Subtropical High Research Articles

Maintenance Mechanisms of the Wintertime Subtropical High over the South Indian Ocean

Abstract Climatologically the surface Mascarene high over the subtropical south Indian Ocean (SIO) shifts westward toward austral winter, and its strength as a planetary-wave component maximizes in late austral winter, unlike its counterpart over other subtropical oceans. The present study investigates the maintenance mechanisms for the wintertime Mascarene high with a linear atmospheric dynamical model (LBM) and an atmospheric general circulation model (AGCM). The LBM experiments reveal the importance of cross-equatorial tropical influences. Deep convection associated with the Asian summer monsoon acts not only to shift the Mascarene high westward as its direct influence but also to enhance midtropospheric subsidence and equatorward surface winds over the central and western portions of the subtropical SIO. The associated near-surface cold advection and subsidence promote (suppress) the formation of low-level (deep convective) clouds. The resultant enhanced radiative cooling and reduced deep condensation heating both reinforce the equatorward portion of the surface high. The LBM experiments also reveal that seasonally enhanced storm-track activity over the SIO is important for maintaining the poleward portion of the Mascarene high through eddy heat and vorticity fluxes. The AGCM experiments demonstrate that the Agulhas Current system and the associated sea surface temperature (SST) front reinforce the high by energizing the storm-track activity. The present study thus proposes that both the Asian summer monsoon and the enhanced storm-track activity maintained by the Agulhas SST front externally modulate the positively coupled system between the wintertime Mascarene high and low-level clouds to realize its unique seasonality.

Concurrent hot extremes and high ultraviolet radiation in summer over the Yangtze Plain and their possible impact on surface ozone

Hot extremes, ultraviolet (UV) radiation, and surface ozone all have prominent effects on human health and ecosystems. Here we show evidence that both hot extremes and high surface UV radiation at noon time occur concurrently in summer over the Yangtze Plain. Composite analysis suggests that hot extremes in summer are primarily caused by the westward extension of the Western Pacific Subtropical High, which leads to less clouds and consequently more downward solar radiation on the surface over the Yangtze Plain. It is found that surface UV radiation may be dominated by cloud variations, instead of stratospheric ozone during the hot extremes. Further analysis indicates that the hot extremes and high UV radiation, which play important roles in photochemistry in the troposphere, may result in more surface ozone. The concurrent hot extremes, strong UV radiation, and severe ozone pollutions over the Yangtze Plain in summer are likely to have dramatical influences on human health, which should be paid more attention.

ENSO effect on hydroclimate changes in southeastern China over the past two millennia

Meteorological observations indicate that both natural and anthropogenic forcing contribute to regional drought/flood in the East Asian summer monsoon (EASM) domain. However, spatiotemporal rainfall pattern and its dynamics during natural climatic variability remains unclear. Here we reconstruct a ∼3 year-resolution EASM precipitation record over the past two millennia, based on 13 230Th dates and 600 δ18O in a stalagmite from Songya Cave, southeastern China. The δ18O sequence shows a long-term decreasing trend, indicating an increasing monsoon precipitation over the past two millennia. A series of centennial-scale fluctuations are superimposed on the long-term trend, with a wetter Little Ice Age than the Medieval Warm Period. The long-term trend and centennial-scale oscillations in EASM rainfall are broadly related to El Niño-Southern Oscillation (ENSO) and Pacific decadal oscillation (PDO) variations, with an increased (decreased) EASM rainfall corresponding to El Niño-like (La Niña-like) conditions and positive (negative) phase of PDO. Comparison of δ18O records from Songya and Wanxiang Cave shows an anti-phased spatial rainfall pattern between southeastern and northwestern China. This spatiotemporal rainfall pattern, consistent with the modern observations, is possibly regulated by the ENSO, through changes in the location and strength of the Western Pacific Subtropical High (WPSH).

July–September rainfall in the Greater Horn of Africa: the combined influence of the Mascarene and South Atlantic highs

July-September rainfall is a key component of Ethiopia’s annual rainfall and is a source of rainfall variability throughout inland Greater Horn of Africa. In this study we investigate the relative influences of the Mascarene (MH) and South Atlantic (AH) highs on July-September rainfall in a covarying region of the Greater Horn of Africa using CHIRPS observed rainfall and the ERA5 reanalysis. We show that a mixed metric using the circulation at 850 hPa of these two subtropical anticyclones (AH-MH), is better correlated with rainfall than individual high circulations. Variations in remote circulation are translated by changes in Central African westerlies and Turkana Jet wind speeds. We apply the AH-MH mixed metric to the CMIP5 and CMIP6 ensembles and show that it is a good indicator of mean July-September rainfall across both ensembles. Biases in circulation are shown to be related to the Hadley circulation in CMIP5 atmosphere-only simulations, while causes of biases in CMIP6 are more varied. Coupled model biases are related to southern ocean warm biases in CMIP5 and western Indian Ocean warm biases in CMIP6. CMIP6 shows an improved relationship between rainfall and Turkana Jet winds and Central African westerlies across the ensemble.

Clustering of large‐scale circulation patterns during tropical cyclone preformation period using self‐organizing maps

AbstractBased on self‐organizing maps (SOM), clustering analysis is performed on the lower‐level large‐scale flow fields centred at the tropical cyclone (TC) genesis points averaged over the 24‐hr TC preformation period. A total of 461 TCs over the western North Pacific (WNP) from June to November in 2000–2019 are classified into five patterns, that is, convergence (CON) pattern, monsoon trough (MT) pattern, pre‐existing cyclone (PC) pattern, subtropical high (SH) pattern and easterly wave (EW) pattern. The most frequent and southmost pattern, the CON pattern, is characterized by a confluent zone of monsoonal southwesterly flows and easterly flows. The second dominant pattern, the MT pattern, occurs the westmost and has a larger cyclonic vorticity in the strongest monsoon trough among the five patterns. In contrast, cyclogenesis in the SH pattern is featured by both the confluent zone and the cyclonic wind shear largely attributed to the subtropical high. The PC pattern features a pre‐existing cyclone to the northwest. The EW pattern with easterly flows in all four adjacent quadrants has the smallest number of TC formation and relatively northeast genesis location. Statistics also shows that the CON pattern dominates in late season, while the other four mainly appear in summertime. Large‐scale environmental conditions are quantified and comparisons with previous studies are also discussed.

Modelling hydrometeorological extremes associated to the moisture transport driven by the Great Plains low-level jet

The Great Plains Low-Level Jet (GPLLJ) system consists of very strong winds in the lower troposphere that transport a huge amount of moisture from the Gulf of Mexico to the American Great Plains. This paper aims to study the extremes of the Transported Moisture (TM) from the GPLLJ source region to the jet domain; and, for low and high TM, to analyze the extremal dependence between the upper tail of the precipitation in the GPLLJ sink region and the lower tail of the tropospheric stability in that region, which is known as tropospheric instability. The declustered extremes of TM were analyzed using Peaks Over Threshold (POT). A non-stationary Exponential model was fitted to the cluster maxima. Estimated return levels show that the extremes of TM are expected to decrease in the future. This is meteorologically congruent with the known displacement of the western edge of the North Atlantic Subtropical High, which controls atmospheric circulation in the North Atlantic, and to a higher scale with the change of phase from negative to positive of the Atlantic Multidecadal Oscillation. Bilogistic and Logistic models were fitted to the extremes of (tropospheric instability, precipitation) for low and high TM, respectively. The extremal dependence between tropospheric instability and precipitation proves to be stronger in the case of high TM. This confirms that dynamical instability is the most important parameter for achieving high values of precipitation once there is a mechanism that allows the continuous supply of large amounts of moisture, such as the derived from a low-level jet system.

Comparison of the Performance of the Observation-Based Hybrid EDMF and EDMF-TKE PBL Schemes in 2020 Tropical Cyclone Forecasts from the Global-Nested Hurricane Analysis and Forecast System

Abstract Better representation of the planetary boundary layer (PBL) in numerical models is one of the keys to improving forecasts of TC structure and intensity, including rapid intensification. To meet this goal, our recent work has used observations to improve the eddy-diffusivity mass flux with prognostic turbulent kinetic energy (EDMF-TKE) PBL scheme in the Hurricane Analysis and Forecast System (HAFS). This study builds on that work by comparing a modified version of EDMF-TKE (MEDMF-TKE) with the hybrid EDMF scheme based on a K-profile method (HEDMF-KP) in the 2020 HAFS-globalnest model. Verification statistics based on 101 cases in the 2020 season demonstrate that MEDMF-TKE improves track forecasts, with a reduction in a large right bias seen in HEDMF-KP forecasts. The comparison of intensity performance is mixed, but the magnitude of low bias at early forecast hours is reduced with the use of the MEDMF-TKE scheme, which produces a wider range of TC intensities. Wind radii forecasts, particularly the radius of maximum wind speed (RMW), are also improved with the MEDMF-TKE scheme. Composites of TC inner-core structure in and above the PBL highlight and explain differences between the two sets of forecasts, with MEDMF-TKE having a stronger and shallower inflow layer, stronger eyewall vertical velocity, and more moisture in the eyewall region. A case study of Hurricane Laura shows that MEDMF-TKE better represented the subtropical ridge and thus the motion of the TC. Finally, analysis of Hurricane Delta through a tangential wind budget highlights how and why MEDMF-TKE leads to faster spinup of the vortex and a better prediction of rapid intensification.

Long‐Lead Predictability of Western North Pacific Subtropical High

AbstractWe present a new assembly of three steppingstones in thorough inquiry of the complex chain reaction from the diverse equatorial sea surface temperature (SST) pattern two‐season ahead to the East Asia summer weather condition, by regarding the western North Pacific subtropical high (WNPSH) as an anchor. We identified three predictors that are discordant with conventional climatic indices and achieve a good skill in the summer WNPSH prediction (temporal correlation coefficient of 0.78) with a two‐season long lead in the leave‐1‐year‐out cross‐validation during 1981–2020, despite the diversity of El Niño‐Southern Oscillation (ENSO) and Indian ocean dipole (IOD) patterns. Specifically, the first predictor captures the initiation and development of both IOD and El Niño via the SST warming over central equatorial Pacific in autumn; the second one serves as a better delegate of IOD by capturing the easterly acceleration over tropical Indian ocean; the third one illustrates the unprecedented informative role of upper‐level high pressure over the entire tropics in the WNPSH prediction with 5months lead time. They mediate the discordance between conventional climate indices and the diverse WNPSH response and caution people to revisit and rethink the complex atmosphere‐ocean coupled response from the diverse tropical SST patterns two seasons ahead to EA summer weather condition.

Magnesium isotopic evidence for staged enhancement of the East Asian Summer Monsoon precipitation since the Miocene

Knowledge of the evolution of the East Asian Summer Monsoon (EASM) provides a valuable opportunity to uncover the dynamic interactions of land-ocean-atmosphere system in the late Cenozoic. However, the evolutionary history of EASM remains debatable, mainly due to the difficulty in separating EASM precipitation and temperature signals. In this study, precipitation proxies of Sr/Ca ratio and δ26Mg value in loess secondary calcite (carbonate nodule (NC) and fine carbonate (FC) (grain size < 4 μm)) are further verified, with high precipitation corresponding to high δ26MgFC but low δ26MgNC and Sr/CaNC ratio, and vice versa. Then, these proxies are investigated in well-researched Chinese loess eolian deposits since ∼22.5 Ma. Results suggests that the EASM precipitation was staged enhancement in the mid-Miocene (∼16.5–14.0 Ma) and the Quaternary (∼2.6–0 Ma). The mid-Miocene enhancement can be compared with the strong EASM intensity and the warmest global temperature since the Miocene. Global warming thus is considered as the dominant force for this enhancement via an expansion in the latitudinal extent of the Indo-Pacific Intertropical Convergence Zone (ITCZ). In contrast, the Quaternary enhancement was accompanied by decreases in global temperature. We propose their linkage via the reinforcement of the Pacific Walker Circulation and a northward shift of the subtropical ridge in the western Pacific. Our study provides a new insight into EASM evolution and its dynamic linkage with global climate changes.

The unexpectedly short Holocene Humid Period in Northern Arabia

The early to middle Holocene Humid Period led to a greening of today’s arid Saharo-Arabian desert belt. While this phase is well defined in North Africa and the Southern Arabian Peninsula, robust evidence from Northern Arabia is lacking. Here we fill this gap with unprecedented annually to sub-decadally resolved proxy data from Tayma, the only known varved lake sediments in Northern Arabia. Based on stable isotopes, micro-facies analyses and varve and radiocarbon dating, we distinguish five phases of lake development and show that the wet phase in Northern Arabia from 8800–7900 years BP is considerably shorter than the commonly defined Holocene Humid Period (~11,000–5500 years BP). Moreover, we find a two century-long peak humidity at times when a centennial-scale dry anomaly around 8200 years BP interrupted the Holocene Humid Period in adjacent regions. The short humid phase possibly favoured Neolithic migrations into Northern Arabia representing a strong human response to environmental changes.

Medium-Range Predictability of Boreal Summer Western North Pacific Subtropical High and Its ENSO Modulation

In boreal summer, variations of intensity and location of the western North Pacific subtropical high (WNPSH) have significant impacts on weather and climate in East Asia. In this study, the medium-range prediction of WNPSH is comprehensively evaluated with various scores based on reforecast data of the National Centers for Environmental Prediction - Global Ensemble Forecast System, and the predictability source of the WNPSH medium-range forecasting is further analyzed by examining how well the model can reproduce the modulation of El Niño-Southern Oscillation (ENSO) on WNPSH as observed. The results show that this system has a systematic bias in the WNPSH forecasts, mainly manifested by the weak strength and the southeastward shifted position, and such a bias further increases with lead time. Effective prediction skills of WNPSH are 10–11 days for its intensity and area, 7 days for its ridge line, but only 1–3 days for its western boundary ridge point in terms of different scores, respectively, which can be improved through developing a bias correction method of prediction. It is demonstrated that the medium-range predictability of WNPSH is mainly originated from ENSO and its significant lagged effects on WNPSH well revealed in observation can be realistically reproduced by this system within the effective prediction lengths. A strong ENSO modulation of the WNPSH prediction skills has been clearly found in terms of the different indices, which depends on the ENSO’s developing and decaying phases. The intensity and area of WNPSH are usually highly predictable due to the ENSO effect being reproduced by the system well while the location indices of WNPSH have relatively low predictability, which are mainly affected by internal variability and difficultly captured by the model. Predictability analysis of WNPSH as modulated by ENSO shows good potential for medium-range forecasting with high skills.

Threshold effect of ecosystem services in response to climate change, human activity and landscape pattern in the upper and middle Yellow River of China

Identifying spatiotemporal heterogeneity and impact mechanism of ecosystem service value (ESV) is critical for enhancing ecosystem management and regional sustainability. However, the existing findings have failed to investigate the synergetic effect of multiple drivers on ESV, nor have considered the threshold effects under different conditions, which is important to understand the complex and frequently interacting ecosystem. In this study, we applied the improved ESV calculation method, Pearson correlation analysis and conditional inference tree model to depict the dynamic changes of ESV, to explore separate and synergetic effects of various factors, and further to identify key thresholds under combined effects in urban agglomerations along the upper and middle Yellow River area (UMYRA). The results showed that: (1) The total ESV of the study area increased by 5936.63 million CNY over the study period. With the exception of the urban agglomeration along the Yellow River in Ningxia (YRNX), four types of sub-services in Hulunbeier-Baotou-Erdos-Yulin urban agglomeration (HBEY) and Taiyuan urban agglomeration (TY) showed a rising trend. Spatially, the greater growth of ESV mainly concentrated in the northern desert belt, whereas the major grain production areas and urban areas presented different degrees of degradation. (2) Climate factors and land use intensity are the main factors affecting the total ESV, followed by economic density and terrain condition, while landscape metrics exert a relatively small impact on ESV. But the influences of these factors on single ESV show distinct internal differences. (3) Synergetic effects of multi-factors showed that: ①When the evapotranspiration and precipitation are at a certain threshold (1277 mm＜PET＜1317 mm and PRE＞417.55 mm), rising temperature can effectively promote the growth of ESV. The same effect is also observed on steep slopes under matched hydrothermal conditions; ②When economic development is at a high level, intensifying human activities within a certain range (0.11＜HAI＜0.29 and 184.32＜LUI＜210.6) could greatly promote the ESV, but the opposite effect was also true when the thresholds were breached; ③Fragmented landscapes with low diversity (SHDI＜0.86) can promote ESV improvement, but high diversity fragmented landscapes (PD＞0.14 or SHDI＞0.85) would lead to ESV decline. Our research sheds light on the key thresholds under synergetic effect of multiple factors on ESV from a novel perspective, which provide a specific and practical guidance for achieving sustainable management.

Wind–SST Dipole Mode in the Caribbean and Gulf of Mexico: large-scale features and drivers

The Dipole Mode (DM) is the leading pattern of springtime wind–SST coupled interannual variability in the Intra-Americas Seas, characterized by SST anomalies of opposite sign between the Caribbean Sea and the Gulf of Mexico. Using the standard deviation (STD) of the wind in a Maximum Correlation Analysis (MCA), this study aims to provide a more dynamic view of the role of the atmosphere in its coupling with the SST. The MCA reveals that the positive phase of the DM is associated with an increase in atmospheric instability, while the negative phase emerges under more stable atmospheric conditions. The DM is preceded by changes in the subtropical high-pressure belt during the previous winter, particularly in the North Atlantic Subtropical High (NASH), and reflects shifts in the latitudinal position of the subtropical jet stream. The DM positive phase tends to occur after an El Niño winter, under negative North Atlantic Oscillation (NAO) conditions. El Niño modulates the DM through a weakening in the meridional pressure gradient and a southward shift of the jet stream. A negative NAO implies a weaker NASH and, therefore, a more irregular circulation over the region. Both El Niño and negative NAO conditions favor the increase in wind STD during the DM positive phase, consistent with an increment in atmospheric disturbances. The DM negative phase responds more to a positive NAO in the previous winter, revealing a stronger NASH acting as an atmospheric block, which justifies the decrease in STD and a more stable circulation.

The Seasonally Altered Atmosphere Moisture Circulations With Rainfall and Rainfall Isotopes in Southwest China

To interpret the climatic signals of precipitation/speleothem δ18O, it is critical to identify the importance of the factors affecting the precipitation δ18O. This study presents new stable isotope data for precipitation δ18O and δD in the site of Shenqi cave, southwest China, from November-2015 to October-2016 (the “Super-El Niño” event), to investigate the regional-scale climate forcing on precipitation δ18O. The precipitation δ18O, δD and d-excess have an obvious seasonality, relatively low values in the wet season and high in the dry season. The further analysis of seasonally altered LMWL and moisture circulations suggested that changes in atmosphere moisture circulations would be the key factor underlying the precipitation/speleothem δ18O fluctuations in our study area at least on seasonal timescales. Combined with the seasonal-monthly variations of the IsoGSM δ18O, GPCP/CRU rainfall and NCEP/NCAR moisture fluxes, we detected that the super-El Niño of 2016 have changed the distributions of monthly rainfall in wet season through the Western Pacific Subtropical High, but not mainly the precipitation isotopic compositions and moisture circulations in our study area.

Detecting Interdecadal Change in Western North Pacific Tropical Cyclone Genesis Based on Cluster Analysis Using pHash + Kmeans

Previous studies have noted an abrupt decrease in western North Pacific (WNP) tropical cyclone (TC) genesis frequency and a westward shift in genesis location since the late 1990s. The recent application of cluster analysis in TC research shows the effect of detecting the contribution of the Western North Pacific Subtropical High (WNPSH) and the interdecadal Pacific oscillation (IPO) on interdecadal change in WNP TCs. In this work, we also apply a clustering algorithm called pHash + Kmeans to group WNP TCs into three classes based on their genesis environmental conditions. The clustering results show that an abrupt decrease after 1998 is related primarily to a decrease in the dominant class (Class3, located mainly in the southern and eastern WNP), and an increase after 2010 occurs because of a new dominant class (Class1, located mainly in the northwestern WNP), which indicates that the WNP environment suppresses Class3 genesis after 1998 and enhances Class1 genesis after 2010. Three periods (P1: 1979–1997, P2: 1998–2010, and P3: 2011–2020) and three regions (SCS: 100°E-120°E, EQ-30°N; WNP1: 120°E-140°E, EQ-30°N; and WNP2: 140°E-160°W, EQ-30°N) are divided to further confirm the above findings. In P1, high (low) mid-level relative humidity (RH), intense (weak) low-level vorticity, and weak (strong) vertical wind shear (VWS) are distributed in WNP2 (SCS and WNP1), indicating suitable environmental conditions for TC genesis in WNP2 but unsuitable conditions in SCS and WNP1. This situation is the opposite in P2, leading to a decrease in genesis frequency and a westward shift in genesis location. In P3, strong low-pressure vorticity and thermodynamic conditions occur in SCS and WNP1, contributing to an increase in TC genesis frequency.

The Intraseasonal Variations of the Leading Mode of Summer Precipitation Anomalies in Meiyu Area of East Asia

The intraseasonal variations of summer precipitation anomalies in the Meiyu area of East Asia are analyzed by applying a combined empirical orthogonal function (CEOF) of the latest meteorological reanalysis data ERA5 of European Center for Medium-Range Weather Forecasts for the period from 1991 to 2020, and the circulation structures and sources of variability of CEOF are also investigated. The first mode of the intraseasonal variations shows an in-phase pattern over the Meiyu area in June, July, and August, accounting for 22.2% of the total variance in the intraseasonal variations of summer precipitation anomalies. The positive (negative) CEOF1 is accompanied by the negative (positive) East Asia/Pacific pattern, including strong westerly wind anomalies in the upper troposphere and southwest monsoon in the lower troposphere, and the Western Pacific Subtropical High extending westward and its ridge line slightly south. The positive CEOF1 is preceded by decay of El Niño episodes, including the abnormal warm sea surface temperature anomalies (SSTAs) in the equatorial Central-Eastern Pacific in spring and warm SSTAs in the equatorial Indian Ocean in summer. The second mode shows an opposite precipitation anomaly in June and July, and the distribution in August is not significant. The corresponding geopotential height circulation of positive CEOF2 shows the large negative anomaly in the region north of 40° N and a positive anomaly over Japan in June, whereas the pattern reverses in July. At the same time, there is a radical reversion from abnormal eastly to westly wind in the upper troposphere. The structure of CEOF2 is somewhat induced by local SSTAs over the Northern Indian Ocean and South China Sea.

Patterns of atmospheric circulation in Western Europe linked to heavy rainfall in Germany: preliminary analysis into the 2021 heavy rainfall episode

The July 2021 heavy rainfall episode in parts of Western Europe caused devastating floods, specifically in Germany. This study examines circulation types (CTs) linked to extreme precipitation in Germany. It was investigated if the classified CTs can highlight the anomaly in synoptic patterns that contributed to the unusual July 2021 heavy rainfall in Germany. The North Atlantic Oscillation was found to be the major climatic mode related to the seasonal and inter-annual variations of most of the classified CTs. On average, wet (dry) conditions in large parts of Germany can be linked to westerly (northerly) moisture fluxes. During spring and summer seasons, the mid-latitude cyclone when located over the North Sea disrupts onshore moisture transport from the North Atlantic Ocean by westerlies driven by the North Atlantic subtropical anticyclone. The CT found to have the highest probability of being associated with above-average rainfall in large part of Germany features (i) enhancement and northward track of the cyclonic system over the Mediterranean; (ii) northward track of the North Atlantic anticyclone, further displacing poleward, the mid-latitude cyclone over the North Sea, enabling band of westerly moisture fluxes to penetrate Germany; (iii) cyclonic system over the Baltic Sea coupled with northeast fluxes of moisture to Germany; (iv) and unstable atmospheric conditions over Germany. In 2021, a spike was detected in the amplitude and frequency of occurrence of the aforementioned wet CT suggesting that in addition to the nearly stationary cut-off low over central Europe, during the July flood episode, anomalies in the CT contributed to the heavy rainfall event.

Characteristics and Cause Analysis of the 1954 Yangtze Precipitation Anomalies

In 1954, the Yangtze River valley was hit by heavy precipitation anomalies, which caused large casualties and economic losses; however, systematic analyses of the causes are lacking. Adopting the latest national historical precipitation data collected by the China Meteorological Administration (CMA) and global sea surface temperature (SST) records, this retrospective study determined the spatial–temporal distribution characteristics of the precipitation in 1954 in Wuhan, a city situated in the Yangtze River valley. The results confirmed that the 1954 precipitation anomalies were characterized by a high volume and a long period of rainfall, plus numerous cloudbursts, with most of the precipitation concentrated during June and July at the mid- and low-Yangtze areas along the Yangtze. An El Niño event caused the West Pacific subtropical highs to continually move southward during the summer, creating a long-term rainband in the drainage basin. Moreover, the continued low SSTs in the Sea of Okhotsk generated an active blocking high that continuously brought high-latitude cold air into the south, boosting precipitation over the drainage basin. This study proposed a new causal model of summertime precipitation across the Yangtze River valley in 1954, whereby the unusual SST changes initially triggered atmospheric circulation anomalies, which caused the precipitation anomalies of 1954.

Large infrequent rain events dominate the hydroclimate of Rapa Nui (Easter Island)

The history of the Polynesian civilization on Rapa Nui (Easter Island) over the Common Era has come to exemplify the fragile relationship humans have with their environment. Social dynamics, deforestation, land degradation, and climatic shifts have all been proposed as important parts of the settlement history and societal transformations on Rapa Nui. Furthermore, climate dynamics of the Southeast Pacific have major global implications. While the wetlands of Rapa Nui contain critical sedimentological archives for reconstructing past hydrological change on the island, connections between the island’s hydroclimate and fundamental aspects of regional climatology are poorly understood. Here we present a hydroclimatology of Rapa Nui showing that there is a clear seasonal cycle of precipitation, with wet months receiving almost twice as much precipitation as dry months. This seasonal cycle can be explained by the seasonal shifts in the location and strength of the climatological south Pacific subtropical anticyclone. For interannual precipitation variability, we find that the occurrence of infrequent, large rain events explains 92% of the variance of the observed annual mean precipitation time series. Approximately one third (33%) of these events are associated with atmospheric rivers, 21% are associated with classic cold-front synoptic systems, and the remainder are characterized by cut-off lows and other synoptic-scale storm systems. As a group, these large rain events are most strongly controlled by the longitudinal position of the south Pacific subtropical anticyclone. The longitudinal location of this anticyclone explains 21% of the variance in the frequency of large rain events, while the remaining variance is left unexplained by any other major atmosphere-ocean dynamics. We find that over the observational era there appears to be no linear relationship between the number of large rain events and any other major climate phenomena. With the south Pacific subtropical anticyclone projected to strengthen and expand westward under global warming, our results imply that Rapa Nui will experience an increase in the number of dry years in the future.

Decadal variation of the precipitation relationship between June and August over South China and its mechanism

This paper investigates the causes for the interdecadal change in the relationships between early and late summer precipitation over South China (SC). It is found that the correlations of the precipitation over SC between June and August shift from weak positive in 1979–1995 to significantly negative in 1996–2019. Further analysis demonstrates that the distinct evolution of sea surface temperature (SST) pattern between the two periods accounts for the interdecadal variations of their relationships. Although the warming of the tropical western Indian Ocean (WIO) in June favors increasing precipitation over the SC via enhancing the Northwest Pacific Subtropical High (NWPSH) during the whole period, the associated SST anomalies in August are rather different between the two periods. Specifically, the WIO warming in June corresponds to slightly positive anomalies over the tropical central-eastern Pacific in August during the 1979–1995, which has weak impact on the NWPSH and results in a weak precipitation correlation between June and August. However, the WIO warming in June corresponds to La Niña’s rapid development in August during the 1996–2019, which favors the enhancement of the NWPSH via increasing the regional Hadley circulation. Due to the climatologically northward movement of NWPSH from June to August, the enhanced NWPSH in August acts to decrease the precipitation over SC, causing a significantly negative correlation between precipitation in June and August. Overall, the distinct evolution of tropical SST pattern is the key factor inducing the change of the relationships between June and August precipitation in the two periods.