[张贴报告]Why a large-scale monsoon does not exist in North America: Orographic effects

Why a large-scale monsoon does not exist in North America: Orographic effects
编号：369 稿件编号：1585 访问权限：仅限参会人 更新：2024-04-10 19:30:02 浏览：759次 张贴报告

报告开始：2024年05月18日 11:04 （Asia/Shanghai）

报告时间：1min

所在会议：[SP] 张贴报告专场 » [sp12] 主题12、大气物理与气象气候

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摘要
To understand why a large-scale monsoon does not exist in North America,
we performed a series of sensitivity simulations to investigate orographic
effects of the Rocky Mountains (RMs) using the Community Atmosphere
Model version 5.1. Results show that the height of the RMs plays a funda
mental role in shaping the monsoon over North America that is confined to
a small area from northwestern Mexico to the southwestern United States.
When the RMs' height is increased by five times their actual height, a larger
part of the non-monsoon region over North America becomes a monsoon
region. The mechanical effects of the RMs uplift dominate in winter, while
thermal effects dominate in summer. During winter the mechanical effects
induce an equivalent barotropic atmospheric response in the troposphere.
With the uplift of the RMs, the ridge and trough located on the western and
eastern sides of Canada are strengthened. Most areas of North America are
influenced by the northerlies during winter such that enhanced descending
motion over the eastern RMs favours a dry winter climate. However, ther
mal effects dominate during summer through enhanced baroclinic atmo
spheric responses. The Mexico high and lower-level cyclonic circulation are
strengthened with the RMs uplift, triggering large-scale ascending motion.
Eastern North America is mainly controlled by the enhanced southerly
wind along the western flank of the North Atlantic subtropical high. Thus,
the enhanced water-vapour transport and upward motion on the eastern
side of the RMs increase summer precipitation. As a result, an obvious sea
sonal variation with the feature of ‘dry winter and wet summer’ finally
develops, indicating that the height of the RMs plays a crucial role in shap
ing the monsoon over the central United States. Additional experiments
show that the base area of the RMs has little effect on the large-scale mon
soon formation over North America.
关键字
large-scale monsoon, modelling, monsoon precipitation index, North American monsoon, Rocky Mountains
报告人
秦如燕
助理讲师 青海理工大学

稿件作者
徐海明 南京信息工程大学
秦如燕 青海理工大学
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