A Coupled Air–Sea Response Mechanism to Solar Forcing in the Pacific Region

Abstract

The 11-yr solar cycle [decadal solar oscillation (DSO)] at its peaks strengthens the climatological precipitation maxima in the tropical Pacific during northern winter. Results from two global coupled climate model ensemble simulations of twentieth-century climate that include anthropogenic (greenhouse gases, ozone, and sulfate aerosols, as well as black carbon aerosols in one of the models) and natural (volcano and solar) forcings agree with observations in the Pacific region, though the amplitude of the response in the models is about half the magnitude of the observations. These models have poorly resolved stratospheres and no 11-yr ozone variations, so the mechanism depends almost entirely on the increased solar forcing at peaks in the DSO acting on the ocean surface in clear sky areas of the equatorial and subtropical Pacific. Mainly due to geometrical considerations and cloud feedbacks, this solar forcing can be nearly an order of magnitude greater in those regions than the globally averaged solar forcing. The mechanism involves the increased solar forcing at the surface being manifested by increased latent heat flux and evaporation. The resulting moisture is carried to the convergence zones by the trade winds, thereby strengthening the intertropical convergence zone (ITCZ) and the South Pacific convergence zone (SPCZ). Once these precipitation regimes begin to intensify, an amplifying set of coupled feedbacks similar to that in cold events (or La Niña events) occurs. There is a strengthening of the trades and greater upwelling of colder water that extends the equatorial cold tongue farther west and reduces precipitation across the equatorial Pacific, while increasing precipitation even more in the ITCZ and SPCZ. Experiments with the atmosphere component from one of the coupled models are performed in which heating anomalies similar to those observed during DSO peaks are specified in the tropical Pacific. The result is an anomalous Rossby wave response in the atmosphere and consequent positive sea level pressure (SLP) anomalies in the North Pacific extending to western North America. These patterns match features that occur during DSO peak years in observations and the coupled models.

Solar Influence on Global and Regional Climates

“…All runs revealed both top-down and bottom-up solar effects, but they account for only a small percentage of the total GMAST variance. A model described by Meehl et al (2003Meehl et al ( , 2008Meehl et al ( , 2009) includes ocean-atmosphere coupling and with the stratosphere and suggests potential resonant connections between UV solar forcing and the ENSO oscillation, as also discussed by Emile-Geay et al (2007) and White and Liu (2008a, b).…”

The effects of changing solar activity on climate: contributions from palaeoclimatological studies

“…A strengthened atmospheric circulation enhances subtropical subsidence, resulting in a positive feedback mechanism of a further reduction in cloud formation and an increased solar absorption at the surface (e.g. Meehl et al 2008Meehl et al , 2009Gray et al 2010). …”

Biogeochemical processes controlling oxygen and carbon isotopes of diatom silica in Late Glacial to Holocene lacustrine rhythmites

“…However, regionally, dry La Niña years and wet El Niño years are not completely uncommon, which indicates that the relationship between SSTs in the tropical Pacific and precipitation anomalies in the central Andes is not straightforward (Garreaud et al 2003). For instance, analysis of observed data from the '11-year Schwabe cycles' shows a reduction of precipitation around the Equator corresponding to anomalously cold SSTs analogous to the pattern that occurs during La Niña years, with lagged El Niño-like conditions a couple of years later (Gray et al, 2010;Meehl et al, 2008Meehl et al, , 2009. However, there is a significant relationship between Andean Altiplano precipitation and the zonal winds modulated by decadal and multidecadal variations in solar activity (Theissen et al, 2008).…”