Barotropic and Nonlinear Decay of the Wintertime Baroclinic Wave Packets over the North Pacific: Energetics Analysis

Based on daily data from the Japanese 55-year Reanalysis (JRA-55) covering the winters (NDJFM) from 1958 to 2018, this study examines the growth and decay mechanisms of the baroclinic wave packet (BWP) inferred from regression analysis over the North Pacific. Day-to-day kinetic energy (KE) and available potential energy (APE) budget analysis suggest that BWP is driven mainly by baroclinic energy conversion (CPB), barotropic energy conversion (CKB), and the nonlinear term (CKE). CPB acts as a predominant APE source for BWP. Part of CPB acts to overcome the APE loss caused by transient eddy flux and most of it acts as a dominant KE source to drive BWP throughout its lifespan. CKB acts as a KE source before day 21, and as a major KE sink to damp BWP afterward, in which the north-south gradient of the climatological meridional flow plays a key role. Similarly, CKE acts as a KE source before day 0 and as a major KE sink afterward. The damping effect of CKE comes mainly from the scale interaction through the advection of highfrequency meridional momentum by the low-frequency zonal flow. It turns out that the vertical geopotential flux divergence also plays an active role in the dynamical coupling of different vertical BWP parts. There is persistent geopotential flux transfer from the middle-tropospheric layer into the lower- and upper-tropospheric layers, which serves as a major KE source to drive the BWP anomalies for the two layers and a major KE sink for the middle-tropospheric layer where the baroclinic energy conversion is the strongest.