Hobie Science: Gary Larson Explains Hurricanes and Hurricane Swell

There is most definitely something special about the transition from the summer into autumn.  The months of September and October seem to always correlate with the best waves of the year, it happens to be in the middle of hurricane season and this year has been quite exceptional. Typically, hurricane seasons starts in early June, when the northern hemisphere begins to receive longer periods of sunlight resulting in warmer temperatures, and ends around the beginning of November when temperatures begin to cool. This year, as of September 24th 2012, the east Pacific basin has produced 13 tropical storms/hurricanes. This is about average in quantity for the year but what is interesting is that 4 of the 13 hurricanes occurred in September, and this month isn’t over yet! Below is a map showing the hurricanes and their paths for the year so far.

Before we go further into the discussion about the most recent hurricane, Miriam, that began forming off the coast of Mexico on September 22nd, it would be helpful if we went over how hurricanes work.

First of all, there are many synonyms for a hurricane. In the Indian and South Pacific oceans they are called tropical cyclones and in the northern hemisphere they are called Typhoons in the west Pacific basin and Hurricanes in the east Pacific basin and Atlantic ocean.

Another common curiosity is how hurricanes are named. Many decades ago the World Meteorological Organization comprised 6 lists of names, both feminine and masculine, and each list contained one name per letter of the alphabet except for the letters Q, U, X, Y, and Z. Only one list is used per year so at the beginning of a new year the next list is used. After the sixth list is used the cycle starts over.  Most recently we observed hurricane Miriam, M, the 13th letter in the alphabet, the 13th hurricane for 2012.

Hurricanes tend to always form between 5˚ and 20˚ north or south latitude. The Coriolis effect is what causes the distinctive spiral of clouds characteristic of hurricanes. At the equator there is no Coriolis effect, therefore hurricanes will never form at the equator nor will they ever cross the equator. Due to the Coriolis effect hurricanes will spin in a counter-clockwise direction in the northern hemisphere and clockwise in the southern hemisphere.

Because hurricane Miriam was recently spinning off the Mexican coast, and moved into the southern California swell window, I find it relevant to use as a reference of how hurricanes form and develop. Looking at the snapshot below, Miriam is easily identifiable just south of Cabo San Lucas. Another feature that is easily seen is the band of clouds that essentially split the northern and southern hemispheres. This band is the inter-tropical convergence zone (ITCZ) and it is where surface winds converge and thunderstorms are the result as the warm humid air rises, up to about 50,000 ft. above the ocean surface. It is along the ITCZ where many, if not most, hurricanes begin to develop.

Surface and atmospheric conditions must be just right in order for a hurricane to form. The ocean temperature needs to be about 80˚F to a depth of about 65ft., winds must be light and humidity must be high and extend up into the troposphere, about 30,000ft. It is only when these conditions are met and upper-level wind conditions are not too strong, a small cluster of clouds and thunderstorms will form. From this localized area of low pressure, the warm, moist air from the ocean surface rises high into the atmosphere where it cools and condenses. As air and moisture rise through convection, the relatively cooler surrounding surface air as well as the descending drier air from the tops of the thunderstorms moves in to fill the void. As this cycle continues, wind speeds will increase and the cluster of thunderstorms (tropical depression) will begin to organize, expand, and spiral into a massive vortex.

Technically, a hurricane can only be identified as a “hurricane” when surface level wind speeds reach 74mph, resulting in a Category 1 hurricane. It isn’t until wind speeds exceed 155mph that a hurricane reaches the highest Category 5.

Hurricanes usually dissipate when one of the fundamental conditions are disrupted. When the storm moves into colder water convection will cease thus stopping the movement of air. Upper atmospheric winds can also increase in speed causing wind sheer resulting in the disorganization of the hurricane. And also common is when the hurricane moves overland, the warm water source is eliminated and the storm quickly breaks down.

So, will hurricane Miriam send us some waves? Well, the answer to this can be fairly complicated. Let’s look at a snapshot of how Miriam was looking about two days after forming.

At this point in time Miriam was a category-3 hurricane with wind speeds around 120mph, and projected to send some significant swell towards southern California. Unfortunately, due to geographic constraints, southern California has a very defined hurricane swell window. Storms that occur off the coast of Mexico must move west to about 118˚W longitude for any of the swell to reach our beaches. So Miriam was looking pretty good at holding a high wind speed and movement.

The above image shows Miriam’s status as of Thursday, September 27th. As can be seen, Miriam kind of stalled out at about 22˚N latitude and wind speeds decreased significantly to about 30mph. The result will be pretty fun, shoulder high waves along the south facing beaches and Friday and Saturday. On the upside, there was a very powerful south Pacific storm earlier this week that will send us some pretty significant swell, head-high plus, for Sunday and Monday.

So, for everyone participating in the Battle of the Paddle this weekend, there will be some surf to contend with; and let’s face it, board carnage is what the spectators are really wanting to witness!

-Gary Larson
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