Geoscience Reference
In-Depth Information
3.1 Moisture Flux into Tropical Wet Regions
Water vapor exerts a powerful physical constraint on the global water cycle. It provides a
strongly positive feedback to climate change (Manabe and Wetherald
1967
; Soden et al.
2005
;
Willett et al.
2008
; Ingram
2010
) but also controls the atmospheric temperature lapse rate and
modulates radiative cooling (e.g., O'Gorman et al.
2012
) and is central to the large-scale
regional responses in precipitation. In the broadest sense, water is transported in the atmo-
sphere by the tropical circulation from the dry, subtropical oceans (net moisture divergence) to
the wet, moisture convergence zones, and also to the higher latitudes (e.g., Bengtsson et al.
2011
) and the continents (e.g., Trenberth et al.
2011
). Rising W with warming (Table
3
)
therefore indicates increased moisture flux (M
F
). Assuming that the magnitude of changes in
atmospheric moisture storage is negligible compared with the other terms, a simple moisture
balance dictates that M
F
balances precipitation minus evaporation (P - E):
P
E
M
F
:
ð
5
Þ
Assuming, for now, that wind flows are unchanged, M
F
will simply vary with atmospheric
moisture, at the rate a * 7 %/K (Table
3
). In fact, to reconcile the energetic constraints
upon global precipitation with the thermodynamic contraints upon water vapor, climate
models simulate a weakening of the tropical Walker circulation that is also evident in
observations and simulations of the twentieth century (Vecchi et al.
2006
), while decadal
variability may cause temporary increases in the Walker circulation (Sohn et al.
2012
).
Nevertheless, as argued by Held and Soden (
2006
), the simple assumptions in (
5
) imply an
enhancement of P - E patterns in climate model simulations (at least over the ocean
where moisture supply is unlimited) with warming
d
ð
P
E
Þrð
adTM
F
Þ
adT
ð
P
E
Þ:
ð
6
Þ
This effect is illustrated in Fig.
5
which shows the changes in M
F
from the dry regions
(column mean downward vertical motion) to the wet regions (column mean upward ver-
tical motion) of the tropics defined by 6-hourly instantaneous fields (Zahn and Allan
2011
).
M
F
is computed at each vertical level along the line dividing wet and dry regions.
(a)
(b)
0
0
M
FC21
- M
FC20
EH5
EH5, C21
EH5, C20
ERA, C20
200
200
400
400
600
600
800
800
1000
1000
-2e+06
-2e+06
0
2e+06 4e+06 6e+06 8e+06 1e+07
0
2e+06
4e+06
6e+06
8e+06
T [kg *s
-1
* hPa
-1
* K
-1
]
T [kg *s
-1
* hPa
-1
* K
-1
]
Δ
M
F
/
Δ
Δ
M
F
/
Δ
Fig. 5 Change in moisture flux from the dry to the wet regions of the tropics per K change in surface
temperature between a warm and cold months in ERA Interim and ECHAM5 (twentieth and twenty-first
centuries time slices) and for b the climate change response from twenty-first minus twentieth century
simulated by ECHAM5 (EH5). In a, moisture fluxes were computed separately for months with above and
below average tropical mean surface temperature; in forming the mean, each month of the year was ascribed
equal weighting. For the climate change response, the mean moisture fluxes were calculated and differenced
between (21C, 2069-2099) and (20C, 1959-1989)
