J. L. Lean

Record‐low thermospheric density during the 2008 solar minimum

[1]xa0We use global-average thermospheric total mass density, derived from the drag effect on the orbits of many space objects, to study the behavior of the thermosphere during the prolonged minimum in solar activity between cycles 23 and 24. During 2007–2009 thermospheric densities at a fiducial altitude of 400 km were the lowest observed in the 43-year database, and were anomalously low, by 10–30%, compared with climatologically expected levels. The density anomalies appear to have commenced before 2006, well before the cycle 23/24 minimum, and are larger than expected from enhanced thermospheric cooling by increasing concentrations of CO2. The height dependence of the mass density anomalies suggests that they are attributable to a combination of lower-than-expected exospheric temperature (−14 K) and reductions in the number density of atomic oxygen (−12%) and other species (−3%) near the base of the diffusive portion of the thermosphere.

Hypothesized climate forcing time series for the last 500 years

A new compilation of annually resolved time series of atmospheric trace gas concentrations, solar irradiance, tropospheric aerosol optical depth, and stratospheric (volcanic) aerosol optical depth is presented for use in climate modeling studies of the period 1500 to 1999 A.D. Atmospheric CO 2 , CH 4 , and N 2 O concentrations over this period are well established on the basis of fossil air trapped in ice cores and instrumental measurements over the last few decades. Estimates of solar irradiance, ranging between 1364.2 and 1368.2 W/m 2 , are presented using calibrated historical observations of the Sun back to 1610, along with cosmogenic isotope variations extending back to 1500. Tropospheric aerosol distributions are calculated by scaling the modern distribution of sulfate and carbonaceous aerosol optical depths back to 1860 using reconstructed regional CO 2 emissions; prior to 1860 the anthropogenic tropospheric aerosol optical depths are assumed to be zero. Finally, the first continuous, annually dated record of zonally averaged stratospheric (volcanic) optical depths back to 1500 is constructed using sulfate flux data from multiple ice cores from both Greenland and Antarctica, in conjunction with historical and instrumental (satellite and pyrheliometric) observations. The climate forcings generated here are currently being used as input to a suite of transient (time dependent) paleoclimate model simulations of the past 500 years. These forcings are also available for comparison with instrumental and proxy paleoclimate data of the same period.

The long‐term variation of the Sun's open magnetic flux

The interplanetary magnetic field (IMF) has its origin in open magnetic regions of the Sun (coronal holes). The location of these regions and their total open flux Φ open can be inferred from current-free extrapolations of the observed photospheric field. We derive the long-term variation of Φ open during 1971-1998 and discuss its causes. Near sunspot minimum, the open flux originates mainly from the large polar coronal holes, whereas at sunspot maximum it is rooted in small, lower-latitude holes characterized by very high field strengths; the total amount of open flux thus remains roughly constant between sunspot minimum and maximum. Through most of the cycle, the variation of Φ open closely follows that of the Suns total dipole strength, showing much less dependence on the total photospheric flux or the sunspot number. However, episodic increases in large-scale sunspot activity lead to strengthenings of the equatorial dipole component, and hence to enhancements in Φ open and the IMF strength lasting typically ∼1 yr.

The effect of increasing solar activity on the Sun's total and open magnetic flux during multiple cycles: Implications for solar forcing of climate

[1]xa0We investigate the relationship between solar irradiance and cosmogenic isotope variations by simulating with a flux transport model the effect of solar activity on the Suns total and open magnetic flux. As the total amount of magnetic flux deposited in successive cycles increases, the polar fields build up, producing a secular increase in the open flux that controls the interplanetary magnetic field which modulates the cosmic ray flux that produces cosmogenic isotopes. Non-axisymmetric fields at lower latitudes decay on time scales of less than a year; as a result the total magnetic flux at the solar surface, which controls the Suns irradiance, lacks an upward trend during cycle minima. This suggests that secular increases in cosmogenic and geomagnetic proxies of solar activity may not necessarily imply equivalent secular trends in solar irradiance. Questions therefore arise about the interpretation of Sun-climate relationships, which typically assume that the proxies imply radiative forcing.

Simulated time‐dependent climate response to solar radiative forcing since 1600

Estimated solar irradiance variations since 1500 have been used to force the GISS atmospheric GCM coupled to a mixed layer q-flux ocean with heat diffusion through the bottom of the mixed layer. The goal is to assess solar-induced climate change in preindustrial and postindustrial epochs. Six simulations and control runs were made to test the effects of different initial conditions, estimates of initial solar forcing conditions, and ocean heat uptake. The results show that an estimated solar forcing increase of 0.25% accounts for a 0.458C temperature increase since 1600 and an increase of about 0.28C over the past 100 years. Global surface temperatures lag solar fluctuations by up to 10 years; the lag is greater over the oceans and so is the correlation due to reduced noise. With only a mixed layer ocean the phase lag is 5 years less. Solar forcing and water vapor feedback each directly account for 35% of the temperature response, with cloud cover changes contributing 20% and sea ice/snow cover 10%. Uncertainty in the initial radiation imbalance or solar forcing affects the surface temperatures for 60 -90 years. Modeled and observed periodicities show dominance of long-period forcing (.50 years), as provided by the solar input in these experiments. Tropical temperatures correlate best with solar forcing, due to the influence of water vapor feedback, especially at these multidecadal periods. Sea ice and extratropical temperatures have less long-period power, while high- frequency fluctuations dominate simulated cloud cover variations, which are relatively independent of solar forcing changes. Global and extratropical precipitation increase as the climate warms, but not low and subtropical precipitation, due to conflicting influences of absolute temperature and temperature gradient changes. Solar forcing by itself was not sufficient to produce the rapid warming during the last several decades. A comparison experiment varying trace gas forcing suggests that if the solar estimate is correct, then negative forcing by tropospheric aerosols (and perhaps volcanoes, ozone, and land use changes) has been about 2 1.2 Wm 22 since 1700, implying approximately equal contribution from direct and indirect tropospheric aerosol effects.

Magnetic Sources of the Solar Irradiance Cycle

Using recently processed Ca K filtergrams, recorded with a 1 A filter at the Big Bear Solar Observatory (BBSO), we quantitatively assess the component of solar irradiance variability attributable to bright magnetic features on the Suns disk. The Ca K filtergrams, flattened by removing instrumental effects and center-to-limb variations, provide information about bright sources of irradiance variability associated with magnetic activity in both active regions and dispersed active region remnants broadly distributed in the supergranule network (termed collectively faculae). Procedures are developed to construct both total and UV spectral solar irradiance variations explicitly from the processed Ca K filtergrams, independently of direct irradiance observations. The disk-integrated bolometric and UV facular brightness signals determined from the filtergrams between late 1991 and mid-1995 are compared with concurrent solar irradiance measurements made by high-precision solar radiometers on the Upper Atmosphere Research Satellite (UARS). The comparisons suggest that active-region and active-network changes can account for the measured variations. This good agreement during a period covering most of the decline in solar activity from the cycle 22 maximum to the impending solar minimum directly implicates magnetic features as the sources of the 11 yr irradiance cycle, apparently obviating the need for an additional component other than spots or faculae.

A new model of solar EUV irradiance variability: 1. Model formulation

We present a new model of solar irradiance variability at extreme ultraviolet wavelengths (EUV, 50–1200 A). In this model, quiet Sun, coronal hole, and active region intensities for optically thin emission lines are computed from emission measure distributions determined from spectrally and spatially resolved observations. For optically thick emission lines and continua, empirical values are used. The contribution of various solar features to the spectral irradiance variability is determined from a simple model of limb-brightening and full-disk solar images taken at the Big Bear Solar Observatory and by the Soft X-Ray Telescope on Yohkoh. To extend our irradiance model beyond the time period covered by the available images, we use correlations with proxies for solar activity. Comparisons with the available irradiance data from the Atmospheric Explorer E (AE-E) spacecraft show that our model is capable of reproducing the rotational modulation of the EUV irradiance near solar maximum. The AE-E data, however, show systematically more solar cycle variability than our model estimates.

Exploring the stratospheric/tropospheric response to solar forcing

[1]xa0We use the new Goddard Institute for Space Studies Global Climate Middle Atmosphere Model 3 with four different resolutions to investigate various aspects of solar cycle influence on the troposphere/stratosphere system. Three different configurations of sea surface temperatures are used to help determine whether the tropospheric response is due to forcing from above (UV variations impacting the stratosphere) or below (total solar irradiance changes acting through the surface temperature field). The results show that the stratospheric response is highly repeatable and significant. With the more active sun, the annual residual circulation change features relative increased upwelling in the Southern Hemisphere and downwelling in the Northern Hemisphere. Stratospheric west wind increases extend down into the troposphere, especially during Southern Hemisphere winter, and in some runs the jet stream weakens and moves poleward. The predominant tropospheric response consists of warming in the troposphere, with precipitation decreases south of the equator and in the Northern Hemisphere subtropics and midlatitudes, with increases north of the equator especially over southern Asia. The tropospheric response is often not significant, but is fairly robust among the different simulations. These features, which have been reported in observations and other model studies, appear to be driven both from the stratosphere and the surface; nevertheless, they account for only a small percentage of the total variance. More accurate simulations of the solar cycle stratospheric ozone response, the quasi-biennial oscillation, and coupled atmosphere-ocean dynamics are necessary before any conclusions can be deemed definitive.

Meridional Flow and the Solar Cycle Variation of the Sun’s Open Magnetic Flux

We simulate the evolution of the Suns large-scale magnetic field during solar cycle 21, including the effect of surface transport processes and active region emergence. As an important new constraint on the model, we have scaled our source fluxes upward to be consistent with the average measured strength of the interplanetary magnetic field (IMF). By adopting a poleward bulk flow of amplitude ~20-25 m s-1 together with a supergranular diffusion rate of ~500 km2 s-1, we are then able to match the observed variation of the Suns polar fields and open magnetic flux. The high meridional flow speeds, peaking at low latitudes, prevent the buildup of an overly strong axisymmetric dipole component at sunspot minimum, while accounting for the giant poleward surges of flux and accompanying polar field fluctuations observed near sunspot maximum. The present simulations also reproduce the large peak in the equatorial dipole and IMF strength recorded in 1982.

ROLE OF A VARIABLE MERIDIONAL FLOW IN THE SECULAR EVOLUTION OF THE SUN'S POLAR FIELDS AND OPEN FLUX

We use a magnetic flux transport model to simulate the evolution of the Suns polar fields and open flux during solar cycles 13 through 22 (1888-1997). The flux emergence rates are assumed to scale according to the observed sunspot-number amplitudes. We find that stable polarity oscillations can be maintained if the meridional flow rate is allowed to vary from cycle to cycle, with higher poleward speeds occurring during the more active cycles. Our model is able to account for a doubling of the interplanetary field strength since 1900, as deduced by Lockwood, Stamper, & Wild from the geomagnetic aa index. We confirm our earlier conclusion that such a doubling of the open flux does not imply that the base level of the total photospheric flux has increased significantly over the last century.