Multi-Scale Impacts of Midlatitude Oceanic Frontal Zones on the Atmosphere (Invited Presentation)

It has recently been realized that, unlike in vast areas of midlatitude ocean basins, midlatitude oceanic frontal zones are the regions where the ocean can potentially influence the overlying atmosphere. Forming between the confluent warm and cool western boundary currents, those frontal zones are zonally elongated and characterized by tight SST gradient and an enormous amount of heat and moisture supply from the warm currents into the atmosphere. Especially so in the cold seasons, this allows effective moisture supply to precipitation systems associated with developing cyclones and efficient restoration of near-surface atmospheric baroclinicity after relaxed by atmospheric disturbances, both of which are crucial for recurrent development of cyclones and thereby the formation of stormtracks and associated polar-front westerly jet (PFJ). The SST front can thus have substantial impacts on the annular mode variability that is represented as the dominant variability of PFJ. Anomalous behavior of the frontal zones and associated SST anomalies also have the potential to change the atmospheric circulation. For example, observations and a realistic climate model integration both indicate that decadal shift of the North Pacific subarctic frontal zone can force a PNA-like atmospheric anomaly pattern in January. An AGCM experiment reveals that similar pattern can be forced by prominent warm SST anomalies observed in October 2011.

In addition to those basin-scale atmospheric anomalies, sharp meridional contrasts in heat and moisture release from the ocean across the fronts along the Kuroshio Extension (KE) and Oyashio are found to leave meso-scale imprints on surface pressure, surface wind convergence, low-level cloudiness and precipitation, whose representation in reanalysis data is found highly sensitive to the resolution of SST field assigned for data assimilation. Interestingly, a surface baroclinic zone along the Oyashio front is maintained mainly by cross-frontal contrasts in sensible heat release, whereas a baroclinic zone along the KE is by advective effects of the converging surface northerlies into the meso-scale pressure trough. These meso-scale imprints of the fronts tend to undergo notable modulations interannually between the stable and unstable regimes of the KE path.

Under the monsoonal northerlies in winter, the warm Kuroshio also organizes shallow convective stratocumulus within the developed unstable mixed layer. The locally enhanced convective updraft acts to augment the super-saturation level, leading to modifications of cloud microphysical properties, including an increase in cloud droplet density and thereby a potential augmentation of cloud albedo.