|Pacific Surface Analysis showing approaching cold front|
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.|
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.|
|January patterns. Red circle approximates Hawai'i's location. Map from here.|
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.|
(1) To be discussed in a future post