low-frequency variability in the Northern Hemisphere extra-tropics. The PNA pattern reflects a quadripole pattern of 500 millibar height anomalies, with anomalies of similar sign located south of the Aleutian Islands and over the southeastern United States. Anomalies with sign opposite to the Aleutian center are located in the vicinity of Hawaii, and over the intermountain region of North America (central Canada) during the winter and fall.
