Polar-midlatitude responses to sea ice reduction from long term coupled simulations

While summer sea ice reduced dramatically/significantly, and the atmospheric warming is amplified over the Arctic, changes in the ocean are less obvious due to its higher inertia. The understanding of the ongoing changes at polar latitudes and its linkages to mid-latitude climate has become a top subject among climate research community. The ocean circulation response to an idealized decline in Arctic sea ice is investigated in a set of novel fully-coupled climate model (AWI-CM) experiments. The atmosphere and thermodynamics is resolved by ECHAM6.3 in a resolution of ca. 180Km, whereas FESOM resolves the ocean and dynamical aspects of the sea ice with resolution ranging from 25 to 150 km. A 250-year reference simulation (REF) is initialized with CORE II and WOA01 data and forced by 1990 greenhouse gases and aerosol concentrations. We conduct a comparative study in which three distinct thermodynamical perturbations are applied on the sea ice to induce a gradual sea ice reduction over 150-year period simulations. Our sensitivity experiments consist of three different approaches to induce an Arctic sea ice reduction: I) the albedo is modified by the increase of snow aging factor; II) reducing the lead closing parameter which resembles a loss of sea ice thickness rather than sea ice area; III) imposing an anomalous heat flux on the sea ice by adding 0.5 W/m2 of long wave radiation. To check the robustness of our results we undertake a second realization of each sensitivity experiment simply by initializing the experiments 30 years later. It is shown that ocean responses establish comparably in all sensitivity experiments. Dynamical adjustments of ocean fluxes and currents are not confined to the polar latitudes. The North Atlantic high-latitude indicates a southward shift of the North Atlantic Current pathway. Although the atmosphere seems to play a secondary role in responding and forcing dynamical changes in the Arctic Ocean, we believe that a negative annular-mode like trend explains the weakening of the westerly winds along the poleward flank of the jet stream, which in turn alters the upper ocean circulation

A simple ocean performance metrics applied to historical CMIP5 simulations

While in atmosphere models it is already common to define objective metrics to investigate how well an atmospheric model performs compared to observations, this is not too common for ocean models. Here we define a simple metrics encompassing the 3D structure of bias and absolute error to estimate the performance of ocean models and we apply it to the historical CMIP5 simulations from 1950 to 2005. Ocean model 3D temperature and salinity fields are compared to the PHC climatology for the major ocean basins. For each 3D grid point of the PHC dataset bias and absolute error of the model climatology are calculated and then volume- averaged over each ocean basin. An average CMIP5 model error is calculated for each ocean basin and used as a reference when investigating a particular model - similarly as has been done for the atmosphere by Reichler and Kim (2008) for CMIP3 models. Ocean surface temperature is generally reasonably well simulated by CMIP5 models and mean absolute errors amount to around 1 K which is comparable to the interannual variability. But in 500 to 1000 m - depending on the ocean basin and on the model - mean absolute errors of up to 4 K are detected which clearly exceed the interannual variability of generally below 1 K. For salinity mean absolute errors are in all levels clearly higher than the interannual variability. For example at the surface the mean absolute error amounts to up to 1 psu while the interannual variability is below 0.2 psu. Even if investigating biases which allows for cancelling out of errors within a basin instead of the mean absolute error this statement still holds in many cases. This means that there is a lot of scope for improvement of the simulation of the vertical structure of the ocean

Quantifying two-way influences between the Arctic and mid-latitudes through regionally increased CO2 concentrations in coupled climate simulations

In which direction is the influence larger: from the Arctic to the mid-latitudes or vice versa? To answer this question, CO2 concentrations have been regionally increased in different latitudinal belts, namely in the Arctic, in the northern mid-latitudes, everywhere outside of the Arctic and globally, in a series of 150 year coupled model experiments with the AWI Climate Model. This method is applied to allow a decomposition of the response to increasing CO2 concentrations in different regions. It turns out that CO2 increase applied in the Arctic only is very efficient in heating the Arctic and that the energy largely remains in the Arctic. In the first 30 years after switching on the CO2 forcing some robust atmospheric circulation changes, which are associated with the surface temperature anomalies including local cooling of up to 1 °C in parts of North America, are simulated. The synoptic activity is decreased in the mid-latitudes. Further into the simulation, surface temperature and atmospheric circulation anomalies become less robust. When quadrupling the CO2 concentration south of 60° N, the March Arctic sea ice volume is reduced by about two thirds in the 150 years of simulation time. When quadrupling the CO2 concentration between 30 and 60° N, the March Arctic sea ice volume is reduced by around one third, the same amount as if quadrupling CO2 north of 60° N. Both atmospheric and oceanic northward energy transport across 60° N are enhanced by up to 0.1 PW and 0.03 PW, respectively, and winter synoptic activity is increased over the Greenland, Norwegian, Iceland (GIN) seas. To a lesser extent the same happens when the CO2 concentration between 30 and 60° N is only increased to 1.65 times the reference value in order to consider the different size of the forcing areas. The increased northward energy transport, leads to Arctic sea ice reduction, and consequently Arctic amplification is present without Arctic CO2 forcing in all seasons but summer, independent of where the forcing is applied south of 60° N. South of the forcing area, both in the Arctic and northern mid-latitude forcing simulations, the warming is generally limited to less than 0.5 °C. In contrast, north of the forcing area in the northern mid-latitude forcing experiments, the warming amounts to generally more than 1 °C close to the surface, except for summer. This is a strong indication that the influence of warming outside of the Arctic on the Arctic is substantial, while forcing applied only in the Arctic mainly materializes in a warming Arctic, with relatively small implications for non-Arctic regions

The earth's climate at the end of the century

Local climate is dependent on the global climate. Here, a global picture on climate change is presented using predictions from the EC-Earth simulations for the end of the century. The results indicate a general rise in annual mean temperature everywhere: 2-4 degrees (global average), 1-6 degrees (over Europe) and 1-4 degrees (Ireland). Changes in precipitation are more varied: large increases (>100%) at high northern latitudes and in the equatorial Pacific but decreases of more than 50% over the subtropics; winters in Europe are predicted to be up to 20% wetter and summers up to 20% drier. Changes in extremes are also presented in this chapter

Using NWP to assess the influence of the Arctic atmosphere on mid-latitude weather and climate

The influence of the Arctic atmosphere on Northern Hemisphere mid-latitude tropospheric weather and climate is explored by comparing the skill of two sets of 14-day weather forecast experiments using the ECMWF model with and without relaxation of the Arctic atmosphere towards ERA-Interim reanalysis data during the integration. Two pathways are identified along which the Arctic influences mid-latitude weather: a pronounced one over Asia and Eastern Europe, and a secondary one over North America. In general, linkages are found to be strongest (weakest) during boreal winter (summer) when the amplitude of stationary planetary waves over the Northern Hemisphere is strongest (weakest). No discernible Arctic impact is found over the North Atlantic and North Pacific region, which is consistent with predominantly southwesterly flow. An analysis of the flow-dependence of the linkages shows that anomalous northerly flow conditions increase the Arctic influence on mid-latitude weather over the continents. Specifically, an anomalous northerly flow from the Kara Sea towards West Asia leads to cold surface temperature anomalies not only over West Asia but also over Eastern and Central Europe. Finally, the results of this study are discussed in the light of potential mid-latitude benefits of improved Arctic prediction capabilities

The influences of the Arctic troposphere on the midlatitude climate variability and the recent Eurasian cooling

Understanding the influence of the Arctic troposphere on the climate at midlatitudes is critical for projecting the impacts of ongoing and anticipated Arctic changes such as Arctic amplification and rapid sea ice decline over the Northern Hemisphere. In this study, we analyze a suite of atmospheric model experiments, with and without atmospheric relaxation toward reanalysis data, to study the impacts of the Arctic troposphere on the midlatitude atmospheric circulation and climate variability. The Arctic troposphere is found to strongly impact the interannual variability of the atmospheric circulation and temperature over the midlatitude continents. The major mechanisms for the impacts of Arctic troposphere include the modulation of the large‐scale atmospheric circulation, the associated heat transport over the continents, and the impacts on synoptic variations in the North Atlantic‐European sector. The impact of the Arctic troposphere on the intensity of the Siberian High is an important factor for how the Arctic can influence temperature variability in south Siberia and East Asia. The trends in the Arctic troposphere in recent decades are closely linked to the recent winter cooling in Northern Eurasia. These recent cooling trends are not driven by the trends in sea surface temperature/sea ice, tropical atmosphere, and the stratosphere. It is argued that the temperature trend pattern of warm Arctic‐cold Eurasia is a manifestation of two possibly independent phenomena and the cooling trend is contributed to by the Arctic troposphere through impacting the large‐scale atmospheric circulation, the atmospheric blocking frequency, and the intensity of the Siberian High

Future sea level contribution from Antarcticainferred from CMIP5 model forcing and itsdependence on precipitation ansatz

Various observational estimates indicate growing mass loss at Antarctica's margins but also heavier precipitation across the continent. In the future, heavier precipitation fallen on Antarctica will counteract any stronger iceberg discharge and increased basal melting of floating ice shelves driven by a warming ocean. Here, we use from nine CMIP5 models future projections, ranging from strong mitigation efforts to business-as-usual, to run an ensemble of ice-sheet simulations. We test, how the precipitation boundary condition determines Antarctica's sea-level contribution. The spatial and temporal varying climate forcings drive ice-sheet simulations. Hence, our ensemble inherits all spatial and temporal climate patterns, which is in contrast to a spatial mean forcing. Regardless of the applied boundary condition and forcing, some areas will lose ice in the future, such as the glaciers from the West Antarctic Ice Sheet draining into the Amundsen Sea. In general the simulated ice-sheet thickness grows in a broad marginal strip, where incoming storms deliver topographically controlled precipitation. This strip shows the largest ice thickness differences between the applied precipitation boundary conditions too. On average Antarctica's ice mass shrinks for all future scenarios if the precipitation is scaled by the spatial temperature anomalies coming from the CMIP5 models. In this approach, we use the relative precipitation increment per degree warming as invariant scaling constant. In contrast, Antarctica gains mass in our simulations if we apply the simulated precipitation anomalies of the CMIP5 models directly. Here, the scaling factors show a distinct spatial pattern across Antarctica. Furthermore, the diagnosed mean scaling across all considered climate forcings is larger than the values deduced from ice cores. In general, the scaling is higher across the East Antarctic Ice Sheet, lower across the West Antarctic Ice Sheet, and lowest around the Siple Coast. The latter is located on the east side of the Ross Ice Shelf

Long-term evolution of ocean eddy activity in a warming world

AbstractMesoscale ocean eddies, an important element of the climate system, impact ocean circulation, heat uptake, gas exchange, carbon sequestration and nutrient transport. Much of what is known about ongoing changes in ocean eddy activity is based on satellite altimetry; however, the length of the altimetry record is limited, making it difficult to distinguish anthropogenic change from natural variability. Using a climate model that exploits a variable-resolution unstructured mesh in the ocean component to enhance grid resolution in eddy-rich regions, we investigate the long-term response of ocean eddy activity to anthropogenic climate change. Eddy kinetic energy is projected to shift poleward in most eddy-rich regions, to intensify in the Kuroshio Current, Brazil and Malvinas currents and Antarctic Circumpolar Current and to decrease in the Gulf Stream. Modelled changes are linked to elements of the broader climate including Atlantic meridional overturning circulation decline, intensifying Agulhas leakage and shifting Southern Hemisphere westerlies.</jats:p