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1. Stratospheric and Tropospheric Flux Contributions to the Polar Cap Energy Budgets

   https://doi.org/10.1175/JCLI-D-20-0722.1  

   Cardinale, Christopher J. ; Rose, Brian E. ; Lang, Andrea L. ; Donohoe, Aaron ( June 2021 , Journal of Climate)

   null (Ed.)

   Abstract The flux of moist static energy into the polar regions plays a key role in the energy budget and climate of the polar regions. While usually studied from a vertically integrated perspective ( F wall ), this analysis examines its vertical structure, using the NASA-MERRA-2 reanalysis to compute climatological and anomalous fluxes of sensible, latent, and potential energy across 70°N and 65°S for the period 1980–2016. The vertical structure of the climatological flux is bimodal, with peaks in the middle to lower troposphere and middle to upper stratosphere. The near-zero flux at the tropopause defines the boundary between stratospheric ( F strat ) and tropospheric ( F trop ) contributions to F wall . Especially at 70°N, F strat is found to be important to the climatology and variability of F wall , contributing 20.9 W m −2 to F wall (19% of F wall ) during the winter and explaining 23% of the variance of F wall . During winter, an anomalous poleward increase in F strat preceding a sudden stratospheric warming is followed by an increase in outgoing longwave radiation anomalies, with little influence on the surface energy budget of the Arctic. Conversely, a majority of the energy input by an anomalous poleward increase in F trop goes toward warming the Arctic surface. Overall, F trop is found to be a better metric than F wall for evaluating the influence of atmospheric circulations on the Arctic surface climate. 

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2. Causes of the Arctic’s Lower-Tropospheric Warming Structure

   https://doi.org/10.1175/JCLI-D-21-0298.1  

   Kaufman, Zachary S. ; Feldl, Nicole ( March 2022 , Journal of Climate)

   Arctic amplification has been attributed predominantly to a positive lapse rate feedback in winter, when boundary layer temperature inversions focus warming near the surface. Predicting high-latitude climate change effectively thus requires identifying the local and remote physical processes that set the Arctic’s vertical warming structure. In this study, we analyze output from the CESM Large Ensemble’s twenty-first-century climate change projection to diagnose the relative influence of two Arctic heating sources, local sea ice loss and remote changes in atmospheric heat transport. Causal effects are quantified with a statistical inference method, allowing us to assess the energetic pathways mediating the Arctic temperature response and the role of internal variability across the ensemble. We find that a step-increase in latent heat flux convergence causes Arctic lower-tropospheric warming in all seasons, while additionally reducing net longwave cooling at the surface. However, these effects only lead to small and short-lived changes in boundary layer inversion strength. By contrast, a step-decrease in sea ice extent in the melt season causes, in fall and winter, surface-amplified warming and weakened boundary layer temperature inversions. Sea ice loss also enhances surface turbulent heat fluxes and cloud-driven condensational heating, which mediate the atmospheric temperature response. While the aggregate effect of many moist transport events and seasons of sea ice loss will be different than the response to hypothetical perturbations, our results nonetheless highlight the mechanisms that alter the Arctic temperature inversion in response to CO₂forcing. As sea ice declines, the atmosphere’s boundary layer temperature structure is weakened, static stability decreases, and a thermodynamic coupling emerges between the Arctic surface and the overlying troposphere.

    

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3. The Increasing Efficiency of the Poleward Energy Transport Into the Arctic in a Warming Climate

   https://doi.org/10.1029/2022GL100834  

   Cardinale, Christopher J. ; Rose, Brian E. J. ( January 2023 , Geophysical Research Letters)

   This study quantifies the contribution to Arctic winter surface warming from changes in the tropospheric energy transport (F_trop) and the efficiency with whichF_tropheats the surface in the RCP8.5 warming scenario of the Community Earth System Model Large Ensemble. A metric for this efficiency,E_trop, measures the fraction of anomalousF_tropthat is balanced by an anomalous net surface flux (NSF). Drivers ofE_tropare identified in synoptic‐scale events during whichF_tropis the dominant driver of NSF.E_tropis sensitive to the vertical structure ofF_tropand pre‐existing Arctic lower‐tropospheric stability (LTS). In RCP8.5, winter‐meanF_tropdecreases from 95.1 to 85.4 W m⁻², whileE_tropincreases by 5.7%, likely driven by decreased Arctic LTS, indicating an increased coupling betweenF_tropand the surface energy budget. The net impact of decreasingF_tropand increasing efficiency is a positive 0.7 W m⁻²contribution to winter‐season surface heating.

    

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4. Cause of the Intense Tropics-Wide Tropospheric Warming in Response to El Niño

   https://doi.org/10.1175/JCLI-D-21-0728.1  

   Hogikyan, A. ; Resplandy, L. ; Fueglistaler, S. ( May 2022 , Journal of Climate)

   During El Niño events, a strong tropics-wide warming of the free troposphere is observed (of order 1 K at 300 hPa). This warming plays an important role for the teleconnection processes associated with El Niño but it remains unclear what initiates this warming. Since convective quasi-equilibrium only holds in regions of deep convection, the strong free-tropospheric warming implies that the warmest surface waters (where atmospheric deep convection occurs) must warm during El Niño. We analyze the evolution of the oceanic mixed layer heat budget over El Niño events as function of sea surface temperature (SST). Data from the ERA5 and an unforced simulation of a coupled climate model both confirm that SSTs during an El Niño event increase at the high end of the SST distribution. The data show that this is due to an anomalous heat flux from the atmosphere into the ocean caused by a decrease in evaporation due anomalously weak low-level winds (i.e., relative to the wind speed observed in the domain of deep convection in the climatological base state). It is hypothesized that the more zonally symmetric circulation during El Niño is responsible for the weakening of low-level winds. The result of a substantial heat flux into the ocean in the domain of atmospheric deep convection (the opposite of the canonical heat flux out of the ocean into the atmosphere observed in the cold eastern Pacific) caused by a decrease in low-level wind speed implies that the prominent tropospheric warming results from mechanical forcing.

    

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5. The Respective Roles of Ocean Heat Transport and Surface Heat Fluxes in Driving Arctic Ocean Warming and Sea Ice Decline

   https://doi.org/10.1175/JCLI-D-23-0399.1  

   Oldenburg, Dylan ; Kwon, Young-Oh ; Frankignoul, Claude ; Danabasoglu, Gokhan ; Yeager, Stephen ; Kim, Who M. ( February 2024 , Journal of Climate)

   Arctic Ocean warming and sea ice loss are closely linked to increased ocean heat transport (OHT) into the Arctic and changes in surface heat fluxes. To quantitatively assess their respective roles, we use the 100-member Community Earth System Model, version 2 (CESM2), Large Ensemble over the 1920–2100 period. We first examine the Arctic Ocean warming in a heat budget framework by calculating the contributions from heat exchanges with atmosphere and sea ice and OHT across the Arctic Ocean gateways. Then we quantify how much anomalous heat from the ocean directly translates to sea ice loss and how much is lost to the atmosphere. We find that Arctic Ocean warming is driven primarily by increased OHT through the Barents Sea Opening, with additional contributions from the Fram Strait and Bering Strait OHTs. These OHT changes are driven mainly by warmer inflowing water rather than changes in volume transports across the gateways. The Arctic Ocean warming driven by OHT is partially damped by increased heat loss through the sea surface. Although absorbed shortwave radiation increases due to reduced surface albedo, this increase is compensated by increasing upwelling longwave radiation and latent heat loss. We also explicitly calculate the contributions of ocean–ice and atmosphere–ice heat fluxes to sea ice heat budget changes. Throughout the entire twentieth century as well as the early twenty-first century, the atmosphere is the main contributor to ice heat gain in summer, though the ocean’s role is not negligible. Over time, the ocean progressively becomes the main heat source for the ice as the ocean warms.

   Arctic Ocean warming and sea ice loss are closely linked to increased ocean heat transport (OHT) into the Arctic and changes in surface heat fluxes. Here we use 100 simulations from the same climate model to analyze future warming and sea ice loss. We find that Arctic Ocean warming is primarily driven by increased OHT through the Barents Sea Opening, though the Fram and Bering Straits are also important. This increased OHT is primarily due to warmer inflowing water rather than changing ocean currents. This ocean heat gain is partially compensated by heat loss through the sea surface. During the twentieth century and early twenty-first century, sea ice loss is mainly linked to heat transferred from the atmosphere; however, over time, the ocean progressively becomes the most important contributor.

    

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