Yen-Ting Hwang

Extratropical Influence on ITCZ Shifts in Slab Ocean Simulations of Global Warming

AbstractRecent studies with climate models have demonstrated the power of extratropical forcing in causing the intertropical convergence zone (ITCZ) to shift northward or southward, and paleoclimate data support the notion that there have been large shifts in the ITCZ over time. It is shown that similar notions apply to slab ocean simulations of global warming. Nine slab ocean model simulations from different modeling centers show a wide range of ITCZ shifts in response to doubling carbon dioxide concentrations, which are experienced in a rather zonally symmetric way in the tropics. Using an attribution strategy based on fundamental energetic constraints, it is shown that responses of clouds and ice in the extratropics explain much of the range of ITCZ responses. There are also some positive feedbacks within the tropics due to increasing water vapor content and high clouds in the new ITCZ location, which amplify the changes driven from the extratropics. This study shows the clear importance of simulating ...

Link between the double-Intertropical Convergence Zone problem and cloud biases over the Southern Ocean

The double-Intertropical Convergence Zone (ITCZ) problem, in which excessive precipitation is produced in the Southern Hemisphere tropics, which resembles a Southern Hemisphere counterpart to the strong Northern Hemisphere ITCZ, is perhaps the most significant and most persistent bias of global climate models. In this study, we look to the extratropics for possible causes of the double-ITCZ problem by performing a global energetic analysis with historical simulations from a suite of global climate models and comparing with satellite observations of the Earth’s energy budget. Our results show that models with more energy flux into the Southern Hemisphere atmosphere (at the top of the atmosphere and at the surface) tend to have a stronger double-ITCZ bias, consistent with recent theoretical studies that suggest that the ITCZ is drawn toward heating even outside the tropics. In particular, we find that cloud biases over the Southern Ocean explain most of the model-to-model differences in the amount of excessive precipitation in Southern Hemisphere tropics, and are suggested to be responsible for this aspect of the double-ITCZ problem in most global climate models.

Interhemispheric Temperature Asymmetry over the Twentieth Century and in Future Projections

The temperature contrast between the Northern and Southern Hemispheres—the interhemispheric temperature asymmetry (ITA)—is an emerging indicator of global climate change, potentially relevant to the Hadley circulation and tropical rainfall. The authors examine the ITA in historical observations and in phases 3 and 5 of the Coupled Model Intercomparison Project (CMIP3 and CMIP5) simulations. The observed annual-mean ITA (northminussouth)hasvariedwithina0.88Crangeandfeaturesasignificantpositivetrendsince1980.TheCMIP multimodel ensembles simulate this trend, with a stronger and more realistic signal in CMIP5. Both ensembles project a continued increase in the ITA over the twenty-first century, well outside the twentieth-century range. The authors mainly attribute this increase to the uneven spatial impacts of greenhouse forcing, which result in amplified warming in the Arctic and northern landmasses. The CMIP5 specific-forcing simulations indicate that, before 1980, the greenhouse-forced ITA trend was primarily countered by anthropogenic aerosols. The authors also identifyan abrupt decrease in theobserved ITA in the late1960s, which is generally not present in the CMIP simulations; it suggests that the observed drop was caused by internal variability. The difference in the strengths of the northern and southern Hadley cells covaries with the ITA in the CMIP5 simulations, in accordance with previous findings; the authors also find an association with the hemispheric asymmetry in tropical rainfall. These relationships imply a northward shift in tropical rainfall with increasing ITA in the twenty-first century, though this result is difficult to separate from the response to global-mean temperature change.

Coupling between Arctic feedbacks and changes in poleward energy transport

[1] The relationship between poleward energy transport and Arcticamplification isexaminedusingclimate modelsand an energy balance model. In 21st century projections, models with large Arctic amplification have strong surface albedo and longwave cloud feedbacks, but only weak increases (or even decreases) in total energy transport into the Arctic. Enhanced Arctic warming weakens the equator‐to‐pole temperature gradient and decreases atmospheric dry static energy transport, a decrease that often outweighs increases from atmospheric moisture transport and ocean heat transport. Model spread in atmospheric energy transport cannot explain model spread in polar amplification; models with greater polar amplification must instead have stronger local feedbacks. Because local feedbacks affect temperature gradients, coupling between energy transports and Arctic feedbacks cannot be neglected when studying Arctic amplification. Citation: Hwang, Y.‐ T., D. M. W. Frierson, and J. E. Kay (2011), Coupling between Arctic feedbacks and changes in poleward energy transport, Geophys. Res. Lett., 38, L17704, doi:10.1029/2011GL048546.

Sensitivity of the Climate Response to the Altitude of Black Carbon in the Northern Subtropics in an Aquaplanet GCM

AbstractThis study explores the dependence of the climate response on the altitude of black carbon in the northern subtropics by employing an atmospheric general circulation model coupled to an aquaplanet mixed layer ocean, with a focus on the pattern changes in the temperature, hydrological cycle, and large-scale circulation. Black carbon added below or within the subtropical low-level clouds tends to suppress convection, which reduces the low cloud amount, resulting in a positive cloud radiative forcing. The warmer northern subtropics then induce a northward shift of the intertropical convergence zone (ITCZ) and a poleward expansion of the descending branch of the northern Hadley cell. As the black carbon–induced local warming is amplified by clouds and is advected by the anomalous Hadley circulation, the entire globe gets warmer. In contrast, black carbon added near the surface increases the buoyancy of air parcels to enhance convection, leading to an increase in the subtropical low cloud amount and a ...