Transcript Document

A new feedback on climate change
from the hydrological cycle
Paul
1
Williams ,
Eric
1,2
Guilyardi ,
Rowan
1
Sutton ,
Jonathan
1,3
Gregory ,
Gurvan
4
Madec
(1) Walker Institute, Department of Meteorology, University of Reading, UK; (2) IPSL / Laboratoire des Sciences du Climat et de l'Environnement, France;
(3) Hadley Centre, Met Office, Exeter, UK; (4) IPSL / Laboratoire d'Océanographie Dynamique et de Climatologie, Université Paris VI, France
1. Introduction
Climate models predict that global
warming will result in an intensified
hydrological cycle [1]. Observations
suggest that this process has
already begun: globally-integrated
rainfall has steadily increased in
recent decades [2]. But changes in
evaporation and precipitation may
themselves
affect
ocean
temperature, giving a possible
feedback on global warming, as
indicated by the dashed arrow:
2. Proposed
,,,,,Feedback
3. Experimental
,,,,,Design
We recently proposed a specific
mechanism by which the feedback
described in Section 1 may
operate [3].
Precipitation and
evaporation maintain salinity and
temperature gradients within the
ocean.
Mixing of waters with
different temperatures causes a
transfer of heat, as shown
schematically below.
Global
warming-induced
changes
in
precipitation and evaporation will
lead to changes in the salinity and
temperature gradients. This could
modify the heat transfer and
potentially alter the temperature
structure of the ocean.
We wish to investigate the sign,
magnitude and linearity of the
feedback proposed in Section 2.
Using a state-of-the-art climate
model, we run two sensitivity
experiments in which the net fresh
water flux from the atmosphere
(consisting
of
evaporation,
precipitation
and
run-off)
is
multiplied by 2 (EPR2) and 0
(EPR0) before being passed to the
ocean. The two experiments are
compared to a control run (CTRL),
and all three are initiated from the
present-day ocean structure of [4]
with constant greenhouse gas
forcing.
We use the SINTEX oceanatmosphere model [5,6], which
o
o
consists of ORCA2 (2 x 0.5-2 with
31 levels) coupled to ECHAM4
(T106).
4. Ocean Temperature
The equilibrated ocean surface
temperature anomalies in EPR0 and
o
EPR2 are shown below ( C). The
responses at low latitudes, where
the vertical component of the heat
transfer discussed in Section 2 is
downward [7], are consistent with
our proposed feedback mechanism.
Decreased gradients in EPR0 give a
decreased downward heat flux and a
o
mean surface warming of 0.6 C,
whereas increased gradients in
EPR2 give an increased downward
heat flux and a mean surface cooling
o
of 0.8 C.
suppressed
hydrological
cycle
6. Conclusions
5. Linearity of the
,,,,,Feedback
The feedback’s characteristics
are summarized in the figure
below,
broken
down
by
geographical region. Because of
atmospheric
feedbacks,
the
freshwater amplification factor in
EPR2 is 1.8 rather than 2. At low
latitudes (<40°N/S) the feedback
is almost linear for amplification
factors in the range studied, but at
higher latitudes (>50°N/S) it is
highly nonlinear.
At low latitudes, our proposed
feedback has negative sign under
an intensified hydrological cycle. It
is linear for multiplying factors in the
range studied. For a multiplying
factor of 1.1, which is of the order
projected to exist at the time of CO2
doubling [1], we estimate by
interpolation a negative ocean
surface temperature anomaly of
o
magnitude around 0.1 C.
We
conclude that an intensification of
the hydrological cycle is likely to
contribute
a
weak
negative
feedback to low-latitude climate
change.
References
EPR2
CTRL
EPR0
amplified
hydrological
cycle
[1] IPCC (2001). Climate Change 2001: The Scientific Basis.
Cambridge University Press.
[2] Dai, A., Fung, I. Y. & Del Genio, A. D. (1997). Surface observed
global land precipitation variations during 1900-88, J. Clim., 10,
2943-2962.
[3] Williams, P. D., Guilyardi, E., Sutton, R. T., Gregory, J. M. & Madec,
G. (2006). On the climate response of the low-latitude Pacific ocean
to changes in the global freshwater cycle, Clim. Dyn., 27(6), 593611.
[4] Levitus, S. (1982). Climatological atlas of the world ocean. NOAA
Professional Paper 13, pp. 173.
[5] Gualdi, S., Navarra, A., Guilyardi, E. & Delecluse, P. (2003).
Assessment of the tropical Indo-Pacific climate in the SINTEX
CGCM, Annals of Geophysics, 46(1), 1-26.
[6] Guilyardi, E., Delecluse, P., Gualdi, S. & Navarra, A. (2003).
Mechanisms for ENSO phase change in a coupled GCM, J. Clim.,
16(8), 1141-1158.
[7] Osborne, T. J. (1998). The vertical component of epineutral
diffusion and the dianeutral component of horizontal diffusion, J.
Phys. Oceanogr., 28, 485-494.