Relationship between Boreal Summer Circulation Trend and Destructive Stationary-Transient Wave Interference in the Western Hemisphere

Abstract:

This study examines the role of the latent heating in exciting the upper-level circulation anomaly which destructively interferes with the climatological stationary wave in the Western Hemisphere during boreal summer. This destructive interference pattern closely resembles the circulation trend which is known to be responsible for surface heat extreme trends. To investigate the mechanism behind this circulation anomaly, daily stationary-transient wave interference and related meteorological variables are analyzed using reanalysis data for the period of 1979-2017.

Numerical model simulations forced by observed heating anomalies indicate that the destructive interference pattern is most effectively excited by latent heating anomalies over the North Pacific and the eastern Canada. The North Pacific heating anomaly drives circulation anomalies that not only project positively onto the destructive interference pattern, but also transports moisture into the eastern Canada. The resulting latent heating over the eastern Canada drives circulation that further reinforces the destructive interference pattern which includes a prominent high pressure system over Greenland.

Tropical heating also plays a role in driving the destructive interference pattern. On intraseasonal time scales, the destructive interference pattern is preceded by suppressed Indo-western Pacific heating and enhanced North American monsoon heating. On decadal time scales, both heating centers have strengthened, but the trend of the North American monsoon heating was greater than that of the Indo-Western Pacific heating. These uneven heating trends help explain the resemblance between the destructive interference pattern and the circulation trend over the Western Hemisphere.