Two Types of California Central Valley Summer Heat waves
Understanding the mechanisms of how Californian Central Valley (CCV) extreme heat waves develop is very important as the events have major impacts on the economy and human safety. This study diagnoses the temporal and spatial evolution as well as the thermodynamics and dynamics of Large Scale Meteorological Patterns (LSMPs) during extreme CCV heat waves. 28 extreme summer heat waves between 1977 to 2010 are roughly divided into two clusters: (1) locally formed heat waves with rapid development, and (2) heat waves developed in Northwest America, which have wider coverage and advects heat over the CCV region. Adiabatic heating of air through the anomalous sinking motion and its horizontal advection is the main local heating mechanism for both clusters, but the spatial distribution of sinking and horizontal flow is often distinct. A ridge-trough-ridge LSMP spanning the North Pacific is built by the wave activity flux traveling across North Pacific in the first cluster. In the second cluster, the LSMP is stationary further north where sinking and advection expand the coast ridge south over the region. Trajectory analysis shows most air parcels sink from the upper layers to the west of the CCV, while the trajectory origins and paths differ between clusters.
Two types of heat waves are examined in historical simulations of 14 climate models in Phase 5 of the Coupled Model Inter-comparison Project (CMIP5). Extreme temperature events in the models are also separated into two clusters. However, models vary quite a bit in the separation between the clusters, as well as the magnitude and the occurrence frequency of the two clusters. In addition, models of higher horizontal resolution or lower vertical top (with relatively poorly resolved stratosphere) tend to better simulate the two types.
Links to tropical convections are explored in terms of boreal summer Madden Julien Oscillation (MJO). It is shown that MJO convection in the Indian Ocean and eastern tropical Pacific is likely followed by CCV heat waves. After strong 2-3 and 8- 1 MJO phase pairs, convergence over divergence vertical structure and a sinking motion over California are part of a remote and lagged response extending into the downstream subtropical central Pacific. The association with precedent MJO convection is more evident in the first cluster than the second cluster.