Impacts of multi-timescale circulations on meridional heat transport

Relative contributions to the zonal mean meridional heat transport by the climatological annual mean, climatological annual variation, synoptic, intra-seasonal and lower-frequency motions were examined based on the ERA-Interim reanalysis data for the period of 1981-2015. The meridional heat transport analysed in this study only includes the component related to meridional wind and temperature. In the tropics, the climatological annual mean circulations dominate the long-term mean meridional heat transport, while the interaction between the climatological annual mean temperature and the seasonal anomalous flow largely contributes to the seasonal variation of the meridional heat transport. In the middle latitudes, the climatological annual mean circulations and transient eddies (mostly synoptic and intra-seasonal eddies) are of roughly equal importance in the poleward heat transport, leading to the maximum poleward heat transport around 50 degrees N/S. The upper- and lower-tropospheric heat transports by the climatological annual mean circulations appear opposite, with the magnitude of the lower-tropospheric transport being greater. The preferred maximum zonal mean heat transport at 50 degrees N by the climatological mean flow is attributed to the maximum zonal mean low-level southerly in situ. The preferred peak latitude of the mid-latitude poleward heat transport by synoptic eddies near 50 degrees N arises from the combined effect of the strong synoptic-scale meridional wind and temperature variabilities in situ and their in-phase relationship. The heat transport by tropical cyclones (TCs) was estimated by applying a statistical relationship between TC intensity and the vertically integrated temperature averaged over the TC core region derived from high-resolution Weather Research and Forecasting (WRF) model simulations. For northern hemisphere summer, TCs contribute about 35% of the total heat transport in the active TC regions, suggesting that TCs play a critical role in the regional meridional heat transport.