The Response Of Midlatitude Atmosphere To Heat And Vorticity Forcing Over North Pacific In Winter

The response of atmosphere to midlatitude SST anomalies is a key problem to understand the mechanism of midlatitude ocean-atmosphere interaction. In this study, we conduct research focusing on the response mechanism of midlatitude atmosphere to anomalous forcing sources related to SST anomalies. With59-year NCEP/NCAR reanalysis data, the distribution characteristics of the climatologic atmospheric diabatic heating, transient eddy heating and transient eddy vorticity forcing in winter are studied, and then the typical patterns of their decadal variations associated with SST anomalies over North Pacific, as well as their configuration relationship with anomalous atmospheric geopotential heights are analyzed. The atmospheric response to the heat and vorticity forcing is investigated with LBM model, and the relative roles of different kinds of atmospheric forcing in the formation of atmospheric response structure are compared. These studies clarify the basic features and determinant factors of atmospheric response to midlatitude SST anomalies, and provide theoretical basis for understanding the mechanism of climate variability and ocean-atmosphere interactions in the middle latitudes.The results illustrate that the diabatic heating is concentrated mostly over tropical region with a center at the middle level of troposphere, while the transient eddy heating is distributed primarily in the middle and high latitudes where the diabatic heating is relatively weak, with maximum at300hPa. The magnitudes of the two types of heating are equivalent. The transient eddy vorticity forcing is only significant in upper troposphere, characterized by basically zonal distribution pattern with positive and negative forcing alternate. It is orthogonal to the eddy-driven jet in meridional direction and thus maintains the jet through vorticity transportation.When the SST over North Pacific becomes cooler on decadal time scale, the associated heat and vorticity forcing both have an enhancement. The total heating anomaly is maximum at the upper level of troposphere, contributed mostly by transient eddy heating, while it is dominated by diabatic heating in the lower level. The maximum heating is downstream to the atmospheric geopotential low. The positive anomaly of transient eddy vorticity forcing is robust mostly in upper troposphere, and it is in phase with the geopotential low, but a little downstream, and thus intensifies the low.A set of LBM model simulations illustrate that the response of atmospheric heights to the midlatitude heat source can either be baroclinic or barotropic, which is sensitive to the background flow, the horizontal location and the vertical profile of heating, and the response to deep idealized heating over east of North Pacific that is similar to the real heat forcing anomaly is probably baroclinic. While the atmospheric response to the midlatitude vorticity forcing is always barotropic, with geopotential lows in phase with the forcing. The atmospheric responses to the real heat and vorticity forcing show similar results, suggesting that all the diabatic heating, transient eddy heating and transient eddy vorticity forcing have effects on atmospheric anomalies. However, the vorticity forcing may play a relatively more important role on the maintenance of the equivalent barotropic structure of atmospheric height anomalies.