Roles of Upper-Level Descending Inflow in Moat Development in Simulated Tropical Cyclones with Secondary Eyewall Formation

This study investigated the effects of upper-level descending inflow(ULDI)associated with inner-eyewall convection on the formation of the moat in tropical cyclones(TCs)with secondary eyewall formation(SEF).In our numerical experiments,a clear moat with SEF occurred in TCs with a significant ULDI,while no SEF occurred in TCs without a significant ULDI.The eyewall convection developed more vigorously in the control run.A ULDI occurred outside the inner-eyewall convection,where it was symmetrically unstable.The ULDI was initially triggered by the diabatic warming released by the inner eyewall and later enhanced by the cooling below the anvil cloud.The ULDI penetrated the outer edge of the inner eyewall with relatively dry air and prevented excessive solid-phase hydrometeors from being advected further outward.It produced extensive sublimation cooling of falling hydrometeors between the eyewall and the outer convection.The sublimation cooling resulted in negative buoyancy and further induced strong subsidence between the eyewall and the outer convection.As a result,a clear moat was generated.Development of the moat in the ongoing SEF prevented the outer rainband from moving farther inward,helping the outer rainband to symmetrize into an outer eyewall.In the sensitivity experiment,no significant ULDI formed since the eyewall convection was weaker,and the eyewall anvil developed relatively lower,meaning the formation of a moat and thus an outer eyewall was less likely.This study suggests that a better-represented simulation of inner-eyewall convective structures and distribution of the solid-phase hydrometeors is important to the prediction of SEF.

The Warming of the Tibetan Plateau in Response to Transient and Stabilized 2.0℃/1.5℃Global Warming Targets

As "the third pole", the Tibetan Plateau(TP) is sensitive to climate forcing and has experienced rapid warming in recent decades. This study analyzes annual and seasonal near-surface air temperature changes on the TP in response to transient and stabilized 2.0℃/1.5℃ global warming targets based on simulations of the Community Earth System Model(CESM). Elevation-dependent warming(EDW) with faster warming at higher elevations is predicted. A surface energy budget analysis is adopted to uncover the mechanisms responsible for the temperature changes. Our results indicate a clear amplified warming on the TP with positive EDW in 2.0℃/1.5℃ warmer futures, especially in the cold season. Mean TP warming relative to the reference period(1961–90) is dominated by an enhanced downward longwave radiation flux, while the variations in surface albedo shape the detailed pattern of EDW. For the same global warming level, the temperature changes under transient scenarios are ~0.2℃ higher than those under stabilized scenarios, and the characteristics of EDW are broadly similar for both scenarios. These differences can be primarily attributed to the combined effects of differential downward longwave radiation, cloud radiative forcing, and surface sensible and latent heat fluxes. These findings contribute to a more detailed understanding of regional climate on the TP in response to the long-term climate goals of the Paris Agreement and highlight the differences between transient and stabilized warming scenarios.

Understanding of the Effect of Climate Change on Tropical Cyclone Intensity:A Review

The effect of climate change on tropical cyclone intensity has been an important scientific issue for a few decades.Although theory and modeling suggest the intensification of tropical cyclones in a warming climate,there are uncertainties in the assessed and projected responses of tropical cyclone intensity to climate change.While a few comprehensive reviews have already provided an assessment of the effect of climate change on tropical cyclone activity including tropical cyclone intensity,this review focuses mainly on the understanding of the effect of climate change on basin-wide tropical cyclone intensity,including indices for basin-wide tropical cyclone intensity,historical datasets used for intensity trend detection,environmental control of tropical cyclone intensity,detection and simulation of tropical cyclone intensity change,and some issues on the assessment of the effect of climate change on tropical cyclone intensity.In addition to the uncertainty in the historical datasets,intertwined natural variabilities,the considerable model bias in the projected large-scale environment,and poorly simulated inner-core structures of tropical cyclones,it is suggested that factors controlling the basin-wide intensity can be different from individual tropical cyclones since the assessment of the effect of climate change treats tropical cyclones in a basin as a whole.

Relationship between cross-equatorial flows over the Bay of Bengal and Australia in boreal summer:Role of tropical diabatic heating

The interannual variability of cross-equatorial flows(CEFs)over the Asian–Australian monsoon(AAM)region during boreal summer was analyzed by applying the empirical orthogonal function(EOF)method to the meridional wind at 925 h Pa.The first mode(EOF1)exhibits an in-phase relationship among different CEF channels over the AAM region,which has received much attention owing to its tight linkage with ENSO.By contrast,the second mode(EOF2)possesses an out-of-phase relationship between the Bay of Bengal(BOB)CEF(90°E)and Australian CEF,among which the New Guinea CEF near 150°E shows the most significant opposite correlation with the BOB CEF.Observational and numerical model results suggest that the equatorially asymmetric heat source(sink)over the western(eastern)Maritime Continent,closely associated with the in-situ sea surface temperature anomaly,can induce cross-equatorial northerly(southerly)flow into the heating hemisphere,which dominates the out-of-phase relationship between the BOB and New Guinea CEFs.Furthermore,an equatorially symmetric heating over the central Pacific may indirectly change the CEFs by modulating the zonal atmospheric circulation near the Maritime Continent.

The Influence of Arctic Sea Ice Concentration Perturbations on Subseasonal Predictions of North Atlantic Oscillation Events

The influence of Arctic sea ice concentration (SIC) on the subseasonal prediction of the North Atlantic Oscillation (NAO) event is investigated by utilizing the Community Atmospheric Model version 4. The optimal Arctic SIC perturbations which exert the greatest influence on the onset of an NAO event from a lead of three pentads (15 days) are obtained with a conditional nonlinear optimal perturbation approach. Numerical results show that there are two types of optimal Arctic SIC perturbations for each NAO event, with one weakening event (marked as type-1) and another strengthening event (marked as type-2). For positive NAO events, type-1 optimal SIC perturbations mainly show positive SIC anomalies in the Greenland, Barents, and Okhotsk Seas, while type-2 perturbations mainly feature negative SIC anomalies in these regions. For negative NAO events, the optimal SIC perturbations have almost opposite patterns to those in positive events, although there are some differences among these SIC perturbations due to different atmospheric initial conditions. Further diagnosis reveals that the optimal Arctic SIC perturbations first modify the surface turbulent heat flux and the temperature in the lower troposphere via diabatic processes. Afterward, the temperature in the low troposphere is mainly affected by dynamic advection. Finally, potential vorticity advection plays a crucial role in the 500-hPa geopotential height prediction in the northern North Atlantic sector during pentad 4, which influences NAO event prediction. These results highlight the importance of Arctic SIC on NAO event prediction and the spatial characteristics of the SIC perturbations may provide scientific support for target observations of SIC in improving NAO subseasonal predictions.

Influence of Arctic Sea-ice Concentration on Extended-range Forecasting of Cold Events in East Asia

Utilizing the Community Atmosphere Model,version 4,the influence of Arctic sea-ice concentration(SIC)on the extended-range prediction of three simulated cold events(CEs)in East Asia is investigated.Numerical results show that the Arctic SIC is crucial for the extended-range prediction of CEs in East Asia.The conditional nonlinear optimal perturbation approach is adopted to identify the optimal Arctic SIC perturbations with the largest influence on CE prediction on the extended-range time scale.It shows that the optimal SIC perturbations are more inclined to weaken the CEs and cause large prediction errors in the fourth pentad,as compared with random SIC perturbations under the same constraint.Further diagnosis reveals that the optimal SIC perturbations first modulate the local temperature through the diabatic process,and then influence the remote temperature by horizontal advection and vertical convection terms.Consequently,the optimal SIC perturbations trigger a warming center in East Asia through the propagation of Rossby wave trains,leading to the largest prediction uncertainty of the CEs in the fourth pentad.These results may provide scientific support for targeted observation of Arctic SIC to improve the extended-range CE prediction skill.

Impact of the Western Pacific Tropical Easterly Jet on Tropical Cyclone Genesis Frequency over the Western North Pacific

Although it is well known that the tropical easterly jet(TEJ)has a significant impact on summer weather and climate over India and Africa,whether the TEJ exerts an important impact on tropical cyclone(TC)activity over the western North Pacific(WNP)remains unknown.In this study,we examined the impact of the TEJ on the interannual variability of TC genesis frequency over the WNP in the TC season(June-September)during 1980-2020.The results show a significant positive correlation between TC genesis frequency over the WNP and the jet intensity in the entrance region of the TEJ over the tropical western Pacific(in brief WP_TEJ),with a correlation coefficient as high as 0.66.The intensified WP_TEJ results in strong ageostrophic northerly winds in the entrance region and thus upper-level divergence to the north of the jet axis over the main TC genesis region in the WNP.This would lead to an increase in upward motion in the troposphere with enhanced low-level convergence,which are the most important factors to the increases in low-level vorticity,mid-level humidity and low-level eddy kinetic energy,and the decreases in sea level pressure and vertical wind shear in the region.All these changes are favorable for TC genesis over the WNP and vice versa.Further analyses indicate that the interannual variability of the WP_TEJ intensity is likely to be linked to the local diabatic heating over the Indian Ocean-western Pacific and the central Pacific El Ni?o-Southern Oscillation.

A brief review of ENSO theories and prediction

Although the El Ni?o-Southern Oscillation(ENSO) originates and develops in the equatorial Pacific, it has substantial climatic impacts around the globe. Thus, the ability to effectively simulate and predict ENSO one or more seasons in advance is of great societal importance, but this remains a challenging task. The main obstacles are the diversity, complexity,irregularity, and asymmetry of ENSO. The purpose of this article is to organically integrate the understanding of ENSO based on current progress on the physical mechanisms, prediction, and connections between the interannual ENSO phenomenon and physical processes on other time and space scales, and to provide guidance for future studies by extracting specific important questions.

Detection and projection of climatic comfort changes in China's mainland in a warming world

Climatic comfort,which refers to the comfort of the human body's thermal sensations,is important for the human habitat.Although considerable efforts have been provided to examine changes in climatic comfort response to global warming from a partial perspective,the trajectory shift in past and future climatic comfort conditions in China's mainland based on uniform indicators has not been revealed.The spatiotemporal pattern of climatic comfort over historical and future periods was investigated in this study,using China's mainland as an example.The temperature-humidity index was adopted on the basis of homogenised meteorological station observations and high-resolution climate model simulations to analyse the trends of comfort/discomfort days from 1960 to 2017 and project changes in climatic comfort under representative concentration pathway scenarios in the late 21st century(2071-2100).Results show a substantial decrease in colduncomfortable days and a moderate increase in comfortable and warm-uncomfortable days from 1960 to 2017.In the late 21st century,the signals of increasing warm-uncomfortable and decreasing cold-uncomfortable days are projected to enhance significantly while the direction of changes in comfortable days exhibits a north-south divergence.The uneven changes in warm-and cold-uncomfortable days and an overall decrease in comfortable days in the late 21st century dominate the future trends in climatic comfort in the densely populated southeast half of China.Effective measures taken for adapting to and mitigating global climate warming can considerably avoid the adverse impact of the projected change.

Understanding the role of SST anomaly in extreme rainfall of 2020 Meiyu season from an interdecadal perspective

Extreme Meiyu rainfall in 2020,starting from early June to the end of July,has occurred over the Yangtze River valley(YRV),with record-breaking accumulated precipitation amount since 1961.The present study aims to examine the possible effect of sea surface temperature(SST)on the YRV rainfall in Meiyu season from the interdecadal perspective.The results indicate that YRV rainfall in June exhibits more significant variability on interdecadal time scale than that in July.The interdecadal-filtered atmospheric circulation in June,compared with the counterpart in July,shows a more predominant and better-organized Western North Pacific Anticyclone(WNPAC)anomaly,which could transport abundant moisture to the YRV by anomalous southwesterly prevailing in northwestern flank of anomalous WNPAC.Both observation and numerical experiment indicate that the interdecadal change of the SST anomaly in tropical western Indian Ocean(TWI)from preceding May to June can significantly affect the anomalous WNPAC,leading to enhanced YRV rainfall in June.The TWI SST anomaly shifts from a cold phase to a warm phase around the early 2000s,with a magnitude of 0.7°C in 2020,which implies that such interdecadal warming might partly contribute to the heavy rainfall in June 2020 by providing a large-scale favorable background flow.

Distribution characteristics and source tracing of petroleum hydrocarbons in the northeastern South China Sea

In recent years,oil spills caused by human activities have occurred frequently,and the resultant oil pollution has received extensive attention worldwide.In this paper,a total of 50 water samples were collected from the northeastern part of the South China Sea,and total petroleum hydrocarbons(TPHs)and n-alkane content in the samples were analyzed by gas chromatography-flame ionization detector(GC-FID)technology.The petroleum hydrocarbon characteristic indices,such as carbon predominance index(CPI)and terrigenous/aquatic ratio(TAR),were calculated to trace the source of petroleum hydrocarbons.The measured value of TPHs ranged from 121.31-603.02μg/L.For surface waters,the TPHs in the no rthern coastal waters and the central waters were higher than that in the far shore.For ve rtical waters,the TPHs sharply decreased at first,and then increased slowly and finally reached a steady state.The n-alkanes in the water samples were concentrated in C10-C38,and they were mainly from terrestrial higher plant.The waters in the near shore,mid-layer and deep sea showed a strong reducing terrestrial characteristic,while the surface waters in the open sea showed an obvious oxidizing endogenous characteristic.

A Diagnostic Study of the Influence of Early Spring Soil Moisture in Southeastern China on Interannual Variability of the East Asian Subtropical Summer Monsoon Onset

The effect of soil moisture(SM)on the onset of East Asian subtropical summer monsoon(EASSM)is investigated based on multiple sets of reanalysis data in the period of 1981–2010.It is found that the EASSM is characterized by persistent 2-m s^(−1) southerly winds for about 3 months in spring at 850 hPa over the subtropical region of East Asia.Considering this feature of the meridional winds,we define the EASSM onset date,and obtain that the climatological onset date is pentad 17.7,around 26 March.On the interannual timescale,the onset date of EASSM exhibits statistically significant correlation with the SM in southeastern China in the month preceding the onset,with wetter(drier)conditions being associated with later(earlier)onset.The physical process by which the preceding SM affects the EASSM onset is further explored by examining the surface energy balance as well as its impacts.Positive(negative)SM anomalies in southeastern China in the month before onset may induce negative(positive)surface temperature anomalies.The decreased(increased)surface temperature in southeastern China before the EASSM onset weakens(strengthens)the zonal sea–land thermal contrast in the surface and low-level atmosphere in the subtropical East Asia.The zonal sea–land thermal contrast in wetter(drier)years induces anomalous northerly(southerly)winds over southeastern China,which tends to delay(advance)the zonal thermal seasonal transition in spring and is conducive to a later(earlier)onset of EASSM.These results are helpful for understanding and prediction of the variability of EASSM and the EASSM onset.

The unusual 2014–2016 El Nino events: Dynamics, prediction and enlightenments

The 2014–2016 El Ni?o events consist of a stalled El Ni?o event in the winter of 2014/2015 and a following extreme El Ni?o event in the end of 2015.Neither event was successfully predicted in operational prediction models.Because of the unusual evolutions of these events that rarely happened in the historical observations,few experience was ready for understanding and predicting the two El Ni?o events when they occurred.Also due to their specialties,considerable attention were attracted with aims to reveal the hidden mechanisms.This article reviews the recent progresses and knowledge that were obtained in these studies.Emerging from these studies,it was argued that the key factor that was responsible for the stalled El Ni?o in 2014 was the unexpected summertime Easterly Wind Surges(EWSs)or the lack of summertime Westerly Wind Bursts(WWBs).Most operational prediction models failed to reproduce such stochastic winds and thus made unrealistic forecasts.The two El Ni?o events awakened the research community again to incorporate the state-of-the-art climate models to simulate the stochastic winds and investigate their roles in the development of El Nino.

The Henan extreme rainfall in July 2021: Modulation of the northward-shift monsoon trough on the synoptic-scale wave train

The synoptic-scale wave train is a dominant pattern of the synoptic variability over the tropical western Pacific and usually affects the extreme weather over South China and Southeast Asia.Whether it could extend its influence and contribute to the Henan extreme rainfall in July 2021 still needs to be unraveled.We found that during the Henan extreme rainfall days a positively synoptic-scale vorticity disturbance dominated Henan province,China,which was embedded in the synoptic-scale wave train that originated from the western North Pacific.Moreover,the propagating pathway of this synoptic-scale wave train located northward and was likely modulated by the latitudinal location change of the monsoon trough over the western North Pacific.A northernmost displacement of the monsoon trough in July 2021(∼23.2°N)would facilitate the synoptic-scale wave train to propagate farther northwestward via shifting the related barotropic conversion northward.Therefore,the synoptic-scale wave train from the tropics could reach Henan,provide the necessary lifting forcing,and supply abundant water vapor associated with the anomalous southerly for the occurrence of Henan extreme rainfall event.The results implicate that the pre-existing synoptic-scale wave train regulated by the location of the monsoon trough may be a potential precursor for heavy rainfalls in northern Central China.