The role of scale interactions in the maintenance of eddy kinetic energy (EKE) during the extreme phases of the intraseasonal oscillation (ISO) is examined through the construction of a new eddy energetics diagnostic tool that separates the effects of ISO and a low-frequency background state (LFBS; with periods longer than 90 days). The LFBS always contributes positively toward the EKE in the boreal summer, regardless of the ISO phases. The synoptic eddies extract energy from the ISO during the ISO active phase. This positive barotropic energy conversion occurs when the synoptic eddies interact with low-level cyclonic and convergent-confluent ISO flows. This contrasts with the ISO suppressed phase during which the synoptic eddies lose kinetic energy to the ISO flow. The anticyclonic and divergent-diffluent ISO flows during the suppressed phase are responsible for the negative barotropic energy conversion.A positive (negative) EKE tendency occurs during the ISO suppressed-to-active (active-to-suppressed) transitional phase. The cause of this asymmetric EKE tendency is attributed to the spatial phase relation among the ISO vorticity, eddy structure, and EKE. The southwest-northeast-tilted synoptic disturbances interacting with cyclonic (anticyclonic) vorticity of ISO lead to a positive (negative) EKE tendency in the northwest region of the maximum EKE center.The genesis number and location and intensification rate of tropical cyclones in the western North Pacific are closely related to the barotropic energy conversion. The enhanced barotropic energy conversion favors the generation and development of synoptic seed disturbances, some of which eventually grow into tropical cyclones.

By analyzing observation-based high-resolution surface air temperature (SAT) data over the Asian monsoon region (here called “monsoon Asia”) for 1981–2007, the modulation by boreal summer intraseasonal oscillation (BSISO) of heat wave (HW) occurrence is examined. Strong SAT variability and a high probability of HW occurrence on intraseasonal time scales are found consistently in the densely populated regions over central India (CI), the Yangtze River valley in China (YR), Japan (JP), and the Korean Peninsula (KP). The two distinct BSISO modes (30–60-day BSISO1 and 10–30-day BSISO2) show different contributions to HW occurrence in monsoon Asia. A significant increase in HW occurrence over CI (YR) is observed during phases 2–3 (8–1) of BSISO2 when the 10–30-day anticyclonic and descending anomaly induce enhanced upward thermal heating and sensible heat flux (warm advection) around the areas. On the other hand, the northeastward propagating BSISO1 is closely connected to the increased HW probability over JP and KP. During phases 7–8 of BSISO1, the 30–60-day subsidence along with the low-level anticyclonic anomaly moves into northeastern Asia, leading to enhanced diabatic (adiabatic) warming near surface in JP (KP). Analysis of a three-dimensional streamfunction tendency equation indicates that diabatic cooling induced by the BSISO-related suppressed convections is the main forcing term of anticyclonic anomaly although it is largely offset by the decreased static stability associated with adiabatic warming. The BSISO-related vorticity advection leads to an anticyclonic (cyclonic) tendency to the northwestern (southeastern) part of the center of anticyclonic anomaly, favoring northwestward development of the BSISO anomalous circulations and thus providing a favorable condition for HW occurrence over the western Pacific–East Asia sector.

A High Resolution Atmospheric Model (HiRAM) at 20-km resolution is adopted to simulate tropical storm (TS) activity over the western North Pacific (WNP) and Taiwan/East Coast of China (TWCN) at the present time (1979 -2003) and future climate (2075 -2099) under the Intergovernmental Panel on Climate Change (IPCC) fifth assessment report (AR5) representative concentration pathway (RCP) 8.5 scenarios. The results show that in contrast to TS simulation activities in most of the low-resolution climate models, TS activities except intensity over the WNP and TWCN region are well simulated by HiRAM at 20-km resolution. The linkage between large-scale environments and TS genesis simulated by HiRAM are dramatically superior to those in low-resolution fifth Coupled Model Intercomparison Project (CMIP5) models. During 2075 -2099, both TS genesis numbers and TS frequency over the WNP and TWCN are projected to decrease consistent with the IPCC AR5 report. However, the rate of decrease (49%) is much greater than that projected in IPCC AR5. The decrease in TC genesis numbers under global warming is primarily attributed to the reduction in mid-level relative humidity and large-scale ascending motion, despite the warmer sea surface temperature (SST) providing more favorable conditions for TS formation. TS intensity and the maximum precipitation rate are projected to increase under global warming. At the end of the 21 st century, the mean precipitation rate within 200 km of TS storm center over the TWCN region is projected to increase by 54%.

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This study formulates a synoptic-scale eddy (SSE) kinetic energy equation by partitioning the original field into seasonal mean circulation, intraseasonal oscillation (ISO), and SSEs to examine the multiscale interactions over the western North Pacific (WNP) in autumn. In addition, the relative contribution of synopticmean and synoptic-ISO interactions to SSE kinetic energy was quantitatively estimated by further separating barotropic energy conversion (CK) into synoptic-mean barotropic energy conversion (CK S2M ) and synoptic-ISO barotropic energy conversion (CK S2ISO ) components.The development of tropical SSE in the lower troposphere is mainly attributed to CK associated with multiscale interactions. Mean cyclonic circulation in the lower troposphere consistently provides kinetic energy to SSEs (CK S2M . 0) during the ISO westerly and easterly phases. However, CK S2ISO during the ISO westerly and easterly phases differs considerably. During the ISO westerly phase, the enhanced ISO cyclonic flow converts energy to SSEs (CK S2ISO . 0). The magnitude of the downscale energy conversion from mean and ISO to SSEs is related to the strength of the SSEs. During the ISO westerly phase, a stronger SSE extracts more kinetic energy from mean and ISO circulation. This positive feedback between SSE-mean and SSE-ISO interactions causes further strengthening of SSEs during the ISO westerly phase.By contrast, upscale energy conversion from SSEs to ISO anticyclonic flow (CK S2ISO , 0) was observed during the ISO easterly phase. The weaker SSE activity during the ISO easterly phase occurred because the mean circulation provides less energy to SSEs and, at the same time, SSEs lose energy to ISO during the ISO easterly phase. The two-way interaction between the ISO and SSEs has considerable effects on the development of tropical SSEs over the WNP in autumn.

This study investigates the northward, and northwestward propagation of 30-60 day oscillation over the western North Pacific (WNP) at upper, and low levels with a three-dimensional streamfunction tendency equation. In the tropical WNP, the surface frictional effect associated with the cyclonic circulation enhances the low-level convergence at the cyclonic vorticity center to the northwest of the convection, causing the 30-60 day convection to develop northwestward. The vorticity advection induces the 30-60 day circulation at upper and low levels to propagate northwestward, with a baroclinic structure. The combined effect of surface frictional-diabatic heating, and vorticity advection, causes the 30-60 day convection and circulation to develop and propagate simultaneously northwestward. After the convection fully develops, increased static stability, associated with adiabatic cooling, reduced solar radiation due to the cloud-radiation effect, and negative land-surface feedbacks on moisture availability, restrict any further development of the 30-60 day convection.A wave train emanating from the South China Sea/western North Pacific (SCS/ WNP) into the extratropical North Pacific, is well established 15-days after the convection reached maximum intensity over the SCS/ WNP. The main process and mechanism responsible for the northwestward propagation of this 30-60 day oscillation in the extratropical WNP is similar to the process proposed for the tropical WNP, except that in the mid-latitudes where the coriolis parameter becomes large, the influence of upper-level vorticity advection extends down to the low levels.

The future changes in the tropical cyclone (TC) intensity and frequency over the western North Pacific (WNP) under global warming remain uncertain. In this study, we investigated such changes using 20-km resolution HiRAM and MRI models, which can realistically simulate the TC activity in the present climate. We found that the mean intensity of TCs in the future (2075−2099) would increase by approximately 15%, along with an eastward shift of TC genesis location in response to the El-Niño like warming. However, the lifetime of future TCs would be shortened because the TCs tend to have more poleward genesis locations and move faster due to a stronger steering flow related to the strengthened WNP subtropical high in a warmer climate. In other words, the enhancement of TC intensity in future is not attributable to the duration of TC lifetime.To understand the processes responsible for the change in TC intensity in a warmer climate, we applied the budget equation of synoptic-scale eddy kinetic energy along the TC tracks in model simulations. The diagnostic results suggested that both the upper level baroclinic energy conversion (CE) and lower-level barotropical energy conversion (CK) contribute to the intensified TCs under global warming. The increased CE results from the enhancement of TC-related perturbations of temperature and vertical velocity over the subtropical WNP, whereas the increased CK mainly comes from synoptic-scale eddies interacting with enhanced zonal-wind convergence associated with seasonal mean and intraseasonal flows over Southeast China and the northwestern sector of WNP.

The interaction between the seasonal mean circulation and the transient eddies over the western North Pacific (WNP) during El Niñ o-Southern Oscillation (ENSO) warm and cold years was investigated by the threedimensional eddy kinetic energy (EKE) and eddy available potential energy (EAPE) budget equations for total eddy, high-frequency (< 10 days) and low-frequency (20-70 days) components. Composites of the energy results indicate that low-level anomalous cyclonic circulation, westerly jet and ascending motion associated with the eastward extension of warm SST during warm ENSO years are favorable for eddy barotropic energy conversion (CK) and eddy baroclinic energy conversions (CE). The enhancement of CK and CE might provide kinetic energy for the growth of high-and low-frequency transient eddies including tropical storms (TSs) from the Philippine Sea to the date line over the tropical WNP during warm ENSO years. In contrast, high-and low-frequency eddies convert EKE to seasonal mean circulation over the subtropical and mid-latitude WNP during warm years. Enhanced eddy baroclinic energy conversion plays an important role in the maintenance and enhancement of the subsequent development of transient eddies including TSs as they propagate northward.The loss of EAPE to EKE due to the eddy baroclinic energy conversion is mainly supplemented by the generation of EAPE associated with eddy diabatic heating. However, the energy conversion from mean available potential energy (MAPE) to EAPE is also important due to the eddy vertical heat transport which is neglected in the two-dimensional EAPE budget equation. It is suggested that high-and low-frequency eddies including TSs may be self-development and intensify through their enhanced diabatic heating and vertical heat transport.