Changes in Arctic sea ice have been proposed to affect midlatitude winter atmospheric circulation, often based on observed coincident variability. However, causality of this covariability remains ...unclear. Here, we address this issue using atmospheric model experiments prescribed with observed sea surface temperature variations and either constant or time‐varying sea ice variability. We show that the observed relationship between late‐autumn Barents‐Kara sea ice and the winter North Atlantic Oscillation can be reproduced by simulated atmospheric internal variability but is not simulated as a forced response to sea ice. Observations and models suggest reduced sea ice is linked to a weaker Aleutian Low. We show that simulated Aleutian Low variability is correlated with observed sea ice variability even in simulations with fixed sea ice, implying that this relationship is not incidental. Instead, we suggest that covariability between sea ice and the Aleutian Low originates from tropical sea surface temperature and rainfall variations and their teleconnections to the extratropics.
Plain Language Summary
Recent dramatic changes in Arctic sea ice due to climate change have been linked to changes in weather patterns across the Northern Hemisphere. Many studies have proposed such links, but correlation does not necessarily imply causality. Here, we explore the causality of this link using atmospheric models run with observed sea surface temperature variations and either constant or time‐varying sea ice. We find that changes in weather patterns over the Atlantic that are correlated with sea ice variations are not caused by changes in sea ice. Instead, the correlation appears to be an incidental occurrence due to internal atmospheric variability. Additionally, we find that changes in weather patterns over the North Pacific, which are also correlated with sea ice variations, are reproduced in model experiments with no knowledge of these sea ice variations. In this case, the correlation appears to arise due to a third factor: rainfall variations over the tropical Pacific Ocean, which can affect midlatitude weather irrespective of sea ice changes.
Key Points
Atmospheric model experiments aid causal interpretation of links between Arctic sea ice and midlatitude winter atmospheric circulation
Observed links between autumn Barents‐Kara sea ice and the winter North Atlantic Oscillation is largely explained by internal variability
Observed links between autumn Barents‐Kara sea ice and the winter Aleutian Low appears to originate from tropical SST and rainfall changes
The Greenland Ice Sheet has been a major contributor to global sea-level rise in recent decades
, and it is expected to continue to be so
. Although increases in glacier flow
and surface melting
have ...been driven by oceanic
and atmospheric
warming, the magnitude and trajectory of the ice sheet's mass imbalance remain uncertain. Here we compare and combine 26 individual satellite measurements of changes in the ice sheet's volume, flow and gravitational potential to produce a reconciled estimate of its mass balance. The ice sheet was close to a state of balance in the 1990s, but annual losses have risen since then, peaking at 345 ± 66 billion tonnes per year in 2011. In all, Greenland lost 3,902 ± 342 billion tonnes of ice between 1992 and 2018, causing the mean sea level to rise by 10.8 ± 0.9 millimetres. Using three regional climate models, we show that the reduced surface mass balance has driven 1,964 ± 565 billion tonnes (50.3 per cent) of the ice loss owing to increased meltwater runoff. The remaining 1,938 ± 541 billion tonnes (49.7 per cent) of ice loss was due to increased glacier dynamical imbalance, which rose from 46 ± 37 billion tonnes per year in the 1990s to 87 ± 25 billion tonnes per year since then. The total rate of ice loss slowed to 222 ± 30 billion tonnes per year between 2013 and 2017, on average, as atmospheric circulation favoured cooler conditions
and ocean temperatures fell at the terminus of Jakobshavn Isbræ
. Cumulative ice losses from Greenland as a whole have been close to the rates predicted by the Intergovernmental Panel on Climate Change for their high-end climate warming scenario
, which forecast an additional 70 to 130 millimetres of global sea-level rise by 2100 compared with their central estimate.
Recent mass loss from ice sheets and ice shelves is now persistent and prolonged enough that it impacts downstream oceanographic conditions. To demonstrate this, we use an ensemble of coupled ...GISS‐E2.1‐G simulations forced with historical estimates of anomalous freshwater, in addition to other climate forcings, from 1990 through 2019. There are detectable differences in zonal‐mean sea surface temperatures (SST) and sea ice in the Southern Ocean, and in regional sea level around Antarctica and in the western North Atlantic. These impacts mostly improve the model's representation of historical changes, including reversing the forced trends in Antarctic sea ice. The changes in SST may have implications for estimates of the SST pattern effect on climate sensitivity and for cloud feedbacks. We conclude that the changes are sufficiently large that model groups should strive to include more accurate estimates of these drivers in all‐forcing historical simulations in future coupled model intercomparisons.
Plain Language Summary
Simulations of recent historical periods are a key test of climate model reliability and skill. These model simulations require an accounting of all the drivers of climate change. We show that the impact of historical changes in freshwater fluxes from ice sheets and ice shelves on the ocean (through changes in salinity and stratification) are detectable in sea surface temperature and sea ice trends, and help improve the match between the modeled climate changes and observations. We recommend that more accurate estimates of these drivers be included in all climate simulations that do not explicitly model ice sheets and ice shelves.
Key Points
The response to anomalous meltwater from ice sheets and shelves is large enough for it to be a forcing in historical climate simulations
When the GISS model includes these drivers, Southern Ocean SST and sea ice trends better match observations
Steric and dynamic impacts on regional sea level in parts of the North Atlantic and coastal Antarctica are significant
In this study, Assaf Yasur-Landau examines the early history of the biblical Philistines who were among the 'Sea Peoples' who migrated from the Aegean area to the Levant during the early twelfth ...century BC. Creating an archaeological narrative of the migration of the Philistines, he combines an innovative theoretical framework on the archaeology of migration with new data from excavations in Greece, Turkey, Cyprus, Syria, Lebanon, and Israel and thereby reconstructs the social history of the Aegean migration to the southern Levant. The author follows the story of the migrants from the conditions that caused the Philistines to leave their Aegean homes, to their movement eastward along the sea and land routes, to their formation of a migrant society in Philistia and their interaction with local populations in the Levant. Based on the most up-to-date evidence, this book offers a new and fresh understanding of the arrival of the Philistines in the Levant.
The Bering Strait oceanic heat transport influences seasonal sea ice retreat and advance in the Chukchi Sea. Monitored since 1990, it depends on water temperature and factors controlling the volume ...transport, assumed to be local winds in the strait and an oceanic pressure difference between the Pacific and Arctic oceans (the “pressure head”). Recent work suggests that variability in the pressure head, especially during summer, relates to the strength of the zonal wind in the East Siberian Sea that raises or drops sea surface height in this area via Ekman transport. We confirm that westward winds in the East Siberian Sea relate to a broader central Arctic pattern of high sea level pressure and note that anticyclonic winds over the central Arctic Ocean also favor low September sea ice extent for the Arctic as a whole by promoting ice convergence and positive temperature anomalies. Month‐to‐month persistence in the volume transport and atmospheric circulation patterns is low, but the period 1980–2017 had a significant summertime (June–August) trend toward higher sea level pressure over the central Arctic Ocean, favoring increased transports. Some recent large heat transports are associated with high water temperatures, consistent with persistence of open water in the Chukchi Sea into winter and early ice retreat in spring. The highest heat transport recorded, October 2016, resulted from high water temperatures and ideal wind conditions yielding a record‐high volume transport. November and December 2005, the only months with southward volume (and thus heat) transports, were associated with southward winds in the strait.
Plain Language Summary
The Chukchi Sea is a focus of resource exploration, and all vessels transiting the Arctic Ocean must pass through it. Sea ice conditions that affect operations in this area are influenced by month‐to‐month variations in how much oceanic heat is brought into the region from the Pacific Ocean via the Bering Strait, the narrow (~85 km wide) channel separating Russia (Chukotka) from the United States (Alaska). The oceanic heat transport depends on both the temperature of the water and volume of water that is transported. The volume transport is in part controlled by surface winds in the strait and along the East Siberian Sea coast. We show that the latter are part of a larger pattern of atmospheric circulation influencing September sea ice extent for the Arctic Ocean as a whole. We use case studies for individual months to document the varying roles on the oceanic heat transport played by water temperature, winds in the East Siberian Sea, and local winds in the Bering Strait.
Key Points
Central Arctic anticyclonic winds favor low September sea ice extent by promoting convergence, warm conditions, and a large oceanic heat flux through Bering Strait
Summertime trends toward higher Arctic Ocean sea level pressure favored increased oceanic heat transports through the Bering Strait from 1980 to 2017
Recent large Bering Strait heat transports coincide with high water temperatures, consistent with early ice retreat and late ice advance in the Chukchi Sea
Despite having some of the world's most densely populated and vulnerable coastlines, Indian Ocean sea level variability over the past century is poorly understood relative to other ocean basins ...primarily, due to the short and sparse observational records. In an attempt to overcome the limitations imposed by the lack of adequate observations, we have produced a 20th century Indian Ocean sea level reconstruction product using a new multivariate reconstruction technique. This technique uses sea level pressure and sea surface temperature in addition to sea level data to help constrain basin‐wide sea level variability by (1) the removal of large spurious signals caused as a result of insufficient tide gauge data specifically during the first half of the 20th century and (2) through its information on large‐scale climate modes such as El Niño‐Southern Oscillation and Indian Ocean Dipole. Basis functions generated by Cyclostationary Empirical Orthogonal Functions are used for the reconstruction. This new multivariate technique provides improved regional sea level variability estimates along with a longer record length in comparison to existing globally reconstructed sea level data. The biggest advantage of using this multivariate reconstruction technique lies in its ability to reconstruct Indian Ocean sea level for the first half of the 20th century, providing a long sea level record for the study of Indian Ocean internal climate variability. This will enable future studies to help improve the understanding of how sea level trends and variability can be modulated by internal climate variability in the Indian Ocean.
Plain Language Summary
Densely populated coastal regions of the countries surrounding the Indian Ocean are becoming increasingly vulnerable to the effects of sea level rise. Accurate predictions of future sea level change will help minimize the social and economic damage posed to these coastal communities. The key to predicting future sea level lies in how well we understand past and present sea level change. However, due to the presence of only a few short and scattered sea level observational records, change in sea level over the Indian Ocean is not well understood. In this paper we have created a new sea level data set by developing new methodology which is particularly well suited for filling data gaps left by the observational record in the Indian Ocean. Additionally, this new technique allows us to reliably extend the observational record to now span the entire 20th century and has been shown to have improved sea level estimates when compared to an older existing sea level data product. Using this new longer sea level data set, we hope that future studies will help shed more light and improve the understanding of Indian Ocean sea level change over long time scales.
Key Points
We have developed a new multivariate sea level reconstruction technique designed to overcome sparse spatio‐temporal tide gauge sampling
Created a new sea level data set with improved 20th century interannual to decadal sea level variability estimates for the Indian Ocean
This sea level reconstruction is used to give context to satellite measured sea level in the Indian Ocean
Drawing upon Ottoman, Russian, and Bulgarian archival sources, this book explores the nexus between the environment, epidemic disease, human mobility, and the centralizing initiatives of the Ottoman ...and Russian states in the late 18th and early 19th centuries.
As part of a broader discussion on Ottoman-Russian diplomacy, this book re-conceptualizes Ottoman-Russian relations in the Black Sea region in the 18th and 19th centuries. In response to significant increases in human mobility and the spread of epidemic diseases, Ottoman and Russian officials - at the imperial, provincial, and local levels - communicated about and coordinated their efforts to manage migratory movements and check the spread of disease in the Black Sea region. By focusing on the settlement of migrants and refugees along the peripheries of the Ottoman and Russian Empires and by foregrounding the role of local and municipal-level state authorities in the management of migration, Migration and Disease in the Black Sea Region contributes to the developing field of provincial studies in Ottoman and Russian history. This is an important book for anyone interested in comparative imperial history, migration, diaspora formation and the spread of epidemic diseases.
Summertime Greenland blocking (GB) can drive melting of the Greenland ice sheet, which has global implications. A strongly increasing trend in GB in the early twenty‐first century was observed but is ...missing in climate model simulations. Here, we analyze the temporal evolution of GB in nearly 500 members from the CMIP6 archive. The recent period of increased GB is not present in the members considered. The maximum 10‐year trend in GB in the reanalysis, associated with the recent increase, lies almost outside the distributions of trends for any 10‐year period in the climate models. GB is shown to be partly driven by the sea surface temperatures and/or sea ice concentrations, as well as by anthropogenic aerosols. Further work is required to understand why climate models cannot represent a period of increased GB, and appear to underestimate its decadal variability, and what implications this may have.
Plain Language Summary
An increasing trend in summertime atmospheric blocking over Greenland was observed during the early twenty‐first century. However, this trend is not reproduced in climate models. This may have important implication for climate change projections, as summertime Greenland blocking drives the melting of its ice sheet which is a major contributing factor to global sea level rise. Here, recent trends in Greenland blocking are assessed in nearly 500 ensemble members from a large archive of state‐of‐the‐art climate models. We find that a recent increasing trend like that observed is absent in all of the ensemble members, and a trend of such magnitude is very unlikely to be simulated in them, which suggests a deficiency in the climate model simulation of Greenland blocking. The model simulations do however suggest that Greenland blocking is partly forced by sea surface temperatures/sea ice concentrations and/or anthropogenic aerosols, but the response of the models to these forcings may be too weak. These results provide new understanding on drivers of Greenland blocking in climate models and offer avenues for model development designed to improve simulations of Greenland climate.
Key Points
The observed rapid increase in summertime Greenland blocking during the first decade of the twenty‐first century has not continued
A period of increased summer Greenland blocking of similar magnitude to observed is rarely reproduced in a large ensemble of climate models
Decadal variability in Greenland blocking in climate models is partly driven by SST/sea ice and/or anthropogenic aerosols
The Southern Ocean is responsible for the majority of the global oceanic heat uptake that contributes to global sea level rise. At the same time, ocean temperatures do not change at the same rate in ...all regions and sea level variability is also affected by changes in salinity. This study investigates 10 years of steric height variability (2008–2017) in the Southern Ocean (30°S to 70°S) by analyzing temperature and salinity variations obtained from the GLORYS‐031 model provided by the European Copernicus Marine Environment Monitoring Service. The thermohaline variability is decomposed into thermohaline modes using a functional Principal Component Analysis. Thermohaline modes provide a natural basis to decompose the joint temperature‐salinity vertical profiles into a sum of vertical modes weighted by their respective principal components that can be related to steric height. Interannual steric height trends are found to differ significantly between subtropical and subpolar regions, simultaneously with a shift from a thermohaline stratification dominated by the first “thermal” mode in the north to the second ’saline’ mode in the South. The Polar Front appears as a natural boundary between the two regions, where steric height variations are minimized. Despite higher melt rates and atmospheric temperatures, steric height in Antarctic waters (0–2,000 m) has dropped since 2008 due to higher salt content in the surface and upper intermediate layer and partially colder waters, while subtropical waters farther north have mostly risen due to increased heat storage.
Plain Language Summary
Sea level variations on longer timescales mainly arise from mass changes or the thermo‐ and halosteric effects of temperature and salinity on water density. Recent variability in steric height in the Southern Ocean was investigated from 2008 to 2017 by analyzing potential temperature and salinity variations obtained from a global ocean reanalysis. The work was performed using a functional approach to a standard Principal Component Analysis that was applied on vertical temperature and salinity profiles (2,000 m). The resulting thermohaline modes contain information about the general temperature and salinity structure and their variations can be attributed to steric height changes. The results have shown that Antarctic waters above 2,000 m have dropped since 2008 due to higher salt content and colder waters, while subtropical waters farther north have mostly risen due to increased heat storage. Those spatial differences in recent steric height trends also display on the total sea level rise (SLR) observed from satellite data, which shows a significantly higher rate of SLR in subtropical waters compared to higher latitudes of the Southern Ocean.
Key Points
Variability of vertical thermohaline modes induces regional patterns in steric height trends in the Southern Ocean
Steric height has risen north of the Polar Front and fallen south of it due to both thermo‐ and halosteric changes
The halosteric effect in the Southern Ocean is nowhere negligible and significantly reduces the rate of sea level rise around Antarctica
During 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.
Plain Language Summary
During the altimeter era, the global mean sea level rose continuously, while the regional sea level trends are not uniform. The atmospheric and oceanic processes associated with climate modes played an important role. The Walker Circulation anomalies, connected closely with the “atmospheric bridge” of the Pacific Decadal Oscillation (PDO), forced a reverse sea level trend in the western and eastern tropical Pacific via propagations of Kelvin/Rossby waves. The sea level signals propagated into the South China Sea (SCS) via the Sibutu Passage and Mindoro Strait, which dominated the sea level variations in the central SCS. Unlike in the tropical Pacific, the decadal variability of sea level in the SCS was associated with PDO through “oceanic bridge.” The results in this study shed light on our understanding on the dynamics of sea level and upper‐layer circulation changes in the SCS and adjacent regions.
Key Points
Sea level trends in the South China Sea (SCS) and western tropical Pacific experienced regime shift during the altimeter era
Remote forcing originated from tropical Pacific dominated the decadal variability of sea level in the SCS
Tropical Pacific conveys its impact on the sea level in the central SCS mainly via the Sibutu Passage and Mindoro Strait
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