Bo Christiansen

Downward propagation of zonal mean zonal wind anomalies from the stratosphere to the troposphere: Model and reanalysis

The connection between the Arctic Oscillation and the stratosphere is investigated on intra-annual timescales. Both the National Centers for Environmental Prediction reanalysis data and a general circulation model simulation are used. In the winter half year November–April the dominant variability in the stratosphere in middle and high latitudes has the form of downward propagation of zonal mean zonal wind anomalies. The strength of the anomalies decays below 10 hPa, but often the anomalies reach the surface. The time for the propagation from 10 hPa to the surface is ∼15 days. When positive anomalies reach the surface, the phase of the Arctic Oscillation tends to be positive. The stratospheric variability and the downward propagation is found to be driven by the vertical component of the Eliassen-Palm flux. This flux propagates from the lower troposphere to the tropopause on a timescale of 5 days. Model and reanalysis compare well in the structure of the stratospheric variability and the connection between the stratosphere and troposphere. However, the strength of the stratospheric variability is ∼25% weaker in the model.

A Surrogate Ensemble Study of Climate Reconstruction Methods: Stochasticity and Robustness

Abstract Reconstruction of the earth’s surface temperature from proxy data is an important task because of the need to compare recent changes with past variability. However, the statistical properties and robustness of climate reconstruction methods are not well known, which has led to a heated discussion about the quality of published reconstructions. In this paper a systematic study of the properties of reconstruction methods is presented. The methods include both direct hemispheric-mean reconstructions and field reconstructions, including reconstructions based on canonical regression and regularized expectation maximization algorithms. The study will be based on temperature fields where the target of the reconstructions is known. In particular, the focus will be on how well the reconstructions reproduce low-frequency variability, biases, and trends. A climate simulation from an ocean–atmosphere general circulation model of the period a.d. 1500–1999, including both natural and anthropogenic forcings, is...

On correlations between the North Atlantic Oscillation, geopotential heights, and geomagnetic activity

We investigate correlations between geomagnetic activity indices, theNorth Atlantic Oscillation (NAO), and stratospheric geopotentialheights. It is shown that the correlation between the geomagnetic indexAp and the NAO index is high and significant since about 1970, that itis significant during winter only, that it was not significant beforeabout 1970, and that the correlations are dominated by quasi-decadalscales of variability. Analysis of the spatial pattern of correlations,restricted to the Northern Hemisphere and wintertime, shows thatsignificant correlations between Ap and sea-level pressures and betweenAp and stratospheric geopotential heights are found for the period1973-2000. However, for the period 1949-1972 no significant correlationsare found at the surface while significant correlations still are foundin the stratosphere. This might indicate that a solar forcing, primarilyacting in the stratosphere, is propagating its influence downward in thelater period but not in the earlier. (Less)

Northern Winter Climate Change: Assessment of Uncertainty in CMIP5 Projections Related to Stratosphere-Troposphere Coupling

Future changes in the stratospheric circulation could have an important impact on northern winter tropospheric climate change, given that sea level pressure (SLP) responds not only to tropospheric circulation variations but also to vertically coherent variations in troposphere-stratosphere circulation. Here we assess northern winter stratospheric change and its potential to influence surface climate change in the Coupled Model Intercomparison Project-Phase 5 (CMIP5) multimodel ensemble. In the stratosphere at high latitudes, an easterly change in zonally averaged zonal wind is found for the majority of the CMIP5 models, under the Representative Concentration Pathway 8.5 scenario. Comparable results are also found in the 1% CO2 increase per year projections, indicating that the stratospheric easterly change is common feature in future climate projections. This stratospheric wind change, however, shows a significant spread among the models. By using linear regression, we quantify the impact of tropical upper troposphere warming, polar amplification, and the stratospheric wind change on SLP. We find that the intermodel spread in stratospheric wind change contributes substantially to the intermodel spread in Arctic SLP change. The role of the stratosphere in determining part of the spread in SLP change is supported by the fact that the SLP change lags the stratospheric zonally averaged wind change. Taken together, these findings provide further support for the importance of simulating the coupling between the stratosphere and the troposphere, to narrow the uncertainty in the future projection of tropospheric circulation changes.

A Surrogate Ensemble Study of Sea Level Reconstructions

Abstract This study investigates the possibility of reconstructing past global mean sea levels. Reconstruction methods rely on historical measurements from tide gauges combined with knowledge about the spatial covariance structure of the sea level field obtained from a shorter period with spatially well-resolved satellite measurements. A surrogate ensemble method is applied based on sea levels from a 500-yr climate model simulation. Tide gauges are simulated by selecting time series from grid points along continental coastlines and on ocean islands. Reconstructions of global mean sea levels can then be compared to the known target, and the ensemble method allows an estimation of the statistical properties originating from the stochastic nature of the reconstructions. Different reconstruction methods previously used in the literature are studied, including projection and optimal interpolation methods based on EOF analysis of the calibration period. This study also includes methods where these EOFs are augm...

Reconstruction of the Extratropical NH Mean Temperature over the Last Millennium with a Method that Preserves Low-Frequency Variability

AbstractA new multiproxy reconstruction of the Northern Hemisphere extratropical mean temperature over the last millennium is presented. The reconstruction is performed with a novel method designed to avoid the underestimation of low-frequency variability that has been a general problem for regression-based reconstruction methods. The disadvantage of this method is an exaggerated high-frequency variability. The reconstruction is based on a set of 40 proxies of annual to decadal resolution that have been shown to relate to the local temperature. The new reconstruction shows a very cold Little Ice Age centered around the 17th century with a cold extremum (for 50-yr smoothing) of about 1.1 K below the temperature of the calibration period, AD 1880–1960. This cooling is about twice as large as corresponding numbers reported by most other reconstructions. In the beginning of the millennium the new reconstruction shows small anomalies in agreement with previous studies. However, the new temperature reconstructi...

Stratospheric Vacillations in a General Circulation Model

The variability in the Northern Hemisphere winter stratosphere is studied with a general circulation model run in perpetual January mode. The variability is of oscillatory nature with a timescale of approximately 100 days. Warmings appear in the upper stratosphere and descend through the stratosphere until they dissipate close to the tropopause. Warming of the upper stratosphere is accompanied by cooling of the lower stratosphere and vice versa. Experiments with time-independent tropospheres show that the vacillations originate from a stratospheric instability when driven by a constant wave forcing at the tropopause. The general circulation model experiments are discussed in the light of low-dimensional models.

Volcanic Eruptions, Large-Scale Modes in the Northern Hemisphere, and the El Niño–Southern Oscillation

Abstract The author analyzes the impact of 13 major stratospheric aerosol producing volcanic eruptions since 1870 on the large-scale variability modes of sea level pressure in the Northern Hemisphere winter. The paper focuses on the Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO) to address the question about the physical nature of these modes. The hypothesis that the phase of the El Nino–Southern Oscillation (ENSO) may control the geographical extent of the dominant mode in the Northern Hemisphere is also investigated, as well as the related possibility that the impact of the eruptions may be different according to the phase of ENSO. The author finds that both the AO and the NAO are excited in the first winter after the eruptions with statistical significance at the 95% level. Both the signal and the significance are larger for the NAO than for the AO. The excitation of the AO and the NAO is connected with the excitation of a secondary mode, which resembles an augmented Pacific–North Ame...

Evidence for Nonlinear Climate Change: Two Stratospheric Regimes and a Regime Shift

Two regimes are found in the interannual variability of the large-scale stratospheric flow in the Northern Hemisphere cold season. The regimes are identified by studying the probability distribution of the leading principal component of the geopotential height, which explains approximately 50% of the variance. The probability distribution has a bimodal structure with two clearly separated peaks corresponding to two circulation regimes. The two regimes are characterized by a strong and a weak vortex, respectively, and they therefore resemble the two phases of the stratospheric part of the Arctic Oscillation. While the upper troposphere and the lower stratosphere are colder in the strong vortex regime than in the weak vortex regime, the lower troposphere is warmer—in particular over the continents. An abrupt regime shift took place in the last half of the 1970s in favor of the more zonal regime. The shift is manifested by a substantial change in the frequencies of the two regimes. Strong statistical significance for the two separate regimes is obtained by a Monte Carlo approach. The regimes and the regime shift are found in two different datasets, reducing the possibility that the results are due to inhomogeneities in the data. The results support the nonlinear dynamical perspective on climate change suggested by T. N. Palmer. According to this idea the response to a weak forcing would be seen mainly in a change of the frequencies of the climate regimes, while the spatial structure of the regimes would be relatively insensitive to the forcing.

Tropospheric response to stratospheric ozone loss

The response to realistic total column ozone trends on the troposphere and the stratosphere as simulated by the ARPEGE General Circulation Model (GCM) has been investigated. In both hemispheres, the lower stratosphere cooled and the polar vortex strengthened significantly during spring/early summer. The cooling trend was weaker than the observed trend in the Northern Hemisphere (NH), but stronger than the observed trend in the Southern Hemisphere (SH). In the troposphere, the changes in geopotential height resembled the positive phase of the Arctic Oscillation (AO) in the NH in March and the positive phase of the Antarctic Oscillation (AAO) in the SH during summer (December–February).