Transcript Manganese Nodules - Ohio State University

Ocean-Atmosphere Interaction: A
Tropical Thermostat for Global
Warming?
Wallace and Clement Papers
Casey Saup
Discussion Plan
 Wallace Paper
 Background
 Purpose/goals, Methods, Figures, Conclusions
 Clement Paper
 Background
 Purpose/goals, Methods, Figures, Conclusions
 Summary
 Future Directions of the Research (IMO)
Wallace Paper Background
 Water vapor in the atmosphere has a
destabilizing effect on climate
 As SSTs increase, more water evaporates, creating a
positive feedback cycle (traps longwave radiation).
 Ramanathan and Collins postulated that cirrus clouds
(associated with deep convection and increase locally
over warm pool with increasing SST) “braked” this
feedback and limited the warm pool (305K)
 Claimed this would explain negative skewness
Wallace Paper Purpose/Goals
 Purpose: To suggest that Ramanathan and Collins’ proposed
thermostat mechanism may not be required to explain SST
distribution.
 Large-scale dynamical processes should maintain uniform tropical tropospheric
temperatures to within ~2K
 Without horizontal temperature contrasts in the atmosphere, a negatively
skewed SST frequency distribution
will develop due to equilibration btw the
atmosphere and SSTs that vary by location.
 Cirrus clouds will not necessarily prevent SST
from rising above 305K even though they
reduce radiation in regions of deep convection.
Wallace Paper Figures/Results
 April climatological mean
geopotential height field on the
200hPa surface—this is
situated in the upper
troposphere at the level of
strongest horizontal gradients.
 The range between the highest and lowest values in the tropics is not more than two
contour intervals (80 m).
 Maps for individual years/months at levels ranging from the surface to the
tropopause (boundary btw the troposphere and the stratosphere) are similar in this
respect.
Wallace Paper Figures/Results
 Vertical scale of tropical circulation systems is comparable to
the depth of the troposphere, the vertically averaged
temperature of the tropical troposphere must be uniform to
within 2 K
Clement Paper Background
 Water vapor in the atmosphere has a
destabilizing effect on climate
 As SSTs increase, more water evaporates, creating a positive
feedback cycle (traps longwave radiation).
 Ramanathan and Collins postulated that cirrus clouds (associated
with deep convection and increase locally over
warm pool with increasing SST) “braked” this
feedback and limited the warm pool (305 K)
 Claimed this would explain negative skeweness
 Wallace pointed out that tropical temperatures
would need to be fairly uniform and that efficient
ocean-atmosphere heat exchange in areas of
deep convection would lead to the negative
skewness in the absence of cirrus cloud cover.
Why did no
one consider
ocean
dynamics?
Clement Paper Background (contd)
 Many other papers were published suggesting
other mechanisms than that of Ramanathan
and Collins, the best of which is
from Pierrehumbert 1995.
 Says that clouds have no net effect on the top of atmosphere
radiation
 Deep convective clouds have no net effect in
stabilizing the tropical climate.
 Relies on “radiator fins”—in areas of deep convection
energy is exported to drier, nonconvecting regions
where it is effectively radiated to space.
 All of these differing views have something in common:
they don’t take “interactive dynamical transports of heat
in the ocean” into account.
Clement Paper Purpose/Goals
 Goal: “To illustrate a possible role for ocean
dynamics in regulating sea surface temperatures
by including ONLY highly idealized atmospheric
thermodynamics.”
Clement Paper Methods
 Zebiak-Cane Model
 Coupled ocean-atmosphere model, solves for perturbations about the
climatological state
 Consists of an atmosphere governed by two shallow-water equations on an
equatorial beta plane and a linear reduced gravity ocean model
 Model domain extends from 29°N to 29°S, 124°E to 80°W
 The Temperature anomaly in the ocean model mixed layer was determined
using the equation below.
 This experiment was intended to stimulate how the coupled tropical system
would respond to a simple forcing.
 In the absence of ocean dynamics, the model would generate an SST
anomaly T=T*
Clement Paper Figures
 Figure 1: Surface temperature anomaly in
April—four model months after the start of the
runs
 The temperature change in the eastern
equatorial (180° to the eastern boundary and
btw 5°S and 5°N) region is less than that of the
surrounding region.
 SST must change so that the surface heat flux
anomaly balances imposed forcing.
 In the eastern region here, the imposed forcing
can be partially balanced by anomalous
horizontal and vertical advection, and the SST
will change less.
 Positive *TE-W temperature gradient
increased, which strengthens the equatorial
easterlies, which will increase upwelling and
cause the thermocline to shoal in the east.
Both of these processes will further cool the
SSTs in the eastern portion of the basin. This
will lead to a coupled interaction that
establishes a new climatology.
Clement Paper Figures/Results
 Resulting annual mean
SSTs for warming and
cooling
 Surprisingly, coupled
interaction causes
temperature anomaly in the
NINO3 region (5S-%N, 90150W) to be the opposite
sign to that of the forcing.
Clement Paper Figures/Results
 Terms in Equation 1 averaged
over the area of the entire
basin.
 The forcing αT* is almost
equally balanced by the
change in heat flux (αT,
dashed) and the vertical
advection of temperature
(change in vertical flux, dotted)
Clement Paper Figures/Results
 Basin and annual mean
temperature anomaly and
NINO3 temperature anomaly
relative to the standard run
as a function of T*.
 Meridional advection
spreads the upwelled water
off of the equator leading to
a basin average temperature
change that is less than
expected.
Clement Paper Figures/Results
 Seasonal cycle of surface
temperature anomaly for the
NINO3 region
 SST response is smaller in the
spring than it is in the fall
 Since the sign of the response
in this region is opposite to that
of the forcing, the seasonal
cycle for warming is enhanced
and weakened for cooling.
Clement Paper Figures/Results
 Representative segments
of time series of the
NINO3 index taken from a
1000-yr run for T*=+2 and
T*=-2
 Variability is dramatic
across T*
 ENSO variability is almost
completely wiped out in
the warming scenario, but
the cooling events
become more regular
Summary
 Wallace Summary:
 Tropical troposphere temperatures are fairly uniform
due to large-scale dynamical processes (within about
2 K)
 Efficient heat exchange between the ocean and the
atmosphere in areas of deep convection will lead to
the negative skewness that Ramanathan and Collins
attributed to cirrus cloud cover (so…no cirrus cloud
cover is necessary for the negative skewness)
Summary
 Clement Summary:
 Everyone else considered the ocean to be
dynamically inactive in their models which is super
wrong.
 Using the Zebiak-Cane model, they discovered that
ocean dynamics alter/affect the mean tropical sea
surface temperature, climatology, amplitude of
seasonal cycle, interannual variability.
 This indicates that you can’t ignore ocean dynamics
when dealing with tropical climate stability, even
though it’s hard 
Future Directions of the Research
 IMO:
 I think it may be beneficial to look into paleoclimatological
records to see if that could shed any light on this area of
research
 I think models that incorporate finer details of wind,
ocean, and atmospheric dynamics and the interactions
between the three should be developed.
References
 Wallace, John M. "Effect of Deep Convection on
the Regulation of Tropical Sea Surface
Temperature." Nature (1992): 230-31. Print.
 Clement, Amy C., Richard Seager, Mark A. Cane,
and Stephen E. Zebiak. "An Ocean Dynamical
Thermostat." Journal of Climate: 2190-196. Print.