We examine the weakening of the Atlantic Meridional Overturning Circulation (AMOC) in response to increasing CO
2
at different horizontal resolutions in a state-of-the-art climate model and in a ...small ensemble of models with differing resolutions. There is a strong influence of the ocean mean state on the AMOC weakening: models with a more saline western subpolar gyre have a greater formation of deep water there. This makes the AMOC more susceptible to weakening from an increase in CO
2
since weakening ocean heat transports weaken the contrast between ocean and atmospheric temperatures and hence weaken the buoyancy loss. In models with a greater proportion of deep water formation further north (in the Greenland-Iceland-Norwegian basin), deep-water formation can be maintained by shifting further north to where there is a greater ocean-atmosphere temperature contrast. We show that ocean horizontal resolution can have an impact on the mean state, and hence AMOC weakening. In the models examined, those with higher resolutions tend to have a more westerly location of the North Atlantic Current and stronger subpolar gyre. This likely leads to a greater impact of the warm, saline subtropical Atlantic waters on the western subpolar gyre resulting in greater dense water formation there. Although there is some improvement of the higher resolution models over the lower resolution models in terms of the mean state, both still have biases and it is not clear which biases are the most important for influencing the AMOC strength and response to increasing CO
2
.
The first acoustic Doppler current profiler current data collected at two locations of the Río de la Plata salt wedge during a period of around 6 months and salinity profiles gathered at and around ...those locations are used to study the vertical structure of currents' response to wind variability in synoptic to intraseasonal timescales and its implications for stratification. Results indicate that the estuary rapidly responds to prevailing southwesterlies/northeasterlies with currents that decay toward the bottom with only little rotation in depth. For the less frequent southeasterlies/northwesterlies the estuary develops a strong vertical structure with a defined inversion in current direction between surface and bottom layers. These patterns derive from the estuary's geometry and bathymetry. Results have important implications for the salinity vertical structure that are verified on the analyzed profiles. First, the combination of the bathymetry and coastline with the prevailing wind variability is highly favorable to the maintenance of a salt wedge structure in this estuary. Second, weakening and eventually breakdown of stratification can only occur for intense and/or persistent southeasterly winds, which even can be very strong, are not frequent. This can explain why the Río de la Plata displays the unusual feature of being an area of spawning and a nursery for a number of coastal species that use the wedge as an essential element for their reproduction. Results show that stratification is highly affected by short‐term wind variability, which is its major characteristic in the area, changing the classical concept of summer‐winter seasonality as the main feature of estuarine variability.
As ice sheets grow or decay, the
net flux of freshwater into the ocean changes and the bedrock adjusts due to isostatic
adjustments, leading to variations in the bottom topography and the oceanic ...boundaries.
This process was particularly intense during the last deglaciation due to the high rates
of ice-sheet melting. It is, therefore, necessary to consider transient ocean bathymetry
and coastlines when attempting to simulate the last deglaciation with Earth system models
(ESMs). However, in most standard ESMs the land-sea mask is fixed throughout simulations
because the generation of a new ocean model bathymetry implies several levels of manual
corrections, a procedure that is hardly doable very often for long runs. This is one of
the main technical problems towards simulating a complete glacial cycle with general
circulation models. For the first time, we present a tool allowing for an automatic computation
of bathymetry and land-sea mask changes in the Max Planck Institute Earth
System Model (MPI-ESM). The algorithms developed in this paper can easily be
adapted to any free-surface ocean model that uses the Arakawa-C grid in the
horizontal and z-grid in the vertical including partial bottom cells. The
strategy applied is described in detail and the algorithms are tested in a
long-term simulation demonstrating the reliable behaviour. Our approach
guarantees the conservation of mass and tracers at global and regional
scales; that is, changes in a single grid point are only propagated
regionally. The procedures presented here are an important contribution to
the development of a fully coupled ice sheet–solid Earth–climate model
system with time-varying topography and will allow for transient simulations
of the last deglaciation considering interactive bathymetry and land-sea
mask.
The Atlantic meridional overturning circulation (AMOC) is an important part of our climate system. The AMOC is predicted to weaken under climate change; however, theories suggest that it may have a ...tipping point beyond which recovery is difficult, hence showing quasi-irreversibility (hysteresis). Although hysteresis has been seen in simple models, it has been difficult to demonstrate in comprehensive global climate models. Here, we outline a set of experiments designed to explore AMOC hysteresis and sensitivity to additional freshwater input as part of the North Atlantic Hosing Model Intercomparison Project (NAHosMIP). These experiments include adding additional freshwater (hosing) for a fixed length of time to examine the rate and mechanisms of AMOC weakening and whether the AMOC subsequently recovers once hosing stops. Initial results are shown from eight climate models participating in the Sixth Coupled Model Intercomparison Project (CMIP6). The AMOC weakens in all models as a result of the freshening, but once the freshening ceases, the AMOC recovers in half of the models, and in the other half it stays in a weakened state. The difference in model behaviour cannot be explained by the ocean model resolution or type nor by details of subgrid-scale parameterisations. Likewise, it cannot be explained by previously proposed properties of the mean climate state such as the strength of the salinity advection feedback. Instead, the AMOC recovery is determined by the climate state reached when hosing stops, with those experiments where the AMOC is weakest not experiencing a recovery.
Meccia, V.L.; Simionato, C.G., and Guerrero, R.A., 2013. The Río de la Plata estuary response to wind variability in synoptic timescale: salinity fields and salt wedge structure. The Río de la Plata ...estuary response to wind variability in a synoptic timescale is studied with the Estuarine, Coastal and Ocean Model forced by tides, runoff, and idealized winds. Ancillary conductivity, temperature, and depth data from synoptic oceanographic cruises are used to validate the conclusions derived from the simulations. Results show that the estuary's response to wind variability can be explained in terms of four characteristic patterns associated with winds that blow with a dominant component either along or across the estuary axis. Cross-estuary winds from the SW produce a northward displacement of the surface salinity front toward the Uruguayan coast and cause downwelling between Montevideo and Punta del Este. Reciprocally, under NE winds, the surface salinity front migrates to the SW and upwelling between Montevideo and Punta del Este occurs. Along-estuary winds with a downstream component produce a net outflow of continental surface waters and inflow of bottom shelf waters, resulting in an intensification of the stratification. In opposition, winds with an upstream component produce a weakening of the vertical structure. Stratification can be destroyed by strong or persistent SE winds, which are not frequent in the region, and it recovers in a relatively short period (10–15 days) after wind relaxation. Consequently, the salt wedge structure is observed in the Río de la Plata during most of the year. This implies that significant vertical mixing events producing exchanges of water and properties between the surface and the bottom waters occur only a few times a year. As a consequence, the Río de la Plata shows strong retentive features, favoring biota by retaining eggs and larvae—and favoring pollutant accumulation.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NMLJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
A long term simulation of the barotropic circulation in the Río de la Plata estuary was performed with the aim of identifying the characteristic patterns of sea surface height (SSH) variability from ...synoptic to inter-annual time scales and their forcing mechanisms. Hamburg Shelf Ocean Model (HamSOM), forced by tides, monthly mean runoffs and 4-daily 10
m winds and surface atmospheric pressure from the National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis was run. The solution was analyzed for the period 1965–2004. Inter-annual variability accounts for almost 10% of the variance. The first EOF mode of SSH variability on this time scale is associated with a mean anomaly of approximately 0.25
m at the upper estuary forced by both runoff and winds, which seems to be strongly associated with the ENSO cycles. Other two modes, with periodicities around 2.5 and 10 years were also found. Even though they are linked to weaker SSH anomalies, they are consistent with inter-annual modes of wind variability reported by other authors. Those modes are important, particularly if they act in phase, because they can provide a background for stronger surges. In contrast with the salinity field, SSH variability on seasonal time scales accounts for a very small percentage of variance and it is the combination of an annual and a semi-annual signal forced by winds and runoff, respectively. Approximately 90% of the variance is due to wind driven variability on sub-annual time scales. The most significant SSH anomalies in this band are associated with cyclogenetic events in the atmosphere, occurring either over Uruguay or over the Patagonian Shelf, whereas the strengthening or weakening of the semi-permanent South Atlantic anticyclone displays a relatively smaller influence. In agreement with previous publications, the estuary's spatial patterns in response to short-scale wind variability seem to be determined by wind direction more than by wind speed.
Understanding long-term committed climate change due to anthropogenic forcing is key to informing climate policies, yet these timescales are still underexplored. We present here a set of ...1000-year-long abrupt stabilization simulations performed with EC-Earth3. Each simulation follows a sudden stabilization of the external forcing at the level specified by CMIP6 for historical (1990) or SSP5-8.5 scenario (2025, 2050, 2065, 2080, 2100) conditions, with a final temperature increase ranging between 1.4 and 9.6 K with respect to the pre-industrial baseline. Remarkably, the simulation stabilized at a greenhouse gas (GHG) level close to the present day (2025) exceeds the Paris Agreement goals of 1.5 and 2° warming above pre-industrial in the long term, and only the 1990 simulation leads to a stabilized climate below 1.5° warming. We first focus on the evolution of the climate response at multi-centennial timescales and its dependence on the level of forcing. We note a decrease in the magnitude of the climate feedback parameter at longer timescales. Conversely, simulations with higher forcing exhibit a larger feedback parameter (in magnitude). Subsequently, the evolution of surface warming patterns over multi-centennial timescales is studied. While the response is generally consistent across simulations, some variations, particularly in the South Pacific and at high latitudes, suggest a certain level of state or forcing dependence. The patterns of precipitation change also evolve during the stabilization runs: the drying trends found in the subtropical oceans and in Mediterranean-like hotspots in the SSP5-8.5 scenario tend to be reduced or even reversed. We finally focus on the rate of heat storage in the global ocean, which is the main driver of the climate response at multi-centennial timescales. We find that the rate of warming of the deep ocean is almost independent from the amplitude of the forcing so that most of the additional heat remains in the upper layers at high forcing. This might be due – at least partly – to a decreased ventilation of the deep ocean, caused by changes in the Meridional Overturning Circulation (MOC). These results highlight the importance of studying multi-centennial timescales of climate change to better understand the response of the deep ocean, which will play a crucial role in determining the final state of the climate system once GHG concentrations are stabilized.
The first long‐period acoustic Doppler current profiler series collected in two locations of the Río de la Plata estuary are explored for barotropic current variability and its connection to wind ...variability in synoptic to intraseasonal timescales. Wind‐driven currents can be explained in terms of two modes, resulting of the estuary's geometry. The first one prevails for winds with a cross‐river component, whereas the second dominates when they blow along the estuary axis. Even though both modes imply that barotropic currents develop in a phase lag with the wind that depends on the location, northeasterly and southwesterly winds generate stronger currents than southeasterly and northwesterly winds. Barotropic response to winds occurs in around 6 hours, and an equilibrium regime is reached for processes with temporal scales more than 4 days. The temporal scales of estuarine variability replicate the wind ones, with activity in bands around 2–8, 10–12, and 18–25 days. Therefore the estuary reaches an equilibrium regime with most of atmospheric processes from synoptic to intraseasonal scales. Intraseasonal variability can be significant and even act in the same direction as the synoptic one, with important effects on circulation. These features imply that the estuary has “weather” and “climate” as the atmosphere does. Our results suggest that conditions classically thought as characteristic of “summer” or “winter” likely occur during any season with high variability. This result has important implications for those coastal species that spawn in the estuary, as they require specific conditions for their successful reproduction, and on estuary's predictability.
Future wintertime atmospheric circulation changes in the Euro–Atlantic (EAT) and Pacific–North American (PAC) sectors are studied from a weather regimes perspective. The Coupled Model Intercomparison ...Project phases 5 and 6 (CMIP5 and CMIP6) historical simulation performance in reproducing the observed regimes is first evaluated, showing a general improvement in the CMIP6 models, which is more evident for EAT. The circulation changes projected by CMIP5 and CMIP6 scenario simulations are analysed in terms of the change in the frequency and persistence of the regimes. In the EAT sector, significant positive trends are found for the frequency and persistence of NAO+ (North Atlantic Oscillation) for SSP2–4.5, SSP3–7.0 and SSP5–8.5 scenarios with a concomitant decrease in the frequency of the Scandinavian blocking and Atlantic Ridge regimes. For PAC, the Pacific Trough regime shows a significant increase, while the Bering Ridge is predicted to decrease in all scenarios analysed. The spread among the model responses is linked to different levels of warming in the polar stratosphere, the tropical upper troposphere, the North Atlantic and the Arctic.
The Río de la Plata Estuary presents a strong bottom salinity front located over a submerged shoal. Apparently favored by retention processes, it is a spawning ground for several coastal fishes. This ...estuary is very shallow and essentially wind driven and, moreover, in time scales relevant to biota, estuarine circulation is wind dominated and highly variable. Two intriguing questions are, therefore, how this system can favor retention and what the involved mechanisms are. This paper qualitatively explores mechanisms involved in the estuary where retention is favored applying numerical simulations in which neutral particles – simulating fish eggs and early larvae – are released along the bottom frontal zone and tracked for different wind conditions. Results suggest that retentive features can be a consequence of estuarine response to natural wind variability acting over bathymetric features. For winds from most directions, particles either remain trapped near their launching position or move northeastward to southwestward along the shoal. As alternation of winds that favor along-shoal motion is the dominant feature of wind variability in the region, a retentive scenario results from prevailing wind variability. Additionally, winds that tend to export particles with a poor chance of being restored to the front are neither frequent nor persistent. Results show, therefore, that physical forcing alone might generate a retentive scenario at the inner part of this estuary. The physical retention mechanism is more effective for bottom than for surface launched particles. Wind statistics indicate that the proposed mechanism has different implications for retention along the seasons. Spring is the most favorable season, followed by summer, when particles would have a larger propensity to reach the southern area of the estuary (Samborombón Bay). Fall and winter are increasingly less favorable. All these features are consistent with patterns observed in the region in organisms having different life history traits.
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