Salinity Anomalies Research Articles

Coral record of reduced El Nino activity in the early 15th to middle 17th centuries

El Nino–Southern Oscillation (ENSO) powers global interannual climate variability through changes in trade wind strength, temperature and salinity anomalies, sea level, and atmospheric circulation patterns. ENSO variability is well characterized in modern times, but instrumental records cannot fully describe natural ENSO variability due to the imprint of anthropogenic climate forcing. ENSO activity may also be affected by solar variability, but the response of ENSO to such changes is difficult to predict. We constructed a continuous, monthly resolved, spliced fossil Porites coral δ18O and Sr/Ca record from Misima Island, Papua New Guinea, in the Western Pacific Warm Pool, spanning 233 yr (1411–1644 CE [Common Era]). The Misima coral record indicates that the surface ocean in this region experienced a small change in hydrologic balance with no change in temperature, extended periods of quiescence in El Nino activity, and no change in average amplitudes of El Nino events relative to signals captured in regional modern records. The reduced El Nino variability occurs during a known change in solar forcing, the initiation of the Little Ice Age. However, there is no clear relationship between the timing of changes in solar forcing and ENSO activity, implying that ENSO variability changes arise from internal dynamics. The century-scale switch between active and inactive El Nino states has not previously been recorded, and provides a new baseline for climate models and reconstructions.

Remote influences on freshwater flux variability in the Atlantic warm pool region

The understanding of freshwater flux variability is both scientifically and socially important. Local freshwater flux response to a large Atlantic warm pool (AWP) is excessive freshwater or negative Evaporation minus Precipitation (EmP) anomalies, whereas the response is deficient to a small AWP. However, the EmP anomalies in the AWP region are also influenced by the SST anomalies in the tropical eastern Pacific and in the tropical South Atlantic. These remote influences operate through the inter‐basin mode represented by the SST gradient between the tropical North Atlantic and eastern Pacific and the Atlantic meridional mode (AMM) defined as the SST gradient between the tropical North and South Atlantic. When either of these two modes is in the negative phase, the EmP and sea surface salinity anomalies in the AWP region can be positive although the AWP is large. This indicates that the remote influences of the inter‐basin mode and/or the AMM can overwhelm the local effect and induce an opposite freshwater response. Additionally, although ENSO and the AMM sometimes coincide with AWP variability, an El Niño in the preceding winter or a positive AMM in the spring does not necessarily follow a large AWP in the summer.

Recent large increases in freshwater fluxes from Greenland into the North Atlantic

Freshwater (FW) fluxes from river runoff and precipitation minus evaporation for the pan Arctic seas are relatively well documented and prescribed in ocean GCMs. Fluxes from Greenland on the other hand are generally ignored altogether, despite their potential impacts on ocean circulation and marine biology. Here, we present a reconstruction of the spatially distributed FW flux from Greenland for 1958–2010. We find a modest increase into the Arctic Ocean during this period. Fluxes into the Irminger Basin, however, have increased by fifty percent (6.3 ± 0.5 km3 yr−2) in less than twenty years. This greatly exceeds previous estimates. For the ice sheet as a whole the rate of increase since 1992 is 16.9 ± 1.8 km3 yr−2. The cumulative FW anomaly since 1995 is 3200 ± 358 km3, which is about a third of the magnitude of the Great Salinity Anomaly (GSA) of the 1970s. If this trend continues into the future, the anomaly will exceed that of the GSA by about 2025.

Characterization of green clay concretions from the Tonggao Formation, South China: Mineralogy, petrogenesis and paleoenvironmental implications1National Natural Science Foundation of China 40825006.

Enigmatic millimetre-scale micro-concretions with pseudocrystal faces and dominated by green clay minerals occur in unfossiliferous siliciclastic mudstone of the Lower Ordovician (479.0–466.0 Ma) Tonggao Formation, South China. The fossil-free mudstone unit is associated with local biodiversity decline. The mineralogy and mineral chemistry of these concretions were unknown previously, and this study comprises a preliminary investigation. The concretions are dominated by Fe-rich phyllosilicate minerals including glauconite and clinochlore, with minor quartz and traces of magnetite. The textural relations between the micro-concretions and the surrounding matrix, and the preservation of original mudstone laminations within the concretions, point to an origin during early diagenesis. The mineralogy and chemistry of these concretions are consistent with an origin in a restricted, hypersaline, relatively deep-water environment, in accordance with stratigraphical and paleonotological data. These micro-concretions provide clues for a stressed environment with poor water circulation and anomalies of salinity and oxygen.

Recent Arctic Climate Change and Its Remote Forcing of Northwest Atlantic Shelf Ecosystems

During recent decades, historically unprecedented changes have been observed in the Arctic as climate warming has increased precipitation, river discharge, and glacial as well as sea-ice melting. Additionally, shifts in the Arctic's atmospheric pressure field have altered surface winds, ocean circulation, and freshwater storage in the Beaufort Gyre. These processes have resulted in variable patterns of freshwater export from the Arctic Ocean, including the emergence of great salinity anomalies propagating throughout the North Atlantic. Here, we link these variable patterns of freshwater export from the Arctic Ocean to the regime shifts observed in Northwest Atlantic shelf ecosystems. Specifically, we hypothesize that the corresponding salinity anomalies, both negative and positive, alter the timing and extent of water-column stratification, thereby impacting the production and seasonal cycles of phytoplankton, zooplankton, and higher-trophic-level consumers. Should this hypothesis hold up to critical evaluation, it has the potential to fundamentally alter our current understanding of the processes forcing the dynamics of Northwest Atlantic shelf ecosystems.

Interannual Variability of the Circulation over the Eastern Canadian Shelf

This study examines the main physical processes affecting the interannual variability of circulation over the eastern Canadian continental shelf (ECS) based on numerical circulation results for the period 1988–2004 produced by a regional ocean–ice model. The regional circulation model is applied to the northwest Atlantic and uses a horizontal curvilinear grid with a horizontal resolution of approximately 1/4°. The model is forced by atmospheric reanalysis fields produced by Large and Yeager (2004) and boundary forcing based on the ocean reanalysis data produced by Smith et al. (2010). In comparison with previous observational and numerical results in the literature, the regional circulation model has reasonable skill in simulating the large-scale circulation and associated seasonal and interannual variability over the ECS. Complex Empirical Orthogonal Function (CEOF) analysis is used to examine the interannual variability in the monthly mean temperature and salinity anomalies in the model. The CEOF analysis of model results demonstrates that the interannual variability over the Labrador and northern Newfoundland shelves is significantly affected by the variability at high latitudes which propagates onto these shelves through the northern open boundary. Over the eastern Newfoundland Shelf, the interannual variability is significantly affected by the non-linear interaction of the Labrador Current with the North Atlantic Current and by the variability propagating from the southern Labrador Shelf. The interannual variability of circulation and hydrography over the Slope Water region off the Scotian Shelf is significantly affected by anomalies advected by the Gulf Stream and by the non-linear dynamics taking place in the deep waters to the south of the Tail of the Grand Banks. RÉSUMÉ [Traduit par la rédaction] La présente étude examine les principaux processus physiques ayant une influence sur la variabilité interannuelle de la circulation dans l'est du plateau continental canadien (ECS) d'après les résultats numériques de circulation pour la période 1988–2004 obtenus d'un modèle régional océan–glace. Le modèle de circulation régional est appliqué au nord-ouest de l'Atlantique et utilise une grille curvilinéaire horizontale ayant une résolution horizontale d'approximativement 1/4°. Le modèle est forcé par des champs de réanalyse atmosphériques produits par Large et Yeager (2004) et subit un forçage aux limites basé sur des données de réanalyse océaniques produites par Smith et coll. (2010). Comparativement aux résultats observationnels et numériques précédents que l'on trouve dans la littérature, le modèle de circulation régional possède une habileté raisonnable pour la simulation de la circulation à grande échelle et de la variabilité saisonnière et interannuelle associée sur l'ECS. Nous utilisons l'analyse par fonctions orthogonales empiriques complexes (CEOF) pour examiner la variabilité interannuelle des anomalies mensuelles moyennes de température et de salinité dans le modèle. L'analyse par CEOF des résultats du modèle montre que la variabilité interannuelle sur les plateaux du Labrador et du nord de Terre–Neuve est significativement influencée par la variabilité aux hautes latitudes qui se propage sur ces plateaux à travers la limite nord ouverte. Sur le plateau de l'est de Terre–Neuve, la variabilité interannuelle est significativement influencée par l'interaction non linéaire du courant du Labrador avec le courant de l'Atlantique Nord et par la variabilité se propageant depuis le plateau du sud du Labrador. La variabilité interannuelle de la circulation et de l'hydrographie dans la région du talus continental au large du plateau néo-écossais est significativement influencée par les anomalies advectées par le Gulf Stream et par la dynamique non linéaire se produisant dans les eaux profondes au sud de la queue des Grands Bancs.

A coral‐based reconstruction of sea surface salinity at Sabine Bank, Vanuatu from 1842 to 2007 CE

Climate variability associated with the El Niño Southern Oscillation (ENSO) results in large sea‐surface temperature (SST) and sea‐surface salinity (SSS) anomalies in many regions of the tropical Pacific Ocean. We investigate interannual changes in SSS driven by ENSO in the southwestern Pacific at Sabine Bank, Vanuatu (SBV, 166.04°E, 15.94°S) using monthly variations in coralδ18O from 1842 to 2007 CE. We develop and apply a coral δ18O‐SSS transfer function, which is assessed using a calibration‐verification exercise (1970–2007 CE). The 165‐year reconstructed SSS record contains a prominent trend toward freshening from 1842 to 2007 CE; mean SSS for 1842–1872 CE is 35.46 ± 0.28 psu, which contrasts with a mean value of 34.85 ± 0.31 psu for 1977–2007 CE, with a freshening trend during the latter part of the 20th century that is not unprecedented with respect to the overall record. Variance in the record is concentrated in the interannual (42%) and interdecadal (29%) bands. The SBV‐SSS record matches well with a similarly reconstructed SSS time series at Malo Channel, Vanuatu, which is located ∼120 km to the east of SBV. This regional signal is likely driven by ENSO‐related changes in the SPCZ and interdecadal changes in surface water advection. The pattern of interdecadal variability at SBV agrees reasonably well with coral records of interdecadal variability from Fiji and Tonga, especially in the pre‐1940 portions of the records, further evidence for the regional extent of the salinity signal at Sabine Bank, Vanuatu.

Observational Study of the Oceanic Surface Parameters in the Eastern Indian Ocean During Two Contrasting Dipole Years 2005 and 2006

The Indian Ocean dipole (IOD) is one of the major interannual climate variation signals in this region through the coupled air–sea interactions. In this letter, the oceanic surface parameters in the eastern Indian Ocean during two consecutive contrasting dipole years 2005 (negative) and 2006 (positive) are examined exclusively from satellite and in situ observations. Results showed that there was sea surface temperature anomaly up to <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\pm 2\ ^{\circ}\hbox{C}$</tex></formula> in the south eastern equatorial Indian Ocean (EEIO) during the mature phase (September–November) of the dipole. Except for the south EEIO, the south of the equator showed very high positive sea level anomaly (up to 45 cm) during the boreal fall (September–November) in 2006 under the influence of the strong positive IOD event. Similar coherent differences were observed in freshwater flux and sea surface salinity anomalies which show the large impact of these dipole events in the south EEIO box. Also, the seasonal rainfall during the northeast Indian monsoon for these two years shows substantial difference over southern India which reveals the paramount impact of IOD on the northeast monsoon rainfall.

Tropical instability waves linked to sea surface salinity anomalies

Stretching westward from the South American coast, a tongue of cold water straddles the equator, reaching at times clear across the Pacific Ocean. The behavior of this cold tongue affects the marine ecosystem within its reach and influences the global climate through the El Niño–Southern Oscillation. The Pacific cold tongue is mirrored in the Atlantic, though that version is much weaker and less well defined. In the Pacific, westward propagating waves can periodically radiate outward from the South American coast. Known as tropical instability waves (TIWs), they give the cold tongue's edges a ribbed appearance when observed from space. TIWs are already associated with anomalous sea surface heights and temperatures, surface winds, chlorophyll concentrations, and ocean circulation patterns. Research by Lee et al. provides the first observational evidence of sea surface salinity anomalies associated with TIWs.

Observations of Freshening in the Northwest Pacific Subtropical Gyre near Luzon Strait

Argo observations reveal that the salinity in the North Pacific subtropical gyre near Luzon Strait gradually declined by 0.2 (practical salinity scale used) from 2003 to 2007 over a depth range of 100 m to 200 m. Such freshening is also found in the outputs of the Estimating the Circulation and Climate of the Ocean (ECCO) model. The possible mechanisms for the freshening are investigated using the surface freshwater flux (E-P) data, the ECCO outputs and a salt budget equation for the upper ocean. Our analysis indicates that the magnitude of the salinity change caused by the surface freshwater flux anomaly is far smaller than observed, suggesting that the surface freshwater flux anomaly is not sufficient to account for the observed freshening. In fact, the salinity anomaly is closely linked to a pronounced freshening at the northeast corner of the study area from 2003 to 2007. The advection of salinity anomalies in the western North Pacific Ocean between 25°N and 35°N via a southwestward flow in the “C–shaped” region associated with the Kuroshio system is the main mechanism responsible for the observed freshening in the study area. RÉSUMÉ [Traduit par la rédaction] Les observations Argo révèlent que la salinité dans le gyre subtropical du Pacifique Nord près du détroit de Luçon a graduellement diminué de 0,2 (en utilisant l’échelle de salinité pratique) entre 2003 et 2007 dans un intervalle de profondeur de 100 à 200 m. Un adoucissement comparable s'observe aussi dans les sorties du modèle ECCO (Estimating the Circulation and Climate of the Ocean). Nous examinons les mécanismes pouvant expliquer l'adoucissement au moyen des données de flux d'eau douce en surface (E–P), des sorties de l'ECCO et de l’équation du bilan du sel pour la couche supérieure de l'océan. Notre analyse indique que l'ampleur du changement de salinité causé par l'anomalie de flux d'eau douce en surface est beaucoup plus petite que ce qui est observé, ce qui suggère que l'anomalie de flux d'eau douce en surface ne suffit pas à expliquer l'adoucissement observé. En fait, l'anomalie de salinité est étroitement liée à un adoucissement prononcé dans le coin nord-est de la zone à l’étude de 2003 à 2007. L'advection des anomalies de salinité dans l'ouest du Pacifique Nord entre 25°N et 35°N par un écoulement vers le sud-ouest dans la région en forme de « C » associée au système du Kuroshio est le principal mécanisme responsable de l'adoucissement observé dans la zone à l’étude.

In pursuit of anomalies—Analyzing the poleward transport of Atlantic Water with surface drifters

We examine the trajectories of 168 drifters as a proxy for Atlantic water flowing through the Nordic Seas. The drifters were released at or passed through the Svinøy section, off the west coast of Norway. For comparison, we generate a set of synthetic trajectories using a stochastic model, with a range of diffusivities.With the both sets, we determine the transit times of both drifters and particles from Svinøy to Arctic gateways: the Barents Sea and Spitsbergen. The mean arrival times to these locations are roughly 200 and 500 days, respectively. This implies ample time for cooling of the surface waters, which increases densities and permits subduction before reaching the Arctic. However a range of transit times is seen; some parcels reach Fram Strait in only 4 months while others are still in the southern Nordic Seas after 2 years.The results do not support the idea that temperature or salinity anomalies can exist as coherent packets. The drifters passing Svinøy, when treated as a group, quickly spread over large distances, mixing with water in the Norwegian and Lofoten Basins. Thus an anomaly entering the Nordic Seas would quickly be obliterated. However, the velocity of the clusters center of mass is consistent with anomaly propagation speeds inferred previously from hydrographic measurements, suggesting the observed variability is advective in nature.

Arctic Ocean freshwater: How robust are model simulations?

The Arctic freshwater (FW) has been the focus of many modeling studies, due to the potential impact of Arctic FW on the deep water formation in the North Atlantic. A comparison of the hindcasts from ten ocean‐sea ice models shows that the simulation of the Arctic FW budget is quite different in the investigated models. While they agree on the general sink and source terms of the Arctic FW budget, the long‐term means as well as the variability of the FW export vary among models. The best model‐to‐model agreement is found for the interannual and seasonal variability of the solid FW export and the solid FW storage, which also agree well with observations. For the interannual and seasonal variability of the liquid FW export, the agreement among models is better for the Canadian Arctic Archipelago (CAA) than for Fram Strait. The reason for this is that models are more consistent in simulating volume flux anomalies than salinity anomalies and volume‐flux anomalies dominate the liquid FW export variability in the CAA but not in Fram Strait. The seasonal cycle of the liquid FW export generally shows a better agreement among models than the interannual variability, and compared to observations the models capture the seasonality of the liquid FW export rather well. In order to improve future simulations of the Arctic FW budget, the simulation of the salinity field needs to be improved, so that model results on the variability of the liquid FW export and storage become more robust.

Variability of the Atlantic Meridional Overturning Circulation in CCSM4

Abstract Atlantic meridional overturning circulation (AMOC) variability is documented in the Community Climate System Model, version 4 (CCSM4) preindustrial control simulation that uses nominal 1° horizontal resolution in all its components. AMOC shows a broad spectrum of low-frequency variability covering the 50–200-yr range, contrasting sharply with the multidecadal variability seen in the T85 × 1 resolution CCSM3 present-day control simulation. Furthermore, the amplitude of variability is much reduced in CCSM4 compared to that of CCSM3. Similarities as well as differences in AMOC variability mechanisms between CCSM3 and CCSM4 are discussed. As in CCSM3, the CCSM4 AMOC variability is primarily driven by the positive density anomalies at the Labrador Sea (LS) deep-water formation site, peaking 2 yr prior to an AMOC maximum. All processes, including parameterized mesoscale and submesoscale eddies, play a role in the creation of salinity anomalies that dominate these density anomalies. High Nordic Sea densities do not necessarily lead to increased overflow transports because the overflow physics is governed by source and interior region density differences. Increased overflow transports do not lead to a higher AMOC either but instead appear to be a precursor to lower AMOC transports through enhanced stratification in LS. This has important implications for decadal prediction studies. The North Atlantic Oscillation (NAO) is significantly correlated with the positive boundary layer depth and density anomalies prior to an AMOC maximum. This suggests a role for NAO through setting the surface flux anomalies in LS and affecting the subpolar gyre circulation strength.

Decadal-timescale changes of the Atlantic overturning circulation and climate in a coupled climate model with a hybrid-coordinate ocean component

A wide range of statistical tools is used to investigate the decadal variability of the Atlantic Meridional Overturning Circulation (AMOC) and associated key variables in a climate model (CHIME, Coupled Hadley-Isopycnic Model Experiment), which features a novel ocean component. CHIME is as similar as possible to the 3rd Hadley Centre Coupled Model (HadCM3) with the important exception that its ocean component is based on a hybrid vertical coordinate. Power spectral analysis reveals enhanced AMOC variability for periods in the range 15–30 years. Strong AMOC conditions are associated with: (1) a Sea Surface Temperature (SST) anomaly pattern reminiscent of the Atlantic Multi-decadal Oscillation (AMO) response, but associated with variations in a northern tropical-subtropical gradient; (2) a Surface Air Temperature anomaly pattern closely linked to SST; (3) a positive North Atlantic Oscillation (NAO)-like pattern; (4) a northward shift of the Intertropical Convergence Zone. The primary mode of AMOC variability is associated with decadal changes in the Labrador Sea and the Greenland Iceland Norwegian (GIN) Seas, in both cases linked to the tropical activity about 15 years earlier. These decadal changes are controlled by the low-frequency NAO that may be associated with a rapid atmospheric teleconnection from the tropics to the extratropics. Poleward advection of salinity anomalies in the mixed layer also leads to AMOC changes that are linked to processes in the Labrador Sea. A secondary mode of AMOC variability is associated with interannual changes in the Labrador and GIN Seas, through the impact of the NAO on local surface density.

Multicentennial variability of the Atlantic meridional overturning circulation and its climatic influence in a 4000 year simulation of the GFDL CM2.1 climate model

We investigate decadal to multicentennial variability of Northern Hemisphere surface air temperature in a 4000‐year control simulation of the GFDL CM2.1 climate model. Spectral analysis shows the presence of a distinct multicentennial timescale of temperature variability. The associated spatial pattern is broad, covering the entire Northern Hemisphere extratropics, but with enhanced amplitude in the Atlantic and Arctic sectors. This variability appears to be driven by interhemispheric fluctuations in oceanic heat transport associated with the Atlantic Meridional Overturning Circulation (AMOC). The AMOC variability is associated with century‐scale propagation of salinity anomalies from the Southern Ocean to the subpolar North Atlantic, with out of phase transport variations between the upper ocean and deeper layers of the Atlantic. When positive (negative) upper ocean salinity anomalies reach the subpolar North Atlantic they strengthen (weaken) the AMOC by modulating upper ocean density and vertical stratification. The large‐scale warming also appears to be enhanced by reductions in surface albedo associated with reduced sea‐ice and low‐level cloudiness, thereby increasing the absorption of shortwave radiation and amplifying the warming from AMOC changes. We speculate that such multicentennial variations in the AMOC could contribute to long‐time scale climate fluctuations in the observed paleo record. This could arise purely as internal variability of the climate system, or through radiatively‐induced changes to atmospheric circulation patterns, such as the NAO, that would in turn influence the AMOC.

A 20-year coupled ocean-sea ice-atmosphere variability mode in the North Atlantic in an AOGCM

In order to understand potential predictability of the ocean and climate at the decadal time scales, it is crucial to improve our understanding of internal variability at this time scale. Here, we describe a 20-year mode of variability found in the North Atlantic in a 1,000-year pre-industrial simulation of the IPSL-CM5A-LR climate model. This mode involves the propagation of near-surface temperature and salinity anomalies along the southern branch of the subpolar gyre, leading to anomalous sea-ice melting in the Nordic Seas, which then forces sea-level pressure anomalies through anomalous surface atmospheric temperatures. The wind stress associated to this atmospheric structure influences the strength of the East Greenland Current across the Denmark Strait, which, in turn, induces near-surface temperature and salinity anomalies of opposite sign at the entrance of the Labrador Sea. This starts the second half of the cycle after approximatively 10 years. The time scale of the cycle is thus essentially set by advection of tracers along the southern branch of the subpolar gyre, and by the time needed for anomalous East Greenland Current to accumulate heat and freshwater anomalies at the entrance of the Labrador Sea. The Atlantic meridional overturning circulation (AMOC) does not play a dominant role in the mode that is confined in the subpolar North Atlantic, but it also has a 20-year preferred timescale. This is due to the influence of the propagating salinity anomalies on the oceanic deep convection. The existence of this preferred timescale has important implications in terms of potential predictability of the North Atlantic climate in the model, although its realism remains questionable and is discussed.

Interannual Variations of Subsurface Spiciness in the Philippine Sea: Observations and Mechanism

Abstract The Philippine Sea (PS) is a key region connecting North Pacific subtropics to the equator via western boundary currents. Using available measurements from Argo profiling floats, satellite altimeters, and research surveys, the authors investigate the characteristics and mechanism of subsurface spiciness variability (represented by salinity changes between 23.5 and 24.5 σθ) in the PS. During the past decade, low-frequency salinity variability was dominated by interannual signals characterized by out-of-phase changes between the southern and northern PS with peak-to-peak amplitudes exceeding 0.1 psu. These salinity anomalies are mainly generated locally by anomalous cross-front geostrophic advections. In 2003, an anomalous cyclonic circulation developed in the PS, which transported greater (less) than normal high-salinity North Pacific Tropical Water to the northern (southern) PS and produced positive (negative) salinity anomalies there. In 2009, an anomalous anticyclone emerged, which produced negative (positive) salinity anomalies in the northern (southern) PS. These year-to-year variations are closely associated with ENSO cycle. During strong El Niño (La Niña) episodes, positive (negative) wind stress curl anomalies between 8° and 18°N evoke westward-propagating upwelling (downwelling) Rossby waves in the central Pacific and positive (negative) anomalous Ekman pumping in the western Pacific, resulting in the observed current and salinity changes in the PS. Further analysis suggests that these locally generated spiciness anomalies disperse quickly while propagating to the equatorial Pacific in the Mindanao Current (MC). In the meantime, anomalies advected from higher latitudes are nearly diminished upon reaching the PS. The western boundary of the North Pacific seems quite efficient in damping extratropical signals.

Long-Term Variability of Volume and Heat Transport in the Nordic Seas: A Model Study

This article presents results from a model study of interannual and decadal variability in the Nordic Seas. Fifty years of simulations were conducted in an initial condition ensemble mode forced with the National Centers for Environmental Prediction (NCEP) reanalysis. We studied two major events in the interannual and interdecadal variability of the Nordic Seas during the past fifty years: the Great Salinity Anomaly in the 1960s and early 1970s and the warming of the Arctic and subarctic oceans in the late 1990s. Previous studies demonstrated that the Great Salinity Anomaly observed in the subarctic ocean in 1960 was originally generated by intensified sea-ice and freshwater inflow from the Arctic Ocean. Our model results demonstrate that the increase in the transport of fresh and cold waters through Fram Strait in the 1960s was concurrent with a reduction in the meridional water exchange over the Greenland–Scotland Ridge. The resulting imbalance in salinity and heat fluxes through the strait and over the...

Interdecadal North-Atlantic meridional overturning circulation variability in EC-EARTH

The Atlantic meridional overturning circulation (AMOC) in a 600 years pre-industrial run of the newly developed EC-EARTH model features marked interdecadal variability with a dominant time-scale of 50–60 years. An oscillation of approximately 2 Sverdrup (1 Sv = 106 m3 s−1) is identified, which manifests itself as a monopole causing the overturning to simultaneously strengthen (/weaken) and deepen (/shallow) as a whole. Eight years before the AMOC peaks, density in the Labrador-Irminger Sea region reaches a maximum, triggering deep water formation. This density change is caused by a counterclockwise advection of temperature and salinity anomalies at lower latitudes, which we relate to the north-south excursions of the subpolar-subtropical gyre boundary and variations in strength and position of the subpolar gyre and the North Atlantic Current. The AMOC fluctuations are not directly forced by the atmosphere, but occur in a delayed response of the ocean to forcing by the North Atlantic Oscillation, which initiates “intergyre”-gyre fluctuations. Associated with the AMOC is a 60-year sea surface temperature variability in the Atlantic, with a pattern and timescale showing similarities with the real-world Atlantic Multidecadal Variability. This good agreement with observations lends a certain degree of credibility that the mechanism that is described in this article could be seen as representative of the real climate system.

On the sub‐decadal variability of South Atlantic Antarctic Intermediate Water

Variability of Antarctic Intermediate Water salinity in the South Atlantic is investigated on interannual and intradecadal timescales. Novel observations of slow, westward propagating salinity anomalies in Argo data are presented. The features have no corresponding signal in temperature and are anomalous in density. Analysis of 40 years of model output supports the existence of these westward propagating salinity anomalies and indicates that they are typical occurrences in time. The features are intensified in a latitude band around 30°S associated with the propagation of Agulhas rings. However, the features are much larger than Agulhas rings and occur on decadal timescales in the model. They propagate westward with speeds of 2.3 cm/s in observations, and 1.7 cm/s in model data. They are more consistent with planetary waves than with the advection of large‐scale salinity anomalies. The observation of these features has implications for the interpretation of salinity anomalies, such as the linking of hydrological cycle changes to salinity changes.