Monday, March 16, 2015

Cyclone Pam, Seattle rain records and the MJO

Last week the small island nation of Vanuatu was devastated by a major tropical cyclone: the Category-5-equivalent Cyclone Pam.  There has been terrible damage to the infrastructure on the islands, with estimates of up to 90% of the buildings in the capital of Port Vila.  Power was lost early on in the storm and is just now being restored in some places.  We can actually see this looking at the weather observations from Port Vila, here from Weather Underground:
You can see that before they lost power late on the 13th, the pressure was plummeting (down to 965 hPa) with wind speeds rapidly increasing to 45 mph.

The CIMSS Satellite blog had a post showing several satellite animations of the cyclone as it hit; I encourage you to take a look at it here.
The cyclone continued south and is giving a glancing blow to New Zealand today, where it has caused some power outages and a lot of rainfall.  Remember that in the southern hemisphere low pressure centers rotate clockwise.  Here's a satellite image from the New Zealand Met Service this morning, showing that the cyclone has basically been sheared apart, but there is still a lot of moisture with strong winds.

As The Weather Channel pointed out in an earlier blog post, Pam isn't the only cyclone we had in the western Pacific over the past week.  There were actually four active at once: Pam, which hit Vanuatu; Olwy, which is affecting the west coast of Australia; Nathan, which is hitting the York Peninsula of Austrlia, and Bavi, which is headed west towards the Philippines, but is expected to weaken before it arrives there.
Is there some uniting factor behind this burst of tropical cyclone formation?  It would appear so---the Madden-Julian Oscillation (MJO) is currently, by many measures, the strongest we have seen it in a very long time.

What is the Madden-Julian Oscillation?  The exact nature of it is still a somewhat poorly-understood topic in meteorology, despite intense research activity in recent years.  Basically, it's an area of enhanced convection (thunderstorm activity) that propagates around the equator every 30-90 days.  (See the Wikipedia article for a decent overview).  We can see this looking at a global satellite loop over the past week.  There's been a lot of cloud cover and convection moving out from Papua New Guinea and northern Australia and into the central Pacific.  You can see that as it moves east, it looks like there are tropical cyclones (Bavi and Pam) that are being "shed" from the convection.

It turns out that the basic structure of the MJO has an equatorial "heat source" being trailed by flanking "gyres" on either side of the equator.  If strong enough, these "gyres" can break away and form tropical cyclones, which is exactly what we saw last week.
How strong is the MJO right now?  Here's a diagram that tracks the strength and position of the MJO as it goes around the globe, called the Wheeler-Hendon diagram.  
You can see that there are different "zones" on the diagram, labeled with different geographical regions (the "Maritime Continent" (Indonesia, New Guinea, etc.) , "Western Pacific", "Western Hemisphere and Africa" and the "Indian Ocean".  So as the MJO moves around the globe, it's supposed to make a big circle around the diagram.  The magnitude of the MJO (how strong the convection associated with it is) is given by how far from the center you are.  The line on the plot shows where the MJO has been for the past 40 days.  You can see that for much of February (the purple part of the line, the MJO was very weak (close to the middle of the diagram) and not clearly being tracked around the globe.  But in March (the red part of the line), the MJO exploded in strength over the Maritime Continent and is now moving out over the western Pacific, getting even stronger.  This is WAY stronger than the MJO has been for a long time!  No wonder we are seeing powerful cyclones.

It turns out that an "active" MJO in the western Pacific can also contribute to heavy rain ("pineapple express") events on the west coast.  Here's a diagram from the MJO wikipedia article describing this connection:
So as the active area of convection on the equator associated with the MJO moves out over the Pacific, the moisture associated with that gets drawn northward to the west coast of North America, causing heavy precipitation.

Well, over the past two days we've had just that here in Seattle.  Here's an animated map of the total column water vapor over the past two days.  You can see the plume of moisture drawn northward from the tropics that brought heavy rain to Seattle.
So much rain fell (2.2") that Sunday was actually the second wettest March day on record in Seattle, and the single wettest day we've had since 2009 (courtesy of Scott Sistek).  It's amazing how a single weather anomaly like the MJO can contribute to both tremendous rain in Seattle and an extreme Cyclone in Vanuatu.

No comments:

Post a Comment