Pacific Decadal Oscillation Research Articles

A Spatiotemporal Assessment of Extreme Cold in Northwestern North America Following the Unidentified 1809 CE Volcanic Eruption

AbstractTwo large volcanic eruptions contributed to extreme cold temperatures during the early 1800s, one of the coldest phases of the Little Ice Age. While impacts from the massive 1815 Tambora eruption in Indonesia are relatively well‐documented, much less is known regarding an unidentified volcanic event around 1809. Here, we describe the spatial extent, duration, and magnitude of cold conditions following this eruption in northwestern North America using a high‐resolution network of tree‐ring records that capture past warm‐season temperature variability. Extreme and persistent cold temperatures were centered around the Gulf of Alaska, the adjacent Wrangell‐St Elias Mountains, and the southern Yukon, while cold anomalies diminished with distance from this core region. This distinct spatial pattern of temperature anomalies suggests that a weak Aleutian Low and conditions similar to a negative phase of the Pacific Decadal Oscillation could have contributed to regional cold extremes after the 1809 eruption.

Increased effective radiative forcing enhanced the modulating effect of Pacific Decadal Oscillation on late Little Ice Age precipitation in the Jiang-Huai region, China

Increased effective radiative forcing enhanced the modulating effect of Pacific Decadal Oscillation on late Little Ice Age precipitation in the Jiang-Huai region, China

Southern California winter precipitation variability reflected in 100-year ocean salinity record

Rainfall in southern California is highly variable, with some fluctuations explainable by climate patterns. Resulting runoff and heightened streamflow from rain events introduces freshwater plumes into the coastal ocean. Here we use a 105-year daily sea surface salinity record collected at Scripps Pier in La Jolla, California to show that El Niño Southern Oscillation and Pacific Decadal Oscillation both have signatures in coastal sea surface salinity. Averaging the freshest quantile of sea surface salinity over each year’s winter season provides a useful metric for connecting the coastal ocean to interannual winter rainfall variability, through the influence of freshwater plumes originating, at closest, 7.5 km north of Scripps Pier. This salinity metric has a clear relationship with dominant climate phases: negative Pacific Decadal Oscillation and La Niña conditions correspond consistently with lack of salinity anomaly/ dry winters. Fresh salinity anomalies (i.e., wet winters) occur during positive phase Pacific Decadal Oscillation and El Niño winters, although not consistently. This analysis emphasizes the strong influence that precipitation and consequent streamflow has on the coastal ocean, even in a region of overall low freshwater input, and provides an ocean-based metric for assessing decadal rainfall variability.

Variability modes of September Arctic sea ice: drivers and their contributions to sea ice trend and extremes

The variability of September Arctic sea ice at interannual to multidecadal time scales in the midst of anthropogenically forced sea ice decline is not fully understood. Understanding Arctic sea ice variability at different time scales is crucial for better predicting future sea ice conditions and separating the externally forced signal from internal variability. Here, we study modes of variability, extreme events and trend in September Arctic sea ice in 100–150 year datasets by using time-frequency analysis. We extract the non-linear trend for sea ice area and provide an estimate for the sea ice loss driven by anthropogenic warming with a rate of ∼−0.25 million km2 per decade in the 1980s and accelerating to ∼−0.47 million km2 per decade in 2010s. Assuming the same accelerating rate for sea ice loss in the future and excluding the contributions of internal variability and feedbacks, a September ice-free Arctic could occur around 2060. Results also show that changes in sea ice due to internal variability can be almost as large as forced changes. We find dominant modes of sea ice variability with approximated periods of around 3, 6, 18, 27 and 55 years and show their contributions to sea ice variability and extremes. The main atmospheric and oceanic drivers of sea ice modes include the Arctic Oscillation and Arctic dipole anomaly for the 3 year mode, variability of sea surface temperature (SST) in the Gulf Stream region for the 6-year mode, decadal SST variability in the northern North Atlantic Ocean for the 18-year mode, Pacific Decadal Oscillation for the 27 year mode, and Atlantic Multidecadal Oscillation for the 55 year mode. Finally, our analysis suggests that over 70% of the sea ice area loss between the two extreme cases of 1996 (extreme high) and 2007 (extreme low) is caused by internal variability, with half of this variability being related to interdecadal modes.

Climate change and pulse migration: intermittent Chugach Inuit occupation of glacial fiords on the Kenai Coast, Alaska

For millennia, Inuit peoples of the Arctic and Subarctic have been challenged by the impacts of climate change on the abundance of key subsistence species. Responses to climate-induced declines in animal populations included switching to alternative food sources and/or migrating to regions of greater availability. We examine these dynamics for the Chugach Inuit (Sugpiat) people of southern coastal Alaska by synthesizing a large body of evidence from archeological sites, including radiocarbon dates and archaeofaunal assemblages, and by applying contemporary knowledge of glaciomarine ecosystems, spatial patterns of resource richness, and ocean-climate induced regime shifts in the Gulf of Alaska. We hypothesize that Chugach groups migrated from Cook Inlet and Prince William Sound to the Kenai Peninsula during periods of low sea surface temperatures (SSTs) to harvest harbor seals, which were seasonally aggregated near tidewater glaciers during pupping season, as well as piscivorous seabirds, Pacific cod, and other species that thrive under cool ocean conditions. During warming phases, the Chugach returned to Cook Inlet and Prince William Sound to fish for salmon and other species that abound during higher SSTs. Drivers of this coupled human-natural system of repeated (pulse) migration include the Pacific Decadal Oscillation (PDO), the dominant pattern of sea surface temperatures in the North Pacific that has been shown to generate step-like regime shifts in the marine food web; and coastal glaciers that structure the functioning of fiord ecosystems and support high levels of biological productivity. The culturally-constructed Chugach niche in the glaciomarine habitat of the Gulf of Alaska was based on intergenerationally transmitted ecological knowledge that enabled a resilient, mobile response to climate and resource variation.

Reconstructed Jing River streamflow from western China: A 399-year perspective for hydrological changes in the Loess Plateau

The Jing River is a secondary tributary of the Yellow River, which flows through the middle of the Loess Plateau in China. Severe water scarcity and soil erosion in the basin have threatened sustainable social and economic development. To assess and solve the region’s water resource problems, it is important to understand its historical hydrological climate change. Accordingly, we used five machine learning models and simple linear regression to reconstruct the January-June streamflow of the Jing River based on the tree ring width of Pinus tabulaeformis and Pinus armandii. By combining six models into an ensemble streamflow reconstruction, we obtained a more accurate reconstruction and streamflow variability information than with a single model. Over the past nearly four centuries, the Jing River has experienced seven high streamflow periods and ten low streamflow periods. The main atmospheric forcing factors driving the streamflow variability are the Pacific Decadal Oscillation and the El Niño-Southern Oscillation, which regulate the climate and hydrology of the region by affecting water vapor fluxes and the Asian monsoon. The different climate scenarios revealed the continued reduction in the future Jing River streamflow and a worsening water resource situation. This new streamflow reconstruction can serve as a valuable reference for analyzing regional hydrology and informing water resource management and policy formulations.

East Asia dust storms in spring 2021: Transport mechanisms and impacts on China

In spring 2021, several strong dust export processes crossed over East Asia, including the combined transport of Mongolia Gobi and Inner Mongolia dust. Based on CAMS forecast systems and satellite sensors, we analyzed the transport mechanisms and impact on China. Our case studies illustrate that in all 4 dust storms, the initial phase of transport was strongly confined by a low-level Mongolian cyclone facilitated by a deep cold-core upper trough. Using a methodology based on MODIS Deep Blue AOD data enables us to estimate the dust optical depth (DOD) and the impact of dust plumes on different areas in China. The result shows that North China was the most affected area in all cases, second only to the source region, with a mean observed DOD >0.8 and depolarization ratio > 0.2. The vertical distribution of the aerosol frequency of occurrence (FoO) was yielded using the CALIOP aerosol classification result. Dust storms were the dominant factor within 6 km (peaked at 2–3 km), while a significant amount of transported smoke aerosols dominated the upper troposphere. Even in dust storms, anthropogenic aerosols still occupied a high proportion of China's atmosphere, especially in South China, indicating the great contribution of human activities. As a subdominant aerosol type, polluted dust showed a profile similar to that of polluted continental aerosols, with peak heights generally lower than that of pure dust, at approximately 1–2 km. The development of cyclone/anticyclone activities was mostly modulated by the atmospheric dynamic background. By discussing the climatic index related to dust storm occurrence in East Asia, we found that the high-frequency years in the 21st century have a satisfactory correspondence with the negative phase of the Pacific Decadal Oscillation (PDO) and the positive phase of the Southern Oscillation Index (SOI). These findings may have positive significance for dust forecasting in East Asia in the future.

Winter cold extremes over the eastern North America: Pacific origins of interannual-to-decadal variability

Winter cold extreme events have been observed to frequently take place over North America mainly over its east side, which show significant interannual and decadal variability and cause huge economic losses in the United States. However, it is unclear what leads to the interannual-decadal variability of winter cold extremes over the eastern North America. In this study, we indicate that the decadal variability of winter cold extremes over the eastern North America, whose period is shortened in the recent decades, is mainly tied to Pacific decadal oscillation (PDO), whereas their interannual variability is mainly regulated by Victoria mode (VM). A positive PDO promotes cold extremes in the lower latitudes of the eastern North America mainly owing to the presence of positive Pacific North American (PNA+) patterns, whereas a positive VM is favorable for intense cold extremes in the higher latitudes of the eastern North America mainly due to the occurrence of negative North Pacific oscillation (NPO−) patterns. Thus, the positive VM and PDO combine to significantly contribute to the interannual-to-decadal variability of winter cold extremes over the eastern North America through changes in the winter NPO− and PNA+ patterns due to the variations of meridional background potential vorticity gradient and basic zonal winds. These new findings can help us understand what are the origins of the interannual-decadal variability of winter cold extremes over the eastern North America.

A 278-Year Summer Minimum Temperature Reconstruction Based on Tree-Ring Data in the Upper Reaches of Dadu River

In the context of global warming, climate change in river headwater regions and its drivers have attracted increasing attention. In this study, tree-ring width (TRW) chronology was constructed using tree-ring samples of fir (Abies faxoniana) in Dadu River Basin in the central part of the western Sichuan Plateau, China. Correlation analysis with climatic factors implies that the radial growth of trees in the region is mainly limited by temperature and has the highest correlation with the mean minimum temperature in summer (June and July) (R = 0.602, p < 0.001). On this basis, the TRW chronology was adopted to reconstruct variations in the mean minimum temperatures in summer from 1733 to 2010 in the upper reaches of Dadu River. The reconstruction equation was stable and reliable and offered a variance explanation rate of 36.2% in the observed period (1962~2010). In the past 278 years, the region experienced nine warm periods and ten cold periods. The warmest and coldest years occurred in 2010 and 1798, respectively, with values of 13.6 °C and 11.0 °C. The reconstruction was highly spatiotemporally representative and verified by temperatures reconstructed using other tree-ring data in surrounding areas. A significant warming trend was found in the last few decades. Moreover, the multi-taper method (MTM) analysis indicated significant periodic changes in quasi-2-year and 21–35-year periods, for which the El Niño Southern Oscillation (ENSO) and the Pacific decadal oscillation (PDO) could be the key controlling factors.

Forest fires and fire protection

According to the data of the Republic Statistical Office of the Republic of Srpska for the period 2000‒2019, forest fires in the Republic of Srpska burned a total of 144,006 ha. Of that, state forests account for 112,965 ha (78.44%) and private forests for 31,041 ha (21.56%). The largest burned areas were recorded in 2012 (33,781 ha), and the lowest in 2018 (166 ha). The growth trend of burned areas is not statistically significant. In the case of deciduous stands in state forests, an increasing trend of burned areas was also found, which is not statistically significant. The maximum was recorded in 2012 (21,226 ha), and the minimum in 2018 (53 ha). The increase in the burned areas of coniferous stands in state forests is somewhat more pronounced, but it is also not statistically significant. The maximum value was recorded in 2017 (2,125 ha) and the minimum in 2014 (29 ha). In mixed stands of deciduous and coniferous forests (state forest), a statistically significant (0.05) trend of decreasing burned areas was recorded. The maximum was recorded in 2001 (3,180 ha) and the minimum in 2014 (12 ha). Of the total burned area in Republika Srpska, low fires account for 95.81% and high fires for 3.99%. It is considered that the main cause of forest fires in Republika Srpska is man, and there are also fires caused by mines left over from the war (1992‒1995). In the forests on the territory of the municipality of Han Pijesak (altitude 762 to 1,537 m) in the period 2003‒2010 total 31 fires were recorded in 2006, with a area of 137 ha burned. In 85% of cases, the cause was man, mines and explosives in 3%, lightning strikes in 2%, while in 10% the cause was not determined. In the Republic of Srpska, the southern part (i.e. the mesoregion of Trebinje) is the most endangered by forest fires. Numerous studies have established a link between climate indices and forest fires. In North America, the El Niño Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), and the Atlantic Decade Oscillation (AMO) operate mainly. The influence of ENSO is evident in parts of South America as well as in Southeast Asia. In Europe, the influence of AMO (France, Portugal), the Mediterranean Oscillation Index (MOI) (Romania), the Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO) (Northern Europe) has been determined. According to the hypothesis of the solar wind as the cause of fire, high-energy particles of the solar wind, originating from active regions and coronal holes on the Sun, penetrate the Earth's magnetic defenses and reach its surface where they cause vegetation fires. This hypothesis has been confirmed for the United States, Portugal, and some parts of Southeast Europe. Research on the relationship between climate indices and forest fires, as well as testing the hypothesis of the solar wind as the cause of fires should be conducted in the Republic of Srpska, and the obtained results could be used in the forecast of forest fires. In order to conduct this research, it is necessary to create a single database on forest fires in the Republic of Srpska. This database should contain the following data: location, time of occurrence, course, completion, extinguishing activities, engaged personnel and equipment, burned area and its structure, causative agent (if known), etc. Proposed measures for improving the state of forest fire protection in the Republic of Srpska include: proper selection of tree species for afforestation, development of fire averages while maintaining existing ones, raising biological fire zones, improving water supply and construction of water reservoirs, improving propaganda measures, improving fire safety equipment, improving fire safety equipment. participants in firefighting actions and the local population and the application of the results of scientific research.

Impacts of Tibetan Plateau sensible heat and El Niño–Southern Oscillation on precipitation over South China under the background of the PDO

This study aims to investigate the impacts of the spring sensible heat (SH) over the Tibetan Plateau (TP) and the El Niño–Southern Oscillation (ENSO) in the preceding wintertime on midsummer (July–August) precipitation over South China under the different Pacific decadal oscillation (PDO) phases. More specifically, eight classifications are adopted at the demarcation point around 1996 when the spring SH over the TP and the midsummer precipitation in South China occurred as well as the PDO phase transition, including positive and negative SHs and ENSOs under a positive PDO phase (1979–1996) and a negative PDO phase (1997–2019), respectively, based on the Niño-3 index and the spring SH calculated from 48 stations over the central and eastern parts of the TP. The results show that both the spring SH and the ENSO in preceding wintertime have a significant impact on the midsummer precipitation over South China; that is, when the two factors are in their respective positive (negative) phase, the midsummer precipitation in South China is generally less (more). Importantly, the phase change of background field PDO can significantly enhance the effect of the SH and the ENSO on summer precipitation over South China. Moreover, compared with the preceding wintertime ENSO, the spring SH over the TP contributes more to the midsummer precipitation in South China based on analyses of their independent and synergistic effects. The main mechanism responsible for the anomalous midsummer precipitation over South China are the combined effects of the South Asian high (SAH) and the western Pacific subtropical high (WPSH), which are controlled by the spring SH anomaly over the TP and the ENSO, respectively. Deep understanding of the dominant factors of the midsummer precipitation over South China will help understand the local climate change and reduce the losses caused by drought and flood disasters.

A 2.5° × 2.5° gridded drought/flood grades dataset for eastern China during the last millennium

Hydroclimate reconstruction for the last millennium is essential to understand the differences in hydroclimate extremes and their causes under cold/warm conditions. In this study, the first gridded drought/flood grades (D/F grades) dataset in eastern China (EC) during the last millennium was generated. This D/F grades dataset mainly consisted of two components. The first section was created by interpolating drought/flood grades from 1500 to 2000 using the angular distance weight method. Sampling error estimates were employed to assess the effects of the interpolated dataset. The second section for the D/F grades dataset during 960–1500 was generated by constructing best subset regression models using selected tree-ring chronologies in the United States through atmospheric teleconnection. The validation parameters of the calibration equations were also derived, including the adjusted R2, predicted R2, RE, and CE. This dataset provides critical support for investigating the characteristics and causes of hydroclimate extremes in EC at various spatiotemporal scales, as well as the relationship with climate modes, such as El Niño-Southern Oscillation, Pacific Decadal Oscillation, and East Asia Summer Monsoon.

Riparian cottonwood mortality following compound impacts from river water withdrawal and hydroclimatic variation

AbstractOriginating in Glacier Park, Montana, the transboundary St. Mary River has provided a focus for international water sharing for a century. It was dammed and diverted in the USA, and more extensively in Alberta, where it supports Canada's centre for agricultural irrigation. Following water withdrawals, the riparian cottonwoods (Populus angustifolia) collapsed, and we assessed the woodland decline downstream from the St. Mary Dam, through eight sets of aerial photographs over seven decades. These revealed 88% woodland loss from 1951 to 1999 (R2 = 0.984), including a steeper decline with the 1980s drought. Following the implementation of an environmental flow regime that increased minimum flows after 1993, the remnant woodlands were stabilized, with a slight recovery by 2022. Analyses of the historical hydrology revealed compound contributions to the woodland mortality. (1) Annual river flows declined with increasing water diversion, and (2) late summer flows were particularly depleted. (3) Dam operations resulted in abrupt stage recessions and irregular spikes. (4) Accompanying climate warming, the spring snowmelt advanced and late summer flows declined. (5) The greatest climatic influence involved multiple‐year clusters with high versus low flows, corresponding with the Pacific decadal oscillation (PDO) from 1920 to 1990 (R2 = 0.365), but less coupled thereafter. This long‐term study demonstrates the coordination between hydroclimatic variation, water management and the fate of riparian woodlands, which reveal ecosystem health. An environmental flow regime provided benefit, and for other regulated rivers in dry ecoregions, we recommend functional flow regimes that provide sufficient minimum flows and avoid abrupt flow recession or irregular spikes.

Role of mean, variability and teleconnection of clouds behind Indian summer monsoon rainfall

AbstractSkilful prediction of the seasonal Indian summer monsoon (ISM) rainfall (ISMR) at least one season in advance has great socio‐economic value. The ISM is a lifeline for about a sixth of the world's population. The ISMR prediction remained a challenging problem with the subcritical skills of the dynamical models due to a limited understanding of the interaction among clouds, convection and circulation. In this study, we have analysed the seasonal mean of high cloud fraction, ice mixing ratio and ice cloud fraction from satellite and reanalysis and demonstrated their importance for ISM. The variability of the mixing ratio of cloud ice in different time scales (3–7 days, 10–20 days and 30–60 days bands) is also examined from reanalysis during ISM. Here, we have shown the teleconnection of different cloud variables over the ISM region with global sea surface temperature. We found that they are tied with slowly varying forcing (e.g., El Niño and Southern Oscillation). Besides, the correlation of cloud ice with different indices (Niño, Pacific Decadal Oscillation, North Atlantic Oscillation and Extratropics) may enhance the potential predictability of ISMR. The representation of deep convective clouds, which involve the ice‐phase processes in a coupled climate model, strongly modulates ISMR variability in association with global predictors. The results from the two sensitivity simulations using coupled global climate model (CGCM) demonstrate the importance of the cloud ice on ISM rainfall predictability. Therefore, this study provides a scientific basis for improving the simulation of the seasonal ISMR by developing the physical processes of the cloud on a subseasonal time scale and motivating further research in this direction.

Regime Shift of the Sea Level Trend in the South China Sea Modulated by the Tropical Pacific Decadal Variability

AbstractDuring the altimeter era, the sea level in the South China Sea (SCS) and western tropical Pacific (WTP) experienced significant decadal variability. The sea level rose during 1993–2009 and fell during 2010–2019. The decadal variability of Walker Circulation associated with the Pacific Decadal Oscillation can explain the sea level variability in the WTP to a great extent. The wind forced westward propagating Rossby waves increased (decreased) the sea level in the WTP during 1993–2009 (2010–2019). However, the interior wind forcing has a negligible contribution to the decadal variability of the sea level in the SCS. The remote forcing from WTP through the oceanic bridge was supposed to play a dominant role. The sensitive experiments of a 1½‐layer model and Regional Oceanic Modeling System suggested that the sea level signals via the Sibutu Passage and Mindoro Strait accounted for the decadal variability of sea level in the central basin of SCS.

Attribution and Causality Analyses of Regional Climate Variability

A two-step attribution and causality diagnostic is designed by employing singular spectrum analysis to unfold the attributed climate time series into a trajectory matrix and then subjected to an empirical orthogonal function analysis to identify the evolving driving forces, which can finally be related to major climate modes through their independent frequencies by wavelet analysis. Application results from the arid and drought-prone southern Intermountain region of North America are compared with the climate or larger scale forcing diagnosed from slow feature analysis using the sources of the water and energy flux balance. The following results are noted: (i) The changes between the subsequent four 20-year periods from 1930 to 2010 suggest predominantly climate-induced forcing by the Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation. (ii) Land cover influences on the changing land cover are of considerably smaller magnitude (in terms of area percentage cover) whose time evolution is well documented from forestation documents. (iii) The drivers of the climate-induced forcings within the last 20 years are identified as the quasi-biennial oscillation and the El Niño–Southern Oscillation by both the inter-annual two-step attribution and the causality diagnostics with monthly scale-based slow feature analysis.

Wave Climate Variability and Trends in Tuvalu Based on a 44‐Year High‐Resolution Wave Hindcast

AbstractWaves are one of the main causes of coastal flooding and shoreline change in low‐lying atoll nations like Tuvalu. A detailed understanding of Tuvalu’s wave climate is therefore critical for decision‐makers, coastal engineers, and disaster‐risk managers. Here, we investigate Tuvalu’s wind‐wave climate, changes due to large‐scale climate variability, and long‐term trends. A 44‐year (1979–2022) high‐resolution wave hindcast was developed using the unstructured version of the wave model Simulating Waves Nearshore (SWAN). The model resolution varied between 20 km offshore in deep water and 300 m close to shore. The model was forced with the European Centre for Medium‐Range Weather Forecasts Reanalysis v5 (ERA5) wind and boundary wave conditions. Northern and Southern Hemisphere winter months produced the largest and most powerful waves. Through the analysis of the directional wave energy spectra, we identified three main wave energy sources: (a) extratropical storms in the Southern Ocean; (b) extratropical storms in the North Pacific; and (c) easterly trade winds. Slightly positive trends in high‐frequency (∼10 s) wave energy from the east and low‐frequency (∼15 s) wave energy from the southwest were linked to an intensification of trade winds and an intensification and poleward displacement of the Southern Ocean storm belt over recent decades. The interannual variability of Tuvalu’s wave climate was strongly linked to large‐scale climate modes such as El Niño Southern Oscillation, Pacific Decadal Oscillation, and Arctic and Antarctic Oscillation. The study builds on previous research and significantly enhances the understanding of Tuvalu’s wave climate variability.

Pacific Decadal Oscillation Influences Tropical Oxygen Minimum Zone Extent and Obscures Anthropogenic Changes

AbstractObservations suggest that the tropical Pacific Ocean has lost oxygen since the 1960s leading to the expansion of its oxygen minimum zone (OMZ). Attribution to anthropogenic forcing is, however, difficult because of limited data availability and the large natural variability introduced by the Pacific Decadal Oscillation (PDO). Here, we evaluate the PDO influence on oxygen dynamics and OMZ extent using observations and hindcast simulations from two global ocean circulation models (NEMO‐PISCES, MOM6‐COBALT). In both models, the tropical Pacific oxygen content decreases by about 30 Tmol.decade−1 and the OMZ volume expands by 1.3 × 105 km3.decade−1 during PDO positive phases, while variations of similar magnitude but opposite sign are simulated during negative phases. Changes in equatorial advective oxygen supply, partially offset by biological demand, control the oxygen response to PDO. Observations which cover 39% of the tropical Pacific volume only partially capture spatio‐temporal variability, hindering the separation of anthropogenic trend from natural variations.

Increasing peak intensity of tropical cyclones passing through the Korean Peninsula

An increasing trend along with an abrupt increase in the peak intensity of tropical cyclones (TCs) passing through the Korean Peninsula (KP) are significantly detected from 1981 to 2020 and since 2003, respectively. Here we present observational evidence that such trend and shift are largely attributed with the increased passages of intense TCs in the KP during the mature boreal autumn (i.e., September–October, SO) and linked with the recent shift of the Pacific Decadal Oscillation (PDO) to its negative phase. A negative PDO during SO is related to environmental changes that are favorable for more intense TC passages in the KP including a weakened East Asian subtropical jet stream, weaker vertical wind shear, warmer subtropical sea surface temperature, and stronger low-level relative vorticity. Such findings are expected to provide new insights on understanding regional TC variability and ultimately, contribute to long range TC prediction initiatives in the KP region.

Northwestward advance of the northern boundary of the East Asian summer monsoon over the 21st century in CMIP6 projections

AbstractThe East Asian summer monsoon (EASM) projection has attracted much attention, whereas there are few investigations on the future changes of the EASM northern boundary. The boundary migration would influence the distribution of precipitation and the related vegetation, and its projection is important for policy development of climate change adaptation. In this study, based on the median of 22 selected Coupled Model Intercomparison Project Phase 6 (CMIP6) models, the linear trends of meridional movement of the EASM northern boundary are found to be 0.45−1.39° of latitude during 2015–2099 based on the three precipitation‐based metrics under three shared socioeconomic pathway scenarios. Spatially, the multimetric climatological EASM northern boundary displays a 70–170 km northwestward advance during 2080–2099 compared to 1981–2010. Such an advance also holds true for most individual models, but the migration magnitudes vary with metrics, scenarios and models. The strengthened EASM in association with the intensified land–sea thermal contrast and the enhanced atmospheric water vapour content in response to global warming account for the northwestward advance of the EASM northern boundary, which is related to the increased possibility of the negative phase of the Pacific decadal oscillation in the future. Additionally, the thermodynamic component due to the increased moisture content contributes more than the dynamic term arising from the reinforced EASM circulations to the intensified precipitation and northwestward migration of the EASM northern boundary. The future advance of the EASM northern boundary favours a “northern flood and southern drought” precipitation pattern over eastern China, which would partly mitigate drought conditions in northern arid regions.