Impact of Rising Greenhouse Gases on Mid-Latitude Storm Tracks and Associated Hydroclimate Variability and Change

The 24 models participating in the Intergovernmental Panel on Climate Change (IPCC) Assessment Report 4 (AR4) robustly predict that the subtropics will dry and expand poleward in the current century as a consequence of greenhouse gas-driven climate change. Recent analysis has shown that a large part of the subtropical drying is caused thermodynamically by increased moisture divergence within divergent flow as specific humidity rises with temperature according to the Clausius-Clapeyron equation. In addition, the Hadley Cell expands poleward and the storm tracks shift poleward and intensify. Essential dynamical changes explain changes in hydroclimate. The poleward moisture transport by transient eddies also increase and shift poleward. As transient eddies supply winter moisture to regions such as southwestern North America, this is important. In this region, drying comes from both increased moisture divergence in the mean flow (a thermodynamic mechanism) and reduced moisture convergence by transient eddies (a dynamic mechanism). The changes in transient eddy moisture fluxes are fundamental mechanisms of subtropical drying and expansion but are not fully understood. A large part of this change can be explained thermodynamically. As temperature rises, spatial gradients of specific humidity also increase--and if transient eddies can be thought of as a mixing process, poleward transient eddy moisture fluxes will also increase. However, this is not a complete explanation. Using the NOAA Geophysical Fluid Dynamics Climate Model 2.1, a detailed breakdown using mixing length theory found the following processes to be important to explaining an increase in transient eddy moist static energy (and moisture) fluxes:

  • Increase in the mean gradients
  • Intensification of transient eddies
  • Increase in the length scale of eddies
  • Increase in the correlation between meridional velocity and specific humidity perturbations within storm systems.

That is, part of the explanation lies in a change in the strength, structure and character of the transient eddies themselves. While there is some work, based around concepts of available potential energy, on how storm track intensity will change in response to rising greenhouse gases (GHGs) there is very little work on how the character and structure of baroclinic eddies will be impacted and how this, in turn, will impact the hydrological cycle and its variability. We propose to take on that problem in this proposal using analysis of the models participating in AR4 and AR5, controlled simulations with GCMs and diagnosis with idealized models. Confidence in projected changes in the hydrological cycle rely on understanding the mechanisms and believing that they are represented faithfully in the models used to make projections. The proposed work on storm tracks and associated hydrological cycle response to radiatively driven climate change primarily fits the following announced area of interest to DOE: ’theoretical understanding leading to increased reliability of climate change projections’. By studying storm tracks, anomalies of which are responsible for weather and climate extremes, it also addresses the DOE goal "to better quantify the frequency, duration, and intensity of extreme events under climate change."

Assessing El Niño-Southern Oscillation Regime Changes in a Changing Climate PI: Fei-Fei Jin

Assessing and understanding future changes of El Niño-Southern Oscillation (ENSO) in response to global warming is essential to addressing issues relating to the interaction of climate change and natural climate variability. The objective of the proposed research is to identify and assess systematic changes ENSO undergoes in response to greenhouse warming. We propose to undertake a critical examination of simulations previously conducted with a number of coupled GCMs and CMIP5 experiments, focusing on the following questions. As anthropogenic greenhouse warming progresses:

  1. How do the changes in the climate background state and in the ocean-atmosphere feedbacks alter ENSO characteristics, such as pattern, amplitude, and period?
  2. Are these changes gradual or abrupt?
  3. Are current state-of-the-art climate models capable of capturing the different “flavors” of ENSO, as well as their transitions?

We will also conduct a 2-parameter space exploration of the ENSO regimes using state-of-art climate system models. This exploration will enable us to identify the existence of co-dimension-2 ENSO bifurcations and associated transitions in the qualitative behavior of ENSO. Two key parameters: greenhouse gas forcing and an important sensitivity parameter for ENSO in the cumulus convection scheme, will be systematically changed. We will then assess the nature of ENSO regime transition in this parameter space captured by different models, and the likelihood of future ENSO regime changes using these coupled models. The proposed research falls in line with the activities outlined by the program announcement: activities that focus on the identification, evaluation, and understanding of low frequency modes, (e.g., ENSO) and how these will change in a changing climate. The project will ultimately support the BER climate science activity’s Long Term Measure (LTM) by improving assessment of the potential response of the Earth's climate to increased greenhouse gas levels.

Project Term: 
2010 to 2014
Project Type: 
University Project