The midlatitude ocean has shown to be important for the maintenance of storm tracks through its restoring effect by sharp sea surface temperature (SST) gradients. In western boundary current (WBC) regions, turbulent heat fluxes exhibit strong high-frequency variability, indicative of vigorous synoptic-scale air-sea interactions. However, the importance of synoptic-scale atmospheric eddies, such as cyclones and anticyclones, for the time-mean air-sea interactions has not yet been sufficiently understood in a quantitative manner. In this study, we quantify the contributions of cyclones and anticyclones to climatological-mean heat and moisture supply from the ocean and rainfall along the two major oceanic frontal zones over the North Pacific and North Atlantic. We show that cyclones and anticyclones play distinct roles in the hydrological cycle, which is climatologically enhanced by the presence of oceanic frontal zones. We also demonstrate that the moisture transport from anticyclones to cyclones is strengthened climatologically with the oceanic frontal zones. These results indicate that oceanic frontal zones climatologically act to strengthen the hydrological cycle between cyclones and anticyclones. Additionally, based on fluxes reconstructed through a bulk formula, we demonstrate that the annual-mean heat fluxes are significantly reduced in extratropics when synoptic-scale atmospheric fluctuations are filtered out. The reduction is particularly pronounced around the WBC regions, reaching up to ~30%. Our findings highlight the essential roles of synoptic-scale atmospheric eddies in shaping time-mean air-sea interactions in midlatitudes and provide a key link to bridge the gap between our understanding of midlatitude air-sea interactions from day-to-day to longer-time scales.

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