Understanding the Time Scales and the Mechanisms of Tropical Responses to Extratropical Forcings

• Main Authors: Kuan-Chen Li, 李冠辰
• Other Authors: 黃彥婷
• Format: Others
• Language: en_US
• Published: 2017
• Online Access: http://ndltd.ncl.edu.tw/handle/4yxr7e

碩士 === 國立臺灣大學 === 大氣科學研究所 === 105 === The intertropical convergence zone (ITCZ) shifts meridionally in response to hemispheric asymmetry extratropical thermal forcings. To understand the mechanisms and the time scales of extratropical forcings influencing the tropics, this study investigates the equilibrium and the transient responses of a time invariant extratropical thermal forcing in an atmospheric model coupled to an aquaplanet slab mixed layer ocean. In equilibrium responses, the midlatitude eddy plays a key role in transporting anomalous heating/cooling equatorward in midlatitudes. The anomalous mean meridional circulation (MMC) accomplishes the changes in energy transports in the tropics, fluxing dry static energy (DSE) toward the cooler hemisphere via the upper branch of the anomalous Hadley Circulation and pushing moisture and precipitation toward the warmer hemisphere. In subtropical upper atmosphere, the anomalous eddy momentum flux divergence (S) is balanced by the anomalous meridional advection of planetary vorticity (f·v) associated with Hadley Circulation. Although the equilibrium responses are similar in the cases with various mixed layer depth (MLD), the time scales of the tropical circulation responding to the imposed extratropical forcings increase in cases with deeper MLD. We report two distinct stages of the responses to the extratropical forcings: (1) When imposing extratropical heating/cooling, the midlatitude warms/cools and decreases/increases temperature gradient rapidly, resulting in weakening/strengthening midlatitude eddies and upper-level subtropical westerly. (2) Once the tropical SST is influenced by the imposed forcings, which takes at least 6 months, eddies in the subtropical upper-level and Hadley cell strength adjust and ITCZ shifts to the warmer hemisphere. Our transient analysis suggests that atmospheric eddies play the main role of transporting anomalous energy and momentum from the extratropics to the edge of the subtropics. Although there are small changes in eddy momentum and eddy heat flux in the subtropical regions occurring before the tropical circulation adjustments, the changes are largely enhanced after the adjustments take place. Surface processes, dominated by wind-evaporation-SST (WES) feedback, appear to play a key role for transporting anomalous warming/cooling from the subtropics into the deep tropics. Since tropical circulation is constrained by SST, the time scales of extratropical forcings influencing the tropics is determined by surface processes, ranging from 8 months for the cases with 50m MLD to about 50 months for the cases with 400m MLD.