On The Characteristics,formation And Simulations Of Boreal Winter Low Frequency Eddy Energies

Transient wave is the most significant feature of the midlatitude atmospheric cir-culations,playing important roles in driving and maintaining the extra-tropical climate variability.A wave is an eddy that satisfies,at least approximately,a dispersion re-lation.The low-frequency waves with the time scale of 10-90 days,are not only as important as the synoptic waves with comparable kinetic energy but also associated with much stronger available potential energies,especially in East Asia,North Amer-ica,and Barents-Kara Sea regions.Low-frequency waves also play an important role in activating and maintaining the variability of the atmospheric circulations and persistent heatwave,drought,and cold surge events.However,the research on the low-frequency wave is limited compare to the high-frequency wave.This study aims to investigate the dynamical mechanism for 10-90 days waves by applying energy budget,diagnos-ing observational data,and carrying out groups of idealized model experiments.Some numerical simulations have been carried out to investigate the effect of large-scale orog-raphy and diabatic heating on the low-frequency eddies.Our results show that:1.The formation of LF EAPE is proposed and further complement the eddy energies' cycleBased on the EKE budget,the local eddy available potential energy(EAPE)and its budget is developed to investigate the low frequency(LF,10-90 days)variations of temperature over the middle-and high-latitude in boreal winter.It is found that the action centers of LF eddy are located at North America,Barents-Kara Sea,and Eurasia,extending downstream along the j et stream.The budget analysis shows the main source of LF EAPE is generated by baroclinic generation term which is associated with the heat flux and baroclinic instability.Further analysis also shows the LF heat flux located downstream to the LF EKE.Applying the lagrangian track analysis,the work further demonstrates the long-lasting cyclones turn northeastward into the Greenland Sea and the cyclones over North Pacific turn northward into the Bering Strait.It is emphasized that the LF EKE enhances the LF baroclinic generation which implies the energy will transfer from LF EKE into LF EAPE.The study on LF EAPE plays a role in linking the mid-latitude temperature anomaly and high-latitude baroclinicity.Furthermore,the budget analysis also completes the Lorenz eddy energies cycle between 2-10 day and 10-90 days time scale.2.A mechanism that the LF EAPE response to the sea ice concentration(SIC)loss over Barents-Kara SeaBased on the LF EAPE formation,the effect of Arctic sea ice loss on mid-latitude LF EAPE is further investigated.Applying the interannual incremental time serial based on the SIC,the work proposes the possible mechanism that the LF EAPE en-hanced response to the SIC loss.Further analysis by the LF EAPE budget shows in-creased the LF EAPE is on account for the enhanced baroclinic generation.Then the generated LF EAPE transports downstream over Eurasia by the advection of basic flow.Moreover,the baroclinic generation is decomposed into the contribution by the meridional temperature gradient and zonal temperature gradient.The analysis shows the enhanced baroclinic generation over the Norwegian Sea and the Greenland Sea is attributed by the contribution of the meridional temperature gradient.But the enhanced baroclinic generation over the Barents-Kara Sea is attributed by the contribution due to the zonal temperature gradient.The Lagrangian analysis further shows the long-lasting tracks number increases significantly associated SIC loss.And the SST is also shown a positive anomaly in SIC-loss-year which lasting for a whole winter.3.A local-forcing-dependent mechanism of LF energy is proposedBased on the proposal that the baroclinic generation contributed by zonal temper-ature gradient is as important as the meridional temperature gradient,the work applies the idealized dry atmospheric model to investigate the mechanism of LF EAPE response to different zonal and meridional temperature gradients.Generally speaking,with in-creased local forcing the LF eddy energies enhanced sharply and the HF eddy energies increase significantly with increased background meridional temperature gradient.Applying the EAPE budget,the work shows both LF and HF EAPE is attributed by the baroclinic generation.With an increased zonal temperature gradient,the LF baroclinic generation contributed by the meridional and zonal temperature gradient both increases significantly.But for the HF EAPE,the baroclinic generation due to the zonal temperature gradient plays little effect.4.The ability of CMIP5 models to simulate LF EAPE in North Atlantic and Barents-Kara Sea is relatively weak.Based on the theoretical work on the LF EAPE formation and its response to the SIC loss,the ability of CMIP5 models simulating the LF EAPE is investigated by com-paring 23 models.Generally speaking,the climatological eddy energies in CMIP5 models have a high correlation(about 95%to observation and a similar amplitude.Further analysis of the energy budget terms shows about 70%correlation.The notable difference between observation and ensemble-averaged LF EAPE in CMIP5 is the energy generation over the Davis Strait,the Norwegian Sea,and the Barents-Kara Sea.Due to the notable lack of LF baroclinic generation contributed by the zonal temperature gradient,the climatological pattern over these regions is less intense.Therefore,this work proposes the hypothesis that the weak ability of CMIP5 simulating the LF EAPE response onto the SIC loss over the Barents-Kara Sea is owing to the weak ability simulating the zonal temperature gradient.