Hindcasting the NAO using diabatic forcing of a simple AGCM (original) (raw)
Related papers
Tropical/Extratropical forcing of the AO/NAO: A corrigendum
Geophysical Research Letters, 2003
In two of our previous papers, we described the separate influence of tropical and extratropical forcing on the North Atlantic (NAO) and the Arctic (AO) Oscillations. Here, we point out a problem with the way the split between tropical and extratropical forcing was carried out. In the corrected model results, we find that extratropical forcing dominates tropical forcing in accounting for interannual variance in both the observed NAO and AO indices. We find that the recent upward trend in the NAO index is driven from the tropics, whereas for the AO index, we now find that extratropical forcing also contributes to the upward trend in the model. It follows that our corrected model results do not support a strong link between tropical forcing and the interannual variability of the wintertime AO, as claimed previously.
Internal variability, external forcing and climate trends in multi-decadal AGCM ensembles
Climate Dynamics, 2004
An atmospheric general circulation model of intermediate complexity is used to investigate the origin and structure of the climate change in the second half of the twentieth century. The variability of the atmospheric flow is considered as a superposition of an internal part, due to intrinsic dynamical variability, and an external part, due to the variations of the sea surface temperature (SST) forcing. The two components are identified by performing a 50-member ensemble of atmospheric simulations with prescribed, observed SSTs in the period 1949–2002. The large number of realizations allows the estimation of statistics of the atmospheric variability with a high confidence level. The analysis performed focuses on interdecadal and interannual variability of 500 hPa geopotential height in the Northern Hemisphere (NH) during winter. The model reproduces well the structure of the observed trend (defined as the difference in the two 25-year intervals 1977–2001 and 1952–1976), particularly in the Pacific region, and about half of the amplitude of the signal. The trend in 500 hPa height projects mainly onto the second empirical orthogonal function (EOF), both in the observations and in the model ensemble. However, differences between the modelled and the observed variability are found in the pattern of the second EOF in the Atlantic sector. SST changes associated with the El Niño southern oscillation (ENSO) are responsible for about 50% of the signal of the 500 hPa height trend in the Pacific. A second 50-member ensemble is used to evaluate the sensitivity of interdecadal variability to an increase in CO2 optical depth compatible with observed concentration changes. In this second experiment, the simulated trend includes a statistically significant contribution from the positive phase of the Arctic oscillation (AO). Such a contribution is also found in observations. Furthermore, the additional CO2 forcing accounts for part of the NH trend in near-surface temperature, and brings the zonal-mean temperature changes in the stratosphere and upper-troposphere closer to observations.
Global SST influence on twentieth century NAO variability
Climate Dynamics, 2003
Recent studies have suggested that sea surface temperature (SST) is an important source of variability of the North Atlantic Oscillation (NAO). Here, we deal with four basic aspects contributing to this issue: (1) we investigate the characteristic time scales of this oceanic influence; (2) quantify the scale-dependent hindcast potential of the NAO during the twentieth century as derived from SST-driven atmospheric general circulation model (AGCM) ensembles; (3) the relevant oceanic regions are identified, corresponding SST indices are defined and their relationship to the NAO are evaluated by means of cross spectral analysis and (4) our results are compared with long-term coupled control experiments with different ocean models in order to ensure whether the spectral relationship between the SST regions and the NAO is an intrinsic mode of the coupled climate system, involving the deep ocean circulation, rather than an artefact of the unilateral SST forcing. The observed year-to-year NAO fluctuations are barely influenced by the SST. On the decadal time scales the major swings of the observed NAO are well reproduced by various ensembles from the middle of the twentieth century onward, including the negative state in the 1960s and part of the positive trend afterwards. A six-member ECHAM4-T42 ensemble reveals that the SST boundary condition affects 25% of total decadal-mean and interdecadal-trend NAO variability throughout the twentieth century. The most coherent NAO-related SST feature is the well-known North Atlantic tripole. Additional contributions may arise from the southern Pacific and the low-latitude Indian Ocean. The coupled climate model control runs suggest only the North Atlantic SST-NAO relationship as being a true characteristic of the coupled climate system. The coherence and phase spectra of observations and coupled simulations are in excellent agreement, confirming the robustness of this decadalscale North Atlantic air-sea coupled mode.
Characteristics of the Recent Eastward Shift of Interannual NAO Variability
Journal of Climate, 2003
Recent observational studies have shown that the centers of action of interannual variability of the North Atlantic Oscillation (NAO) were located farther eastward during winters of the period 1978-97 compared to previous decades of the twentieth century. In this study, which focuses on the winter season (December-March), new diagnostics characterizing this shift are presented. Further, the importance of this shift for NAO-related interannual climate variability in the North Atlantic region is discussed. It is shown that an NAO-related eastward shift in variability can be found for a wide range of different parameters like the number of deep cyclones, near-surface air temperature, and turbulent surface heat flux throughout the North Atlantic region. By using a near-surface air temperature dataset that is homogenous with respect to the kind of observations used, it is shown that the eastward shift is not an artifact of changes in observational practices that took place around the late 1970s. Finally, an EOF-based Monte Carlo test is developed to quantify the probability of changes in the spatial structure of interannual NAO variability for a relatively short (20 yr) time series given multivariate ''white noise.'' It is estimated that the likelihood for differences in the spatial structure of the NAO between two independent 20-yr periods, which are similar (as measured by the angle and pattern correlation between two NAO patterns) to the observed differences, to occur just by chance is about 18%. From the above results it is argued that care has to be taken when conclusions about long-term properties of NAO-related climate variability are being drawn from relatively short recent observational data (e.g., 1978-97).
Geophysical Research Letters, 2001
Recent studies claim that useful predictability of the North Atlantic Oscillation (NAG) up to several years in advance may be possible using atmospheric models in which the sea surface temperature (SST) is specified from observations. Achieving this goal requires that such models adequately capture the observed variation in the NAG at interannual as well as interdecadal timescales. We investigate whether this is the case by comparing interannual variability in the HadAM3 atmospheric model with observations in the SGC air-sea flux dataset for 1980-1995.
Advances in Space Research, 2007
General atmospheric circulation is the system of atmospheric motions over the Earth on the scale of the whole globe. Two main types of circulation have been identified: zonal-characterized by low amplitude waves in the troposphere moving quickly from west to east, and meridional with stationary high amplitude waves when the meridional transfer is intensified. The prevailing type of circulation is related to global climate. Based on many years of observations, certain "circulation epochs" have been defined when the same type of circulation prevails for years or decades. Here we study the relation between long-term changes in solar activity and prevailing type of atmospheric circulation, using NAO index reconstructed for the last four centuries as a proxy for large-scale atmospheric circulation. We find that when the southern solar hemisphere is more active, increasing solar activity in the secular solar cycle results in increasing zonality of the circulation, while when the northern solar hemisphere is more active, increasing solar activity increases meridional circulation. In an attempt to explain the observations, we compare the shortterm reaction of NAO and NAM indices to different solar drivers: powerful solar flares, high speed solar wind streams, and magnetic clouds.
Who controls the long-term NAO variability?
2019
The regionality of climate response to the homogeneous external forcing (i.e. solar variability and enhanced density of greenhouse gases) is one of the challenging frontiers of climate research. A reference to the existing climatic modes, as factors modifying regionally climate changes, simply raises a new question – What are the factors deter- mining internal climatic modes? We have conducted a comparative analysis of NAO and lower stratospheric ozone variability, which reveals their covariance during last century (1900-2010). We show that observed coherence is their spatial-temporal evolution is due to the ozone’s influence on the surface temperature and pressure, which determine the alternating multi-decadal changes of NAO phase. In addition, the factors determining ozone’s centennial variability will be also discussed.