Mechanisms of the Boreal Summer Circulation Trend Pattern and Extreme Surface Temperature
Restricted (Penn State Only)
- Author:
- Kim, Dong Wan
- Graduate Program:
- Meteorology and Atmospheric Science
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 02, 2023
- Committee Members:
- David Stensrud, Program Head/Chair
Sukyoung Lee, Chair & Dissertation Advisor
Steven Feldstein, Major Field Member
Andrew Carleton, Outside Unit & Field Member
Laifang Li, Major Field Member - Keywords:
- Summer Circulation Dynamics
Extreme Surface Temperature
Long-term Trend - Abstract:
- In this dissertation, I investigate the mechanisms behind the boreal summer circulation trend pattern, which is the spatial pattern of the linear regression coefficient between streamfunction and time at each grid point, and the characteristics of regional surface temperature extremes are investigated. The summer circulation trend pattern in the western hemisphere is found to be induced by extratropical latent heating anomalies with additional support from the tropical heating. The North Pacific heating anomalies, which are aided by the moisture flux originating from the tropical western Pacific, initiate the circulation trend pattern. Then, the excited anomalous circulation induces another moisture flux originating from the Atlantic that releases latent heating anomalies over eastern Canada which further amplify the circulation trend pattern. The Eurasian trend pattern, which accompanies strong temperature anomalies over central Europe, is excited by the quasi-stationary dipole structure of enhanced latent heating over the North Sea region and suppressed heating over the Caspian Sea region. I find that without such quasi-stationary latent heating anomalies, the anomalous circulation is transient and therefore does not strongly influence the surface temperature. These latent heating anomalies not only induce the circulation anomalies on the intraseasonal timescale, but the decadal trend of the dipole heating structure also contributes to the associated wave pattern emerging in the long-term trend. Similarly, the Russian Far East trend pattern, which includes the upper-level anticyclone near Kamchatka Peninsula, is excited by latent heating anomalies over the Yakutia region located to the west of Kamchatka Peninsula. The trend pattern over this region is unlikely a downstream development of the Eurasian trend pattern but rather an independent teleconnection pattern, as the Yakutia heating-induced circulation anomalies occur regardless of the upstream condition. However, I found that the strength of surface temperature anomaly does depend on the characteristics of the upstream Eurasian circulation. When the upstream circulation resembles the Eurasian trend pattern, the circulation initiates the temperature anomaly over Russian Far East through temperature advection. Then, the anticyclone of the Russian Far East further amplifies the surface temperature. However, without such an upstream condition, this Russian Far East anticyclone alone cannot induce strong surface temperature anomalies because of its relatively transient characteristics. The findings of these consecutive studies indicate that the extratropical latent heating anomalies at multiple locations play an important role in inducing the boreal summer circulation anomalies that are closely linked to the surface heat extremes. I also investigate the characteristics of the regional extreme surface temperature anomaly development by using the thermodynamic budget equation. On intraseasonal timescales, I found that the horizontal temperature advection and the adiabatic vertical motion contribute to the extreme temperature growth over the extratropics and regions with steep surface topography, respectively. The diabatic process, which is a combination of radiative heating, latent heating, and vertical mixing, rather reveals the land-sea contrast pattern as the heating by the combination of latent and vertical mixing generally contributes to the anomaly growth over the land while longwave radiative heating strongly dampens the anomaly growth over the ocean. Lastly, I examine the role of intraseasonal timescale mechanisms in inducing the long-term trend of the zonally asymmetric skin temperature trend pattern using the surface energy balance equation. The zonally asymmetric temperature trend pattern includes hotspots of strong temperature trend over Europe and western North America. The trend over Europe is mainly aided by the shortwave flux trend driven by an anticyclonic circulation trend, while the trend over western North America is mostly contributed by the suppressed cooling trend of the latent heat flux associated with the trend in the soil moisture deficit. The findings indicate that the pronounced temperature trends over these two regions are driven by different types of the intraseasonal timescale processes.