Geomagnetic activity forcing of the Northern Annular Mode via the stratosphere (original) (raw)
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Geomagnetic perturbations on stratospheric circulation in late winter and spring
Journal of Geophysical Research, 2008
1] This study investigates if the descent of odd nitrogen, generated in the thermosphere and the upper mesosphere by energetic particle precipitation (EPP-NO x ), has a detectable impact on stratospheric wind and temperature in late winter and spring presumably through the loss of ozone and reduction of absorption of solar UV. In both hemispheres, similar downward propagating geomagnetic signals in the extratropical stratosphere are found in spring for those years when no stratospheric sudden warming occurred in mid-winter. Anomalous easterly winds and warmer polar regions are found when the 4-month averaged winter Ap index (A p ) is high, and the signals become clearer when solar F10.7 is low. In May, significant geomagnetic signals are obtained in the Northern Hemisphere when the data are grouped according to the phase of the stratospheric equatorial QBO. The magnitudes of changes in spring stratospheric wind and temperatures associated with A p signals are in the range of 10-20 m s À1 and 5-10 K, which are comparable with those of the 11-yr SC signals typically found in late winter. The spring A p signals show the opposite sign to that expected due to in situ cooling effects caused by catalytic destruction of stratospheric ozone by descending EPP-NO x . Thus it is unlikely that the in situ chemical effect of descending EPP-NO x on stratospheric ozone would have a dominant influence on stratospheric circulation. Instead, we suggest that the detected A p signals in the extratropical spring stratosphere may be an indirect consequence of geomagnetic and solar activity, dynamically induced by changes in wave ducting conditions.
Atmospheric Chemistry and Physics, 2011
The atmospheric chemistry general circulation model ECHAM5/MESSy is used to simulate polar surface air temperature effects of geomagnetic activity variations. A transient model simulation was performed for the years 1960-2004 and is shown to develop polar surface air temperature patterns that depend on geomagnetic activity strength, similar to previous studies. In order to eliminate influencing factors such as sea surface temperatures (SST) or UV variations, two nine-year long simulations were carried out, with strong and weak geomagnetic activity, respectively, while all other boundary conditions were held to year 2000 levels. Statistically significant temperature effects that were observed in previous reanalysis and model results are also obtained from this set of simulations, suggesting that such patterns are indeed related to geomagnetic activity. In the model, strong geomagnetic activity and the associated NO x (= NO + NO 2 ) enhancements lead to polar stratospheric ozone loss. Compared with the simulation with weak geomagnetic activity, the ozone loss causes a decrease in ozone radiative cooling and thus a temperature increase in the polar winter mesosphere. Similar to previous studies, a cooling is found below the stratopause, which other authors have attributed to a decrease in the mean meridional circulation. In the polar stratosphere this leads to a more stable vortex. A strong (weak) Northern Hemisphere vortex is known to be associated with a positive (negative) Northern Annular Mode (NAM) index; our simulations exhibit a positive NAM index for strong ge-Correspondence to: A. J. G. Baumgaertner (work@andreas-baumgaertner.net) omagnetic activity, and a negative NAM for weak geomagnetic activity. Such NAM anomalies have been shown to propagate to the surface, and this is also seen in the model simulations. NAM anomalies are known to lead to specific surface temperature anomalies: a positive NAM is associated with warmer than average northern Eurasia and colder than average eastern North Atlantic. This is also the case in our simulation. Our simulations suggest a link between geomagnetic activity, ozone loss, stratospheric cooling, the NAM, and surface temperature variability. Further work is required to identify the precise cause and effect of the coupling between these regions.
The summertime annular mode in the Northern Hemisphere and its linkage to the winter mode
Journal of Geophysical Research, 2004
1] The seasonal variations of the Northern Hemisphere annular mode (NAM) are investigated through empirical orthogonal function analysis of the zonally averaged geopotential height fields for each individual calendar month. Patterns of the winter and summer NAMs differ not only in the geopotential height fields but also in the mean meridional circulation and eddy structure. The summer NAM has a smaller meridional scale and is displaced poleward as compared to the winter NAM. The antinode on the lower-latitude side in the summer NAM is at the nodal latitude of the winter NAM. The summer NAM is more strongly related to surface air temperatures over Eurasia than the original Arctic Oscillation. The summer NAM is a wave-driven internal atmospheric mode that is maintained by both stationary and transient waves. The summer NAM is associated with the Arctic front, polar jet, and storm track around the Arctic Ocean. The winter-to-summer linkage described by M. Ogi et al. can be interpreted as a preferred transition from one polarity of the winter annular mode to the same polarity of the summer annular mode. The spring cryosphere, i.e., snow in Eurasia and sea ice in the Barents Sea, plays a supporting role in this transition.
2004
Influence of El Niño/Southern Oscillation (ENSO) on the stratosphere-troposphere dynamical linkage during stratospheric sudden warming (SSW) events is investigated based upon the composite analysis using 44 winter record of NCEP-NCAR reanalysis data. Stratospheric waves with zonal wavenumber 1 (2), which are linked with tropospheric teleconnections over the Pacific-North America-Atlantic sector (the Eurasian sector), are important for the onset of SSWs during the warm (cold) phase of ENSO. During the cold phase, easterly anomalies associated with SSWs penetrate into the troposphere, and annular-mode-like height anomalies appear subsequently in the troposphere. On the other hand, during the warm phase, easterly anomalies are restricted in the stratosphere, and zonally asymmetric height anomalies are observed in the troposphere about 10 days after the warming peak in the stratosphere. Modulation of storm track activities due to lower stratospheric anomalies is found to be important for the downward penetration of zonal wind anomalies into the troposphere.
Journal of Geophysical Research: Space Physics, 2019
The magnetic field records of the magnetometer networks in the American, East Asian-Australian, and European-African sectors were employed in this present work. We used them to investigate equatorial electrojet (EEJ), counter electrojet (CEJ), tidal variability in EEJ strength and ionospheric current during the 2005/2006 and 2008/2009 sudden stratospheric warming (SSW) events. In addition to the well-investigated tidal variability in EEJ strength over the American and East Asian sectors, we investigated that of the African sector for the first time. Interestingly, the tidal components in EEJ strength during both SSW events clearly exhibit marked longitudinal differences with high, moderate, and low amplitudes in the American, East Asian, and African sectors, respectively. An exception found around day 71 in the African sector after the 2008/2009 SSW event had higher solar diurnal tidal component as compared to that of the Asian sector. Over the American sector, solar and lunar semidiurnal tides were strongly associated with CEJ current during both SSW events, whereas at the African and East Asian sectors such variabilities are not evident. A solar diurnal tidal component was strongly related to a reduction in the EEJ strength over the East Asian sector. In addition, a prolonged period of CEJ occurrence that begins during the SSW precondition and ends when the SSW was evolving characterized the African sector during both SSW events. There is a steady shift in phase at later hours when both SSW events are evolving.
Stratospheric variability and tropospheric annular-mode timescales
Geophysical Research Letters, 2011
Climate models tend to exhibit much too persistent Southern Annular Mode (SAM) circulation anomalies in summer, compared to observations. Theoretical arguments suggest this bias may lead to an overly strong model response to anthropogenic forcing during this season, which is of interest since the largest observed changes in Southern Hemisphere high-latitude climate over the last few decades have occurred in summer, and are congruent with the SAM. The origin of this model bias is examined here in the case of the Canadian Middle Atmosphere Model, using a novel technique to quantify the influence of stratospheric variability on tropospheric annular-mode timescales. Part of the model bias is shown to be attributable to the too-late breakdown of the stratospheric polar vortex, which allows the tropospheric influence of stratospheric variability to extend into early summer. However, the analysis also reveals an enhanced summertime persistence of the model's SAM that is unrelated to either stratospheric variability or the bias in model stratospheric climatology, and is thus of tropospheric origin. No such feature is evident in the Northern Hemisphere. The effect of stratospheric variability in lengthening tropospheric annular-mode timescales is evident in both hemispheres. While in the Southern Hemisphere the effect is restricted to late-spring/early summer, in the Northern Hemisphere it can occur throughout the winter-spring season, with the seasonality of peak timescales exhibiting considerable variability between different 50 year sections of the same simulation.
Bridging the annular mode and North Atlantic Oscillation paradigms
Journal of Geophysical …, 2007
1] The annular nature of the leading patterns of the Northern Hemisphere winter extratropical circulation variability is revisited. The analysis relies on principal component analysis (PCA) of tropospheric geopotential height fields and lagged correlations with the stratospheric polar vortex strength and with a proxy of midlatitude tropospheric zonal mean zonal momentum anomalies. Results suggest that two processes, occurring at different times, contribute to the Northern Annular Mode (NAM) spatial structure. Polar vortex anomalies appear to be associated with midlatitude tropospheric zonal mean zonal wind anomalies occurring before the stratospheric anomalies. After the polar vortex anomalies, zonal mean zonal wind anomalies of the same sign are observed in the troposphere at high latitudes. The timescale separation between the two signals is about 2 weeks. It is suggested that the leading tropospheric variability patterns found in the literature represent variability associated with both processes. The tropospheric variability patterns which appear to respond to the polar vortex variability have a hemispheric scale but show a dipolar structure only over the Atlantic basin. The dipole resembles the North Atlantic Oscillation pattern (NAO), but with the node line shifted northward.
Geomagnetic activity signatures in wintertime stratosphere wind, temperature, and wave response
We analyzed ERA-40 and ERA Interim meteorological re-analysis data for 4 signatures of geomagnetic activity in zonal mean zonal wind, temperature, 5 and Eliassen-Palm flux in the Northern Hemisphere extended winter (November-6 March). We found that for high geomagnetic activity levels the stratospheric 7 polar vortex becomes stronger in late winter, with more planetary waves be-8 ing refracted equatorward. The statistically significant signals first appear 9 in December and continue until March, with poleward propagation of the 10 signals with time, even though some uncertainty remains due to the limited 11 amount of data available (∼50 years). Our results also indicated that the ge-12 omagnetic effect on planetary wave propagation has a tendency to take place 13 when the stratosphere background flow is relatively stable, or when the po-14 lar vortex is stronger and less disturbed in early winter. These conditions typ-15 ically occur during high solar irradiance cycle conditions, or westerly Quasi-16 Biennial Oscillation conditions.