Weakened Sea‐Land Breeze in a Coastal Megacity Driven by Urbanization and Ocean Warming

Abstract

AbstractThe sea‐land breeze (SLB) occurs regularly in coastal areas and influences the local climate and atmospheric environment. Fifteen‐year observations (2005–2019) from urban, coastal, and marine stations were collected to examine the trend in the number of SLB days (SLBD) in Tianjin, a coastal megacity in China. The number of SLBD in Tianjin generally showed a decreasing trend with an annual average decline rate of 3.89%. The average land breeze speed decreased by 0.20 m·s−1 and sea breeze speed decreased by 0.69 m·s−1 in the examined 15 years. Simultaneously, the distance of influence of SLB in summer was reduced by ∼30 km. The Weather Research and Forecasting model developed with high‐resolution land use data and variations in the sea surface temperature (SST) was applied to investigate the mechanisms leading to the weakening of SLB. The increasing SST led to a reduction in the temperature difference between sea and land, which was found to be the dominant factor driving the weakening of SLB in summer. The changes in the density and height of buildings due to urbanization altered the wind direction in the coastal areas, which in turn drive the reduction of the sea breeze during fall, while increased surface temperature at night in urban areas led to the reduction of the land breeze. With projections indicating ocean warming and increasing urbanization, the SLB will become weaker and occur less frequently in the future, probably leading to more frequent air pollution due to poor ventilation in coastal cities.