13:45 〜 15:15
[ACG30-P15] An Energetic Study on Atmospheric Internal and External Teleconnections over the North Pacific in Winter
キーワード：テレコネクション、エネルギー変換、大気内部変動
Low-frequency atmospheric variabilities exhibit spatial inhomogeneity. Studies have identified dominant teleconnection patterns with large magnitudes or strong remote covariability. The western Pacific (WP) pattern, North Pacific Oscillation (NPO), and the Pacific-North American (PNA) pattern are well-known teleconnection patterns over the wintertime North Pacific, which are characterized by a meridional dipole of height anomalies over the basin. Although the formation/maintenance mechanisms of those dominant teleconnection patterns have been examined in terms of external forcing (e.g., sea surface temperature variability) and atmospheric internal processes (e.g., energy conversion processes from the background climatological mean state), the relative importance of many possible processes has not been well understood. Another challenge is to explain why the dominant teleconnection patterns are dominant over other possible patterns, i.e., to comprehensively understand the key factors for those teleconnection patterns to dominate over the basin. This issue has not been addressed because previous studies focused on the mechanisms of a specific teleconnection pattern.
In this study, we systematically extracted 286 meridional teleconnection patterns anchored to various locations over the North Pacific in winter, from d4PDF large-ensemble simulations with an atmospheric general circulation model. We then evaluated efficiency of individual energy conversion processes with the climatological mean state or modulated high-frequency eddy activity. This efficiency is compared among different teleconnection patterns for comprehensive understanding on the energetics of those patterns.
For all the 286 internally-driven meridional teleconnection patterns obtained from d4PDF, net adiabatic energy conversion efficiency is positive, indicating that meridional teleconnection patterns thus obtained have structures that can gain energy from climatological mean flow and through modulations of high-frequency eddy activity. Among energy conversion processes, baroclinic energy conversion (CP), which arises from the vertically phase-tilted structures in height anomalies embedded in the baroclinic background state, has the largest contribution. Barotropic energy conversion (CK) plays a secondary role.
The net adiabatic efficiency is highest for the teleconnection patterns whose key zonal wind anomalies are located over the subtropical or subpolar North Pacific. This sensitivity of the efficiency on different teleconnection patterns mostly comes from that of CK plus CP. The subtropical efficiency maximum corresponds to the PNA pattern, while subpolar one corresponds to the WP pattern or NPO. Indeed, a systematic examination of the 286 patterns reveals that total energy (kinetic and available potential energy) associated with a pattern tends to be larger when the pattern has higher net adiabatic efficiency (Fig. a), which suggests that energy conversion from the extratropical background state determines dominant meridional teleconnection patterns.
By contrast, externally-driven teleconnection patterns obtained from d4PDF generally have lower net adiabatic efficiency compared to internal variability, mostly due to reduction in CP efficiency. Dominance in the external teleconnection patterns is not apparently determined by energy conversion efficiency (Fig. b). Instead, other processes such as energy influx from the tropics or inhomogeneous forcing from extratropical sea surface temperature variability are likely the key factor for the dominance.
In this study, we systematically extracted 286 meridional teleconnection patterns anchored to various locations over the North Pacific in winter, from d4PDF large-ensemble simulations with an atmospheric general circulation model. We then evaluated efficiency of individual energy conversion processes with the climatological mean state or modulated high-frequency eddy activity. This efficiency is compared among different teleconnection patterns for comprehensive understanding on the energetics of those patterns.
For all the 286 internally-driven meridional teleconnection patterns obtained from d4PDF, net adiabatic energy conversion efficiency is positive, indicating that meridional teleconnection patterns thus obtained have structures that can gain energy from climatological mean flow and through modulations of high-frequency eddy activity. Among energy conversion processes, baroclinic energy conversion (CP), which arises from the vertically phase-tilted structures in height anomalies embedded in the baroclinic background state, has the largest contribution. Barotropic energy conversion (CK) plays a secondary role.
The net adiabatic efficiency is highest for the teleconnection patterns whose key zonal wind anomalies are located over the subtropical or subpolar North Pacific. This sensitivity of the efficiency on different teleconnection patterns mostly comes from that of CK plus CP. The subtropical efficiency maximum corresponds to the PNA pattern, while subpolar one corresponds to the WP pattern or NPO. Indeed, a systematic examination of the 286 patterns reveals that total energy (kinetic and available potential energy) associated with a pattern tends to be larger when the pattern has higher net adiabatic efficiency (Fig. a), which suggests that energy conversion from the extratropical background state determines dominant meridional teleconnection patterns.
By contrast, externally-driven teleconnection patterns obtained from d4PDF generally have lower net adiabatic efficiency compared to internal variability, mostly due to reduction in CP efficiency. Dominance in the external teleconnection patterns is not apparently determined by energy conversion efficiency (Fig. b). Instead, other processes such as energy influx from the tropics or inhomogeneous forcing from extratropical sea surface temperature variability are likely the key factor for the dominance.