The Intraseasonal Oscillation and the Energetics of Summertime Tropical Western North Pacific Synoptic-Scale Disturbances

Abstract

The tropical intraseasonal oscillation (ISO) causes variations in the large-scale flow over the western North Pacific during June?August that strongly impact the energetics of tropical depression (TD)-type disturbances. An energetics analysis is conducted with NCEP?NCAR reanalysis data during June?August of 1979?2001. Composite TD-type disturbance perturbation kinetic energy (PKE) is significantly higher during ISO 850-hPa westerly periods than during ISO 850-hPa easterly periods. ISO westerly periods are associated with enhanced barotropic conversion and enhanced perturbation available potential energy (PAPE) to PKE conversion. ISO easterly periods are characterized by diminished TD-type PKE, negligible barotropic conversion, and weakened PAPE to PKE conversions, as compared to composite TD-type disturbances during ISO westerly periods and the entire June?August record. Barotropic conversion accounts for a larger fraction of the PKE generation during ISO westerly periods than during the entire June?August record, and vertically averaged barotropic conversion during ISO westerly periods is 3?4 times that during ISO easterly periods. Barotropic conversion during ISO westerly periods maximizes in the lower troposphere, coincident with the maximum in TD-type disturbance kinetic energy. PAPE to PKE conversion maximizes in the upper troposphere, where it is redistributed to the lower-troposphere and tropopause levels, and horizontally, by the perturbation geopotential flux. PAPE is primarily generated through convective heating associated with the TD-type disturbances and is converted to PKE through the negative correlation of pressure velocity and temperature. The effect of western Pacific ISO flow variations on the energy budgets of TD-type disturbances may help explain the ISO-related modulation of tropical cyclones observed by Liebmann et al. Energetic TD-type disturbances during ISO westerly periods may provide suitable seed disturbances from which tropical cyclones may form. June?August TD-type disturbance structure and energetics (unstratified by ISO phase) were compared to the results of Lau and Lau, who used a different analysis product, lower-resolution dataset, and shorter data record. TD-type disturbance structure and energetics are consistent with those shown in Lau and Lau. The largest deviation in the present analysis from that of Lau and Lau is the strong destruction of PKE found at 150 hPa, a level not resolved in their study. Although the sign of the 150-hPa signal is consistent with southwest?northeast-tilted TD-type disturbances interacting with strongly sheared easterly flow aloft, the nonlinear nature of the energy budget calculations may also amplify the effects of unrelated variability.