Section 6.2: Physical Basis for Sea-Surface Temperature
97
and condenses moisture can lead to a positive feed-back intensifying the circulation. This clearly will tend to occur only in regions of significant deep
penetrative convection with ample moisture supply, and these regions tend to
be in regions of SST maxima. Secondly, and related, convergence of moisture
is perhaps most crucial for the large scale atmospheric response. Convergence of moisture will be influenced by the SLP pattern, but also by such
factors as the magnitude of the SST (higher SST leads to a warmer nearsurface atmosphere that can hold more water in gaseous form), the air-mass
characteristics such as stability and the wind strength.
The release of latent heat by condensation of moisture into cloud droplets
in tropical regions of strong convection can lead to remote atmospheric responses. This has been shown in simple models of the tropical atmosphere
(e.g. Gill, 1982). So an anomalous SST maximum in the western Pacific may
lead to enhanced convection and latent heat release locally, which in turn can
influence atmospheric variability in many regions of the globe. Connections
of tropical heating to extratropical circulation have also been proposed (e.g.
Hoskins and Karoly, 1981).
6.2.2 Extratropics
The impact of extratropical SST variations on atmospheric variability is
more controversial. Compared to the tropics, atmospheric variability in midlatitudes seems to have a stronger component generated by internal atmospheric dynamics, with the SST often in the main responding to, rather than
significantly altering, the low frequency atmospheric variability. Nonetheless,
evidence has been presented for the influence of variations in SST gradients
on atmospheric baroclinicity in important cyclogenesis regions such as near
Newfoundland (Palmer and Sun, 1985). These ideas are not developed in
this chapter, but evidence is presented of an SST fluctuation that involves
changes in the relative temperature of the Northern Hemisphere compared
to the Southern Hemisphere. It has been suggested that such swings in this
interhemispheric temperature gradient lead to changes in the location and intensity of the inter-tropical convergence zone (ITCZ) (Folland et al., 1986aj
Street-Perrott and Perrott, 1990). A warmer Northern Hemisphere draws
the ITCZ furt her north and/or makes it more active, bringing more rain to
N orthern Hemisphere semi-arid regions such as the Sahel region of Africa.
97
and condenses moisture can lead to a positive feed-back intensifying the circulation. This clearly will tend to occur only in regions of significant deep
penetrative convection with ample moisture supply, and these regions tend to
be in regions of SST maxima. Secondly, and related, convergence of moisture
is perhaps most crucial for the large scale atmospheric response. Convergence of moisture will be influenced by the SLP pattern, but also by such
factors as the magnitude of the SST (higher SST leads to a warmer nearsurface atmosphere that can hold more water in gaseous form), the air-mass
characteristics such as stability and the wind strength.
The release of latent heat by condensation of moisture into cloud droplets
in tropical regions of strong convection can lead to remote atmospheric responses. This has been shown in simple models of the tropical atmosphere
(e.g. Gill, 1982). So an anomalous SST maximum in the western Pacific may
lead to enhanced convection and latent heat release locally, which in turn can
influence atmospheric variability in many regions of the globe. Connections
of tropical heating to extratropical circulation have also been proposed (e.g.
Hoskins and Karoly, 1981).
6.2.2 Extratropics
The impact of extratropical SST variations on atmospheric variability is
more controversial. Compared to the tropics, atmospheric variability in midlatitudes seems to have a stronger component generated by internal atmospheric dynamics, with the SST often in the main responding to, rather than
significantly altering, the low frequency atmospheric variability. Nonetheless,
evidence has been presented for the influence of variations in SST gradients
on atmospheric baroclinicity in important cyclogenesis regions such as near
Newfoundland (Palmer and Sun, 1985). These ideas are not developed in
this chapter, but evidence is presented of an SST fluctuation that involves
changes in the relative temperature of the Northern Hemisphere compared
to the Southern Hemisphere. It has been suggested that such swings in this
interhemispheric temperature gradient lead to changes in the location and intensity of the inter-tropical convergence zone (ITCZ) (Folland et al., 1986aj
Street-Perrott and Perrott, 1990). A warmer Northern Hemisphere draws
the ITCZ furt her north and/or makes it more active, bringing more rain to
N orthern Hemisphere semi-arid regions such as the Sahel region of Africa.