Tuesday, January 21, 2014

A Storm is Coming: Winter Weather Patterns In Hawaii

Pacific Surface Analysis showing approaching cold front
One of the main reasons that millions of tourists flock to Hawai'i each year is because the weather year round is fairly pleasant and predictable, and not as subject to the seasonal shifts that characterize the climate at higher latitudes.  But if you've spent any time in Hawai'i you've likely noticed that there are indeed seasonal shifts.  In the "summer" it's usually a little bit warmer, but the refreshing tradewinds blow a good bit more regularly, which helps to cool us off and brings rain to windward and mauka areas.  In the "winter" the trades aren't as reliable, and we have more frequent kona winds.  The Hawaiians, being excellent geographers, have names for these two seasons.  The warmer season is called Kau and generally lasts from approximately mid to late April until October, whereas the cooler season is called Ho'oilo and lasts from mid to late October until April.  The changes that come with Ho'oilo are the subject of this blog post.

The Big Picture...

One major feature of Ho'oilo is the periodic occurrence of thunderstorms, which in general are relatively rare in Hawai'i due to the tradewind temperature inversion.   However, in the winter months, cold air and low pressure systems sweep down from the north, bringing occasionally severe weather along with the massive swells that the North Shore is so famous for.  But did you know that these storms are a part of the global system of atmospheric circulation?  It all begins with the earth-sun relationship, which you can read about in a previous post.  Since the earth is tilted, the point on the earth's surface that receives the sun's energy directly shifts over the course of the year, which basically means that the latitude that receives the most energy migrates over the course of the year.  This spot, called the subsolar point, is loosely tied to the Inter-Tropical Convergence Zone (ITCZ), an area of convection (rising air) and thunderstorms that helps to drive the entire global atmospheric circulation system!  You've probably learned in geography class about the ITCZ, which is part of the three cell model of circulation (1).

As with most everything in life, whatever goes up must come down.  This is true for air that rises in coriolis effect (to be discussed in a future post), which twists the path of the air (to the right in the northern hemisphere, to the left in the southern hemisphere.  This part of the global atmospheric circulation is referred to as the Hadley Cell, and there are two of them, one to the north of the ITCZ and one to the south.  You can see the general pattern in the figure below, which shows the circulation when it is summer in the northern hemisphere.
Three Cell Model diagram from here.
the ITCZ.  Once it reaches the top of the troposphere (the lowest layer of the atmosphere where virtually all weather happens), it diverges and circulates to the north and the south, sinking at approximately 30 degrees north and south of the equator, but the latitude at which the air sinks shifts along with the ITCZ and the subsolar point over the course of the year.  The places where this air sinks are high pressure areas, because the sinking air is exerting force on anything below it.  The ITCZ, conversely, is a low pressure area because the air is rising there.  Because of the rotation of the earth, the sinking air is subject to the

How this Affects Hawai'i...

As you can see, a major area of sinking air is usually located to the northeast of Hawaii.  Here in Hawaii we call this high pressure area the "Hawaiian High", but in general it referred to as the Northern Pacific Subtropical Anticyclone.  Anticyclones are areas of sinking air where the wind circulates outward from the high in a clockwise direction.  Note from the graphic the direction that the wind blows coming out of the high.  You should notice that our islands are right in the path of the wind!  This is the source of the tradewinds, which blow about 80% of the time in the Kau season.

July patterns.  Approximately location of Hawai'i denoted with red circle.  Map from here.
When it is winter in the northern hemisphere it is summer in the southern hemisphere, since the subsolar point and ITCZ shift to the south.  Along with this travels the Hadley cells.  Another characteristic of the northern hemisphere winter months is that the Hawaiian High tends to weaken, and so the tradewinds are less consistent.  At the same time, the storm-producing polar front (another part of the global atmospheric circulation), moves to the south.  One major characteristic of the polar front is that it produces low pressure systems that drive cold fronts and produce heavy rainfall and severe weather.  These are the same types of systems that generally bring high snowfall totals to the continent in the winter months.  Hawaii is much further south (and surrounded by the ocean), so with the exceptions of Mauna Kea, Mauna Loa, and Haleakala we don't get any snow.  But a few times a year the cold fronts do sweep down and roll over Kaua'i, Oahu, and the other islands, moving from west to east.

January patterns.  Red circle approximates Hawai'i's location.  Map from here.
When this happens there is a fairly noticeable sequence of atmospheric events that will, if you know what to look for, help you to predict the weather over the next couple of days and amaze your friends.  The first thing that will happen is that the wind will start blowing from the south (Kona).  This happens because the wind blows roughly parallel to an approaching cold front, heading in the direction of the low pressure area that is at the center of the storm system.  The wind will gradually strengthen.  You may also notice a very characteristic cloud progression.  The first clouds you notice will arrive a day or two ahead of the front (depending on how fast the front is moving).  These clouds will be very high (cirrus) clouds and will cover much of the sky.  Then as the front continues to move towards your island, you'll see lower and lower (and thicker, more ominous) clouds appear, until finally the sky is socked in by low cumulus clouds.  The reason that this happens is that the cold air that is approaching is abruptly pushing up the warmer, moist air in front of it.  This causes the air to cool, which leads to cloud formation.

When the front arrives it will bring with it significant rainfall and pretty heavy winds in some cases.  Sometimes the fronts pass quickly, but sometimes they may stick around for a couple of days.  After the front passes, you should notice clear skies, and the direction of the wind will shift; instead of coming from the south it will be coming from the west or northwest.  Then after a day or two if high pressure conditions return to the north of the islands, the trade winds will return.

The entire north Pacific at the time this post was written.  The symbols point in the direction the wind is blowing.  From National Weather Service.
That pretty much sums up winter cold fronts in Hawai'i.  These don't happen in the summer time because the polar front, which is the source of the disturbances, moves northward in the summer time.  So the next time the wind starts to blow from the south, keep your eyes on the sky, and you may be able to apply what you've learned here and in class.  And when you do, you can remember the kilo lani, or "sky watchers", who were special kahunas in Old Hawai'i that had a tremendous amount of knowledge about their natural environment, including the atmospheric conditions and signs that helped them to predict the weather.



(1)  To be discussed in a future post

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