02.05.11
The Rum Runner
Have you ever heard of the Pineapple Express?
The Pineapple Express is a term for a plume of moisture that originates around the east central Pacific (often near Hawaii) and travels up to the West Coast of the United States. This band of moisture is often responsible for heavy rain, snow, and/or flooding events from the Pacific Northwest to southern California.
Now, have you ever heard of the Rum Runner?
The Rum Runner is a similar moisture plume that runs from the eastern Caribbean up to western Europe. This atmospheric river of moisture is also responsible for many of the heavier rain and flooding events from Ireland and the United Kingdom to Spain.
Examples
The following map (click to enlarge), from Jason Cordeira’s website, shows global 250 mb heights (the “dips” in the northern hemisphere are troughs of low pressure, while the “mountains” are ridges of high pressure…the reverse is true for the southern hemisphere) and precipitable water (the amount of water that would be measured if all the moisture in the atmosphere at that point fell out as rain). The important part to note here is that the colors are showing moisture, and the bigger the number, the greater the moisture.
Note the plume of moisture being transported from the central Pacific up into British Columbia and Washington State…this is the Pineapple Express.
Note also the plume of moisture being transported from the western Atlantic up into Ireland and United Kingdom…this is the Rum Runner.
The infrared satellite image below, from the U.K. Met Office website, also shows evidence of the Rum Runner in the thin band of clouds that stretch across the image into southern Scandinavia.
The Rum Runner is stronger when there is a strong westerly jet stream, as there is now. The stronger jet stream is a result of a stronger Icelandic Low and Azores High (for more on the North Atlantic Oscillation, see this blog post and this Wikipedia).
The above map shows surface pressure (contoured) and the 300-mb jet stream (filled colors), from Weather Online UK, for 12Z this morning. Note the low pressure over northern Europe and the high pressure over Spain, contributing to the higher wind speeds in the jet stream. This jet stream is transporting the deep plume of moisture into northern Europe (see also below, including precipitation).
I recommend checking out the animation of the above precipitable water map, to get a better appreciation for how these atmospheric rivers move. It’s pretty cool, and can be found at the website of Jason Cordeira, student at SUNY Albany.