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Ocean dynamical thermostat
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Ocean dynamical thermostat
Ocean dynamical thermostat is a physical mechanism through which changes in the mean radiative forcing influence the gradients of sea surface temperatures in the Pacific Ocean and the strength of the Walker circulation. Increased radiative forcing (warming) is more effective in the western Pacific than in the eastern where the upwelling of cold water masses damps the temperature change. This increases the east-west temperature gradient and strengthens the Walker circulation. Decreased radiative forcing (cooling) has the opposite effect.
The process has been invoked to explain variations in the Pacific Ocean temperature gradients that correlate to insolation and climate variations. It may also be responsible for the hypothesized correlation between El Niño events and volcanic eruptions, and for changes in the temperature gradients that occurred during the 20th century. Whether the ocean dynamical thermostat controls the response of the Pacific Ocean to anthropogenic global warming is unclear, as there are competing processes at play; potentially, it could drive a La Niña-like climate tendency during initial warming before it is overridden by other processes.
The equatorial Pacific is a key region of Earth in terms of its relative influence on the worldwide atmospheric circulation. A characteristic east-west temperature gradient is coupled to an atmospheric circulation, the Walker circulation, and further controlled by atmospheric and oceanic dynamics. The western Pacific features the so-called "warm pool", where the warmest sea surface temperatures (SSTs) of Earth are found. In the eastern Pacific conversely an area called the "cold tongue" is always colder than the warm pool even though they lie at the same latitude, as cold water is upwelled there. The temperature gradient between the two in turn induces an atmospheric circulation, the Walker circulation, which responds strongly to the SST gradient.
One important component of the climate is the El Niño-Southern Oscillation (ENSO), a mode of climate variability. During its positive/El Niño phase, waters in the central and eastern Pacific are warmer than normal while during its cold/La Niña they are colder than normal. Coupled to these SST changes the atmospheric pressure difference between the eastern and western Pacific changes. ENSO and Walker circulation variations have worldwide effects on weather, including natural disasters such as bushfires, droughts, floods and tropical cyclone activity. The atmospheric circulation modulates the heat uptake by the ocean, the strength and position of the Intertropical Convergence Zone (ITCZ), tropical precipitation and the strength of the Indian monsoon.
Already in May 1996 Sun and Liu published a hypothesis that coupled interactions between ocean winds, the ocean surface and ocean currents can limit water temperatures in the western Pacific. As part of that study, they found that increased equilibrium temperatures drive an increased temperature gradient between the eastern and western Pacific.
The ocean dynamical thermostat mechanism was described in a dedicated publication by Clement et al. 1996 in a coupled ocean-atmosphere model of the equatorial ocean. Since in the western Pacific SSTs are only governed by stored heat and heat fluxes, while in the eastern Pacific the horizontal and vertical advection also play a role. Thus an imposed source of heating primarily warms the western Pacific, inducing stronger easterly winds that facilitate upwelling in the eastern Pacific and cool its temperature - a pattern opposite that expected from the heating. Cold water upwelled along the equator then spreads away from it, reducing the total warming of the basin. The temperature gradient between the western and eastern Pacific thus increases, strengthening the trade winds and further increasing upwelling; this eventually results in a climate state resembling La Niña. The mechanism is seasonal as upwelling is least effective in boreal spring and most effective in boreal autumn; thus it is mainly operative in autumn. Due to the vertical temperature structure, ENSO variability becomes more regular during cooling by the thermostat mechanism, but is damped during warming.
The model of Clement et al. 1996 only considers temperature anomalies and does not account for the entire energy budget. After some time, warming would spread to the source regions of the upwelled water and in the thermocline, eventually damping the thermostat. The principal flaw in the model is that it assumes that the temperature of the upwelled water does not change over time.
Later studies have verified the ocean dynamical thermostat mechanism for a number of climate models with different structures of warming and also the occurrence of the opposite response - a decline in the SST gradient - in response to climate cooling. In fully coupled models a tendency of the atmospheric circulation to intensify with decreasing insolation sometimes negates the thermostat response to decreased solar activity. Liu, Lu and Xie 2015 proposed that an ocean dynamical thermostat can also operate in the Indian Ocean, and the concept has been extended to cover the Indo-Pacific as a whole rather than just the equatorial Pacific.
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Ocean dynamical thermostat
Ocean dynamical thermostat is a physical mechanism through which changes in the mean radiative forcing influence the gradients of sea surface temperatures in the Pacific Ocean and the strength of the Walker circulation. Increased radiative forcing (warming) is more effective in the western Pacific than in the eastern where the upwelling of cold water masses damps the temperature change. This increases the east-west temperature gradient and strengthens the Walker circulation. Decreased radiative forcing (cooling) has the opposite effect.
The process has been invoked to explain variations in the Pacific Ocean temperature gradients that correlate to insolation and climate variations. It may also be responsible for the hypothesized correlation between El Niño events and volcanic eruptions, and for changes in the temperature gradients that occurred during the 20th century. Whether the ocean dynamical thermostat controls the response of the Pacific Ocean to anthropogenic global warming is unclear, as there are competing processes at play; potentially, it could drive a La Niña-like climate tendency during initial warming before it is overridden by other processes.
The equatorial Pacific is a key region of Earth in terms of its relative influence on the worldwide atmospheric circulation. A characteristic east-west temperature gradient is coupled to an atmospheric circulation, the Walker circulation, and further controlled by atmospheric and oceanic dynamics. The western Pacific features the so-called "warm pool", where the warmest sea surface temperatures (SSTs) of Earth are found. In the eastern Pacific conversely an area called the "cold tongue" is always colder than the warm pool even though they lie at the same latitude, as cold water is upwelled there. The temperature gradient between the two in turn induces an atmospheric circulation, the Walker circulation, which responds strongly to the SST gradient.
One important component of the climate is the El Niño-Southern Oscillation (ENSO), a mode of climate variability. During its positive/El Niño phase, waters in the central and eastern Pacific are warmer than normal while during its cold/La Niña they are colder than normal. Coupled to these SST changes the atmospheric pressure difference between the eastern and western Pacific changes. ENSO and Walker circulation variations have worldwide effects on weather, including natural disasters such as bushfires, droughts, floods and tropical cyclone activity. The atmospheric circulation modulates the heat uptake by the ocean, the strength and position of the Intertropical Convergence Zone (ITCZ), tropical precipitation and the strength of the Indian monsoon.
Already in May 1996 Sun and Liu published a hypothesis that coupled interactions between ocean winds, the ocean surface and ocean currents can limit water temperatures in the western Pacific. As part of that study, they found that increased equilibrium temperatures drive an increased temperature gradient between the eastern and western Pacific.
The ocean dynamical thermostat mechanism was described in a dedicated publication by Clement et al. 1996 in a coupled ocean-atmosphere model of the equatorial ocean. Since in the western Pacific SSTs are only governed by stored heat and heat fluxes, while in the eastern Pacific the horizontal and vertical advection also play a role. Thus an imposed source of heating primarily warms the western Pacific, inducing stronger easterly winds that facilitate upwelling in the eastern Pacific and cool its temperature - a pattern opposite that expected from the heating. Cold water upwelled along the equator then spreads away from it, reducing the total warming of the basin. The temperature gradient between the western and eastern Pacific thus increases, strengthening the trade winds and further increasing upwelling; this eventually results in a climate state resembling La Niña. The mechanism is seasonal as upwelling is least effective in boreal spring and most effective in boreal autumn; thus it is mainly operative in autumn. Due to the vertical temperature structure, ENSO variability becomes more regular during cooling by the thermostat mechanism, but is damped during warming.
The model of Clement et al. 1996 only considers temperature anomalies and does not account for the entire energy budget. After some time, warming would spread to the source regions of the upwelled water and in the thermocline, eventually damping the thermostat. The principal flaw in the model is that it assumes that the temperature of the upwelled water does not change over time.
Later studies have verified the ocean dynamical thermostat mechanism for a number of climate models with different structures of warming and also the occurrence of the opposite response - a decline in the SST gradient - in response to climate cooling. In fully coupled models a tendency of the atmospheric circulation to intensify with decreasing insolation sometimes negates the thermostat response to decreased solar activity. Liu, Lu and Xie 2015 proposed that an ocean dynamical thermostat can also operate in the Indian Ocean, and the concept has been extended to cover the Indo-Pacific as a whole rather than just the equatorial Pacific.