Possible mechanism of abrupt jump in winter surface air temperature in the late 1980s over the Northern Hemisphere.
Possible cause of an abrupt warming in winter mean surface air temperature in the midlatitudes of the Northern Hemisphere in the late 1980s is investigated using observation and reanalysis data. To determine the timing of abrupt warming, we use a regime shift index based on detection of the largest significant differences between the mean values of two contiguous periods. Results show that the abrupt warming occurred in association with a regime shift after the 1980's in which the zonal mean sea level pressure (SLP) is significantly increased (decreased) at the latitude 25-35°N (60-70°N), in the form of north-south dipole-like SLP anomaly spanning the subtropics and high latitude. The dipole SLP anomaly can be attributed to a northward expansion of Hadley cell, a poleward broadening and intensification of the Ferrel cell, coupled with a collapse of polar cell. During the abrupt warming, strong anomalous southerly warm advection at the surface was induced by an enhanced and expanded Ferrel circulation, in association with a northward and downward shift of maximum center of northward eddy heat flux over the midlatitudes. An intensification of polar jet subsequent to regime shift may be instrumental in sustaining the warming up to more than 5 years.
School of Environmental Science and Engineering Pohang University of Science and Technology Pohang South Korea.
Yeon-Hee
YH
Kim
Department of Atmospheric Science Kongju National University Gongju South Korea.
Maeng-Ki
MK
Kim
Earth System Science Interdisciplinary Center University of Maryland College Park Maryland USA.
William K M
WK
Lau
Laboratory for Atmospheres NASA Goddard Space Flight Center Greenbelt Maryland USA.
Kyu-Myong
KM
Kim
National Institute of Meteorological Sciences Seogwipo South Korea.
Chun-Ho
CH
Cho
Ferrel cell; Hadley cell; abrupt warming; eddy heat flux; regime shift