We use surface air temperature to evaluate the decadal forecast skill of the fully coupled Max Planck Institut Earth System Model (MPI‐ESM) initialized using only surface wind stress applied to the ...ocean component of the model (Modini: Model initialization by partially coupled spin‐up). Our analysis shows that the greenhouse gas forcing alone results in a significant forecast skill on the 2–5 and 6–9 year range even for uninitialized hindcasts. For the first forecast year, the forecast skill of Modini is generally comparable with previous initialization procedures applied to MPI‐ESM. But only Modini is able to generate a significant skill (correlation) in the tropical Pacific for a 2–5 year (and to a lesser extent for a 6–9 year) hindcast. Modini is also better able to capture the observed hiatus in global warming in hindcast mode than the other methods. Finally, we present forecasts for 2015 and the average of years 2016–2019 and 2020–2024, predicting an end to the hiatus.
Key Points
Modini initialization has decadal hindcast skill in the Pacific
Modini initialization has skill at hindcasting the recent hiatus
Forecast out to 2024 using Modini initialization suggests an end to the hiatus period
Sea ice concentration is a fundamental property of the Arctic ice‐ocean‐atmosphere system reflecting both dynamics and thermodynamics. Concentration integrates across space and time and is useful for ...characterizing both observed and numerically simulated systems. Concentration is reasonably well measured by remote sensing, and several high‐quality sea ice concentration data sets exist beginning with the satellite era. In this paper we examine the simulated sea ice concentration from nine ice‐ocean numerical models that are part of the coordinated experiments of the Arctic Ocean Model Intercomparison Project (AOMIP). Spatial patterns of means and differences between models and observations, and among models, are compared for a multiyear record and for the September sea ice minimum. Interannual variations are assessed on data with monthly climatology removed. As a proxy for the annual cycle of open water for each model, the total areas with concentration less than 10% are compared among models. Mean ice statistics are computed for grid points with greater than 1% and greater than 10% concentrations. The results show that the models have similar characteristics for the winter months when 100% cover is produced, and most models reproduce an observed minimum in sea ice concentration for 1990. The compared observational data sets use the NASA Team algorithm (Goddard Space Flight Center data, the adjusted or Walsh data, and the Hadley Centre data) and the Bootstrap algorithm. Variability in sea ice concentration is less among the four observational records than among models.
Variability and trends of Fram Strait sea ice area and volume exports are examined for the period of 1990–2010. Simulations from a high-resolution version of the MPIOM model (STORM project) reproduce ...area and volume export well when compared with NSIDC and ICESat satellite data and in-situ ice thickness observations. The fluxes derived from ice thickness and drift satellite products vary considerably, indicating a high uncertainty in these estimates which we mostly assign to the drift observations. The model captures the observed average seasonal cycles and interannual variability of ice export. The simulated mean annual sea ice area export is 860 × 10
3
km
2
a
− 1
(1990–2010), and the correlation with the NSIDC-based area fluxes is
r
= 0.67. The simulated mean annual volume export is 3.3 × 10
3
km
3
a
− 1
(1990–2010), close to the ICESat/ULS values, with a correlation of
r
= 0.58. The simulated monthly area export has a significant positive trend of + 10% per decade, explained by wind forcing. The major contribution to the robust trend in area export between June and September. Fram Strait ice volume export variability is mainly controlled by ice drift with a dominant role of the Transpolar Drift and, to a lesser extent thickness variability. The area export increase reflects increasing ice-drift speed, but is balanced with a reduced thickness over time when it comes to volume export, giving no significant trend in volume export. The spatial variability of ice drift indicates that the export influences a large area upstream in the Trans-Polar Drift stream, and that high volume export events lead to a thinner thickness there. The central Arctic is well connected drift-wise to the Fram Strait via the Transpolar Drift while for thickness, the region north of Greenland is dominated and controlled by the Fram Strait ice export.
Abstract
The uniqueness of optimal parameter sets of an Arctic sea ice simulation is investigated. A set of parameter optimization experiments is performed using an automatic parameter optimization ...system, which simultaneously optimizes 15 dynamic and thermodynamic process parameters. The system employs a stochastic approach (genetic algorithm) to find the global minimum of a cost function. The cost function is defined by the model–observation misfit and observational uncertainties of three sea ice properties (concentration, thickness, drift) covering the entire Arctic Ocean over more than two decades. A total of 11 independent optimizations are carried out to examine the uniqueness of the minimum of the cost function and the associated optimal parameter sets. All 11 optimizations asymptotically reduce the value of the cost functions toward an apparent global minimum and provide strikingly similar sea ice fields. The corresponding optimal parameters, however, exhibit a large spread, showing the existence of multiple optimal solutions. The result shows that the utilized sea ice observations, even though covering more than two decades, cannot constrain the process parameters toward a unique solution. A correlation analysis shows that the optimal parameters are interrelated and covariant. A principal component analysis reveals that the first three (six) principal components explain 70% (90%) of the total variance of the optimal parameter sets, indicating a contraction of the parameter space. Analysis of the associated ocean fields exhibits a large spread of these fields over the 11 optimized parameter sets, suggesting an importance of ocean properties to achieve a dynamically consistent view of the coupled sea ice–ocean system.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Large freshwater anomalies clearly exist in the Arctic Ocean. For example, liquid freshwater has accumulated in the Beaufort Gyre in the decade of the 2000s compared to 1980–2000, with an extra ...≈5000km3 — about 25% — being stored. The sources of freshwater to the Arctic from precipitation and runoff have increased between these periods (most of the evidence comes from models). Despite flux increases from 2001 to 2011, it is uncertain if the marine freshwater source through Bering Strait for the 2000s has changed, as observations in the 1980s and 1990s are incomplete. The marine freshwater fluxes draining the Arctic through Fram and Davis straits are also insignificantly different. In this way, the balance of sources and sinks of freshwater to the Arctic, Canadian Arctic Archipelago (CAA), and Baffin Bay shifted to about 1200±730km3yr−1 freshening the region, on average, during the 2000s. The observed accumulation of liquid freshwater is consistent with this increased supply and the loss of freshwater from sea ice. Coupled climate models project continued freshening of the Arctic during the 21st century, with a total gain of about 50,000km3 for the Arctic, CAA, and Baffin Bay (an increase of about 50%) by 2100. Understanding of the mechanisms controlling freshwater emphasizes the importance of Arctic surface winds, in addition to the sources of freshwater. The wind can modify the storage, release, and pathways of freshwater on timescales of O(1–10) months. Discharges of excess freshwater through Fram or Davis straits appear possible, triggered by changes in the wind, but are hard to predict. Continued measurement of the fluxes and storage of freshwater is needed to observe changes such as these.
•Unprecedented volumes of freshwater have accumulated in the Arctic Ocean since 2000.•Models suggest that the extra freshwater derives from increased precipitation and runoff, and a smaller volume of sea ice.•The net export flux of Arctic freshwater to the North Atlantic Ocean has not changed significantly in the last decade.•Climate projections suggest increased inflow, storage, and outflow of freshwater to the Arctic in the 21st century.•It is possible that large freshwater discharges to the Atlantic could occur, triggered by changes in Arctic winds.
Radar altimetry measurements of the current satellite mission CryoSat‐2 show an increase of Arctic sea ice thickness in autumn 2013, compared to previous years but also related to March 2013. Such an ...increase over the melting season seems unlikely and needs to be investigated. Recent studies show that the influence of the snow cover is not negligible and can highly affect the CryoSat‐2 range retrievals if it is assumed that the main scattering horizon is given by the snow‐ice interface. Our analysis of Arctic ice mass balance buoy records and coincident CryoSat‐2 data between 2012 and 2014 adds observational evidence to these findings. Linear trends of snow and ice freeboard measurements from buoys and nearby CryoSat‐2 freeboard retrievals are calculated during accumulation events. We find a positive correlation between buoy snow freeboard and CryoSat‐2 freeboard estimates, revealing that early snow accumulation might have caused a bias in CryoSat‐2 sea ice thickness in autumn 2013.
Key Points
CryoSat‐2 range measurements correlate with buoy‐measured snow depth changes
Assuming that CryoSat senses the ice freeboard is not justified for thick snow
Arctic wide snow depth data are required
Abstract
Improvement and optimization of numerical sea ice models are of great relevance for understanding the role of sea ice in the climate system. They are also a prerequisite for meaningful ...prediction. To improve the simulated sea ice properties, we develop an objective parameter optimization system for a coupled sea ice–ocean model based on a genetic algorithm. To take the interrelation of dynamic and thermodynamic model parameters into account, the system is set up to optimize 15 model parameters simultaneously. The optimization is minimizing a cost function composed of the model–observation misfit of three sea ice quantities (concentration, drift, and thickness). The system is applied for a domain covering the entire Arctic and northern North Atlantic Ocean with an optimization window of about two decades (1990–2012). It successfully improves the simulated sea ice properties not only during the period of optimization but also in a validation period (2013–16). The similarity of the final values of the cost function and the resulting sea ice fields from a set of 11 independent optimizations suggest that the obtained sea ice fields are close to the best possible achievable by the current model setup, which allows us to identify limitations of the model formulation. The optimized parameters are applied for a simulation with a higher-resolution model to examine a portability of the parameters. The result shows good portability, while at the same time, it shows the importance of the oceanic conditions for the portability.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Arctic climate system change has accelerated tremendously since the beginning of this century, and a strikingly extreme sea‐ice loss occurred in summer 2007. However, the greenhouse‐gas‐emissions ...forcing has only increased gradually and the driving role in Arctic climate change of the positively‐polarized Arctic/North Atlantic Oscillation (AO/NAO) trend has substantially weakened. Although various contributing factors have been examined, the fundamental physical process, which orchestrates these contributors to drive the acceleration and the latest extreme event, remains unknown. We report on drastic, systematic spatial changes in atmospheric circulations, showing a sudden jump from the conventional tri‐polar AO/NAO to an unprecedented dipolar leading pattern, following accelerated northeastward shifts of the AO/NAO centers of action. These shifts provide an accelerating impetus for the recent rapid Arctic climate system changes, perhaps shedding light on recent arguments about a tipping point of global‐warming‐forced climate change in the Arctic. The radical spatial shift is a precursor to the observed extreme change event, demonstrating skilful information for future prediction.
•The Weddell Gyre circulation has an elongated, double-cell structure.•The eastern cell is stronger than the western cell.•Range in previous gyre transport estimates due to zonal variation in gyre ...strength.•Enhanced baroclinic shear is found at the northern and eastern gyre peripheries.•A large volume of water circulates the eastern cell omitting the western gyre.
Argo floats provide both hydrographic and trajectory data, affording the opportunity to investigate surface to mid-depth ocean dynamics. Here, Argo float data are used to determine the absolute geostrophic velocity field of the upper 50–2000 m of the Weddell Gyre, from which the overall circulation pattern is investigated. The Weddell Gyre plays a pivotal role in the modification of climate by advecting heat towards the Antarctic ice shelves and by modifying the water masses that feed into the lowest limb of the global ocean overturning circulation. Warm Deep Water, the source water mass that delivers heat to the Weddell Gyre, is conveniently located within the upper 2000 m domain covered by the floats; we investigate its volume transport as it circulates the gyre. Full depth volume transports are estimated by applying a quadratic function to extrapolate the relative dynamic height field component to the full ocean depth, using CTD profiles from ship-based surveys to determine an extrapolation error. Major new insights are provided by this study. There is an established double-gyre structure to the circulation, with a strong eastern cell and a weaker western cell. Regional variation of the baroclinic component of the flow field is revealed, indicating a northeast-to-southwest reduction in the baroclinic flow, along with a strong meridional gradient of baroclinic flow along the northern limb of the gyre, especially east of ∼25°W. The zonal mean gyre strength away from the shelf edge is 32 ± 5 Sv (1 Sv = 1 × 106 m3 s−1), of which 13 ± 3 Sv are associated with the advection of Warm Deep Water. There is a considerable amount of recirculation within the gyre interior, where water does not traverse the full zonal extent of the gyre. The recirculation is stronger in the eastern cell of the observed double-gyre structure. The interior circulation cells partly explain the large variations in previous gyre strength estimates. We provide an extensive review of previous estimates in context of the new results obtained.
The use of a coupled ocean–atmosphere–sea ice model to hindcast (i.e., historical forecast) recent climate variability is described and illustrated for the cases of the 1976/77 and 1998/99 climate ...shift events in the Pacific. The initialization is achieved by running the coupled model in partially coupled mode whereby global observed wind stress anomalies are used to drive the ocean/sea ice component of the coupled model while maintaining the thermodynamic coupling between the ocean/sea ice and atmosphere components. Here it is shown that hindcast experiments can successfully capture many features associated with the 1976/77 and 1998/99 climate shifts. For instance, hindcast experiments started from the beginning of 1976 can capture sea surface temperature (SST) warming in the central-eastern equatorial Pacific and the positive phase of the Pacific decadal oscillation (PDO) throughout the 9 years following the 1976/77 climate shift, including the deepening of the Aleutian low pressure system. Hindcast experiments started from the beginning of 1998 can also capture part of the anomalous conditions during the 4 years after the 1998/99 climate. The authors argue that the dynamical adjustment of heat content anomalies that are present in the initial conditions in the tropics is important for the successful hindcast of the two climate shifts.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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