Changes in the Dynamics of a Blocking Event with Warming

Many previous studies have demonstrated that synoptic-scale transients play an important role in the formation and maintenance of blocking or persistent anomaly (PA) events. From a potential-vorticity (PV) perspective, PA events can be viewed as volumes of anomalous PV in the upper troposphere, or equivalently, as regions of anomalous potential temperature on the dynamic tropopause. Transients contribute to the onset and maintenance of these anomalies both via advective and diabatic processes. Here, we explore the general question of whether, and how, climate warming may affect the dynamics and character of PAs, and specifically, we consider possible changes in the dynamical contribution of transients to the onset and maintenance of PAs in a changing climate.

We selected a long-lived, impactful blocking event from November, 1985 over the North Pacific Ocean for a detailed case study. Control simulations with both the Weather, Research, and Forecasting (WRF) model and the Model for Prediction Across Scales (MPAS) were able to capture the onset and initial amplification of this event. With the WRF model, a pseudo-global warming (PGW) simulation was conducted, along with runs in which condensational heating was eliminated. This suite of simulations allows us to quantify the effect of diabatic processes and climate change on the transient interactions. An analogous set of simulations was conducted with the global MPAS model, though the MPAS PGW experiment is not identical to that with WRF owing to the lack of lateral boundary conditions. This experiment involved applying warming to initial conditions and lower boundary conditions, and altering trace gas concentrations consistently with the RCP8.5 CMIP5 experiments.

Based on previous work, one logical hypothesis is that the diabatic component of initiation and maintenance of PAs will increase with warming, owing to increased water vapor content and precipitation rates. This effect could result in larger PA amplitude and duration, though compensating processes may intervene. We test this hypothesis using both the limited-area and global model simulations, via comparison of control, no-latent-heating, and warming runs. Our analysis involves decomposition of the flow into time-averaged and transient components, and computation of a potential temperature budget for the dynamic tropopause in the vicinity of the blocking event. Though our analysis of diabatic and advective transient contributions is not yet complete, preliminary analysis of the WRF simulations demonstrates that the PGW simulation yields a longer-lived and larger amplitude blocking event relative to the current climate.