Regime Shifts in the North Pacific Simulated by a COADS-driven Isopycnal Model

The Miami Isopycnal Coordinate Ocean Model (MICOM) is adopted to simulate the intevdecadal variability in the Pacific Ocean with most emphasis on regime shifts in the North Pacific. The computational domain covers 60°N to 40°S with an enclosed boundary condition for momentum flux, whereas there are thermohalirie fluxes across the southern end as a restoring term. In addition, sea surface salinity of the model relaxes to the climatological season cycle, which results in climatological fresh water fluxes. Surface forcing functions from January 1945 through December 1998 are derived from the Comprehensive Ocean and Atmospheric Data Set (COADS). Such a numerical experiment reproduces the observed evolution of the interdecadal variability in the heat content over the upper 400-m layer by a two-year lag. Subduction that occurs at the ventilated thermocline in the central North Pacific is also been simulated and the subducted signals propagate from 35°N to 25°N, taking about 8 to 10 years, in agreement with the expendable Bathy Thermograph observation over recent decades. Interdecadal signals take a southwest-ward and downward path rather than westward propagation, meaning they are less associated with the baroclinic planetary waves. During travel, the signals appear to conserve potential vorticity. Therefore, the ventilated thermocline and related subduction are probably the fundamental physics for interdecadal variability in the mid-latitude subtropics of the North Pacific.

Rolling Gaussian Process Regression with Application to Regime Shifts

Gaussian Process Regression (GPR) can be applied to the problem of estimating a spatially-varying field on a regular grid, based on noisy observations made at irregular positions. In cases where the field has a weak time dependence, one may desire to estimate the present-time value of the field using a time window of data that rolls forward as new data become available, leading to a sequence of solution updates. We introduce “rolling GPR” (or moving window GPR) and present a procedure for implementing that is more computationally efficient than solving the full GPR problem at each update. Furthermore, regime shifts (sudden large changes in the field) can be detected by monitoring the change in posterior covariance of the predicted data during the updates, and their detrimental effect is mitigated by shortening the time window as the variance rises, and then decreasing it as it falls (but within prior bounds). A set of numerical experiments is provided that demonstrates the viability of the procedure.

Lake regime shift from submerged macrophyte to phytoplankton affected phosphorus speciation in sediment and eutrophic state in Caohai Lake,Guizhou,China

To understand the effect of regime shift in Caohai Lake in Yunnan-Guizhou Plateau,SW China from submerged macrophyte dominance to phytoplankton dominance on the specification and distribution of phosphorus and on ecological and environmental states,changes in phosphorus specification in the sediments and water were studied.The form,composition,and distribution of phosphorus in sediment were sampled in July 2020(before regime shift)and July 2021(after regime shift)were analyzed.Results reveal that phosphorus content in sediment was lower than that those of Erhai Lake and Dianchi Lake,Yunnan,SW China,on the same plateau,and was lower than those of Taihu Lake,Chaohu Lake,and Poyang Lake in the middle-lower Changjiang(Yangtze)River Plain.Organic phosphorus(Or-P)was the main form(up to 60%),followed by inactive phosphorus(Ina-P),and the active phosphorus(Act-P),the least,which is opposite to those of Taihu Lake and Poyang Lake in the middle-lower Changjiang River Plain in the eastern China.Or-P content was high,indicating a high potential risk of phosphorous release.After the regime shift,the total phosphorus in sediment decreased from 0.87±0.13 to 0.70±0.13 g/kg.The proportion of Or-P and Act-P decreased from 68.23% to 65.32% and from 5.35% to 4.69%,respectively.In contrast,the proportion of Ina-P increased from 26.42% to 29.99%.The Moran’s I index revealed that the heterogeneity of the spatial distributions of the total phosphorus(S-TP)and Act-P in the sediments before regime shift was significant(P<0.1).However,the heterogeneity of the spatial distributions of S-TP and the various forms of phosphorus after regime shift was not significant(P>0.05).The regime shift aggravated the eutrophication of the lake,the trophic level index(TLI)increased from 48.42 to 54.49(P<0.01),and the previously mesotrophic lake became a mildly eutrophic lake.The results of this study revealed the impact of regime shift in the lake from submerged macrophyte dominance to phytoplankton dominance on the composition and spatial distribution of phosphorus in sediments and provided a basis for the restoration of eutrophicated and aquatic ecosystem degraded lakes.

Covariation of the Indonesian Throughflow and South China Sea Throughflow Associated with the 1976/77 Regime Shift

Changes in the Indonesian Throughflow （ITF） and the South China Sea throughflow-measured by the Luzon Strait Transport （LST）-associated with the 1976/77 regime shift are analyzed using the Island Rule theory and the Simple Ocean Data Assimilation dataset. Results show that LST increased but ITF transport decreased after 1975. Such changes were induced by variations in wind stress associated with the regime shift. The strengthening of the easterly wind anomaly east of the Luzon Strait played an important role in the increase of LST after 1975, while the westerly wind anomaly in the equatorial Pacific contributed significantly to the decrease in ITF transport after 1975; accounting for 53% of the change. After 1975, the Kuroshio Current strengthened and the Mindanao Current weakened in response to a decrease in the total transport of the North Equatorial Current. Both the North Equatorial Countercurrent and the South Equatorial Current weakened after 1975, and an anomalous cyclonic circulation in the western equatorial Pacific prevented the tropical Pacific water from entering the Indian Ocean directly.

Cloudiness regime shift during 1946—1992 recorded by coral in the South China Sea

By extracting and comparing long-term trend components of coral gray value, sea surface temperature, cloudiness, and net heat flux for the period of 1946-1992, the relationships among them are addressed. There exists a prominent regime shift in the cloudiness associated with the corresponding variabilities of sea surface temperature and net heat flux occurred in the mid-1960s, which can be successfully recorded by coral gray value, a climatic proxy. Long-term cloudiness variations in the South China Sea are completely opposite to the equatorial western Pacific in the past five decades, whereas they share a similar trend to that over Asian monsoon prevailing waters. The fact that the coral gray value is highly correlated to cloudiness provides a unique perspective on utilizing this coral to study cloudiness variations in the pre-instrumental period.

Precipitation Regime Shift Associated with the Pacific Decadal Oscillation in the Maritime Continent

Recent changes in precipitation regime in South-East Asia are a subject of ongoing discussion. In this article, for the first time, evidence of a precipitation regime shift during the mid-1970s in the Northern Hemispheric part of South-East Asia is demonstrated. The detection of regime shifts is made possible by using a new comprehensive dataset of daily precipitation records (South-East Asian Climate Assessment and Dataset) and applying a novel Bayesian approach for regime shift detection. After the detected regime shift event in the mid-1970s, significant changes in precipitation distribution occurred in the Northern Hemispheric regions—Indochina Peninsula and the Philippines. More specifically, dry days became up to 10% more frequent in some regions. However, no precipitation regime shift is detected in Southern Hemisphere regions—Java and Northern Australia, were the number of observed dry days increased gradually.

The regime shift in the 1960s and associated atmospheric change over the southern Indian Ocean

The change of sea surface temperature（SST） in the southern Indian Ocean（SIO） during the recent six decades has been analyzed based on oceanic reanalysis and model, as well as atmospheric data. The results show that a thermal regime shift in SIO during the 1960 s, which is not caught enough attentions, has been of equal magnitude to the linear warming since 1970. Empirical Orthogonal Function（EOF） analyses reveal that a thermal shift is combined with atmospheric changes such as the weakening of westerly during the period of 1960–1967. Inner dynamic connections can be defined that when the westerly winds turn weak, the anticyclonic wind circulation between westerly winds and the trade winds decreases, which further reduces the SST to a negative peak in this period. It is noted that the shifts in the 1960 s are also evident for Southern Hemisphere. For example, subtropical high and the entire westerly winds belt at high latitudes both change dramatically in the 1960 s. This large-scaled process maybe link to the change of southern annular mode（SAM）.

Climate-Related Variations in Autumn Cohort of Japanese Common Squid Todarodes pacificus

The Japanese common squid Todarodes pacificus is widely distributed in the Sea of Japan,the East China Sea and the Northwest Pacific Ocean,and is largely exploited by the Asia-Pacific countries.In this study,the impacts of regime shift of the Pacific Decadal Oscillation(PDO)during 1977-2015 on the autumn cohort of T.pacificus were assessed by examining the variations in vertical water temperatures at depths of 0 m(Temp_0 m),50 m(Temp_50 m)and 100 m(Temp_100 m)and the distribution pattern of preferred spawning ground(SSG).Spatial and temporal correlation analysis revealed that the PDO index exhibited a significantly negative relationship with vertical water temperatures at different depths and was negatively related to catch-per-unit-effort(CPUE)and stock biomass of T.pacificus.In addition,water temperatures in the specific three water layers were positively associated with CPUE.The PDO regime shift strongly affected the thermal condition and spawning ground of T.pacificus.Comparing to the negative PDO,the waters from the surface to the deep became cool in the positive PDO phase;correspondingly,the area of SSG largely contracted at different depths.Consequently,the CPUE and stock biomass of T.pacificus profoundly decreased.From the results it can be concluded that the PDO regime shift-driven changes in vertical thermal condition and SSG ranges can yield substantial impacts on T.pacificus abundance.

Extreme events as ecosystems drivers： Ecological consequences of anomalous Southern Hemisphere weather patterns during the 2001/2002 austral spring-summer

The frequency and severity of extreme events associated with global change are both forecast to increase with a concomitant increase expected in perturbations and disruptions of fundamental processes at ecosystem, community and population scales, with potentially catastrophic consequences. Extreme events should thus be viewed as ecosystem drivers, rather than as short term deviations from a perceived ＇norm＇. To illustrate this, we examined the impacts associated with the extraordinary weather pattern of the austral spring/summer of 2001/2002, and find that patterns of ocean-atmosphere interactions appear linked to a suite of extreme events in Antarctica and more widely across the Southern Hemisphere. In the Antarctic, the extreme events appear related to particular ecological impacts, including the substantial reduction in breeding success of Ade1ie penguins at sites in the Antarctic Peninsula as well as for Adelie penguin and snow petrel colonies in East Antarctica, and the creation of new benthic habitats associated with the disintegration of the Larsen B Ice Shelf. Other major impacts occurred in marine and terrestrial ecosystems at temperate and tropical latitudes. The suite of impacts demonstrates that ecological consequences of extreme events are manifested at fundamental levels in ecosystem processes and produce long-term, persistent effects relative to the short-term durations of the events. Changes in the rates of primary productivity, species mortality, community structure and inter-specific interactions, and changes in trophodynamics were observed as a consequence of the conditions during the 2001/2002 summer. Lasting potential consequences include reaching or exceeding tipping points, trophic cascades and regime shifts.

An Abrupt Increase in the Summer High Temperature Extreme Days across China in the mid-1990s

Based on the daily maximum surface air temperature records from an updated homogenized temperature dataset for 549 Chinese stations during 1960-2008,we reveal that there is an abrupt increase in the number of days with high temperature extremes （HTEs,an HTE day is defined when the maximum temperature exceeds the 95th percentile of the daily maximum temperature distributions） across China in the mid-1990s.Before this regime shift,the average number of HTE days is about 2.9 d yr 1 during the period from the 1970s to the early 1990s,while it rocketed to about 7.2 d yr 1 after the mid-1990s.We show that the significant HTE day increase occurs uniformly across the whole of China after the regime shift.The observational evidence raises the possibility that this change in HTE days is associated with global-scale warming as well as circulation adjustment.Possible causes for the abrupt change in the HTE days are discussed,and the circulation adjustment is suggested to play a crucial role in the increase in HTE days in this region.

Self-stabilizing maintenance process in plant communities of alpine meadows under different grazing intensities

Backgrounds:Grazing prohibition and reduced grazing intensity,as two important“vegetation close-to-nature recovery”methods,have been suggested as economical and effective technologies for enhancing forage production.However,numerous studies have found that the yield of forage could be increased by removing or reducing grazing in a short time in some steady stage of alpine Kobresia meadows,but not in others.To reveal the mechanism behind this phenomenon,we proposed a series of experiments.Methods:We monitored the plant and soil characteristics in the key steady stages of Kobresia meadows under reduced and prohibited grazing conditions in the same geographic and climatic environments in the northeastern Qinghai-Tibet Plateau for 6 years.We estimated the relationships between the plant community and soil nutrients and obtained the following results.Results:All measured variables were positively correlated with each other.The plant community structure had higher path coefficients to aboveground biomass,soil organic matter,total nitrogen,and nitrate nitrogen than to other factors.The plant community structure played an important role in response to grazing intensity.Different plant functional groups(PFGs)had different responses to grazing intensity,which led to plant community re-establishment or re-organization under different grazing intensities.Poaceae and Kobresia were more sensitive to grazing intensity than other PFGs,and the ratio of Kobresia biomass(including Kobresia humilis and Kobresia pygmaea)to the total biomass of Poaceae and Kobresia could be used as an indicator of regime shifts within plant communities.With Kobresia pygmaea as the dominant species,the prohibition of grazing was not an efficient approach to increase the yield in the steady stages because this treatment needed more time to recover aboveground biomass.If Poaceae is the dominant PFG,grazing should only be prohibited for 3 years in the steady stages because the aboveground biomass will decrease if grazing is prohibited for more than 3 years.Conclusions:Therefore,the different steady stages of alpine meadows require different recovery methods to increase recovery efficiency and speed.

A restoration-promoting integrated floating bed and its experimental performance in eutrophication remediation

Numerous studies on eutrophication remediation have mainly focused on purifying water first, then restoring submerged macrophytes. A restoration-promoting integrated floating bed （RPIFB） was designed to combine the processes of water purification and macrophyte restoration simultaneously. Two outdoor experiments were conducted to evaluate the ecological functions of the RP1FB. Trial 1 was conducted to compare the eutrophication purification among floating bed, gradual-submerging bed （GSB） and RPIFB technologies. The results illustrated that RPIFB has the best purification capacity. Removal efficiencies of RPIFB for TN, TP,NH4＋-N, NO3-N, CODcr, Chlorophyll-a and turbidity were 74.45%, 98.31%, 74.71%, 88.81%, 71.42%, 90.17% and 85%, respectively. In trial 2, influences of depth of GSB and photic area in RPIFB on biota were investigated. When the depth of GSB decreased and the photic area of RPIFB grew, the height of Potamogeton crispus Linn. increased, but the biomass of Canna indica Linn. was reduced. The mortalities of Misgurnus anguillicaudatus and Bellamya aeruginosa in each group were all less than 7%. All results indicated that when the RPIFB was embedded into the eutrophic water, the regime shift from phytoplankton-dominated to macrophyte-dominated state could be promoted. Thus, the RPIFB is a promising remediation technology for eutrophication and submerged macrophyte restoration.

Ecohydrological advances and applications in plant–water relations research:a review

Aims The field of ecohydrology is providing new theoretical frameworks and methodological approaches for understanding the complex interactions and feedbacks between vegetation and hydrologic flows at multiple scales.Here we review some of the major scientific and technological advances in ecohydrology as related to understanding the mechanisms by which plant–water relations influence water fluxes at ecosystem,watershed and landscape scales.Important Findings We identify several cross-cutting themes related to the role of plant–water relations in the ecohydrological literature,including the contrasting dynamics of water-limited and water-abundant ecosystems,transferring information about water fluxes across scales,understanding spatiotemporal heterogeneity and complexity,ecohydrological triggers associated with threshold behavior and shifts between alternative stable states and the need for long-term data sets at multiple scales.We then show how these themes are embedded within three key research areas where improved understanding of the linkages between plant–water relations and the hydrologic cycle have led to important advances in the field of ecohydrology:upscaling water fluxes from the leaf to the watershed and landscape,effects of plant–soil interactions on soil moisture dynamics and controls exerted by plant water use patterns and mechanisms on streamflow regime.In particular,we highlight several pressing environmental challenges facing society today where ecohydrology can contribute to the scientific knowledge for developing sound management and policy solutions.We conclude by identifying key challenges and opportunities for advancing contributions of plant–water relations research to ecohydrology in the future.

Patterns and trajectories of macrophyte change in East China’s shallow lakes over the past one century

Macrophytes play important roles in maintaining ecosystem health and stability of shallow lakes.Better understanding of their long-term dynamics has important theoretical and practical significance for both lake ecosystem restoration and eutrophication control.However,the knowledge about the historical status and changing patterns of macrophytes in China’s shallow lakes is still controversial and lacks systematic research.Here,we reviewed and synthesized the published records of submerged macrophytes from 14 typical shallow lakes in the eastern plain covering the past 100 years.The results suggest that submerged macrophytes have experienced three clear stages of change:rare period(the 1900s–the 1950s),growth period(the 1950s–the 1980s),and recession period(the 1980s–now).This finding is different from the traditional understanding that submerged macrophytes were abundant in the early 20th century and have been degrading since then.On this basis,we proposed the possible evolution pattern(less-more-less)of submerged macrophytes in the eastern plain lake region over the past 100 years,which provides new perspectives about the long-term evolution process of macrophytes in shallow lakes.Furthermore,we found that the decline of submerged macrophytes during the regime shift shows a gradual process at the interdecadal scale;this finding contradicts the classical regime shift theory that macrophytes decline sharply during the critical transition.This study has important theoretical value for the restoration of the eastern plain lakes in China from“turbid lake”to“clear lake”,especially for establishing the historical reference condition and restoration path of macrophytes.