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We study the sensitivity of South Asian Summer Monsoon (SASM) precipitation to Southern Hemisphere (SH) subtropical Absorbed Solar Radiation (ASR) changes using Community Earth System Model 2 simulations. Reducing positive ASR biases over the SH subtropics impacts SASM, and is sensitive to the ocean basin where changes are imposed. Radiation changes over the SH subtropical Indian Ocean (IO) shifts rainfall over the equatorial IO northward causing 1–2 mm/day drying south of equator, changes over the SH subtropical Pacific increases precipitation over northern continental regions by 1–2 mm/day, and changes over the SH subtropical Atlantic have little effect on SASM precipitation. Radiation changes over the subtropical Pacific impacts the SASM through zonal circulation changes, while changes over the IO modify meridional circulation to bring about changes in precipitation over northern IO. Our findings suggest that reducing SH subtropical radiation biases in climate models may also reduce SASM precipitation biases. Plain Language Summary: Precipitation from South Asian Summer Monsoon (SASM) is of high significance to the livelihoods of over a billion people. As the global climate continues to evolve, it is essential to have a clear understanding of the possible future changes to the SASM. However, current state‐of‐the‐art climate models have difficulties in simulating climatological mean SASM precipitations. Here we study sensitivity of SASM precipitation to subtropical southern ocean radiation as one of the possible causes of SASM precipitation bias. Our experiments indicate that SASM precipitation is sensitive to southern hemisphere (SH) subtropical radiation changes particularly to those in subtropical Pacific. These findings imply that improving SH subtropical radiation biases might improve SASM precipitation simulations in climate models. Key Points: We test if biases in southern hemisphere shortwave radiation contributes to biases in South Asian summer monsoon precipitation in the CESM2Reducing incoming shortwave radiation in the subtropical southern hemisphere reduces dry biases over continental South AsiaThis effect is mostly due to forcing in the South Pacific, with less impact from the Atlantic or Indian Ocean [ABSTRACT FROM AUTHOR]

The magnitude of the aerosol forcing remains among the largest unknowns when assessing climate sensitivity over the historical period. Here, we quantify and explain a crucial but often overlooked source of uncertainty in aerosol forcing: the temporal variability of aerosol emissions. We show that time‐variability in biomass burning (BB) emissions weakens the time‐averaged total aerosol forcing, particularly in the Northern Hemisphere mid‐ to high‐latitudes. BB emissions variability produces weaker (less negative) mean effective radiative forcing (ERF) compared to scenarios with no interannual variability in emissions. Satellite‐estimated BB emissions (and associated variability) result in a June–September absolute ERF (relative to zero BB emissions) of −7.7 W m−2 from 50° to 70°N, compared to −10.4 W m−2 when no emissions variability is used in the Community Earth System Model version 2 (CESM2). This difference in forcing is attributable to nonlinear aerosol‐cloud interactions. Aerosol forcing will be overestimated (i.e., more negative) if emissions are temporally‐smoothed. Plain Language Summary: Aerosols are small particles in the air that can impact the climate, but we don't know exactly how much. One reason for this uncertainty is that the amount of aerosols can change a lot over time, especially when it comes from burning things like wood or plants (i.e., biomass burning emissions). When the amount of these biomass burning aerosols changes a lot from year‐to‐year, it has less of an impact on the climate. This is because clouds react less strongly when aerosol concentrations are high compared to when they are low. If we smooth out the changes in aerosol amounts over time (as is done with most Earth System Model simulations), we will overestimate their cooling impact on the climate. Key Points: The radiative forcing due to aerosols is overestimated if emissions are temporally‐smoothedInterannual variability in aerosol emissions leads to a weakened time‐average cloud radiative effectChanges in time‐average cloud radiative effect with differing aerosol variability are due to nonlinear aerosol‐cloud interactions [ABSTRACT FROM AUTHOR]

The HIV-1 reverse transcriptase-RNA/RNA-dCTP ternary complex has a different conformation than complexes involving DNA/DNA and DNA/RNA. Because of this, dCTP incorporation is significantly less than that for DNA/DNA and DNA/RNA substrates.

A novel decomposition of the ocean heat energy that contributes to sea ice melt and growth (ocean‐ice and frazil heat) into components that are driven by surface heat flux and ocean circulation changes is used to isolate the evolving roles of the atmosphere and ocean in the Arctic sea ice loss from CO2 increases. A sea ice volume budget analysis is used to separate the impacts of the anomalous frazil/ocean‐ice heat from those of atmosphere‐ice heat on the evolving Arctic sea ice volume. The role of atmosphere‐ocean coupling in augmenting or curtailing the atmosphere‐ and ocean‐driven sea ice losses is further isolated by comparing the ice volume budget and the anomalous frazil/ocean‐ice heat components in partially and fully coupled experiments. Atmosphere‐ice heat fluxes drive most of Arctic sea ice loss in the first decade following CO2 increase by increasing the sea ice top face melt in summer, while ocean circulation changes drive the loss over the longer term through the anomalous increase of heat transport into the Arctic, which drive decreases in frazil ice growth and sea ice extent in winter. Atmosphere‐ocean coupling in the subpolar Atlantic further supports a negative feedback that attenuates the ocean‐driven sea ice losses over time; by accelerating the weakening of the Atlantic meridional overturning circulation, it causes a large cooling of the subpolar Atlantic and attenuation of the anomalous heat transport into the Arctic in winter, allowing for a seasonal Arctic sea ice in the fully coupled experiment, while the Arctic completely becomes ice free in the partially coupled experiment. Plain Language Summary: The interactions between the atmosphere, ocean, and sea ice are disentangled in order to isolate their individual roles in driving Arctic sea ice losses and how these roles change over time. We introduce a new decomposition of the ocean heat that contributes to melting and growing sea ice into parts that are caused by changes in the surface heat input into the ocean and ocean circulation. We analyze the contributions of the ocean heat and atmosphere heat to the overall Arctic sea ice volume changes, both in an experiment where a two‐way interaction between the atmosphere and ocean is allowed and in another where this two‐way interaction is not allowed. Our results suggest that the atmosphere causes most of Arctic sea ice loss (within a decade) by increasing sea ice top face melt, while the ocean causes the long‐term sea ice changes in the Arctic by decreasing winter bottom face growth. The interactions between the atmosphere and ocean further enhance the ocean circulation weakening and surface cooling in the subpolar Atlantic and thereby stop the winter heat transport increase into the Arctic and eventually curtail the ocean‐driven sea ice loss and prevent the total loss of Arctic sea ice. Key Points: A novel decomposition isolates how ocean surface heating, circulation changes, and air‐sea interactions drive sea ice loss from CO2 increasesAtmosphere changes determine Arctic sea ice loss within the first decade; ocean circulation changes drive longer term ice lossesAtmosphere‐ocean coupling feedbacks on ocean circulation change in the subpolar Atlantic stabilizes Arctic sea ice loss in the long term [ABSTRACT FROM AUTHOR]

Numerical water tracers implemented in a global climate model are used to study how polar hydroclimate responds to CO2-induced warming from a source-receptor perspective. Although remote moisture sources contribute substantially more to polar precipitation year-round in the mean state, an increase in locally sourced moisture is crucial to the winter season polar precipitation response to greenhouse gas forcing. In general, the polar hydroclimate response to CO2-induced warming is strongly seasonal: over both the Arctic and Antarctic, locally sourced moisture constitutes a larger fraction of the precipitation in winter, while remote sources become even more dominant in summer. Increased local evaporation in fall and winter is coincident with sea ice retreat, which greatly augments local moisture sources in these seasons. In summer, however, larger contributions from more remote moisture source regions are consistent with an increase in moisture residence times and a longer moisture transport length scale, which produces a robust hydrologic cycle response to CO2-induced warming globally. The critical role of locally sourced moisture in the hydrologic cycle response of both the Arctic and Antarctic is distinct from controlling factors elsewhere on the globe; for this reason, great care should be taken in interpreting polar isotopic proxy records from climate states unlike the present. [ABSTRACT FROM AUTHOR]

The Last Millennial Reanalysis (LMR) employs a data assimilation approach to reconstruct climate fields from annually-resolved proxy data over years 0-2000CE. We use the LMR to examine Atlantic Multidecadal Variability (AMV) over the last two millennia, and find several robust thermodynamic features associated with a positive Atlantic Multidecadal Oscillation (AMO) index that reveal a dynamically-consistent pattern of variability: the Atlantic and most continents warm; sea ice thins over the Arctic and retreats over the Greenland-Iceland-Norwegian Seas; and equatorial precipitation shifts northward. The latter is consistent with anomalous southward energy transport mediated by the atmosphere. Net downward shortwave radiation increases at both the top-of-atmosphere and surface, indicating a decrease in planetary albedo, likely due to a decrease in low clouds. Heat is absorbed by the climate system and the oceans warm. Wavelet analysis of the AMV time series shows a reddening of the frequency spectrum at the 50-to-100-year time scale, but no evidence of a distinct multidecadal or centennial spectral peak. This latter result is insensitive to both choice of prior model and calibration dataset used in the data assimilation algorithm, suggesting that the lack of a distinct multidecadal spectral peak is a robust result. [ABSTRACT FROM AUTHOR]