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The Role of Atmospheric and Oceanic Feedbacks in Orbital and Millennial Timescale Variability of the Coupled Climate-Carbon System /

Record Type:	Electronic resources : Monograph/item
Title/Author:	The Role of Atmospheric and Oceanic Feedbacks in Orbital and Millennial Timescale Variability of the Coupled Climate-Carbon System // Fangze Zhu.
Author:	Zhu, Fangze,
Description:	1 electronic resource (124 pages)
Notes:	Source: Dissertations Abstracts International, Volume: 85-07, Section: B.
Contained By:	Dissertations Abstracts International85-07B.
Subject:	Climate change. -
Online resource:	https://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=30989456
ISBN:	9798381444698

The Role of Atmospheric and Oceanic Feedbacks in Orbital and Millennial Timescale Variability of the Coupled Climate-Carbon System /
Zhu, Fangze,

The Role of Atmospheric and Oceanic Feedbacks in Orbital and Millennial Timescale Variability of the Coupled Climate-Carbon System /Fangze Zhu. - 1 electronic resource (124 pages)

Source: Dissertations Abstracts International, Volume: 85-07, Section: B.

An intriguing mechanism of the Pleistocene glacial-interglacial cycles is the phase locking of internal climate oscillations to external forcings, suggesting the leading role of the natural climate variability in the past climate changes on orbital timescales. Similarly, the millennialscale variability over the last glacial period may be attributed to unforced climate dynamics. One of the central themes of the climate sciences is to acquire a fundamental understanding of climate feedbacks over a broad range of timescales. The main objective of this thesis is to identify the characteristic natural variability in a fully coupled climate-carbon model from orbital to millennial timescales, and the essential feedback processes contributing to such behaviors, such as the role of the oceanic carbon cycle in the dynamical structures of the warm climates.Four stable climatic equilibria are identified in an idealized and fully coupled energy-conserving model (MITgcm) with a prognostic carbon cycle, using identical external forcings and global carbon inventory, but with different initial conditions. These steady states are called Warm, Cold, Waterbelt, and Snowball states. A fifth, small ice cap state similar to the present-day climate is found to be barely unstable. Radiative kernels are developed and the state dependence of radiative feedbacks is analyzed for three non-Snowball steady states. The systematic decrease in surface albedo feedback from Cold to Warm states is offset by an increase in longwave water vapor feedback. The key role of the carbon cycle is a dramatic lengthening of the adjustment time near the Warm state, a timescale comparable to orbital forcings. The bifurcation structure of the model prohibits a hysteresis loop between Warm and Cold states, thus raising possible scenarios for Warm/Cold transitions triggered by orbital forcings with timescales of the slow carbon cycle adjustment, which has important implications for glacial-interglacial transitions.The orbital-scale variability in the coupled climate-carbon model (MITgcm) is overlaid with several millennial-scale oscillations in the vicinity of the Warm equilibrium, almost ice-free. The oscillations first appear due to gradual loss of deep polar ocean stratification as a result of bottom warming, which triggers the onset of a subpolar deep Meridional Overturning Circulation (MOC). The subpolar deep MOC further invigorates the subtropical deep MOC, increasing global air-sea CO2 fluxes and atmospheric pCO2 through upwelling of deep Dissolved Inorganic Carbon (DIC). The increasing atmospheric pCO2 leads to global surface warming that further warms and stratifies the subsurface polar ocean, shutting down the subpolar deep MOC and completing a delayed negative feedback which reverses the oscillation phase. Mechanism-denial experiments reveal that CO2 radiative feedback and time-varying MOC and polar stratification are all essential components of the oscillator mechanism, while winter sea ice and time-varying biological nutrients are not. The millennial timescale is determined by the delayed response of atmospheric pCO2 to variations in polar stratification and subpolar deep MOC. The study of the unforced millennial-scale oscillations under the ice-free conditions provides a unique perspective for understanding millennial-scale variability in a warming scenario.To validate the mechanism of the internal millennial-scale oscillations, a linear delayed oscillator model consisting of a one-dimensional Delay Differential Equation (DDE) and two diagnostic equations is constructed. The only prognostic variable is the atmospheric pCO2, the slowest component in the studied climate-carbon system. The equation of pCO2 depicts two delayed negative feedbacks and another negative feedback, which are essentially the coupling between pCO2 and ocean carbon reservoir in the presence of switching on and off of the deep MOC, through differential local air-sea CO2 fluxes and the carbonate chemistry of sea waters. When tuned to MITgcm, the simple model captures the major characteristics of the oscillatory phenomenon, thereby supporting the mechanism of the unforced oscillator. Stability analysis indicates that the timescales of the stable oscillations are dominated by the two delay parameters and the delayed feedbacks associated with them, while the positive and negative feedbacks determine the peak values. The model is an essential step toward understanding the physical mechanisms of the three-dimensional MITgcm from a mathematical perspective, and paves the way for developing low-order dynamical systems for studying past and future climate changes.

English

ISBN: 9798381444698Subjects--Topical Terms:

2079509
Climate change.
Subjects--Index Terms:

Carbon cycle

The Role of Atmospheric and Oceanic Feedbacks in Orbital and Millennial Timescale Variability of the Coupled Climate-Carbon System /
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520 $a An intriguing mechanism of the Pleistocene glacial-interglacial cycles is the phase locking of internal climate oscillations to external forcings, suggesting the leading role of the natural climate variability in the past climate changes on orbital timescales. Similarly, the millennialscale variability over the last glacial period may be attributed to unforced climate dynamics. One of the central themes of the climate sciences is to acquire a fundamental understanding of climate feedbacks over a broad range of timescales. The main objective of this thesis is to identify the characteristic natural variability in a fully coupled climate-carbon model from orbital to millennial timescales, and the essential feedback processes contributing to such behaviors, such as the role of the oceanic carbon cycle in the dynamical structures of the warm climates.Four stable climatic equilibria are identified in an idealized and fully coupled energy-conserving model (MITgcm) with a prognostic carbon cycle, using identical external forcings and global carbon inventory, but with different initial conditions. These steady states are called Warm, Cold, Waterbelt, and Snowball states. A fifth, small ice cap state similar to the present-day climate is found to be barely unstable. Radiative kernels are developed and the state dependence of radiative feedbacks is analyzed for three non-Snowball steady states. The systematic decrease in surface albedo feedback from Cold to Warm states is offset by an increase in longwave water vapor feedback. The key role of the carbon cycle is a dramatic lengthening of the adjustment time near the Warm state, a timescale comparable to orbital forcings. The bifurcation structure of the model prohibits a hysteresis loop between Warm and Cold states, thus raising possible scenarios for Warm/Cold transitions triggered by orbital forcings with timescales of the slow carbon cycle adjustment, which has important implications for glacial-interglacial transitions.The orbital-scale variability in the coupled climate-carbon model (MITgcm) is overlaid with several millennial-scale oscillations in the vicinity of the Warm equilibrium, almost ice-free. The oscillations first appear due to gradual loss of deep polar ocean stratification as a result of bottom warming, which triggers the onset of a subpolar deep Meridional Overturning Circulation (MOC). The subpolar deep MOC further invigorates the subtropical deep MOC, increasing global air-sea CO2 fluxes and atmospheric pCO2 through upwelling of deep Dissolved Inorganic Carbon (DIC). The increasing atmospheric pCO2 leads to global surface warming that further warms and stratifies the subsurface polar ocean, shutting down the subpolar deep MOC and completing a delayed negative feedback which reverses the oscillation phase. Mechanism-denial experiments reveal that CO2 radiative feedback and time-varying MOC and polar stratification are all essential components of the oscillator mechanism, while winter sea ice and time-varying biological nutrients are not. The millennial timescale is determined by the delayed response of atmospheric pCO2 to variations in polar stratification and subpolar deep MOC. The study of the unforced millennial-scale oscillations under the ice-free conditions provides a unique perspective for understanding millennial-scale variability in a warming scenario.To validate the mechanism of the internal millennial-scale oscillations, a linear delayed oscillator model consisting of a one-dimensional Delay Differential Equation (DDE) and two diagnostic equations is constructed. The only prognostic variable is the atmospheric pCO2, the slowest component in the studied climate-carbon system. The equation of pCO2 depicts two delayed negative feedbacks and another negative feedback, which are essentially the coupling between pCO2 and ocean carbon reservoir in the presence of switching on and off of the deep MOC, through differential local air-sea CO2 fluxes and the carbonate chemistry of sea waters. When tuned to MITgcm, the simple model captures the major characteristics of the oscillatory phenomenon, thereby supporting the mechanism of the unforced oscillator. Stability analysis indicates that the timescales of the stable oscillations are dominated by the two delay parameters and the delayed feedbacks associated with them, while the positive and negative feedbacks determine the peak values. The model is an essential step toward understanding the physical mechanisms of the three-dimensional MITgcm from a mathematical perspective, and paves the way for developing low-order dynamical systems for studying past and future climate changes.
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650 4 $a Ocean engineering. $3 660731
653 $a Carbon cycle
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653 $a Feedback
653 $a Internal variability
653 $a Nonlinear dynamics
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710 2 $a State University of New York at Albany. $b Atmospheric and Environmental Sciences. $e degree granting institution. $3 3816808
720 1 $a Rose, Brian $e degree supervisor.
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