Based on daily data from the Japanese 55-year Reanalysis (JRA-55) covering the springs from 1958 to 2018, this study examines the formation mechanisms and climate impacts of springtime western Pacific (WP) pattern as subseasonal climate variability over North Pacific. Results suggest that the springtime WP pattern arises from a weak dipole-like disturbance over North Pacific and disturbances over East Asia. The energetic analysis suggests that the baroclinic energy conversion acts as an important energy source to balance the available potential energy loss caused by transient eddies and diabatic heating and acts as a kinetic energy (KE) source for the WP pattern. For the feedback forcing by total transient eddies, it acts as a major KE source for the WP pattern before day 0 and acts as a strong KE sink after day 0. It turns out that the barotropic energy conversion makes only weak KE contribution to the WP pattern.Once the WP pattern forms, East Asia and North America experience strong surface air temperature anomalies of opposite signs, while strong sea surface temperature anomalies are found to occur over mid-latitude and tropical North Pacific at the same time. Concurrently, the Pacific jet and the storm track shift north-southward around their climatological position. In addition, a dipole-like shallow convective anomaly appears over mid-latitude North Pacific, and a band of anomalous deep convection tends to occur in the tropics as the energy of the WP pattern propagates into the region.

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Based on unfiltered daily Japanese 55 year reanalysis covering the 60 winters in 1958–2018, a new teleconnection pattern called the zonal North Pacific Oscillation (ZNPO) pattern has been detected. The ZNPO pattern describes a mass oscillation in the troposphere between the eastern and western North Pacific, persisting for a week or so. It is shown that the ZNPO pattern is a high-impact teleconnection pattern that brings the wintertime North Pacific and North America severe weather and hydroclimate events. It may cause rapid surface air temperature drop or rise over the northern North Pacific and North America, remarkable sea ice concentration anomalies over the northeastern Bering Sea, and strong convective anomalies in the lower troposphere over the eastern and western midlatitude North Pacific. The ZNPO pattern arises from two westward-moving geopotential height disturbances over the North Pacific and North America and is driven mainly by baroclinic energy conversion and feedback forcing by transient eddies. The baroclinic energy conversion acts to overcome the available potential energy (APE) loss caused by the heat flux of transient eddies and at the same time acts as a major kinetic energy (KE) source to maintain the ZNPO pattern. The barotropic feedback forcing by transient eddies acts as a major KE source to drive the ZNPO pattern during the growing stage of the ZNPO pattern and as a major KE sink to heavily damp the ZNPO pattern during the decaying stage.

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 −1, and as a major KE sink to damp BWP afterwards, 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 afterwards. The damping effect of CKE comes mainly from the scale interaction through the advection of high-frequency 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.