1,721,004 research outputs found
The decrease in ocean heat transport in response to global warming
Full text linkThe ocean is taking up additional heat but how this affects ocean circulation and heat transport is unclear. Here, using coupled model intercomparison project phase 5/6 (CMIP5/6) climate projections, we show a future decrease in poleward ocean heat transport (OHT) across all Northern Hemisphere latitudes and south of 10° S. Most notably, the CMIP5/6 multimodel mean reduction in poleward OHT for the Atlantic at 26.5° N and Indo-Pacific at 20° S is 0.093–0.304 PW and 0.097–0.194 PW, respectively, dependent on scenario and CMIP phase. These changes in OHT are driven by decline in overturning circulation dampened by upper ocean warming. In the Southern Ocean, the reduction in poleward OHT at 55° S is 0.071–0.268 PW. The projected changes are stronger in CMIP6, even when corrected for its larger climate sensitivity. This is especially noticable in the Atlantic Ocean for the weaker forcing scenarios (shared socioeconomic pathway SSP 1-2.6/representative concentration pathways RCP 2.6), where the decrease is 2.5 times larger at 26.5° N due to a stronger decline in the Atlantic meridional overturning circulation
Changing water cycle and freshwater transports in the Atlantic Ocean in observations and CMIP5 models
Full text linkObservations over the last 40 years show that the Atlantic Ocean salinity pattern has amplified, likely in response to changes in the atmospheric branch of the global water cycle. Observational estimates of oceanic meridional freshwater transport (FWT) at 26.5° N indicate a large increase over the last few decades, during an apparent decrease in the Atlantic Meridional Overturning Circulation (AMOC). However, there is limited observation based information at other latitudes. The relative importance of changing FWT divergence in these trends remains uncertain. Ten models from the Coupled Model Intercomparison Project Phase 5 are analysed for AMOC, FWT, water cycle, and salinity changes over 1950–2100. Over this timescale, strong trends in the water cycle and oceanic freshwater transports emerge, a part of anthropogenic climate change. Results show that as the water cycle amplifies with warming, FWT strengthens (more southward freshwater transport) throughout the Atlantic sector over the 21st century. FWT strengthens in the North Atlantic subtropical region in spite of declining AMOC, as the long-term trend is dominated by salinity change. The AMOC decline also induces a southward shift of the Inter-Tropical Convergence Zone and a dipole pattern of precipitation change over the tropical region. The consequent decrease in freshwater input north of the equator together with increasing net evaporation lead to strong salinification of the North Atlantic sub-tropical region, enhancing net northward salt transport. This opposes the influence of further AMOC weakening and results in intensifying southward freshwater transports across the entire Atlantic
Multiple timescales of stochastically forced North Atlantic Ocean variability: A model study
Full text linkThe Atlantic meridional overturning circulation (AMOC) and the subpolar gyre (SPG) are important elements in mechanisms for multidecadal variability in models in the North Atlantic Ocean. In this study, a 2000-year long global ocean model integration forced with the atmospheric patterns associated with a white noise North Atlantic Oscillation (NAO) index is shown to have three distinct timescales of North Atlantic Ocean variability. First, an interannual timescale with variability shorter than 15 years, that can be related to Ekman dynamics. Second, a multidecadal timescale, on the 15- to 65-year range, that is mainly concentrated in the SPG region and is controlled by constructive interference between density anomalies around the gyre and the changing NAO forcing. Finally, the centennial timescales, with variability longer than 65 years, that can be attributed to the ocean being in a series of quasi-equilibrium states. The relationship between the ocean’s response and the NAO index differs for each timescale; the 15-year and shorter timescales are directly related to the NAO of the same year, 15- to 65-year timescales are dependent on the NAO index in the last 25–30 years in a sinusoidal sense while the 65-year and longer timescales relate to a sum of the last 50–80 years of the NAO index
Dynamical evolution of ENSO in a warming background: A review of recent trends & future projections
No full textThe wide-spread implications of El Niño-Southern Oscillation (ENSO) on global and regional climate necessitates a better understanding of how the underlying interannual dynamics have changed over recent years. Year-to-year changes in ENSO impact terrestrial and marine habitats, water availability, food security and social stability (Santoso et al., 2017). With abundant evidence of a warming climate, it is imperative to understand how a large-scale climatic oscillation such as ENSO is evolving and influencing changes in large-scale atmospheric circulation patterns (Alizadeh et al., 2022; Cai et al., 2021). Furthermore, quantifying the influence of the ocean on changes in this climatic pattern is an interesting and important question to answer. Evaluating the ability of models to appropriately represent the underlying physics and dynamical changes impacting the spatiotemporal extent and the intensity of ENSO is crucial to understanding ocean-climate teleconnections and changes in climatic extremes. In this study, we review and evaluate the representation of ENSO in several high-resolution CMIP6 and HighResMIP models and forced ocean-only simulations focusing on the ability of current state-of-the-art models to represent central equatorial pacific warming and cooling. This evaluation involves looking at the development and propagation of warm temperature anomalies on surface and sub-surface levels in the equatorial Pacific and understanding the differences in simulating surface heat budget and exchange with the overlying atmosphere and the deeper ocean. Surface and sub-surface (up to 200m depth) temperature anomalies in the Niño 3.4 region were calculated from modelled data and were then compared with anomalies from observational and reanalysis temperature datasets (like EN4, ORAS5). We find good agreement in the timing and vertical structure of surface/sub-surface temperature anomalies in the forced model simulations, particularly during strong ENSO years. Moreover, the genesis of sub-surface anomalies and their further propagation to the surface was well simulated in the forced simulations. The vertical coherence of temperature anomalies was relatively more pronounced in forced ocean-only simulations than in coupled high-resolution model runs. Furthermore, we comment on the shortcomings and suggest potential improvements that can be made in the models that could improve the model's ability to capture ENSO strength and variability
Characterising surface discrepancies and vertical coherence of ocean temperature anomalies in CMIP6 HighResMIP during ENSO events
No full textConsidering the widespread implications of El Niño-Southern Oscillation (ENSO) on global and regional climate, through atmospheric and oceanic teleconnections, understanding its changing inter-annual dynamics in the context of a warming climate is crucial. Therefore, evaluating the ability of current generation high-resolution climate models to accurately simulate the spatio-temporal characteristics and underlying dynamics of ENSO, is essential for improving future projections. In this study, we review and evaluate the representation of ENSO in several high-resolution coupled climate model simulations from the CMIP6 HighResMIP project, along with two ocean-only simulations forced with surface fluxes from atmospheric reanalyses. Our evaluation concentrates on the ability of current state-of-the-art models to represent central equatorial Pacific warming and cooling. We assess the representation of surface and sub-surface spatio-temporal characteristics of the equatorial Pacific Ocean temperature anomalies that define ENSO events against observation-based ocean temperature datasets. We observe a strong alignment in both the timing and vertical structure of temperature anomalies in the ocean-only model simulations with observations, particularly during strong ENSO events. The genesis of sub-surface anomalies and their further vertical propagation to the surface is well simulated in the atmosphere-forced ocean-only model runs. However, several high-resolution coupled model runs underestimated the magnitude of sub-surface temperature anomalies and showed significant diversity in representing typical ENSO characteristics. The vertical coherence of temperature anomalies was more pronounced in forced ocean-only simulations compared to coupled model runs. The underestimation of sub-surface temperature anomalies and the large diversity in characteristics in coupled model runs indicate potential shortcomings in accurately representing the genesis and evolution of temperature anomalies. Furthermore, potential hypotheses have been discussed to explain the observed model diversity and shortcomings of coupled model runs compared to the ocean-only model simulations
Stable AMOC off state in an eddy-permitting coupled climate model
Full text linkShifts between on and off states of the Atlantic Meridional Overturning Circulation (AMOC) have been associated with past abrupt climate change, supported by the bistability of the AMOC found in many older, coarser resolution, ocean and climate models. However, as coupled climate models evolved in complexity a stable AMOC off state no longer seemed supported. Here we show that a current-generation, eddy-permitting climate model has an AMOC off state that remains stable for the 450-year duration of the model integration. Ocean eddies modify the overall freshwater balance, allowing for stronger northward salt transport by the AMOC compared with previous, non eddy-permitting models. As a result, the salinification of the subtropical North Atlantic, due to a southward shift of the intertropical rain belt, is counteracted by the reduced salt transport of the collapsed AMOC. The reduced salinification of the subtropical North Atlantic allows for an anomalous northward freshwater transport into the subpolar North Atlantic dominated by the gyre component. Combining the anomalous northward freshwater transport with the freshening due to reduced evaporation in this region helps stabilise the AMOC off state
Comparing observed and modelled components of the Atlantic Meridional Overturning Circulation at 26°N
Full text linkThe Coupled Model Intercomparison Project (CMIP) allows the assessment of the representation of the Atlantic Meridional Overturning Circulation (AMOC) in climate models. While CMIP Phase 6 models display a large spread in AMOC strength, the multi-model mean strength agrees reasonably well with observed estimates from RAPID1, but this does not hold for the AMOC s various components. In CMIP Phase 6 (CMIP6), the present-day AMOC is characterized by a lack of lower North Atlantic Deep Water (lNADW) due to the small scale of Greenland Iceland Scotland Ridge overflow and too much mixing. This is compensated for by increased recirculation in the subtropical gyre and more Antarctic Bottom Water (AABW). Deep-water circulation is dominated by a distinct deep western boundary current (DWBC) with minor interior recirculation compared with observations. The future decline in the AMOC of 7 Sv by 2100 under a Shared Socioeconomic Pathway 5-8.5 (SSP5-8.5) emission scenario is associated with decreased northward western boundary current transport in combination with reduced southward flow of upper North Atlantic Deep Water (uNADW). In CMIP6, wind stress curl decreases with time by 14 % so that wind-driven thermocline recirculation in the subtropical gyre is reduced by 4 Sv (17 %) by 2100. The reduction in western boundary current transport of 11 Sv is more than the decrease in wind-driven gyre transport, indicating a decrease over time in the component of the Gulf Stream originating from the South Atlantic.</p
A 30-year reconstruction of the Atlantic meridional overturning circulation shows no decline
Full text linkA decline in Atlantic meridional overturning circulation (AMOC) strength has been observed between 2004 and 2012 by the RAPID-MOCHA-WBTS (RAPID – Meridional Overturning Circulation and Heatflux Array – Western Boundary Time Series, hereafter RAPID array) with this weakened state of the AMOC persisting until 2017. Climate model and paleo-oceanographic research suggests that the AMOC may have been declining for decades or even centuries before this; however direct observations are sparse prior to 2004, giving only “snapshots” of the overturning circulation. Previous studies have used linear models based on upper-layer temperature anomalies to extend AMOC estimates back in time; however these ignore changes in the deep circulation that are beginning to emerge in the observations of AMOC decline. Here we develop a higher-fidelity empirical model of AMOC variability based on RAPID data and associated physically with changes in thickness of the persistent upper, intermediate, and deep water masses at 26∘ N and associated transports. We applied historical hydrographic data to the empirical model to create an AMOC time series extending from 1981 to 2016. Increasing the resolution of the observed AMOC to approximately annual shows multi-annual variability in agreement with RAPID observations and shows that the downturn between 2008 and 2012 was the weakest AMOC since the mid-1980s. However, the time series shows no overall AMOC decline as indicated by other proxies and high-resolution climate models. Our results reinforce that adequately capturing changes to the deep circulation is key to detecting any anthropogenic climate-change-related AMOC decline
Effect of AMOC collapse on ENSO in a high resolution general circulation model
No full textWe look at changes in the El Niño Southern Oscillation (ENSO) in a high-resolution eddy-permitting climate model experiment in which the Atlantic Meridional Circulation (AMOC) is switched off using freshwater hosing. The ENSO mode is shifted eastward and its period becomes longer and more regular when the AMOC is off. The eastward shift can be attributed to an anomalous eastern Ekman transport in the mean equatorial Pacific ocean state. Convergence of this transport deepens the thermocline in the eastern tropical Pacific and increases the temperature anomaly relaxation time, causing increased ENSO period. The anomalous Ekman transport is caused by a surface northerly wind anomaly in response to the meridional sea surface temperature dipole that results from switching the AMOC off. In contrast to a previous study with an earlier version of the model, which showed an increase in ENSO amplitude in an AMOC off experiment, here the amplitude remains the same as in the AMOC on control state. We attribute this difference to variations in the response of decreased stochastic forcing in the different models, which competes with the reduced damping of temperature anomalies. In the new high-resolution model, these effects approximately cancel resulting in no change in amplitude
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