Interannual And Decadal Variations Of Tropical Upper Tropospheric Trough Over The Northern Pacific In Summer And Their Causes

Based on the NCEP/NCAR reanalysis, HadlSST and monthly surface air temperature datasets of University of Delaware from 1951 to 2010, a new TUTT intensity index and a TUTT position index at 200hPa over the northern Pacific in boreal summer have been well defined. The interannual and decadal variations of TUTT and their causes, together with their relationship with the surface air temperature over the North America, are investigated. The main conclusions are as follows:(1) Results indicate that the TUTT intensity experiences a marked interannual variation during the period of 1979-2010 with its two main periodicities of quasi-4 and quasi-5 years. Results also show that the TUTT intensity is highly negative correlated with the surface air temperature over the southeast part of North America. As the summer TUTT increases (decreases) in intensity, the geopotential height in the upper-middle troposphere over southeast part of North America decreases (increases) and thus the high which dominates over the southeast part of North America is correspondingly weakened (enhanced), leading to a decrease (an increase) in surface air temperature there.In addition, the interannual variation of TUTT is both highly related to the SST anomalies over the tropical Indian Ocean and SST anomalies over the tropical western Atlantic Ocean in summer, however, the former acts to play a dominated role. The SST anomalies over the tropical Indian Ocean in summer affect the geopotential heights in upper troposphere over the northeastern Pacific and TUTT mainly by heating the overlying atmosphere, and exciting eastward-propagating Kelvin waves to the east of the equatorial Indian Ocean.(2) TUTT intensity exhibits an abrupt change in the late 1970s during the period of 1951-2010.Correspondingly the relationship between the summer TUTT and surface air temperature over Mexican Plateau and Great Lakes of North America has experienced an inter-decadal variation before and after the abrupt change. When the summer TUTT experiences a strong-to-weak change, the geopotential height in the upper-middle troposphere over Mexican Plateau(Great Lakes) of North America increases(decreases), thus the high which dominates over the southeast part of North America is correspondingly enhanced(weakend), leading to a increase (an decrease) in surface air temperature there. Besides, the decadal variation of TUTT intensity may be impacted by SST decadal variability over the tropical central eastern Pacific.(3) The location of TUTT mainly displayed an interannual variation during the period of 1951-2010 with its two main periodicities of 2-4 years and quasi-8 years. Results also show that the TUTT position is significantly negative correlated with the surface air temperature over the northwest part of North America. As the summer TUTT anomaly moves westward, the surface air temperature over northwest part of North America decreases and vice versa. Simultaneously, the variation of TUTT position is negatively correlated to the SST anomalies over the tropical western Atlantic Ocean in summer. As the summer SST anomalies over the tropical western Atlantic Ocean is anomalous lowness, the TUTT position over the northern Pacific in summer is westward than normal and vice versa.