of the storms in the East Pacific and North Atlantic studied by Knaff and Kossin, all exhibited easterly winds and cold air in the upper troposphere. In addition to strong outflow, suggesting that the conditions that give rise to annular tropical cyclones are most optimal towards the equatorward side of a subtropical ridge and within the tropics. However, warmer sea surface temperatures (SSTs) are not required for annular tropical cyclones, with annular characteristics developing only within a narrow range of modest SSTs, ranging from . Conditions favorable for annular typhoon development in the Western North Pacific are localized within two areas within a zonal belt between 20°N−30°N; one of these areas lies over the central part of the basin, while the other is located east of Taiwan. Within the Eastern North Pacific, such conditions were present only 3 percent of the time between 1998 and 1999. In the same timeframe, the North Atlantic basin only exhibited conducive conditions for annular development 0.8 percent of the time. See also Tropical cyclogenesis Tropical cyclone forecasting Central dense overcast Coriolis force References Tropical cyclone meteorology