Mid-latitude North Pacific Ocean Research Articles

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

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.

Predictability of Sea Surface Temperature and Sea Level Pressure Anomalies over the North Pacific Ocean

Nonseasonal variability of sea level pressure (SLP) and sea surface temperature (SST) in the mid-latitude North Pacific Ocean is examined. The objective is examination of the basic scales of the variability and determination of possible causal connections which might allow prediction of short-term climatic (time scales between a month and a year) variability. Using empirical orthogonal function descriptions of the spatial structure, it is found that SLP variability is concentrated in a few large-scale modes but has a nearly white frequency spectrum. SST variability is spatially complex (being spread over many spatial modes, some of which have small-scale changes) but is dominated by low-frequency changes. The use of linear statistical estimators to examine predictability is discussed and the importance of limiting the number of candidate data used in a correlation starch is underscored. Using linear statistical predictors, it is found that (A) SST anomalies can be predicted from SST observations several months in advance with measurable skill, (B) the anomalous SLP variability can be specified from simultaneous SST data with significant skill, thus showing the fields are related, and (C) future SLP anomalous variability cannot be predicted from SST data although previous SLP can be specified. The fact that previous SLP variability is better specified by SST data than is simultaneous SLP variability, coupled with a complete inability to predict future SLP anomalies, suggests that, in the region studied and on the time scales of a month to a year, the observed connection between SST and SLP variabilities is the result of the atmosphere driving the ocean.

Testing the Statistical Significance of the Response of the NCAR General Circulation Model to North Pacific Ocean Surface Temperature Anomalies

The response of the six-layer NCAR atmospheric general circulation model (GCM) to mid-latitude North Pacific Ocean surface temperature anomalies is examined. In particular, January and winter season averages of the 1.5 km temperature fields generated by the GCM with prescribed ocean surface temperatures (i.e., the control experiment) are subtracted from January and winter season averages of three separate prescribed change experiments in which ocean surface temperature anomalies are superimposed on the ocean surface temperature patterns of the control case. These differences between prescribed change and control experiments are compared at each grid point to the standard deviations of the 1.5 km temperature field constructed from five independent GCM generated January simulations. The statistical significance of the results is evaluated using a methodology developed for the GCM by Chervin and Schneider based on the classical Student's t-test. The results suggest that a statistically significant signal can be detected in the immediate vicinity of large ocean surface temperature anomalies, but that teleconnections of this signal downstream over the United States can be identified only with a small statistical confidence. Some implications of these results and suggestions for further work are presented.