Kona winds are defined by a change in wind direction, representing a reversal from the prevailing northeasterly trade winds across the Hawaiian Islands. The term “kona” is a Hawaiian word meaning leeward, describing the south or southwest direction from which these winds originate. These episodic events transform the climate, bringing conditions different from the consistent weather patterns usually associated with the islands.
The Mechanism of Kona Wind Formation
The development of Kona winds is contingent upon the disruption of the semi-permanent North Pacific High pressure system. This vast high-pressure cell, normally situated to the northeast of Hawaii, is the atmospheric engine that drives the consistent trade winds. When this high-pressure system weakens or moves far to the north, the atmospheric conditions become favorable for a shift in the prevailing flow.
This shift is triggered by the approach of a low-pressure system, often a cold-core cyclone known as a “Kona low.” These systems usually develop in the North Central Pacific and track within approximately 500 miles northwest of the islands. The counter-clockwise circulation around this approaching low pressure draws in air from the south and southwest, directly opposing the usual northeasterly flow. This change in the pressure gradient forces the wind direction to reverse, initiating the Kona wind event.
Seasonal Frequency and Timing
Kona wind events occur most frequently during the cooler, wetter period of the year, generally spanning from October through April. This is when the influence of the North Pacific High is at its weakest. During the summer months, the trade winds are strongest and dominate the weather, making Kona wind occurrences rare.
Kona winds are episodic events that interrupt the trade wind regime. An individual event usually lasts only a few days to a week, though larger Kona low systems can influence the weather for longer periods. On average, the islands typically experience between two and three significant Kona low events annually during this cool season. This frequency varies year-to-year, with some winters passing without a single major event, while others may see four or five storms.
Distinct Weather Characteristics
The arrival of Kona winds shifts the weather pattern. The change in wind direction means that the normally dry leeward sides of the islands suddenly become the windward sides. These winds draw moisture from the warm tropical ocean, which is then forced up by the mountains, leading to heavy rainfall on the leeward coasts.
These events are often called Kona storms, bringing substantial precipitation, flash flooding, and strong, gusty winds. The wind flow can also create localized, extreme wind conditions, particularly where air is channeled through mountain gaps or funnels. The change in wind direction also causes an increase in humidity and warmer temperatures across the islands. The southerly flow often transports volcanic smog, or vog, from the Kilauea volcano on the Big Island across the other islands, resulting in hazy conditions until the trade winds return.