Sang-Ik Shin

Modeling El Niño and its tropical teleconnections during the last glacial‐interglacial cycle

(1) Simulations with the NCAR Climate System Model (CSM), a global, coupled ocean-atmosphere-sea ice model, for the last glacial-interglacial cycle reproduce recent estimates, based on alkenones and Mg/Ca ratios, of sea surface temperature (SST) changes and gradients in the tropical Pacific and predict weaker El Ninos/La Ninas compared to present for the Holocene and stronger El Ninos/La Ninas for the Last Glacial Maximum (LGM). Changes for the LGM (Holocene) are traced to a weakening (strengthening) of the tropical Pacific zonal SST gradient, wind stresses, and upwelling and a sharpening (weakening) of the tropical thermocline. Results suggest that proxy evidence of weaker precipitation variability in New Guinea and Ecuador are explained not only by changes in El Nino/ La Nina but also changes in the atmospheric circulation and hydrologic cycle. INDEX TERMS: 3344 Meteorology and Atmospheric Dynamics: Paleoclimatology; 4267 Oceanography: General: Paleoceanography; 4522 Oceanography: Physical: El Nino; 3337 Meteorology and Atmospheric Dynamics: Numerical modeling and data assimilation; 9604 Information Related to Geologic Time: Cenozoic. Citation: Otto-Bliesner, B. L., E. C. Brady, S.-I. Shin, Z. Liu, and C. Shields, Modeling El Nino and its tropical teleconnections during the last glacial-interglacial cycle, Geophys. Res. Lett., 30(23), 2198, doi:10.1029/2003GL018553, 2003.

Tropical cooling at the last glacial maximum and extratropical ocean ventilation1

An optical device, useful for three-dimensional stereo viewing not requiring any mechanical shuttering, including two liquid crystal lenses, each comprising: (1) a thin layer of liquid crystal material, preferably nematic liquid crystal; (2) two plates of conductively-coated transparent material enclosing the liquid crystal, each plate being connectable to a source of voltage, and each plate being insulated from the other; (3) a front polarizing sheet forming the outside surface of the crystal lens; and (4) a rear polarizing sheet forming the inner surface of the crystal lens. The axes of polarization of the front and rear polarizing sheets are at right angles to each other, with the result that maximum light transmission through each of the lenses occurs when no voltage is applied across the conductively-coated plates, and minimum light transmission occurs when a voltage is applied across the coated plates.

Understanding the mid-holocene climate

Abstract Paleoclimatic evidence suggests that during the mid-Holocene epoch (about 6000 yr ago) North America and North Africa were significantly drier and wetter, respectively, than at present. Modeling efforts to attribute these differences to changes in orbital parameters and greenhouse gas (GHG) levels have had limited success, especially over North America. In this study, the importance of a possibly cooler tropical Pacific Ocean during the epoch (akin to a permanent La Nina–like perturbation to the present climate) in causing these differences is emphasized. Systematic sets of atmospheric general circulation model experiments, with prescribed sea surface temperatures (SSTs) in the tropical Pacific basin and an interactive mixed layer ocean elsewhere, are performed. Given the inadequacies of current fully coupled climate models in simulating the tropical Pacific climate, this intermediate coupling model configuration is argued to be more suitable for quantifying the contributions of the altered orbit...

Sensitivity of the Northern Hemisphere climate system to extreme changes in Holocene Arctic sea ice

The extent of seasonal and perennial sea ice changed dramatically through the Late Quaternary and these changes influenced both the ocean and atmosphere by controlling the exchange of energy, moisture and gases between them, and by altering the planetary albedo. Reconstructing the changing patterns of sea ice distribution in the recent past remains one of the outstanding challenges to the paleo-community. To evaluate the importance of these reconstructions we performed sensitivity tests using NCARs Community Climate Model (CCM3), and a series of prescribed sea ice extents designed to capture the full range of Arctic sea ice variability under interglacial (Holocene) and full glacial (Last Glacial Maximum) boundary conditions. Our simulations indicate that surface temperatures and sea level pressures in winter (DJF) are most sensitive to changes in sea ice, and that these changes are propagated over the surrounding land masses in the North Atlantic, but that equivalent changes in sea ice produce smaller corresponding changes in temperature or sea level pressure in the North Pacific region. A comparison between CLIMAP (Map Chart Series MC-36, Geological Society of America, Boulder, CO, 1981) and a more realistic assessment of LGM sea ice yields dramatic changes in winter temperatures and precipitation patterns across Eurasia. These differences, forced only by changed sea ice conditions, reinforce the need to develop accurate maps of past sea ice to correctly simulate Late Quaternary environments. Such reconstructions also will be essential to validate the next generation of sea ice models.

On thermocline ventilation of active and passive tracers

The ventilation propagation of temperature and passive tracer anomalies are compared in a numerical model. Although both anomalies tend to have pathways close to the mean ventilation flow, the temperature anomaly could have a slower propagation speed and a more complex spatial structure. The temperature propagation is better understood in terms of baroclinic planetary waves.

Optimal Tropical Sea Surface Temperature Forcing of North American Drought

Abstract The optimal anomalous sea surface temperature (SST) pattern for forcing North American drought is identified through atmospheric general circulation model integrations in which the response of the Palmer drought severity index (PDSI) is determined for each of 43 prescribed localized SST anomaly “patches” in a regular array over the tropical oceans. The robustness and relevance of the optimal pattern are established through the consistency of results obtained using two different models, and also by the good correspondence of the projection time series of historical tropical SST anomaly fields on the optimal pattern with the time series of the simulated PDSI in separate model integrations with prescribed time-varying observed global SST fields for 1920–2005. It is noteworthy that this optimal drought forcing pattern differs markedly in the Pacific Ocean from the dominant SST pattern associated with El Nino–Southern Oscillation (ENSO), and also shows a large sensitivity of North American drought to ...

Response of the Equatorial Thermocline to Extratropical Buoyancy Forcing

Abstract The GFDL Modular Ocean Model and the Miami Isopycnal Ocean Model are used to investigate the response of the equatorial thermocline to extratropical buoyancy forcing. Passive tracers and analytical theories are also used to shed light on the dynamics of the thermocline response. The major findings are the following. (i) The midlatitude region seems to be the optimal region for surface buoyancy forcing to affect the equatorial thermocline. This occurs because, first, thermocline anomalies in the midlatitudes can penetrate into the equator very efficiently; second, buoyancy forcing generates a strong local response in the midlatitudes. (ii) Dynamic waves as well as thermocline ventilation contribute to the response in the equatorial thermocline. Consequently, equatorward penetration is substantially greater for a temperature anomaly than for a passive tracer. (iii) Midlatitude forcing generates a significant temperature response in the equatorial thermocline for forcing periods longer than decadal....

Dynamical and observational constraints on tropical Pacific sea surface temperatures at the last glacial maximum

Asynchronously coupled atmosphere and ocean general circulation model simulations are used to examine the consequences of changes in the west/east sea-surface temperature (SST) gradient across the equatorial Pacific at the last glacial maximum (LGM). Simulations forced by the CLIMAP SST for the LGM, where the west/east SST gradient across the Pacific is reduced compared to present, produce a reduction in the strength of the trade winds and a decrease in the west/east slope of the equatorial thermocline that is incompatible with thermocline depths newly inferred from foraminiferal assemblages. Stronger-than-present trade winds, and a more realistic simulation of the thermocline slope, are produced when eastern Pacific SSTs are 2°C cooler than western Pacific SSTs. Our study highlights the importance of spatial heterogeneity in tropical SSTs in determining key features of the glacial climate.

Tropical Climate Regimes and Global Climate Sensitivity in a Simple Setting

Multiple tropical climate regimes are found in an atmospheric general circulation model (AGCM) coupled to a global slab ocean when the model is forced by different values of globally uniform insolation. Even in this simple setting, convection organizes into an intertropical convergence zone (ITCZ) solely due to the effect of planetary rotation, as was found in Kirtman and Schneider, for a single value of insolation. Here the response to a range of insolation values is explored, and surprisingly, multiple climate regimes characterized by radically different ITCZ structures are found. In order from the coldest to warmest climates, these are a symmetric double ITCZ, a near-symmetric equatorial ITCZ, a transient asymmetric ITCZ, and a stable, strongly asymmetric ITCZ. The model exhibits hysteresis in the transition from the near-symmetric to the strongly asymmetric ITCZ regimes when insolation is increased and then decreased. The initial transition away from symmetry can occur in the absence of air–sea coupling; however, the coupling is essential for the establishment and maintenance of the strongly asymmetric ITCZ. Wind–evaporation–SST feedback as well as the longwave radiative effects of clouds and water vapor on SSTs appear to be important in maintaining the asymmetric regime. The existence of multiple regimes in a single AGCM, and the dependence of these regimes on SST feedbacks, may have a bearing on the ITCZ simulation errors of current coupled climate models. The sensitivity of the global mean surface temperature generally decreases with increasing insolation, a consequence primarily of increasingly negative shortwave cloud forcing. Climate sensitivity measured across a regime transition can be much larger than the sensitivity within a single regime.

Realism of local and remote feedbacks on tropical sea surface temperatures in climate models

[1] An important emerging issue in climate research is the degree to which a sea surface temperature (SST) change in one tropical ocean basin affects the SST in other basins. In this study, the SST interactions among eight broadly defined regions of coherent SST variability in the tropical Pacific, Indian, and Atlantic oceans are estimated using 3 observational and 76 climate model simulation data sets of the 20th century. The eight-dimensional SST feedback matrix is estimated separately using each data set by constructing a Linear Inverse Model based on the lag-covariance statistics of the 100 year monthly SST time series. The simulated feedback matrices are found to differ in several key respects from the observed matrices and also from one another. In particular, the influence of the eastern Pacific ENSO region on other regions and of the other regions on the ENSO region is found to vary considerably from model to model. The representation of remote interactions with the Indo-Pacific Warm Pool region is also found to be highly variable. It is argued that these large errors/differences arise mainly from differences in the representation of the remote atmospheric teleconnective feedbacks, and to a lesser extent the local radiative-thermodynamic feedbacks, on the SSTs in the models, whereas differences in the representation of the tropical oceanic wave dynamics are likely less important.