Lecture 8 N. American ClimateLecture 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 MassesImportant 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 熱帶氣旋帶

Trewartha Climatic Classification System

冬季北美大陸冷氣團 (cP; Canadian high),強度不及東亞的西伯利亞冷氣團 (Siberian high)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 

http://trmm.gsfc.nasa.gov/images_dir/avg_rainrate.html

WinterSummer

http://trmm.gsfc.nasa.gov/images_dir/avg_rainrate.html

WinterSummer

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 10-12ºNThe 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 DroughtThe 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 MonsoonIt 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

* Area with very high frequency of long path F3 to F5 tornadoes includes northern Mississippi, northern Alabama and western Tennessee.* Four most prominent tornado alleys: south-central Mississippi, east-central to northeast Oklahoma, southeast Nebraska and area from western Tennessee to northeast Mississippi and northern Alabama.

(Source: Higgens 2004)

Plateau monsoon in North America