Browse

You are looking at 1 - 10 of 121,545 items for

  • Refine by Access: All Content x
Clear All
Min-Gyu Seong
,
Seung-Ki Min
, and
Xuebin Zhang

Abstract

Recent studies showed that anthropogenic greenhouse gas (GHG) increase is a major driver of the observed increases in extreme temperatures at global and regional scales using an optimal fingerprint (OF) method, which is a frequentist approach based on linear regression. Here, a Bayesian decision method is employed, which finds the most probable cause of the observed changes by comparing likelihoods of different forcings in view of observations. To quantify individual forcing contributions, a new modified attribution procedure based on Bayesian decision is proposed, i.e., computing the likelihood ratio [Bayes factor (BF)] between different forcings. First, the contribution of anthropogenic forcing (ANT) is measured by BF between anthropogenic-plus-natural forcing (ALL) and natural forcing (NAT) using a threshold for “substantial” evidence (lnBF ≥ 1). Similarly, the NAT contribution is assessed by BF between ALL and ANT. Further, the GHG contribution to the detected ANT is quantified by BF between ANT and anthropogenic aerosols (AA), and the AA contribution is evaluated by BF between ANT and GHG. The devised Bayesian approach is applied to HadEX3 observations and CMIP6 multimodel simulations for extreme temperature intensities (warmest day/night and coldest day/night) for global, continental, and regional domains following previous studies. Bayesian attribution results indicate that the ANT signal is detected in many continental and subregions for all extremes indices. This is generally consistent with OF-based results but with less frequent detection, indicating that the Bayesian method is slightly stricter than the OF method. However, GHG contributions to the detected ANT are identified over more subregions in the Bayesian attribution, suggesting its potential advantage over conventional methods in case of low signal-to-noise ratio and high collinearity.

Restricted access
Bing Pu
,
Qinjian Jin
,
Paul Ginoux
, and
Yan Yu

Abstract

California is one of the nation’s top agriculture producers and is vulnerable to extreme events such as droughts and heat waves. Concurrent extreme events may further stress water and energy resources, exerting greater adverse socioeconomic, environmental, and health impacts than individual events. Here we examine the features of compound drought, heat wave, and dust events in California during spring and summer. From 2003 to 2020, 16 compound events are found in warm seasons, with a mean duration of ∼4 days. Compound events are characterized by enhanced surface temperature up to 4.5°C over northern and western California, reduced soil moisture and vegetation density, and an increase in dust optical depth (DOD) by 0.05–0.1 over central and southern California. The enhanced DOD is largely associated with severe vegetation dieback that favors dust emissions and southeasterly wind anomalies that support northward transport of dust from source regions in southern California. Surface fine dust and PM2.5 concentrations also increase by more than 0.5 and 5 μg m−3, respectively, during compound events associated with both enhanced dust emissions and a relatively stable atmosphere that traps pollutants. The development of the compound events is related to an anomalous high over the west coast in the lower to middle troposphere, which is a pattern favoring sinking motion and dry conditions in California. The anomalous high is embedded in a wave train that develops up to 7 days before the events. In comparison with heat wave extremes alone, compound events show significantly higher DOD and lower vegetation density associated with droughts.

Open access
Guang-Bing Yang
,
Changshui Xia
,
Xia Ju
,
Quanan Zheng
,
Yeli Yuan
,
Xue-Jun Xiong
, and
Fangli Qiao

Abstract

Previous in situ observations have suggested that bottom water temperature variations in shelf seas can drive significant ocean bottom heat flux (BHF) by heat conduction. The BHF-driven bottom water temperature variations, however, have been overlooked in ocean general circulation models. In this study, we established a sea-sediment fully coupled model through incorporating the BHF. The coupled model included a sediment temperature module/model, and the BHF was calculated based on the sediment heat content variations. Meanwhile, we applied temporally varying BHF in the calculation of the bottom water temperature, which further determined the sediment temperature. The two-way coupled BHF process presents a more complete and physically reasonable heat budget in the ocean model and a synchronously varying sediment temperature profile. The coupled model was validated using a one-dimensional test case, and then it was applied in a domain covering the Bohai and Yellow Seas. The results suggest that when a strong thermocline exists, the BHF can change the bottom water temperature by more than 1°C because the effects of the BHF are limited to within a shallow bottom layer. However, when the water column is well mixed, the BHF changes the temperature of the entire water column, and the heat transported across the bottom boundary is ventilated to the atmosphere. Thus, the BHF has less effect on water temperature and may directly affect air–sea heat flux. The sea-sediment interactions dampen the amplitude of the bottom water temperature variations, which we propose calling the seabed dampening ocean heat content variation mechanism (SDH).

Open access
Olivier Arzel
,
Thierry Huck
,
Antoine Hochet
, and
Alexandre Mussa

Abstract

Identifying the primary drivers of North Atlantic interdecadal climate variability is crucial for improving climatic prediction over the coming decades. Here the effect of thermal coupling on the leading energy sources of the interdecadal variability of the ocean–atmosphere system is examined by means of a stochastically forced idealized coupled model. The effect of coupling is quantified from a comparison of the buoyancy variance budget of coupled and uncoupled model configurations. The simplicity of the model allows us to contrast the effect of coupling between a supercritical regime where the deterministic ocean dynamics drive the variability and a damped regime where noise forcing is central to its existence. The results show that changes in surface buoyancy fluxes act as a sink of temperature variance in the supercritical regime, and only become a source in the strongly damped regime. By contrast, internal ocean dynamics associated with the interaction of transient buoyancy fluxes with mean buoyancy gradients always act as a source of interdecadal variability. In addition to the reduced thermal damping effect in coupled integrations, thermal coupling with the atmosphere is shown to significantly increase the role of internal ocean dynamics in the variability, particularly in the regime where interdecadal modes are damped. Only for oceanic background states in the strongly damped regime do changes in surface buoyancy fluxes play a leading role in the upper-ocean variability. A stochastically forced coupled box model is proposed that captures the basic effect of thermal coupling on atmospheric and oceanic energy sources of variability.

Significance Statement

The purpose of this study is to better understand the impact of ocean–atmosphere thermal coupling on the leading energy sources of Atlantic interdecadal variability. Increasing our understanding of the physical mechanisms driving climate variability at interdecadal time scales is important to improve climate prediction. We show that the effect of ocean–atmosphere thermal coupling, as measured by the atmospheric feedback on sea surface temperature anomalies, is to substantially increase the role of internal ocean dynamics in the low-frequency variability of the upper-ocean heat content and sea surface temperature. Atmospheric stochastic forcing only becomes the primary driver of the oceanic temperature variability in the large dissipative limit, when internal ocean modes are strongly damped.

Restricted access
Zhun Guo
,
Kalli Furtado
,
Tianjun Zhou
,
Vincent E. Larson
, and
Ling Zhang

Abstract

During the winter and subsequent spring of an El Niño year, the East Asian marginal sea (EAMS) exhibits positive sea surface temperature anomalies (SSTAs) and fewer low clouds, while the western North Pacific experiences negative SSTAs. In this study, we suggest that the positive SSTAs in EAMS are maintained by a positive low cloud–SST feedback. In neutral winters and springs, the EAMS is covered by low clouds, which have a cooling effect on surface temperatures. During an El Niño year, a western North Pacific anomalous anticyclone is established, and along its northwestern flank, there are favorable conditions for convergence of moisture and weaker surface latent heat flux over the EAMS. Once a positive SSTA has been established, a further reduction of turbulent mixing results in less low cloud and enhanced solar heating of the ocean mixed layer; this reinforces and maintains both the positive SSTA and the lack of low cloud via a positive feedback mechanism. The concurrent increase of low cloud–SST feedback and anticyclone circulation strengths is evident in the coupled-model simulations from phase 6 of the Coupled Model Intercomparison Project. Furthermore, sensitivity experiments, performed with the atmospheric components of Community Earth System Model (CESM2), reveal that a positive SSTA helps to maintain the western North Pacific anomalous anticyclone. Four pacemaker-coupled experiments by CESM2, with sea surface temperature in the equatorial Pacific restored to the observational anomalies plus the model climatology and altered low cloud feedback over EAMS, suggest that the low cloud–SST feedback results in more than the maintenance of a positive SSTA over the EAMS: the positive feedback is also a previously overlooked mechanism for the maintenance of the western North Pacific anomalous anticyclone.

Significance Statement

The East Asian marginal sea (EAMS) and western North Pacific are important areas that bridge El Niño and the climate of East Asia. Unlike the cold sea surface temperature anomaly (SSTA) over the western North Pacific during El Niño, the positive SSTA over EAMS, which is covered by winter low cloud, has received less attention. We suggest that a “low cloud–SST” feedback—namely, one in which decreasing low-level clouds allows more sunlight to strike the ocean surface and favors higher SST—maintains the positive SSTA over EAMS. We also configure a widely used atmospheric model with a set of preset SSTA patterns that mimic different climate patterns. Our experiments with different climate patterns and CMIP6 historical runs show that the low cloud–SST feedback (through the positive SSTA) is a possible supplementary mechanism for reinforcing the WNP anomalous anticyclone.

Restricted access
Daehyun Kang
,
Daehyun Kim
,
Stephanie Rushley
, and
Eric Maloney

Abstract

This study investigates why the major convective envelope of the Madden–Julian oscillation (MJO) detours to the south of the Maritime Continent (MC) only during boreal winter [December–March (DJFM)]. To examine processes affecting this MJO detour, the MJO-related variance of precipitation and column-integrated moisture anomalies in DJFM are compared with those in the seasons before [October–November (ON)] and after [April–May (AM)]. While MJO precipitation variance is much higher in the southern MC (SMC) during DJFM than in other seasons, the MJO moisture variance is comparable among the seasons, implying that the seasonal locking of the MJO’s southward detour cannot be explained by the magnitude of moisture anomalies alone. The higher precipitation variance in the SMC region is partly explained by the much higher moisture sensitivity of precipitation in DJFM than in other seasons, resulting in a more efficient conversion of anomalous moisture to anomalous precipitation. DJFM is also distinguishable from the other seasons by stronger positive wind–evaporation feedback onto MJO precipitation anomalies due to the background westerly wind in the lower troposphere. It is found that the seasonal cycle of moisture–precipitation coupling and wind–evaporation feedback in the SMC region closely follows that of the Australian monsoon, which is active exclusively in DJFM. Our results suggest that the MJO’s southward detour in the MC is seasonally locked because it occurs preferentially when the Australian monsoon system produces a background state that is favorable for MJO development in the SMC.

Open access
John R. Christy

Abstract

Time series of snowfall observations from over 500 stations in Oregon (OR) and Washington (WA) were generated for subregions of these states. Data problems encountered were as follows: 1) monthly totals in printed reports prior to 1940 that were not in the digital archive, 2) archived data listed as “missing” that were available, 3) digitized reports after 2010 eliminated good data, and 4) “zero” totals incorrectly entered in the official archive rather than “missing,” especially after 1980. Though addressing these was done, there is reduced confidence that some regional time series are representative of true long-term trends, especially for regions with few systematically reporting stations. For most regions characterized by consistent monitoring and with the most robust statistical reproducibility, we find no statistically significant trends in their periods of record (up to 131 years) for November–April seasonal totals through April 2020. This result includes the main snowfall regions of the Cascade Range. However, snowfall in some lower-elevation areas of OR and WA appear to have experienced declining trends, consistent with an increase in northeastern Pacific Ocean temperatures. Finally, previously constructed time series through April 2011 for regions in California are updated through April 2020 to include the recent, exceptionally low seasonal totals on the western slopes of the Sierra Nevada. This update indicates 2014/15 was the record lowest, 2013/14 was the 5th lowest, and 2012/13 was the 14th lowest of 142 years. Even so, the 1879–2020 linear trend in this key watershed region, though −2.6% decade−1, was not significantly different from zero due to high interannual variability and reconstruction uncertainty.

Restricted access
Zhangqi Dai
,
Bin Wang
,
Ling Zhu
,
Jian Liu
,
Weiyi Sun
,
Longhui Li
,
Guonian Lü
,
Liang Ning
,
Mi Yan
, and
Kefan Chen

Abstract

Atlantic multidecadal variability (AMV) is a cornerstone for decadal prediction and profoundly influences regional and global climate variability, yet its fundamental drivers remain an issue for debate. Studies suggest that external forcing may have affected AMV during the Little Ice Age (AD 1400–1860). However, the detailed mechanism remains elusive, and the AMV’s centennial to millennial variations over the past 2000 years have not yet been explored. We first show that proxy-data reconstructions and paleo-data assimilations suggest a significant 60-yr AMV during AD 1250–1860 but not during AD 1–1249. We then conducted a suite of experiments with the Community Earth System Model (CESM) to unravel the causes of the changing AMV property. The simulation results under all external forcings match the reconstructions reasonably well. We find that the significant 60-yr AMV during 1250–1860 arises predominantly from the volcano forcing variability. During the period 1–1249, the average volcanic eruption intensity is about half of the 1250–1860 intensity, and a 20–40-yr internal variability dominates the AMV. The volcanic radiative forcing during 1250–1860 amplifies AMV and shifts the internal variability peak from 20–40 years to 60 years. The volcano forcing prolongs AMV periodicity by sustaining Arctic cooling, delaying subpolar sea ice melting and atmospheric feedback to reduce surface evaporation. These slow-response processes over the subpolar North Atlantic results in a persisting reduction of sea surface salinity, weakening the Atlantic overturning circulation, and warm water transport from the subtropical North Atlantic. The results reveal the cause of the nonstationary AMV over the past two millennia and shed light on the AMV’s response to external forcing.

Significance Statement

AMV plays an important role in the regional and global climate variability. The purpose of this study is to better understand the secular change of AMV during the past 2000 years and its response to the external forcing. Proxy data and model simulation consistently show a significant 60-yr AMV during AD 1250–1860 that is absent during AD 1–1249. Active volcanic eruptions during 1250–1860 amplify the AMV and shift its intrinsic 20–40-yr to a prominent 60-yr variance peak. Volcanoes prolong AMV periodicity by sustaining Arctic cooling, delaying subpolar sea ice melting, reducing evaporation, and increasing surface salinity. These results help us better understand nonstationary AMV and highlight the role of external forcing over the past two millennia.

Open access
Luke Grant
,
Lukas Gudmundsson
,
Edouard L. Davin
,
David M. Lawrence
,
Nicolas Vuichard
,
Eddy Robertson
,
Roland Séférian
,
Aurélien Ribes
,
Annette L. Hirsch
, and
Wim Thiery

Abstract

Land-use and land-cover changes (land use) alter climates biogeophysically by affecting surface fluxes of energy and water. Yet, near-surface temperature responses to land use across observational versus model-based studies and spatial-temporal scales can be inconsistent. Here we assess the prevalence of the historical land use signal of daily maximum temperatures averaged over the warmest month of the year (tLU) using regularized optimal fingerprinting for detection and attribution. We use observations from the Climatic Research Unit and Berkeley Earth alongside historical simulations with and without land use from the Coupled Model Intercomparison Project Phase 6 to reconstruct an experiment representing the effects of land use on climate, LU. To assess the signal of land use at spatially resolved continental and global scales, we aggregate all input data across reference regions and continents, respectively. At both scales, land use does not comprise a significantly detectable set of forcings for two of four Earth system models and their multi-model mean. Furthermore, using a principal component analysis, we find that tLU is mostly composed of the non-local effects of land use rather than its local effects. These findings show that, at scales relevant for climate attribution, uncertainties in Earth system model representations of land use are too high relative to the effects of internal variability to confidently assess land use.

Restricted access
Alejandro Cáceres-Euse
,
Anne Molcard
,
Natacha Bourg
,
Dylan Dumas
,
Charles-Antoine Guérin
, and
Giovanni Besio

Abstract

To assess the contribution of wind drag and Stokes drift on the near-surface circulation, a methodology to isolate the geostrophic surface current from high-frequency radar data is developed. The methodology performs a joint analysis utilizing wind field and in situ surface currents along with an unsupervised neuronal network. The isolation method seems robust in the light of comparisons with satellite altimeter data, presenting a similar time variability and providing more spatial detail of the currents in the coastal region. Results show that the wind-induced current is around 2.1% the wind speed and deflected from the wind direction in the range [18°, 23°], whereas classical literature suggests higher values. The wave-induced currents can represent more than 13% of the ageostrophic current component as function of the wind speed, suggesting that the Stokes drift needs to be analyzed as an independent term when studying surface sea currents in the coastal zones. The methodology and results presented here could be extended worldwide, as complementary information to improve satellite-derived surface currents in the coastal regions by including the local physical processes recorded by high-frequency radar systems. The assessment of the wave and wind-induced currents have important applications on Lagrangian transport studies.

Restricted access