69 research outputs found
Reduced viscosity steadily weakens oceanic currents
The viscosity of both air and water is temperature dependent. A rising temperature leads to an increased viscosity for air but a decreased viscosity for water. As climate becomes warmer, this increased air viscosity can partly inhibit the reduction of wind stress over the ocean, and the reduced water viscosity causes less downward momentum and heat transport. As these opposing effects of warming on air and water viscosity are not included in the state-of-the-art climate models, the understanding of their potential impacts on the response of the climate system to the anthropogenic warming is lacking. Here, via analyzing the Simple Ocean Data Assimilation oceanic reanalysis dataset, we show that the ocean heat content increases at a rate of ~1.3 × 1022 J/yr over 35 years, which leads to a continuous reduction of oceanic viscosity. As a result, the ocean vertical shear enhances with a shoaling of the mixed layer depth and a reduced vertical linkage in the ocean. Our calculations show a reduction of the oceanic kinetic energy at a rate of ~2.4 × 1016 J/yr. Potentially, this could generate far-reaching impacts on the energy storage of the climate system and, hence, could pace the global warming. Thus, it is important to include the temperature-dependent viscosity in our climate models. Freshwater discharged from polar ice sheets and mountain glaciers also contributes to the reduction in oceanic viscosity but, at present, to a lesser extent than that in oceanic warming. Reduced oceanic viscosity, therefore, is an important, but hitherto overlooked, response to a warming climate and contributes to many recent weather extremes including heavier rainfall rates in hurricanes, slackening of the polar vortex, and oceanic heat waves
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Role of Perturbing Ocean Initial Condition in Simulated Regional Sea Level Change
Multiple lines of observational evidence indicate that the global climate has been getting warmer since the early 20th century. This warmer climate has led to a global mean sea level rise of about 18 cm during the 20th century, and over 6 cm for the first 15 years of the 21st century. Regionally the sea level rise is not uniform due in large part to internal climate variability. To better serve the community, the uncertainties of predicting/projecting regional sea level changes associated with internal climate variability need to be quantified. Previous research on this topic has used single-model large ensembles with perturbed atmospheric initial conditions (ICs). Here we compare uncertainties associated with perturbing ICs in just the atmosphere and just the ocean using a state-of-the-art coupled climate model. We find that by perturbing the oceanic ICs, the uncertainties in regional sea level changes increase compared to those with perturbed atmospheric ICs. Thus, in order for us to better assess the full spectrum of the impacts of such internal climate variability on regional and global sea level rise, approaches that involve perturbing both atmospheric and oceanic initial conditions are necessary
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What causes the spread of model projections of ocean dynamic sea-level change in response to greenhouse gas forcing?
Sea levels of different atmosphere-ocean general circulation models (AOGCMs) respond to climate change forcing in different ways, representing a crucial uncertainty in climate change research. We isolate the role of the ocean dynamics in setting the spatial pattern of dynamic sea-level (zeta) change by forcing several AOGCMs with prescribed identical heat, momentum (wind) and freshwater flux perturbations. This method produces a zeta projection spread comparable in magnitude to the spread that results from greenhouse gas forcing, indicating that the differences in ocean model formulation are the cause, rather than diversity in surface flux change. The heat flux change drives most of the global pattern of zeta change, while the momentum and water flux changes cause locally confined features. North Atlantic heat uptake causes large temperature and salinity driven density changes, altering local ocean transport and zeta. The spread between AOGCMs here is caused largely by differences in their regional transport adjustment, which redistributes heat that was already in the ocean prior to perturbation. The geographic details of the zeta change in the North Atlantic are diverse across models, but the underlying dynamic change is similar. In contrast, the heat absorbed by the Southern Ocean does not strongly alter the vertically coherent circulation. The Arctic zeta change is dissimilar across models, owing to differences in passive heat uptake and circulation change. Only the Arctic is strongly affected by nonlinear interactions between the three air-sea flux changes, and these are model specific.Peer reviewe
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Sea level extremes and compounding marine heatwaves in coastal Indonesia
Low-lying island nations like Indonesia are vulnerable to sea level Height EXtremes (HEXs). When compounded by marine heatwaves, HEXs have larger ecological and societal impact. Here we combine observations with model simulations, to investigate the HEXs and Compound Height-Heat Extremes (CHHEXs) along the Indian Ocean coast of Indonesia in recent decades. We find that anthropogenic sea level rise combined with decadal climate variability causes increased occurrence of HEXs during 2010–2017. Both HEXs and CHHEXs are driven by equatorial westerly and longshore northwesterly wind anomalies. For most HEXs, which occur during December-March, downwelling favorable northwest monsoon winds are enhanced but enhanced vertical mixing limits surface warming. For most CHHEXs, wind anomalies associated with a negative Indian Ocean Dipole (IOD) and co-occurring La Niña weaken the southeasterlies and cooling from coastal upwelling during May-June and November-December. Our findings emphasize the important interplay between anthropogenic warming and climate variability in affecting regional extremes.
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Changes in the Arctic and their impact on the oceanic meridional overturning circulation
Variations of the sea ice condition in the Arctic and its adjacent seas could significantly influence the earth\u27s climate. Recent observations show that both sea ice and oceanic properties in the polar and sub-polar seas are undergoing significant changes. In this study, by applying a coupled sea ice-ocean model---the Miami Isopycnic Coordinate Ocean Model and the elastic-viscous-plastic dynamic-thermodynamic sea ice model and the NCEP/NCAR reanalysis data, the changes of the Arctic sea ice caused by the NAO-related atmospheric anomalies and the response of the oceanic Meridional Overturning Circulation (MOC) to these changes are investigated.Model solutions indicate that the Arctic sea ice varies with the atmospheric transients. The summer minimum sea ice extent in the high NAO case reduces about 17% of that in the low NAO case. The largest reduction in multi-year ice extent is along the Siberian coast region. Horizontally, ice is about 1 to 2 m thicker (thinner) at Eurasian coast (Canadian) side of the Arctic in low NAO years relative to that in high NAO years.Sea ice export from Arctic through Fram Strait is about 5474 km 3 per year in high NAO years, more than doubled of that in low NAO years. This high efflux is mainly caused by the increased strength of the wind forcing. The rate of the net sea ice production in the high NAO case is about 10 times as that in the low NAO case along the Siberian and Alaskan coasts, and 2 to 3 times in the other regions. The high rate of ice production is related to the efficient sea ice transport and the low ice compactness. It is worth mentioning that in the model solution, a net sea ice influx from the Barents Sea to the Arctic basin makes up 15 to 18% of the ice efflux at Fram Strait. The ice efflux at Fram Strait follows the NAO transients without any noticeable time lag.The strength of the MOC is 16.2, 13.4 and 12.3 Sv in the high NAO, climatic and low NAO cases, respectively. The rate of dense water formation in the high NAO case is about 3 Sv higher than that in the low NAO case in the Labrador Sea and south of the Denmark Strait region, and 1 Sv weaker in the Greenland Sea. The overall dense water formation is almost the same in the ice related marginal seas. Model solutions also show that the longterm persistent atmospheric anomalies are important for generating systematic MOC variations. MOC also responds quickly to the decadal timescale atmospheric fluctuations. Because the adjustment timescale of the MOC is long, the response of the MOC to the NAO transients is sensitive to the initial state of the forcing fields
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