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  • 1
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    Stratospheric Response to the 11-Yr Solar Cycle: Breaking Planetary Waves, Internal Reflection, and Resonance (2017)
    American Meteorological Society
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    Publication Date: 2017-08-08
    Description: Breaking planetary waves (BPWs) affect stratospheric dynamics by reshaping the waveguides, causing internal wave reflection, and preconditioning sudden stratospheric warmings. This study examines observed changes in BPWs during the northern winter resulting from enhanced solar forcing and the consequent effect on the seasonal development of the polar vortex. During the period 1979–2014, solar-induced changes in BPWs were first observed in the uppermost stratosphere. High solar forcing was marked by sharpening of the potential vorticity (PV) gradient at 30°–45°N, enhanced wave absorption at high latitudes, and a reduced PV gradient between these regions. These anomalies instigated an equatorward shift of the upper-stratospheric waveguide and enhanced downward wave reflection at high latitudes. The equatorward refraction of reflected waves from the polar upper stratosphere then led to enhanced wave absorption at 35°–45°N and 7–20 hPa, indicative of a widening of the midstratospheric surf zone. The stratospheric waveguide was thus constricted at about 45°–60°N and 5–10 hPa in early boreal winter; reduced upward wave propagation through this region resulted in a stronger upper-stratospheric westerly jet. From January, the regions with enhanced BPWs acted as “barriers” for subsequent upward and equatorward wave propagation. As the waves were trapped within the stratosphere, anomalies of zonal wavenumbers 2 and 3 were reflected poleward from the stratospheric surf zone. Resonant excitation of some of these reflected waves resulted in rapid growth of wave disturbances and a more disturbed polar vortex in late winter. These results provide a process-oriented explanation for the observed solar cycle signal. They also highlight the importance of nonlinearity in the processes that drive the stratospheric response to external forcing.
    Print ISSN: 0894-8755
    Electronic ISSN: 1520-0442
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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  • 2
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    Downward Wave Reflection as a Mechanism for the Stratosphere–Troposphere Response to the 11-Yr Solar Cycle (2017)
    American Meteorological Society
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    Publication Date: 2017-01-20
    Print ISSN: 0894-8755
    Electronic ISSN: 1520-0442
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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  • 3
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    A Comparative Study of Wave Forcing Derived from the ERA-40 and ERA-Interim Reanalysis Datasets (2015)
    American Meteorological Society
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    Publication Date: 2015-03-13
    Description: The Eliassen–Palm (E-P) flux divergences derived from ERA-40 and ERA-Interim show significant differences during northern winter. The discrepancies are marked by vertically alternating positive and negative anomalies at high latitudes and are manifested via a difference in the climatology. The magnitude of the discrepancies can be greater than the interannual variability in certain regions. These wave forcing discrepancies are only partially linked to differences in the residual circulation but they are evidently related to the static stability in the affected regions. Thus, the main cause of the discrepancies is most likely an imbalance of radiative heating. Two significant sudden changes are detected in the differences between the eddy heat fluxes derived from the two reanalyses. One of the changes may be linked to the bias corrections applied to the infrared radiances from the NOAA-12 High-Resolution Infrared Radiation Sounder in ERA-40, which is known to be contaminated by volcanic aerosol from the 1991 eruption of Mt. Pinatubo. The other change may be due in part to the use of uncorrected radiances from the NOAA-15 Advanced Microwave Sounding Units by ERA-Interim since 1998. These sudden changes have the potential to alter the wave forcing trends in the affected reanalysis, suggesting that extreme care is needed when one comes to extract trends from the highly derived wave forcing quantities.
    Print ISSN: 0894-8755
    Electronic ISSN: 1520-0442
    Topics: Geography , Geosciences , Physics
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  • 4
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    Dynamical Response to the QBO in the Northern Winter Stratosphere: Signatures in Wave Forcing and Eddy Fluxes of Potential Vorticity (2015)
    American Meteorological Society
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    Publication Date: 2015-11-19
    Description: Wave–mean flow interactions associated with the Holton–Tan effect (HTE), whereby the tropical quasi-biennial oscillation (QBO) modulates the Northern Hemisphere wintertime stratospheric polar vortex, are studied using the ERA-Interim dataset. Significant evidence of the HTE in isentropic coordinates is found, with a weaker and warmer polar vortex present when the lower-stratospheric QBO is in its easterly phase (QBOe). For the first time, the authors quantify the QBO modulation of wave propagation, wave–mean flow interaction, and wave decay/growth via a calculation of potential vorticity (PV)-based measures, the zonal-mean momentum budget, and up-/downgradient eddy PV fluxes. The effect of the tropospheric subtropical jet on QBO modulation of the wave activity is also investigated. In the subtropical-to-midlatitude lower stratosphere, QBOe is associated with an enhanced upward flux of wave activity, and corresponding wave convergence and wave growth, which leads to a stronger poleward zonal-mean meridional circulation and consequently a warmer polar region. In the middle stratosphere, QBOe is associated with increased poleward wave propagation, leading to enhanced wave convergence and in situ wave growth at high latitudes and contributing to the weaker polar vortex. In agreement with recent studies, the results suggest that the critical-line effect cannot fully account for these wave anomalies associated with the HTE. Instead, it is suggestive of a new, additional mechanism that hinges on the QBO-induced meridional circulation effect on the latitudinal positioning of the subtropical jet. Under QBOe, the QBO-induced meridional circulation causes a poleward shift of the subtropical jet, encouraging more waves to propagate into the stratosphere at midlatitudes.
    Print ISSN: 0022-4928
    Electronic ISSN: 1520-0469
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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  • 5
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    Inhomogeneity of the surface air temperature record from Halley, Antarctica (2021)
    American Meteorological Society
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    Publication Date: 2021-03-18
    Description: Commencing in 1956, observations made at Halley Research Station, Antarctica provide one of the longest continuous series of near-surface temperature observations from the Antarctic continent. Since few other records of comparable length are available, the Halley record has been used extensively in studies of long-term Antarctic climate variability and change. The record does not, however, come from a single location but is a composite of observations from a sequence of seven stations, all situated on the Brunt Ice Shelf, that range from around 10 km to 50 km distance from the coast. Until now, it has generally been assumed that temperature data from all of these stations could be combined into a single composite record with no adjustment. Here, we examine this assumption of homogeneity. Application of a statistical change point algorithm to the composite record detects a sudden cooling associated with the move from Halley IV to Halley V station in 1992. We show that this temperature step is consistent with local temperature gradients measured by a network of automatic weather stations and with those simulated by a high-resolution atmospheric model. These temperature gradients are strongest in the coastal region and result from the onshore advection of maritime air. The detected inhomogeneity could account for the weak cooling trend seen in the uncorrected composite record. In future, studies that make use of the Halley record will need to account for its inhomogeneity.
    Print ISSN: 0894-8755
    Electronic ISSN: 1520-0442
    Topics: Geography , Geosciences , Physics
    Published by American Meteorological Society
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