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On the turbulent heat fluxes: A comparison among satellite-based estimates, atmospheric reanalyses, and in-situ observations during the winter climate over Arctic sea ice
Full text linkEn sammenligning mellom satellittbaserte estimater, atmosfæriske reanalyser, og in situ observasjoner under vinterklimaet over arktisk sjøis I Polhavet og tilgrensende områder finnes det svært få direkte målinger av fysiske forhold, særskilt om vinteren. Derfor må vi ofte ta i bruk atmosfæriske modeller, reanalyser, satellittdata, og klimamodeller for å studere klimavariasjoner og -endringer. I reanalyser blir det brukt observasjoner fra hele verden sammen med en værmodell for å beregne et estimat for atmosfærens tilstand, også for områder hvor det er mangel på direkte målinger. Derfor er data i reanalyser mer usikre i områder med manglende observasjoner, noe som er viktig å ta hensyn til under forsøk på å verifisere og forbedre klima- og værmodeller. I vår studie benyttet vi unike atmosfæriske observasjoner fra den NP-ledede ekspedisjonen N-ICE2015, som foregikk i Polhavet nord for Svalbard fra januar til juni i 2015, til å evaluere data fra reanalyser og satellitt. Dette ga en sjelden mulighet til å gjøre en evaluering for dette området om vinteren. Resultatene er varierende, og noen langvarige problemer finnes fortsatt i nye produkter, inkludert betydelige avvik. Generelt sett virker reanalyser bedre enn satellittdataprodukter for overflaten av sjøis, men begge har atskillige avvik fra våre direkte observasjoner. Våre resultater viser også et problem knyttet til hvordan reanalysedata presenteres – oftest som gjennomsnittverdier over et stort område med en blanding av is og åpent hav. Selv om det bare er noen få prosent av området som er åpent hav, har dette stor påvirkning på gjennomsnittlig utveksling av varme og vanndamp. At det ikke skilles mellom verdier for isdekke og for åpent hav i reanalyse-datasett, medfører at det er vanskelig å evaluere reanalysene eller bruke dem til å modellere sjøis. Våre resultater gir nyttig kunnskap om reanalyser, hvilke produkter som egner seg best i fremtidige studier, og hvilke parametere som det bør vies mest forsiktig med. Vi tror resultatene også vil bli benyttet til å forbedre eksisterende reanalyse- og satellittdataproduktene.The surface energy budget is crucial for Arctic sea ice mass balance calculation and climate systems, among which turbulent heat fluxes significantly affect the air–sea exchanges of heat and moisture in the atmospheric boundary layer. Satellite observations (e.g. CERES and APP-X) and atmospheric reanalyses (e.g., ERA5) are often used to represent components of the energy budget at regional and pan-Arctic scales. However, the uncertainties of the satellite-based turbulent heat fluxes are largely unknown, and cross-comparisons with reanalysis data and in-situ observations are limited. In this study, satellite-based turbulent heat fluxes were assessed against in-situ observations from the N-ICE2015 drifting ice station (north of Svalbard, January–June 2015) and ERA5 reanalysis. The turbulent heat fluxes were calculated by two approaches using the satellite-based ice surface temperature and radiative fluxes, surface atmospheric parameters from ERA5, and snow/sea ice thickness from the pan-Arctic Ice Ocean Modeling and Assimilation System (PIOMAS). We found that the bulk-aerodynamic formula based results could better capture the variations of turbulent heat fluxes, while the maximum entropy production based estimates are comparable with ERA5 in terms of root-mean-square error (RMSE). CERES-based estimates outperform the APP-X-based ones but ERA5 performs the best in all seasons (RMSE of 18 and 7 W m−2 for sensible and latent heat flux, respectively). The air–ice temperature/humidity differences and the surface radiation budget were found the primary driving factors in the bulk-formula method and maximum entropy production (MEP) method, respectively. Furthermore, errors in the surface and near-surface temperature and humidity explain almost 50% of the uncertainties in the estimates based on the bulk-formula, whereas errors in the net radiative fluxes explain more than 50% of the uncertainties in the MEP-based results.publishedVersio
Regime shift in Arctic Ocean sea ice thickness
Full text linkManifestations of climate change are often shown as gradual changes in physical or biogeochemical properties1. Components of the climate system, however, can show stepwise shifts from one regime to another, as a nonlinear response of the system to a changing forcing2. Here we show that the Arctic sea ice regime shifted in 2007 from thicker and deformed to thinner and more uniform ice cover. Continuous sea ice monitoring in the Fram Strait over the last three decades revealed the shift. After the shift, the fraction of thick and deformed ice dropped by half and has not recovered to date. The timing of the shift was preceded by a two-step reduction in residence time of sea ice in the Arctic Basin, initiated first in 2005 and followed by 2007. We demonstrate that a simple model describing the stochastic process of dynamic sea ice thickening explains the observed ice thickness changes as a result of the reduced residence time. Our study highlights the long-lasting impact of climate change on the Arctic sea ice through reduced residence time and its connection to the coupled ocean–sea ice processes in the adjacent marginal seas and shelves of the Arctic Ocean.publishedVersio
Upper ocean warming and sea ice reduction en the East Greenland current from 2003 to 2019
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Divergence in Climate Model Projections of Future Arctic Atlantification
Full text linkThe Arctic Ocean is strongly stratified by salinity in the uppermost layers. This stratification is a key attribute of the region as it acts as an effective barrier for the vertical exchanges of Atlantic Water heat, nutrients, and CO2 between intermediate depths and the surface of the Eurasian and Amerasian basins (EB and AB, respectively). Observations show that from 1970 to 2017, the stratification in the AB has strengthened, whereas, in parts of the EB, the stratification has weakened. The strengthening in the AB is linked to freshening and deepening of the halocline. In the EB, the weakened stratification is associated with salinification and shoaling of the halocline (Atlantification). Simulations from a suite of CMIP6 models project that, under a strong greenhouse gas forcing scenario (ssp585), the overall surface freshening and warming continue in both basins, but there is a divergence in hydrographic trends in certain regions. Within the AB, there is agreement among the models that the upper layers will become more stratified. However, within the EB, models diverge regarding future stratification. This is due to different balances between trends at the surface and trends at depth, related to Fram Strait fluxes. The divergence affects projections of the future state of Arctic sea ice, as models with the strongest Atlantification project the strongest decline in sea ice volume in the EB. From these simulations, one could conclude that Atlantification will not spread eastward into the AB; however, models must be improved to simulate changes in a more intricately stratified EB correctly.publishedVersio
