Effects of Synoptic-Scale Control on Long-Term Declining Trends of Summer Fog Frequency over the Pacific Side of Hokkaido Island

Abstract

In this study, long-term visibility data for the Pacific Ocean side of Hokkaido Island, northeast Japan, are investigated to clarify the relationship between interannual variation in summer fog frequency (FF) and largescale circulation patterns. At Kushiro, a significant FF decrease is found during 1931-2010 even without the influence of the observatory's relocation after 2000. In particular, since the late 1970s, a linear declining trend has accelerated, as evidenced by an increased number of years with very low FF in July and August. To clarify the climatological factor causing the summer FF declining trend at Kushiro, atmospheric vertical conditions in the planetary boundary layer and large-scale circulation are examined during 1989-2009 and 1958-2002, respectively, using available datasets. Composite analyses that are based on radiosonde observations reveal that the shallow fog layer is covered with a strong inversion layer during fog days whereas the inversion layer is absent during nonfog days. Composite circulation anomalies for the low-FF years at Kushiro show an intensified Okhotsk high (OH) pressure feature and southward shrinking of the North Pacific high (NPH) in July, in addition to the eastward displacement or shrinking of the NPH in August. These anomalous synoptic circulation patterns cause weakening in the southerly-southeasterly wind, which reduces sea-fog advection toward Kushiro and prevents the formation of stable stratification over the sea-fog layer. The authors suggest that the interannual variation in summer FF with the recent accelerated decline at Kushiro is primarily controlled by changes in the synoptic circulation associated with the OH and NPH development.

What Causes Observed Fog Trends: Air Quality or Climate Change?

“…Mainly radiation fog was affected by the trend, and ongoing urbanization with a decrease of the nightly radiative cooling rates was identified as most likely cause. In Japan, decreasing fog trends are reported for several stations, ongoing urbanization (Yamashita and Tsuji, 2007) and systematic changes of the regional circulation patterns (Sugimoto et al 2013) are discussed to have contributed or even caused the observed trends. Al-Fenadi (2001) shows that in Lybia, some stations show upwards trends and some other stations downwards trends, while the year-to-year variability is high at all sites.…”

Synoptic climatology of fog in selected locations of southern Poland (1966–2015)

“…Klemm and Lin (2016) found that a change of ten percent in aerosol concentration had about the same effect on fog occurence as a change of 0.1°C in air temperature. Reduction in fog frequency over the last 30 years has been reported for more than 300 stations in Europe (Vautard et al 2009) and other localities (Gomez and Smith 1984;Koepke 1995, 1997;Kokkola et al 2003;Shi et al 2008;Witi and La Dochy 2008;van Oldenborgh et al 2010;Sugimoto et al 2013;Fu et al 2014). An increase in the frequency of fog events due to growing industrial activity and rising atmospheric pollution concentration is observed in India (Singh and Dey 2012;Shrivastava et al 2016) and other parts of South Asia (Syed et al 2012).…”

Unusually High Fog Frequency at Kushiro, Japan, during Early Autumn in 2012

“…For example, interannual variability in midsummer fog frequency as observed at Kushiro, Hokkaido Island ( Fig. 1), has shown a long-term decline since 1931 (Sugimoto et al 2013). This is attributed to large-scale circulation changes in association with the NPH, which control the advection of sea fog onto land.…”

“…3) Sea surface temperature (SST) around Hokkaido Island also increased by approximately 3.5 and 4.5 K in early and mid-September 2012, respectively, due to the suppression of oceanic vertical mixing and the surface heating associated with the NPH extension. The strengthened warm moist southerly flow of the NPH extension during early autumn in 2012 may have favored advection of sea fog over Kushiro, as shown by Gultepe et al (2009) and Sugimoto et al (2013). In addition, the difference in the warming of the air and sea surface might have affected the likelihood of sea fog generation over the ocean due to changes in the stability of nearsurface stratification (Pilié et al 1979;Dorman et al 1998;Cho et al 2000;Koracin et al 2001;Lewis et al 2003).…”