Wonsun Park

The variability of the East Asian summer monsoon and its relationship to ENSO in a partially coupled climate model

The variability of the East Asian summer monsoon (EASM) is studied using a partially coupled climate model (PCCM) in which the ocean component is driven by observed monthly mean wind stress anomalies added to the monthly mean wind stress climatology from a fully coupled control run. The thermodynamic coupling between the atmospheric and oceanic components is the same as in the fully coupled model and, in particular, sea surface temperature (SST) is a fully prognostic variable. The results show that the PCCM simulates the observed SST variability remarkably well in the tropical and North Pacific and Indian Oceans. Analysis of the rainfall-SST and rainfall-SST tendency correlation shows that the PCCM exhibits local air-sea coupling as in the fully coupled model and closer to what is seen in observations than is found in an atmospheric model driven by observed SST. An ensemble of experiments using the PCCM is analysed using a multivariate EOF analysis to identify the two major modes of variability of the EASM. The PCCM simulates the spatial pattern of the first two modes seen in the ERA40 reanalysis as well as part of the variability of the first principal component (correlation up to 0.5 for the model ensemble mean). Different from previous studies, the link between the first principal component and ENSO in the previous winter is found to be robust for the ensemble mean throughout the whole period of 1958–2001. Individual ensemble members nevertheless show the breakdown in the relationship before the 1980’s as seen in the observations.

Southern Ocean Decadal Variability and Predictability

The Southern Ocean featured some remarkable changes during the recent decades. For example, large parts of the Southern Ocean, despite rapidly rising atmospheric greenhouse gas concentrations, depicted a surface cooling since the 1970s, whereas most of the planet has warmed considerably. In contrast, climate models generally simulate Southern Ocean surface warming when driven with observed historical radiative forcing. The mechanisms behind the surface cooling and other prominent changes in the Southern Ocean sector climate during the recent decades, such as expanding sea ice extent, abyssal warming, and CO2 uptake, are still under debate. Observational coverage is sparse, and records are short but rapidly growing, making the Southern Ocean climate system one of the least explored. It is thus difficult to separate current trends from underlying decadal to centennial scale variability. Here, we present the state of the discussion about some of the most perplexing decadal climate trends in the Southern Ocean during the recent decades along with possible mechanisms and contrast these with an internal mode of Southern Ocean variability present in state-of-the art climate models.

Influence of Model Bias on Simulating North Atlantic Sea Surface Temperature During the Mid‐Pliocene

Climate models generally underestimate the pronounced warming in the sea surface temperature (SST) over the North Atlantic during the mid‐Pliocene that is suggested by proxy data. Here we investigate the influence of the North Atlantic cold SST bias, which is observed in many climate models, on the simulation of mid‐Pliocene surface climate in a series of simulations with the Kiel Climate Model. A surface freshwater‐flux correction is applied over the North Atlantic, which considerably improves simulation of North Atlantic Ocean circulation and SST under present‐day conditions. Using reconstructed mid‐Pliocene boundary conditions with closed Bering and Arctic Archipelago Straits, the corrected model depicts significantly reduced model‐proxy SST discrepancy in comparison to the uncorrected model. A key factor in reducing the discrepancy is the stronger and more sensitive Atlantic Meridional Overturning Circulation and poleward heat transport. We conclude that simulations of mid‐Pliocene surface climate over the North Atlantic can considerably benefit from alleviating model biases in this region.

Internal Southern Ocean Centennial Variability: Dynamics, Impacts and Implications for Global Warming. Climate Change: Multidecadal and Beyond

It is well established that centennial climate variability can be externally forced by, e.g., quasi-oscillatory fluctuations of the solar constant or slowly varying atmospheric aerosol concentrations in association with changes of volcanic activity. Climate models recently suggested that substantial centennial variability can be also produced internally, and different competing mechanisms have been proposed. This paper deals with the internal centennial variability originating in the Southern Ocean Sector simulated by the Kiel Climate Model (KCM). In that model, the Southern Ocean centennial variability (SOCV) is linked to Weddell Sea deep convection activity and drives regional as well as global climate variations, as witnessed, for example, by coherent changes in Antarctic sea ice extent and globally averaged surface air temperature (SAT). Furthermore, the SOCV is associated with changes in deep Southern Ocean temperature in the KCM. Interestingly, a warming of the abyssal Southern Ocean has been observed during the recent decades, suggesting a contribution from SOCV. Another important impact of the SOCV in the model concerns the Atlantic Meridional Overturning Circulation (AMOC). The AMOC strengthens and deepens after the cessation of Weddell Sea deep convection and Antarctic Bottom Water (AABW) formation with a time delay of several decades to a century. Internal North Atlantic sea level variations can be as large as ± 15cm/century in the model with a strong contribution from the SOCV. Such regional sea level variations are of the same order of magnitude as the observed globally averaged 20th century sea level rise amounting to about 15-20cm. Finally, the KCM simulation suggests that the SOCV may have contributed to the current hiatus in global warming through an enhanced deep ocean heat uptake.