Warm North Atlantic Research Articles

Global Monsoon Responses to Decadal Sea Surface Temperature Variations during the Twentieth Century: Evidence from AGCM Simulations

AbstractMultidecadal variations in the global land monsoon were observed during the twentieth century, with an overall increasing trend from 1901 to 1955 that was followed by a decreasing trend up to 1990, but the mechanisms governing the above changes remain inconclusive. Based on the outputs of two atmospheric general circulation models (AGCMs) forced by historical sea surface temperature (SST) covering the twentieth century, supplemented with AGCM simulations forced by idealized SST anomalies representing different conditions of the North Atlantic and tropical Pacific, evidence shows that the observed changes can be partly reproduced, particularly over the Northern Hemisphere summer monsoon (NHSM) domain, demonstrating the modulation of decadal SST changes on the long-term variations in monsoon precipitation. Moisture budget analysis is performed to understand the interdecadal changes in monsoon precipitation, and the dynamic term associated with atmospheric circulation changes is found to be prominent, while the contribution of the thermodynamic term associated with humidity changes can lead to coincident wetting over the NHSM domain. The increase (decrease) in NHSM land precipitation during 1901–55 (1956–90) is associated with the strengthening (weakening) of NHSM circulation and Walker circulation. The multidecadal scale changes in atmospheric circulation are driven by SST anomalies over the North Atlantic and the Pacific. A warmer North Atlantic together with a colder eastern tropical Pacific and a warmer western subtropical Pacific can lead to a strengthened meridional gradient in mid-to-upper-tropospheric thickness and strengthened trade winds, which transport more water vapor into monsoon regions, leading to an increase in monsoon precipitation.

Decadal–Multidecadal Variations of Asian Summer Rainfall from the Little Ice Age to the Present

Abstract Features of decadal–multidecadal variations of the Asian summer rainfall are revealed by analysis of the reconstructed Asian summer precipitation (RAP) dataset from 1470 to 2013. Significant low-frequency periodicities of the all-Asian rainfall (AAR) index (AARI) are found on decadal (8–10 yr), quasi-bidecadal (22 yr), and multidecadal (50–54 yr) time scales, as well as centennial time scales. The decadal and multidecadal peaks are mainly from the “monsoon Asia” area and the Maritime Continent, while the 22-yr peak is from the “arid Asia” area. A remarkable change of leading frequency from multidecadal to decadal after AD 1700 is detected across the entire Asian landmass. The leading empirical orthogonal function (EOF) modes on the decadal and multidecadal time scales exhibit a uniform structure similar to that on the interannual time scale, suggesting a cross-time-scale, in-phase variation of the rainfall across continental Asia and the Maritime Continent. Enhanced AAR on a decadal time scale is found associated with the mega-La Niña sea surface temperature (SST) pattern over the Pacific. The AARI–mega-ENSO (El Niño–Southern Oscillation) relationship is persistently significant except from 1820 to around 1900. Enhanced decadal AAR is also found to be associated with extratropical North Atlantic warming. The AARI–AMO (Atlantic multidecadal oscillation) relationship, however, is nonstationary. On the multidecadal time scale, the AAR is significantly related to the AMO. Mechanisms associated with the decadal–multidecadal variability of AAR are also discussed.

An Atlantic-driven rapid circulation change in the North Pacific Ocean during the late 1990s

Interbasin interactions have been increasingly emphasized in recent years due to their roles in shaping climate trends and the global warming hiatus in the northern hemisphere. The profound influence from the North Atlantic on the Tropical Pacific has been a primary focus. In this study, we conducted observational analyses and numerical modeling experiments to show that the North Atlantic has also strongly influenced the Extratropical North Pacific. A rapid and synchronous change in the atmospheric and oceanic circulations was observed in the North Pacific during the late 1990s. The change was driven by the transbasin influence from the Atlantic Ocean. During the positive phase of the Atlantic Multidecadal Oscillation (AMO) since the 1990s, the anomalously warm North Atlantic triggers a series of zonally symmetric and asymmetric transbasin teleconnections involving the Inter-tropical Convergence Zone (ITCZ), Walker and Hadley circulations, and Rossby wave propagation that lead to a decrease in wind stress curls over the Pacific subtropics, resulting in an abrupt weakening in the North Pacific subtropical gyre (NPSG) and the Kuroshio Current.

Geochemical Signatures of Paleoclimate Changes in the Sediment Cores from the Gloria and Snorri Drifts (Northwest Atlantic) over the Holocene-Mid Pleistocene

A multiproxy study of the sediment cores taken from the Snorri Drift, formed under the influence of the Iceland–Scotland bottom contour current, and from the Gloria Drift, located southward Greenland at the boundary of Irminger and Labrador Seas, was performed. This area undergoes a variable mixing of polar waters with the warm North Atlantic current, whose intensity and direction seemed to change dramatically with the alteration of warming and cooling periods during the six marine isotope stages MIS 1-6. The relative age of this core does not exceed 190,000 cal yr BP; the average sedimentation rate was 1.94 and 2.45 cm/kyr in the Gloria and Snorri Drifts core respectively. In both the cores, the sediment records showed the downcore co-variation of ice-rafted debris (IRD); and terrigenous elements, such as Si, Al, Ti, Cr, and Zr, were revealed; their values were clearly higher in the glacial periods (MIS 2, 4, and 6) compared to interglacials (MIS 1, 3, and 5). The downcore rhythmic distributions of these elements, as well as Al/Si, Ti/Al, Fe/Al ratios exhibit an opposite trend with that of δ18O values, biogenic components (CaCO3, BioSiO2), and Si/Fe and Mn/Fe ratios.

Possible impact of North Atlantic warming on the decadal change in the dominant modes of winter Eurasian snow water equivalent during 1979–2015

An east–west dipole mode of winter Eurasian snow water equivalent (SWE) is found during the period of 1979–2015. It accounts for about 23.4% of the total variance, and displayed a significant decadal change in the early-2000s. The basin warming footprint of the North Atlantic likely exerted an influence on this decadal change, and the observation-based evidence is reproduced by numerical experiments using the Community Atmosphere Model (CAM3.1). A basin-wide warming of North Atlantic sea surface temperature induced atmospheric anomalies by exciting a stationary Rossby wave train, which prorogates from the subtropical North Atlantic to the mid-to-high latitudes of the Eurasian continent. Along with the Rossby wave train, an enhanced upper-level ridge occurs over the Ural Mountain, and a deepened upper-level trough appears over the eastern Siberian Plateau, which promotes heavy snowfall over the eastern Siberian Plateau and light snowfall to its west. Thus, it is plausible that the North Atlantic warming plays a role in exciting the Rossby wave train to modulate the decadal change in the east–west dipole SWE mode of the extratropical Eurasian continent. The possible moisture transport paths associated with the decadal change in the east–west dipole SWE mode are also discussed.

Lichens and vascular plants in Duvefjorden area on Nordaustlandet, Svalbard

Floristic check-lists were compiled for the first time for Duvefjorden Bay on Nordaustlandet, Svalbard, based on field work in July 2012 and on data from literature and herbaria. The check-lists include 172 species of lichens and 51 species of vascular plants. Several species rare in Svalbard and in the Arctic were discovered: Candelariella borealis was new to Svalbard. 51 lichen species were newly recorded on Nordaustlandet and 131 lichen species were observed in the Duvefjorden area for the first time. Among lichen species rare in Svalbard and in the Arctic the following can be mentioned: Caloplaca magni-filii, C. nivalis, Lecidea silacea, Phaeophyscia nigricans, Polyblastia gothica, Protothelenella sphinctrinoidella, Rinodina conradii, Stenia geophana, and Tetramelas pulverulentus. Two species of vascular plants, Saxifraga svalbardensis and S. hyperborea, were found new to the Duvefjorden area. The investigated flora is represented mostly by species widespread in Svalbard and in the Arctic. Although Duvefjorden area is situated in the northernmost part of Svalbard, its flora is characterized by relatively high diversity of vascular plants and lichens. Apparently this is due to the influence of the warm North Atlantic Current.

Impacts of the North Atlantic Warming Hole in Future Climate Projections: Mean Atmospheric Circulation and the North Atlantic Jet

AbstractIn future climate simulations there is a pronounced region of reduced warming in the subpolar gyre of the North Atlantic Ocean known as the North Atlantic warming hole (NAWH). This study investigates the impact of the North Atlantic warming hole on atmospheric circulation and midlatitude jets within the Community Earth System Model (CESM). A series of large-ensemble atmospheric model experiments with prescribed sea surface temperature (SST) and sea ice are conducted, in which the warming hole is either filled or deepened. Two mechanisms through which the NAWH impacts the atmosphere are identified: a linear response characterized by a shallow atmospheric cooling and increase in sea level pressure shifted slightly downstream of the SST changes, and a transient eddy forced response whereby the enhanced SST gradient produced by the NAWH leads to increased transient eddy activity that propagates vertically and enhances the midlatitude jet. The relative contributions of these two mechanisms and the details of the response are strongly dependent on the season, time period, and warming hole strength. Our results indicate that the NAWH plays an important role in midlatitude atmospheric circulation changes in CESM future climate simulations.

Contrasting Responses of Midlatitude Jets to the North Pacific and North Atlantic Warming

AbstractMidlatitude atmospheric circulation is projected to shift poleward, yet the Northern Hemisphere jet shift is absent. Competing thermodynamic responses between tropical and Arctic warming have opposing influences on the jets and increase the uncertainties in future projections. This study shows, however, that sea surface temperature (SST) warming in the midlatitude is a major driver for the future midlatitude jet. Coupled Model Intercomparison Program phase 5 models indicate different SST warming between the midlatitude oceans, which induces a weakening of the North Pacific jet and a poleward shift of the North Atlantic jet. Our atmospheric model experiments enable to quantify the relative roles of Arctic, midlatitude, and tropical warming. The competing effects of midlatitude and tropical warming play a substantial role in the future midlatitude jet, hindering any poleward shift of the North Pacific jet, whereas for the North Atlantic jet, midlatitude SST warming is likely to win the competition.

Impact of the Atlantic Multidecadal Oscillation on the Pacific North Equatorial Current bifurcation

Variability in surface currents is mainly induced by the time-varying wind forcing. Recent studies have revealed robust pronounced changes in the atmospheric circulation over the Pacific came with the Pacific Decadal Oscillation (PDO). However, the PDO is a climate index based on sea surface temperature (SST) variations, and may not be appropriately considered as a climate forcing. Here, we suggest the Atlantic SST variability is the ultimate forcing leading to changes in the atmospheric circulation and surface winds over the North Pacific. Anomalous warm North Atlantic and cold South Atlantic leads to weak Hadley cell in the Northern Hemisphere, resulting in a northward displacement of the ITCZ as well as a positive wind stress curl anomaly in the Pacific subtropical region, which would reduce the North Pacific subtropical gyre (NPSG). Associated with reduced sea surface height in the subtropics by the weakened NPSG, the North Equatorial Current (NEC) is weakened based on geostrophy. Changes in basin-scale winds further result in the southward migration of the tropical gyre and consequential downstream ocean circulation.

An Externally Forced Decadal Rainfall Seesaw Pattern Over the Sahel and Southeast Amazon

AbstractBy analyzing observations and model simulations, here we show that there exists a significant anticorrelation on interannual to multidecadal time scales between the Sahel and southeast Amazon rainfall during July‐August‐September. This rainfall seesaw, which is strongest on decadal to multidecadal scales, is due to an anomalous meridional gradient of sea surface temperatures across the tropical Atlantic that pushes the Intertropical Convergence Zone and its associated rain belt toward the anomalously warm hemisphere. Large ensemble model simulations suggest that the seesaw pattern is likely caused by decadal changes in anthropogenic and volcanic aerosols, rather than internal climate variability. Our results suggest that the recent decadal to multidecadal climate variations in and around the North Atlantic basin are largely externally forced and that projected large North Atlantic warming could lead to a wetter Sahel but drier Amazon in the future.

Climate and the Global Famine of 1876–78

AbstractFrom 1875 to 1878, concurrent multiyear droughts in Asia, Brazil, and Africa, referred to as the Great Drought, caused widespread crop failures, catalyzing the so-called Global Famine, which had fatalities exceeding 50 million people and long-lasting societal consequences. Observations, paleoclimate reconstructions, and climate model simulations are used 1) to demonstrate the severity and characterize the evolution of drought across different regions, and 2) to investigate the underlying mechanisms driving its multiyear persistence. Severe or record-setting droughts occurred on continents in both hemispheres and in multiple seasons, with the “Monsoon Asia” region being the hardest hit, experiencing the single most intense and the second most expansive drought in the last 800 years. The extreme severity, duration, and extent of this global event is associated with an extraordinary combination of preceding cool tropical Pacific conditions (1870–76), a record-breaking El Niño (1877–78), a record strong Indian Ocean dipole (1877), and record warm North Atlantic Ocean (1878) conditions. Composites of historical analogs and two sets of ensemble simulations—one forced with global sea surface temperatures (SSTs) and another forced with tropical Pacific SSTs—were used to distinguish the role of the extreme conditions in different ocean basins. While the drought in most regions was largely driven by the tropical Pacific SST conditions, an extreme positive phase of the Indian Ocean dipole and warm North Atlantic SSTs, both likely aided by the strong El Niño in 1877–78, intensified and prolonged droughts in Australia and Brazil, respectively, and extended the impact to northern and southeastern Africa. Climatic conditions that caused the Great Drought and Global Famine arose from natural variability, and their recurrence, with hydrological impacts intensified by global warming, could again potentially undermine global food security.

Extreme Drought Events over the Amazon Basin: The Perspective from the Reconstruction of South American Hydroclimate

The Amazon basin has experienced severe drought events for centuries, mainly associated with climate variability connected to tropical North Atlantic and Pacific sea surface temperature anomalous warming. Recently, these events are becoming more frequent, more intense and widespread. Because of the Amazon droughts environmental and socioeconomic impacts, there is an increased demand for understanding the characteristics of such extreme events in the region. In that regard, regional models instead of the general circulation models provide a promising strategy to generate more detailed climate information of extreme events, seeking better representation of physical processes. Due to uneven spatial distribution and gaps found in station data in tropical South America, and the need of more refined climate assessment in those regions, satellite-enhanced regional downscaling for applied studies (SRDAS) is used in the reconstruction of South American hydroclimate, with hourly to monthly outputs from January 1998. Accordingly, this research focuses on the analyses of recent extreme drought events in the years of 2005 and 2010 in the Amazon Basin, using the SRDAS monthly means of near-surface temperature and relative humidity, precipitation and vertically integrated soil moisture fields. Results from this analysis corroborate spatial and temporal patterns found in previous studies on extreme drought events in the region, displaying the distinctive features of the 2005 and 2010 drought events.

Estimation of possible climate change impact on methane hydrate in the Arctic Ocean

We study the potential impact of a possible warming in the Arctic Ocean in the 21st century on the methane hydrates stability zone. In order to assess the space-time variability of the ocean bottom temperature, we employ a regional version of a coupled ice-ocean model that has been developed at the Institute of Computational Mathematics and Mathematical Geophysics, Siberian Brunch of the Russian Academy of Sciences. This study is based on a combination of the coupled ocean-ice model and a one-dimensional thermal diffusion sediment model. As an atmospheric forcing, some results obtained with CMIP5 climate models simulated with the RCP8.5 scenario (from 2006-2100) are used. We have found that warm North Atlantic water will have a major influence on the Arctic gas hydrates. In such regions as the Barents Sea, the West Svalbard continental margin, and the continental shelf of Norway methane hydrates may exist in shallow waters, where the strongest warming occurs. For this reason, these regions are most vulnerable to releasing methane into the ocean and the atmosphere when the sea water temperature is increased by approximately 2-3 °C. According to our estimates, the seafloor water warming in these areas during the next 100 years may lead to a shift in the upper boundary of the gas hydrates stability zone by 10-110 m.

High latitude winter sightings of common minke whale calves (Balaenoptera acutorostrata) in the Northeast Atlantic

Although common minke whales occur in relatively high numbers in the North Atlantic, relatively little is known about their seasonal movements or the location of their breeding and calving grounds. This lack of knowledge compromises our ability to protect potentially important areas for this species. Here we report high latitude winter sightings of unweaned minke whale calves recorded in offshore waters in the Northeast Atlantic. We discuss how such sightings may contribute to our understanding of the winter distribution and movements of this species; raising the possibility that a proportion of the population may over-winter at high latitudes in offshore waters, and that calving may occasionally occur in the warm North Atlantic Drift waters of western Europe’s continental shelf.

Future Drying in Central America and Northern South America Linked With Atlantic Meridional Overturning Circulation

AbstractMany climate models from the fifth phase of the Coupled Model Intercomparison Project predict decreases in precipitation in Central America and northern South America by the year 2100 for the Representative Concentration Pathway 8.5 scenario. Here we show that the fifth phase of the Coupled Model Intercomparison Project models able to more accurately simulate warm North Atlantic sea surface temperatures (SSTs) for the present climate (strong Atlantic meridional overturning circulation models) are more likely to project a larger precipitation decrease. Drought amplification from the slowdown of Atlantic meridional overturning circulation is more significant during the wet season in the Northern Hemisphere, with an SST‐constrained model estimate yielding a 73% larger decrease in precipitation (−1.11 mm/day) than the multimodel mean (−0.64 mm/day). Since most Earth system models underestimate contemporary SSTs in the North Atlantic, the use of the multimodel mean for impact analysis likely underestimates drought stress and the vulnerability of neotropical forests to increasing drought from climate change.

North Atlantic Natural Variability Modulates Emergence of Widespread Greenland Melt in a Warming Climate

AbstractRecord‐breaking melt over Greenland in recent decades is linked not only to climate change but also to natural variability, including persistent atmospheric high‐pressure conditions in the negative phase of the North Atlantic Oscillation and warm North Atlantic Ocean temperatures during the positive phase of the Atlantic Multidecadal Oscillation. However, the relative importance of natural variability for Greenland melt under varying degrees of greenhouse forcing is still unclear. Using reanalysis data and a large ensemble of climate model simulations, we find that a negative North Atlantic Oscillation and positive Atlantic Multidecadal Oscillation consistently promote heightened summer melt under various forcing conditions. Moreover, timing of widespread 21st century Greenland melt varies considerably between ensemble members due to different phasing of these modes of natural variability. These results indicate the importance of natural modes of variability across a range of external forcing conditions for interannual melt variability and the emergence of widespread Greenland melt.

Beryllium-10 chronology of early and late Wisconsinan moraines in the Revelation Mountains, Alaska: Insights into the forcing of Wisconsinan glaciation in Beringia

We present 32 new cosmogenic 10Be exposure ages from a moraine sequence deposited during the Wisconsinan glaciation in the Swift River valley, Revelation Mountains, western Alaska Range. 10Be ages from an early Wisconsinan [Marine Isotope Stage 4] moraine average 59.7 ± 3.6 ka (n = 9; excluding one outlier), and 10Be ages from a late Wisconsinan [Marine Isotope Stage 2] terminal moraine, inboard moraines, and two recessional end moraines average 21.3 ± 0.8 ka (n = 3; excluding two outliers), 20.2 ± 1.0 ka (n = 7; excluding three young outliers), 19.6 ± 1.0 ka (n = 2; excluding one outlier) and 17.7 ± 0.8 ka (n = 4; excluding two outliers), respectively. The early Wisconsinan moraine age coincides with the close of MIS 4, consistent with previous chronologies from Beringia. Whereas Southern Hemisphere glaciers retreated prior to the onset of Heinrich Stadial 6 (64–60 ka), the MIS 4 glacier advance in the Swift River valley terminated at the close of Heinrich Stadial 6, possibly in response to abrupt North Atlantic warming. Our late Wisconsinan ages indicate significant initial glacier retreat between ca. 21.3–17.7 ka (prior to any significant increase in CO2) likely in response to amplified warming from rising insolation. Preservation of MIS 4 moraines in Beringia has traditionally been ascribed to limited MIS 2 glacier advance modulated by low regional moisture availability, stemming from the exposed Bering Land Bridge. However, we hypothesize that atmospheric re-organization forced by the Laurentide Ice Sheet resulted in relatively higher temperatures and drier conditions in Beringia during MIS 2 than MIS 4, allowing for the preservation of early Wisconsinan moraines.

Mechanisms Governing the Development of the North Atlantic Warming Hole in the CESM-LE Future Climate Simulations

AbstractA warming deficit in North Atlantic sea surface temperatures is a striking feature in global climate model future projections. This North Atlantic warming hole has been related to a slowing of the Atlantic meridional overturning circulation (AMOC); however, the detailed mechanisms involved in its generation remain an open question. An analysis of the Community Earth System Model Large Ensemble simulations is conducted to obtain further insight into the development of the warming hole and its relationship to the AMOC. It is shown that increasing freshwater fluxes through the Arctic gates lead to surface freshening and reduced Labrador Sea deep convection, which in turn act to cool Labrador Sea sea surface temperatures. Furthermore, the resulting changes in surface ocean circulation lead to enhanced transport of cooled Labrador Sea surface waters into the interior of the subpolar gyre and a more zonal orientation of the North Atlantic Current. As a result, there is an increase in ocean advective heat flux divergence within the center of the subpolar gyre, causing this warming deficit in North Atlantic sea surface temperatures. These local changes to the ocean circulation affect the AMOC and lead to its slowdown.

Predictive models using randomForest regression for distribution patterns of meiofauna in Icelandic waters

The exchange of cold polar waters from the north and warm North Atlantic waters from the south is limited by the Greenland-Iceland-Scotland Ridge (GSR) which then may act as a barrier for faunal exchange from the deep Artic and deep Atlantic basins. We investigated the meiofauna density and distribution patterns from different regions north and south of the GSR at water depths between 307 and 2749 m. A total of 84 multicore samples were examined collected during the IceAGE1 (Icelandic marine Animals: Genetics and Ecology) project in summer 2011. We used a gradientForest approach to assess the magnitude of compositional change and the thresholds of remarkable community changes along environmental variables. randomForest regression was applied to predict the meiofauna on a spatial contiguous scale with a set of 23 environmental variables. Meiofauna density ranged between 187 and 3185 individuals per 10 cm2 with the highest densities observed north of the GSR. Nematoda was the most abundant taxon among the meiofauna community, followed by Copepoda and Nauplii. In the Irminger Basin, Gastrotricha was the third most abundant taxon, while in all other regions, Ostracoda was the third most abundant. Food supply, water depth, bottom water oxygen, and hydrographic activity being the most important variables for community changes explain up to 86% of the variation observed in the meiofauna communities.

Increased risk of a shutdown of ocean convection posed by warm North Atlantic summers

A shutdown of ocean convection in the subpolar North Atlantic, triggered by enhanced melting over Greenland, is regarded as a potential transition point into a fundamentally different climate regime1–3. Noting that a key uncertainty for future convection resides in the relative importance of melting in summer and atmospheric forcing in winter, we investigate the extent to which summer conditions constrain convection with a comprehensive dataset, including hydrographic records that are over a decade in length from the convection regions. We find that warm and fresh summers, characterized by increased sea surface temperatures, freshwater concentrations and melting, are accompanied by reduced heat and buoyancy losses in winter, which entail a longer persistence of the freshwater near the surface and contribute to delaying convection. By shortening the time span for the convective freshwater export, the identified seasonal dynamics introduce a potentially critical threshold that is crossed when substantial amounts of freshwater from one summer are carried over into the next and accumulate. Warm and fresh summers in the Irminger Sea are followed by particularly short convection periods. We estimate that in the winter 2010–2011, after the warmest and freshest Irminger Sea summer on our record, ~40% of the surface freshwater was retained. North Atlantic Ocean convection may be impeded by increased freshwater input or reduced surface heat losses. This study shows that warm, fresh surface layers in summer are followed by reduced heat losses in winter, shortening the time for convection.