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Linear and nonlinear barotropic vorticity model frameworks are constructed to understand the formation of the monsoon trough in boreal summer over the western North Pacific. The governing equation is written with respect to specified zonal background flows, and a wave perturbation is prescribed in the eastern boundary. Whereas a uniform background mean flow leads no scale contraction, a confluent background zonal flow causes the contraction of zonal wavelength. Under linear dynamics, the wave contraction leads to the development of smaller scale vorticity perturbations. As a result, there is no upscale cascade. Under nonlinear dynamics, cyclonic (anticyclonic) wave disturbances shift northward (southward) away from the central latitude due to the vorticity segregation process. The merging of small-scale cyclonic and anticyclonic perturbations finally leads to the generation of a pair of large-scale cyclonic and anti-cyclonic vorticity gyres, straddling across the central latitude. The large-scale cyclonic circulation due to nonlinear upscale cascade can be further strengthened through a positive convection-circulation feedback.

The present study investigates relative contributions of different time-scale variations of environmental factors to the tropical cyclone (TC) genesis over the western North Pacific (WNP) during July–August–September–October (JASO). Distinct from previous studies that are concerned with large-scale spatial patterns during a certain period, the present study focuses on local and instantaneous conditions of the TC genesis. Analysis shows that the contribution of convection and lower-level vorticity to the TC genesis is mainly due to intraseasonal and synoptic variations. The contribution of vertical wind shear is largely related to synoptic variations. The contribution of midlevel specific humidity is almost 2 times more from intraseasonal variations than from synoptic variations. The contribution of sea surface temperature (SST) to the TC genesis is mainly due to interannual and intraseasonal variations. The barotropic energy for synoptic-scale disturbances during the TC genesis comes mainly from climatological mean flows over the southwest quadrant and from intraseasonal wind variations over the northeast quadrant of the WNP, respectively. The contribution of interannual variations to the TC genesis is enhanced over the southeast quadrant of the WNP. More TCs form under weak easterly and westerly vertical shears, respectively, during El Niño developing and decaying JASO. The contribution of interannual variations of SST tends to be larger during El Niño decaying than during developing JASO.

The ARW Model is used to investigate the sharp northward turn of Super Typhoon Megi (2010) after it moved westward and crossed the Philippines. The NCEP analyzed fields during this period are separated into a slowly varying background-flow component, a 10–60-day low-frequency component representing the monsoon gyre, and a 10-day high-pass-filtered component representing Megi and other synoptic-scale motion. It appears that the low-frequency (10–60 day) monsoon gyre interacted with Megi and affected its track. To investigate the effect of the low-frequency mode on Megi, numerical experiments were designed. In the control experiment, the total fields of the analysis are retained in the initial and boundary conditions, and the model is able to simulate Megi’s sharp northward turn. In the second experiment, the 10–60-day monsoon gyre mode is removed from the initial and lateral boundary fields, and Megi moves westward and slightly northwestward without turning north. Tracks of the relative positions between the Megi and the monsoon gyre centers suggest that a Fujiwhara effect may exist between the monsoon gyre and Megi. The northward turning of both Megi and the monsoon gyre occurred when the two centers were close to each other and the beta drift was enhanced. A vorticity budget analysis was conducted. It is noted that the Megi moves toward the maximum wavenumber-1 vorticity tendency. The sharp change of the maximum vorticity tendency direction before and after the track turning point is primarily attributed to the change of the horizontal vorticity advection. A further diagnosis shows that the steering of the vertically integrated low-frequency flow is crucial for the change of the horizontal advection tendency.

The impact of different vertical structures of a nearby monsoon gyre (MG) on a tropical cyclone (TC) track is investigated using idealized numerical simulations. In the experiment with a relatively deeper MG, the TC experiences a sharp northward turn at a critical point when its zonal westward-moving speed slows down to zero. At the same time, the total vorticity tendency for the TC wavenumber-1 component nearly vanishes as the vorticity advection by the MG cancels the vorticity advection by the TC. At this point, the TC motion is dominated by the beta effect, as in a no-mean-flow environment, and takes a sharp northward turn. In contrast, the TC does not exhibit a sharp northward turn with a shallower MG nearby. In the case with a deeper MG, a greater relative vorticity gradient of the MG promotes a quicker attraction between the TC and MG through the vorticity segregation process. In addition, a larger outer size of the TC also favors a faster westward propagation from its initial position, thus having more potential to collocate with the MG. Once the coalescence is in place, the Rossby wave energy dispersion associated with the TC and MG together is enhanced and rapidly strengthens the southwesterly flow on the eastern flank of both systems. The steering flow from both the beta gyre and the Rossby wave dispersion leads the TC to take a sharp northward track when the total vorticity tendency is at its minimum. This study indicates the importance of good representations of the TC structure and its nearby environmental flows in order to accurately predict TC motions.

This study investigates the relative contributions of components of flows on multiple time scales to westward and eastward-moving tropical cyclones (TCs) over the South China Sea (SCS) from a local and instantaneous perspective of TC position along the TC tracks during June to October from 1965 to 2015. The total steering flows obtained by vertical integration of winds from 850 to 300 hPa averaged along the entire TC tracks are separated into climatological mean flows, interannual, intraseasonal, and synoptic time scales. For westward-moving TCs, the zonal steering flows are contributed dominantly by climatological mean easterly, whereas the meridional steering flows are contributed dominantly by climatological mean and synoptic-scale southerly winds. In contrast, for eastward-moving TCs, the zonal steering flows are contributed equally by the intraseasonal and synoptic components, while the meridional steering flows are contributed dominantly by climatological mean southerly winds along with the secondary contribution of synoptic-scale southerly winds. This work provides a better understanding of relative contributions of different time scale components of steering flows to the TC motions over the SCS.

The prediction of the tropical cyclone (TC) intensity remains a difficult issue. This study analyzes local instantaneous barotropic kinetic energy conversion in the lower troposphere in rapid intensifying (RI) and rapid decaying (RD) TCs over the western North Pacific (WNP) during June through November from 1979 to 2017. The kinetic energy conversion to the synoptic eddies is separated for climatological mean, interannual and intraseasonal flows. It is found that the intraseasonal cyclonic flows display a northwest‐southeast orientation in the RI TCs, but a circular feature in the RD TCs. Intraseasonal and climatological mean zonal flows contribute together to the positive kinetic energy conversion in the northwest quadrant of the RI TCs. Intraseasonal meridional flows induce positive (negative) kinetic energy conversion in the northeast (south) part of the RD TCs. The kinetic energy conversion associated with interannual flows is small for both the RI and RD TCs. Plain Language Summary The damage caused by the tropical cyclones (TCs) is closely related to their intensity. Revealing the factors of the intensification of the TCs may help to improve the skill of prediction of the TC intensity and reduce the damage of the TCs. From the energetic point of view, the intensification of the TCs requires the conversion of energy from the background flows. Here, we compare the barotropic kinetic energy conversion in the rapid intensifying (RI) and decaying (RD) TCs over the western North Pacific during June through November. Our analysis reveals that the intraseasonal flows have the largest contribution to both the RI and decaying TCs through the kinetic energy conversion to the synoptic scale eddies. The climatological mean flows have a secondary contribution to the RI TCs. The contribution of the interannual flows to the kinetic energy conversion is small for both the RI and RD TCs. Our results signify the importance of the configuration and magnitude of the background flows relative to the synoptic scale disturbances in the barotropic kinetic energy conversion for the TC intensity changes. Key Points Intraseasonal cyclonic flows feature a northwest‐southeast and circular distribution for rapid intensifying (RI) and decaying (RD) tropical cyclones (TCs), respectively Barotropic energy from intraseasonal and mean zonal flows contributes to the RI TCs Barotropic energy conversion from intraseasonal flows are opposite in northeast and south parts of the RD TCs

This work compares the contributions of synoptic, intraseasonal, and interannual components of large-scale parameters to tropical cyclone (TC) genesis over the North Indian Ocean (NIO) from April to December from 1979 to 2020. A composite analysis is employed with respect to TC genesis time and location. It is shown that most TCs occur when the total sea surface temperature (SST) is between 28 and 30 °C and SST anomalies in three time ranges are small (with the magnitude less than 0.2 °C). The TCs form mostly when the anomalies of vertical zonal wind shear are between −6 and 6 m s−1 and total vertical zonal wind shear falls within −12 and −3 m s−1, with the synoptic component being a positive contributor. The intraseasonal component of vorticity and convergence in the low level, vertical motion and specific humidity in the middle level, and convection contributes dominantly to the TC genesis. Synoptic-scale tropical disturbances obtain barotropic kinetic energy from the climatological mean and intraseasonal flows, with the former dominant in the southeastern sector, and the latter dominant in the northwestern sector. The contributions of the three temporal components of environmental factors are compared for TC genesis between the Arabian Sea (AS) and Bay of Bengal (BOB) and between the early season (April through June) and late season (September through December). The relative contributions of the three temporal components of factors are also compared for the TC formation among the NIO, northern tropical Atlantic Ocean (NTA), Northwestern Pacific (WNP), and Northeastern Pacific (ENP).

We investigated the sensitivity of the size of a tropical cyclone (TC) to warming or cooling sea surface temperatures (SST) in its outer region by simulating the SST beyond a radius of 200 km from the TC center. Sensitivity experiments showed that an increased SST outside the core region of the TC had a negative effect on its size. Warming in the outer region contributed to the local enhancement of the latent heat flux from sea surface, which promoted the development of small-scale convection and warmed the lower and midtroposphere. This warming altered the local pressure gradient force in the upper and lower troposphere in such a way that it weakened the secondary circulation of the TC and led to suppression of the spiral rainbands outside the eyewall. Further analysis showed that the outward-propagating rainband structure favored an increase in the size of the TC. The diabatic heat released by the rainbands induced an inflow at lower levels, facilitating expansion of the TC. The greater the distance of the rainbands from the center of the TC, given the same amplitude of diabatic heating, the stronger the forced inflow, resulting in a faster increase in the size of the TC.

The present study investigates the relationship of tropical cyclone (TC) genesis among the western North Pacific (WNP), eastern North Pacific (ENP), and tropical North Atlantic Ocean (TNA) during July–October (JASO) from 1979 to 2019. A coherent interannual variation is found for TC genesis among the southeastern part of the WNP (SEW for brevity), ENP and TNA. When there are more TCs generated over the SEW, TC geneses are more than normal over the ENP, but less than normal over the TNA. Correlation analysis shows there is a significant positive (negative) relationship between the SEW/ENP (TNA) TC genesis and a tripole zonal sea surface temperatures (SST) anomaly distribution among the central and eastern Pacific Ocean, western Pacific and tropical Atlantic Ocean. The zonal tripole SST anomalies provide favorable genesis potential index (GPI) anomalies for TC genesis over the SEW and ENP, but unfavorable GPI anomalies over the TNA. The diagnoses display that the absolute vorticity has the greatest contribution and the contributions of the relative humidity and vertical wind shear terms are secondary to GPI anomalies over the SEW. The positive GPI anomalies are mainly due to a positive contribution of the vertical wind shear and potential intensity over the ENP. The major term contributing to the negative GPI anomalies is the vertical wind shear over the TNA. This study may help to improve seasonal prediction of the TC genesis over the WNP, ENP and TNA.

The Advanced version of the Weather Research and Forecasting (WRF-ARW) Model is used to investigate the influence of an easterly wave (EW) on the genesis of Typhoon Hagupit (2008) in the western North Pacific. Observational analysis indicates that the precursor disturbance of Typhoon Hagupit (2008) is an easterly wave (EW) in the western North Pacific, which can be detected at least 7 days prior to the typhoon genesis. In the control experiment, the genesis of the typhoon is well captured. A sensitivity experiment is conducted by filtering out the synoptic-scale (3–8-day) signals associated with the EW. The absence of the EW eliminates the typhoon genesis. Two mechanisms are proposed regarding the effect of the EW on the genesis of Hagupit. First, the background cyclonic vorticity of the EW could induce the small-scale cyclonic vorticities to merge and develop into a system-scale vortex. Second, the EW provides a favorable environment in situ for the rapid development of the typhoon disturbance through a positive moisture–convection feedback.