Mechanism for the Sudden Rise in U.S. Temperatures
Marty Hoerling, ESRL, Physical Sciences Division
This study examines mechanisms by which oceans may be contributing to the sudden rise in U.S. temperature during 1977-2006. Using atmospheric climate models, we consider the separate impacts of tropical and extratropical SST trends during the last 30 years. The atmospheric models indicate that the recent U.S. warming during October-March is consistent with an observed increase in extratropical SSTs. The simultaneous warming of both North Pacific and North Atlantic SSTs is shown to induce a poleward displacement of the wintertime westerlies, causing widespread U.S. warming. In contrast, an increase in SSTs over much of the tropical oceans observed during recent decades, while producing an annular-like atmospheric response in the tropics, induces a wavetrain extratropical response that yields little net warming of the U.S.. While it is difficult to separate the effects of internal ocean variability from the effects of external radiative forcing, additional simulations are presented that contrast the dynamical consequences of externally forced versus observed SST changes.
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