We compare vertical wind fluctuations observed by VHF radar wind profilers in the tropical troposphere over a large, mountainous island (Pohnpei, at 7°N, 158°E) and a large, low-profile atoll (Christmas Island, at 2°N, 157°W). The major difference in the two datasets appears to be due to the frequent occurrence of mountain waves over Pohnpei and to their absence over Christmas Island. These waves are generated continuously at low levels over Pohnpei by the steady ENE trade winds acting on the 800 m high terrain, and can extend at least into the lower stratosphere. We find the occurrence of mountain waves at middle- and upper-tropospheric levels over Pohnpei to be governed primarily by a “critical-layer” relationship between the winds aloft and the near-surface winds: a reversal in the direction of the upper level winds relative to the lower-level trade winds precludes the upward propagation of mountain waves, whereas a nonreversed wind profile allows the waves to propagate freely into the lower stratosphere, and possibly to much higher heights, depending on the phase of the QBO.

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