The impact of oceanic vertical mixing on the near-surface vertical structure of the Arctic Ocean is investigated in a global ice–ocean model with a passive tracer. Lowering surface background ...vertical diffusivity and ignoring the effects of surface wave breaking under sea ice improves the model simulation of the horizontal Arctic river water distribution. This improvement is largely responsible for the freshening of the Arctic surface salinity in the model. Although these modifications in the model vertical mixing scheme are applied over the whole global ocean, the change in the surface salinity over the Arctic is larger than that in the rest of the global ocean by one to two orders of magnitude. In contrast, when a reduced background vertical diffusivity is used at all depths, the Arctic vertical salinity stratification is improved below the surface as well as in the surface layer, but the vertical structure and deep circulation in the rest of the global ocean are also strongly affected. Weaker surface vertical mixing in the Arctic Ocean also causes sea ice to thicken even without changes in the parameters for the sea ice component.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The formation and spreading of dense deepwater in the polar regions play a key role in one of the most important climate systems, namely ocean meridional overturning circulation, and the deepwater ...formation is projected to decrease under the global warming. However, the impact of the reduced deepwater formation on the climate system has not been explored in detail. Here, we performed a series of numerical experiments with a climate model where the downward water mass transport through the bottom boundary layer is artificially reduced to quantitatively evaluate its impacts on the transient ocean and climate responses. It is demonstrated that changes in deepwater formation have non‐negligible impacts on not only ocean heat content but also the Earth's radiation budget at the top of the atmosphere: reduction in deepwater formation in high‐latitude oceans causes warming of bottom water, cooling of the ocean surface, and a subsequent decrease in outgoing longwave radiation.
Plain Language Summary
The sinking and spreading of cold, dense water into the ocean deep layers at high latitudes plays a crucial role in large‐scale ocean circulation, closely linked to the climate system. In this study, we use a climate model to investigate the impact of the reduced dense deepwater formation on the heat budget of the climate system, aiming at a comprehensive understanding of deepwater formation in a warming climate transition. The results show that changes in the dense water formation have a non‐negligible effect on the heat budget of the atmosphere as well as on the heat content of the ocean. A decrease in deepwater formation in the high‐latitude oceans leads to warming of the bottom waters, cooling the ocean surface, and a concomitant decrease in outgoing longwave radiation.
Key Points
We evaluate the impacts of reduced deepwater formation in a climate model by adjusting the parameters of the bottom boundary layer
Weakening the downward water mass transport through the bottom boundary layer leads to increased net heat transport to the deep ocean
Changes in deepwater formation have non‐negligible impacts on ocean heat content and the atmospheric heat balance
The sixth version of the Model for Interdisciplinary Research on Climate
(MIROC), called MIROC6, was cooperatively developed by a Japanese modeling
community. In the present paper, simulated mean ...climate, internal
climate variability, and climate sensitivity in MIROC6 are evaluated and
briefly summarized in comparison with the previous version of our climate
model (MIROC5) and observations. The results show that the overall
reproducibility of mean climate and internal climate variability in MIROC6
is better than that in MIROC5. The tropical climate systems (e.g.,
summertime precipitation in the western Pacific and the eastward-propagating
Madden–Julian oscillation) and the midlatitude atmospheric circulation
(e.g., the westerlies, the polar night jet, and troposphere–stratosphere
interactions) are significantly improved in MIROC6. These improvements can
be attributed to the newly implemented parameterization for shallow
convective processes and to the inclusion of the stratosphere. While there
are significant differences in climates and variabilities between the two
models, the effective climate sensitivity of 2.6 K remains the same because
the differences in radiative forcing and climate feedback tend to offset
each other. With an aim towards contributing to the sixth phase of the
Coupled Model Intercomparison Project, designated simulations tackling a
wide range of climate science issues, as well as seasonal to decadal climate
predictions and future climate projections, are currently ongoing using
MIROC6.
The Southern Ocean is of great importance for the global stratification and biological carbon storage because it is connected to the global ocean conveyor by which atmospheric information absorbed in ...the Southern Ocean is redistributed globally and buffered over centuries. Therefore, understanding what controls the Southern Ocean climate, the global ocean conveyor, and links between them is a key to quantifying uncertainties in future climate projections. Based on a set of climate model experiments, here we show that the tide-induced micro-scale mixing in the Pacific deep ocean has significant impacts on the wintertime Southern Ocean climate through basin-scale reorganization of ocean stratification and resultant response of the global ocean conveyor. Specifically, Pacific deep water, which is modified by the deep ocean mixing while travelling south, reinforces the subsurface stratification and suppresses deep convection in the Southern Ocean. Resultant increase of the Ross Sea sea-ice leads to decrease of incoming shortwave radiation and strengthening of the westerly and storms. Because the Southern Ocean could regulate the global warming progress through its role as heat and carbon sink, our study implies that better representation of deep ocean mixing in climate models contributes to reliability improvement in regional-to-global climate projections.
•A new surface dataset for driving ocean-sea ice models (JRA55-do) is produced.•This dataset aims to replace the dataset currently used in the CORE/OMIP framework.•The merits of JRA55-do are the high ...horizontal resolution ( ≈ 55 km) and temporal interval (3 h).•JRA55-do corrects JRA-55 using satellite and other atmospheric reanalysis products.•Assessment shows that JRA55-do can suitably replace the current CORE/OMIP dataset.
We present a new surface-atmospheric dataset for driving ocean–sea-ice models based on Japanese 55-year atmospheric reanalysis (JRA-55), referred to here as JRA55-do. The JRA55-do dataset aims to replace the CORE interannual forcing version 2 (hereafter called the CORE dataset), which is currently used in the framework of the Coordinated Ocean-ice Reference Experiments (COREs) and the Ocean Model Intercomparison Project (OMIP). A major improvement in JRA55-do is the refined horizontal grid spacing ( ∼ 55 km) and temporal interval (3 hr). The data production method for JRA55-do essentially follows that of the CORE dataset, whereby the surface fields from an atmospheric reanalysis are adjusted relative to reference datasets. To improve the adjustment method, we use high-quality products derived from satellites and from several other atmospheric reanalysis projects, as well as feedback on the CORE dataset from the ocean modelling community. Notably, the surface air temperature and specific humidity are adjusted using multi-reanalysis ensemble means. In JRA55-do, the downwelling radiative fluxes and precipitation, which are affected by an ambiguous cloud parameterisation employed in the atmospheric model used for the reanalysis, are based on the reanalysis products. This approach represents a notable change from the CORE dataset, which imported independent observational products. Consequently, the JRA55-do dataset is more self-contained than the CORE dataset, and thus can be continually updated in near real-time. The JRA55-do dataset extends from 1958 to the present, with updates expected at least annually. This paper details the adjustments to the original JRA-55 fields, the scientific rationale for these adjustments, and the evaluation of JRA55-do. The adjustments successfully corrected the biases in the original JRA-55 fields. The globally averaged features are similar between the JRA55-do and CORE datasets, implying that JRA55-do can suitably replace the CORE dataset for use in driving global ocean–sea-ice models.
A new version of the atmosphere–ocean general circulation model cooperatively produced by the Japanese research community, known as the Model for Interdisciplinary Research on Climate (MIROC), has ...recently been developed. A century-long control experiment was performed using the new version (MIROC5) with the standard resolution of the T85 atmosphere and 1° ocean models. The climatological mean state and variability are then compared with observations and those in a previous version (MIROC3.2) with two different resolutions (medres, hires), coarser and finer than the resolution of MIROC5.
A few aspects of the mean fields in MIROC5 are similar to or slightly worse than MIROC3.2, but otherwise the climatological features are considerably better. In particular, improvements are found in precipitation, zonal mean atmospheric fields, equatorial ocean subsurface fields, and the simulation of El Niño–Southern Oscillation. The difference between MIROC5 and the previous model is larger than that between the two MIROC3.2 versions, indicating a greater effect of updating parameterization schemes on the model climate than increasing the model resolution. The mean cloud property obtained from the sophisticated prognostic schemes in MIROC5 shows good agreement with satellite measurements. MIROC5 reveals an equilibrium climate sensitivity of 2.6 K, which is lower than that in MIROC3.2 by 1 K. This is probably due to the negative feedback of low clouds to the increasing concentration of CO₂, which is opposite to that in MIROC3.2.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
The Arctic region has warmed faster than the global mean in past decades. Future climate change projections also suggest this Arctic warming amplification will continue. Here, using ...50-member historical and future scenario simulations by a single climate model, we find that Arctic warming amplification is stronger in a low-emission scenario, compared to a high-emission scenario, after the mid-2040s. This is because in the low-emission scenario, sea ice continues to exist beyond 2040 and the ice-albedo feedback therefore maintains Arctic warming, unlike other latitudes. By contrast, in the high-emission scenario summer sea ice melts away by about 2050. Multi-model analyses show that the strength of Arctic amplification in the low-emission scenario is highly correlated with the amount of sea-ice reduction, whereas this relationship weakens in the high-emission scenario. Our results indicate that climate change mitigation may have a side effect because Arctic warming persists even if the global warming is stabilized.
The impact of April sea-ice thickness (SIT) initialization on the predictability of September sea-ice extent (SIE) is investigated based on a series of perfect model ensemble experiments using the ...MIROC5.2 climate model. Ensembles with April SIT initialization can accurately predict the September SIE for greater lead times than in cases without the initialization – up to 2 years ahead. The persistence of SIT correctly initialized in April contributes to the skilful prediction of SIE in the first September. On the other hand, errors in the initialization of SIT in April cause errors in the predicted sea-ice concentration and thickness in the Pacific sector from July to September and consequently influence the predictive skill with respect to SIE in September. The present study suggests that initialization of the April SIT in the Pacific sector significantly improves the accuracy of the September SIE forecasts by decreasing the errors in sea-ice fields from July to September.
Abstract Initialization of sea ice and the upper halocline in the Arctic Ocean is crucial for sea-ice prediction, but their representation in climate models still remains biased. Here, using ...historical and four different simulations by a single climate model, we find that constraining the sea-ice momentum by surface wind stress contributes to a better representation of the sea-ice velocity, area, and concentration. Moreover, the wind-constrained sea-ice drift modifies the underlying ocean structure via ice-ocean stress, leading to an improved climatological halocline’s vertical structure in the Canada Basin. This is because the excessively represented negative wind and ice-ocean stress curl in the climate model is weakened when constraining the sea-ice momentum and consequently the downward vertical speed, including the Ekman pumping, is also weakened at depths of 0–500 m, alleviating the deepening of isohalines. From these results, the improvement of sea-ice and ocean states by constraining sea-ice momentum is expected to make sea-ice prediction more accurate.
A dataset of historical river discharge into oceans was created using the CaMa-Flood global river routing model and adjusted runoff from the land component of JRA-55. The major rivers were well ...resolved with a 0.25° horizontal resolution. The total runoff on each drainage basin exhibits a distinctive bias on decadal time scales. The input runoff data were modified using 5-year low-pass-filtered multiplicative factors to fit the annual mean climatology and decadal variations in the reference dataset. The model incorporated data from 1958 to 2016. The yearly and seasonal variations of the major rivers are well represented by the model.
PDF
