Subpolar Atlantic Ocean mixed layer heat content variability is increasingly driven by an active ocean

Cold conditions in the upper layer of the subpolar North Atlantic Ocean, at a time of pervasive warming elsewhere, have provoked significant debate. Uncertainty arises both from potential causes (surface heat loss and ocean circulation changes) and characteristic timescales (interannual to multidecadal). Resolution of these uncertainties is important as cold conditions have been linked to recent European weather extremes and a decline in the Atlantic overturning circulation. Using observations, supported by high resolution climate model analysis, we show that a surprisingly active ocean regularly generates both cold and warm interannual anomalies in addition to those generated by surface heat exchange. Furthermore, we identify distinct sea surface temperature patterns that characterise whether the ocean or atmosphere has the strongest influence in a particular year. Applying these new insights to observations, we find an increasing role for the ocean in setting North Atlantic mixed layer heat content variability since 1960

Spatial and temporal scales of variability in Tropical Atlantic sea surface salinity from the SMOS and Aquarius satellite missions

Taking advantage of the spatially dense, multi-year time series of global Sea Surface Salinity (SSS) from two concurrent satellite missions, the spatial and temporal decorrelation scales of SSS in the Tropical Atlantic 30°N–30°S are quantified for the first time from SMOS and Aquarius observations. Given the dominance of the seasonal cycle in SSS variability in the region, the length scales are calculated both for the mean and anomaly (i.e. seasonal cycle removed) SSS fields. Different 7–10 days composite SSS products from the two missions are examined to explore the possible effects of varying resolution, bias corrections and averaging characteristics. With the seasonal cycle retained, the SSS field is characterized by strongly anisotropic spatial variability. Homogeneous SSS variations in the Tropics have the longest zonal scales of over ~ 2000 km and long temporal scales of up to ~ 70–80 days, as shown by both SMOS and Aquarius. The longest meridional scales, reaching over ~ 1000 km, are seen in the South Atlantic between ~ 10°–25°S, most discernible in Aquarius data. The longest temporal scales of SSS variability are reported by both satellites to occur in the North-West Atlantic region 15°–30°N, at the Southern end of the Sargasso Sea, with SSS persisting for up to 150–200 days. The removal of the seasonal cycle results in a noticeable decrease in the spatio-temporal decorrelation scales over most of the basin. Overall, with the exception of the differences in the South Atlantic, there is general agreement between the spatial and temporal scales of SSS from the two satellites and different products, despite differences in individual product calibration and resolution characteristics. These new estimates of spatio-temporal decorrelation scales of SSS improve our knowledge of the processes and mechanisms controlling the Tropical Atlantic SSS variability, and provide valuable information for a wide range of oceanographic and modelling applications

Declining winter heat loss threatens continuing ocean convection at a Mediterranean dense water formation site

A major change in winter sea surface heat loss between two key Mediterranean dense water formation sites, the North-west Mediterranean (NWMed) and the Aegean Sea, since 1950 is revealed using atmospheric reanalyses. The NWMed heat loss has weakened considerably (from -154 Wm-2 in 1951-1985 to -137 Wm-2 in 1986-2020) primarily because of reduced latent heat flux. This long-term weakening threatens continued dense water formation, and we show by evaluation of historical observations that winter-time ocean convection in the NWMed has declined by 40% from 1969 to 2018. Extension of the heat flux analysis reveals changes at other key dense water formation sites that favour an eastward shift in the locus of Mediterranean convection towards the Aegean Sea (where heat loss has remained unchanged at -172 Wm-2). The contrasting behaviour is due to differing time evolution of sea-air humidity and temperature gradients. These gradients have weakened in the NWMed due to more rapid warming of the air than the sea surface but remain near-constant in the Aegean. The different time evolution reflects the combined effects of global heating and atmospheric circulation changes which tend to offset heating in the Aegean but not the NWMed. The shift in heat loss has potentially significant consequences for dense water formation at these two sites and outflow to the Atlantic. Our observation of differential changes in heat loss has implications for temporal variations in the balance of convection elsewhere e.g., the Labrador-Irminger-Nordic Seas nexus of high latitude Atlantic dense water formation sites

Global water cycle amplifying at less than the Clausius-Clapeyron rate

A change in the cycle of water from dry to wet regions of the globe would have far reaching impact on humanity. As air warms, its capacity to hold water increases at the Clausius-Clapeyron rate (CC, approximately 7% °C−1). Surface ocean salinity observations have suggested the water cycle has amplified at close to CC following recent global warming, a result that was found to be at odds with state-of the art climate models. Here we employ a method based on water mass transformation theory for inferring changes in the water cycle from changes in three-dimensional salinity. Using full depth salinity observations we infer a water cycle amplification of 3.0 ± 1.6% °C−1 over 1950–2010. Climate models agree with observations in terms of a water cycle amplification (4.3 ± 2.0% °C−1) substantially less than CC adding confidence to projections of total water cycle change under greenhouse gas emission scenarios

Extreme variability in Irminger Sea winter heat loss revealed by ocean observatories initiative mooring and the ERA5 reanalysis

Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 46(1), (2018): 293-302, doi:10.1029/2018GL080956.Ground‐breaking measurements from the ocean observatories initiative Irminger Sea surface mooring (60°N, 39°30′W) are presented that provide the first in situ characterization of multiwinter surface heat exchange at a high latitude North Atlantic site. They reveal strong variability (December 2014 net heat loss nearly 50% greater than December 2015) due primarily to variations in frequency of intense short timescale (1–3 days) forcing. Combining the observations with the new high resolution European Centre for Medium Range Weather Forecasts Reanalysis 5 (ERA5) atmospheric reanalysis, the main source of multiwinter variability is shown to be changes in the frequency of Greenland tip jets (present on 15 days in December 2014 and 3 days in December 2015) that can result in hourly mean heat loss exceeding 800 W/m2. Furthermore, a new picture for atmospheric mode influence on Irminger Sea heat loss is developed whereby strongly positive North Atlantic Oscillation conditions favor increased losses only when not outweighed by the East Atlantic Pattern.We are grateful to Meric Srokosz and the two reviewers for helpful comments on this work. S. J. acknowledges the U.K. Natural Environment Research Council ACSIS programme funding (Ref. NE/N018044/1). M. O. acknowledges support from EU Horizon 2020 projects AtlantOS (grant 633211) and Blue Action (grant 727852). G. W. K. M. acknowledges support from the Natural Sciences and Engineering Research Council of Canada. Support for the Irminger Sea array of the ocean observatories initiative (OOI) came from the U.S. National Science Foundation. Thanks to the WHOI team and ships' officers and crew for the field deployments and to Nan Galbraith for processing the data and computing the air‐sea fluxes. Support for this processing, and making available and sharing the OOI data, came from the National Science Foundation under a Collaborative Research: Science Across Virtual Institutes grant (82164000) to R. A. W. Data used are available from the following sites: NOAA Climate Prediction Center NAO and EAP indices ftp://ftp.cpc.ncep.noaa.gov/wd52dg/data/indices/tele_index.nh, ECMWF Reanalysis 5 (ERA5) https://www.ecmwf.int/en/forecasts/datasets/archive‐datasets/reanalysis/datasets/era5, and ocean observatories initiative Irminger Mooring https://ooinet.oceanobservatories.org/.2019-06-1

Observed and projected changes in North Atlantic seasonal temperature reduction and their drivers

The autumn-winter seasonal temperature reduction (STR) of the surface North Atlantic Ocean is investigated with control and climate change simulations of a coupled model and an observation-based sea surface temperature (SST) data set. In the climate change simulation, an increase in the magnitude of the STR is found over much of the North Atlantic, and this change is particularly marked in sea-ice affected regions and the subpolar gyre. Similar results for the mid-high latitude North Atlantic are obtained in the observational analysis. In particular, both the observation and climate model based results show that the STR has increased in magnitude by up to 0.3°C per decade in the subpolar gyre over the period 1951–2020. Drivers for the stronger STR are explored with a focus on potential contributions from increases in either ocean heat loss or the sensitivity of SST to heat loss. Over a large part of the mid-high latitude North Atlantic surface heat loss is found to have weakened in recent decades and is therefore not responsible for the stronger STR (exceptions to this are the near-coastal areas where sea-ice loss is important). In contrast, analysis of daily sensible and latent heat flux data reveals that the sensitivity of SST to heat loss has increased indicating that this term has played a major role in the stronger STR. Areas of greater SST sensitivity (and greater STR) are associated with increased surface stratification brought about predominantly by warming of the northern ocean regions

Role of air–sea fluxes and ocean surface density in the production of deep waters in the eastern subpolar gyre of the North Atlantic

Wintertime convection in the North Atlantic Ocean is a key component of the global climate as it produces dense waters at high latitudes that flow equatorward as part of the Atlantic Meridional Overturning Circulation (AMOC). Recent work has highlighted the dominant role of the Irminger and Iceland basins in the production of North Atlantic Deep Water. Dense water formation in these basins is mainly explained by buoyancy forcing that transforms surface waters to the deep waters of the AMOC lower limb. Air–sea fluxes and the ocean surface density field are both key determinants of the buoyancy-driven transformation. We analyze these contributions to the transformation in order to better understand the connection between atmospheric forcing and the densification of surface water. More precisely, we study the impact of air–sea fluxes and the ocean surface density field on the transformation of subpolar mode water (SPMW) in the Iceland Basin, a water mass that “pre-conditions” dense water formation downstream. Analyses using 40 years of observations (1980–2019) reveal that the variance in SPMW transformation is mainly influenced by the variance in density at the ocean surface. This surface density is set by a combination of advection, wind-driven upwelling and surface fluxes. Our study shows that the latter explains ∼ 30 % of the variance in outcrop area as expressed by the surface area between the outcropped SPMW isopycnals. The key role of the surface density in SPMW transformation partly explains the unusually large SPMW transformation in winter 2014–2015 over the Iceland Basin

Impact of the atmospheric climate modes on wave climate in the North Atlantic

Trabajo presentado en la EGU General Assemby 2014, celebrada del 27 de abril al 2 de mayo de 2014 en Viena (Austria)This study establishes the relationships between the mean modes of atmospheric variability in the North Atlantic and present wave climate. The modes considered, namely the North Atlantic Oscillation (NAO), the East Atlantic pattern (EA), the East Atlantic Western Russian pattern (EA/WR) and the Scandinavian pattern (SCAN), are obtained from the NOAA Climate Prediction Centre. The wave data sets used consist of buoy records and two high-resolution simulations of significant wave height (SWH), mean wave period (MWP) and mean wave direction (MWD) forced with ERA-40 (1958-2002) and ERA-INTERIM (1989-2008) wind fields. The results show the winter impact of each mode on wave parameters which are discussed regionally. The NAO and EA pattern increase winter SWH up to 1 m per unit index at the Scottish and Spanish coasts, respectively, during their positive phase; while EA pattern causes clockwise changes of winter MWD up to more than 60 degrees per unit index at the Bay of Biscay during its negative phase. EA/WR and SCAN patterns have a weaker impactPeer Reviewe

Atlantic deep water formation occurs primarily in the Iceland Basin and Irminger Sea by local buoyancy forcing

The Atlantic Meridional Overturning Circulation (AMOC), a key mechanism in the climate system, delivers warm and salty waters from the subtropical gyre to the subpolar gyre and Nordic Seas, where they are transformed into denser waters flowing southward in the lower AMOC limb. The prevailing hypothesis is that dense waters formed in the Labrador and Nordic Seas are the sources for the AMOC lower limb. However, recent observations reveal that convection in the Labrador Sea contributes minimally to the total overturning of the subpolar gyre. In this study, we show that the AMOC is instead primarily composed of waters formed in the Nordic Seas and Irminger and Iceland basins. A first direct estimate of heat and freshwater fluxes over these basins demonstrates that buoyancy forcing during the winter months can almost wholly account for the dense waters of the subpolar North Atlantic that are exported as part of the AMOC

Maintenance and broadening of the ocean’s salinity distribution by the water cycle

The global water cycle leaves an imprint on ocean salinity through evaporation and precipitation. It has been proposed that observed changes in salinity can be used to infer changes in the water cycle. Here salinity is characterized by the distribution of water masses in salinity coordinates. Only mixing and sources and sinks of freshwater and salt can modify this distribution. Mixing acts to collapse the distribution, making saline waters fresher and fresh waters more saline. Hence, in steady state, there must be net precipitation over fresh waters and net evaporation over saline waters. A simple model is developed to describe the relationship between the breadth of the distribution, the water cycle, and mixing—the latter being characterized by an e-folding time scale. In both observations and a state-of-the-art ocean model, the water cycle maintains a salinity distribution in steady state with a mixing time scale of the order of 50 yr. The same simple model predicts the response of the salinity distribution to a change in the water cycle. This study suggests that observations of changes in ocean salinity could be used to infer changes in the hydrological cycle