2:45 PM - 3:00 PM
[ACG42-05] Latitudinal Differences of Last 40years Changes in Outgoing Longwave Radiation
Keywords:Earth's energy budget, Outgoing longwave radiation, Feedbacks
The Earth’s energy budget is a fundamental physical constraint on the heat balance of the climate system. However, its controlling factors are complex, with contributions from numerous atmospheric components such as well-mixed greenhouse gases, water vapor, clouds and aerosols. The balance between net incoming insolation and outgoing thermal energy, OLR (outgoing longwave radiation), determines the gain or loss of energy in the entire climate system. OLR consists of emissions from Earth’s atmosphere and surface. Many studies have attempted to accurately quantify the absolute value and trends of OLR on a global scale and zonal mean scale. However, research on the factors controlling the latitudinal differences of clear-sky OLR changes over time using hyperspectral radiative transfer are limited. These controlling factors elucidate the mechanisms of how the Earth is responding regionally to global warming in the form of radiative energy changes.
Increases in well mixed greenhouse gases (e.g. CO2, CH4, N2O) forcings act to reduce OLR, which is expected to be enhanced by water vapor increase but offset by increased thermal emission from a warmer atmosphere and surface. However, there is absence of studies exploring how these competing factors influence the zonal-mean change in OLR over the past 40 years.
By employing the 4-decade, 43-year ERA5 reanalysis dataset and conducting line-by-line radiative transfer calculation, this study provides a quantitative understanding of the observed evolution of OLR temporal change across latitudes, including a diagnostic analysis on how a critical component of the Earth’s energy balance is being perturbed from the tropics to the Arctic over time.
Our results reveal a transition in the sign of OLR changes between low and high latitudes, with a “crossover“ point located between the tropics and extratropics, around 30ºN. To the best of our knowledge, our study is the first to explore the differing sign trends across from the tropics to high-latiudes in the Northern Hemisphere over the past 40 years.
Our breakdown by radiative components reveals that the latitudinal gradient of the change in OLR is mainly driven by a large positive contribution from surface temperature change at high latitudes and a negative contribution by water vapor in the tropics. The former is attributable to both Arctic warming amplification and a larger sensitivity of OLR to dTs due to a more spectrally ‘open’ atmospheric window. The latter is attributable to both a larger upper-tropospheric water vapor increase and a greater sensitivity of OLR to H2O change in the tropics. Changes in well mixed greenhouse gases partly contribute to the latitudinal gradient of OLR change because of larger GHGs’ forcings in the tropics, consistent with the results of previous studies. The contributions of both stratospheric and tropospheric temperatures are approximately uniform across all latitudes. This is due to the compensation between the larger sensitivity of OLR to dTa for the entire atmosphere in the tropics, and a deeper stratosphere and bigger Ta increase in lower troposphere in the Arctic. The combination of all these contributions shapes the latitudinal gradient of change in OLR.
The temporal variability of relative humidity, meridional difference in dTs, and the nuanced relationship between local Ta and water vapor feedbacks, as well as the spatial differences in OLR sensitivity to dTs and GHGs’ forcings delineate the cross-over point latitude, highlighting a critical threshold at which the sign of the OLR change shifts.
What could we say under the assumption that global warming driven by well mixed greenhouse gases continue to increase in the future? The change in well mixed greenhouse gases (CO2, CH4, N2O, CFCs etc) would continue to have a negative influence at all latitudes, but remain stronger in the tropics. This would push down and the cross-over point would shift poleward. However, both temperature and H2O would also be changing. According to our CM4 simulation up to 2100, Arctic amplification will be enhanced. Thus, our results suggest an even larger latitudinal gradient of the change in OLR in the future. However, the uncertainty of the changes in the meridional profile of water vapor and Ta changes over latitude and with time makes it difficult to conclude in a quantitative manner. Regarding the extent of compensation between the impact of Ts, Ta, H2O and GHGs’ forcings, further investigation through future simulations and continuous monitoring of these variables are important to quantify the trajectory of the cross-over point with time.
Increases in well mixed greenhouse gases (e.g. CO2, CH4, N2O) forcings act to reduce OLR, which is expected to be enhanced by water vapor increase but offset by increased thermal emission from a warmer atmosphere and surface. However, there is absence of studies exploring how these competing factors influence the zonal-mean change in OLR over the past 40 years.
By employing the 4-decade, 43-year ERA5 reanalysis dataset and conducting line-by-line radiative transfer calculation, this study provides a quantitative understanding of the observed evolution of OLR temporal change across latitudes, including a diagnostic analysis on how a critical component of the Earth’s energy balance is being perturbed from the tropics to the Arctic over time.
Our results reveal a transition in the sign of OLR changes between low and high latitudes, with a “crossover“ point located between the tropics and extratropics, around 30ºN. To the best of our knowledge, our study is the first to explore the differing sign trends across from the tropics to high-latiudes in the Northern Hemisphere over the past 40 years.
Our breakdown by radiative components reveals that the latitudinal gradient of the change in OLR is mainly driven by a large positive contribution from surface temperature change at high latitudes and a negative contribution by water vapor in the tropics. The former is attributable to both Arctic warming amplification and a larger sensitivity of OLR to dTs due to a more spectrally ‘open’ atmospheric window. The latter is attributable to both a larger upper-tropospheric water vapor increase and a greater sensitivity of OLR to H2O change in the tropics. Changes in well mixed greenhouse gases partly contribute to the latitudinal gradient of OLR change because of larger GHGs’ forcings in the tropics, consistent with the results of previous studies. The contributions of both stratospheric and tropospheric temperatures are approximately uniform across all latitudes. This is due to the compensation between the larger sensitivity of OLR to dTa for the entire atmosphere in the tropics, and a deeper stratosphere and bigger Ta increase in lower troposphere in the Arctic. The combination of all these contributions shapes the latitudinal gradient of change in OLR.
The temporal variability of relative humidity, meridional difference in dTs, and the nuanced relationship between local Ta and water vapor feedbacks, as well as the spatial differences in OLR sensitivity to dTs and GHGs’ forcings delineate the cross-over point latitude, highlighting a critical threshold at which the sign of the OLR change shifts.
What could we say under the assumption that global warming driven by well mixed greenhouse gases continue to increase in the future? The change in well mixed greenhouse gases (CO2, CH4, N2O, CFCs etc) would continue to have a negative influence at all latitudes, but remain stronger in the tropics. This would push down and the cross-over point would shift poleward. However, both temperature and H2O would also be changing. According to our CM4 simulation up to 2100, Arctic amplification will be enhanced. Thus, our results suggest an even larger latitudinal gradient of the change in OLR in the future. However, the uncertainty of the changes in the meridional profile of water vapor and Ta changes over latitude and with time makes it difficult to conclude in a quantitative manner. Regarding the extent of compensation between the impact of Ts, Ta, H2O and GHGs’ forcings, further investigation through future simulations and continuous monitoring of these variables are important to quantify the trajectory of the cross-over point with time.