[세미나] 송하준 교수님, 김정훈 교수님

연세대-서울대 공동세미나

Title :The Southern Ocean changes in response to the anthropogenic influences in the atmospheric circulation

The Southern Ocean is the place where the ocean interior meets the atmosphere allowing the exchange of heat and gases between them. Anthropogenic influences have led to a strengthening and poleward shift of westerly winds over the Southern Ocean, especially during austral summer. These changes of westerly winds are attributed to the cooling of the surface temperature in the last few decades. In response to the wind changes, previous work hypothesized a two-time scale response for sea surface temperature: Initially, Ekman transport cools the surface before sustained upwelling causes warming on decadal time scales. The fast response is robust across our models and the observations: We find Ekman-driven cooling in the mixed layer, mixing-driven warming below the mixed layer, and small upwelling-driven warming at the temperature inversion. The long- term response is inaccessible from observations. Neither of our models shows a persistent upwelling anomaly, or long-term, upwelling-driven subsurface warming. Mesoscale eddies act to oppose the anomalous wind-driven upwelling, through a process known as eddy compensation, thereby preventing long-term warming. Our results align with the recent trend in the air-sea carbon dioxide flux over the Southern Ocean.


[서울대 김정훈 교수님]

Title : Numerical Simulations of Tropospheric Disturbances Responsible for Upper-Level Turbulence in East Asia

Energies from mid- and upper- tropospheric disturbances induced by synoptic and mesoscale forcings cascade down to smaller-scale, which triggers micro-scale vertical mixings. This is of importance not only for understanding the Stratospheric-Tropospheric Exchange (STE), but also for predicting upper-level turbulence in Upper Troposphere and Lower Stratosphere (UTLS). When a cruising aircraft encountered turbulence unexpectedly, it causes in-flight injuries, structural damage, and flight delays, which results in extra costs for aviation users. Possible sources for those turbulence encounters in UTLS include various synoptic and meso scale weather phenomena like upper-level jet and frontogenesis with tropopause folding, inertial instability in anti-cyclonic shear/curvature flows, and interactions with convections and other physics. The presentation will show the recent studies on several turbulence events in East Asia, which has been examined by the Weather and Research Forecasting (WRF) model with multinested domains that capture large to small scale processes responsible for moderate or greater turbulence encounters. First case study for a turbulence outbreak in South Korea showed that upper-level frontogenesis is induced by thermally direct ageostrophic circulation in jet entrance region, which results in strong tropopause folding and turbulence in west coast of South Korea. Near Jeju Island turbulence is associated with inertial instability in strong anti-cyclonic shear side of upper-level jet. Dry run confirms that the mesoscale complex system (MCS) in mid China enhances those upper-level jet/frontal system across South Korea. Second study is about a severe turbulence just above the banded cirrus anvil cloud shield, which is associated with the mid-latitude cyclone in the northwestern Pacific Ocean. WRF model simulation with a finest horizontal grid spacing of 370 m reproduced well the banded structures within the southern edge of the anvil cirrus, which is consistent with the characteristics of the cirrus bands observed in infrared satellite data. Generation mechanism of the cirrus bands is similar to the horizontal convective roll in boundary layer. Difference here is that the convective instability is strongly driven by the radiative cooling at cloud top and warming at bottom of the layer within the cirrus anvil under the strong wind shear near anti-cyclonic upper-level ridge. The estimated value of the cube root of Eddy Dissipation Rate (EDR) within the bands corresponds to the severe level of turbulence experienced by a large aircraft.