Friday, August 8, 2014

Why are Hurricanes Rare in Hawaii?

Today, tropical storm Iselle is hitting the big island of Hawaii and the much stronger Hurricane Julio is right behind (see infrared satellite image below).  Julio, the more eastern storm, has a far more defined structure, and possesses a nice eye and substantial spiral rainbands.


Tropical storms are relatively rare in Hawaii, although direct hits have occurred, such as the devastating landfall of Hurricane Iniki on Kauai in 1992.  FAR more hurricanes occur at similar latitudes in the Caribbean.

A plot of historical hurricanes tracks shows the story clearly (see image).  Hurricanes move way north in the Caribbean and eastern Atlantic.  The same for the western Pacific.  But over the eastern Pacific they die quickly as they approach Hawaii.


A blow-up image illustrates this better:


What is it about Hawaii that works against their getting strong tropical storms and hurricanes?

The main reasons:  the  water temperatures are generally too cool and there is often too much vertical wind shear, the change in horizontal winds with height.

Cool sea surface temperatures

Anyone who has taken a vacation in Hawaii knows the story.  The waters of the Pacific are relatively cool and you know it when you snorkel or swim.   70s are typical, ten degrees or more cooler than the Caribbean for example.

The minimum temperature generally conducive to hurricane formation is 26C (79F), and often the waters around Hawaii during the tropical cyclone season (mid summer to early fall) are cooler than
that.   During the past few months the sea surface temperature (SST) has been above normal in the eastern Pacific around Hawaii, something that is illustrated by the SST anomalies (differences from climatology) shown below.  This has pushed the 26C sea surface temperature north of Hawaii.

 Why do hurricanes care about the temperature of the ocean surface?  Because the heat and moisture (evaporation) from the ocean surface is the fuel for hurricane development and maintenance.

Vertical Wind Shear

The change in horizontal wind with height is called vertical wind shear.  Such shear is bad for tropical storms, since it tends to distort the vertical structures necessary for the development of these systems.There is often a lot of shear near Hawaii, with easterly trade winds at low levels and westerly  winds aloft.
        Here is a plot of the vertical wind shear over the eastern Pacific and western U.S. yesterday.  Reds are high shear. The two red dots indicate Iselle and Julio.   You will notice that the shear was relatively low over their locations...that allowed these storms to develop and to retain key structures.  The vertical wind shear over them is less than normal.

So the combination of warmer than normal sea surface temperatures and reduced wind shear has helped tropical storms to develop and approach Hawaii.

    Tropical Storm Iselle is dumping heavy precipitation on some of the Hawaiian islands, but it now appears that Julio will move north of the island chain, reducing its impacts substantially (see graphics from the National Hurricane Center)


One place that will NEVER get tropical storms is the Pacific Northwest.  Our 50F water is a barrier that will never let them approach our coast.






8 comments:

ip said...

Thanks Cliff. I find it interesting that the three hurricanes to make an impact on Hawaii since 1950 all came from the south. No east to west storms have made a landfall as a hurricane. All direct impacts were on Kauai as well.

Zathras said...

ip--the close up view of storm tracks that Cliff posted shows three or four storms over the years tracked into the islands from the ESE or SE. None seen to have come straight up from the south.

Candy B said...

Could we get the moisture though? ie. the pineapple express?

Justin Wilkerson said...

Zathras, Hurricane Iniki, which cruised over Kauai and became one of the most devastating hurricanes to hit Hawaii, came from the south. In the close up view, if you look at Kauai you will see one track in particular go almost directly S-N, follow it backward and you will see it made a very abrupt track change, nearly a 90 degree turn.

When you major in Meteorology at the University of Hawaii, you learn a lot about Iniki and its impacts.

eprman said...

The SST anomaly in the far north looks impressive. Will this contribute to more arctic ice melt this summer?

Zathras said...

Ah, the Jurassic Park storm, I see it now:
http://www.ndbc.noaa.gov/hurricanes/1992/iniki/image002.gif

Carl Dinse said...

I heard a rumor that the 1962 Columbus Day storm was the remains or old core of a pacific typhoon or hurricane.

Brendan Lane Larson said...

Aloha Cliff,

Nice write up! Let's encourage current, future and past politicians to take some meteorology courses, whether at UW or Univ. of Hawaii or any of the fine university meteorology programs offered in the U.S. or overseas! This is my recent suggestion to some politicians in Hawaii to do just that:

https://plus.google.com/114212124546673371823/posts/61qBLrCk3t8

Cheers,

-Brendan Lane Larson