Effects of ~27-Day Solar Ultraviolet Variations on Tropical Precipitation and the Madden-Julian Oscillation

Composite and regression analyses using global reanalysis data and NOAA satellite-based outgoing longwave radiation data have been conducted for the maximum periods of four solar cycles to investigate the existence and origin of lagged responses of tropical precipitation and the Madden-Julian Oscillation (MJO) to short-term (~27-day) solar ultraviolet (UV) variations. Previous work has found a lagged tropospheric temperature response to short-term solar UV peaks and minima as well as a lagged increase in the occurrence rate of strong MJO days following 27-day solar UV minima. The hypothesized mechanism involves relative downwelling, a slowing of the mean meridional (Brewer-Dobson) stratospheric circulation, and an increase of static stability in the tropical tropopause region following solar UV peaks and their attendant increased ozone production and radiative heating in the tropical upper stratosphere. Opposite conditions occur following UV minima.

Here, 137 peaks and 128 minima in ~27-day solar activity occurring during four solar maximum periods (1979-83; 1989-93; 1999-03; 2013-15) are analyzed. Evidence is obtained for a ~4-day lagged mean response of tropical precipitation such that precipitation in the central and eastern tropical Pacific is, on average, reduced following solar UV peaks and increased following solar UV minima. As shown in Figure 1, the amplitude of the precipitation response is larger and more statistically significant if only days when the MJO is active (OMI > 1 standard deviation; about 60-65% of all days) are considered. An opposite behavior is found in the equatorial Indian ocean with increased precipitation on average following UV peaks and reduced precipitation following UV minima. The origin of these precipitation differences appears to be related to the ability of the MJO to propagate across the Maritime Continent barrier. Considering only those days with an active MJO, only 9 of 78 UV minimum days were spent in MJO phases 3 and 4 (eastern Indian Ocean and western Maritime Continent) compared to 22 of 86 UV peak days. The number of days spent in phases 5 and 6 (eastern Maritime Continent and West Pacific) following UV peaks and minima are comparable. In all, these results suggest that there is more effective MJO propagation across the Maritime Continent barrier following UV minima, leading to increased precipitation in the western and central Pacific. A similar influence of the stratospheric quasi-biennial oscillation (QBO) on the MJO has previously been found with more effective propagation across the Maritime Continent barrier during the easterly QBO phase relative to the westerly phase.

Previous work has found that an active MJO in phases 2 and 3 (centered over the Indian Ocean) is associated with a poleward shift of the North Pacific storm track while an active MJO in phases 6 through 8 (western and central Pacific Ocean) is associated with an equatorward shift. Shifts in position of the storm track and North Pacific jet have been shown to influence the frequency of extreme weather, such as temperature extremes, in North America. Therefore, given the significant influence of ~27-day solar UV cycles on the MJO, we suggest that the occurrence of solar rotational cycles should be considered in addition to the QBO when predicting the MJO and its influence on the midlatitude storm track and attendant extreme weather on extended range to subseasonal timescales.