Increased Winter Precipitation Research Articles

Hydroclimatic anomalies detected by a sub-decadal diatom oxygen isotope record of the last 220 years from Lake Khamra, Siberia

Abstract. Northern latitudes have been significantly impacted by recent climate warming, which has increased the probability of experiencing extreme weather events. To comprehensively understand hydroclimate change and reconstruct hydroclimatic anomalies such as drought periods, appropriate proxy records reaching further back in time beyond meteorological measurements are needed. Here we present a 220-year (2015–1790 CE), continuous, stable oxygen isotope record of diatoms (δ18Odiatom) from Lake Khamra (59.99° N, 112.98° E) in eastern Siberia, an area highly sensitive to climate change and for which there is a demand for palaeohydrological data. This high-resolution proxy record was obtained from a 210Pb–137Cs-dated sediment short core and analysed to reconstruct hydroclimate variability at a sub-decadal scale. The interpretation of the δ18Odiatom record is supported by meteorological data, modern isotope hydrology and geochemical analyses of the same sediment, which is indicative of the conditions in the lake and catchment. A comparison with meteorological data going back to 1930 CE revealed that the δ18Odiatom record of Lake Khamra is primarily influenced by regional precipitation changes rather than the air temperature. We identified winter precipitation, which enters the lake as isotopically depleted snowmelt water, as the key process impacting the diatom isotope variability. We related the overall depletion of δ18Odiatom in recent decades to an observed increase in winter precipitation in the area, likely associated with the global air temperature rise, Arctic sea ice retreat and increased moisture transport inland. Available palaeoclimate proxy records, including a fire reconstruction for the same lake, support the idea that the new record is a valuable hydroclimate proxy that is indicative of precipitation deficits and excludes solar insolation and air temperature as primary driving forces, even before the first meteorological recordings. We propose two possible hydroclimatic anomalies that were detected in the Lake Khamra δ18Odiatom record: one at the beginning of the 19th century and a second prominent event in the 1950s. Both are interpreted as prolonged dry periods associated with enriched δ18Odiatom values likely caused by reduced winter precipitation, which coincide with phases of reconstructed severe wildfires in the region. Despite the apparent pristine lake area, we observed a three- to fourfold increase in mercury concentrations and accumulation rates within the sediment record since the early 20th century, which is partly attributed to human air pollution.

Influence of the Pacific Decadal Oscillation on Winter Temperatures and Precipitation Over the Southern Tibetan Plateau

AbstractWe used observational data and a long‐term piControl simulation from the Community Earth System Model Version 2 to investigate the influence of the Pacific Decadal Oscillation (PDO) on the winter climate over the Tibetan Plateau. The results showed that changes in the phase of the PDO have a significant effect on winter temperatures and precipitation over the southern Tibetan Plateau. Changes in the sea surface temperature (SST) during the positive PDO can weaken the Walker circulation and increase the SST in the Indian Ocean. Our analyses of the moist static energy showed that warming of the tropical troposphere over the Indian Ocean caused by the increased SST has resulted in the horizontal advection of anomalous moist enthalpy by the climatological zonal winds, which was responsible for anomalous ascending motion over the Tibetan Plateau. The additional moisture budget suggests that enhanced vertical motion contributes to the increase in winter precipitation and related total cloud cover over the Tibetan Plateau, leading to the increase of snow depth. The increased total cloud cover and snow depth, in turn, reduces net surface shortwave radiation. The surface air temperature of the Tibetan Plateau is then decreased as a result of the reduction in the net surface shortwave radiation. The PDO therefore has an important modulating role in the interdecadal variability of the winter climate over the Tibetan Plateau. We therefore need to focus on changes in the PDO in research related to the decadal prediction of the climate over the Tibetan Plateau.

Influence of snowmelt on increasing Arctic river discharge: numerical evaluation

Snow is the most important component of the Arctic climatic and hydrological system and is directly vulnerable to climate change. In recent decades, observations have indicated significant decreases in the Arctic snow cover and snowfall rate, whereas water discharge from circumpolar Arctic river basins into the Arctic Ocean has increased. To evaluate the contribution, not well quantified, of snow to the river discharge increase, we conducted sensitivity simulations with surface air temperature and precipitation as climatic treatment variables, combining a land surface model and a distributed discharge model. Variables were treated assuming higher climate variations in the Arctic cold season in 1979–2018. The surface and subsurface runoffs simulated by the land surface model were set as inflows in the discharge model to estimate river discharge. Snowmelt mostly converted to surface runoff, accounting for 73.6% of the anomalous surface runoff increase and inducing the simulated peak discharge in spring and early summer. This relationship was enhanced by the winter precipitation increase. Snow loss induced by higher air temperature contributed to the decrease in the peak and annual discharges, but caused the peak discharge to occur earlier. Additionally, warmer temperature increased the proportion of rainfall in the partitioning of precipitation, causing more subsurface runoff, particularly in autumn and winter. These results provide a first separate evaluation of factors influencing Arctic water discharge, including seasonal hydrographs, and illustrate the influence of climate warming-induced snowfall and rainfall variations on the circumpolar Arctic river discharge.

Assessing the hydroclimate changes in Western Himalayas during the Little Ice Age

The Little Ice Age (LIA) was a period of most recent glacial advancement and had pronounced cooling effect in the North Atlantic region. Synchronous hydroclimate changes are also reported from the Himalayas, however owing to the heterogeneity within the proxy reconstructions, their relationship with LIA cooling is unclear. Varied topography, huge glacial mass, and multiple moisture sources (both from the Indian Summer Monsoon (ISM) and the Westerlies) makes understanding of the impact of LIA cooling on this region ambiguous. In this study, we review and assess the existing paleoclimatic proxy records for a comprehensive analysis of the regional response of the Western Himalayas to LIA cooling. Using the existing meteorological reanalysis data for back trajectory analysis for the last 20 years, the Western Himalayan region was classified into three different zones based on the relative percentage of moisture-bearing-wind-source contribution. The upper Western Himalayas receive most of the moisture from the Westerlies, both Middle and Lower Western Himalayas receive majority of rainfall from the ISM, with a relatively higher contribution of Westerlies in the Lower Western Himalayas. Comparison of reconstructions using Principal Component Analysis reveal consistent high moisture conditions during the LIA, with increased winter precipitation and decreased summer precipitation coherently in all the records. Spectral analysis of the available proxy records and various climate forcing for LIA cooling show similar dominant frequency, attesting that the LIA cooling drove the hydroclimate changes in the Western Himalayan region. External forcings such as decreased solar activity and increased volcanic activity caused cooling, influenced the Inter Tropical Convergence Zone, and resulted in weaker summer rainfall during the LIA. Synchronous changes in the North Atlantic Oscillations and El Niño–Southern Oscillation records with precipitation records suggest a link between “monsoon breaks” and enhanced Westerly intensity and an intensified winter precipitation in this region.

Climate Change Will Impact Surface Water Extents and Dynamics Across the Central United States.

Climate change is projected to impact river, lake, and wetland hydrology, with global implications for the condition and productivity of aquatic ecosystems. We integrated Sentinel-1 and Sentinel-2 based algorithms to track monthly surface water extent (2017-2021) for 32 sites across the central United States (U.S.). Median surface water extent was highly variable across sites, ranging from 3.9% to 45.1% of a site. To account for landscape-based differences (e.g., water storage capacity, land use) in the response of surface water extents to meteorological conditions, individual statistical models were developed for each site. Future changes to climate were defined as the difference between 2006-2025 and 2061-2080 using MACA-CMIP5 (MACAv2-METDATA) Global Circulation Models. Time series of climate change adjusted surface water extents were projected. Annually, 19 of the 32 sites under RCP4.5 and 22 of the 32 sites under RCP8.5 were projected to show an average decline in surface water extent, with drying most consistent across the southeast central, southwest central, and midwest central U.S. Projected declines under surface water dry conditions at these sites suggest greater impacts of drought events are likely in the future. Projected changes were seasonally variable, with the greatest decline in surface water extent expected in summer and fall seasons. In contrast, many north central sites showed a projected increase in surface water in most seasons, relative to the 2017-2021 period, likely attributable to projected increases in winter and spring precipitation exceeding increases in projected temperature.

Convection-Permitting Future Climate Simulations for Bulgaria under the RCP8.5 Scenario

In recent decades, climate change has become a critical global issue with far-reaching consequences for regional climates and ecosystems. While regional climate models provide valuable information, there is a growing need for high-resolution simulations to assess local impacts. This paper addresses this gap by presenting the first simulation of a 3 km convection-permitting (CP) scenario simulation for Bulgaria. The main aim of this study is to assess different precipitation indices and their future changes for Bulgaria under the Representative Concentration Pathway 8.5 (RCP8.5) scenario following the Coordinated Regional Climate Downscaling Experiment Flagship Pilot Study protocol. The simulations are evaluated against high-resolution observations. We downscale Coupled Model Intercomparison Project 5 Global Climate Model (CMIP5 GCM) data for historical (1995–2004) and future (2089–2098) periods using a regional climate model (RCM) at 15 km grid spacing and parametrized convection. We use these fields as initial and boundary conditions for convection-permitting kilometer-scale simulations. The 15 km grid spacing driving model is used as a reference to assess the added value of the kilometer-scale simulation. Additionally, the 3 km seasonal mean and projected 2 m temperature and the winter snow water equivalent are presented. The results show that the kilometer-scale simulation shows better performance of wet-hour intensity in all seasons, wet-hour frequency in the spring, fall, and winter, and extreme precipitation (99.9th percentile of all precipitation events, p99.9) in the winter and fall. The kilometer-scale simulation improves the projected precipitation distribution and modifies the signal of the precipitation frequency, intensity, and heavy precipitation change over some areas. A positive projected change in the wet-hour intensity is expected in all seasons (13.86% in spring, MAM, 17.48% in summer, JJA, 1.97% in fall, SON, and 17.43% in winter, DJF) and in the heavy precipitation in the spring (13.14%) and winter (31.19%) in the kilometer-scale experiment. The projected increase in mean winter precipitation is accompanied by a significant decrease in mean winter snowfall over lowlands (50−70%). The convection-permitting Regional Climate Model, version 4.7.1 (RegCM4.7.1) suggests an increase in winter snowfall over the highest parts of the country, but a significant increase in the 2 m temperatures there. The results of this study are encouraging and may be of interest to the community of climate scientists and users of climate data for making reliable estimates of the local impacts of future climate change.

Characterization of the natural resource potential of the agricultural zone of the Krasnoyarsk territory

The aim of the study was to determine the natural resource potential of the agricultural territories in the agricultural regions of the Krasnoyarsk Territory and to investigate the fundamental characteristics of climatic conditions. Location and time of the study. The study utilized multi-year data (from 1978 to 2020) from 22 meteorological stations located in the natural districts of the Krasnoyarsk Territory to analyze climatic conditions. Methods. A georesource database was used to assess the natural resource potential (NRP), including information about soils, agroclimatic parameters, and their impact on agricultural crops. Soils and climatic parameters were ranked on a scale of 5 to 100. The NRP assessment was categorized as follows (points): 1–20 – low; 21–40 – below average; 41–60 – average; 61–80 – above average; 81–100 – high. Climatic conditions were analyzed based on multi-year data from meteorological stations in different natural regions of the Krasnoyarsk Territory, using trend analysis and data smoothing methods to identify dynamics. Results. The natural resource potential values vary from 32.5 points for the northern forest-steppe podzolic soils in the Kansk Natural District to 62.5 points for chernozems (black soils) in the northern forest-steppe of the South Minusinsk Natural District. Weighted average NRP values for the agricultural territories of the Krasnoyarsk Krai increase in the following sequence (points): subtaiga (38.0) – Kansk Natural District (45.4) –Krasnoyarsk Natural District (46.3) – Achinsk-Bogotol Natural District (47.4) – Chulymo-Yenisei Natural District (47.8) – Nazarovo Natural District (49.4) – South Minusinsk Natural District (54.3). Conclusions. Climate warming, accompanied by significant increases in summer and winter precipitation, milder winters, and prolonged growing seasons favorable for agricultural crops and varieties, will have a positive impact on the NRP of the region's agrolandscapes, promoting the development of commercial agriculture, especially in the forest-steppe zone. The range of agricultural crops and varieties may expand. Cultivating winter and hard wheat varieties is expected to become more effective. Improved natural conditions will contribute to bringing long abandoned lands into agricultural production. Changes in soil regimes and properties will lead to the transformation of agricultural crop cultivation technologies.

An interdisciplinary model chain quantifies the footprint of global change on reservoir sedimentation

Global change alters hydro-climatic conditions, affects land use, and contributes to more frequent droughts and floods. Large artificial reservoirs may effectively alleviate hydro-climatic extremes, but their storage capacities are threatened by sedimentation processes, which in turn are exacerbated by land use change. Envisioning strategies for sustainable reservoir management requires interdisciplinary model chains to emulate key processes driving sedimentation under global change scenarios. Therefore, we introduce a model chain for the long-term prediction of complex three-dimensional (3d) reservoir sedimentation considering concurrent catchment, hydro-climatic, and land-use conditions. Applied to a mountainous Mediterranean catchment, the model chain predicts increased sediment production and decreased discharge for high and medium emission pathways. Increased winter precipitation, accompanied by a transition from snowfall to rainfall, is projected to aggravate reduced summer precipitation, emphasizing a growing need for reservoirs. Additionally, higher winter precipitation proliferates sediment production and reservoir sedimentation. Land use change can outweigh the increased reservoir sedimentation originating from hydro-climatic change, which highlights the significance of localized actions to reduce sediment production. Finally, a 3d hydro-morphodynamic model provides insights into interactions between global change and reservoir sedimentation with spatially explicit information on future sedimentation patterns facilitating the implementation of management strategies.

The spatiotemporal extent of the Green Sahara during the last glacial period

The Sahara Desert, one of today's most inhospitable environments, has known periods of enhanced precipitation that supported pre-historic humans. However, the Green Sahara timing and moisture sources are not well known due to limited paleoclimate information. Here, we present a multi-proxy (δ18O, δ13C, Δ17O, and trace elements) speleothem-based climate record from Northwest (NW) Africa. Our data document two Green Sahara periods during Marine Isotope Stage (MIS) 5a and the Early to Mid-Holocene. Consistency with paleoclimate records across North Africa highlights the east-west geographical extent of the Green Sahara, whereas millennial-scale North Atlantic cooling (Heinrich) events consistently resulted in drier conditions. We demonstrate that an increase in westerly-originating winter precipitation during MIS5a resulted in favorable environmental conditions. The comparison of paleoclimate data with local archaeological sequences highlights the abrupt climate deterioration and the decline in human density in NW Africa during the MIS5-4 transition, which suggests climate-forced dispersals of populations, with possible implications for pathways into Eurasia.

The 2018 west-central European drought projected in a warmer climate: how much drier can it get?

Abstract. Projections of changes in extreme droughts under future climate conditions are associated with large uncertainties, owing to the complex genesis of droughts and large model uncertainty in the atmospheric dynamics. In this study we investigate the impact of global warming on soil moisture drought severity in west-central Europe by employing pseudo global warming (PGW) experiments, which project the 1980–2020 period in a globally warmer world. The future analogues of present-day drought episodes allow for investigation of changes in drought severity conditional on the historic day-to-day evolution of the atmospheric circulation. The 2018 west-central European drought is the most severe drought in the 1980–2020 reference period in this region. Under 1.5, 2 and 3 ∘C global warming, this drought episode experiences strongly enhanced summer temperatures but a fairly modest soil moisture drying response compared to the change in climatology. This is primarily because evaporation is already strongly moisture-constrained during present-day conditions, limiting the increase in evaporation and thus the modulation of the temperature response under PGW. Increasing precipitation in winter, spring and autumn limits or prevents an earlier drought onset and duration. Nevertheless, the drought severity, defined as the cumulative soil moisture deficit volume, increases considerably, with 20 % to 39 % under 2 ∘C warming. The extreme drought frequency in the 1980–2020 period strongly increases under 2 ∘C warming. Several years without noticeable droughts under present-day conditions show very strong drying and warming. This results in an increase in 2003-like drought occurrences, compounding with local summer temperature increases considerably above 2 ∘C. Even without taking into account a (potentially large) dynamical response to climate change, drought risk in west-central Europe is strongly enhanced under global warming. Owing to increases in drought frequency, severity and compounding heat, a reduction in recovery times between drought episodes is expected to occur. Our physical climate storyline provides evidence complementing conventional large-ensemble approaches and is intended to contribute to the formulation of effective adaptation strategies.

Propagation and Characteristics of Hydrometeorological Drought Under Changing Climate in Irish Catchments

AbstractHydrometeorological droughts are complex hazards expressed through the relative deviation in water availability relative to long‐term average conditions. The development and propagation of drought is governed by hydrological processes at different spatial scales including precipitation, evapotranspiration, overland flow, soil moisture, groundwater storage and discharge. Detailed investigation is thus required to evaluate associated linkages among various types of hydrometeorological drought to understand the likely impacts of climate change on drought characteristics (magnitude, frequency, duration and propagation). This study explores the impact of climate change on hydrometeorological drought for 10 Irish catchments, using standardized drought indices representing different components of the hydrological system. We employ 12 Global Climate Models from the Coupled Model Inter‐comparison Project phase 6 (CMIP6), forced with SSP370 and bias corrected to catchment conditions to force a conceptual hydrological model to generate hydrological projections for each catchment. The results indicate substantial drying during summer with associated increases in summer drought magnitude and frequency. However, simulations show a wide range of change, especially for hydrological drought (discharge and baseflow). Only modest changes in the magnitude and frequency of hydrological drought events were found, with increases in winter and spring precipitation offsetting summer dryness. Similarly, the probability of meteorological drought propagating to hydrological events (Standardized Streamflow Index/Standardized Baseflow Index), shows modest increases under the climate change projections considered. Findings highlight that drought, especially during summer are a critical climate change risk for adaptation.

Paleoclimatic reconstruction of northwest Himalaya since CE 475 using lake sediments from Tadag Taal, Kumaun, India

The Kumaun region of India in the northwestern Himalaya experiences complex rainfall patterns, receiving precipitation both during the summer monsoon (June to September) as well as during winter months dominated by westerlies (October to March). In this paper, we present a multi-proxy palaeoclimate record spanning the last ∼1500 years from Tadag Taal, an intermontane lake in Kumaun, India. Our dataset (stable isotope ratios, grain size, trace element ratios) suggests the occurrence of four long-term precipitation cycles of high and low rainfall coinciding with the Dark Ages Cold Period (DACP, ∼475–850 CE), Medieval Climate Anomaly (MCA ∼900–1300 CE), Little Ice Age (LIA, ∼1450–1800 CE) and Current Warm Period (CWP, ∼1850 CE to Present). We infer multi-decadal to centennial-scale shifts in the Indian summer monsoon co-varying with changes in El Niño-Southern Oscillation (ENSO) activity superimposed by changes in solar insolation. To correlate our time-series from the northwestern Himalaya with the Core Monsoon Zone and understand coherency in precipitation between the two regions, we applied the Wavelength Coherency Test. Our data suggest wet conditions during the early phase of the LIA (∼1450–1640 CE) and dry conditions in the later phase of the LIA from ∼1640–1800 CE. The wet conditions in the early LIA were perhaps driven by increased winter precipitation. This is in contrast to Central India that witnessed dry conditions during the entire duration of the LIA. This study thus suggests that monsoon strength was different during earlier and later phase of the LIA. Further, the LIA did not witness uniform climate variability through its duration as has been reported in some European and Chinese proxy records.

Multidecadal data indicate increase of aquatic insects in Central European streams

In recent years, declining insect biodiversity has sparked interest among scientists and drawn the attention of society and politicians. However, our understanding of the extent of this decline is incomplete, particularly for freshwater insects that provide a key trophic link between aquatic and terrestrial ecosystems, but that are also especially vulnerable to climate change. To investigate the response of freshwater insects to climate change, we quantified shifts in insect abundance and diversity across 7264 samples covering Central Europe during 1990–2018 and related these changes to annual data on temperature and precipitation. We observed both increases in richness (10.6 %) and abundance (9.5 %) of freshwater insects over the past three decades. These changes were related to increases in summer temperature and summer precipitation, which had negative effects on species richness, and to increases in winter temperature and precipitation, which had positive effects. Further we found that increased temperature was generally related to increased abundance, whereas increased precipitation was associated with declines, thus highlighting the particularly varying impacts on differing insect orders. Given that freshwater insects have been more severely affected by global change than marine and terrestrial species, the observed increases are a positive sign, but the overall situation of freshwater invertebrates is still critical.

Deepened snow in combination with summer warming increases growing season nitrous oxide emissions in dry tundra, but not in wet tundra

Impacts of increased winter snowfall and warmer summer air temperatures on nitrous oxide (N2O) dynamics in arctic tundra are uncertain. Here we evaluate surface N2O dynamics in both wet and dry tundra in West Greenland, subjected to field manipulations with deepened winter snow and summer warming. The potential denitrification activity (PDA) and potential net N2O production (N2Onet) were measured to assess denitrification and N2O consumption potential. The surface N2O fluxes averaged 0.49 ± 0.42 and 2.6 ± 0.84 μg N2O–N m−2 h−1, and total emissions were 212 ± 151 and 114 ± 63 g N2O–N scaled to the entire study area of 0.15 km2, at the dry and wet tundra, respectively. The experimental summer warming, and in combination with deepened snow, significantly increased N2O emissions at the dry tundra, but not at the wet tundra. The deepened snow increased winter soil temperatures and growing season soil N availability (DON, NH4+-N or NO3−-N), but no main effect of deepened snow on N2O fluxes was found at either tundra ecosystem. The mean PDA was 5- and 121-fold higher than the N2Onet at the dry and wet tundra, respectively, suggesting that N2O might be reduced and emitted as dinitrogen (N2). Overall, this study reveals modest but evident surface N2O fluxes from tundra ecosystems in Western Greenland, and suggests that projected increases in winter precipitation and summer air temperatures may increase N2O emissions, particularly at the dry tundra dominating in this region.

Synergistic Effect of El Niño and Negative Phase of North Atlantic Oscillation on Winter Precipitation in the Southeastern United States

Abstract This study reveals that the significant increase of winter precipitation over the southeastern United States (SEUS) is associated with El Niño and the negative phase of North Atlantic Oscillation (NAO−). Diagnosis of large-scale dynamics shows that El Niño and NAO− have a synergistic effect on the enhancement of transient eddies and stationary waves in the eastern Pacific, the southern United States, and the North Atlantic. These enhanced transient eddies are associated with subtropical jet stream acceleration and maintenance of subtropical low from the eastern Pacific to the Atlantic, influencing stronger stationary waves propagation from the tropical Pacific to the SEUS and North Atlantic, and from the northwestern Atlantic to the SEUS and Pacific. This favors a positive phase of the meridional dipole in geopotential height anomalies between the tropics and subtropics in the Western Hemisphere (WTSD) during the co-occurrence of El Niño and NAO−. The strong positive WTSD-like pattern, accompanied by a zonally extended and southerly shifted subtropical westerly jet, induces a northward-tilted secondary circulation with ascending motion over the SEUS and Gulf of Mexico. Simultaneously, an intensified trough over the SEUS promotes moisture and warm advection from the subtropical and tropical Pacific converging with cold advection from the northwestern North Atlantic in the adjacent Gulf of Mexico, which is beneficial for front and cyclone generation, and induces precipitation in the SEUS. This study suggests that the synergistic effects of El Niño and NAO− help to understand the variability of winter SEUS precipitation.

Palaeoclimatic and regional implications of Older Dryas and Younger Dryas local glacier activity in the low‐Arctic valley Finnkongdalen, Andøya, northern Norway

Continuous glacier margin and equilibrium‐line altitude fluctuations of a former glacier on central Andøya, northern Norway, are reconstructed during the Lateglacial based on moraines and AMS 14C‐dated sediments from the distal glacier‐fed lake Ner‐Finnkongdalsvatnet. The results indicate that a valley glacier occupied the entire valley during the Last Glacial Maximum (before 21 970±620 cal. a BP). The glacier remained large throughout the early Lateglacial until a significant glacier retreat took place about 14 300±330 cal. a BP. Major advances occurred during the Older Dryas (OD) and during the Younger Dryas (YD), while minor advances are suggested to have taken place during the Intra Allerød Cold Period and the Late Allerød Cooling. Additionally, three smaller glacier retreats/re‐advances within the YD are suggested to have taken place, the latter being the largest. The glacier re‐formations/advances during the Lateglacial are consistent with increases in temperature, and they are thus suggested to be the result of increased winter precipitation. Comparing the results with relevant glacier and sea‐surface temperature records, a south–north migration of storm tracks may have occurred between 12 100–11 810±220 cal. a BP. The high temporal resolution of local glacier activity in Finnkongdalen improves our understanding of the climate forcing of the regional glacier fluctuations of the northwestern sector of the Scandinavian Ice Sheet during the Skarpnes‐ (OD) and Tromsø‐Lyngen (YD) re‐advances.

Effect of anthropogenic forcing on increased winter precipitation in Southeast Asia from 1979 to 2014

Observations indicate that the winter precipitation over Southeast Asia (SEAP) shows significantly increased trends from 1979 to 2014, which can be partly reproduced under the historical all-forcing simulations from the 12 Coupled Model Intercomparison Project Phase 6 (CMIP6) climate models. By analyzing separate external forcings run with the Detection and Attribution experiments, we find that the effects of anthropogenic forcing rather than natural variation play a primary role in driving the increasing of SEAP trends simulated in the historical all-forcing experiments. Further analysis indicate that the observed increasing trend is closely associated with the decadal shift of the Southeast Asian precipitation after the mid-1990s, which could be also driven by anthropogenic forcing to some extent. Anthropogenic forcing can favor a La Niña-like pattern of sea surface temperature (SST) warming in the tropical Pacific and result in the decadal increase of SEAP via the enhanced zonal SST gradient and the Pacific Walker circulation. This study thus provides some evidence of the impacts of anthropogenic forcing to drive recent changes in the winter SEAP.

Revisit the Upper Portion of the Japan Sea Proper Water: A Recent Structural Change and Freshening in the Formation Area

AbstractLong‐term transition in the deep water of the Japan Sea was investigated to understand how oceans respond to climate change. A basin‐scale density decrease in the upper portion of the Japan Sea Proper Water (UJSPW) by 0.01 σθ was detected from the period 1964–1985 (Period I) to that of 2001–2019 (Period II) using Argo‐ and ship‐based data sets. The density decrease was mainly due to a temperature increase of 0.3°C. Based on the pycnostad density, the UJSPW was redefined as the water mass of 27.30–27.33 σθ, along with the core density of 27.30–27.31 σθ. The dissolved oxygen (DO) and thickness in the UJSPW showed a notable decrease between the periods, along with the isopycnal surfaces flattening. Warming down to 700 m and freshening in the upper 300 m were observed in the winter UJSPW formation area in the northwestern Japan Sea. An influence of the hemispheric‐scale regime shift occurred in 1988/1989 on the UJSPW warming was suggested, along with the long‐term trend in the winter air temperature. The freshening in the formation area was attributable to the increase in winter precipitation in the Amur River basin and the decrease in winter sea‐ice extents in the Okhotsk and northern Japan Seas. The intensified stratification in the UJSPW formation area brought a decrease in the amount of the UJSPW formation through the prevention of winter deep convection, which resulted in the thinner thickness and lower DO in the UJSPW in Period II, along with the temperature increase.

Changing Spring Phenology of Northeast China Forests during Rapid Warming and Short-Term Slowdown Periods

The vast forests of Northeast China are under great pressure from climate change. Understanding the effects of changing climate conditions on spring phenology is of great significance to assessing the stability of regional terrestrial ecosystems. Using Normalized Difference Vegetation Index data from 1982 to 2013, this paper investigated the changes in the start date of the vegetation growing season (SOS) of two main forest types in Northeast China, analyzing the changes in temporal and spatial patterns of forest spring phenology before and during the recent short-term warming slowdown, and exploring the effects of day and night temperatures and precipitation on the start of the growing season. The results showed that, during the rapid warming period (1982–1998), the SOS of deciduous needleleaf forests (DNF) was significantly advanced (−0.428 days/a, p < 0.05), while the rate of advance of SOS of deciduous broadleaf forests (DBF) was nonsignificant (−0.313 days/a, p > 0.10). However, during the short-term slowdown (1998–2013), the SOS of DBF was strongly delayed (0.491 days/a, p < 0.10), while the change in SOS of DNF was not significant (0.169 days/a, p > 0.10). The SOS was sensitive to spring maximum temperature for both forest types during the analysis period. Increases in winter precipitation influenced the SOS during the rapid warming period for DNF; this combined with the increase in the spring maximum temperature contributed to the advance in SOS. The decrease in the spring maximum temperature during the short-term slowdown, combined with a decrease in the previous summer maximum temperature, contributed to the rapid delay of SOS for DBF. DBF SOS was also more influenced by lagged effects of prior conditions, such as previous autumn to spring precipitation during the rapid warming period and previous summer maximum temperature during the short-term slowdown. In general, SOS was mainly determined by changes in daytime thermal conditions; DNF is more sensitive to temperature increases and DBF is more sensitive to decreases. Different regional climate conditions lead to differences in the distribution of DNF and DBF, as well as in the response of SOS to climate change during the rapid warming and short-term slowdown periods.

Variations of Summertime SSTA Independent of ENSO in the Maritime Continent and Their Possible Impacts on Rainfall in the Asian–Australian Monsoon Region

Abstract The sea surface temperature anomalies (SSTA) in the Maritime Continent (MC) region are mainly related to local variability, ENSO, and the Indian Ocean dipole. Using the reanalysis data from NOAA and NCEP–NCAR, by employing the empirical orthogonal function (EOF) analysis, we have explored the principal mode of ENSO-independent summertime SSTA in the MC and its associations with regional climate anomalies. After ENSO signals have been removed, the leading mode of SSTA in the MC exhibits a uniformly signed pattern, which mainly varies on an interannual time scale. The maintenance mechanisms of the ENSO-independent SSTA are different in different subregions, especially over the region south of Java and the tropical northwestern Pacific. When the time coefficient of the first leading EOF mode (EOF1) is positive, warmer SSTAs are observed in the area south of Java. The oceanic dynamic heating there facilitates the warmer SSTA. Thus, the Gill-type response of the atmosphere is found over the region south of Java. The diabatic cooling in the atmosphere is dominant over the tropical northwestern Pacific where the warmer SSTA is maintained by the absorption of solar radiation due to less cloud cover there. A tilted vertical circulation is hence formed, linking the tropical southeastern Indian Ocean with the tropical northwestern Pacific. The anomalous circulations in the Asian–Australian monsoon region are affected by this ENSO-independent SSTA mode, resulting in decreased summer rainfall anomaly in the region near the southeast coast of China and increased winter precipitation anomaly over the extratropical region of Australia.