Strong Anticyclone Research Articles

A Preliminary Fuzzy Inference System for Predicting Atmospheric Ozone in an Intermountain Basin

High concentrations of ozone in the Uinta Basin, Utah, can occur after sufficient snowfall and a strong atmospheric anticyclone creates a persistent cold pool that traps emissions from oil and gas operations, where sustained photolysis of the precursors builds ozone to unhealthy concentrations. The basin’s winter-ozone system is well understood by domain experts and supported by archives of atmospheric observations. Rules of the system can be formulated in natural language (“sufficient snowfall and high pressure leads to high ozone”), lending itself to analysis with a fuzzy-logic inference system. This method encodes human expertise as machine intelligence in a more prescribed manner than more complex, black-box inference methods such as neural networks, increasing user trustworthiness of our model prototype before further optimization. Herein, we develop an ozone forecasting system, Clyfar, informed by an archive of meteorological and air-chemistry measurements. This prototype successfully demonstrates proof-of-concept despite rudimentary tuning. We describe our framework for predicting future ozone concentrations if input values are drawn from numerical weather prediction forecasts rather than observations as Clyfar initial conditions. We evaluate inferred values for one winter, finding our prototype demonstrates mixed performance but promise after optimization to deliver useful forecast guidance for decision-makers when forecast data are used as input. This early version model is the basis of ongoing optimization through machine learning.

Synoptic and Mesoscale Dynamics of Cold Surges Over the South China Sea and Their Control on Extreme Rainfall

AbstractWe investigate the synoptic and mesoscale dynamics of two wet and two dry cold surges in January 2021 using a combination of observations, reanalysis, and convective‐scale model forecasts from the Met Office Unified Model (MetUM). We focus on the wet surges, and particularly the wettest days which are locally extreme over Singapore and the surrounding region (i.e., the daily mean and area‐averaged rainfall over 20 years exceeds the 99th percentile). On the large scale, the wet surges are characterized by an anomalously strong anticyclone over Siberia prior to their onset. The anticyclone and resultant surge winds are stronger than those of the dry surges. There is also a relatively moist (dry) environment prior to the onset of the wet (dry) surges, with the Madden‐Julian Oscillation (MJO) being in Phase 3 (Phase 6). On the mesoscale, the combination of the cold surge and a local tropical low produce strong, moist north‐easterly winds and convection over the Singapore region. The equatorward advection of positive anomalies of equivalent potential temperature resembles a weak gravity‐current‐like structure at its head, although the spatial scale is much too large for a gravity current. There is a moist bias in the model forecasts, although the precipitation is underestimated regionally during the wet surges and particularly on the extreme rainfall days. Overall, the model forecasts perform well synoptically but not in the details of mesoscale convection.

Comparative Analysis of Characteristics and Physical Mechanisms for Typical Summer Extreme Precipitation in Pakistan

Abstract The 2022 floods in Pakistan resulted in severe losses and garnered global attention. This study aims to enhance the understanding of extreme precipitation (EP) events in Pakistan by examining the characteristics and mechanisms behind the persistent EP during summer, utilizing daily precipitation data from the Climate Prediction Center (CPC). Results showed that the monsoon precipitation in 2010, 2020 and 2022 are the highest three years on record. Notably, these peak events in 2010 (concentrating in the north) and 2022 (concentrating in the south) spanned from July through August. Conversely, the extreme precipitation in August 2020 was concentrated in northern Pakistan. For the circulation patterns, the intensification of the South Asian High and the western Pacific subtropical high with a strong Indian monsoon is a unifying feature, but the Iranian high and monsoon low-pressure system on the south of Pakistan was different. Additionally, the EP in July 2010 and August 2022 were also influenced by the teleconnection associated with European Blocking. La Niña events and the negative-phase Indian Ocean Dipole (IOD) also played a role in affecting summer EP, with the strongest La Niña occurring in 2010 and a notable triple-dip La Niña coinciding with a significant negative IOD phase in 2022. La Niña contributed to the formation of an anomalously strong anticyclone over the northwest Pacific and easterly winds along the southern Himalayas, impacting moisture transport to Pakistan. Conversely, the negative IOD phase amplified EP in Pakistan by enhancing the northward movement of convective systems and westerly winds over the Indian Ocean. Furthermore, reduced snow cover on the Tibetan Plateau in the springs of 2010 and 2022 likely induced a stronger thermal dynamical effect, acting as a heat source in summer and increasing precipitation in Pakistan.

Vortex dynamics in rotating Rayleigh–Bénard convection

We investigate the spatial distribution and dynamics of the vortices in rotating Rayleigh–Bénard convection in a reduced Rayleigh number range $1.3\le Ra/Ra_{c}\le 83.1$ . Under slow rotations ( $Ra\approx 80\,Ra_{c}$ ), the vortices are distributed randomly, which is manifested by the size distribution of the Voronoi cells of the vortex centres being a standard $\varGamma$ distribution. The vortices exhibit Brownian-type horizontal motion in the parameter range $Ra\gtrsim 10\,Ra_{c}$ . The probability density functions of the vortex displacements are, however, non-Gaussian at short time scales. At modest rotating rates ( $4\,Ra_{c}\le Ra\lesssim 10\,Ra_{c}$ ), the centrifugal force leads to radial vortex motions, i.e. warm cyclones (cold anticyclones) moving towards (outwards from) the rotation axis. The horizontal scale of the vortices decreases with decreasing $Ra/Ra_c$ , and the size distribution of their Voronoi cells deviates from the $\varGamma$ distribution. In the rapidly rotating regime ( $1.6\,Ra_{c}\le Ra\le 4\,Ra_{c}$ ), the vortices are densely distributed. The hydrodynamic interaction of neighbouring vortices results in the formation of vortex clusters. Within clusters, cyclones exhibit inverse-centrifugal motion as they submit to the outward motion of the strong anticyclones, and the radial velocity of the anticyclones is enhanced. The radial mobility of isolated vortices, scaled by their vorticity strength, is shown to be a simple power function of the Froude number. For all flow regimes studied, we show that the number of vortices with a lifespan greater than $t$ decreases exponentially as $\exp ({-t/{\tau }})$ for large time, where $\tau$ represents the characteristic lifetime of long-lived vortices.

Weakened Subtropical Westerlies Reduced Early Spring Precipitation in the Southeast Tibetan Plateau

Abstract The Southeast Tibetan Plateau (SETP) is a major region where many low-latitude glaciers are located, with spring precipitation being a major input of the glacier mass balance. This study shows that early spring precipitation has decreased significantly since 1999, which is attributed to declined moisture contribution from the far-field sources (west of 70°E) induced by the weakened subtropical westerlies. The possible physical mechanism underlying this change has also been revealed. It is found that snow-cover extent (SCE) in March reduced in midlatitude Eurasia after 1999; meanwhile, strong solar radiation during this month may have exacerbated snow melting through snow albedo–radiation interactions. These two processes led to warming and caused a strong anticyclone over midlatitude Eurasia that weakened the subtropical westerlies near 30°N. This decadal change in the subtropical westerlies led to a decrease in moisture transport upstream. As a result, the windward slopes of large terrain along the latitudinal belt near 30°N received less precipitation, and the decrease in SETP precipitation was part of this change. A further analysis shows that the positive correlation between the westerlies and precipitation has weakened since 1999. Significance Statement The purpose of this study is to reveal the decreased early spring precipitation and explore its possible physical mechanism in the Southeast Tibetan Plateau (SETP), which is crucial to understand the shrinkage of the local glacier. Our results indicate that the reduction of snow cover in midlatitude Eurasia since 1999 and the strong solar radiation in March contributed to the weakening subtropical westerlies, which further resulted in the decreasing precipitation in the SETP and other windward slopes of large terrain along the latitudinal 30°N belt in Eurasia.

Characteristics of Surface “Melt Potential” over Antarctic Ice Shelves based on Regional Atmospheric Model Simulations of Summer Air Temperature Extremes from 1979/80 to 2018/19

Abstract We calculate a regional surface “melt potential” index (MPI) over Antarctic ice shelves that describes the frequency (MPI-freq; %) and intensity (MPI-int; K) of daily maximum summer temperatures exceeding a melt threshold of 273.15 K. This is used to determine which ice shelves are vulnerable to melt-induced hydrofracture and is calculated using near-surface temperature output for each summer from 1979/80 to 2018/19 from two high-resolution regional atmospheric model hindcasts (using the MetUM and HIRHAM5). MPI is highest for Antarctic Peninsula ice shelves (MPI-freq 23%–35%, MPI-int 1.2–2.1 K), lowest (2%–3%, <0 K) for the Ronne–Filchner and Ross ice shelves, and around 10%–24% and 0.6–1.7 K for the other West and East Antarctic ice shelves. Hotspots of MPI are apparent over many ice shelves, and they also show a decreasing trend in MPI-freq. The regional circulation patterns associated with high MPI values over West and East Antarctic ice shelves are remarkably consistent for their respective region but tied to different large-scale climate forcings. The West Antarctic circulation resembles the central Pacific El Niño pattern with a stationary Rossby wave and a strong anticyclone over the high-latitude South Pacific. By contrast, the East Antarctic circulation comprises a zonally symmetric negative Southern Annular Mode pattern with a strong regional anticyclone on the plateau and enhanced coastal easterlies/weakened Southern Ocean westerlies. Values of MPI are 3–4 times larger for a lower temperature/melt threshold of 271.15 K used in a sensitivity test, as melting can occur at temperatures lower than 273.15 K depending on snowpack properties.

The synergistic effect of the summer NAO and northwest pacific SST on extreme heat events in the central–eastern China

Extreme heat events happen frequently in East Asia, and may cause great damage to the ecosystem and society. The synergistic effect of the summer positive North Atlantic Oscillation (pNAO) and positive northwest Pacific (pNWP) sea surface temperature anomaly (SSTA) on the interannual variability of the extreme heat events in the central–eastern China (CEC) is investigated in this study. The two factors act synergistically in strengthening the extreme heat events in the CEC via a series of atmospheric bridges, and the CEC is likely to experience a hotter summer when both the summer pNAO and pNWP SSTA occur. The pNWP SSTA increases the strength of pNAO via the eastward propagating Rossby wave from the western Pacific. The enhanced pNAO induces a stronger eastward Rossby wave propagation across the Eurasian continent, and leads to a strong anomalous anticyclone over the CEC. The significantly increased atmosphere thickness increases the air temperature of the layer, and favors the extreme heat events in the CEC. Besides, the pNWP SSTA also has adjacent effect on the atmospheric circulation over the CEC, which could lead to a positive geopotential height anomaly. Therefore, the summer pNAO and pNWP SSTA act synergistically in influencing the atmospheric circulation over the CEC, and thereby significantly increase the extreme heat events in the CEC.

Large-scale background and role of quasi-biweekly moisture transport in the extreme Yangtze River rainfall in summer 2020

A severe flooding hit southern China along the Yangtze River in summer 2020. The floods were induced by extreme rains, and the associated dynamic and thermodynamic conditions are investigated using daily gridded rainfall data of China and NCEP-NCAR reanalysis. It is found that the June–July rainfall over the Yangtze River Basin (YRB) experienced pronounced subseasonal variation in 2020, dominated by a quasi-biweekly oscillation (QBWO) mode. The southwestward-moving anomalous QBWO circulation was essentially the fluctuation of cold air mass related to the tropospheric polar vortex or trough-ridge activities over the mid-high latitude Eurasian in boreal summer. The southwestward-transport of cold air mass from mid-high latitudes and the northeastward-transport of warm and moist air by the strong anomalous anticyclone over the western North Pacific provided important large-scale circulation support for the extreme rainfall in the YRB. The analysis of streamfunction of water vapor flux demonstrates that a large amount of water vapor eastward zonal transport from the Bay of Bengal and Indo-China and northward transport from the South China Sea provided the background moisture supply for the rainfall. The quasi-biweekly anomalies of potential and divergent component of vertically integrated water vapor flux played an important role in maintaining the subseasonal variability of rainfall in June–July of 2020. The diagnosis of moisture tendency budget shows that the enhanced moisture closely related to the quasi-biweekly fluctuated rainfall was primarily attributed to the moisture convergence. Further analysis of time-scale decomposition in the moisture convergence indicates that the convergence of background mean specific humidity by the QBWO flow and convergence of QBWO specific humidity by the mean flow played dominant roles in contributing to the positive moisture tendency. In combination with adiabatic ascent over the YRB induced by the warm temperature advection, the boundary layer moisture convergence strengthened the upward transport of water vapor to moisten the middle troposphere, favoring the persistence of rainfall during June–July. The vertical moisture transport associated with boundary layer convergence was of critical importance in causing low-level tropospheric moistening. By comparison, the horizontal moisture advection played a secondary important role in the quasi-biweekly oscillation of rainfall in June–July 2020.

Influence of Meteorological Parameters on the Urban Heat Island in Moscow

The urban heat island (UHI) intensity in Moscow and the influence of various meteorological parameters are discussed using weather station data. The maximal and average in-space UHI intensities, i.e., a comparison of air temperature T either in the city centre or in the whole urban area together with rural zone have averaged 1.9 and 0.9 °C, respectively, in recent years. The UHI in Moscow has stabilized over the past decade and is not growing. Under conditions of a strong anticyclone, the maximal UHI intensity in space and time reaches 11–12 °C. Low cloudiness and amplitudes of diurnal air temperature, as well as surface temperature, demonstrate the closest relationship with the UHI intensity among other parameters with the correlation coefficient of up to −0.67 for low cloudiness and the maximal UHI intensity. The effect of wind speed, total cloudiness and relative humidity on the UHI is slightly weaker, but still significant. The relationships of all meteorological parameters with the maximal UHI intensity are closer than those with the average one. The multiple correlation coefficient between the maximal UHI intensity and both parameters (low cloudiness and average daily wind speed) is 0.76–0.82. The UHI intensity function of air temperature has a minimum in the range from −4 to 0 °C; its growth both at lower and higher T is due to the influence of anticyclonic weather. The UHI intensity function of wind speed decreases with wind strength. The threshold value at which this function asymptotically approaches its lower limit is 10 m/s in the 40–200 m air layer. The UHI intensity functions of both total and low cloudiness decrease with increasing cloudiness and the differences between them are significant if the cloud cover is more than 50%.

Madden–Julian Oscillation Impacts on Australian Temperatures and Extremes

Abstract We assess seasonally varying impacts of the Madden–Julian oscillation (MJO) on Australian maximum and minimum temperature anomalies and extremes, and their modulation by El Niño–Southern Oscillation (ENSO), for the period June 1974–May 2022. Our composite-based approach uses observed temperatures from the Australian Gridded Climate Data, and 850-hPa wind data from the NCEP–NCAR reanalysis, to show how relationships to temperature and circulation evolve over the eight-phase life cycle of the MJO, which we derive from the real-time multivariate MJO index. The MJO has significant impacts on Australian temperatures and winds in all parts of the country at various times throughout the year, and to varying degrees. Two of the most pronounced impacts are 1) daytime warming across southeastern Australia in MJO phase 3 during spring associated with a strong anomalous anticyclone and 2) nighttime cooling over Queensland in MJO phase 7 during winter associated with anomalous advection of cool dry continental air. La Niña acts to significantly lessen both of these impacts, while El Niño enhances both the phase 3 warming over southern Australia in spring and the phase 7 overnight cooling over southern Queensland in winter. We show how the MJO can combine with El Niño and La Niña to have strong compounding influences, thus highlighting the importance of understanding interactions between multiple modes of climate variability and how they relate to Australian temperatures and extremes.

April 2022 Floods over East Coast South Africa: Interactions between a Mesoscale Convective System and a Coastal Meso-Low

Extreme rainfall occurred over the South African east coast during 10–12 April 2022, leading to over 500 deaths and massive damage. This study seeks to understand a key feature, namely the progression of the rainfall maxima from the northern KwaZulu-Natal (KZN) coast during the night of 10–11 April by ~550 km to the Eastern Cape coast about 48 h later. The large-scale circulation was dominated by a cut-off low over the South African interior with a strong ridging anticyclone southeast of the country in the South Indian Ocean. Satellite, rain gauge, and ERA5 reanalysis are used to show that the timing and location of the heaviest rainfall are closely tied to, firstly, the formation of a Mesoscale Convective System (MCS) at ~0300UTC April 11 on the northern KZN coast which tracked south and then offshore, and secondly, a meso-trough which formed a coastal meso-low by ~2100 UTC April 11. The meso-low tracked south over the warm waters of the Agulhas Current before strengthening into Subtropical Depression Issa. Mesoscale interaction between the MCS and coastal meso-low determined the positioning and timing of the strongest onshore low-level jet, moisture convergence and uplift, and hence the progression of the heavy rainfall down the coast. Such mesoscale interaction has not previously been documented in southern Africa or the Southern Hemisphere.

Relationship between the South Asian High and Western North Pacific tropical cyclone genesis

This study highlights a significant inter-decadal modulation of the South Asian high (SAH) on the meridional distribution of western North Pacific (WNP) tropical cyclone genesis frequency (TCGF). There is an obvious inter-decadal change in the first leading SAH mode, with an abrupt weakening since the early 1990s. During 1979–1993, when there was a stronger SAH, TCs were suppressed over the northern WNP and enhanced over the southern WNP in response to inter-annual SAH changes. The northern WNP TCGF had a significant negative correlation with the SAH (r = −0.69), while the southern WNP TCGF had a significant positive correlation with the SAH (r = 0.78). This north-south seesaw pattern driven by the SAH weakened during 1994–2019.This inter-decadal modulation of the inter-annual SAH-WNP TC genesis relationship can be explained by changes in large-scale environmental parameters. There is anomalous ascending (descending) motion and negative (positive) vorticity centered at 850-hPa as well as less (more) moisture at 600-hPa in the northern WNP (southern WNP) during 1979–1993. The maintenance of a strong low-level anomalous anticyclone accompanied by the concurrent influence of the SAH and East Asian upper-tropospheric jet stream decreases northern WNP TCGF. Linking to an El Niño-like SST pattern, the cyclonic circulation forced by a Gill-type forcing and enhanced by equatorial low-level westerly anomalies increases southern WNP TCGF. In contrast, we find a relatively weak inter-annual relationship of large-scale factors to the SAH since 1994. This weakening relationship is largely due to the inter-decadal weakening of the SAH and East Asian upper-tropospheric jet stream. Inter-decadal changes of the East Asian summer monsoon and the phase shift of the Atlantic Multi-decadal Oscillation appear to be two important factors modulating the inter-annual relationship between the SAH and the meridional distribution of WNP TCGF.

An environmental synoptic analysis of tropical transitions in the central and Eastern North Atlantic

A tropical transition (TT) is the process whereby a baroclinic, high-to-moderate vertical wind shear, extratropical or subtropical cyclone is transformed into a warm-core, low vertical wind shear, tropical cyclone. Thirty TT events were identified over the central and eastern North Atlantic basin during the period 1979–2019. The TT process is here studied from a synoptic storm-centered composite climatology and an environmental classification perspective. The aim is to study their common features and highlight their differences. The storm-centered composite analysis reveals that a westerlies meridional trough with quasigeostrophic forcig acts as precursor. TT environments are characterized by a trough at 300 hPa geopotential and the increase of the 1000–500 hPa thickness, i.e., the system evolves into a warm-core, and a strong anticyclone is located north of the surface cyclone. The transition is accompanied by a large latent heat release which promotes the vertical redistribution of potential vorticity and a reduction of the 850–300 hPa vertical wind shear. The identified TTs in the central North Atlantic predominantly developed in environments with warm sea surface temperatures (> 25 °C) and low-to-moderate wind shear (10–15 m s−1). In contrast, the eastern North Atlantic TTs transitioned in low sea surface temperature values (< 25 °C) and high wind shear (> 15 m s−1). Finally, the statistically significant differences in the environmental classification encouraged further analysis of their environments via storm-centered composites, revealing that eastern North Atlantic cyclones have a more defined extratropical structure, while central North Atlantic cyclones show more tropical characteristics.

Summertime Marine Boundary Layer Cloud, Thermodynamic, and Drizzle Morphology over the Eastern North Atlantic: A Four-Year Study

Abstract Summertime remote sensor and in situ data from 2016 to 2019 collected at the ARM Eastern North Atlantic (ENA) Observatory are combined with aircraft measurements from the Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) campaign to quantify marine boundary layer (MBL) cloud, thermodynamic, and drizzle morphology in the region. A radar reflectivity–rainfall rate relationship (Z–R) is developed from aircraft data and 6-h cloud morphological regimes are identified from ENA data using a k-means algorithm driven by three independent inputs quantifying cloud thickness, drizzle intensity, and cloud field geometric complexity. Four separate MBL structural regimes representing non- or weakly drizzling single-layer stratocumulus, drizzling stratocumulus and cumulus-coupled stratocumulus, deep convection, and broken clouds embedded in northerly flow are identified. Single-layer stratocumulus is indicated when weak subtropical anticyclones are significantly west of the ENA site, and the MBL is cooler and drier than when drizzling and cumulus-coupled stratocumulus and broken clouds are observed. Drizzling and cumulus-coupled stratocumulus clouds are observed on the eastern flank of strong subtropical anticyclones in deep warm moist air masses with wind speeds exceeding 7 m s−1 and strong near-surface wind shear. Broken clouds exhibit strong wind shear near the inversion, while single-layer stratocumulus clouds have lower wind speeds and minimal shear. Net latent heat fluxes in the subcloud layer resulting from a combination of the ocean surface heat flux and evaporating drizzle average near zero over long periods in drizzling and cumulus-coupled stratocumulus. The ECMWF reanalysis version 5 (ERA5) is found to accurately represent single-layer stratocumulus properties, while producing significant discrepancies when drizzling stratocumulus and cumulus-coupled stratocumulus are observed.

Contributions of Various Sources to the Higher-Concentration Center of CO within the ASM Anticyclone Based on GEOS-Chem Simulations

Satellite observations show that carbon monoxide (CO) concentration centers exist in the tropopause region of the Tibetan Plateau, while their sources and formation mechanism still remain uncertain. In this paper, the 3-D chemical transport model GEOS-Chem is used to conduct sensitivity analysis in 2016. Combined with the analysis data and satellite data, the contribution of three important emission sources (South Asia, East Asia and Southeast Asia) and two important chemical reaction species (CH4 and nonmethane volatile organic compounds (NMVOCs)) to CO in the upper troposphere and lower stratosphere (UTLS) are studied. The results show that in the Asian monsoon region CO emissions originating from the surface are transported to the upper troposphere via a deep convection process and then enter the Asian Summer Monsoon (ASM) anticyclone. The strong ASM anticyclone isolates the mixing process of air inside and outside the anticyclone, upon entry of carbon monoxide-rich air. In the lower stratosphere, the intensity of the ASM anticyclone declines and the air within the anticyclone flows southwestward with monsoon circulation. We found that in the summer Asian monsoon region, South Asia exhibited the highest carbon monoxide concentration transported to the UTLS. CH4 imposed the greatest influence on the CO concentration in the UTLS region. According to the model simulation results, the CO concentrations in the Asian monsoon region at 100 hPa altitudes were higher than those in other regions at the same latitudes. Regarding effects, 43.18% originated from CH4 chemical reactions, 20.81% originated from NMVOC chemical reactions, and 63.33% originated from surface CO emissions, while sinks yielded a negative contribution of −27.32%. Regarding surface CO emissions, East Asia contributed 13.56%, South Asia contributed 39.27%, and Southeast Asia contributed 7.15%.

Response of East Asian Summer Precipitation to Intermediate SST Anomalies while El Niño Decays and Dependence on Type of Events

Abstract Impacts of El Niño on the following summer East Asian precipitation are dependent on its peak-time characteristics. Three types are identified and denoted as eastern Pacific (EP), mixed-type Pacific (MP), and central Pacific (CP) El Niño. For EP, excessive rainfall occurs in the Yangtze River valley, in relation to an anomalous anticyclone over the western tropical Pacific. For MP, a dipolar precipitation with anomalous wet conditions in northern China and dry conditions in east-central China is mainly due to an anomalous cyclone over Northeast Asia and a weak anticyclone over the Sea of Japan. For the case of CP El Niño, wetness along the Yangtze–Huaihe River valley and dryness in Southeast China are ascribed to an anomalous cyclone in Northeast China and a strong anticyclone over the western tropical Pacific. It is also revealed that the precipitation anomalies are fundamentally caused by different intermediate sea surface temperature (SST) anomalies from other basins while El Niño decays in the following summer. In the case of EP El Niño, the joint action from warm SST in the tropical Indian Ocean, the Niño-1.2 region, and the North Atlantic is responsible for the delayed effect of El Niño. For MP, the main delayed effect comes from warm SST in the northeast subtropical Pacific, developing La Niña, and cold SST in the southern tropical Atlantic. Finally, CP El Niño exerts its delayed influence through warm SST in the northern tropical Atlantic.

The Cooling Over Northeast Asia in June Over the Most Recent Decade: A Possible Response to Declining Bering Sea Sea Ice in March

AbstractIn sharp contrast to rapid global warming, the surface air temperature (SAT) in Northeast Asia (NEA) has exhibited strong cooling in June since 2010. Our results show that this cooling is a possible response to declining Bering Sea sea ice concentrations (BSICs) in March. A physical process analysis shows that decreasing BSICs provoke a south‐eastward‐propagating Rossby wave train and induce a strong anomalous anticyclone over the midlatitude Northwest Atlantic, resulting in tripolar anomalous sea surface temperature (SST) pattern with warm anomalies in the midlatitude Northwest Atlantic. These persistent tripolar SST anomalies generate a new Rossby wave across Eurasia in June, which enhances the cyclonic atmosphere circulation anomaly over NEA and leads to the cold SAT anomalies observed in this area. Correspondingly, in response to the sharp decline in BSIC in March, the NEA has exhibited an unprecedented cooling trend in June over the most recent decade.

Mechanism of interannual variability of ocean bottom pressure in the South Pacific

The study of ocean bottom pressure (OBP) is useful for understanding the variability that contributes to sea level change. Previous studies have reported the strong OBP anomalies in the Southern Ocean on different timescales. In this study, the characteristic and mechanisms of the energetic interannual OBP variability in the southeastern Pacific are examined using 14 years of Gravity Recovery and Climate Experiment (GRACE) data. It is found that the OBP anomalies are positive (negative) related to the convergence (divergence) of Ekman transport forced by local winds variability. Such local winds are attributed to atmospheric teleconnections, particularly the second Pacific-South American (PSA2). The sea level pressure (SLP) anomalies associated with the positive phase of PSA2 shows a wavenumber-3 structure in the high latitude of South Pacific, which benefits a strong and persistent anticyclone over the southeastern Pacific, leading to the positive OBP anomalies there. Moreover, El Niño–Southern Oscillation (ENSO) plays an important role in the concurrent OBP variability during austral spring (August–November) and leads the austral autumn (March–June) OBP variability by 1 season. These results highlight the influence of atmospheric teleconnections on interannual OBP variability and are validated by a mass conservation (non-Boussinesq) ocean model, which is expected to not only better understanding of OBP mechanisms in a longer time, but also predict OBP variation in the global scale.

Strengthened influence of the East Asian trough on spring extreme precipitation variability over eastern Southwest China after the late 1980s

The relationship between variations in the East Asian trough (EAT) intensity and spring extreme precipitation over Southwest China (SWC) during 1961–2020 is investigated. The results indicate that there is an interdecadal increase in the relationship between the EAT and spring extreme precipitation over eastern SWC around the late 1980s. During the latter period, the weak (strong) EAT corresponds to a strong and large-scale anomalous anticyclone (cyclone) over the East Asia–Northwest Pacific region. The EAT-related anomalous southerlies (northerlies) dominate eastern SWC, leading to significant upward (downward) motion and moisture convergence (divergence) over the region, providing favorable (unfavorable) dynamic and moisture conditions for extreme precipitation over eastern SWC. In contrast, during the former period, the EAT-related circulation anomalies are weak and cover a relatively smaller region, which cannot significantly affect the moisture and dynamic conditions over eastern SWC; therefore, the response in extreme precipitation over eastern SWC to EAT is weak over the period. The interdecadal change in the relationship between eastern SWC spring extreme precipitation and the EAT could be related to the interdecadal change in the EAT variability. The large (small) variability of the EAT is associated with significant (insignificant) changes in spring extreme precipitation over eastern SWC during the latter (former) period.摘要本文研究表明东亚大槽强度与中国西南地区东部春季极端降水的关系在20世纪80年代末后显著增强, 这可能与东亚大槽自身变率的年代际变化有关. 在80年代末之后, 东亚大槽的变率显著增强, 其对应的大气环流异常也偏强, 范围偏大, 可以显著影响西南地区东部的水汽和动力条件, 从而引起该地区春季极端降水的显著变化. 而在80年代末之前, 东亚大槽的变率偏弱, 其对应的大气环流异常也偏弱, 范围偏小, 因此不能对西南地区东部春季极端降水的变化产生显著影响.

The role of Indian Ocean warming on extreme rainfall in central China during early summer 2020: without significant El Niño influence

This study investigates the roles of water vapor transport and sea surface temperature (SST) warming in the tropical Indian Ocean (TIO) on the heavy rainfall in central China during the boreal early summer (May–June–July). In the past four decades, four significant rainfall events, in 1983, 1998, 2016, and 2020, occurred in central China and caused severe floods, and the year 2020 has the most extreme event. All four events are associated with significant TIO SST warming and a strong anomalous anticyclone on the western North Pacific (WNPAC). The anomalous winds in the northwestern flank of the WNPAC bring excess water vapor into central China. The water vapor, mainly carried from the western tropical Pacific, converges in central China and result in heavy rainfall. A theory of regional ocean–atmosphere interaction can well explain the processes, called the Indo-Western Pacific Ocean Capacitor (IPOC) effect. The WNPAC is usually associated with strong El Niño-Southern Oscillation (ENSO), except for the 2020 case. The 2020 event is extraordinary without a significant El Niño occurred in the previous winter. In 2020, the significant TIO warming sustained the anomalous WNPAC, inducing the most significant extreme rainfall event in central China. This study reveals that the IPOC effect can dramatically influence the East Asian climate even without involving the ENSO in the Pacific.