Cyclones in the Pacific

Cyclones in the Pacific
  • Credit:

    Image produced by Hal Pierce (SSAI/NASA GSFC) and caption by Steve Lang (SSAI/NASA GSFC).

Two cyclones, Olaf a powerful Category 4 cyclone and Nancy a weaker but still strong Category 2 cyclone, are together threatening the Cook Islands and the islands of Samoa in the South Pacific. This spectacular image from TRMM, the Tropical Rainfall Measuring Mission satellite, shows Cyclone Olaf in the upper left as it is approaching the islands of Samoa and Cyclone Nancy in the lower right as it heads towards Rarotonga in the Cook Islands.

The image was taken at 15:34 UTC on February 15, 2005, and shows the horizontal distribution of rain intensity as seen from above by the TRMM satellite. Rain rates in the center swath are from the TRMM Precipitation Radar (PR), the first and only precipitation radar in space, while rain rates in the outer swath are from the TRMM Microwave Imager (TMI). The rain rates are overlaid on infrared (IR) data from the TRMM Visible Infrared Scanner (VIRS).

At the time of this image, Olaf was a Category 4 storm with maximum sustained winds estimated at 120 knots (138 mph) by the Joint Typhoon Warning Center and was in the process of intensifying. Nancy, meanwhile, was a Category 2 storm with maximum sustained winds estimated at 95 knots (109 mph) and was beginning to weaken. These characterizations are supported by TRMM, which shows that Olaf has much better banding in the rain field as evidenced by the green arcs of moderate rain intensity and a tight, closed eye (green circle). Nancy exhibits no well-defined eye, and the banding is much less evident.

When two cyclones come within close proximity and their circulations begin to interact with each other, two possible interactions can occur. One interaction is known as the Fujiwara effect, wherein the cyclones begin circling around each another. The other possibility is that the outflow from one storm impedes the outflow from the other storm, weakening it.

TRMM is a joint mission between NASA and the Japanese space agency JAXA.

Images & Animations


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  • Data Date:

    February 15, 2005
  • Visualization Date:

    February 16, 2005
  • Sensor(s):

NASA - National Aeronautics and Space Administration