Heat Wave Frequency Variability over North America: Two Distinct Leading Modes

Seasonal prediction of heat wave variability is a scientific challenge and of practical importance. This study investigates the heat wave frequency (HWF) variability over North America (NA) during the past 53 summers (19582010). It is found that the NA HWF is dominated by two distinct modes, namely, the inter-decadal (ID) mode and the inter-annual (IA) mode. The ID mode primarily depicts a HWF increasing pattern over most of the NA continent except some western coastal areas. The IA mode resembles a tri-pole HWF anomaly pattern with three centers over the northwestern, central and southern NA, respectively. The two leading modes have different dynamic structures and predictability sources. The ID mode is closely associated with the prior spring sea surface temperature anomaly (SSTA) in the tropical Atlantic and tropical western Pacific that can persist throughout the summer, whereas the IA mode is linked to the development of El NiñoSouthern Oscillation (ENSO). A simplified general circulation model is utilized to examine the possible physical mechanism. For the ID mode, the tropical Atlantic SSTA can induce a Gill- type response which extends to NA, while the northwestern Pacific SSTA excites a Rossby wave train propagating eastward towards NA. These two flow patterns jointly contribute to the formation of the large-scale circulation anomalies associated with the ID mode. For the IA mode, the corresponding circulation anomalies are basically similar to a Pacific-North American (PNA) pattern. The subsidence associated with high pressure anomalies warms and dries the boundary layer, inhibiting cloud formation. The resulting surface radiative heating further warms the surface. For the low pressure anomalies, the situation is just opposite. Through such processes, these SSTAs can exert profound influences to the HWF variability over NA.