Role of Interhemispheric Heat Transport and Global Atmospheric Cooling in Multidecadal Trends of Northern Hemisphere Precipitation

Abstract

Changes in the hydrological cycle in the northern hemisphere (NH) have a significant socioeconomic impact. Thus, quantifying past changes and projecting future changes in this cycle are important. We analyzed the positive and negative multidecadal trends in the NH mean precipitation since the 1950s that were accurately reproduced by a state‐of‐the‐art Earth system model. Results showed that changes in the interhemispheric heat transport play a major role in the NH atmospheric heat budget, accounting for 59% of the negative‐to‐positive trend change in NH precipitation through a north–south shift in the tropical precipitation zone, and were strongly linked to Hadley circulation anomaly. The remaining 41% corresponded to changes in atmospheric cooling with small interhemispheric differences, which were primarily related to clear‐sky longwave radiation. These findings quantitatively agreed with the results from a multimodel analysis.

Factors Driving Past Trends in Arctic Precipitation and Their Future Changes

“…In the tropics, the north-south asymmetric trend in Period I with a decrease in NH and an increase in SH (Figure 3a) could be attributed to aerosol forcing (Figure S2a, b). The southward shift of the intertropical convergence zone reproduced by the CMIP6 models (Yukimoto et al, 2022) reflected this past change in the prevailing forcing.…”

“…(1) (Muller and O'Gorman, 2011;Dagan & Stier, 2020;Yukimoto et al, 2022), where L is the latent heat of evaporation (2.5 × 10 6 J/kg), P is precipitation (kg/m 2 /s), ∇ H S is the horizontal divergence of the column-integrated DSE (W/m 2 ), and F SH is the surface sensible heat flux (W/m 2 , upward positive). The total DSE divergence ∇S can be expressed as ∇S = ∇ H S − F SH ; that is, the downward surface sensible heat flux corresponds to the DSE vertical divergence.…”

Factors Contributing to Historical and Future Trends in Arctic Precipitation

“…(1) (Bonan et al, 2023;Dagan & Stier, 2020;Muller & O'Gorman, 2011;Pithan & Jung, 2021;Yukimoto et al, 2022), where L is the latent heat of evaporation (2.5 × 10 6 J/kg), P is precipitation (kg/m 2 /s), ∇ H DSE is the horizontal divergence of the column-integrated DSE (W/m 2 ), and F SH is the surface sensible heat flux (W/m 2 , upward positive). The downward surface sensible heat flux corresponds to the DSE vertical divergence.…”

“…If we ignore changes in the atmospheric heat storage because we are dealing with long‐term changes on an annual mean basis, the column‐integrated dry static energy (DSE) balance of the atmosphere can be expressed as $$$LP=-R+{\nabla }_{\mathrm{H}}\,\text{DSE}-{F}_{\text{SH}}$$$ (Bonan et al., 2023; Dagan & Stier, 2020; Muller & O’Gorman, 2011; Pithan & Jung, 2021; Yukimoto et al., 2022), where L is the latent heat of evaporation (2.5 × 10 6 J/kg), P is precipitation (kg/m 2 /s), ∇ H DSE is the horizontal divergence of the column‐integrated DSE (W/m 2 ), and F SH is the surface sensible heat flux (W/m 2 , upward positive). The downward surface sensible heat flux corresponds to the DSE vertical divergence.…”

Changes in External Forcings Drive Divergent AMOC Responses Across CESM Generations

“…Physically, a more reflective northern hemisphere would indicate a hemispheric difference in solar heating, which would require anomalous cross-equatorial heat transport by the AMOC to maintain energy balance. This metric is not an exact proxy for the AMOC because atmospheric processes are also able to transport anomalous energy across the equator to balance the hemispheric difference in solar heating (Chiang & Bitz, 2005;Donohoe et al, 2013;Bischoff & Schneider, 2014;Lembo et al, 2019;Irving et al, 2019;Yukimoto et al, 2022;Pearce & Bodas-Salcedo, 2023;Needham & Randall, 2023). Nonetheless the metric is a useful measure of the energetic influences on the AMOC and the wider climate over the twentieth century.…”