New Zealand is located under a maritime tropical air mass (mT). This indicates abundant precipitation and relatively warm temperatures due to mid-latitudes and its close proximity with the Pacific Ocean. The warmer sub-tropical air results in a higher saturation quantity and therefore higher specific humidity. However, using a hygrometer we would find some sharp variations in humidity between the coastal regions and colder southern mountain ranges.
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| Map of Annual Rainfall in New Zealand Note the contrast in rainfall in the south Island and the resulting rain shadow to the East of the mountains. Image courstesy of: http://www.niwa.co.nz/education-and-training |
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| Map of Mean Temperature in New Zealand Being in the Southern Hemisphere temperatures approach sub-tropical in the North Island, and cooler temperate in the South Island. Image courtesy of: http://www.niwa.co.nz/education-and-training |
Another major factor in New Zealand's climate is the interaction between the persistent Westerly winds and the 10,000ft plus mountains that span it's length. As the saturated air over the Tasman Sea is forced East it collides with the windward side of the mountains. This results in an adiabatic process called Orographic Lifting where the air moves upwards to the peak of the mountains. The higher elevation cools the air due to lower pressure and quickly raises the relative humidity to 100% as it passes the Lifting Condensation Level, resulting in precipitation. When the winds reach the eastern or leeward side of the Southern Alps they are much drier and are commonly known as foehn winds. Because much of the remaining water vapor evaporates as the foehns reach the lowlands, the land is left dry and arid in the south east.
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| Effective Visualization of Orographic Lifting Thanks to our professor we can see the essential properties of an Adiabatic Process. Image Courtesy of: Dr. Casey Allen |
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| Graph of Annual Precipitation versus Distance Inland from the West Coast of the South Island Note the majority of rainfall takes place before crossing the highest peak. Also note the steep drop in precipitation on the eastern side of the mountains. Image Courtesy of: http://www.briangwilliams.com/weather-climate/orographic-precipitation.html |
New Zealand experiences its summer months from November to Februrary. Excess latent heat from higher levels of Summer insolation contribute to high winds, low pressure cells and Cyclogenesis. Tropical Cyclones are the most devastating of storms to hit New Zealand, commonly originating in the warmer equatorial waters to the North.
The combination of warm tropical sea surface temperatures near Fiji, plentiful water vapor and an ideal latitude of 16-18 degrees south of the equator helped create Cyclone Bola on Feb. 23rd, 1988. When a cold front from the Westerly winds penetrated a warm front from the South Trade winds an Occluded front was formed. This resulted in much lower pressures at the convergence of the two fronts and the warm air and water from the north fueled Bola's development.
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| Satellite Image of Cyclone Bola, 1988 |
Formed in the maritime equatorial (mE) waters near Fiji at the north of the image,
Bola moved Southward changing from a Tropical to Extra-tropical Cyclone.
Image courtesy of:http://en.wikipedia.org/wiki/Cyclone_Bola
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| Map of Cyclone Bola's Path From Fiji to New Zealand |
The blue represents the early stages as well as the transition to an Extra-Tropical Cyclone.
The yellow is Bola's Tropical Cyclone period as well as first land fall in Fiji.
Image Courtesy of:http://en.wikipedia.org/wiki/Cyclone_Bola
After reaching its peak power as a Tropical Cyclone, Bola devastated more than 3,000 homes and destroyed multiple bridges in Fiji before weakening on its way to New Zealand. When it reached the North Island it dropped a record 917mm of rain over just a few days resulting in heavy floods. The storm damaged roads causing 3 deaths and left thousands out of power due to downed power lines. In all it was the most expensive recovery effort in New Zealand's history at over 160 million dollars in damage.
Sources:
http://en.wikipedia.org/wiki/Cyclone_Bola
http://www.new-zealand-nz.net/weather.html
Dr. Casey Allen
http://weatherwatch.co.nz/
