My research efforts aim to understand the stability of the climate system under anthropogenic forcing, addressing questions such as whether we will have stronger and more frequent El Nino events, or whether the whole planet may undergo a Venus-style, run-away instability in response to anthropogenic forcing. These efforts may be subgrouped into the following three interconnected areas:
I have chosen two climate phenomena which I believe typify the seemingly conflicting behaviors of the climate system, and therefore understanding of them may provide the necessary light for us to peer deeper into the nature of the climate system. The phenomenon that typifies the stability of the climate system is the stability of the tropical maximum SST--or the SST in the western Pacific warm-pool. It has never exceeded much beyond 303 K or 30 oC over the known history of the climate system. The lack of change in the upper bound of the tropical maximum SST suggests that the net feedback of all the involved physical processes must be strongly negative, but how this negative feedback is constituted and executed is an unsolved problem. The phenomenon that typifies the instability of the climate system or its proneness to substantial changes is the El Niño Southern Oscillation . Every 2-7 years, the western Pacific warm-pool extends to the east, resulting in anomalous warming in the central and eastern Pacific. The warming in turn causes changes in the large-scale circulation in the atmosphere and ocean and thereby affects the climate worldwide including the United States of America . The recurrent occurrence of El Niño warming suggests that the coupled tropical ocean-atmosphere is often unstable, but what are the fundamental forces that so steadfastly push the tropical ocean-atmosphere to an unstable state, is still an open question. I am particularly attracted to the hypothesis that the aforementioned two phenomena--the lack of change in the upper bound of the tropical maximum SST and the recurrent occurrence of El Niño-- may be linked: the lack of change in the upper bound of the tropical maximum SST may be due to the recurrent occurrence of El Niño, and vice versa!
Understanding the relationship between the tropical maximum SST and El Niño is also fundamental for addressing the question of whether ENSO would become more energetic in response to global warming --a question that has drawn ever increasing societal concern. Because of the profound effect of the tropical SST on the statistics of the weather events, understanding the response of ENSO to global climate change is in turn fundamental for addressing questions concerning how the statistics of weather events over the continental US may respond to global warming, questions such as: Will we have more heat waves and/or ice storms as global warming progresses? Will we be subject to a greater risk for more floods and droughts? Thus the seemingly very theoretical interest that I have in the two basic aspects of the tropical climate has in fact direct implications for addressing some very practical questions concerning the physical and economical wellbeing of ordinary citizens.
My tools are mathematical models of varying complexity. The simplest model that I have constructed for the coupled tropical ocean-atmosphere system is an analytical model in which the atmosphere and ocean are represented by a few boxes. The most complex model that I am running is the NCAR Community Climate System Model (CCSM) consisting of a general circulation model for the atmosphere and a general circulation model for the ocean. I also make use of observations and data from assimilation systems. I place emphasis on the use of simplified models in tandem with general circulation models (GCMs). The simplified models can be understood completely. The insights gained from the simplified models are then tested with the use of GCMs. I also use simplified models in conjunction with GCM experiments to understand outstanding biases in GCM simulations. A point that cannot be overemphasized, but often overlooked, is that a comprehensive climate model such as the NCAR CSM, is not necessarily closer to nature in all aspects than much simpler models. The overall performance of a comprehensive climate model may be better, but some aspects are known not reliable, such as water vapor and cloud feedbacks. This in turn speaks of the importance of diagnostic studies of GCM simulations, and the importance of using simplified and/or more empirically based models in conjunction with the use of GCMs in addressing our environmental concerns.