Winter low-pressure and high-pressure activity in the North Pacific mid-latitudes: 1950-2100.

Summary Using a new automatic scheme, mid-latitude North Pacific synoptic low and high trajectories were constructed from daily sea level pressure (SLP) data over the period 1950-2001. Over the eastern North Pacific, high pressure strength decreases and their frequency becomes more variable, while lows intensify and exhibit more southerly trajectories from the mid-1970s. Thus, winter low-pressure and high-pressure activity are significantly anti-correlated. These changes in transient activity result in weaker PNMs at the Aleutian Low, consistent with the positive phase of the North Pacific Oscillation, inducing positive ocean surface temperature (OST) anomalies along the west coast of North America and negative OST anomalies over the central basin. These conditions are associated with warmer temperatures and more precipitation from Alaska to Mexico and over the southwestern United States. According to the simulations of the Coupled Global Model of the National Center for Meteorological Research version 3, the winter frequency of lows could be significantly reduced in a context of strong increase of greenhouse gases. This modification could be favorable to anticyclonic conditions over the East, inducing a weighting of anthropogenic warming along the North American west coast and a reduction/increase in precipitation over the regions located south and north of 45°N respectively.