A Northern Hemispheric Wave Train Associated with Interannual Variations in the Bermuda High during Boreal Summer

Abstract

AbstractThe processes that lead to the spatial and temporal evolution of the Bermuda high (BH) during July and August (JA) are investigated on the basis of linear regression analysis. The analysis is based on a Bermuda high index (BHI): the difference in standardized, deseasonalized, and detrended sea level pressure (SLP) between northeast of Bermuda (40°N, 60°W) and New Orleans (30°N, 90°W). Negative values of BHI indicate a westward expansion of the Bermuda high relative to its climatological-mean location and reduced precipitation in the southeastern United States (SEUS), whereas positive values correspond to BH contraction and enhanced precipitation in the SEUS. Linear regression of the 200-hPa geopotential height based on the BHI reveals the existence of a Rossby wave train that extends zonally from the eastern North Pacific to the eastern North Atlantic. The troughs and ridges associated with this wave train are spatially collocated with the climatological-mean jet stream, indicating that the jet serves as their waveguide. Anomalous troughing in the SEUS associated with this wave train is linked to the contraction of the Bermuda high during JA. The enhanced precipitation is associated with anomalous ascent to the east and south of this trough where anomalous warm advection is observed. Based on these results, it is hypothesized that this Rossby wave train may partially explain the occurrence of suppressed precipitation tied to midsummer drought in the SEUS during July and August. It is found that the BHI has trended from negative to positive in recent decades, suggesting that it may be influenced by low-frequency variability.