Lecture 8 N. American Climate Physical Settings Circulation controls North American Monsoon (NAM) and Great Plains LLJ (GPLLJ)

四面環海, 與亞、非洲 相比, 受海洋 / 洋流影響 最大◦ 阿拉斯加洋流 加州洋流 北赤道洋流 墨西哥灣流 拉布拉多洋流

Important North American Air Masses 阿留申囚錮性氣旋帶 大西洋温帶氣旋帶 熱帶氣旋帶

Trewartha Climatic Classification System

冬季北美大陸冷氣團 (cP), 強度不及東亞的西伯利亞 冷氣團 亞洲大陸面積較大 北美內陸水體面積較大 東西伯利亞多山脈 一為氣旋帶的頭, 一為氣旋帶的尾

Climatological precipitation for Jun–Sep.

ITCZ seasonal migration ITCZ has an average latitude of 5ºN in Winter, during that season the rain over the coast of Mexico is low; as long as the ITCZ starts to move northward at the beginning of May, the rain increases gradually in the southern coast of Mexico. In summer the average latitude of the ITCZ is 10-12ºN The interannual variability of the position and intensity of the ITCZ is strongly associated with the SST meridional gradient. During El Niño Summers, this meridional gradient is really weak, and the ITCZ is likely to be located away from the south coast of Mexico. 1998, one of the strongest El Niño year was one of the driest years ever in Mexico.

ITCZ blocks the solar radiation over the Warm Pool in the southern coast of Mexico, The SST decreases, the SST meridional gradient decreases, and the ITCZ moves southward giving origin to the MSD. Once the sky is clear over the coast of Mexico, the solar radiation reaches increases the SST, and the ITCZ has energy again to move northward. Mid Summer Drought The intensity and location of the ITCZ has also an intra-seasonal variation. There’s a diminution of rain in July and August called the Mid Summer Drought (MSD) Why?

Hurricanes (1998) Hurricanes are a big issue in Mexico too, as they present in both oceans surrounding Mexico, Gulf of Mexico (Caribbean) and East Pacific Ocean. Atlantic Ocean has 8 tropical storms per year. Pacific Ocean has 15 tropical storms per year.

North American Monsoon It is experienced as a pronounced increase in rainfall from an extremely dry June to a rainy July over large areas of the southwestern United States and northwestern Mexico. The combination of seasonally warm land surfaces in lowlands and elevated areas together with atmospheric moisture supplied by nearby maritime sources is conducive to the formation of a monsoonlike system.

北美季風肇始前後的濕度和 500-mb 高度場變化

(Source: Higgens 2004)