Response of the NCAR General Circulation Model to Prescribed Changes in Ocean Surface Temperature Part I: Mid-Latitude Changes

Abstract

Four numerical experiments are analyzed to determine the three-dimensional response of the NCAR general circulation model to large prescribed changes in mid-latitude North Pacific Ocean surface temperature. The ocean surface temperature (OST) boundary conditions are subjected to changes of opposite sign in the eastern and west-central portions of the North Pacific Ocean. The maximum amplitude of the temperature changes is either 12°C or 4°C. The model atmosphere response in the North Pacific sector includes changes in amplitude and vertical tilt of the long waves, an increased direct thermal circulation (i.e., warm air rises over the positive OST change and cold air sinks over the negative OST change), and locally enhanced westerlies to the north of the positive OST change. Cyclones form and/or intensify over the positive OST change and tend to be absent or weak over the negative OST change. The mid-tropospheric response extends downstream from the prescribed change region, and the response both over and downstream from the region depends strongly on the longitude of the prescribed changes. Many features of the response are statistically significant, although generally not over the continental United States. The amplitude and phase of the mid-tropospheric long waves (zonal wavenumbers 1–4) are also affected. The prescribed change response is largest and of greatest statistical significance when the prescribed change is very large (12°C maximum amplitude) but is also frequently detectable when the prescribed change is one-third as large (4°C maximum amplitude). A comparable experiment involving a prescribed North Atlantic OST change produces a similar mid-tropospheric response.