[A-CG29] Extratropical oceans and atmosphere

[ACG29-P03] A breakdown of the diabatic genesis/loss of polar cold air masses and its relationship with air-sea interactions

*Yuki Kanno¹, Toshiki Iwasaki² (1.Central Research Institute of Electic Power Industry, 2.Department of Gephysics, Graduate School of Science, Tohoku University)

Keywords:polar cold air mass, surface heat fluxes, diabatic heating

In winter, polar cold air masses generated in high-latitudes outflow into the mid-latitudes intermittently and then lose its coldness due to diabatic heating. However, it is not clear which diabatic processes contribute to the genesis and loss of polar cold air masses. This study quantitatively clarifies which diabatic processes contribute to the genesis and loss of polar cold air masses in the boreal winter and the relationship between diabatic loss of polar cold air masses and surface heat fluxes. We utilize the Negative Heat Content (NHC), proposed by Iwasaki et al. (2014), below potential temperature of 280 K as an indicator of polar cold air masses. Because only diabatic heating and cooling within NHC contribute to its genesis and loss, NHC facilitates the quantitative understanding of the thermal aspect of polar cold air masses. Over the mid- and high latitudes, the NHC is generated by the long-wave radiation. Over the mid- and high latitude Oceans, on the other hand, the NHC is lost primary by the vertical diffusion in the mixing layers and secondary by moist processes. This indicates that polar cold air masses are lost through the air-sea heat exchanges. As the air-sea heat exchanges contribute to lose NHC, we have studied the relationship between the loss of NHC and surface heat fluxes. Over most of the mid- and high latitude Oceans, the loss of NHC have a linear relation with surface heat fluxes. A composite analysis for large NHC loss and large surface heat flux events shows that cold air outbreaks are triggers for the air-sea heat exchanges in these events. However, deviations from the linear relation are found over the Kuroshio Extension region. A composite analysis for these deviations shows that, although the NHC almost the same with winter climatological mean values, strong surface wind is the main driver of strong air-sea heat exchange. These findings suggest that the Kuroshio Extension region has a different air-sea interaction process among the mid- and high latitude Oceans.