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A. G. Marshall, A. A. Scaife, and S. Ineson

vortex leads to increased refraction of planetary waves, which in turn decreases the deceleration of the vortex (e.g., Stenchikov et al. 2006 ). Thus, the impact of volcanic aerosols leads to an enhanced positive phase of the Arctic Oscillation (AO) and associated North Atlantic Oscillation (NAO; Thompson and Wallace 1998 ) that is most prominent in boreal winter and persists for up to 2 yr after each eruption (e.g., Robock and Mao 1992 ; Stenchikov et al. 2006 ). The observed AO response to

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Florence Colleoni, Simona Masina, Annalisa Cherchi, and Doroteaciro Iovino

between K115 and K229 (cf. Fig. 5l with Fig. 5f ). While GHG have a homogeneous impact on Northern Hemisphere mean annual temperature, the impact of orbital parameters shows a larger spatial variability ( Figs. 5g–i ). Groll et al. (2005) and Groll and Widmann (2006) suggest that the orbital configuration of the simulated time period determines the structure of temperature-related teleconnections in the northern high latitudes, specifically the Arctic Oscillation. According to Thompson and

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Christopher J. Goodman and Jennifer D. Small Griswold

environmental conditions that impact DA. By understanding which airports are likely to see changes in the expected DAs as a result of El Niño–Southern Oscillation (ENSO) and the Arctic Oscillation (AO), scheduling of aircraft, especially those with heavy payloads, could be altered to take off or land during lower DA times, adjust the payload of flights, utilize aircraft with better performance, or plan for increased or decreased delays. 2. Global Impacts of ENSO and AO on density altitude a. El Niño

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Min Wen, Song Yang, Arun Kumar, and Peiqun Zhang

monsoon and the temperature and precipitation over China in winter have also been linked to the influence of the Arctic Oscillation (AO; Thompson and Wallace 1998 ). When the AO is positive, the monsoon is weak and temperature and precipitation increase over China ( Wu and Wang 2002 ; Gong and Wang 2003 ; Jeong and Ho 2005 ; Chen and Kang 2006 ). While the increase in precipitation occurs over a large part of the country, the warming is mainly limited to northern and northeastern China (e

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Jin-Song von Storch

1. Introduction Thompson and Wallace (1998 , 2000) found that the primary modes of the geopotential height field in each hemisphere are remarkably similar. Both modes, referred to as the Antarctic and the Arctic Oscillations (AAO and AO), are related to meridional dipoles in zonal-mean zonal wind field and to seesaws in atmospheric mass in the polar regions and the low-latitude zonal rings. While the similarity between the primary modes in the two hemispheres became more apparent in the

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Ran Zhang, Jiabei Fang, and Xiu-Qun Yang

-day lead ( Figs. 2a,f ), featuring a negative-phase Arctic Oscillation (AO) pattern ( Thompson and Wallace 1998 ). With time evolution (lead time from day −80 to day 0), the regional negative (positive) geopotential height anomaly centers over both North Pacific and southeast North America (northwest North America and around Hawaii) become clear, resembling a positive-phase Pacific–North American (PNA) pattern. Meanwhile, a negative-phase North Atlantic Oscillation (NAO) anomaly pattern gradually

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Ignatius G. Rigor, John M. Wallace, and Roger L. Colony

Oscillation (AO), whose index is defined as the leading principal component (PC) of Northern Hemisphere SLP ( Thompson and Wallace 1998 ). The AO can be characterized as an exchange of atmospheric mass between the Arctic Ocean and the surrounding zonal ring centered ∼45°N. The observed trend in the AO toward its “high index” polarity (i.e., toward stronger westerlies at subpolar latitudes and lower SLP over the Arctic) is a way of interpreting the observed decrease in SLP over the North Pole and the

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XiaoJing Jia, Hai Lin, and Xia Yao

regions nearby ( Rogers 1990 ; Hurrell 1995 , 1996 ). Although there exists some debate, many regard NAO as a regional display of the Arctic Oscillation (AO), which was defined first by Thompson and Wallace (1998) . It is well accepted that the AO/NAO is, to a significant degree, an internal mode of variability of the atmospheric circulation. The spatial structure and amplitude of the AO/NAO can be well simulated in atmospheric general circulation models (AGCM) forced with fixed external forcing

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Marika M. Holland

appear to be interrelated and to be associated with variability and trends in the North Atlantic Oscillation (NAO) or the closely related Arctic Oscillation (AO; Thompson and Wallace 1998 ). The North Atlantic Oscillation is the dominant mode of variability in the North Atlantic region and represents a redistribution of atmospheric mass between centers of action located near the Azores high and the Icelandic low. A high NAO phase indicates a strengthening of the Azores high and the Icelandic low

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Maria Flatau and Young-Joon Kim

1. Introduction The Madden–Julian oscillation (MJO) is primarily a tropical phenomenon, with its influence extending to the extratropics through the teleconnection reaching the polar regions in both hemispheres. This interaction appears to be stronger in the Northern Hemisphere (NH), where the largest MJO amplitudes coincide with the winter circulation. There is some evidence that the MJO influences the southern as well as northern polar regions. An MJO influence on the Arctic Oscillation (AO

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