Dimming/brightening over the Iberian Peninsula: Trends in sunshine duration and cloud cover and their relations with atmospheric circulation

Abstract

[1] This study analyzes the spatial and temporal changes in sunshine duration (SunDu) and total cloud cover (TCC) over the Iberian Peninsula (IP) and four subregions during 1961-2004 using high-quality, homogenized data sets. The analyses confirm that over most of the IP and in most seasons, SunDu and TCC variations are strongly negatively correlated, with absolute values $0.8-0.9. Somewhat weaker correlations (0.5-0.6) are found in the southern portion of the IP in summer. A large discrepancy between the SunDu and TCC records occurs from the 1960s until the early 1980s when the SunDu series shows a decrease that it is not associated with an increase in TCC. This negative trend or ''dimming'' is even more pronounced after removing the effects of TCC via linear regression. Since the early 1980s, the SunDu and TCC residual SunDu series exhibit an upward trend or ''brightening.'' In addition to the long-term dimming and brightening, the volcanic eruptions of El Chichon and Mount Pinatubo are clearly evident in the TCC residual SunDu record. The TCC and SunDu records over the IP are well correlated with sea level pressure (SLP), with above normal TCC and below normal SunDu corresponding to below normal SLP locally in all seasons. The TCC and SunDu related SLP changes over the IP in winter and spring are part of a larger-scale north-south dipole pattern that extends over the entire Euro-Atlantic sector. Other more regional atmospheric circulation patterns, identified from rotated principal component analysis, are also linked to TCC and SunDu variations over the IP. Finally and perhaps surprisingly, the TCC residual SunDu series exhibits a statistically significant relationship with a regional atmospheric circulation pattern during spring, summer, and autumn.

Atmospheric impacts on climatic variability of surface incident solar radiation

“…SunDu has been regarded as a measure of cloud cover. However, studies have shown that aerosols can also reduce SunDu (Horseman et al, 2008;Kaiser and Qian, 2002;Raschke et al, 2006;Sanchez-Lorenzo et al, 2008Tang et al, 2011). This paper further demonstrates that SunDu can be used to estimate R s globally with sufficient accuracy to establish its year-to-year variations.…”

“…The inter-annual variation of aerosols was not included. This inter-annual variation can be captured by the observed SunDu (Horseman et al, 2008;Kaiser and Qian, 2002;Raschke et al, 2006;SanchezLorenzo et al, 2008;Sanchez-Lorenzo et al, 2009;Tang et al, 2011). Yang et al (2006) used one year of SunDu data in Japan and R s to derive a 0 , a 1 , and a 2 in Eq.…”

“…Aerosols can only reduce SunDu at low solar elevations. Although, it has been questioned (Stanhill and Cohen, 2005) as to whether SunDu would detect the impact of aerosols on R s , existing studies have already shown that a reduction in SunDu correlates with increases in atmospheric aerosols (Horseman et al, 2008;Kaiser and Qian, 2002;Qian et al, 2006;Sanchez-Lorenzo et al, 2009). Figure 9 shows that decadal variability of SunDu-derived R s is dominated by the changes in aerosols, especially those over Europe and China.…”

“…Several previous studies have established good correlations between SunDu and R s (Essa and Etman, 2004;Hoyt, 1977;Sanchez-Lorenzo et al, 2009;Stanhill and Cohen, 2005). SunDu has been regarded as a measure of cloud cover.…”

Cloud and aerosol contributions to variation in shortwave surface irradiance over East Asia in July during 2001 and 2007

“…Riihimaki et al [4] studied the contribution of aerosols to the direct irradiance in Oregon, USA, for almost three decades. Sanchez-Larenzo et al [5] have tried to relate dimming/brightening to sunshine duration, cloud cover, and atmospheric dynamics over the Iberian Peninsula for more than four decades. Therefore, it is important to quantify how individual atmospheric parameters such as clouds, aerosols, and water vapor contribute to variation in S. However, few studies have estimated the contributions of each parameter.…”