角運動量、1日の長さ、および季節内低周期現象

Abstract

In this paper some global aspects of the intraseasonal oscillations on the time scale of 30 to 50 days are explored. Noting that the variability of zonal flow of the monsoon, the atmospheric angular momentum and the length of day are strongly correlated on this time scale, we have made an effort to examine the global variability using the length of day as a point of reference. The scenario of this cycle is presented starting from a super cloud cluster at the near equatorial latitudes. This seems to be accompanied with an acceleration of zonal flows, an increase of the atmospheric angular momentum and an increase in the length of day. The transfer of westerly angular momentum from the earth to the atmosphere occurs over regions of the surface easterlies to the east of the super cloud clusters resulting in an increase in the length of day. During this transition from a mean length of day to a maximum length of day, an active phase of the Indian summer monsoon is noted. The interesting aspect of the length of day transition occurs on its return cycle when the near equatorial cloud cover eases or moves away from the equator with a decrease in the monsoonal zonal flows and a reduction of this component of atmospheric angular momentum. The length of day does not simply go back to an equilibrium value, but the long term data from the laser ranger shows an overshooting beyond that to a minimum value. This transition is characterized in general by monsoon break-like conditions, counter monsoon flows in the low levels and by a transition from high index to low index conditions in the upper troposphere of the middle latitudes. Phenomenologically, some blocking situations have been noted over the higher middle latitudes during this transition. The reduction of the angular momentum is attributed to the transfer of the westerly angular momentum from the atmosphere to the earth via frictional and mountain torques. These torques exhibit a clear relationship to the changes in the atmospheric angular momentum on this time scale. The behavior of the middle latitude low frequency variability is also in part explained by the meridional wave energy flux. That problem is examined in this context with the full non-linear equations in the frequency domain. It is shown that unlike the linear problems where such fluxes are inhibited beyond the critical latitude, the nonlinear problem permits the temporal oscillations of zonal flows on this time scale. As a consequence, a significant tropical-middle latitude coupling is noted by this process.A simple mathematical model of the oscillation is also presented. It is a local theory in which ocean and atmosphere interact. Initially, the atmosphere is stably stratified with weak winds at the sea surface and stronger winds aloft; the ocean has a surface mixed layer of temperature Ts lying over deep cold water. Solar heating gradually increases Ts which leads to atmospheric convection with associated transport of horizontal momentum and increased winds at the sea surface. Increased winds lead to deepening of the mixed layer and a drop in Ts because of mixing of deep cold water with surface waters. Convection ceases, winds decay, and the cycle repeats only after solar heating has once more increased Ts. The period of this oscillation is shown to be on the order of 30 days.