Lower-tropospheric Circulation Research Articles

Climatology of sea breeze over Jiangsu coast: An overview from the ERA5 reanalysis

AbstractThe sea breeze characteristics for 30 years of summer (1981–2010) in the coastal region of Jiangsu Province were analysed in this study, using the four‐step filter method based on ECMWF Fifth‐Generation Reanalysis (ERA5) data. The results showed that the filtering method reasonably identified three types of sea breeze. Additionally, approximately 23% of the total summer days remained; the corkscrew type dominates the three types of sea breeze; and the backdoor sea breeze was the least common. Except for the type of backdoor sea breeze, the other two types had relatively fixed dominant weather types on three coastlines. The synoptic weather patterns in the 500‐hPa level corresponding to each sea breeze type were different. Further, the details of the diurnal cycle of lower tropospheric circulation indicated the strength of the onshore wind and the location of the large potential temperature gradient at noon being influenced by sea breeze type and coastal location. The wind strength for the three types of sea breeze on the middle coastline was stronger than that on the other two coastlines. On the three coastlines, strong onshore winds were concentrated in areas near the coastline, where they reached their maximum strengths around noon. On the southern coastline, strong onshore winds moved further inland.

Interdecadal Change in the Covariability of the Tibetan Plateau and Indian Summer Precipitation and Associated Circulation Anomalies

This study investigates the interdecadal change in the covariability between the Tibetan Plateau (TP) east–west dipole precipitation and Indian precipitation during summer and primarily explores the modulation of atmospheric circulation anomalies on the covariability. The results reveal that the western TP precipitation (WTPP), eastern TP precipitation (ETPP), and northwestern Indian precipitation (NWIP) have covariability, with an in-phase variation between the WTPP and NWIP and an out-of-phase variation between the WTPP and ETPP. Moreover, this covariability was unclear during 1981–2004 and became significant during 2005–2019, showing a clear interdecadal change. During 2005–2019, a thick geopotential height anomaly, which tilted slightly northward, governed the TP, forming upper- and lower-level coupled circulation anomalies (i.e., anomalous upper-level westerlies over the TP and lower-level southeasterlies and northeasterlies around the southern flank of the TP). As such, the upper- and lower-tropospheric circulation anomalies synergistically modulate the summer WTPP, ETPP, and NWIP, causing the covariability of summer precipitation over the TP and India during 2005–2019. The upper- or lower-level circulation anomalies cannot independently result in significant precipitation covariability. During 1981–2004, the upper- and lower-level circulation anomalies were not strongly coupled, which caused precipitation non-covariability. The sea surface temperature anomalies (SSTAs) in the western North Pacific (WNP) and tropical Atlantic (TA) may synergistically modulate the upper- and lower-level coupled circulation anomalies, contributing to the covariability of the WTPP, ETPP, and NWIP during 2005–2019. The modulation of the WNP and TA SSTs on the coupled circulation anomalies was weaker during 1981–2004, which was therefore not conducive to this precipitation covariability. This study may provide valuable insights into the characteristics and mechanisms of spatiotemporal variation in summer precipitation over the TP and its adjacent regions, thus offering scientific support for local water resource management, ecological environment protection, and social and economic development.

Robust changes in global subtropical circulation under greenhouse warming

The lower tropospheric subtropical circulation (SC) is characterized by monsoons and subtropical highs, playing an important role in global teleconnections and climate variability. The SC changes in a warmer climate are influenced by complex and region-specific mechanisms, resulting in uneven projections worldwide. Here, we present a method to quantify the overall intensity change in global SC, revealing a robust weakening across CMIP6 models. The weakening is primarily caused by global-mean surface warming, and partly counteracted by the direct CO2 effect. The direct CO2 effect is apparent in the transient response but is eventually dominated by the surface warming effect in a slow response. The distinct response timescales to global-mean warming and direct CO2 radiative forcing can well explain the time-varying SC changes in other CO2 emission scenarios. The declined SC implies a contracted monsoon range and drying at its boundary with arid regions under CO2-induced global warming.

Influence of the Southern Indian Ocean Dipole on the following spring climate in China: A synthetic observational and numerical study

The influence of the Southern Indian Ocean Dipole (SIOD) on the following spring climate in China is examined, using a 42-year simulation of the Community Earth System Model, version 1.0.4 (CESM1.0.4) and 42-year observation-based ocean-atmosphere analyses (1979–2021). It is shown that the SIOD, an inverse phase pattern of SST anomalies (SSTAs) in the southwest and northeast of the Southern Indian Ocean (SIO), is the dominant mode of observed interannual variability. Composite analyses based on observational data suggest that when a positive SIOD signal occurs, the surface temperature in the ensuing spring is warmer in southwest China and colder in northeast China, and the precipitation in southeast China decreases significantly. The response phase of the surface temperature and precipitation in China under a negative SIOD signal is the opposite of that under a positive SIOD signal, and the response intensity is also greater under a negative SIOD signal. Numerical experiments with CESM can also well reproduce the distribution of the surface temperature and precipitation the following spring over China under positive and negative SIOD signals, respectively. Analysis of the mechanism by which the SIOD influences the subsequent precipitation in China shows that it may occur through interaction between the mid to high latitudes and low latitudes, as well as upper and lower tropospheric circulations. During negative SIOD years, due to the remote response generated by the boreal winter SSTA in the SIO, a strong positive anomaly at 500 hPa geopotential height extending from northeast China to the northwest Pacific Ocean at mid to high latitudes corresponds to a significant low pressure in south and northwest China, and this low pressure helps southerly winds bring more water vapor to southeast China. Meanwhile, the westward extension and northward elevation of the western Pacific subtropical high lead to the convergence of water vapor in south China. In addition, westerly anomalies over the equatorial Indian Ocean and easterly anomalies over the equatorial western Pacific change the local Walker circulation, which weakens the northwest Pacific anticyclonic circulation and subtropical westerly jet. The abnormally warm SST in the southern equatorial Indian Ocean promotes the formation of the Hadley circulation, enhances the upward movement, and changes the water vapor convergence in south China. These factors together lead to an increase in precipitation in southeast China in the following spring in both observations and CESM numerical experiments. At the same time, during positive SIOD years, both observations and simulations show that the causes affecting precipitation are almost the opposite of those in negative SIOD years. Therefore, the SIOD during boreal winter can be an important predictor of precipitation variability over south and southeast China during the following spring.

The Key Role of the Vertical Structure of the Stratospheric Quasi‐Biennial Oscillation in the Variations of Asian Precipitation in Summer

AbstractBased on observations and numerical simulations, this study explores the responses of Asian precipitations in summer to the vertical structure of Stratospheric quasi‐biennial oscillation (QBO), and the relevant mechanisms. Compared to the QBO phase defined by a particular level traditionally, considering QBO's vertical structure leads to more significant precipitation responses. When the tropical zonal winds exhibit easterlies (westerlies) at 70 hPa and westerlies (easterlies) at both 30 and 50 hPa, there are downwellings (upwellings) over tropics and upwellings (downwellings) over mid‐latitudes in the upper troposphere and lower stratosphere. The meridional movement of the subtropical westerly jet is related to this secondary circulation, accompanied by anomalies of the South Asian High. These circulation anomalies lead to anomalous lower tropospheric circulations, causing abnormal vertical velocities and moisture transports, resulting in increased precipitation over South Asia and decreased precipitation in the Yangtze River basin.

Seasonality in the Amplitude of Decadal Variability

Abstract As the understanding of decadal variability in climate systems deepens, there is a growing interest in investigating the decadal variability of seasonal-mean or monthly mean variables. This study aims to understand the seasonality observed in the amplitude of decadal variability. To accomplish this, we analyze the decadal variability of the monthly mean North Atlantic Oscillation (NAO) index and North Pacific index (NPI) over the past decades using two different calculating processes: the full smoothing (F) process and the seasonal-specific (SS) process. Our findings suggest that the F process only captures the decadal variability of annual-mean variables, whereas the SS process is suited for capturing the seasonality of decadal variability. We find that the seasonality in decadal variability aligns with the seasonality in interannual variability. Additionally, we explore the seasonality in decadal variability in atmospheric and oceanic variables. The seasonality in oceanic decadal variability, including sea surface temperature and salinity, is found to be weak and small. The amplitude of decadal variability in the Pacific decadal oscillation (PDO) is similar across different months, indicating weak seasonality in the PDO. On the other hand, decadal variability of lower-tropospheric atmospheric circulation, including horizontal wind, geopotential height, and surface air temperature, exhibits significant seasonality in the extratropics, with the strongest decadal variability occurring in winter. Moreover, the significant seasonality in decadal variability of precipitation is observed in the tropics, with the strongest decadal variability occurring in summer. Our study provides insights into understanding the seasonality of decadal variability, which can aid in the improvement of decadal prediction of climate variability. Significance Statement The amplitude of decadal variability of seasonal-mean or monthly mean variables exhibits seasonality. Our results show that the seasonality in decadal variability is consistent with the seasonality in interannual variability. We also identified that the seasonality in oceanic decadal variability is weak and smaller than that in atmospheric decadal variability. Decadal variability of lower-tropospheric atmospheric circulation exhibits significant seasonality in the extratropics, with the strongest decadal variability occurring in winter. However, the significant seasonality in decadal variability of precipitation occurs in the tropics, with the strongest decadal variability occurring in summer. Our study provides insights into the seasonality of decadal variability in climate systems.

Global monsoon systems and their modulation by the equatorial Quasi-Biennial Oscillation

Monthly-mean data of ERA-Interim reanalysis, precipitation, outgoing longwave radiation (OLR) and sea surface temperature(SST) are investigated for 40 years (1979-2018) to reveal the modulation of the global monsoon systems by the equatorial quasi-biennial oscillation (QBO), focusing only on the neutral El Niño-Southern Oscillation (ENSO) periods (in total 374 months). First, the climatology of the global monsoon systems is viewed with longitude-latitude plots of the precipitation, its proxies and lower tropospheric circulations for the annual mean and two solstice seasons, together with the composite differences between the two seasons. In addition to seasonal variations of Intertropical Convergence Zones (ITCZs), several regional monsoon systems are well identified with an anti-phase of the annual cycle between the two hemispheres. Precipitation-related quantities (OLR and specific humidity), surface conditions [i.e., mean sea level pressure (MSLP) and SST] and circulation fields related to moist convection systems show fundamental features of the global monsoon systems. After introducing eight QBO phases based on the leading two principal components of the zonal-mean zonal wind variations in the equatorial lower-stratosphere, the statistical significance of the composite difference in the precipitation and tropospheric circulation is evaluated for the opposite QBO phases. The composite differences show significant modulations in some regional monsoon systems, dominated by zonally asymmetric components, with the largest magnitudes for specific QBO-phases corresponding to traditional indices of the equatorial zonal-mean zonal wind at 20 and 50 hPa. Along the equator, significant QBO influence is characterized by the modulation of the Walker circulation over the western Pacific. In middle latitudes during boreal summer, for a specific QBO-phase, statistically significant modulation of low-pressure cyclonic perturbation is obtained over the Northern-Hemisphere western Pacific Ocean associated with statistically significant features of heavier precipitation over the eastern side of the cyclonic perturbation and the opposite lighter precipitation over the western side. During boreal winter, similar significant low-pressure cyclonic perturbations were found over the Northern-Hemisphere eastern Pacific and Atlantic Oceans for specific phases.

Quasi-Biweekly Oscillation over the Western North Pacific in Boreal Winter and Its Influence on the Central North American Air Temperature

Abstract This study investigates the characteristics and climate impacts of the quasi-biweekly oscillation (QBWO) over the western North Pacific (WNP) in boreal winter based on observational and reanalysis data and numerical experiments with a simplified model. The wintertime convection over the WNP is dominated by significant biweekly variability with a 10–20-day period, which explains about 66% of the intraseasonal variability. Its leading mode on the biweekly time scale is a northwestward-propagating convection dipole over the WNP, which oscillates over a period of about 12 days. When the convection-active center of this QBWO is located to the east of the Philippines, it can generate an anticyclonic vorticity source to the south of Japan via inducing upper-tropospheric divergence and excite a Rossby wave train propagating toward North America along the Pacific rim. The resultant lower-tropospheric circulation facilitates cold advection and leads to cold anomalies over central North America in the following week. This result highlights a cause–effect relationship between the WNP convection and the North American climate on the quasi-biweekly time scale and may provide some prediction potential for the North American climate. Significance Statement This study establishes a cause–effect relationship between the wintertime western North Pacific convection and the central North American air temperature on the quasi-biweekly time scale. In boreal winter, the convection over the western North Pacific oscillates significantly with a 10–20-day period. When the convection is active, it can disturb the atmosphere to the south of Japan and excite a midlatitude Rossby wave train. The latter propagates along the North Pacific rim and leads to cold spells over central North America within one week. This information connects the climate variability across the Pacific and provides an additional subseasonal-to-seasonal prediction potential for the North American winter climate.

Mechanisms of the subseasonal influences of Scandinavian events on winter surface air temperature over Eastern China

Based on the daily Japanese 55-year Reanalysis dataset from 1958 to 2020, this study reveals that positive-phase Scandinavian (SCA) teleconnection events can cause significant negative surface air temperature (SAT) anomaly over the whole eastern China from 6 to 12 days after the peak day of SCA events. Clearly, the timing of this significant impact exceeds the e-folding timescale of approximately 6 days of SCA events by approximately one week. During the first stage (from peak day to 6 days after peak day), the vertical coupling between the upper and lower tropospheric circulation anomalies over Eurasian continent facilitates the accumulation of cold air masses from the higher latitude to the north of the Tibetan Plateau, which becomes a precursory signal of persistent negative SAT anomaly affecting eastern China. During the second stage (from 7 to 12 days after peak day), the diabatic heating anomaly caused by precipitation anomaly becomes an important factor in the persistence of the negative SAT anomaly over eastern China. Firstly, the persistent negative diabatic heating anomaly near Lake Baikal before day 12 is conducive to the maintenance of anomalously enhanced Siberian High over eastern China. Secondly, corresponding to the gradually southward movement of the positive precipitation anomaly over East Asia, significant negative diabatic heating anomaly gradually substitutes the positive diabatic heating anomaly over East Asia, which is also favorable for the maintenance of the local anticyclonic anomaly. Eventually, the sustained anticyclonic anomaly can induce the persistent negative SAT anomaly over eastern China.

Reduction of mid-summer rainfall in northern India after the late-1990s induced by the decadal change of the Silk Road pattern

Variation of summer rainfall can pose substantial challenges to water resources and agriculture in northern India, which includes the most fertile land in India, i.e. the Ganges Plain. This study identifies a significant rainfall reduction in northern India during its peak rainy season (July and August) after the late-1990s, based on observational and reanalysis data since 1958. This rainfall reduction is about 0.86 mm d−1, equivalent to 8.3% of the climatological mean. We suggest that the decadal reduction of northern Indian rainfall is induced by the decadal change of the Silk Road pattern (SRP), which is an atmospheric teleconnection pattern along the upper-tropospheric Asian westerly jet. After the late-1990s, corresponding with the decadal phase shift of the SRP, there is a cyclonic anomaly over West Asia, which can reduce rainfall in northern India. Furthermore, this rainfall reduction is unprecedented since the beginning of the 20th century, when observational data are available. Possible roles of the lower-tropospheric circulation changes and the Atlantic Multidecadal Oscillation are discussed.

Different Configurations of Interannual Variability of the Western North Pacific Subtropical High and East Asian Westerly Jet in Summer

This study investigates the circulation and precipitation anomalies associated with different configurations of the western North Pacific subtropical high (WNPSH) and the East Asian westerly jet (EAJ) in summer on interannual timescales. The in-phase configuration of the WNPSH and EAJ is characterized by the westward (eastward) extension of the WNPSH and the southward (northward) shift of the EAJ, which is consistent with the general correspondence between their variations. The out-of-phase configuration includes the residual cases. We find that the in-phase configuration manifests itself as a typical meridional teleconnection. For instance, there is an anticyclonic (cyclonic) anomaly over the tropical western North Pacific and a cyclonic (anticyclonic) anomaly over the mid-latitudes of East Asia in the lower troposphere. These circulation anomalies are more conducive to rainfall anomalies over the Yangtze River basin and south Japan than are the individual WNPSH or EAJ. By contrast, for the out-of-phase configuration, the mid-latitude cyclonic (anticyclonic) anomaly is absent, and the lower-tropospheric circulation anomalies feature an anticyclonic (cyclonic) anomaly with a large meridional extension. Correspondingly, significant rainfall anomalies move northward to North China and the northern Korean Peninsula. Further results indicate that the out-of-phase configuration is associated with the developing phase of ENSO, with strong and significant sea surface temperature (SST) anomalies in the tropical central and eastern Pacific which occur simultaneously during summer and persist into the following winter. This is sharply different from the in-phase configuration, for which the tropical SSTs are not a necessity.

The climatological impact of the upper-tropospheric Rossby wave propagation on the Red Sea trough in winter

There is a prominent feature in the climate of the northeast Africa that, once activated, may cause heavy rainfall occurrence in large parts of the eastern Mediterranean (EM) and the Arabian Peninsula. The feature is Red Sea Trough (RST) which is an inverted trough of low pressure at lower troposphere wave over the northeast Africa and the Red Sea. This study investigates the way Rossby wave propagation in the upper troposphere may change the circulation of the lower troposphere and thus the surface weather in the northeast Africa, the Red Sea and the EM regions on a monthly time scale. The interaction between the wave activity flux at the 300 hPa level and the lower tropospheric circulation is investigated with consideration of the meteorological quantities at different levels of the troposphere in a wide region from the western North Atlantic to the EM. For this, two critical phases of the wave activity flux are defined. The critical positive (negative) phase corresponds to the months during which the amount of wave activity entering the region of study is higher (lower) than the climatological mean plus (minus) one standard deviation. The results show that during the critical positive phase, in the northeast Africa, the mean meridional circulation is strengthened throughout the troposphere. Further consequences of the positive phase are the strengthening and interconnection of the divergence cores of the Atlantic and Mediterranean storm tracks, strengthening of the RST and thus locally-unstable conditions leading to higher monthly precipitation in large parts of the study region.

Direct and semi-direct radiative forcing of biomass-burning aerosols over the southeast Atlantic (SEA) and its sensitivity to absorbing properties: a regional climate modeling study

Abstract. Simulations are performed for the period 2000–2015 by two different regional climate models, ALADIN and RegCM, to quantify the direct and semi-direct radiative effects of biomass-burning aerosols (BBAs) in the southeast Atlantic (SEA) region. Different simulations have been performed using strongly absorbing BBAs in accordance with recent in situ observations over the SEA. For the July–August–September (JAS) season, the single scattering albedo (SSA) and total aerosol optical depth (AOD) simulated by the ALADIN and RegCM models are consistent with the MACv2 climatology and MERRA-2 and CAMS-RA reanalyses near the biomass-burning emission sources. However, the above-cloud AOD is slightly underestimated compared to satellite (MODIS and POLDER) data during the transport over the SEA. The direct radiative effect exerted at the continental and oceanic surfaces by BBAs is significant in both models and the radiative effects at the top of the atmosphere indicate a remarkable regional contrast over SEA (in all-sky conditions), with a cooling (warming) north (south) of 10 ∘S, which is in agreement with the recent MACv2 climatology. In addition, the two models indicate that BBAs are responsible for an important shortwave radiative heating of ∼0.5–1 K per day over SEA during JAS with maxima between 2 and 4 km a.m.s.l. (above mean sea level). At these altitudes, BBAs increase air temperature by ∼0.2–0.5 K, with the highest values being co-located with low stratocumulus clouds. Vertical changes in air temperature limit the subsidence of air mass over SEA, creating a cyclonic anomaly. The opposite effect is simulated over the continent due to the increase in lower troposphere stability. The BBA semi-direct effect on the lower troposphere circulation is found to be consistent between the two models. Changes in the cloud fraction are moderate in response to the presence of smoke, and the models differ over the Gulf of Guinea. Finally, the results indicate an important sensitivity of the direct and semi-direct effects to the absorbing properties of BBAs. Over the stratocumulus (Sc) region, DRE varies from +0.94 W m−2 (scattering BBAs) to +3.93 W m−2 (most absorbing BBAs).

Upper- and Lower-tropospheric Circulation Anomalies Associated with Interannual Variation of Pakistan Rainfall during Summer

This study investigated the large-scale circulation anomalies, in both the upper and lower troposphere, associated with the interannual variation of rainfall in Pakistan during summer, using the station observation data in this country and circulation data of the NCEP−NCAR reanalysis from 1981 to 2017. Results showed that the upper- and lower-tropospheric circulation anomalies associated with monthly rainfall variability exhibit similar features from June to August, so analyses were performed on June−August circulation and Pakistan rainfall data. The analyzed results indicated that summer rainfall in Pakistan is enhanced when there is an anticyclonic anomaly to the northwest of Pakistan in the upper troposphere and easterly anomalies along the southern foothills of the Himalayas in the lower troposphere, and vice versa. These upper- and lower-tropospheric circulation anomalies were found to be related, but show unique features. The upper-tropospheric anticyclonic anomaly is closely related to the Silk Road Pattern along the Asian westerly jet, while the lower-tropospheric easterly anomalies are related to the cyclonic anomaly to the south of Pakistan, i.e., intensified South Asian monsoon trough. The results presented here suggest that the interannual variability of summer rainfall in Pakistan is a combined result of upper- and lower-tropospheric circulation anomalies, and of extratropical and tropical circulation anomalies.

Climatological seasonal changes of wind and rainfall in the Philippines

AbstractClimatological seasonal changes of rainfall and lower tropospheric circulation in and around the Philippines are analysed by utilizing the TRMM 3B42 to obtain the 5‐day mean rainfall and ERA‐Interim wind data for the period 1998–2013. In particular, climatological onset and withdrawal processes of the southwest (SW) and northeast (NE) monsoon are investigated. It is found that the onset of the SW monsoon occurs abruptly in mid‐May almost simultaneously over the Philippines except in the southern region. The start of the SW monsoon in the Philippines occurs earlier in the north than in the south. In addition, a climatological increase in rainfall on the west coast precedes the SW monsoon arrival by approximately two pentads, due to tropical cyclone influences in some years. After the SW monsoon onset, the monsoon trough is located over the southern Philippines. In mid‐June, the monsoon trough begins to deepen, and migrates northward to the central Philippines, and easterly winds intrude shortly to the northern Philippines. Then, the entire Philippines is covered by the summer monsoon westerly in early July, which is followed by the rainfall peak over the west coast region occurring in early August. The SW monsoon begins to retreat from the north in mid‐September, and fully retreats from the Philippines rather suddenly in late September. In general, the rainfall amount in the west coastal region remains high for approximately 2 months after this wind reversal, showing relatively larger post monsoon rainfall due mainly to tropical cyclone effects. During the NE monsoon season, the rainfall centre in the east coast is located in the northern and central region until mid‐December. Afterwards it is anchored in the southern region from late December to mid‐March, indicating the fully established NE monsoon season.

Interannual Variability of the Tropical Cyclone Landfall Frequency over the Southern and Northern Regions of East Asia in Autumn

Abstract This study focused on the interannual variability of tropical cyclone (TC) activity over the western North Pacific in autumn. The results show that the frequencies of TC landfalls in the southern and northern coastal regions of East Asia are roughly independent, implying that they are affected by different factors and should be studied separately. Further analysis indicates that the frequency of TC landfall in the southern region is closely related to El Niño–Southern Oscillation, which affects both the upper- and lower-tropospheric circulation over the western North Pacific and East Asia and induces changes in the steering flow. By contrast, the frequency of TC landfall over the northern region has a close connection with a teleconnection pattern in the upper troposphere over the Eurasian continent, which seems to be triggered by an anomalous Rossby wave source over the North Atlantic. This teleconnection pattern leads to anomalous meridional winds over the western North Pacific and East Asia and induces significant changes in the steering flow.

Assessment of climatological tropical cyclone activity over the north Indian Ocean in the CORDEX-South Asia regional climate models

This study evaluates the ability of the regional climate models (RCMs) in the Coordinated Regional Climate Downscaling Experiment for South Asia (CORDEX-SA) to simulate the tropical cyclone activity in the north Indian Ocean. The RCMs used in the CORDEX-SA are Rossby Centre regional atmospheric model version 4 (RCA4) and Regional Climatic Model version 4 (RegCM4) forced by the ECMWF Interim Reanalysis (ERA-Interim) from 1989 to 2008. Both models have reasonable skill in capturing the observed climatological seasonal and spatial distribution of the genesis and track of the tropical cyclones. However, the two issues faced are (i) in RegCM4 model, higher number of tropical cyclones throughout the year compared to observations in addition to the unrealistic simulation of tropical cyclones during winter and (ii) in the RCA4 model, relatively higher tropical cyclone activity over the Arabian Sea compared to the Bay of Bengal. In the RCA4, the anomalous large-scale lower tropospheric cyclonic circulation observed over the southern Arabian Sea might be responsible for the overestimation in the number of tropical cyclones over there. In both models, the relationship between minimum pressure and the maximum wind is well simulated even though there is an underestimation in the intensity of the tropical cyclones. Quantitative analysis of large-scale environmental parameters that influence the tropical cyclone activity using an index called genesis potential index showed that in the RegCM4 model, the model simulates seasonal variation (enhancement/suppression) of environmental conditions fairly good as seen in the observation. However, the model has higher number of tropical cyclones in all the seasons, which is due to the enhanced large scale dynamical and thermodynamical parameters. The cold and wet bias in the middle troposphere causes the reduction in the mid-layer saturation deficit throughout the year, which in turn favours an unrealistic high number of tropical cyclone genesis. Therefore, the changes in the large scale dynamics and relatively saturated middle troposphere in the model might be responsible for the higher number of tropical cyclones.

ENSO Features, Dynamics, and Teleconnections to East Asian Climate as Simulated in CAMS-CSM

This study evaluates the performance of CAMS-CSM (the climate system model of the Chinese Academy of Meteorological Sciences) in simulating the features, dynamics, and teleconnections to East Asian climate of the El Nino–Southern Oscillation (ENSO). In general, fundamental features of ENSO, such as its dominant patterns and phase-locking features, are reproduced well. The two types of El Nino are also represented, in terms of their spatial distributions and mutual independency. However, the skewed feature is missed in the model and the simulation of ENSO is extremely strong, which is found—based on Bjerknes index assessment—to be caused by underestimation of the shortwave damping effect. Besides, the modeled ENSO exhibits a regular oscillation with a period shorter than observed. By utilizing the Wyrtki index, it is suggested that this periodicity bias results from an overly quick phase transition induced by feedback from the thermocline and zonal advection. In addition to internal dynamics of ENSO, its external precursors—such as the North Pacific Oscillation with its accompanying seasonal footprinting mechanism, and the Indian Ocean Dipole with its 1-yr lead correlation with ENSO—are reproduced well by the model. Furthermore, with respect to the impacts of ENSO on the East Asian summer monsoon, although the anomalous Philippine anticyclone is reproduced in the post-El Nino summer, it exhibits an eastward shift compared with observation; and as a consequence, the observed flooding of the Yangtze River basin is poorly represented, with unrealistic air–sea interaction over the South China Sea being the likely physical origin of this bias. The response of wintertime lower-tropospheric circulation to ENSO is simulated well, in spite of an underestimation of temperature anomalies in central China. This study highlights the dynamic processes that are key for the simulation of ENSO, which could shed some light on improving this model in the future.

The role of air\u2013sea interactions in regulating the thermal effect of the Tibetan\u2013Iranian Plateau on the Asian summer monsoon

The thermal effect of the Tibetan–Iranian Plateau (TIP) on the Asian summer monsoon and the role of air–sea interactions over the Indian Ocean in regulating the effects of the TIP are explored. The results demonstrate that the direct thermal effect of the TIP produces a lower troposphere cyclonic circulation in the area surrounding the TIP and increases the continental precipitation over South and East Asia. It also decreases the precipitation over the tropical Indian Ocean and increases the sea-surface temperature (SST) of the tropical Indian Ocean with a large gradient zone located along 10°N but decreases SST of the western coast of Indonesia. In the lower troposphere, the air–sea interaction induced by the TIP thermal forcing produces an anticyclonic circulation surrounding the TIP and a stronger westerly flow to the south of the anticyclone. A circulation dipole thus forms to the south of the TIP. Together with this horizontal dipole, a meridional circulation dipole is generated to the south of the TIP, which is characterized by strong air ascent from 10 to 15°N where a strong westerly flow occurs, and the descent of air over the southern slope of the TIP and south Hemisphere. These results demonstrate that the indirect effect of the air–sea interaction over the Indian Ocean induced by the TIP thermal forcing is to counteract its direct effect on the Asian summer monsoon. The uncertainty of this indirect effect is also discussed.

Seasonal Variations of Aerosol Optical Depth over East China and India in Relationship to the Asian Monsoon Circulation

Seasonal variation features of aerosol optical depth (AOD) over East China and India in association with the Asian monsoon system are investigated, based on the latest AOD data derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Terra satellite, the NCEP Final (FNL) Operational Global Analysis data, the Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) data, and the NCEP/NCAR reanalysis data from March 2000 to February 2017. The results indicate that AOD in East China is significantly larger than that in India, especially in spring. The seasonal mean AOD in East China is high in both spring and summer but low in fall and winter. However, the AOD averaged over India is highest in summer and lower in spring, fall, and winter. Analysis reveals that AOD is more closely related to changes in surface wind speed in East China, while no obvious relation is found between precipitation and the AOD distribution on the seasonal timescale. As aerosols are mainly distributed in the atmospheric boundary layer (ABL), the stability of the ABL represented by Richardson number (Ri) is closely correlated with spatial distribution of AOD. The upper and lower tropospheric circulation patterns significantly differ between East China and India, resulting in different effects on the AOD. The effect of advection associated with lower tropospheric circulation on the AOD and the influence of convergence and divergence on the AOD distribution play different roles in maintaining the AOD in East China and India. These results improve our understanding of the mechanism responsible for and differences among the aerosol changes in East China and India.