Reading focus questions
Bjerknes
(1969): J. Criscio
- What are the fundamental components of Bjerknes' coupled
ocean-atmosphere system, and how do they explain tropical
teleconnections? What was his mechanism for extratropical
teleconnections?
- What components of ENSO events could Bjerknes not explain?
Discussion summary: Bjerknes used both atmospheric and
oceanographic data available from the late 1940s through the mid-1960s
to demonstrate the coupling between the zonal gradient in sea surface
temperature and surface air pressure across the tropical Pacific basin.
Surface easterlies drive eastern equatorial Pacific cold tongue
upwelling; surface atmospheric convergence over the warm western Pacific
drive convection and subsidence aloft over the eastern equatorial
Pacific in a zonal cell he called the Walker Circulation. During
the El Nino warm phase events of 1957-8, 1963-4 and 1965-6, the Walker
Circulation was displaced eastward and weakened, displacing the other
major centers of tropical convection. The tropical-subtropical
meridional overturning circulation (the Hadley Circulation), and
therefore the midlatitude westerlies over the North Pacific, were
strengthened. Bjerknes observed that SST anomalies associated with
ENSO events led atmospheric anomalies, but he could not explain
variations in event amplitude, duration and return interval, nor how
events were initiated or terminated (although this is discussed in his
1963 and 1966 papers). The latter problem may be partly explained
by delayed oscillator theory (see animation linked in web pages from
Barsugli). The mechanism by which anomalous atmospheric latent
heating occurs is probably larger-scale than Bjerknes implied. We
will see how well the Bjerknes mechanism explains almost 20 years of
TOGA/TAO observations (McPhaden, 1999), and how successful its explicit
incorporation into a predictive numerical model (Zebiak and Cane, 1987)
has been. (MNE)
[1/27/06] By popular vote this is the
first and last summary.
Zebiak and
Cane (1987): M. Decker
- What aspects of ENSO are captured by the model? Which ones
aren't?
- What elements are crucial to the interannual oscillations?
- Given that the model is sensitive to a variety of parameters, not
only the coupling but the defined mean fields, to what degree did the
desired outcome define the model?
[1/30/06]
McPhaden et al.
(1999): MNE
- Do the 1997-8 observations support or challenge 'delayed
oscillator' theory as presented by McPhaden?
- What is the Madden-Julian Oscillation (MJO), and how is it linked
to ENSO events, especially initiation?
- Did models successfully predict this event?
- Given your review of current TAO array observations, ZC model
predictions, and one other model prediction (TAO data
exercise), what is your prognosis for Fall 2006? How much
uncertainty do you ascribe to your prognosis?
[2/6/06]
Tribbia
(1991): J. Conroy, S. Bieda
- Tribbia mentions seasonal variability in the strength of NH
teleconnections. Why are NH teleconnections stronger in the winter than
in the summer?
- From previous discussions and Tribbia, what is a Rossby wave? How
does it conserve absolute vorticity?
- How much of a contribution does the Rossby wave play in NH
teleconnections?
- What is the role and significance of the discovery and use of the
SWB mode?
- Do the winter 2005-6 teleconnection patterns observed in the data
exercise meet expectations based on Tribbia and other readings?
[2/13/06]
Gutzler (2002),
Higgins et al. (1999): S. Bieda, J. Conroy
- What is the relationship between warm and cold ENSO events and
seasonal percent departure from normal precipitation values in AZNM,
NWMX, and SWMX? What is the proposed physical reason for the
statistically significant correlation between SWMX seasonal percent
departure from normal precipitation values and warm/cold events?
- What is the relationship between wet and dry summer monsoons in
AZNM and conditions in the SW US and Pacific NW during the preceding
winter? Why does this relationship exist?
- According to Gutzler et al., does the PDO provide any predictive
skill for southwest precipitation? If so, how? If not, why
not?
- Of all parameters related to ENSO that we have read (including
Gutzler et al. and Higgins et al.), which one parameter (if any) is best
correlated to above normal, normal, or below normal North American
Monsoon activity?
[2/18/06]
Fedorov and
Philander (2000): T. Ault
- What (three) parameters do the authors use to describe the
tropical Pacific Ocean? How do these parameters interact to create
different "modes of instability"? What are these modes?
- In the context of Fedorov and Philander, "decadal variability"
has essentially two meanings. What are they? How is decadal variability
linked to mean climate? How is it linked to the delayed oscillator?
- In the terms of the parameters used by Fedorov and Philander,
what is the current state of the ENSO system? Has the state remained
stable over time? What is the biggest problem with assessing the
stability of the ENSO system over time?
- Supposing Fedorov and Philander are correct, what type of
variability should we expect in a greenhouse gas warmed world?
[2/27/06]
Gedalof and
Smith (2001); Evans et al. (2001): MNE
- Why should these indices represent decadal variability in the
Pacific? Why might they not
represent decadal variability in the Pacific?
- What are the most important sources of error in these
reconstructions?
- How well-supported by their results are conclusions made by the
authors about the characteristic timescale(s) and mechanisms underlying
Pacific decadal variability?
- If you could have perfect data, what would you require to
determine the mechanism and timescale (or lack thereof) underlying
decadal variability in the Pacific Basin?
[3/5/06]
Karspeck and
Cane (2002): T. Ault; Schneider and Cornuelle (2005): P. Shaw
- Why does KC02 focus on TDA instead of SST or wind stress?
How do the authors separate the ENSO signal from the decadal? Is this a
valid approach? How could this approach introduce bias?
- How does KC02 eliminate the possibility that extratropical winds
play a role in tropical variability?
- According to KC02, which forcing is more important for generating
patterns of decadal variability: SST or wind stress?
- What evidence does KC02 cite to suggest decadal variability might
be a seasonally sensitive phenomenon?
- What processes are shown in SC05 to statistically influence the
variability in the PDO? On what time scales?
- Is the PDO a single dynamical mode, or something else? Does
your answer reinforce what we've learned about ENSO and the PDO?
[3/28/06]
Venzke et al.
(2000): S. Bieda; Pierce (2001): P. Shaw
- Do the authors believe the 1976-1977 event was itself a shift, or
an actual event that could be predicted?
- What significance does wind stress play in the whole scheme of
decadal predictability, if any?
- Does the model used in this paper consider atmospheric and
oceanic parameters? If not, which one parameter was
excluded? If so, were both parameters given equal consideration?
- How much skill does the model used in this paper have in
"hindcasting" not only the 1976-1977 event, but other events?
- As an oceanographer/climatologist/meteorologist, would you use
this model for consideration as a predictive tool?
- Does white noise forcing affect variability on decadal time
scales?
- Do coupled atmosphere ocean models support or refute this idea?
- How do regime shifts relate to noise forcing?
- Can decadal variability be predicted, or is it just a response to
chaotic atmospheric forcing?
[4/3/06]
Cane et al.
(1997): J. Criscio
- What is the proposed mechanism by which a uniform radiative
heating from above affects the mean state and variability of the
tropical Pacific coupled ocean-atmosphere system? How is the
mechanism similar to and different from the ENSO physics we've discussed
over the semester?
- What are the various observational and model results that support
the operation of the ocean dynamical thermostat over the past
century? What are their various strengths and weaknesses?
- What do you expect to see from this mechanism as atmospheric CO2
concentrations continue to climb?
[4/11/06]
Khatiwala et
al. (2001): N. Johnson
- What are the characteristics of the unforced Lorenz system? What
do the terms state space, fixed point, regime, and stability mean in the
context of dynamical systems?
- Does the forcing of the model change the mean persistance time,
PDF maxima, and the PDF of persistance? Why or why not?
- Can this model be used to help predict ENSO? How would
anthropogenically forced climate changes be seen in ENSO?
[4/17/06]
Timmermann et
al. (1999); Collins (2005): M. Decker, N. Johnson
- What differences/similarities exist between the model control
ENSO and ENSO observations described in T99?
- What feedback mechanisms described in T99 may be responsible for
altering the mean state of the tropical Pacific under greenhouse gas
warming? Does this model address these mechanisms?
- In T99, what changes in the Pacific cause the changes in the
interannual variability of ENSO? Does this agree with previous
results?
- Which of the coupled model control simulations reported in C05
produces results that look most like observations? Least like
observations?
- How does Collins reach the conclusion that the most likely
scenario is for no large-amplitude change towards mean El
Niño-like or La Niña-like conditions?
- What are some of the criticisms that Collins admits this study
has? Are there others?
[4/25/06]
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