11:00 AM - 1:00 PM
[U02-P03] Climate impacts and ocean responses to historical high-amplitude interannual fluctuations of biomass burning aerosol emissions in the CMIP6 protocol
We consider the impacts on the global ocean in response to a rectifier effect with high-amplitude interannual variability in CMIP6 biomass burning (BB) emissions at high latitudes in the Northern Hemisphere, and explore the implications for the mean climate state through multi-decadal memory residing in the ocean’s heat reservoir.
This is investigated utilizing the 100-member CESM2 Large Ensemble simulation (Rodgers et al., 2021, ESD), which largely follows CMIP6 standard historical and SSP3-7.0 forcing protocols over 1850-2100. Two separate sets of aerosol emissions from BB are used to force the first and second sets of 50 members. For the first set, the original CMIP6 protocols are applied (CMIP6-50). The second set is forced by temporally smoothed CMIP6 BB emissions (SMBB-50) that were designed to conserve total emissions relative to CMIP6-50 (Figure). Smoothing is applied using an 11-year running-mean filter over the period that incorporates the Global Fire Emissions Database (GFED) in the CMIP6 forcing protocols (1997–2014), impacting the variability of the BB fluxes over the years 1990–2020.
Differences in the temporal variations of the BB emissions result in additional heat absorption into the Earth system in the CMIP6-50 simulations during the GFED period, through a rectification effect due to nonlinearities in sea-ice/land-ice processes. The globally integrated heat input during the GEFD period for the CMIP6-50 members relative to the SMBB-50 members is comparable in amplitude to the reduction of heat uptake during a major historical volcanic eruption, despite the net amount of BB aerosol emissions being nearly conserved between CMIP6-50 and SMBB-50. The inter-hemispheric thermal imbalance in the atmosphere is eliminated immediately after the end of the GFED period as the oceans will absorb most of the additional heat.
The heat warms the upper ocean in both the North Pacific and North Atlantic, and the two ocean basins respond in distinct ways. In the Atlantic, enhanced stratification in the subpolar region due to the warming weakens the AMOC, resulting in compensation of the temperature anomaly by the reduced northward heat transport from low latitudes. On the other hand, in the Pacific, surface trade wind anomalies associated with a northward shift of the ITCZ position in response to the interhemispheric atmospheric temperature gradient drive anomalous upper ocean circulation over the low-/mid-latitudes of both hemispheres. Weakened north subtropical circulation reduces northward heat transport along the western boundary current, resulting in compensation of the positive temperature anomaly. In the South Pacific, anomalous Ekman pumping due to enhanced trade winds carries heat into the ocean interior. Subducted heat in the South Pacific then reaches the sub-surface of the tropical Pacific, resulting in a long-term multi-decadal non-local rectified response to the interannual fluctuations that extends well beyond the domain of the BB perturbations.
This is investigated utilizing the 100-member CESM2 Large Ensemble simulation (Rodgers et al., 2021, ESD), which largely follows CMIP6 standard historical and SSP3-7.0 forcing protocols over 1850-2100. Two separate sets of aerosol emissions from BB are used to force the first and second sets of 50 members. For the first set, the original CMIP6 protocols are applied (CMIP6-50). The second set is forced by temporally smoothed CMIP6 BB emissions (SMBB-50) that were designed to conserve total emissions relative to CMIP6-50 (Figure). Smoothing is applied using an 11-year running-mean filter over the period that incorporates the Global Fire Emissions Database (GFED) in the CMIP6 forcing protocols (1997–2014), impacting the variability of the BB fluxes over the years 1990–2020.
Differences in the temporal variations of the BB emissions result in additional heat absorption into the Earth system in the CMIP6-50 simulations during the GFED period, through a rectification effect due to nonlinearities in sea-ice/land-ice processes. The globally integrated heat input during the GEFD period for the CMIP6-50 members relative to the SMBB-50 members is comparable in amplitude to the reduction of heat uptake during a major historical volcanic eruption, despite the net amount of BB aerosol emissions being nearly conserved between CMIP6-50 and SMBB-50. The inter-hemispheric thermal imbalance in the atmosphere is eliminated immediately after the end of the GFED period as the oceans will absorb most of the additional heat.
The heat warms the upper ocean in both the North Pacific and North Atlantic, and the two ocean basins respond in distinct ways. In the Atlantic, enhanced stratification in the subpolar region due to the warming weakens the AMOC, resulting in compensation of the temperature anomaly by the reduced northward heat transport from low latitudes. On the other hand, in the Pacific, surface trade wind anomalies associated with a northward shift of the ITCZ position in response to the interhemispheric atmospheric temperature gradient drive anomalous upper ocean circulation over the low-/mid-latitudes of both hemispheres. Weakened north subtropical circulation reduces northward heat transport along the western boundary current, resulting in compensation of the positive temperature anomaly. In the South Pacific, anomalous Ekman pumping due to enhanced trade winds carries heat into the ocean interior. Subducted heat in the South Pacific then reaches the sub-surface of the tropical Pacific, resulting in a long-term multi-decadal non-local rectified response to the interannual fluctuations that extends well beyond the domain of the BB perturbations.