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Jingliang Huangfu
,
Wen Chen
,
Ronghui Huang
, and
Juan Feng

Abstract

This paper investigates how La Niña Modoki modulates the impacts of the warm Indian Ocean basin mode (IOBM) on the boreal summer climate and the genesis of tropical cyclones (TCs) over the northwest Pacific (NWP). The results showed that the influence of the Indian Ocean sea surface temperature (SST) on TC genesis is the primary mechanism during the boreal summer, while La Niña Modoki exerts a secondary influence. However, although the summertime index of the IOBM shows a high negative correlation with the number of TCs generated over the NWP, warm IOBM events without La Niña Modoki have only limited influences on the boreal summertime circulations and TC genesis. The present study showed that when warm IOBM events and La Niña Modoki coexisted, the average location of TC genesis shifted westward, and the annual number of generated TCs substantially decreased. La Niña Modoki–related cold sea surface temperature anomalies over the central Pacific further suppressed convective activities over the eastern NWP compared with warm IOBM events without La Niña Modoki. Upper-level convergence and enlarged tropospheric vertical wind shears both contributed to the weakening of the low-level relative vorticity in the coupled cases, leading to a suppressed NWP monsoon trough. Additionally, together with the weaker moisture supply, the impacts of warm IOBM cases were significantly enhanced under the modulation of La Niña Modoki, leading to poorer TC genesis conditions over the eastern NWP. In addition, the energy conversion processes in the aforementioned modulation showed that joint cases will provide fewer initial disturbance seedlings for TC genesis. These results are useful for further understanding the role of warm IOBM cases in TC genesis over the NWP.

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Yulian Tang
,
Jingliang Huangfu
,
Ronghui Huang
, and
Wen Chen

Abstract

This study assesses the relative impacts of model resolutions, tropical cyclone (TC) trackers, and ocean coupling on simulating TC climatology over the western North Pacific (WNP) based on six Coupled Model Intercomparison Project phase 6 (CMIP6) High-Resolution Model Intercomparison Project (HighResMIP) models from 1979 to 2014. The HighResMIP multimodel ensemble (MME) analysis shows that the high resolution has a higher Taylor skill score II (S 2) in both temporal and spatial patterns of TC genesis frequency and accumulated cyclone energy (ACE) than the low resolution. In contrast, the TempestExtremes tracker (coupled run) results in a higher S 2 in temporal patterns but a lower S 2 in spatial patterns than the TRACK tracker (uncoupled run). Among the three factors, increased resolution leads to the greatest improvement in S 2 in both temporal and spatial patterns. Furthermore, this study investigates the projections of future TC activity over the WNP by HighResMIP under the SSP5–8.5 scenario. Overall, HighResMIP MMEs project a decrease in the genesis frequency, track density, and ACE of all TCs, with the high-resolution, TRACK tracker, and uncoupled run showing greater magnitude. The high-resolution MMEs, using both trackers, project an increase in the genesis frequency and ACE of intense TCs in the coupled run. Moreover, TC track density and ACE show a larger poleward migration in the coupled run than in the uncoupled run, consistent with the significant surface warming in the northern WNP.

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Lin Wang
,
Ronghui Huang
,
Lei Gu
,
Wen Chen
, and
Lihua Kang

Abstract

Interdecadal variations of the East Asian winter monsoon (EAWM) and their association with the quasi-stationary planetary wave activity are analyzed by using the 40-yr European Centre for Medium-Range Weather Forecasts Re-Analysis dataset and the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis dataset. It is found that the EAWM experienced a significant weakening around the late 1980s; that is, the EAWM was strong during 1976–87 and became weak after 1988. This leads to an obvious increase in the wintertime surface air temperature as well as a decrease in the frequency of occurrence of cold waves over East Asia. The dynamical process through which the EAWM is weakened is investigated from the perspective of quasi-stationary planetary waves. It is found that both the propagation and amplitude of quasi-stationary planetary waves have experienced obvious interdecadal variations, which are well related to those of the EAWM. Compared to the period 1976–87, the horizontal propagation of quasi-stationary planetary waves after 1988 is enhanced along the low-latitude waveguide in the troposphere, and the upward propagation of waves into the stratosphere is reduced along the polar waveguide. This results in a weakened subtropical jet around 40°N due to the convergence of the Eliassen–Palm flux. The East Asian jet stream is then weakened, leading to the weakening of the EAWM since 1988. In addition, the amplitude of quasi-stationary planetary waves is significantly weakened around 45°N, which is related to the reduced upward propagation of waves from the lower boundary after 1988. This reduced amplitude may weaken both the Siberian high and the Aleutian low, reduce the pressure gradient in between, and then weaken the EAWM. Further analyses indicate that zonal wavenumber 2 plays the dominant role in this process.

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Xuefeng Cui
,
Hans-F. Graf
,
Baerbel Langmann
,
Wen Chen
, and
Ronghui Huang

Abstract

The hydrological impact of forest removal on the southeast Tibetan Plateau during the second half of the last century is investigated in this study using an atmospheric general circulation model. The effects of deforestation are investigated by examining the differences between the forest replacement and control experiments. Model results demonstrate that deforestation of the southeast Tibetan Plateau would influence the local and the remote climate as well. It would lead to decreased transpiration and increased summer precipitation in the deforested area and a wetter and warmer climate on the Tibetan Plateau in summer. This may produce more runoff into the rivers originating from the Tibetan Plateau and worsen flooding disasters in the downstream areas. The numerical experiments also show that deforestation would remotely impact Asian climate, and even global climate, although the statistical significance is small. A strong drought is found at middle and lower reaches of the Yellow River, where livelihoods and economics have suffered from recent droughts. Ecosystem research on the Tibetan Plateau is a relatively new topic and needs further interdisciplinary investigation.

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Tao Feng
,
Xiu-Qun Yang
,
Jia-Yuh Yu
, and
Ronghui Huang

Abstract

Tropical-depression (TD)-type waves are synoptic-scale disturbances embedded with deep convection over the western North Pacific. Studies of these disturbances began over six decades ago; however, some properties of these disturbances remain vague, e.g., the coupling mechanism between the deep convection and the waves. This two-part study aims to examine the rainfall progression in TD-type disturbances and associated tropospheric moisture controlling convective rainfall. Part I investigates the rainfall and moisture characteristics of TD-type waves using TRMM-derived rainfall products and the ERA-Interim data during the period of June–October 1998–2013. The rainfall features a north–south asymmetrical pattern with respect to a TD-type disturbance, with enhanced convective and stratiform rainfall occurring in the southern portion. Along with the northwestward propagation, deep convective and stratiform rainfall occur in phase with the TD-type disturbance without significant preceding shallow convective rainfall. Following the deepest convection, shallow convective rainfall increases in the anomalous southerlies. Such a rainfall progression differs from the paradigm from shallow to deep convection, then to stratiform rainfall, which is suggested in other convectively coupled equatorial waves. The rainfall progression and the atmospheric moisture anomaly are phase locked to the TD-type disturbances such that the relative displacements change little when the disturbances propagate northwestward. The latent heat release in deep convection, which is obtained from the TRMM 3G25 dataset, superposes with a broad warm anomaly in the mid- to upper troposphere, suggesting wave growth through the generation of available potential energy from diabatic heating.

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Tao Feng
,
Jia-Yuh Yu
,
Xiu-Qun Yang
, and
Ronghui Huang

Abstract

The companion of this paper, Part I, discovered the characteristics of the rainfall progression in tropical-depression (TD)-type waves over the western North Pacific. In Part II, the large-scale controls on the convective rainfall progression have been investigated using the ERA-Interim data and the TRMM 3B42 precipitation-rate data during June–October from 1998 to 2013 through budgets of moist static energy (MSE) and moisture. A buildup of column-integrated MSE occurs in advance of deep convection, and an export of MSE occurs following deep convection, which is consistent with the MSE recharge–discharge paradigm. The MSE recharge–discharge is controlled by horizontal processes, whereby horizontal moisture advection causes net MSE import prior to deep convection. Such moistening by horizontal advection creates a moist midtroposphere, which helps destabilize the atmospheric column, leading to the development of deep convective rainfall. Following the heaviest rainfall, negative horizontal moisture advection dries the troposphere, inhibiting convection. Such moistening and drying processes explain why deep convection can develop without preceding shallow convection. The advection of moisture anomalies by the mean horizontal flow controls the tropospheric moistening and drying processes. As the TD-type waves propagate northwestward in coincidence with the northwestward environmental flow, the moisture, or convective rainfall, is phase locked to the waves. The critical role of the MSE import by horizontal advection in modulating the rainfall progression is supported by the anomalous gross moist stability (AGMS), where the lowest AGMS corresponds to the quickest increase in the precipitation rate prior to the rainfall maximum.

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Tao Feng
,
Xiu-Qun Yang
,
Wen Zhou
,
Ronghui Huang
,
Liang Wu
, and
Dejian Yang

Abstract

Tropical depression (TD)-type waves are the dominant mode of synoptic-scale fluctuations over the western North Pacific. By applying spatiotemporal filters to the observed OLR data and the NCEP–DOE AMIP-II reanalysis data for 1979–2013, this study reveals the characteristics and energetics of convectively coupled TD-type waves under the effects of different circulation patterns in association with vertical wind shear. Results exhibit that different ambient sheared flows significantly affect the vertical structure of westward-propagating TD-type waves, with a lower-tropospheric mode in an easterly sheared background and an upper-tropospheric mode in a westerly sheared background. Energetic diagnoses demonstrate that when the disturbance is trapped in the lower (upper) level by easterly (westerly) shear, the horizontal mean flow in the lower (upper) level favors wave growth by converting energy from the shear of the zonal mean flow (from the convergence of the meridional mean flow). During the penetration of a westward-propagating synoptic-scale disturbance from a westerly sheared flow into an easterly sheared flow, the upper-level disturbance decays, and the lower-level disturbance intensifies. Meanwhile, the upper-level kinetic energy is transferred downward, but the effect induces the wave growth only confined to the midlevels. Consequently, the low-level growth of the westward-propagating upper-level synoptic-scale disturbance is mainly attributed to the barotropic conversion of horizontal mean flow in the lower troposphere.

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Liang Wu
,
Chia Chou
,
Cheng-Ta Chen
,
Ronghui Huang
,
Thomas R. Knutson
,
Joseph J. Sirutis
,
Stephen T. Garner
,
Christopher Kerr
,
Chia-Jung Lee
, and
Ya-Chien Feng

Abstract

A high-resolution regional atmospheric model is used to simulate present-day western North Pacific (WNP) tropical cyclone (TC) activity and to investigate the projected changes for the late twenty-first century. Compared to observations, the model can realistically simulate many basic features of the WNP TC activity climatology, such as the TC genesis location, track, and lifetime. A number of spatial and temporal features of observed TC interannual variability are captured, although observed variations in basinwide TC number are not. A relatively well-simulated feature is the contrast of years when the Asian summer monsoon trough extends eastward (retreats westward), more (fewer) TCs form within the southeastern quadrant of the WNP, and the corresponding TC activity is above (below) normal over most parts of the WNP east of 125°E. Future projections with the Coupled Model Intercomparison Project phase 3 (CMIP3) A1B scenario show a weak tendency for decreases in the number of WNP TCs, and for increases in the more intense TCs; these simulated changes are significant at the 80% level. The present-day simulation of intensity is limited to storms of intensity less than about 55 m s−1. There is also a weak (80% significance level) tendency for projected WNP TC activity to shift poleward under global warming. A regional-scale feature is a projected increase of the TC activity north of Taiwan, which would imply an increase in TCs making landfall in north China, the Korean Peninsula, and parts of Japan. However, given the weak statistical significance found for the simulated changes, an assessment of the robustness of such regional-scale projections will require further study.

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