North East / South West Monsoon

Eastern Asia is defined by a shift in winds from the winter months to the summer months. During the winter the Asiatic high dominates the weather pattern while in the summer month a reversal of winds and the pressure gradient sets up as heat lows develop across Eastern Asia. The image below is the general synoptic pattern during the North East Monsoon. This setup typically begins in early September in Northern China then gradually works south becoming strongest in Mid-January. The Northeast Monsoon is often characterized numerous cold surges that bring freezing temperatures as far south as Hong Kong. These cold surges are often accompanied by gusty winds, quick temperature drops, heavy snowfalls, severe frost, and sandstorms. As the summer month start to approach in eastern Asia the dominating Asiatic high begins to break down and weaken. As the high pressure lifts to the north a thermal low starts to take its place.  At the same time the West Pacific High strengthens farther to the west. This transition is marked by a seasonal belt of persistent rain showers first starting in mid may in the Indo-China Peninsula and ending in July in Mongolia. With the south west monsoon in effect tropical systems have more moisture to develop in the western pacific. Furthermore the ITCZ lifts farther north over the Philippines where it brings persistent heavy rain showers during the wet season.  The Polar front jet also moves farther north reducing vertical wind shear in the tropics.

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