Research is conducted on simulations performed with the superparameterized (SP) version of the Community Climate System Model (CCSM, version 3), or SP-CCSM. The atmospheric component model used in a coupled climate model typically uses a parameterization for convection, in which the effects of convection are estimated without actually resolving the convection. In superparameterization, the parameterization is replaced by a cloud resolving model at each point in the model's horizontal grid. In this way the climate model gains some of the benefits of a global cloud resolving model without the enormous computational expense of running such a high-resolution (7km or less) nonhydrostatic model over the globe. Based on a 20-year simulation performed as preliminary work, SP-CCSM produces a better tropical simulation than CCSM3, including improvements in the periodicity of El Nino/Southern Oscillation events, reduction in the classical double-ITCZ bias, and a more credible simulation of the Madden-Julian Oscillation (MJO).
Two research objectives are addressed in this research: first, SP-CCSM and its component atmospheric model are used to evaluate the roles of surface fluxes, air-sea interactions, and small-scale convective processes in producing MJO events. This research also considers the mechanisms through which the MJO affects the Asian summer monsoon. Second, the work seeks to understand the effect of the MJO on El Nino/Southern Oscillation events in the equatorial Pacific.
The work has broader impacts through its potential to forecasts of MJO events, which have consequences for the Asian summer monsoon and weather in the United States. Research results may also lead to improvements in the parameterization of tropical convection in weather and climate models. In addition, the project supports a postdoctoral researcher, thereby providing for the next generation of scientists in this field.
The Madden-Julian Oscillation (MJO) is a large-scale, slowly-evolving convective disturbance that impacts weather throughout the tropics. The cloudy part of the disturbance (i.e., the convection) is focused on the equator and spans ~5000 km of longitude (more than the width of North America). As this convective envelope slowly shifts eastward, it regulates rainy and dry periods (such as monsoons), spawns hurricanes, and sometimes initiates El Nino events. The disturbance generates so much atmospheric heating that is effects are often experienced at higher latitudes in the form of planetary waves that impact the jet stream. Because the disturbance repeats every 30~60 days, predicting MJO evolution can help improve global weather forecasts. Unfortunately, many models have poor MJO prediction skill. Recent research has taught us that the large spatial scale and slow movement of the MJO is sensitive to how convection interacts with atmospheric humidity. Other studies show that models can better simulate the MJO if changes to the underlying ocean are taken into account. Winds and cloudiness cool the ocean during the cloudy, or "active" phase of the disturbance, and calm conditions and clear skies warm the ocean during the "suppressed" phase. During the suppressed (clear sky) phase, energy from the sun accumulates in the upper layer of the ocean until the arrival of MJO convection, at which point it is rapidly transferred to the atmosphere by wind-driven evaporation. While we have a good understanding of the MJO surface energy exchange, we know less about how it impacts MJO convection, and how it might be involved in the convection-humidity feedbacks described above. This study used a new type of model that has highly realistic cloud-humidity interactions to study the role of the ocean on the MJO. We ran simulations where the ocean was allowed to interact with the atmosphere (a "coupled" model) and compared it to simulations where the ocean was fixed to a mean climatological state (an "uncoupled" model). Both coupled and uncoupled models produced a good MJO, but it was more realistic in the coupled model. Detailed budget studies of the sources of moisture (humidity) for the MJO suggest that the release of energy from the upper ocean helps to organize and intensify MJO convection, which heats the atmosphere and induces a circualtion response. The stronger circulation response in the coupled model enhances atmospheric humidity east of MJO convection, where it invigorates cloud-humidity interactions, allowing the MJO to propagate eastward. Similar experiments using models with less realistic cloud-humidity interactions showed that ocean coupling did not significantly improve the MJO. This supports the conclusions of many other studies that cloud-humidity interactions are critical to simulating the MJO, while ocean feedbacks--although potentially helpful--are secondary. Because the MJO is linked to the timing of monsoon rainfall in India, southeast Asia, and northern Australia, the more realistic MJO in the coupled simulation also produced more realistic monsoon rain events than the uncoupled simulation. This study advanced our knowledge of the role of ocean feedbacks onto the MJO. We found that ocean feedbacks play a role in the organization and intensification of MJO convection, which helps moisten the downstream environmental air, aiding the slow eastward propagation. The broader impacts of this work include the improved ability to model a disturbance that impacts weather worldwide, as well as our ability to forecast its global impacts.