An Island in the South Pacific

Fiji being a South Pacific island doesn't have as many air mass influences as we have seen in Colorado. The island is East of Australia and Indonesia with Suva sitting at about 18 degrees South of the equator. This puts it in the prevailing winds of the Southeast Trades. This causes the air masses to be mT or maritime tropical over the entirety of the year. However, even though it is mT throughout the year there are other factors that effect the island that create significant changes in the weather showing up in seasonal weather patterns.

The general prevailing winds of the world. Sitting above the Tropic of Capricorn , you can see the East  to West winds of the Southeast Trades. Image : http://www.fccj.us/gly1001/tests/10Ch18L.htm

The Intertropical Convergence Zone (ITCZ) has great effect on Fiji. Residing in the South Pacific with little land to heat up and move the ITCZ results in a very significant affect on the weather of Fiji. It is known as the South Pacific Convergence Zone (SPCZ). It is a branch off the ITCZ. It has very little dependence on land heating and thus tends to stay in the same location year round. Convergence of the trade winds to the monsoon winds cause the convergence zone to form. 

The SPCZ over the Pacific Ocean in February. There is massive convergence over the islands in the South Pacific. This is  where Fiji resides and is what causes the seasonal weather that the islands face. Image: http://www.nrlmry.navy.mil/~chu/chap2img/fig204.jpg

This is the SPCZ during August over the Pacific Ocean. You can still see that it is still occurring, but is not as big. Image:  http://www.nrlmry.navy.mil/~chu/chap2img/fig204.jpg

From this we can see how the ITCZ effects Fiji. Large amounts of rainfall occur from November to April, and it tends to be dry from May to October. This coincides with the ITCZ. There are other things that determine the weather of the main island, Viti Levu, with the capital Suva on it. 

Being formed by volcanic processes the island is essentially a large mountain in the ocean. The ridge sits in the center of the island and causes localized weather on either side. Suva sits on the windward side of the mountain ridge and the storms also flow from East to West on the island. This causes large amounts of rain to fall on the East side of the mountain ridge causing it to be much more tropical, while the leeward side of the mountain ridge has a climate that is much dryer. This is very similar to what we experience in the lee of the Rockies. 

There may be very slight adiabatic heated winds on the lee side of the mountain ridge in Nadi, but they are probably less extreme than the Rockies. The highest point sits at 4340' so there is height for the air to heat up on the decent. Sitting in the SPCZ is unique, being the largest water convergence zone and it causes most of the weather in Fiji. 

This shows the SPCZ during the rainy months of the year. It passes right through the South Pacific and shows the convergence area. It shows the very low sea pressure areas. Storms form with this convergence.  Image: http://www.niwa.co.nz/sites/default/files/imported/__data/assets/image/0010/37945/olr_600.gif

These factors explain why Fiji experiences the weather it does. Being a volcanic island with a large mountain ridge residing in the middle, with mT air masses year round, and residing the area of the SPCZ, the weather and climate of Fiji is governed by these factors.