Retreat Of Monsoon Research Articles

Present Climate and Future Changes in the Annual Cycle of TC Activity in the WNP Investigated by HighResMIP GCMs

Abstract Atmospheric general circulation models (AGCMs) and coupled general circulation models (CGCMs) in the High Resolution Model Intercomparison Project (HighResMIP) were evaluated on their ability to simulate tropical cyclone (TC) activity in the western North Pacific (WNP) over its annual cycle. Specifically, we examined these models’ ability to simulate the south–north migration of the mean TC genesis location. The results revealed that both types of models realistically captured TC numbers and the south–north migration of TC genesis locations associated with the meridional migration of the WNP subtropical high ridge in response to the annual cycle. However, TC numbers decreased less rapidly in the AGCMs than in both the CGCMs and observed data during the monsoon retreat period (after September). This bias was probably attributed to a low-tropospheric cyclonic anomaly over the Philippine Sea in response to La Niña–like sea surface temperature (SST) differences between the AGCMs and the CGCMs. Because of these differences, the TC genesis frequency in the AGCMs over the Philippine Sea was overestimated. The cyclonic anomaly occurred when the northeasterly trade wind arose and was maintained through wind–evaporation–SST feedback. In future climate (2021–50), the main changes occurred during the monsoon retreat period. A more rapid decrease in TC numbers shown in AGCMs was likely attributed to the decrease (increase) of low-level vorticity (vertical wind shear) modulated by enhanced WNP subtropical high and subsidence anomalies. Conversely, a cyclonic circulation and ascending anomalies projected by CGCMs were identified off-equator, which favored TC genesis location shifting northward. Significance Statement Model performance and future changes in the annual cycle of tropical cyclone (TC) activity in the western North Pacific are investigated. We found that high-resolution uncoupled and coupled models realistically captured the TC numbers and meridional migration of TC genesis locations associated with the meridional migration of subtropical high ridge. However, TC numbers decreased more gradually in the uncoupled models than in the coupled models after September. This lower skill may stem from differences between the uncoupled and coupled models in simulating a cyclonic anomaly over the Philippine Sea in response to the La Niña–like sea surface temperature difference. In future climate (2021–50), TC numbers are significantly projected to decrease more rapidly during the monsoon retreat period in uncoupled models.

A comparison of intense rainfall characteristics and mechanisms between monsoon onset and retreat over the Yangtze River Basin

ABSTRACT The thermodynamic and convective characteristics of the seasonal progression of the monsoon over eastern China are examined, with the aim of understanding why regional heavy rainfall events (RHREs) over the Yangtze River Basin (YRB) are more frequent and intense during the monsoon progression from mid-June to mid-July than during the retreat from mid-August to mid-September. During the monsoon progression, the southerly monsoon flow at low and mid-levels intensifies and moves northward, while the Western Pacific subtropical high shifts northeastward. These changes result in enhanced moisture advection over eastern China. The stronger southerly flow brings warmer and moist air into the YRB, leading to a higher equivalent potential temperature (θE) and favouring convection. Conversely, during the monsoon retreat, the northerly flow becomes stronger and drier, causing lower θE air over the YRB. Additionally, as the Western Pacific subtropical anticyclone retreats, easterly prevailing winds prevail over eastern China, causing reduced specific humidity. The monsoon progression period exhibits higher convective available potential energy but also higher convective inhibition, which is overcome by the presence of the monsoon front providing sufficient dynamical uplift. Understanding the severity of RHREs, particularly over the YRB, is crucial for improving forecasting capabilities and reducing societal vulnerability.

The relationship between the Bay of Bengal summer monsoon retreat and early summer rainfall in East Asia

As the upstream region of the Asian summer monsoon, the Bay of Bengal summer monsoon (BOBSM) system has impacts on rainfall patterns in East Asia. In this study, we investigate the impact of the interannual variability of the BOBSM retreat on China precipitation in early summer (June) of the following year. When the BOBSM retreat occurs earlier in the previous year, we find enhanced rainfall in both the northeastern and eastern parts of China. Conversely, when the retreat of the BOBSM is delayed in the previous year, there is a tendency for decreased rainfall in most of northeastern and eastern China, while rainfall in the northern part of the Taiwan island region tends to increase. Statistical analysis demonstrates the co-variability between China’s June precipitation anomalies and preceding wind anomalies in the eastern Bay of Bengal. The results indicate a strong relationship between the preceding BOBSM retreat and China precipitation anomalies in the following June. Furthermore, the analysis suggests that the BOBSM retreat is more of an independent signal rather than modulated by an Indian Ocean Dipole event.

Climatological Madden-Julian Oscillation during boreal spring leads to abrupt Australian monsoon retreat and Asian monsoon onsets

Abrupt monsoon onsets/retreats are indispensable targets for climate prediction and future projection, but the origins of their abruptness remain elusive. This study establishes the existence of three climatological Madden-Julian Oscillation (CMJO) episodes contributing to the rapid Australian summer monsoon retreat in mid-March, the South China Sea (or East Asian) summer monsoon onset in mid-May, and the Indian summer monsoon onset in early June. The CMJO displays a dynamically coherent convection-circulation structure resembling its transitionary counterpart, demonstrating its robustness as a convectively coupled circulation system and the tendency of the transient MJOs’ phase-lock to the annual cycle. The CMJO is inactive during the boreal winter due to destructive year-to-year modulations of El Niño-Southern Oscillation. We hypothesize that the interaction between atmospheric internal variability (MJO) and the insolation-forced slow annual cycle generates the sudden monsoon withdrawal/onset during the boreal spring. Understanding the factors determining the timing and location of the MJO’s phase-locking and its variability is vital for monsoon forecasting and climate projection.

Australian Summer Monsoon Bursts: A Moist Static Energy Budget Perspective

AbstractThe Australian monsoon's wet season is associated with sequences of wet and dry conditions known as bursts and breaks, which usually have timescales of a week or two. There are several hypotheses for the physical processes involved in monsoon bursts, ranging from the effects of the Madden‐Julian Oscillation to extratropical influences. We analyze rainfall bursts in Northern Australia using a moist static energy (MSE) budget framework. We separate the bursts into three types and further separate them into pre‐monsoon, active monsoon, and post‐monsoon. We then apply ERA5 data to calculate the MSE budget for each burst and construct composite bursts for each of the three types. We find that the horizontal advection of MSE over the tropical northern Australian convergence zone is the most critical term in the budget for the day‐to‐day precipitation variation. MSE‐related gross moist stability analysis reveals that each type has distinct characteristics. The burst type associated with active monsoon bursts displays convective growth and decay patterns both before and after the precipitation peak, which is typical of oceanic convection. A low‐rainfall burst type, which is often seen before monsoon onset, is linked to coastal convection and has minimal rainfall decay after the burst. Finally, a sustained high‐rainfall burst type, mostly observed at the end or after the monsoon retreat, has growth characteristics similar to the typical monsoon burst but does not decay, maintaining very high rainfall for at least a week due to a rearrangement of the circulation and local thermodynamic structure of the atmosphere.

An Origin of North American Monsoon Retreat Biases in Climate Models

Abstract Many coupled climate models suffer from a late retreat bias in North American monsoon (NAM) simulations, which is manifested by overestimated precipitation in October. The overestimated precipitation has long been attributed to the negative sea surface temperature (SST) biases in the tropical Atlantic and insufficient model resolution to resolve mesoscale features. However, we found little correlation between CMIP6 model resolutions and the simulated NAM retreat-season precipitation in October. Instead, we showed that tropical eastern North Pacific SST biases and the associated large-scale circulation biases play a dominant role in inducing the retreat-season biases, with SST biases in other ocean basins playing a secondary role. As revealed by simulations using a hierarchy of models, the positive SST biases in the tropical eastern North Pacific enhance local convection and lead to positive diabatic heating biases throughout the troposphere; the diabatic heating biases generate a Matsuno–Gill type of response that strengthens the subtropical high over the North Atlantic and weakens the subtropical high over the North Pacific, enhancing the low-level northward moisture transport from the tropics to the NAM region. The conclusion is robust across phase 6 of CMIP (CMIP6) models. The precipitation seasonality in the NAM region is used to constrain future projection. The “good” CMIP6 models project that the timing of the NAM peak season remains the same, but the peak-season precipitation is reduced and monsoon retreat is delayed, while the “poor” CMIP6 models project a delayed monsoon peak season with slightly enhanced peak-season precipitation. Both model groups project a drier dry season in the NAM region.

Rapid retreat of the East Asian summer monsoon on the western Chinese loess Plateau during the middle to late Holocene and its environmental and societal implications

Spatiotemporal changes in the Holocene climate of the arid region of China were principally caused by shifts in the intensity and location of the East Asian summer monsoon (EASM). However, our knowledge of these EASM variations is limited, especially since the middle Holocene. We investigated two aeolian sedimentary sections (the Linxia/LX and Gulang/GL sections) on the western Chinese Loess Plateau (CLP), to explore the variations in the intensity and location of the EASM and their impact on the regional environment and human cultural development during the middle to late Holocene. The longitudes of these two loess sections are similar, but the GL section is located on the northern edge of the CLP, in the eastern Hexi Corridor, and it is situated further north than the LX section. The chronologies of both sections are based on accelerator mass spectrometry 14C dating, and environmental magnetic parameters and grain size end-members were used to reconstruct their moisture histories. The results suggest that the climate at the LX section was relatively dry before 3.5 ka, after which it became wetter. In contrast, at the GL section, the climate was also relatively dry before 3.5 ka, with the wettest conditions occurring during 3.5–2 ka, but subsequently there was an abrupt shift to a dry climate. We suggest that these spatiotemporal patterns reflect the westward movement and rapid southward retreat of the EASM and the linked monsoon rainfall belt. TraCE-21 ka climate simulation results support our inference. We also found that, compared with other ancient cultures in the eastern Hexi Corrido, sites of the Shajing culture, dated to ∼2.4–2.8 ka, are predominantly located in areas that are arid at the present-day, and relatively distant from rivers. We suggest that this distribution pattern reflected the occurrence of a humid climate during the period of the Shajing culture, thus demonstrating the impact of the EASM on cultural development in this region.

Diagnosis of atmospheric circulation shifts in the central Tibetan Plateau: Evidence from stable isotopes

The productivity of the alpine meadows in the central Tibetan Plateau (TP) is usually linked to atmospheric circulation shifts between the westerlies and Indian monsoon. Hence, it is critical to identify the dates of these circulation shifts, i.e., the Indian monsoon onset and retreat. However, the exact timing of the Indian monsoon onset in the central TP is highly uncertain. Here, we diagnose the atmospheric circulation shifts in the central TP during 2016–2018 using hydrogen and oxygen stable isotopes. Our results show that the Indian monsoon onset was marked by sharp decreases of the stable isotopes in both water vapor and precipitation (δ2Hv and δ2Hp) in mid-late June, which is approximately one month later than previous findings. In the subsequent period, the Indian monsoon replaced the westerlies across the central TP, where water vapor and precipitation were characterized by persistently lower stable isotope values. Substantial increases of the δ2Hv and δ2Hp values in late September denoted the Indian monsoon retreat. The δ2Hv and δ2Hp values also record fluctuations of the Indian monsoon intensity and the occasional intrusions of the westerlies even during the monsoon season. Our findings propose that stable isotopes can be used as indicators of atmospheric circulation shifts and may act as potential indices to monitor the vegetation growth in the central TP.

Combined Effects of Tropical Indo-Pacific–Atlantic SST Anomalies on Record-Breaking Floods over Central-North China in September 2021

Abstract The rainy season typically terminates over the area of central northern China (CNC) in late August with the rapid retreat of the East Asian summer monsoon (EASM). However, the CNC suffered continuous heavy rainfall in September 2021. The monthly accumulative precipitation amount broke its historical record since 1961 and caused severe floods. The present study ascribes this extreme rainfall event to the westward inland intrusion of the western North Pacific subtropical high (WNPSH). Both case study and statistical analysis indicate that westward extension of the WNPSH is significantly positively correlated with convection over the Maritime Continent (MC) and northern Indian Peninsula (NIP) under the influence of sea surface temperature anomalies (SSTAs). Atmospheric general circulation model (AGCM) experiments validate the individual and combined effects of SSTAs in the tropical Indo-Pacific–Atlantic Oceans on WNPSH and CNC rainfall. In particular, the warm SSTAs in the Indo-Pacific warm pool increase the probability of extremely westward extension of the WNPSH and above-normal rainfall over the CNC. The cold SSTAs in the equatorial central-eastern Pacific enlarge the zonal SSTA gradient, strengthen the anomalous convection over the MC and NIP, and intensify the WNPSH and CNC rainfall anomalies. Meanwhile, the warmer tropical Atlantic anchors the anomalous WNPSH and CNC rainfall in their observed positions. A statistical model based on the tropical SSTAs in August can skillfully predict the westward extension of the WNPSH in September 2021, suggesting that the combined effects of tropical Indo-Pacific–Atlantic SSTAs in late summer possibly tend to prolong rainy season of the EASM until early autumn.

Climatological characteristics of the East Asian summer monsoon retreat based on observational analysis

Climatological characteristics of the East Asian summer monsoon retreat based on observational analysis

Responses of the Optical Properties and Distribution of Aerosols to the Summer Monsoon in the Main Climate Zones of China

The influence of aerosols on climate varies greatly within different spatial zones. China has a very prominent summer monsoon climate and summer monsoon activity basically determines the climate distribution pattern. Consequently, we need to understand the aerosol optical properties and spatial distribution under the background of summer monsoon activity in China, which is the basis for further research on the impact of aerosols on the climate system. Based on Moderate Resolution Imaging Spectroradiometer (MODIS) and Modern-Era Retrospective Analysis for Research and Applications (MERRA-2) data, the spatial response of the high aerosol optical depth (AOD) region in China to the advance and retreat of summer monsoon was analyzed. The main types of aerosol and the contribution of each type of aerosol particles to the total AOD were discussed. The results showed that before the landing of summer monsoon, the high value areas of AOD were distributed in the eastern Sichuan Basin, Changsha, Wuhan and Pearl River Delta regions. With the northward advance of the monsoon, the high value areas moved to the transition region affected by the summer monsoon and the AOD in this region was highly sensitive to the summer monsoon. The main aerosol types were dust and sulfate in this region and the contribution to total AOD was 27% and 57%, respectively; before the monsoon onset, the contribution of dust to total AOD was 16%, and that of sulfate was 18%; after the monsoon onset, the contribution of dust decreased by half to 8%, while the contribution of sulfate aerosol increased to 20%.

Association of retreating monsoon and extreme air pollution in a megacity

The world's top ranked mega city Delhi is known for deteriorated air quality. However, the analysis of air pollution data of 5 years (2014–2018) reveals that years 2016 and 2017, which were marked by an unusual delayed withdrawal of monsoon, witnessed an unprecedented extreme levels of toxic PM2.5 particles (≤2.5 µm in diameter) touching a peak level of ∼760 µg/m3 (24 hr average), immediately after the monsoon retreat, surpassing WHO standards by ∼30 time and Indian national standards by ∼12 times, jeopardising lives of its citizens. However, the normal monsoon withdrawal years do not show such extreme levels of pollution. The high resolution WRF-Chem model along with meteorological data are used in this work to understand that how the delayed monsoon withdrawal and associated vagarious anti-cyclonic circulation resulted in trapping externally generated pollutants ceaselessly under colder conditions, leading to historic air quality crisis in landlocked mega city in these selected years. The sensitivity analysis confirmed that when WRF-chem model forced the climatology of normal monsoon year (2015) to simulate the pollution scenario of 2016 and 2017 for the above time period, the crisis subsided. Present findings suggest that such unusual monsoon patterns are on the hook to spur extreme pollution events in recent time.

Delayed retreat of the summer monsoon over the Indochina Peninsula linked to surface warming trends

AbstractThe Indochina Peninsula (ICP) is dominated by a tropical monsoon climate, and changes of the prevailing winds here have a marked impact on moisture and aerosol transport. September to early October is within the monsoon retreat period that is preceded by a change in the zonal wind from westerly to easterly. Here, we show the long‐term change of the monsoon retreat date and its related climate factors. Climatologically, the zonal wind is primarily controlled by the meridional land–sea temperature gradient through the geostrophic balance. A simple physical model suggests that a 1 K increase in this gradient is associated with a zonal wind speed change of 2.17 m·s−1, and this estimate is consistent with reanalyses. Over the most recent 40 years there has been a positive trend of 0.107 K·decade−1 in the gradient, and linear regression shows that the average rate of delay of the summer monsoon retreat date over the ICP associated with trend in land–sea contrast is 23.5 day·K−1. The mixing ratio gradient also makes a contribution to the monsoon retreat but its significance is lower than the land–sea temperature contrast. The ICP is located in a transitional zone bounded by three sub‐regions of Asia‐Pacific monsoon system; the delayed ICP monsoon retreat is most consistent with changes of the East Asian‐Western North Pacific monsoon.

Large–scale seasonal control of air quality in Taiwan

By investigating data during 2004–2015, this study explored the dynamic mechanisms of seasonal control of air quality over southern and northern Taiwan. Overall, higher (lower) pollution frequencies were observed in dry (wet) stages. Analysis of seasonality further suggested that pollution over southern Taiwan exhibited a symmetric characteristic, and that tropical modulation was a driving factor, whereas pollution over northern Taiwan displayed an asymmetric seasonality and likely followed the asymmetric East Asian seasonality. With the retreat of summer monsoon, air quality in southern Taiwan entered a poor phase rapidly; by contrast, good air quality was observed over northern Taiwan, which was due to the gradual southward migration of the western North pacific (WNP) high. Detailed dynamic processes contributing to atmospheric conditions and large-scale circulation were further compared between pollution and nonpollution cases. During dry stages, the midlatitude perturbations observed as wavy patterns had downstream effects on the atmospheric condition in Taiwan. The downstream effect of the midlatitude wave activity on air pollution was obviously found in the Dry Northerly stage in northern Taiwan and the Dry Southerly stage in southern Taiwan. In contrast to poor air quality during dry stages, air pollution was rarely observed during wet stages. The strength and location of the WNP high, which can be involved in the formation of nearby tropical cyclones, may mainly influence air quality in Taiwan.

Climatological characteristics of the synoptic changes accompanying South China Sea summer monsoon withdrawal

This manuscript analyses the climatological characteristics (especially the synoptic phenomena and changes) of the South China Sea (SCS) summer monsoon withdrawal (SCSSMW) based on monsoon retreat dates from the National Climate Center of China Meteorological Administration. The SCSSMW is mainly defined as the westerly to easterly shift in the zonal winds due to the westwards intrusion of the western North Pacific (WNP) subtropical high in the northern SCS region. At the low level (850 hPa), the weakening and retreat of the intertropical convergence zone (ITCZ) and rain belt in the SCS–WNP are pertinent, and the southwesterly winds over the north Indian Ocean and the SCS also weaken and retreat. The anomalous anticyclone centred over the northern SCS resembles a Rossby wave response to the reduced precipitation over the SCS and the Philippine Sea. Changes in the upper‐level (200 hPa) circulations include the deceleration of the tropical easterly stream (from the Indo‐China Peninsula to the Arabian Sea) and northerly cross‐equatorial flow (around the equatorial eastern Indian Ocean and Maritime Continent). At the mid‐level (500 hPa), the ascending centre moves equatorwards, shifting from the northern to the southern SCS. There appears an anomalous vertical circulation cell (descending/ascending in the northern/southern SCS), which links the anomalous horizontal circulation through the Sverdrup balance. In addition, the upper‐level divergent centre is shifted southeastwards, as is the generating centre of tropical cyclones. Moreover, an anomalous convergent centre over Taiwan Island is prominent in the upper level. The time series of several atmospheric variables (e.g., zonal wind, precipitation, convection) also exhibited abrupt changes during the SCSSMW. The reasons for not simultaneous retreat of monsoonal westerlies and rainy season in the SCS are also explored.

Effect of Horizontal Resolution on the Representation of the Global Monsoon Annual Cycle in AGCMs

The sensitivity of the representation of the global monsoon annual cycle to horizontal resolution is compared in three AGCMs: the Met Office Unified Model-Global Atmosphere 3.0; the Meteorological Research Institute AGCM3; and the Global High Resolution AGCM from the Geophysical Fluid Dynamics Laboratory. For each model, we use two horizontal resolution configurations for the period 1998–2008. Increasing resolution consistently improves simulated precipitation and low-level circulation of the annual mean and the first two annual cycle modes, as measured by the pattern correlation coefficient and equitable threat score. Improvements in simulating the summer monsoon onset and withdrawal are region-dependent. No consistent response to resolution is found in simulating summer monsoon retreat. Regionally, increased resolution reduces the positive bias in simulated annual mean precipitation, the two annual-cycle modes over the West African monsoon and Northwestern Pacific monsoon. An overestimation of the solstitial mode and an underestimation of the equinoctial asymmetric mode of the East Asian monsoon are reduced in all high-resolution configurations. Systematic errors exist in lower-resolution models for simulating the onset and withdrawal of the summer monsoon. Higher resolution models consistently improve the early summer monsoon onset over East Asia and West Africa, but substantial differences exist in the responses over the Indian monsoon region, where biases differ across the three low-resolution AGCMs. This study demonstrates the importance of a multi-model comparison when examining the added value of resolution and the importance of model physical parameterizations for simulation of the Indian monsoon.

Response of dune mobility and pedogenesis to fluctuations in monsoon precipitation and human activity in the Hulunbuir dune field, northeastern China, since the last deglaciation

The Hulunbuir dune field is located at the northern margin of the temperate monsoon zone in East Asia, and changes in dune activity and pedogenesis in the dune field are highly sensitive to the advance and retreat of the East Asian summer monsoon (EASM) and to human activity. Thus, the stratigraphic sequences of paleosol and aeolian sand of the dune field have great potential for reconstructing the history of dune activity and pedogenesis and their response to past fluctuations in monsoon precipitation and the intensity of human activity. However, our knowledge of the evolution of the landscape and paleoclimate of the dune field is limited. Here, we present the results of analyses of quartz optically stimulated luminescence (OSL), grain size and magnetic susceptibility (MS) from 13 representative sections in the Hulunbuir dune field. The grain-size analysis indicates that the sand layers are composed of typical aeolian sand, and the MS variations of the aeolian sand and sandy paleosol sequences mainly reflect changes in the intensity of pedogenesis. We combine our new OSL dating results with previously published OSL ages to refine the chronology of sand dune development. An OSL age hiatus in both aeolian sand and paleosol during the 5–7 ka indicates that aeolian deposition in Hulunbuir ceased almost completely during this period, which corresponds to the peak in EASM strength. Thus, our study provides new evidence for a delayed response of the EASM maximum to peak insolation forcing (11–10 ka) in the mid-latitude monsoon margin. In addition, we speculate that peaks in probability density (PD) from 1.2–0.7 ka and from 0.25–0 ka of aeolian sand accumulation correspond to population increases and the development of cultivation and animal husbandry in the Hulunbuir dune field.

Role of Madden–Julian Oscillation in Modulating Monsoon Retreat

The Madden–Julian oscillation (MJO) is the major fluctuation in tropical weather on a seasonal scale. The impact of MJO on different epochs, viz., onset, advance and active break is well known. There can be several MJO events in a season and it may enhance/suppress the retreat process. The present study aims to find the MJO-modulated retreat of monsoon. The results suggest that the fastest retreat of monsoon occurred in the years 2007 and 2008, while slowest retreat of monsoon occurred in the year 1979. The retreat features of the Indian summer monsoon (ISM) are investigated with the MJO phase and amplitude variations. The daily MJO indices for the retreat period 1979–2016 are used. The results reveal that the MJO strength decreases during the transition phase (i.e., summer monsoon to winter monsoon transition). The monsoon retreat is most favored by strong MJO phase 4 and phase 5. The fastest retreat of monsoon occurred in the years 2007 and 2008, while the slowest retreat of monsoon occurred in the year 1979. There exists a weak positive correlation between the MJO amplitude and the retreat period of monsoon. The monsoon retreat is most favored by strong MJO phase 4 and phase 5. The MJO amplitude variations during MJO phases 1–8 suggest that the MJO amplitude decreases with increase in retreat period. The MJO-modulated retreat results in slow retreat of monsoon, whereas fast and normal retreat of monsoon is seen on rare occasions. Weak MJO events lead to normal retreat of monsoon.

Radio refractivity gradients in the lowest [formula omitted] of the atmosphere over Lagos, Nigeria in the rainy-harmattan transition phase

Radio engineers and researchers in conjunction with the International Telecommunication Union (ITU) have established the pivotal role of radio refractivity to the propagation of electromagnetic energy in the troposphere. In particular, the refractivity gradient statistics for the lowest 100m in the troposphere are used to determine the probability of occurrence of anomalous propagation conditions known as ducting. The major challenge to characterising the propagation condition over any environment is accessing the data of the lowest boundary layer of the atmosphere, which is highly dynamic and turbulent in evolution. High resolution radiosonde data from the Nigerian Meteorological Agency (NiMet) were used for a synoptic study of the rain-harmattan transition phase. The rain-harmattan transition phase marks the onset of the dry season due to the movement of the intertropical convergence zone interplay between (north-easterly and south-westerly) trade winds and monsoonal circulation. The lowest 100m data were analysed to determine the frequency of ducting per month. Progressive increase in the occurrence of ducting was observed during the rain-harmattan transition phase, which coincides with the West African Monsoon retreat. The results show significant divergence from previous studies, which reported that the tropospheric condition over Lagos (Geo. 6.5°N, 3.3°E), Nigeria, is predominantly super-refractive.

Rapid retreat of the East Asian summer monsoon in the middle Holocene and a millennial weak monsoon interval at 9 ka in northern China

Knowledge of hydroclimatic dynamics in the East Asian monsoon region during the Holocene was hindered by few absolutely-dated and decadally-resolved proxy records in northern China. Here we present replicated carbonate δ18O records of six stalagmites with sub-decadal to multi-decadal resolutions from the Lianhua cave to reveal a detailed evolution of the East Asian Summer Monsoon (EASM) intensity in northern China since 11.5 thousand years before present (ka BP, before 1950 CE). Our composite record shows that solar forcing dominated hydroclimatic changes regionally, including an intensified monsoon at the Holocene Optimum from the termination of Younger Dryas to 6.5 ka BP, and a subsequent multi-millennial weakening monsoon, that agrees with cave records in central and southern China. However, the EASM has retreated southwards more rapidly than the Indian summer monsoon after ∼6.5 ka BP, resulting in aridity conditions occurring at 4.0 ka BP in northern China, which is almost 2000-year earlier than that in central and southern China. This north–south asynchroneity is likely related to the different regional responses among the coupling of the EASM, Indian summer monsoon, the solar forcing, and the differences in thermal forcing due to complex geographical configurations. In addition, a relative enrichment of 1‰ in 18O data of the Lianhua record from 9.5 to 8.1 ka BP shows that the Holocene Optimum was punctuated by a millennial-long weakening monsoon interval, which is not registered among previous cave records in central and southern China. The fresh water-induced cold climate conditions in the North Atlantic region could create stronger East Asian winter monsoon, and induce a weakened EASM and a southward shift of rain belt in northern China. Therefore, it shall not be surprised that there are strong heterogeneities among regional hydroclimatic conditions across monsoonal China, given the complex interplay between external and internal forcing mechanisms over the entire Holocene.