Mechanisms of decadal North Atlantic climate variability and implications for the recent cold anomaly

Decadal sea surface temperature (SST) fluctuations in the North Atlantic Ocean influence climate over adjacent land areas and are a major source of skill in climate predictions. However, the mechanisms underlying decadal SST variability remain to be fully understood. This study isolates the mechanisms driving North Atlantic SST variability on decadal time scales using low-frequency component analysis, which identifies the spatial and temporal structure of low-frequency variability. Based on observations, large ensemble historical simulations, and preindustrial control simulations, we identify a decadal mode of atmosphere–ocean variability in the North Atlantic with a dominant time scale of 13–18 years. Large-scale atmospheric circulation anomalies drive SST anomalies both through contemporaneous air–sea heat fluxes and through delayed ocean circulation changes, the latter involving both the meridional overturning circulation and the horizontal gyre circulation. The decadal SST anomalies alter the atmospheric meridional temperature gradient, leading to a reversal of the initial atmospheric circulation anomaly. The time scale of variability is consistent with westward propagation of baroclinic Rossby waves across the subtropical North Atlantic. The temporal development and spatial pattern of observed decadal SST variability are consistent with the recent observed cooling in the subpolar North Atlantic. This suggests that the recent cold anomaly in the subpolar North Atlantic is, in part, a result of decadal SST variability.publishedVersio

A Direct Estimate of Volume, Heat, and Freshwater Exchange Across the Greenland‐Iceland‐Faroe‐Scotland Ridge

The flow of warm salty water toward the Nordic Seas is of fundamental importance to the climate of central and northern Europe. In an effort to gain an improved quantitative assessment of these fluxes a program was started in 2008 to measure upper-ocean currents from the high-seas ferry Norröna, which operates out of the Faroes to Iceland and Denmark. The current measurements were made with an acoustic Doppler current profiler mounted in the Norröna\u27s hull. Starting in fall of 2013 monthly deployments of Expendable BathyThermographs give comprehensive information on the temperature field. These velocity and temperature data can be combined to estimate mean volume and temperature fluxes (referenced to 0 °C) for the two sections. Archived hydrographic data give us the corresponding salt transport. Thanks to an array of 12 tall moorings across the Blosseville Basin that measured currents, temperature, and salinity net transport of volume, temperature, and salt between Iceland and Greenland can also be estimated. By combining the velocity data from these three sections the strength of the Nordic Seas branch of the meridional overturning circulation is estimated to be 7.7 ± 0.8 Sv where 1 Sv = 106 m3/s. Imposing the constraint of zero net volume and salt flux, the corresponding heat and freshwater fluxes are estimated to be 264 ± 27 TW (1 TW = 1012 W) and −0.104 ± 0.01 Sv, respectively. The uncertainties in heat and freshwater fluxes are largely governed by volume fluxes. The Norröna program is ongoing

Sea-level variability and change along the Norwegian coast between 2003 and 2018 from satellite altimetry, tide gauges, and hydrography

Sea-level variations in coastal areas can differ significantly from those in the nearby open ocean. Monitoring coastal sea-level variations is therefore crucial to understand how climate variability can affect the densely populated coastal regions of the globe. In this paper, we study the sea-level variability along the coast of Norway by means of in situ records, satellite altimetry data, and a network of eight hydrographic stations over a period spanning 16 years (from 2003 to 2018). At first, we evaluate the performance of the ALES-reprocessed coastal altimetry dataset (1 Hz posting rate) by comparing it with the sea-level anomaly from tide gauges over a range of timescales, which include the long-term trend, the annual cycle, and the detrended and deseasoned sea-level anomaly. We find that coastal altimetry and conventional altimetry products perform similarly along the Norwegian coast. However, the agreement with tide gauges in terms of trends is on average 6 % better when we use the ALES coastal altimetry data. We later assess the steric contribution to the sea level along the Norwegian coast. While longer time series are necessary to evaluate the steric contribution to the sea-level trends, we find that the sea-level annual cycle is more affected by variations in temperature than in salinity and that both temperature and salinity give a comparable contribution to the detrended and deseasoned sea-level variability along the entire Norwegian coast. A conclusion from our study is that coastal regions poorly covered by tide gauges can benefit from our satellite-based approach to study and monitor sea-level change and variability

Mechanisms of decadal North Atlantic climate variability and implications for the recent cold anomaly

Decadal sea surface temperature (SST) fluctuations in the North Atlantic Ocean influence climate over adjacent land areas and are a major source of skill in climate predictions. However, the mechanisms underlying decadal SST variability remain to be fully understood. This study isolates the mechanisms driving North Atlantic SST variability on decadal time scales using low-frequency component analysis, which identifies the spatial and temporal structure of low-frequency variability. Based on observations, large ensemble historical simulations, and preindustrial control simulations, we identify a decadal mode of atmosphere–ocean variability in the North Atlantic with a dominant time scale of 13–18 years. Large-scale atmospheric circulation anomalies drive SST anomalies both through contemporaneous air–sea heat fluxes and through delayed ocean circulation changes, the latter involving both the meridional overturning circulation and the horizontal gyre circulation. The decadal SST anomalies alter the atmospheric meridional temperature gradient, leading to a reversal of the initial atmospheric circulation anomaly. The time scale of variability is consistent with westward propagation of baroclinic Rossby waves across the subtropical North Atlantic. The temporal development and spatial pattern of observed decadal SST variability are consistent with the recent observed cooling in the subpolar North Atlantic. This suggests that the recent cold anomaly in the subpolar North Atlantic is, in part, a result of decadal SST variability

Wintertime fCO2 variability in the subpolar North Atlantic since 2004

Winter data of surface ocean temperature (SST), salinity (SSS) and CO2 fugacity (fCO2) collected on the VOS M/V Nuka Arctica in the subpolar North Atlantic between 2004 and 2017 are used to establish trends, drivers, and interannual variability. Over the period, waters cooled and freshened, and the fCO2 increased at a rate similar to the atmospheric CO2 growth rate. When accounting for the freshening, the inferred increase in dissolved inorganic carbon (DIC) was found to be approximately twice that expected from atmospheric CO2 alone. This is attributed to the cooling. In the Irminger Sea, fCO2 exhibited additional interannual variations driven by atmospheric forcing through winter mixing. As winter fCO2 in the region is close to the atmospheric, the subpolar North Atlantic has varied between being slightly supersaturated and slightly undersaturated over the investigated period

Wintertime fCO2 variability in the subpolar North Atlantic since 2004

Winter data of surface ocean temperature (SST), salinity (SSS) and CO2 fugacity (fCO2) collected on the VOS M/V Nuka Arctica in the subpolar North Atlantic between 2004 and 2017 are used to establish trends, drivers, and interannual variability. Over the period, waters cooled and freshened, and the fCO2 increased at a rate similar to the atmospheric CO2 growth rate. When accounting for the freshening, the inferred increase in dissolved inorganic carbon (DIC) was found to be approximately twice that expected from atmospheric CO2 alone. This is attributed to the cooling. In the Irminger Sea, fCO2 exhibited additional interannual variations driven by atmospheric forcing through winter mixing. As winter fCO2 in the region is close to the atmospheric, the subpolar North Atlantic has varied between being slightly supersaturated and slightly undersaturated over the investigated period