On How the Middle Atmospheric Residual Circulation Responds to the Solar Cycle Close to the Solstices

Abstract

During high solar activity, the atmosphere receives more energy from the sun, particularly in the form of shortwave radiation. Most notable is the effect in the middle and upper atmosphere, which in general shows a positive temperature response due to physical and chemical processes that are intensified at high solar activity. It is thus surprising that a clear solar cycle signal is absent in the summer polar mesosphere region in spite of it being illuminated around the clock. In this study, it is investigated how the circulation in the summer mesosphere is affected by changes in the solar flux using a 30-yr run from the nudged version of the Canadian Middle Atmosphere Model (CMAM30). It is found that—in July—the solar cycle signal from direct solar heating is counteracted by an enhanced residual circulation, which adiabatically cools the region at a higher rate when the solar activity is above average. The dynamical cooling is partly initiated in the Southern Hemisphere winter stratosphere.

The Missing Solar Cycle Response of the Polar Summer Mesosphere

“…The temperature response is significant in the upper mesosphere during July and January (Figures a and b), with the exception of a slightly reduced solar response at the highest latitudes in summer. Model studies (Gan et al, ; Karlsson & Kuilman, ; Qian et al, ) have shown a far stronger reduction in the T‐Lα correlation near the polar summer mesopause than is indicated by the observations in Figure , a discrepancy that is currently not understood. The H 2 O‐Lα anticorrelation is significant in the upper mesosphere during July and January (Figures c and d), except for latitudes poleward of ~55° where a weak positive correlation appears.…”

“…Future efforts will benefit from model studies to verify the patterns reported here and understand their underlying causes. Given the model results (e.g., Karlsson & Kuilman, ) indicating dynamical feedbacks that damp out solar cycle influences in the polar summer mesosphere, the greater mystery may be the seemingly large solar cycle response before ~2002 and not the recent absence of one. In this regard, there are two relevant but untested suggestions in the literature.…”

“…This seasonal difference was confirmed by model simulations and attributed to intensified upwelling in the polar summer mesosphere during solar maximum. The model results of Karlsson and Kuilman () confirm a stronger mesospheric residual circulation at solar maximum and show that this is due to changes in stratospheric zonal winds which affect the propagation of gravity waves.…”

“…Despite this steady correlation, the solar cycle change in T (ΔT SS ) is reduced by ~40%, with HALOE indicating ΔT SS at 68°N (68°S) of 4.2 (5.9) K compared to 2.5 (3.8) K for the later years. Karlsson and Kuilman () compared the modeled solar cycle response in polar summer T for 1979–1996 and 1997–2008, and also found a weaker response in the later period. Their results, however, did not uncover the ultimate cause of this change.…”

“…The mystery deepens as some models predict a strong solar cycle response in PMCs, T, and H 2 O after 1979 (e.g., Berger & Lübken, ; Lübken et al, , ), while others indicate a missing solar cycle response in T near the polar summer mesopause (Gan et al, ; Karlsson & Kuilman, ; Qian et al, ). Hartogh et al () showed that the anticorrelation between solar Lyman‐α (Lα) and H 2 O measured in the polar upper mesosphere was present in winter but not summer (1996–2006 at 69°N).…”