Tropical Cyclone Pola Gets 'Cracked' in the South Pacific

The precarious feature, called a standing wave, appeared in Tropical Cyclone Pola on Tuesday as the storm hovered east of Fiji.

Standing waves are ripples in the atmosphere that stay in one spot for long periods of time. Remember that the atmosphere is essentially a fluid layer of the Earth that works much the same way the ocean does.

When talking about standing waves, we're usually looking at a lenticular cloud that forms when the atmosphere's liquid is forced up and over mountains. They "stand" still because the mountains don't move, and only dissipate when winds die down.

But standing waves can also form in other ways.

In this case, a wave in the atmosphere, called a gravity wave, approached Pola from the west until it couldn't get any closer to the storm due to strong upper level winds.

Winds at 30,000 feet and higher are flowing out of a hurricane or tropical cyclone and away from its eye. In this case, the winds were flowing westward at cloud-top level.

These winds slowed down the gravity wave and caused it to stand still until it dissipated about 15 hours after it formed.

How This Tropical Cyclone 'Cracked'

This inflow was moving from southwest to northeast near Fiji, according to weather balloons that were launched that morning.

The difference in wind directions from eastward winds in the lower levels of the atmosphere to westward winds higher up incited a horizontal roll.

Whenever you see this kind of rolling atmosphere, one side of the roll must have upward-moving air and the other side has to have sinking air.

In this case, the crack develops in the sinking air on the westward side of the roll.

Clouds, moisture and air flowed westward through the crack, but in a diminished state, as you can see by the slightly warmer and lower cloud tops on the left side of the image above.

The stronger the storm, the higher its clouds will tower and the colder its cloud tops will be.

You can see the storms west of the crack (or to the left of the image) are actually weaker than those east of it. This was caused by some sinking air incurred by the combination of the horizontal roll in the atmosphere and the standing wave itself.

This effect would have led to less rainfall west of the standing wave, or generally less rainfall for Fiji for the time that the standing wave was active.