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- Author or Editor: Mark Lander x
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Abstract
In its simplest description, the large-scale low-level circulation of summer over the western North Pacific Ocean can be described in terms of low-latitude southwesterlies, a monsoon trough, and a subtropical ridge. When the axis of the monsoon trough is in its normal orientation (NW-SE), tropical cyclones tend to move northwestward on tracks close to those expected from climatology. As an episodic event, the axis of the monsoon trough extends farther north and east than normal and acquires a reverse (SW-NE) orientation. When the monsoon trough becomes reverse oriented, tropical cyclones within it tend to exhibit north-oriented motion and other specific unusual motions such as eastward motion at low latitude and binary interactions with other tropical cyclones along the trough axis. Approximately 80% of the tropical cyclones that are associated with a reverse-oriented monsoon trough move on north-oriented tracks. A tropical cyclone track type, defined herein as the “S”-shaped track, is primarily associated with reverse orientation of the monsoon trough: 23 of 35 cases (66%) of S motion during the period 1978–94 occurred in association with a well-defined reverse-oriented monsoon trough.
Abstract
In its simplest description, the large-scale low-level circulation of summer over the western North Pacific Ocean can be described in terms of low-latitude southwesterlies, a monsoon trough, and a subtropical ridge. When the axis of the monsoon trough is in its normal orientation (NW-SE), tropical cyclones tend to move northwestward on tracks close to those expected from climatology. As an episodic event, the axis of the monsoon trough extends farther north and east than normal and acquires a reverse (SW-NE) orientation. When the monsoon trough becomes reverse oriented, tropical cyclones within it tend to exhibit north-oriented motion and other specific unusual motions such as eastward motion at low latitude and binary interactions with other tropical cyclones along the trough axis. Approximately 80% of the tropical cyclones that are associated with a reverse-oriented monsoon trough move on north-oriented tracks. A tropical cyclone track type, defined herein as the “S”-shaped track, is primarily associated with reverse orientation of the monsoon trough: 23 of 35 cases (66%) of S motion during the period 1978–94 occurred in association with a well-defined reverse-oriented monsoon trough.
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The evolution of the pattern of the deep convective cloud is presented for three selected cases of tropical cyclone twins symmetrical with respect to the equator. In each case, the pattern evolution is similar and can be separated into four distinct stages.
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The evolution of the pattern of the deep convective cloud is presented for three selected cases of tropical cyclone twins symmetrical with respect to the equator. In each case, the pattern evolution is similar and can be separated into four distinct stages.
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El Niño-Southern Oscillation (ENSO)-related variations in the observed character (e.g., annual storm totals, preferred genesis region, etc.) of the tropical cyclone activity in the western North Pacific are sought. With respect to annual storm totals, no ENSO signal is found; with respect to genesis region, a strong relationship is found between ENSO indices and the zonal displacement of the annual mean genesis locations. ENSO indices during the first half of the calendar year were found to be weakly predictive of the number and genesis locations of tropical cyclones occurring from July through December.
Abstract
El Niño-Southern Oscillation (ENSO)-related variations in the observed character (e.g., annual storm totals, preferred genesis region, etc.) of the tropical cyclone activity in the western North Pacific are sought. With respect to annual storm totals, no ENSO signal is found; with respect to genesis region, a strong relationship is found between ENSO indices and the zonal displacement of the annual mean genesis locations. ENSO indices during the first half of the calendar year were found to be weakly predictive of the number and genesis locations of tropical cyclones occurring from July through December.
Abstract
This paper describes the character and evolution of the low-level wind, sea level pressure, and satellite-observed cloudiness over the western North Pacific (WNP) during August 1991 when the low-level monsoon circulation there became organized as a monsoon gyre. The specific configuration of the monsoon circulation, which herein is called a monsoon gyre, is an episodic event—occurring roughly once per year, for two or three weeks during July, August, or September. As a monsoon gyre, the low-level circulation of the WNP becomes organized as a large cyclonic vortex associated with a nearly circular 2500-km-wide depression in the contours of the sea level pressure. A cyclonically curved band of deep convective clouds rims the southern through eastern periphery of this large vortex. Once this pattern is established, it becomes a prolific generator of mesoscale vortices that emerge from the downstream end of the major peripheral cloud band. These mesoscale vortices form the seed disturbances for midget or small-sized tropical cyclones. The large area encompassed by the outermost closed isobar of the monsoon gyre of August 1991 (the centroid of which moved slowly westward along 20°N) was the site of genesis for two tropical depressions, two tropical storms, and two typhoons during its 20-day westward journey. Initially, small tropical cyclones formed in the peripheral circulation of the gyre and later, the gyre itself evolved into a very large tropical cyclone; this is suggestive of two distinct modes of tropical cyclogenesis: one mode operates to produce small tropical cyclones in the eastern periphery of the gyre, and the other mode operates to accelerate the winds of the monsoon gyre until it becomes a giant tropical cyclone.
Abstract
This paper describes the character and evolution of the low-level wind, sea level pressure, and satellite-observed cloudiness over the western North Pacific (WNP) during August 1991 when the low-level monsoon circulation there became organized as a monsoon gyre. The specific configuration of the monsoon circulation, which herein is called a monsoon gyre, is an episodic event—occurring roughly once per year, for two or three weeks during July, August, or September. As a monsoon gyre, the low-level circulation of the WNP becomes organized as a large cyclonic vortex associated with a nearly circular 2500-km-wide depression in the contours of the sea level pressure. A cyclonically curved band of deep convective clouds rims the southern through eastern periphery of this large vortex. Once this pattern is established, it becomes a prolific generator of mesoscale vortices that emerge from the downstream end of the major peripheral cloud band. These mesoscale vortices form the seed disturbances for midget or small-sized tropical cyclones. The large area encompassed by the outermost closed isobar of the monsoon gyre of August 1991 (the centroid of which moved slowly westward along 20°N) was the site of genesis for two tropical depressions, two tropical storms, and two typhoons during its 20-day westward journey. Initially, small tropical cyclones formed in the peripheral circulation of the gyre and later, the gyre itself evolved into a very large tropical cyclone; this is suggestive of two distinct modes of tropical cyclogenesis: one mode operates to produce small tropical cyclones in the eastern periphery of the gyre, and the other mode operates to accelerate the winds of the monsoon gyre until it becomes a giant tropical cyclone.
Abstract
The observed tendency for tropical cyclones to meander about a longer-term track with periods of several days and amplitudes around 100 km is investigated. An analysis of 26 cyclones in the western North Pacific Ocean does not support the theories by Syono and Futi that tropical cyclone track oscillations occur from excitation of inertial oscillations. The observations and related numerical modeling studies also do not support the vortex patch and rotating cylinder theories by Yeh and Kuo. It is suggested that many meanders occur from interactions with mesoscale vortices and convective systems within the cyclone circulation. This hypothesis is supported by a case study of the effects of mesoscale convective complexes that developed in Typhoon Sarah (1989).
Abstract
The observed tendency for tropical cyclones to meander about a longer-term track with periods of several days and amplitudes around 100 km is investigated. An analysis of 26 cyclones in the western North Pacific Ocean does not support the theories by Syono and Futi that tropical cyclone track oscillations occur from excitation of inertial oscillations. The observations and related numerical modeling studies also do not support the vortex patch and rotating cylinder theories by Yeh and Kuo. It is suggested that many meanders occur from interactions with mesoscale vortices and convective systems within the cyclone circulation. This hypothesis is supported by a case study of the effects of mesoscale convective complexes that developed in Typhoon Sarah (1989).
