Solar Influences on Climate
Some of the major points from a comprehensive review paper, 'Solar Influences on Climate,' which has just been submitted to the Reviews of Geophysics, are presented. Among these points are the following. 1. Recent analysis lets us discriminate between different reconstructions of total solar irradiance during the period of satellite observations. 2. Although estimates of the direct effects of solar variations on climate give small effects, recent modeling work suggests that total solar irradiance influences on the hydrologic cycle and solar UV variations produce significant climate effects that qualitatively agree with observations. 3. As knowledge of solar physics has increased, reconstructions of recent past solar variations suggest less substantial variations occurred. 4. Some of the evidence for galactic cosmic ray influences on cloud cover is questionable. 5. Our ability to model climate responses to solar variations has increased greatly in recent years and will continue to improve.
Global-scale teleconnections in the Earth's middle atmosphere
The global-scale circulation of the Earth's middle atmosphere is driven by angular momentum transfers effected by waves propagating up from the more turbulent, thermally-driven troposphere. The resulting effects on the middle atmosphere are largest in polar regions. This 'mechanical forcing' is an indirect response to the direct thermal forcing of the atmosphere by the Sun, and can act in a thermally-indirect manner, i.e. as a refrigerator. As it involves wave propagation, it can also act anti-diffusively, and non-locally. The basic physics of the process is described and examples given of how it can lead to global-scale teleconnections, both vertically and latitudinally. Parallels with the dynamics of the Sun will be mentioned.
A Mechanism for the Influence of Solar Variability on Tropospheric Climate
It is well known that solar variability plays an important role in determining the thermodynamic structure of the upper atmosphere, due to absorption of shortwave ultraviolet radiation, but until recently quantitative measures of variations in near-UV radiation were not available. Satellite instruments are now providing high spectral resolution measurements and it is clear that even variations in the 200-320nm range are proportionately much larger than those in total, spectrally-integrated, irradiance. These wavelengths are predominantly absorbed in the stratosphere where they play a determining role in the ozone and heat budgets. We have been investigating mechanisms whereby solar-induced changes in the thermal structure of the lower stratosphere might, through dynamical coupling processes, influence the climate of the troposphere. We impose heating perturbations in the lower stratosphere of a simple climate model. We find that the mid- latitude jets are weaker when the stratosphere is warmer and that their position is shifted depending on the latitudinal gradient of the heating perturbation. Heating imposed mainly in the tropical lower stratosphere causes the jets and storm tracks to move polewards, while uniform or polar heating causes an equatorial shift. We can explain this behaviour in terms of how the imposed changes in thermal structure near the tropopause influence the refraction of upward propagating planetary waves and the subsequent impact on the momentum budget and circulation of the troposphere. Analysis of observational data shows a significant warming of the tropical lower stratosphere in response to solar activity, and a weakening and poleward shift of the jets - entirely consistent with our modelling studies. Data analysis also shows a solar signal in polar annular modes of variability which is much stronger when the phase of the quasi-biennial oscillation is taken into account. As the QBO is largest in the tropical lower stratosphere it is plausible that the coupling of solar and QBO effects in that region may influence tropospheric circulations and polar vortex through the same mechanism; his is the subject of ongoing research.
Atmospheric circulation and its variability on the giant planets
The giant planets Jupiter, Saturn, Uranus, and Neptune constitute immense natural experiments in planetary fluid dynamics. They obey the same fluid mechanics laws as Earth and exhibit many similar phenomena, but the details differ because of their greater sizes, faster rotation rates, lack of solid surfaces, and weaker solar- energy absorption as compared to Earth. At the cloud level, the dynamics of the giant planets are dominated by multiple east-west (zonal) jet streams whose speeds range from 100 m/sec on Jupiter to 400 m/sec on Saturn and Neptune. Most of these jets have remained nearly steady over 30 years of observations, with some notable exceptions. All four planets also exhibit numerous time-variable clouds, vortices, and turbulent regions that probably play important roles in the planetary energy cycle. Jupiter and Saturn sport hundreds of compact vortices ranging in size from Jupiter's Great Red Spot (dimensions 20,000 km by 10,000 km) to the resolution limit of current images (hundreds of km or less). Neptune and Uranus are less active than Jupiter and Saturn but nevertheless exhibit several interesting vortices and other cloud features. Giant planet vortices have been observed to merge, split, eject filaments, orbit other vortices, oscillate in shape and position, migrate in longitude and latitude, change color and albedo, and interact with jets in a variety of ways. Lifetimes range from >100 years for the Great Red Spot to only days for some of the smaller structures. Jupiter and perhaps Saturn also exhibit a stratospheric thermal oscillation that seems to be similar to the Quasi-Biennial Oscillation (QBO) on Earth, which is driven by absorption in the stratosphere of waves propagating upward from the troposphere. In addition to these phenomena, the observational record demonstrates that Jupiter exhibits multi-year-long, quasiperiodic variations in the banded cloud patterns, which remain poorly understood but may have analogy to interannual and interdecadal climate oscillations on the Earth. In my talk, I will survey the observational record, briefly summarize our current theoretical understanding, and highlight possible ways that the study of giant planets and Earth climate/dynamics can synergistically cross-fertilize.
The Characteristics and Climate Forcing Implication of the Multi-time Scale Fluctuations in the Air Temperature in Xiamen, China From 1954 to 2007
Xiamen (also known as Amoy) is a coastal city with a population of over 1.5 millions located in the southeast Fujian Province across the Taiwan Strait from Taiwan. The monthly and annual surface air temperature (SAT) time series from 1954 on from its meteorological observatory (24.48 °N, 118.07 °E, 139.0m) are available at the National Meteorological Information Centre of China Meteorological Administration and the Global Historical Climatology Network online database. Over the 54-year period from 1954 to 2007, Xiamen experienced diverse seasonal trends in SAT: warming at a rate of 0.14 K/10a in winter and 0.05 K/10a in spring, as opposed to cooling at -0.04 and -0.03 K/10a in summer and fall, respectively. Overall, the trend in the annual SAT series over the 54-year period is an insignificant warming rate of 0.03 K/a, which is substantially smaller than the nation-wide and global averages. Nevertheless, the fluctuations in the SAT may bear important clues about climate forcing in this region. Our Morlet wavelet analysis reveals three principle time scales -- 42-year, 30-year, and 13-year -- in the Xiamen annual series. Interestingly, the periodic component of around 13-year, ranging from 9 to 15 years, exhibits not only in the annual series, but also in the four seasonal series. Moreover, the components of roughly same scale have been detected from several nearby meteorological stations including Fuzhou and Nanping in Fujian Province, and Tainan, Kaohsiung and Hengchun in Taiwan. The possible attribution of this 9- to 15-year temperature fluctuation to the solar activity cycling remains under investigation.
System Studies of Long-Term Variability of the Middle and Upper Atmosphere
The terrestrial middle and upper atmosphere is at the interface between interplanetary and lower-atmospheric processes and plays a uniquely important role in the solar-terrestrial system. The spatial and temporal variability of this region of the atmosphere directly reflects all aspects of the cause-and-effect chains in linking the Sun, heliosphere, and magnetosphere to the Earth's upper and lower atmosphere. Consequently it has been one of the most intriguing problems facing the upper atmospheric research community. Despite several decades of theoretical and observational studies, a detailed quantitative understanding of this variability, however, has not been fully achieved due to the lack of long-term systematic observational data needed to fully characterize the state of the atmosphere and its changes. Understanding subtle changes arising from solar- and human-induced processes also requires a long-term dataset with large geographic coverage. At present, various U.S. and international spaceborne and ground-based observational programs together have provided almost three decades of measurements that potentially can be used to evaluate the relative contributions of naturally and anthropogenically induced changes. In this paper, we will discuss the issues associated with these long-term datasets (e.g. sampling schemes, measurement precision and bias) in the context of upper atmosphere climate studies.