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Based on an area-averaged time series (the same as those in section 3.2), monthly anomalies of the frequency of specific cloud types or phases are defined as the monthly time series minus the 11-year average for that month, and they are used to examine their general interannual variability in the Arctic (Fig. 4). Linear regression was used in conjunction with a least square method and an F-test to identify any persistent and significant increasing or decreasing trends. We applied a 13-point Gaussian filter to the monthly anomalies—after filling in the missing months by temporal linear interpolation—to demonstrate the continuous variation.
Figure 4. Monthly anomalies (June 2006–May 2017) of different cloud frequencies: (a) total cloud, (b)–(i) eight cloud types, and (j)–(l) three cloud phases. Gray bars are the monthly cloud frequency anomalies. Blue lines are the linear trend of the monthly anomalies. Red lines are the 13-point Gaussian filtered anomalies.
The trends of altostratus (Fig. 4c), altocumulus (Fig. 4d), and cumulus clouds (Fig. 4g) were particularly significant (Table 1). Altostratus clouds showed a significant decreasing trend, while altocumulus and cumulus clouds showed a significant increasing trend. The total cloud cover did not change much (Fig. 4a), though overall, there was a slight decreasing trend. High clouds had a certain decreasing trend (p< 0.05). Stratus clouds displayed a slight increasing trend (Fig. 4e). Nimbostratus clouds showed a slight decreasing trend (Fig. 4h). Deep convection clouds showed a weak downward trend (Fig. 4i). However, the changes in total, stratus, stratocumulus, nimbostratus, and deep convection clouds were not statistically significant. Ice clouds (Fig. 4j) significantly decreased and conversely, water clouds (Fig. 4l) significantly increased (Table 1). Mixed clouds showed a slight upward trend (Fig. 4k).
Cloud type or phase Linear trends (yr–1) Significance p Total –4.45 × 10–4 3.74 × 10–1 High –1.30 × 10–3 4.58 × 10–2 ** Altostratus –1.50 × 10–3 5.65 × 10–6 *** Altocumulus 7.33 × 10–4 7.11 × 10–4 *** Stratus 1.08 × 10–4 1.74 × 10–1 Stratocumulus 2.41 × 10–4 6.12 × 10–1 Cumulus 2.68 × 10–4 7.41 × 105 *** Nimbostratus –2.97 × 10–4 2.77 × 10–1 Deep Convection –4.79 × 10–6 4.83 × 10–1 Ice –2.91 × 10–3 1.70 × 10−5 *** Mixed 2.04 × 10–4 6.50 × 10−1 Water 1.65 × 10–3 5.37 × 10–4 *** Table 1. Linear trends and significance of monthly anomalies (June 2006–May 2017) of total clouds, eight cloud types, and three cloud phases frequencies. *, **, and *** represent an F-test significant at p < 0.1, 0.05, and 0.01, respectively.
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Considering the differences in cloud distribution relative to the sea–land distribution and different features of continents, such as those discussed in section 3, it is necessary to further analyze the spatial distribution of linear trends to prevent the significant increase and decrease in some areas from being offset and ignored in spatial averaging.
For clouds in different phases, the spatial distribution is prone to a significant increase in water clouds occurs over a wide range (Fig. 5l) and the signs of the trends are consistent, as is the significant decrease in ice clouds (Fig. 5j). Significant increasing points, albeit of lesser magnitude, are found for mixed clouds (Fig. 5k) and total clouds (Fig. 5a) near the inner parts of the Arctic. It is noteworthy that significant linear trends are primarily located over the oceans. These distributions offer support for Arctic amplification theory (Jone, 2011; Zhao and Garrett, 2015), where positive feedback exists to some extent among the increase in water clouds, melting of sea ice, and Arctic warming.
Figure 5. Linear trends (shading area) of monthly anomalies (June 2006–May 2017) of different cloud frequencies: (a) total cloud, (b)–(i) eight cloud types, (j)–(l) three cloud phases. The shading unit is 10–2 yr–1. The black points represent statistical significance at p < 0.01 according to an F-test.
For different cloud types, the high clouds (Fig. 5b), altostratus (Fig. 5c), altocumulus (Fig. 5d), and cumulus (Fig. 5g) also have significant trends over a wide range with the same trend signs as they have in the regionally averaged results (Fig. 4, Table 1). One exception is stratocumulus clouds (Fig. 5f), which significantly increase inside the Arctic Ocean over wide ranges but significantly decrease in Greenland and the Atlantic Ocean. Transformations of cloud types may explain the above-mentioned decrease in stratocumulus, aside from Greenland, where other low clouds such as cumulus and nimbostratus significantly increase. However, solely applying the geographic distributions and relative relationships of different clouds is insufficient for explaining the physical mechanisms which drive the interannual trends. The possible causes of these differences are discussed in the following section combined with the introduction of more physical variables.
Cloud type or phase | Linear trends (yr–1) | Significance p |
Total | –4.45 × 10–4 | 3.74 × 10–1 |
High | –1.30 × 10–3 | 4.58 × 10–2 ** |
Altostratus | –1.50 × 10–3 | 5.65 × 10–6 *** |
Altocumulus | 7.33 × 10–4 | 7.11 × 10–4 *** |
Stratus | 1.08 × 10–4 | 1.74 × 10–1 |
Stratocumulus | 2.41 × 10–4 | 6.12 × 10–1 |
Cumulus | 2.68 × 10–4 | 7.41 × 105 *** |
Nimbostratus | –2.97 × 10–4 | 2.77 × 10–1 |
Deep Convection | –4.79 × 10–6 | 4.83 × 10–1 |
Ice | –2.91 × 10–3 | 1.70 × 10−5 *** |
Mixed | 2.04 × 10–4 | 6.50 × 10−1 |
Water | 1.65 × 10–3 | 5.37 × 10–4 *** |