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“In Hawaii, hurricanes hardly happen”

Aug 14, 2013

Apologies to George Bernard Shaw, and Lerner and Loewe for the title. But, in fact, hurricanes are not that common in Hawaii, despite the location of the island state in the middle of the largest ocean on the planet and solidly in the tropical latitudes.

At this writing we have moved through the early stages of the hurricane season for the North Atlantic and the eastern and central North Pacific. So far, the eastern North Pacific (defined as the region from the west coast of central and north America west to 140°W) is ahead of the Atlantic as far as number of storms is concerned. This is not unusual, although in recent years the Atlantic has been more active. The eastern Pacific has already had eight named systems, while the Atlantic has had only four, and there are signs (again as of this writing — early August) that more named systems are imminent in the eastern Pacific while nothing appears imminent in the Atlantic. It’s still early, though.

The central Pacific region (from 140°W to 160°E) includes Hawaii, and on average sees fewer named tropical systems than either the eastern Pacific or the Atlantic. One of the main reasons for this has to do with sea surface temperatures in the eastern Pacific. The California current flows southward along the West Coast of the U.S. and into tropical latitudes, and this is a cold current. This means that tropical cyclones which develop in the coastal waters of central America and track west or west-northwest eventually move into a region of colder water and this almost always leads to significant weakening of these systems, and frequently dissipation. Thus systems that form in the eastern Pacific region and track west usually do not survive to reach the central Pacific region. This is shown very clearly in the statistics: on average the eastern Pacific region sees 15 named tropical cyclones each season while the central Pacific region averages just four or five, and this includes tropical depressions, which are not named.

Some of the tropical cyclones which impact the central Pacific region move into the region from the eastern Pacific if they survive their transit over the colder water, and others develop within the region. In both cases, the presence of El Nino (warmer ocean temperatures in the equatorial Pacific) correlates well with more active hurricane seasons in this region. This season El Nino is not present, although neither is La Nina (colder than normal ocean temperatures) with a neutral situation prevailing.

So far this season, of the eight eastern Pacific systems noted above, three have survived to move into the central Pacific, which is somewhat unusual particularly without an El Nino present. This illustrates that warm sea surface temperatures are not the only criteria for tropical storm development. One of these three systems, tropical storm Flossie, passed close enough to Hawaii for warnings to be issued in late July, and this was the first time warnings were needed for any portion of Hawaii since the 2007 season. Ironically enough, the system which affected the state in 2007 also carried the name Flossie. While the effects of Flossie were not extreme, tropical storm force winds did impact some areas leading to some minor damage, and significant rainfall amounts were recorded at some locations as well. Additionally, sea states in the waters around the islands were well above normal and some minor coastal flooding occurred. Here is a preliminary summary of this system: (www.prh.noaa.gov/hnl/pages/examples/TS_Flossie_Review_v2.pdf).

The most serious system to impact Hawaii in recent history was Hurricane Iniki in 1992, when a strong El Nino was present. This system formed in the lower latitudes of the western portion of the eastern Pacific region, then strengthened as it moved west into the central Pacific region where it received its name (systems that are named in the eastern region retain their name when they move into the central region). Due to an unusual weakness in the subtropical oceanic high to the north of Hawaii, Iniki turned northward once reaching Hawaii’s longitude, and struck the state from the south as a Category 4 hurricane with devastating impacts on Kauai.

One of the main messages here is that even though, as noted above, “hurricanes hardly happen in Hawaii,” the potential impact from these strong weather systems cannot be discounted. Very rarely in meteorology do we use the word “never” when talking about weather systems, certainly not in this case. Mariners making passages to and from Hawaii need to understand the climatology of tropical cyclones in the central and eastern Pacific, and should be aware of the risks. It is critical to regularly obtain the most current forecast information when voyaging through this area during the hurricane season.

Much more information can be found by visiting the Central Pacific Hurricane Center website: www.prh.noaa.gov/hnl/cphc/

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