Seong-Joong Kim

Weakening of the stratospheric polar vortex by Arctic sea-ice loss

Successive cold winters of severely low temperatures in recent years have had critical social and economic impacts on the mid-latitude continents in the Northern Hemisphere. Although these cold winters are thought to be partly driven by dramatic losses of Arctic sea-ice, the mechanism that links sea-ice loss to cold winters remains a subject of debate. Here, by conducting observational analyses and model experiments, we show how Arctic sea-ice loss and cold winters in extra-polar regions are dynamically connected through the polar stratosphere. We find that decreased sea-ice cover during early winter months (November-December), especially over the Barents-Kara seas, enhances the upward propagation of planetary-scale waves with wavenumbers of 1 and 2, subsequently weakening the stratospheric polar vortex in mid-winter (January-February). The weakened polar vortex preferentially induces a negative phase of Arctic Oscillation at the surface, resulting in low temperatures in mid-latitudes.

The Melting Arctic and Midlatitude Weather Patterns: Are They Connected?*

AbstractThe potential of recent Arctic changes to influence hemispheric weather is a complex and controversial topic with considerable uncertainty, as time series of potential linkages are short (<10 yr) and understanding involves the relative contribution of direct forcing by Arctic changes on a chaotic climatic system. A way forward is through further investigation of atmospheric dynamic mechanisms. During several exceptionally warm Arctic winters since 2007, sea ice loss in the Barents and Kara Seas initiated eastward-propagating wave trains of high and low pressure. Anomalous high pressure east of the Ural Mountains advected Arctic air over central and eastern Asia, resulting in persistent cold spells. Blocking near Greenland related to low-level temperature anomalies led to northerly flow into eastern North America, inducing persistent cold periods. Potential Arctic connections in Europe are less clear. Variability in the North Pacific can reinforce downstream Arctic changes, and Arctic amplification...

Recent recovery of the Siberian High intensity

This study highlights the fast recovery of the wintertime Siberian High intensity (SHI) over the last two decades. The SHI showed a marked weakening trend from the 1970s to 1980s, leading to unprecedented low SHI in the early 1990s according to most observational data sets. This salient declining SHI trend, however, was sharply replaced by a fast recovery over the last two decades. Since the declining SHI trend has been considered as one of the plausible consequences of climate warming, the recent SHI recovery seemingly contradicts the continuous progression of climate warming in the Northern Hemisphere. We suggest that alleviated surface warming and decreased atmospheric stability in the central Siberia region, associated with an increase in Eurasian snow cover, in the recent two decades contributed to this rather unexpected SHI recovery. The prominent SHI change, however, is not reproduced by general circulation model (GCM) simulations used in the IPCC AR4. The GCMs indicate the steady weakening of the SHI for the entire 21st century, which is found to be associated with a decreasing Eurasian snow cover in the simulations. An improvement in predicting the future climate change in regional scale is desirable.

The Impact of Southern Ocean Sea Ice in a Global Ocean Model

Abstract Most of the Southern Ocean (SO) is marginally stably stratified and thus prone to enhanced convection and possibly bottom-water formation whenever the upper ocean is cooled or made more saline by ice formation. Sea ice modifies the heat and freshwater fluxes, which in turn constitute a critical surface condition in this sensitive region of intense vertical exchange. The authors investigate the effect of SO sea ice in modifying these fluxes in a global, coarse-resolution, primitive-equation ocean general circulation model, which has been coupled to a comprehensive dynamic–thermodynamic sea ice model. Specifically, the long-term impact of a series of modifications in the formulation of the sea ice model and its forcing on quantities such as the overturning circulation, the deep ocean water-mass characteristics, the sea ice thickness, the strength of convection, as well as the strength of the major volume transports are investigated. The results indicate that the rate of Antarctic bottom-water forma...

On the Role of Sea Ice and Convection in a Global Ocean Model

Abstract An earlier estimate regarding the possible impact of sea ice on deep-ocean water mass properties and the global thermohaline circulation in a coupled sea ice–ocean general circulation model (OGCM) is updated. Compared to the earlier application, the main upgrade is a subgrid-scale plume-convection parameterization that replaces conventional grid-cell-wide convective adjustment. The different treatment of convection leads to some noticeable differences in some of the repeated sensitivity experiments. For example, in an experiment where sea ice salinity is assumed to be that of the upper ocean, thus neglecting the primary effect of sea ice formation and melting on the oceans buoyancy forcing, Antarctic Bottom Water formation comes essentially to a halt, the global deep-ocean properties and thermohaline circulation thus being almost solely determined by North Atlantic Deep Water. The much weaker impact in the earlier estimate turns out to be mainly due to excessive open-ocean convection in the Sout...

Increased Pliocene North Atlantic Deep Water: Cause or consequence of Pliocene warming?

Raymo et al. [1996] suggested that the mid-Pliocene (∼3 Ma) warm period was associated with increased North Atlantic Deep Water (NADW) production. Is this circulation change a cause or consequence of Pliocene warming? We test the hypothesis that increased strength of NADW was a consequence of the warming around Antarctica affecting deep Antarctic outflow. A sensitivity experiment with an ocean general circulation model with Pliocene surface conditions changed only over the Southern Ocean (SO) indicates that warmer temperatures around Antarctica result in lower rates of sea ice formation and SO deep water outflow. The decreased abyssal density gradient in the SO directly leads to about a 20% increase in NADW outflow at 30°S, a 10% increase in NADW overturning in the subpolar North Atlantic, and a 20% increase in poleward heat transport in the North Atlantic. We postulate that the largest initial Pliocene climate change was in the SO because the greater sea ice area in this region is more sensitive to inferred slightly higher CO2 levels in the mid-Pliocene.

Impacts of Snow Initialization on Subseasonal Forecasts of Surface Air Temperature for the Cold Season

AbstractThe present study examines the impacts of snow initialization on surface air temperature by a number of ensemble seasonal predictability experiments using the NCAR Community Atmosphere Model version 3 (CAM3) AGCM with and without snow initialization. The study attempts to isolate snow signals on surface air temperature. In this preliminary study, any effects of variations in sea ice extent are ignored and do not explicitly identify possible impacts on atmospheric circulation. The Canadian Meteorological Center (CMC) daily snow depth analysis was used in defining initial snow states, where anomaly rescaling was applied in order to account for the systematic bias of the CAM3 snow depth with respect to the CMC analysis. Two suites of seasonal (3 months long) ensemble hindcasts starting at each month in the colder part of the year (September–April) with and without the snow initialization were performed for 12 recent years (1999–2010), and the predictability skill of surface air temperature was estima...

Major cause of unprecedented Arctic warming in January 2016: Critical role of an Atlantic windstorm.

In January 2016, the Arctic experienced an extremely anomalous warming event after an extraordinary increase in air temperature at the end of 2015. During this event, a strong intrusion of warm and moist air and an increase in downward longwave radiation, as well as a loss of sea ice in the Barents and Kara seas, were observed. Observational analyses revealed that the abrupt warming was triggered by the entry of a strong Atlantic windstorm into the Arctic in late December 2015, which brought enormous moist and warm air masses to the Arctic. Although the storm terminated at the eastern coast of Greenland in late December, it was followed by a prolonged blocking period in early 2016 that sustained the extreme Arctic warming. Numerical experiments indicate that the warming effect of sea ice loss and associated upward turbulent heat fluxes are relatively minor in this event. This result suggests the importance of the synoptically driven warm and moist air intrusion into the Arctic as a primary contributing factor of this extreme Arctic warming event.

Impact of Subgrid-Scale Convection on Global Thermohaline Properties and Circulation

Abstract In most ocean general circulation models the simulated global-scale deep-ocean thermohaline properties appear to be chronically colder and fresher than observed. To some extent, this discrepancy has been known to be due to excessive open-ocean deep convection in the Southern Ocean (SO) caused by crude “convective adjustment” parameterizations on scales typically two orders of magnitude larger than the actual convection scale. To suppress the strength of open-ocean convection and to thereby eventually improve the global deep-ocean water properties, the authors first reduced convection in the SO in an ad hoc manner by activating it every 10 days rather than every model time step (20 hours). Second, a more physically based subgrid-scale convection in the SO was introduced by applying the penetrative plume convection scheme of Paluszkiewicz and Romea. With both treatments, SO convection decreased by about 30%, and the globally averaged deep-ocean potential temperature and salinity increased substanti...

Connecting early summer cloud‐controlled sunlight and late summer sea ice in the Arctic

This study demonstrates that absorbed solar radiation (ASR) at the top of the atmosphere in early summer (May–July) plays a precursory role in determining the Arctic sea ice concentration (SIC) in late summer (August–October). The monthly ASR anomalies are obtained over the Arctic Ocean (65°N–90°N) from the Clouds and the Earths Radiant Energy System during 2000–2013. The ASR changes primarily with cloud variation. We found that the ASR anomaly in early summer is significantly correlated with the SIC anomaly in late summer (correlation coefficient, r ≈ −0.8 with a lag of 1 to 4 months). The region exhibiting high (low) ASR anomalies and low (high) SIC anomalies varies yearly. The possible reason is that the solar heat input to ice is most effectively affected by the cloud shielding effect under the maximum TOA solar radiation in June and amplified by the ice-albedo feedback. This intimate delayed ASR-SIC relationship is not represented in most of current climate models. Rather, the models tend to over-emphasize internal sea ice processes in summer.