Propagation of the Arctic Oscillation from the stratosphere to the troposphere

Abstract

Abstract. Geopotential anomalies ranging from the Earth's surface to the middle stratosphere in the northern hemisphere are dominated by a mode of variability known as the Arctic Oscillation (AO). The AO is represented herein by the leading mode (the first empirical orthogonal function) of low-frequency variability of wintertime geopotential between 1000 and 10 hPa. In the middle stratosphere the signature of the AO is a nearly zonally symmetric pattern representing a strong or weak polar vortex. At 1000 hPa the AO is similar to the North Atlantic Oscillation, but with more zonal symmetry, especially at high latitudes. In zonalmean zonal wind the AO is seen as a north-south dipole centered on 40ø-45øN; in zonalmean temperature it is seen as a deep warm or cold polar anomaly from the upper troposphere to --10 hPa. The association of the AO pattern in the troposphere with modulation of the strength of the stratospheric polar vortex provides perhaps the best measure of coupling between the stratosphere and the troposphere. By examining separately time series of AO signatures at tropospheric and stratospheric levels, it is shown that AO anomalies typically appear first in the stratosphere and propagate downward. The midwinter correlation between the 90-day low-pass-filtered 10-hPa anomaly and the 1000-hPa anomaly exceeds 0.65 when the surface anomaly time series is lagged by about three weeks. The tropospheric signature of the AO anomaly is characterized by substantial changes to the storm tracks and strength of the midtropospheric flow, especially over the North Atlantic and Europe. The implications of large stratospheric anomalies as precursors to changes in tropospheric weather patterns are discussed.

Reduced North Atlantic Central Water formation in response to early Holocene ice‐sheet melting

“…Today, an augmented stratospheric circulation strengthens the positive mode of the Arctic Oscillation [Baldwin and Dunkerton, 1999;Shindell et al, 2001], augments the vigor and size of the SPG [Häkkinen and Rhines, 2004], and thus the export of fresher and cooler subpolar waters to the SPMW formation region in the eastern SPG. Assuming this link can be applied to longer timescales, a stronger polar vortex would also have led to an intensification of the Trade Winds.…”

Residence time distributions of the Northern Annular Mode

“…The solid lines in the lower panels mark the 1σ significance level of the differences. This figure is available in colour online at www.interscience.wiley.com/ijoc circulation (Thompson and Wallace, 1998;Baldwin and Dunkerton, 1999) thus affecting the NAM. In our study, the 'high' and 'low' solar activity are separated by the median value of the solar F10.7 cm flux.…”

“…We calculate the durations of their positive and negative phases using the daily NAM indices calculated for 17 atmospheric heights (1000-10 hPa) for 1958-1997 using the NCEP Reanalysis data (see Baldwin and Dunkerton (1999), the later extension to 2006 is made by M. Baldwin.)…”

“…Its time behavior is characterized by the NAM indices defined as principal components of the first EOFs of the geopotential height anomalies at every atmospheric level (Baldwin and Dunkerton, 1999). Feldstein (2000) has interpreted the principal components of the North Atlantic Oscillation (NAO) in the troposphere at 300 hPa, equivalent to the NAM index at that height, as describing a highfrequency stochastic process subjected to a weak external forcing.…”

Evidence from Wavelet Lag Coherence for Negligible Solar Forcing of Climate at Multi-year and Decadal Periods

“…Kodera and Kuroda (2002) interpreted re-analyses data from 1979 to 1998 and proposed a mechanism for the dynamical and radiative forcing of the stratosphere by the solar cycle, while the analysis is provocative, there must be doubt to its statistical robustness as less than two whole solar cycles are included in the data set. While it is clear that stratospheric anomalies can penetrate downwards to the troposphere, it is a rather atypical phenomenon (Baldwin and Dunkerton 1999;2001), and in general the troposphere drives the stratosphere. However, it is clear that from both observational and modeling studies that the stratosphere can provide an efficient and fast transport mechanism for linking tropical and polar climate (Baldwin and Dunkerton 2005;Jevrejeva et al 2004), thus the stratosphere provides a bridge between the annular modes and ENSO phenomena, and so we may expect it be one factor that it is especially sensitive to the solar cycle.…”