Tim Merlis

“Response of tropical climate to greenhouse gas changes and orbital forcing”

This talk explores the climate dynamics that are relevant to understanding the tropics in past and future climates.  Idealized climate modeling experiments are used to reveal the physical processes that underly the tropical climate's response to changes in radiation.  Specifically, I examine the response of the equatorial east-west surface temperature contrast and the off-equator precipitation maximum to changes in longwave radiation, such as greenhouse gases, and the response of tropical precipitation to orbital precession.

Amy Clement

“Can we predict decadal fluctuations in the tropical Pacific climate?”

Seasonal to interannual fluctuations of the tropical Pacific Ocean have well-known connections to North American hydroclimate. During cold La Nina years in the tropical Pacific, the southern part of the US has a higher likelihood of dry conditions, particularly in wintertime. The mechanisms that produce these La Nina (or their opposite, El Nino) events are relatively well-understood: Dynamical coupling between the ocean and atmosphere leads to a positive feedback that allows climate anomalies to grow. As a consequence of this, the development of events is generally preceded by changes in the subsurface ocean, observations of which can give predictive skill for tropical Pacific climate anomalies 3-6 months  in advance. However, there are numerous examples of multi-year or decadal droughts in North America with devastating impacts. The ongoing drought in Texas, for example, is part of a longer-term trend, with impacts growing as the length of the drought persists. This drought coincides with a long-term shift in the Pacific climate to more La Nina-like conditions over the last 30 years. What is the cause of the persistent climate anomalies in the tropical Pacific ocean, and what are their associated impacts? How might these shifts be related to anthropogenic climate change?

The international community is embarking on a coordinated effort to evaluate to what extent such decadal shifts in regional climates are predictable. However, we have very little understanding of the potential sources of decadal predictability, i.e. what are the physical mechanisms that can produce such persistent changes in climate? In this seminar I will show recent work using a hierarchy of coupled climate models in which we identify processes that do (and don’t) produce persistence. I will focus in particular on the equatorial ocean thermocline and on marine stratocumulus clouds. The ocean thermocline is thought to be the source of predictability on seasonal to interannual timescales, but I will show that fluctuations in the thermocline actually reduce persistence of tropical Pacific climate anomalies on decadal scales. Marine stratocumulus clouds, on the other hand, appear to be a source of persistence of such anomalies, and I will show that these clouds may be an important factor in the generating the decadal timescale climate anomalies that have contributed to the ongoing drought in Texas. I will then discuss the implications of these findings for the observational and modeling system needed to predict decadal climate fluctuations in the tropical Pacific, and also discuss the inherent limitations of such a system for providing probabilistic information about the climate of the coming decades.

Kiori Obuse

“Long-time behavior of zonal flows in two-dimensional turbulence on a rotating sphere”

In forced two-dimensional turbulence on a rotating sphere, it is known that a multiple zonal-band structure, i.e. a structure with alternating eastward and westward jets, emerges in the course of time development. The multiple zonal-band structure then experiences intermittent mergers and disappearances of zonal jets, and a structure with only a few large-scale zonal jets is realized as an asymptotic state (Obuse et al., 2010).

With the view of understanding the long-time behavior, especially the disappearing and merging processes of the zonal jets, described above, we consider large-scale zonal flows superposed upon a small-scale deterministic non-zonal background base flow on a β plane, which is the model originally introduced by Manfroi and Young (1999). We analytically derive steady isolated zonal jet solutions of the evolution equation of such zonal flows described in Manfroi and Young’s model. Then the disappearing process of zonal jets is discussed by examining the linear stability of the steady isolated zonal jet solutions and their nonlinear time evolution. The Merging process of zonal jets, on the other hand, is investigated by considering the weak interaction between two steady isolated zonal jet solutions placed apart.

Dave Schneider

“Assessing Antarctic warming, Arctic amplification, and the Bipolar Seesaw:

Does it matter which data you use?” 

The assessment of observational data sets is vital to the integrity of observational, modeling and prediction studies of climate variability and change. Disagreements among data sets of the same variable have been hyped in public debates involving climate change skeptics, yet there numerous, more subtle examples in mainstream science where spurious trends and variability in data have been interpreted as real, physical signals. For example, the primary mechanism of Arctic amplification was once proposed to be due to increasing poleward heat transport, but this result was shown to be an artifact of the reanalysis employed. More recent data have led to an emerging consensus (but not uniform agreement) that the primary mechanisms are feedbacks arising from Arctic sea ice loss. In another example from the polar regions, some investigators proposed a 20th-Century bipolar seesaw—when the Arctic warms, the Antarctic cools, and vice versa. This result however is based on very sparse data, and does not hold up under scrutiny of independent instrumental and paleoclimate records. When scientists take the time to assess and compare different data sets, and publish their findings, many of these miss-interpretations may be avoided. I will show a couple of examples of such assessments, one involving the depiction of Antarctic surface mass balance in reanalyses, and the other from my own work involving warming in West Antarctica as seen in several temperature data sets and reflected in related variables including sea ice and atmospheric circulation. Finally, I will discuss the Climate Data Guide (https://climatedataguide.ucar.edu/) as a platform for discussing the strengths and limitations of climate data sets, and their applications to climate analyses and model evaluation.

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Paul Kusher   

“Climate Sensitivity and Climate Models”

The climate sensitivity parameter measures how much the Earth surface warms for a doubling of atmospheric carbon dioxide concentration, under conditions of radiative equilibrium. It is arguably the most important number to pin down in order to predict the future of Earth's climate under global warming.  In this talk I will aim to build physical intuition about climate sensitivity, and the climate feedbacks that contribute to it, using simple models. This intuition will help us interpret the relative importance and uncertainty of the feedbacks in comprehensive climate models. I will highlight the relatively simple snow albedo feedback, which is surprisingly uncertain in climate models. This uncertainty has local consequences for North American summertime climate and remote consequences, through long-range spatial correlations, for the atmosphere-ocean circulation of the Northern Hemisphere.

Frank Giraldo

“A Summary on the Development of Next-Generation

Climate and Weather Models”

The current vision at operational numerical weather prediction centers is to develop unified (global to mesoscale) models that can be used for global NWP as well as for regional modeling. We have seen this trend already in Canada (with GEM) and the UK (with the Unified Model) but are also seeing it in Japan (with NICAM) and Germany (with ICON), although these last two systems are primarily focused on the climate problem. In the U.S., there is currently a joint effort between NOAA (National Oceanic and Atmospheric Admininstration) and the Navy to develop a unified model for both NWP (5-7 day forecasts) and for decadal simulations (10-30 years). The unifying theme in all of these efforts is “multi-scale” modeling, meaning that both operational NWP and climate research centers are focusing on developing a model that is applicable across a disparate range of scales; the reason for this is economics (the reduced cost in maintaining only one model per organization). From a science perspective, this opens the door for the construction of radically new models that must use fewer underlying physical simplifications (e.g., cannot use the hydrostatic approximation). In addition, computing power has reached a sufficiently high-level that it is now possible to propose the use of the anelastic, non-hydrostatic, or pseudo-incompressible equations instead of the hydrostatic equations that have been typically used for global models in both climate and weather.

In this talk, I will describe our strategy for constructing such unified models. It turns out that addressing this issue from the standpoint of a modeling framework simplifies the model construction significantly. What I mean by a modeling framework is that the model is constructed by increasing levels of complexity that are all built within the same infrastructure. This simplifies the validation by adding new components to the existing ones in a hierarchical approach. With this approach in mind, we can construct numerical models that are valid across various flow regimes (from hydrostatic to non-hydrostatic) and applicable to both mesoscale (regional or limited-area) to global modeling.

We feel that we have the governing equations and numerical methods for solving these equations sorted out, however, what is not so clear to many of us is how to improve the sub-grid scale parameterizations in order to reap the full benefits of our sophisticated numerical models. I will briefly describe the class of problems that we must tackle in the future in order to understand how we can improve future weather and climate models.

Paul O’Gorman

“What controls the intensity of the midlatitude storm tracks in different climates?”

The extratropical storm tracks play an important role in the climate system by transporting momentum, energy, and water in the atmosphere.  Changes in the extratropical storm tracks under global warming would impact these transports, the ocean circulation and carbon cycle, and society through changing weather patterns.  A warmer and moister atmosphere has greater internal, potential, and latent energy, but only a small fraction of this increased energy is available to be converted into the kinetic energy associated with circulations. I will discuss how changes in available energy can be used to help understand the varied responses of the extratropical storm tracks to climate change. The implications for storm tracks in past climates will also be discussed.

Bette Otto-Bleisner

“Modeling the Climate Evolution and Abrupt Changes over the Last 21,000 Years”

Climate models have been used since the 1970s to understand past climates. The first paleoclimate simulations, for example by COHMAP and PMIP, were time-slice simulations for key periods in the past. In concert with paleoenvironmental data syntheses, they have allowed testing of hypotheses on the different climatic factors that have shaped our environment and improved our understanding of the mechanisms of climate change. These paleoclimate simulations have been a key contribution to past IPCC assessments allowing evaluation of the capability of state-of-the-art models to reproduce different climates. With the increase in computing power, we are also now able to consider transient simulations of global climate change, for the Last Millennium, Holocene, deglaciation, or even longer. These transient simulations provide an opportunity to test the forcing of past abrupt climate events and an assessment of the leads and lags of the climate responses. They also provide a more natural comparison to paleoclimate proxy records.

In this talk, I will provide a brief description of climate models and history of how they have been used to study the Last Glacial Maximum (LGM) to present. I will then hightlight two results from our synchronously coupled NCAR Community Climate System Model (CCSM3) transient simulation from the LGM to present.  The first example will discuss the simulation of the deglacial evolution to the Bolling-Allerod warming that occurred about 15,000 years ago, and in particular will describe the important roles of the ocean circulation and meltwater in the climate system response. The second example will discuss the extension of the simulation to the start of the Holocene (11,600 years ago) and its implications for a synchronous African Humid Period in the Sahel and southeast equatorial Africa. Contributions by the changing orbital parameters and increasing atmospheric carbon dioxide concentrations will be shown to both be important.