The annual mean number of tropical cyclones in the Central North Pacific (CNP) is approximately three. Although this number is low in comparison to other basins, the interannual variability of tropical cyclones, which includes tropical storms and hurricanes, is high. For the period 1966-1997, the annual number ranges from 0 to 10 tropical cyclones.There is a large and positive correlation between CNP tropical cyclone counts and El Niñ o 3.4 region SST anomalies with a 95% significance level. In the El Niñ o Hurricane Season (ENHS), a greater number of cyclones formed in the CNP and more cyclones propagated into this area from the east. The monsoon trough, low level relative vorticity, and tropospheric vertical wind shear in the CNP undergo pronounced changes during warm and cold phases of ENSO.For instance, the 1000 hPa relative vorticity values within the CNP in an El Niñ o autumn composite are double the values in a corresponding La Niñ a composite. The El Niñ o autumn composite of tropospheric vertical wind shear shows a two to three times reduction equatorward of 16 N-17 N when compared to the La Niñ a autumn composite. The increased values of the dynamic potential term in Gray's (1977) seasonal genesis parameter correspond well with the increased cyclone frequency in the CNP for an ENHS composite. Furthermore, a majority of initial detection points of named storms is found within a band of relatively large values of dynamic potential. This suggests that this term can be used to diagnose favorable areas for tropical cyclogenesis on a seasonal time scale.

Rotating yarn loops, which are called yarn balloons in the textile industry, play an important role in establishing yarn tension in textile yarn-manufacturing processes such as ring spinning and two-for-one twisting. Recent theoretical work has brought the computational simulation of these processes to a high degree of refinement.In this paper, a simple experimental system, consisting of a loop of yarn rotating about a fixed axis, without twist insertion, is described. This system exhibits a rich variety of bifurcation behaviours as the length of yarn in the loop is varied.It is shown that the theoretical bifurcation curves (which plot guide-eye tension versus the unstretched yarn length in the rotating loop) can be fitted to the experimentally obtained curves by an appropriate choice for the value of the air-drag parameter. For the almost inextensible yarns considered here, the value of the elasticity parameter has only a very slight effect on the theoretical results.In particular it is shown that the 'fluttering' oscillations of the experimental balloons, corresponding to certain sections of the experimental bifurcation curve, can be identified with the limit-cycle behaviour of the theoretical balloons between Hopf bifurcation points on the theoretical bifurcation curve.

Tropical cyclone activity (tropical depressions, tropical storms, and hurricanes combined) in the central North Pacific has been found to be on the rise and this increase amounts to about 3.2 cyclones over the last 32 years (1966-97). An examination of time series of tropical storms and hurricanes and hurricane records alone also reveals an increasing trend in both series since 1966. The upward trend is characterized by decadal-scale variability as manifested by fewer cyclones during the first half of the record (1966-81) and more during the second half of the record (1982-97). The maximum hurricane intensity has also increased in the central North Pacific, as well as the number of intense hurricanes from the first to the second half of the record. Relative to 1966-81, sea surface temperatures in the tropical Pacific and relative vorticity near the surface to the south of Hawaii have increased dramatically during 1982-97. The changes in the frequency and intensity of tropical cyclones in the central North Pacific appear to be modulated by decadal-scale variability of the basic state of SST, which transitioned from a cold to a warm phase in the late 1970s; this warm phase may have resulted in stronger and more frequent El Nino events as seen during the second half of the record, leading to more cyclones in the central North Pacific.

In this paper a simple experimental system consisting of a length of cable, fixed to the edge of a rotating disc at its upper end, and free at its lower end or with a point mass ( drogue ) attached there, is described. This system exhibits a rich variety of bifurcation behaviours as the length of cable, angular speed of the fixed end, mass of the drogue and elasticity of the cable is varied. Bifurcation diagrams for the quasistationary configurations (cable shapes that appear stationary with respect to the rotating reference frame) are described. Linearized stability analyses of these quasistationary balloons are compared with solutions to the full time–dependent equations of motion. It is shown that there is an exchange of stability at the turning points of the quasi–stationary bifurcation curves, and that Hopf bifurcations occur at otherwise undistinguished points of these curves. It is shown that limit–cycle oscillations of the system occur at angular speeds corresponding to points on the bifurcations curves in the neighbourhood of the Hopf bifurcation points. These oscillations have been observed experimentally.