| 1. |
- Bender, Frida, et al.
(författare)
-
Response to the eruption of Mount Pinatubo in relation to climate sensitivity in the CMIP3 models
- 2010
-
Ingår i: Climate Dynamics. - : Springer. - 0930-7575 .- 1432-0894. ; 35:5, s. 875-886
-
Tidskriftsartikel (refereegranskat)abstract
- The radiative flux perturbations and subsequent temperature responses in relation to the eruption of Mount Pinatubo in 1991 are studied in the ten general circulation models incorporated in the Coupled Model Intercomparison Project, phase 3 (CMIP3), that include a parameterization of volcanic aerosol. Models and observations show decreases in global mean temperature of up to 0.5 K, in response to radiative perturbations of up to 10 W m−2, averaged over the tropics. The time scale representing the delay between radiative perturbation and temperature response is determined by the slow ocean response, and is estimated to be centered around 4 months in the models. Although the magniude of the temperature response to a volcanic eruption has previously been used as an indicator of equilibrium climate sensitivity in models, we find these two quantities to be only weakly correlated. This may partly be due to the fact that the size of the volcano-induced radiative perturbation varies among the models. It is found that the magnitude of the modelled radiative perturbation increases with decreasing climate sensitivity, with the exception of one outlying model. Therefore, we scale the temperature perturbation by the radiative perturbation in each model, and use the ratio between the integrated temperature perturbation and the integrated radiative perturbation as a measure of sensitivity to volcanic forcing. This ratio is found to be well correlated with the model climate sensitivity, more sensitive models having a larger ratio. Further, if this correspondence between “volcanic sensitivity” and sensitivity to CO2 forcing is a feature not only among the models, but also of the real climate system, the alleged linear relation can be used to estimate the real climate sensitivity. The observational value of the ratio signifying volcanic sensitivity is hereby estimated to correspond to an equilibrium climate sensitivity, i.e. equilibrium temperature increase due to a doubling of the CO2 concentration, between 1.7 and 4.1 K. Several sources of uncertainty reside in the method applied, and it is pointed out that additional model output, related to ocean heat storage and radiative forcing, could refine the analysis, as could reduced uncertainty in the observational record, of temperature as well as forcing.
|
|
| 2. |
- Björk, Göran, 1956, et al.
(författare)
-
On the Arctic Ocean ice thickness response to changes in the external forcing
- 2012
-
Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 39:12, s. 3007-3018
-
Tidskriftsartikel (refereegranskat)abstract
- Submarine and satellite observations show that the Arctic Ocean ice cover has undergone a large thickness reduction and a decrease in the areal extent during the last decades. Here the response of the Arctic Ocean ice cover to changes in the poleward atmospheric energy transport, F wall, is investigated using coupled atmosphere-ice-ocean column models. Two models with highly different complexity are used in order to illustrate the importance of different internal processes and the results highlight the dramatic effects of the negative ice thickness—ice volume export feedback and the positive surface albedo feedback. The steady state ice thickness as a function of F wall is determined for various model setups and defines what we call ice thickness response curves. When a variable surface albedo and snow precipitation is included, a complex response curve appears with two distinct regimes: a perennial ice cover regime with a fairly linear response and a less responsive seasonal ice cover regime. The two regimes are separated by a steep transition associated with surface albedo feedback. The associated hysteresis is however small, indicating that the Arctic climate system does not have an irreversible tipping point behaviour related to the surface albedo feedback. The results are discussed in the context of the recent reduction of the Arctic sea ice cover. A new mechanism related to regional and temporal variations of the ice divergence within the Arctic Ocean is presented as an explanation for the observed regional variation of the ice thickness reduction. Our results further suggest that the recent reduction in areal ice extent and loss of multiyear ice is related to the albedo dependent transition between seasonal and perennial ice i.e. large areas of the Arctic Ocean that has previously been dominated by multiyear ice might have been pushed below a critical mean ice thickness, corresponding to the above mentioned transition, and into a state dominated by seasonal ice.
|
|
| 3. |
- Caron, Louis-Philippe, et al.
(författare)
-
Multi-year prediction skill of Atlantic hurricane activity in CMIP5 decadal hindcasts
- 2014
-
Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 42:9-10, s. 2675-2690
-
Tidskriftsartikel (refereegranskat)abstract
- Using a statistical relationship between simulated sea surface temperature and Atlantic hurricane activity, we estimate the skill of a CMIP5 multi-model ensemble at predicting multi-annual level of Atlantic hurricane activity. The series of yearly-initialized hindcasts show positive skill compared to simpler forecasts such as persistence and climatology as well as non-initialized forecasts and return anomaly correlation coefficients of similar to 0.6 and similar to 0.8 for five and nine year forecasts, respectively. Some skill is shown to remain in the later years and making use of those later years to create a lagged-ensemble yields, for individual models, results that approach that obtained by the multi-model ensemble. Some of the skill is shown to come from persisting rather than predicting the climate shift that occur in 1994-1995. After accounting for that shift, the anomaly correlation coefficient for five-year forecasts is estimated to drop to 0.4, but remains statistically significant up to lead years 3-7. Most of the skill is shown to come from the ability of the forecast systems at capturing change in Atlantic sea surface temperature, although the failure of most systems at reproducing the observed slow down in warming over the tropics in recent years leads to an underestimation of hurricane activity in the later period.
|
|
| 4. |
- Caron, Louis-Philippe, et al.
(författare)
-
On the relationship between cloud-radiation interaction, atmospheric stability and Atlantic tropical cyclones in a variable-resolution climate model
- 2013
-
Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 40:5-6, s. 1257-1269
-
Tidskriftsartikel (refereegranskat)abstract
- We compare two 28-year simulations performed with two versions of the Global Environmental Multiscale model run in variable-resolution mode. The two versions differ only by small differences in their radiation scheme. The most significant modification introduced is a reduction in the ice effective radius, which is observed to increase absorption of upwelling infrared radiation and increase temperature in the upper troposphere. The resulting change in vertical lapse rate is then observed to drive a resolution-dependent response of convection, which in turn modifies the zonal circulation and induces significant changes in simulated Atlantic tropical cyclone activity. The resulting change in vertical lapse rate and its implication in the context of anthropogenic climate change are discussed.
|
|
| 5. |
- Caron, Louis-Philippe, et al.
(författare)
-
Understanding and simulating the link between African easterly waves and Atlantic tropical cyclones using a regional climate model : the role of domain size and lateral boundary conditions
- 2012
-
Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 39:1-2, s. 113-135
-
Tidskriftsartikel (refereegranskat)abstract
- Using a suite of lateral boundary conditions, we investigate the impact of domain size and boundary conditions on the Atlantic tropical cyclone and african easterly Wave activity simulated by a regional climate model. Irrespective of boundary conditions, simulations closest to observed climatology are obtained using a domain covering both the entire tropical Atlantic and northern African region. There is a clear degradation when the high-resolution model domain is diminished to cover only part of the African continent or only the tropical Atlantic. This is found to be the result of biases in the boundary data, which for the smaller domains, have a large impact on TC activity. In this series of simulations, the large-scale Atlantic atmospheric environment appears to be the primary control on simulated TC activity. Weaker wave activity is usually accompanied by a shift in cyclogenesis location, from the MDR to the subtropics. All ERA40-driven integrations manage to capture the observed interannual variability and to reproduce most of the upward trend in tropical cyclone activity observed during that period. When driven by low-resolution global climate model (GCM) integrations, the regional climate model captures interannual variability (albeit with lower correlation coefficients) only if tropical cyclones form in sufficient numbers in the main development region. However, all GCM-driven integrations fail to capture the upward trend in Atlantic tropical cyclone activity. In most integrations, variations in Atlantic tropical cyclone activity appear uncorrelated with variations in African easterly wave activity.
|
|
| 6. |
- Colleoni, Florence, et al.
(författare)
-
The sensitivity of the Late Saalian (140 ka) and LGM (21 ka) Eurasian ice sheets to sea surface conditions
- 2011
-
Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 37:3-4, s. 531-553
-
Tidskriftsartikel (refereegranskat)abstract
- This work focuses on the Late Saalian (140 ka) Eurasian ice sheets’ surface mass balance (SMB) sensitivity to changes in sea surface temperatures (SST). An Atmospheric General Circulation Model (AGCM), forced with two preexisting Last Glacial Maximum (LGM, 21 ka) SST reconstructions, is used to compute climate at 140 and 21 ka (reference glaciation). Contrary to the LGM, the ablation almost stopped at 140 ka due to the climatic cooling effect from the large ice sheet topography. Late Saalian SST are simulated using an AGCM coupled with a mixed layer ocean. Compared to the LGM, these 140 ka SST show an inter-hemispheric asymmetry caused by the larger ice-albedo feedback, cooling climate. The resulting Late Saalian ice sheet SMB is smaller due to the extensive simulated sea ice reducing the precipitation. In conclusion, SST are important for the stability and growth of the Late Saalian Eurasian ice sheet.
|
|
| 7. |
- Cook, Edward R., et al.
(författare)
-
Tree-ring reconstructed summer temperature anomalies for temperate East Asia since 800 CE
- 2013
-
Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 41:11-12, s. 2957-2972
-
Tidskriftsartikel (refereegranskat)abstract
- We develop a summer temperature reconstruction for temperate East Asia based on a network of annual tree-ring chronologies covering the period 800-1989 C.E. The East Asia reconstruction is the regional average of 585 individual grid point summer temperature reconstructions produced using an ensemble version of point-by-point regression. Statistical calibration and validation tests indicate that the regional average possesses sufficient overall skill to allow it to be used to study the causes of temperature variability and change over the region. The reconstruction suggests a moderately warm early medieval epoch (ca. 850-1050 C.E.), followed by generally cooler 'Little Ice Age' conditions (ca. 1350-1880 C.E.) and 20th century warming up to the present time. Since 1990, average temperature has exceeded past warm epochs of comparable duration, but it is not statistically unprecedented. Superposed epoch analysis reveals a volcanic forcing signal in the East Asia summer temperature reconstruction, resulting in pulses of cooler summer conditions that may persist for several years. Substantial uncertainties remain, however, particularly at lower frequencies, thus requiring caution and scientific prudence in the interpretation of this record.
|
|
| 8. |
- Garreta, Vincent, et al.
(författare)
-
A method for climate and vegetation reconstruction through the inversion of a dynamic vegetation model
- 2010
-
Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 1432-0894 .- 0930-7575. ; 35:2-3, s. 371-389
-
Tidskriftsartikel (refereegranskat)abstract
- Climate reconstructions from data sensitive to past climates provide estimates of what these climates were like. Comparing these reconstructions with simulations from climate models allows to validate the models used for future climate prediction. It has been shown that for fossil pollen data, gaining estimates by inverting a vegetation model allows inclusion of past changes in carbon dioxide values. As a new generation of dynamic vegetation model is available we have developed an inversion method for one model, LPJ-GUESS. When this novel method is used with high-resolution sediment it allows us to bypass the classic assumptions of (1) climate and pollen independence between samples and (2) equilibrium between the vegetation, represented as pollen, and climate. Our dynamic inversion method is based on a statistical model to describe the links among climate, simulated vegetation and pollen samples. The inversion is realised thanks to a particle filter algorithm. We perform a validation on 30 modern European sites and then apply the method to the sediment core of Meerfelder Maar (Germany), which covers the Holocene at a temporal resolution of approximately one sample per 30 years. We demonstrate that reconstructed temperatures are constrained. The reconstructed precipitation is less well constrained, due to the dimension considered (one precipitation by season), and the low sensitivity of LPJ-GUESS to precipitation changes.
|
|
| 9. |
- Graversen, Rune G., et al.
(författare)
-
Warm winds from the Pacific caused extensive Arctic sea-ice melt in summer 2007
- 2011
-
Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 36:11-12, s. 2103-2112
-
Tidskriftsartikel (refereegranskat)abstract
- During summer 2007 the Arctic sea-ice shrank to the lowest extent ever observed. The role of the atmospheric energy transport in this extreme melt event is explored using the state-of-the-art ERA-Interim reanalysis data. We find that in summer 2007 there was an anomalous atmospheric flow of warm and humid air into the region that suffered severe melt. This anomaly was larger than during any other year in the data (1989-2008). Convergence of the atmospheric energy transport over this area led to positive anomalies of the downward longwave radiation and turbulent fluxes. In the region that experienced unusual ice melt, the net anomaly of the surface fluxes provided enough extra energy to melt roughly one meter of ice during the melting season. When the ocean successively became ice-free, the surface-albedo decreased causing additional absorption of shortwave radiation, despite the fact that the downwelling solar radiation was smaller than average. We argue that the positive anomalies of net downward longwave radiation and turbulent fluxes played a key role in initiating the 2007 extreme ice melt, whereas the shortwave-radiation changes acted as an amplifying feedback mechanism in response to the melt.
|
|
| 10. |
- Graversen, Rune, et al.
(författare)
-
Greenland’s contribution to global sea-level rise by the end of the 21st century
- 2011
-
Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 37, s. 1427-1442
-
Tidskriftsartikel (refereegranskat)abstract
- The Greenland ice sheet holds enough water to raise the global sea level with *7 m. Over the last few decades, observations manifest a substantial increase of the mass loss of this ice sheet. Both enhanced melting and increase of the dynamical discharge, associated with calving at the outlet-glacier fronts, are contributing to the mass imbalance. Using a dynamical and thermodynamical ice-sheet model, and taking into account speed up of outlet glaciers, we estimate Greenland’s contribution to the 21stcentury global sea-level rise and the uncertainty of this estimate. Boundary fields of temperature and precipitation extracted from coupled climate-model projections used for the IPCC Fourth Assessment Report, are applied to the icesheet model. We implement a simple parameterization for increased flow of outlet glaciers, which decreases the bias of the modeled present-day surface height. It also allows for taking into account the observed recent increase in dynamical discharge, and it can be used for future projections associated with outlet-glacier speed up. Greenland contributes 0–17 cm to global sea-level rise by the end of the 21st century. This range includes the uncertainties in climate-model projections, the uncertainty associated with scenarios of greenhouse-gas emissions, as well as the uncertainties in future outlet-glacier discharge. In addition, the range takes into account the uncertainty of the ice-sheet model and its boundary fields.
|
|
