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23 Apr 14 12:03

Early this year (Jan 2014), you may have read about Arctic air making its way to Florida. But what about Hawaiian water as a source for the recent mudslides in Washington, flash flooding in California, or the rest of the west coast?

Within narrow regions (~300-500 kilometers wide) of the atmosphere, large amounts of water vapor can be transported from the tropics towards the poles. These areas have been called Atmospheric Rivers (ARs) by scientists. In fact, the rate of transport within these ARs can be 7.5 to 15 times the water flow reaching the mouth of the Mississippi River!

The impact of the ARs on the US west coast is fairly well-established. Over the period between 1997 and 2006, California experienced approximately 42 separate AR events. Even this year's unusually wet March season, which resulted in the catastrophic landslide in Oso, WA, can be attributed to these ARs (see Figure 1).

The moisture source is provided by the Madden Julian Oscillation (MJO), which is characterized by areas of thunderstorms and dry air which move from west to east in the tropics. If the jet stream is oriented from southwest to northeast when the thunderstorm phase of the MJO is near Hawaii, the jet stream will transport much of the moisture associated with these thunderstorms northward towards the west coast of the US.

Figure 2 shows an enhanced satellite image of an atmospheric river from 2010. This extreme event dumped over 20 inches of rain in Northern California and about 17 feet of snow in the Sierra Nevada over the course of 5 days. These events have been colloquially known as the Pineapple Express (nothing to do with the movie).

The ARs are a valuable water supply for the region, and if a Pineapple Express occurred today, there would be some relief for drought stricken California. However, they also pose a threat in terms of extreme flash flooding and heavy snowfall. It would not take much for a pineapple express to turn into an extreme precipitation event like the East Coast's "Snowmageddon" of 2010. During that storm, 2 to 3 feet of snow fell over a 2-day period, leading to widespread property damage, business interruption and total economic losses exceeding $500 million.

In 2011, the U.S. Geological Survey (USGS) released a report highlighting a hypothetical scenario for an Atmospheric River with the intensity associated with a 1000 year scenario. The experts termed these winter storms as ARkStorms (for Atmospheric River 1000). A detailed impact analysis, considering the primary (wind, floods) and the secondary (landslides) hazards, showed that the total direct property loss could be as high as $400 billion. The insured property claims for such an event would add up to about $20 to $30 billion. But more importantly… Imagine …All the vineyards washed away and no more Californian wine!

Figure 1: An Atmospheric River focused on US Pacific North West; an unusually wet March was observed for 2014 in the State of Washington. (Source: NOAA NESDIS)

Figure 2: A strong atmospheric river from December 2010 making landfall over Northern California. (Source: NASA Jet Propulsion Lab; Continents are colored in black.)


Category: Climate/natural disasters: Drought, Floods/storms

Location: United States


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