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

The water mass transformation framework and variability in hurricane activity

Hurricane activity has been higher since 1995 than in the 1970s and 1980s. This rise in activity has been linked to a warming Atlantic. In this study, we consider variability of the volume of water warmer than 26.5 ºC, considered widely to be the temperature threshold crucial to hurricane development. We find the depth of the 26.5 ºC isotherm better correlated with seasonal hurricane counts than SST in the early part of the Atlantic hurricane season in some regions. The volume of water transformed by surface heat fluxes to temperatures above 26.5 ºC is directly calculated using the Water Mass Transformation framework. This volume is compared with the year-to-year changes in the volume of water of this temperature to see how much of the volume can be explained using this calculation. In some years, there is notable correspondence between transformed and observed volume anomalies, but anomalies in other years must be largely associated with other processes, such as the divergence of horizontal heat transport associated with the AMOC. This technique provides evidence that, in a given year, coordinated physical mechanisms are responsible for the build-up of anomalous ocean heat; not only net surface heat exchange but also the convergence of horizontal heat transport from ocean currents, to provide fuel for larger numbers of intense hurricanes

Drivers of exceptionally cold North Atlantic Ocean temperatures and their link to the 2015 European heat wave

The North Atlantic and Europe experienced two extreme climate events in 2015: exceptionally cold ocean surface temperatures and a summer heat wave ranked in the top ten over the past 65 years. Here, we show that the cold ocean temperatures were the most extreme in the modern record over much of the mid-high latitude North-East Atlantic. Further, by considering surface heat loss, ocean heat content and wind driven upwelling we explain for the first time the genesis of this cold ocean anomaly. We find that it is primarily due to extreme ocean heat loss driven by atmospheric circulation changes in the preceding two winters combined with the re-emergence of cold ocean water masses. Furthermore, we reveal that a similar cold Atlantic anomaly was also present prior to the most extreme European heat waves since the 1980s indicating that it is a common factor in the development of these events. For the specific case of 2015, we show that the ocean anomaly is linked to a stationary position of the Jet Stream that favours the development of high surface temperatures over Central Europe during the heat wave. Our study calls for an urgent assessment of the impact of ocean drivers on major European summer temperature extremes in order to provide better advance warning measures of these high societal impact events

Impact of model resolution on Arctic sea ice and North Atlantic Ocean heat transport

Arctic sea-ice area and volume have substantially decreased since the beginning of the satellite era. Concurrently, the pole-ward heat transport from the North Atlantic Ocean into the Arctic has increased, partly contributing to the loss of sea ice. Increasing the horizontal resolution of general circulation models (GCMs) improves their ability to represent the complex interplay of processes at high latitudes. Here, we investigate the impact of model resolution on Arctic sea ice and Atlantic Ocean heat transport (OHT) by using five different state-of-the-art coupled GCMs (12 model configurations in total) that include dynamic representations of the ocean, atmosphere and sea ice. The models participate in the High Resolution Model Intercomparison Project (HighResMIP) of the sixth phase of the Coupled Model Intercomparison Project (CMIP6). Model results over the period 1950–2014 are compared to different observational datasets. In the models studied, a finer ocean resolution drives lower Arctic sea-ice area and volume and generally enhances Atlantic OHT. The representation of ocean surface characteristics, such as sea-surface temperature (SST) and velocity, is greatly improved by using a finer ocean reso-lution. This study highlights a clear anticorrelation at interannual time scales between Arctic sea ice (area and volume) and Atlantic OHT north of 60 ◦N in the models studied. However, the strength of this relationship is not systematically impacted by model resolution. The higher the latitude to compute OHT, the stronger the relationship between sea-ice area/volume and OHT. Sea ice in the Barents/Kara and Greenland–Iceland–Norwegian (GIN) Seas is more strongly connected to Atlantic OHT than other Arctic seas

Re-emergence of North Atlantic subsurface ocean temperature anomalies in a seasonal forecast system

A high-resolution coupled ocean atmosphere model is used to study the effects of seasonal re-emergence of North Atlantic subsurface ocean temperature anomalies on northern hemisphere winter climate. A 50-member control ensemble is integrated from 1 September 2007 to 28 February 2008 and compared with a parallel ensemble with perturbed ocean initial conditions. The perturbation consists of a density-compensated subsurface Atlantic temperature anomaly corresponding to the observed subsurface temperature anomaly for September 2010. The experiment is repeated for two atmosphere horizontal resolutions (~ 60 km and ~ 25 km) in order to determine whether the sensitivity of the atmosphere to re-emerging temperature anomalies is dependent on resolution. A wide range of re-emergence behavior is found within the perturbed ensembles. While the observations seem to indicate that most of the re-emergence is occurring in November, most members of the ensemble show re-emergence occurring later in the winter. However, when re-emergence does occur it is preceded by an atmospheric pressure pattern that induces a strong flow of cold, dry air over the mid-latitude Atlantic, and enhances oceanic latent heat loss. In response to re-emergence (negative SST anomalies), there is reduced latent heat loss, less atmospheric convection, a reduction in eddy kinetic energy and positive low-level pressure anomalies downstream. Within the framework of a seasonal forecast system the results highlight the atmospheric conditions required for re-emergence to take place and the physical processes that may lead to a significant effect on the winter atmospheric circulation

Easterly Waves over Africa. Part I: The Seasonal Cycle and Contrasts between Wet and Dry Years

The nature of African easterly waves (AEWs) and the easterly wave season over West Africa are examined using the National Centers for Environmental Prediction (NCEP) reanalysis. The study is carried out with two objectives in mind. The first goal is to describe the seasonal cycle of wave activity and to determine if it has a distinctive signature. To achieve this, the temporal evolution of wave period, amplitude, and structure are examined with wavelet analysis. This analysis is carried out at a grid point (158N, 08) that is at located where AEWs are well developed. The second goal is to determine differences in the wave characteristics and the wave season between wet and dry years. Regarding the seasonal cycle, it was found, in accordance with previous research, that AEW activity typically occurs between May and October. Disturbances have a periodicity between 4 and 8 days. At the level (600 mb) of the African easterly jet (AEJ) there was a broad peak in the magnitude of the variance over the months of July, August, and September. This may be due to the increased horizontal shear and barotropic instability, which also peaks at that time. There is a greater contribution of variance from longer periods (6.25–7.5 days) in the later half of the summer. At lower levels, the disturbances appear to be confined to periods of 3.75–5.0 days, with the maximum variance in the mean occurring in July. This may be a response to the change in the magnitud

Estimates of meridional overturning circulation variability in the North Atlantic from surface density flux fields

A method developed recently by Grist et al. (2009) is used to obtain estimates of variability in the strength of the meridional overturning circulation (MOC) at various latitudes in the North Atlantic. The method employs water mass transformation theory to determine the surface buoyancy forced overturning circulation (SFOC) using surface density flux fields from both the Hadley Centre Coupled Model version 3 (HadCM3) and National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) reanalysis observational data set. The previous application of the method was at 48°N using 100 years of model output, and here we show from a longer 400 year data set that it can be extended to provide useful estimates of the MOC variability in the range 35–65°N. The method relies on averaging of the SFOC over an interval prior to that at which the MOC estimate is required. The length of this interval increases as the latitude decreases from about 6 years at 65°N to 15 years at 36°N. Values for the correlation coefficient between the HadCM3 SFOC and MOC time series of 0.60, 0.64, and 0.39 are obtained at 60°N, 48°N, and 36°N. Thus, the SFOC approach may provide valuable complementary information about MOC variability in the middle-high-latitude North Atlantic to that determined from the Rapid array at 26°N but it becomes less useful as latitude decreases. The method is then applied using the NCEP/NCAR reanalysis to estimate MOC variability in the middle-high-latitude North Atlantic for the past 50 years. <br/