All this incredible snow is caused by something, right?! Everyone is so quick to say La Nina and Pineapple Express. But what exactly do these terms mean?
Pineapple Express is a non-technical term for a meteorological phenomenon characterized by a strong and persistent flow of atmospheric moisture and associated heavy rainfall from the waters adjacent to the Hawaiian Islands and extending to any location along the Pacific coast of North America. A Pineapple Express is an example of an atmospheric river, which is a more general term for such narrow corridors of enhanced water vapor transport at mid-latitudes around the world.
A Pineapple Express is driven by a strong, southern branch of the Polar jetstream and is usually marked by the presence of a surface frontal boundary which is typically either slow or stationary, with waves of low pressure traveling along its axis. Each of these low pressure systems brings enhanced rainfall.
The conditions are often created by the Madden-Julian oscillation, an equatorial rainfall pattern which feeds its moisture into this pattern. They are also present during an El Niño episode.
The composition of moisture-laden air, atmospheric dynamics, and orographic enhancement resulting from the passage of this air over the mountain ranges of the western coast of North America causes some of the most torrential rains to occur in the region. In British Columbia especially, Pineapple Express systems typically generate heavy snowfall in the mountains and Interior Plateau, which often melts rapidly because of the warming effect of the system. After being drained of their moisture, the tropical air masses reach the Canadian Prairies as a Chinook wind or simply “a Chinook”, a term which is also synonymous on the Coast with the Pineapple Express.
La Niña is a coupled ocean-atmosphere phenomenon that is the counterpart of El Niño as part of the broader El Niño-Southern Oscillation climate pattern. During a period of La Niña, the sea surface temperature across the equatorial Eastern Central Pacific Ocean will be lower than normal by 3-5 degrees Celsius. In the United States, an episode of La Niña is defined as a period of at least 5 months of La Niña conditions. The name La Niña originates from Spanish, meaning “the girl”, analogous to El Niño meaning “the boy”.
La Niña, sometimes informally called “anti-El Niño”, is the opposite of El Niño, where the latter corresponds instead to a higher sea surface temperature by a deviation of at least 0.5 °C, and its effects are often the reverse of those of El Niño. El Niño is famous due to its potentially catastrophic impact on the weather along both the Chilean and Australian coasts. La Niña is often preceded by a strong El Niño.