3.1. PDO/PNA Index
 Figure 2 shows the temporal evolution of the annual mean PDO index from 1961 to 2005. The shift that occurred from a negative to a positive mode in 1976 was clearly evident in the unsmoothed time series. The exact month when this shift occurred was during June 1976 from the monthly mean time series (not shown). In the annual mean time series, the index values increased from −1.10 during 1975 to 0.01 in 1976. It has been proposed that the positive phase of the PDO lasted from about 1977 to about 2001 [Hartmann and Wendler, 2005], but within that period the annual mean time series showed other additional characteristics that are important to mention. From Figure 2, there clearly were other minor shifts present, though less prominent than the one in 1976, which occurred in 1989 where the index has fallen from 0.53 in 1988 to −0.18 in 1989 and in 1999 where the index decreased from 0.25 in 1998 to −1.06 in 1999.
Figure 2. Time series of the annual mean Pacific Decadal Oscillation (PDO) (light red line) and the Pacific North American (PNA) index (light green line) from 1961 to 2005. A 5 year moving average is applied to both the PDO (dark red line) and PNA (dark green line) time series.
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 For the winter (DJF) mean time series of the PDO (Figure 3) the shift occurred during the winters of 1975 and 1976 with index values increasing from −1.53 to 1.32. The difference of the indices from these two winters was greater than the difference from the shift found in the annual mean series. A minor change in index found before from the annual time series was again present during winter and occurred between the winters of 1988 and 1989 with index values from 1.15 to −0.8, respectively. The second additional shift occurred between 1998 and 1999 with index values that fell from 1.02 to −0.47. Moreover, by comparing the winter mean to the annual series for these two additional shifts during 1989 and 1999, the winter mean series showed a much more pronounced change in the indices.
 In addition to the PDO, the annual (Figure 2) and winter mean (Figure 3) series of the PNA indices were also analyzed. The shift of 1976 was almost nonexistent in the annual mean evolution of the PNA, but their relationship between these indices was still strong and was more evident in the winter average. For the unsmoothed original values of the indices, the correlation was 0.58, which increased to 0.78 using a 5 year moving average. Although the PNA affects climate in North America over most of the year, temperature variations in the Northwest and Southwest United States are strongly related to the PNA in winter and spring. For the winter series of the PNA, there was an indication for a shift that occurred in the late 1970s with minor changes that occurred around 1989 and 1999. Moreover, the correlation was greater during winter between the PNA and PDO with a value of 0.69 for the original series and 0.80 for the smoothed series. All correlations for the annual and winter mean time series showed a statistical significance at a confidence level greater than 95%.
3.2. Regional Surface Solar Radiation and Cloud Cover Trends
 The annual and seasonal mean DSW for the six regions of Alaska (southeast, south-central, southwest, west, interior, and Arctic) are shown in Figures 4–9. The red line represents the annual or seasonal average series and the black line is the 5 year moving average. Though these time series showed interannual variability, the overall characteristic of these regions (except the Arctic) displayed a decrease in DSW during the late 1970s to the 1990s, which closely followed the variability of the PDO time series. Particularly for the southeast region in winter (Figure 4), the sharp decrease that occurred in the late 1970s was apparent, which is in agreement with the winter time series of the PDO, though they are anticorrelated with one another. Additional changes that were also evident in almost all of the time series was during 1989 and 1998, which is in line with the changes that occurred in the PDO index.
Figure 4. Time series of (top and middle) seasonal and (bottom) annual mean model-derived all-sky downward surface shortwave radiation (red line) with 5 year moving averages (black line) for the southeast region.
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 In the south-central region the changes in DSW during 1976 and 1989 were quite strong but only in the winter season (Figure 5). The sharp decrease around 1998 for this region was apparent in all time series with the strongest signal in spring (MAM), summer (JJA), and in the annual mean series. Also, after the year 2000 a strong reversal in the DSW occurred in all seasons and in the annual. For the southwest region (Figure 6), the sharp decrease during the late 1970s was present in all time series with a much stronger change in 1989, especially in winter. After about 1990 the DSW became more stable throughout the period.
 The next region is located in the western part of Alaska, which showed similar characteristics among all the seasons as well as in the annual mean series (Figure 7). The DSW decreased in the late 1970s until the 1990s with another change found only in winter during 1989. Throughout the rest of the 1990s a sharp decrease occurred in 1998 and was easily visible in all the time series.
 The interior region of Alaska only showed downward changes of the late 1970s occurring in winter; however, the 1998 change strongly occurred during the spring, summer, and in the annual mean series (Figure 8). Finally, for the Arctic region the typical decrease in DSW from the late 1970s through the 1990s did not occur (Figure 9). What was apparent in this region was the sharp decrease in DSW that occurred in the late 1980s in all the time series except in winter.
 To investigate the impact that the 1976 climate shift had on the quantitative changes in the DSW, trends were computed before and after the shift (Table 1). The greatest changes occurred in the southeast region in winter during 1961–1975 with a trend of 1.67% yr−1 before the shift and −1.07% yr−1 during 1977–1991 after the shift. Almost all trends before the shift in winter, except in the Arctic, were statistically significant at the 95% confidence level. Followed by the southeast, the southwest and the western regions displayed the greatest changes occurring in the summer and autumn (SON) with almost all trends in these two regions being statistically significant. The least amount of change occurred in the south-central and interior region and almost one half of the trends from all the seasons in this region were statistically significant. In general, from all of the regions, the spring and summer showed the least amount of change from the trends before and after the shift. Also, all regions and time periods (except Arctic winter, and the south-central spring, summer, and autumn) showed the same reversal in trend with positive values before and negative values after the shift with maximum changes at the shift occurring in winter. The regions with exceptions, however, still showed a decrease in the DSW after the shift.
Table 1. Regional Alaskan Trends of Annual and Seasonal Means From Two Periods, 1961–1975 and 1977–1991, of All-Sky Downward Surface Solar Radiation Based Upon Linear Least Squares Regressiona
|Region||Annual (% yr−1)||Winter (DJF) (% yr−1)||Spring (MAM) (% yr−1)||Summer (JJA) (% yr−1)||Autumn (SON) (% yr−1)|
 Figures 10–16 show the temporal evolution of cloud cover for each station that had available data for the entire 1961–2005 period. In Figure 10 the time series of the cloud cover is shown for the Annette station, which is located in the southeast region of Alaska. Large increases were found until about 1988 for all seasons (only winter is shown) as well as in the annual mean except in the winter and spring, which was in agreement with the variability of the PDO time series. The changes that occurred in cloud cover in 1998 were also evident in all the time series but from this point onward, it showed disagreement in phase between the cloudiness and the DSW; the cloud cover decreased while the DSW increased. Moreover, compared to all the time series of the DSW, the interannual variability in cloudiness was far greater.
Figure 10. Time series of the opposite of (top) annual and (bottom) winter (DJF) mean model-derived all-sky downward surface shortwave radiation (light red line) and cloud cover observations (light blue line) with smoothed (5 year moving average) (dark blue line) and unsmoothed (dark red line) correlations for Annette. Units are standardized anomalies and are dimensionless.
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 The Anchorage site (south central) displayed the increase in cloud cover during the 1980s during winter (Figure 11) and summer only. The 1989 change was also clearly visible in the annual, spring, and summer time series. All of the time series showed the change that occurred in 1999 with the strongest signals occurring in the annual, spring, and summer. In addition, there existed a high anticorrelation between the annual mean DSW and the cloud cover; the correlation was −0.70 for the original time series and −0.73 for the 5 year moving average or smoothed series, which were both statistically significant. The highest correlation was found in the autumn season with −0.72 for the unsmoothed series and −0.80 for the smoothed series, followed by the summer and winter and lastly the spring. All correlations were statistically significant.
 Another site that is included in the south-central region is Talkeetna (Figure 12). This site was clearly affected by the changes that occurred in the PDO during the late 1970s in winter and also in spring. The annual mean time series clearly revealed the change during 1989 followed by winter and then spring. The change at this time was evident but was not as strong as the one in Anchorage. Also, the correlation between these two parameters, though significant, was not as high as the one found in the Anchorage site, so there was less agreement for all time series.
 In the southwest region shown by the site of Saint Paul Island, the changes in DSW during the late 1970s were clearly seen in the winter (Figure 13) and autumn time series. Also, the magnitude of the cloud cover for this shift was greater than the DSW but there was still a high significant correlation of −0.71 and −0.92 for the winter mean smoothed and unsmoothed time series, respectively. The distinct change that occurred in 1989 was also evident but only in the spring and summer mean time series. During 1999 all the time series showed this abrupt change in cloud cover except the winter mean series.
 The west region includes the site of Kotzebue, with the change in the late 1970s only seen in the annual series (Figure 14). The other turning points occurred during 1989 and 1998, which were easily noticeable and were among the clearest and strongest signals from all the regions. It was also important to note that there was an exceptionally good agreement during 1998 between the DSW and the cloud cover. The correlation was high and significant, especially in summer with a maximum of −0.85 for the smoothed time series.
 For the Interior region, which was represented by the Fairbanks site, only the change in 1998 was clearly defined in these time series, especially in the annual, winter (Figure 15), and summer. Again the agreement between the DSW and the cloud cover was high during this change with a maximum significant correlation of −0.80 during the winter mean smoothed time series The Arctic site of Barrow did not display the typical variations that were discovered in the other regions; however, the change that occurred in 1998 was evident in the winter time series, which showed good agreement with the DSW at this time (Figure 16).
 Trends in cloud cover for all stations were also analyzed (not shown). The greatest changes before and after the shift occurred in sites that represented the south-central region (Anchorage and Talkeetna) followed by the interior and western Alaska. The only location that showed a clear change from a negative to positive trend was the Interior (Fairbanks) during winter with −0.06 and 0.76% yr−1, respectively. The least amount of changes in cloud cover occurred in the southeast, southwest, and Arctic regions where most sites were not statistically significant.
3.3. Effects From Circulation Patterns (PDO and PNA)
 Figure 17 shows the location of each site analyzed for the correlation between the winter mean DSW and the winter mean PDO/PNA index for the whole series (1961–2005). Overall the correlation for the DSW and PDO was good to moderate with a maximum and significant value of −0.43 found in Big Delta (interior). The lowest correlations were located in some of the sites in the western region and in Barrow (Arctic), which were not statistically significant. For the PNA, a much stronger relationship in all sites was displayed, except for Saint Paul Island. A maximum value of −0.66 was found in Anchorage (south central) and was statistically significant. Also, in Anchorage the PNA explained 36% of the winter-to-winter variability in DSW while the PDO and PNA together explained 43%. In Big Delta, for example, the PNA explained 30% of the winter-to-winter variability in DSW and the PNA and PDO explained 30%. Weaker correlations with no significance were found in some of the sites in the western region and in the Arctic; these were the only sites that were not significant.
Figure 17. Site-by-site correlation analysis for 1961–2005 between the model-derived all-sky downward surface shortwave radiation (DSW) and the Pacific Decadal Oscillation (PDO)/Pacific North American (PNA) index and between the cloud cover observations and the PDO/PNA index. Upper left values are the comparison with the fluxes and the PDO, and upper right values are with the PNA. Lower left values are the correlations with the cloud cover observations and the PDO, and lower right are the values with the PNA. If no lower correlations are present, then the upper values represent the ones for the fluxes. Correlation coefficients underlined in red are statistically significant at the 95% confidence level.
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 The correlation between the cloud cover and the PDO/PNA is also displayed in Figure 17. What was noticeable was the increased influence from circulation changes on the cloud cover for all sites except Fairbanks (interior), Bethel (west), and Kotzebue (west). Note that these were locations with the lowest correlations, which were also not statistically significant. Sites contained in the other regions showed higher values of correlation and were statistically significant with a maximum in Anchorage of 0.48 and 0.71 for the PDO and PNA, respectively.
 Figure 18 is similar to the previous figure, but represents the correlations computed from 1961 to 1990. This period was chosen in order to determine if changes occurred in the correlations among the different sites. In addition, the end of this period corresponded to one of the minor shifts in the PDO and PNA found in 1989, discussed earlier. Referring to the DSW and the PDO/PNA relationship, a maximum of −0.63/−0.78 occurred in the Kodiak site (southwest) and was statistically significant. Also, as before the PNA relationship with the DSW was much greater for all sites except for Saint Paul Island. Low correlations were found in some of the sites in the western and Arctic regions and were not significant.
 The correlations among the clouds and the PDO/PNA in this period were highest in Anchorage (south central) with maximum values of 0.57/0.74. This was followed by the Talkeetna site (south central) with values of 0.55/0.70, which were all statistically significant. Also note the weaker values found in Kotzebue and Fairbanks, which were not statistically significant.