Extreme air–sea interaction over the North Atlantic subpolar gyre during the winter of 2013–2014 and its sub-surface legacy

Exceptionally low North American temperatures and record-breaking precipitation over the British Isles during winter 2013–2014 were interconnected by anomalous ocean evaporation over the North Atlantic subpolar gyre region (SPG). This evaporation (or oceanic latent heat release) was accompanied by strong sensible heat loss to the atmosphere. The enhanced heat loss over the SPG was caused by a combination of surface westerly winds from the North American continent and northerly winds from the Nordic Seas region that were colder, drier and stronger than normal. A distinctive feature of the air–sea exchange was that the enhanced heat loss spanned the entire width of the SPG, with evaporation anomalies intensifying in the east while sensible heat flux anomalies were slightly stronger upstream in the west. The immediate impact of the strong air–sea fluxes on the ocean–atmosphere system included a reduction in ocean heat content of the SPG and a shift in basin-scale pathways of ocean heat and atmospheric freshwater transport. Atmospheric reanalysis data and the EN4 ocean data set indicate that a longer-term legacy of the winter has been the enhanced formation of a particularly dense mode of Subpolar Mode Water (SPMW)—one of the precursors of North Atlantic Deep Water and thus an important component of the Atlantic Meridional Overturning Circulation. Using particle trajectory analysis, the likely dispersal of newly-formed SPMW is evaluated, providing evidence for the re-emergence of anomalously cold SPMW in early winter 2014/2015

On the mechanisms governing gas penetration into a tokamak plasma during a massive gas injection

A new 1D radial fluid code, IMAGINE, is used to simulate the penetration of gas into a tokamak plasma during a massive gas injection (MGI). The main result is that the gas is in general strongly braked as it reaches the plasma, due to mechanisms related to charge exchange and (to a smaller extent) recombination. As a result, only a fraction of the gas penetrates into the plasma. Also, a shock wave is created in the gas which propagates away from the plasma, braking and compressing the incoming gas. Simulation results are quantitatively consistent, at least in terms of orders of magnitude, with experimental data for a D 2 MGI into a JET Ohmic plasma. Simulations of MGI into the background plasma surrounding a runaway electron beam show that if the background electron density is too high, the gas may not penetrate, suggesting a possible explanation for the recent results of Reux et al in JET (2015 Nucl. Fusion 55 093013)

The impact of aerosol loading on estimates of the surface shortwave flux in the SOC climatology. COAPEC Project - Balancing the Atlantic Heat and Freshwater Budgets, Report No. 2

The uncertainty in empirical estimates of the shortwave flux in the Southampton Oceanography Centre (SOC) air-sea flux climatology due to neglect of tropospheric aerosols is investigated. The SOC shortwave flux fields were derived using a formula originally developed from data in an area of low aerosol loading and do not take into account the variability of aerosol content over the global ocean. We use the method of Tragou et al. (1999) to estimate the reduction that should be applied in order to correct for the effects of aerosol loading. Our results suggest that the mean global effect of not specifying atmospheric aerosol content is that the SOC shortwave flux is an overestimate, but by no more than 2 Wm-2. This equates to less than 7% of the global bias in the original climatology. At a regional level, the effect of the aerosols may be up to 40 Wm-2 in the tropical Atlantic and the Arabian Sea. Independent measurements of the shortwave flux from research buoys and satellites provide some support for our results. However, further analyses are required as only a few independent buoy measurements are currently available in regions of high aerosol loading

Balancing the SOC climatology using inverse analysis with spatially fixed parameter adjustments. COAPEC Project - Balancing the Atlantic Heat and Freshwater Budgets, Report No. 1

Early results from a project which has the aim of obtaining a balanced version of the SOC climatology using linear inverse analysis techniques are discussed. In particular, we investigate whether a set of balanced fields can be obtained using spatially fixed analysis parameter adjustments which satisfies the requirements of a.) global heat budget closure; b.) consistency with hydrographic estimates of regionally averaged surface heat fluxes, and c.) agreement with independent research buoy measurements. Results of analyses obtained using two formulations of the inverse method with up to ten ocean heat transport constraints distributed throughout the Atlantic and North Pacific oceans are presented. The first formulation is an established technique which utilises the heat transport estimates directly as constraints. The second is a novel application in which pairs of heat transport estimates are used to derive area averaged heat fluxes which are then employed as constraints. The solutions obtained in each case are found to be sensitive to the choice of location of the heat transport estimates when only a small number (less than 5) of constraints are applied. Consequently, we have focused on solutions obtained with the full set of ten hydrographic constraints both with and without the additional requirement that the globally averaged het flux should equal zero. Without this requirement solutions are obtained which have a net heat loss to the atmosphere of between 5 and 7 Wm-2. In order to close the global heat budget exactly it is necessary to specify it as an additional constraint. However, in this case the solution obtained with the heat transport method is not acceptable according to the criterion of Isemer et al. (1989) which requires the magnitude of the parameter adjustments to be smaller than the assumed error range for each. This criterion is satisfied if the requirement of exact closure is relaxed such that the global net heat flux is constrained to be 0±2 Wm-2. In the latter case, the inverse analysis adjustments to the different components of the heat flux are increases of 19% to the latent heat flux, 7% to the sensible heat flux, 9% to the longwave flux and a reduction of 6% to the shortwave flux. Comparison of the adjusted fluxes with measurements made by various WHOI research buoys confirm the suggestion of Josey et al. (1999) that spatially fixed parameter adjustments lead to poorer agreement with the buoys than was found to be the case with the original SOC fluxes. This result indicates that spatially dependent adjustments of the free parameters in the inverse analysis are necessary in order to obtain a solution which is satisfactory in the sense that it meets the three requirements listed above

Closing the heat budget of the SOC climatology through spatially dependent inverse analysis parameter adjustment. COAPEC Project - Balancing the Atlantic Heat and Freshwater Budgets, Report No. 3

Previous research aimed at closing the heat budget of the Southampton Oceanography Centre (SOC) air-sea flux climatology through the method of inverse analysis is extended to include spatially dependent parameter adjustments and error specification. In earlier analyses, a balanced solution was achieved using globally fixed parameter adjustments, primarily an increase of 19% to the latent heat flux and a reduction of 6% to the shortwave flux. In the new method, the global ocean is divided into various sub-regions in order to allow the parameter adjustments to vary spatially. With this approach a balanced version of the SOC climatology is obtained that requires smaller adjustments, in the range 2-12%, to the latent heat flux than previously but larger changes to the shortwave, up to 18%, depending on region. In addition to enabling direct spatial dependence of the parameter adjustments we have also explored the possibility of making the parameter error spatially dependent both by sub-region and through a dependency on observation density. The various solutions obtained have been evaluated both through the large scale implied ocean heat transport and local comparisons with research buoy measurements. Some improvement is found in the level of agreement of the heat transport with the applied constraints but the buoy comparisons reveal similar problems to those obtained in our previous research. Further, the larger adjustment to the shortwave flux with the new solutions leads to significant differences with respect to satellite based estimates of this component of the flux. We conclude that the earlier solution in which the latent heat flux is increased by 19% is in better agreement with independent estimates than the new spatially dependent solutions. It is thus our preferred means of closing the SOC climatology heat budget imbalance through inverse analysis

The impact of surface flux anomalies on the mid-high latitude Atlantic Ocean Circulation in HADCM3. RAPID Project – The Role of Air-Sea Forcing in Causing Rapid Changes in the North Atlantic Thermohaline Circulation Report No. 1

forcing in the Hadley Centre coupled ocean-atmosphere model (HadCM3) are reported; the study forms part of a Natural Environment Research Council (NERC) Rapid programme project. An analysis of 100 years of the HadCM3 control run indicates that deep convection occurs in the Greenland Sea, the Irminger Basin and the Labrador Sea. However, a composite analysis of mixed layer depth only reveals a clear connection between deep convection and air-sea flux anomalies in the Greenland Sea, and we have focused on this region in our subsequent analysis. Evaluation of the different components of the density flux in the Greenland Sea shows that the net heat flux is a more important influence on surface density than both net evaporation and ice melt. A composite analysis of the ocean circulation was carried out for years with anomalously strong and weak heat loss over the Greenland Sea. Years of strong heat loss are associated with increased Greenland Sea convection and a rapid increase in the southward flow through the Denmark Strait by about 30%. Evidence of more widespread changes in the circulation at mid-high latitudes was also found but we have not yet established whether they are directly linked to the anomalous Greenland Sea forcing

Clues to rainfall variability in West Africa

A study of the easterly wave season over Africa showed systematic differences between wet and dry years that may help our understanding of rainfall variability in West Africa. Earlier observational studies had not found clear connections between wave activity and the interannual variability of rainfall. The new study, using NCEP reanalysis, agreed with previous research that African Easterly Waves (AEWs) typically form between May and October. Disturbances generally recur every 4 to 8 days. At the level (600mb) of the African Easterly Jet (AEJ), the waves peak in July, August, and September. This may be due to the increased horizontal wind shear, which also peaks at that time. At lower levels, the disturbances appear to be confined to periods of 3.75-5.0 days, with the strongest amplitudes occurring in July. Wet years had longer easterly wave seasons and stronger waves at 600mb. These stronger waves may be due to the stronger shear around the AEJ in wet years. At a lower (925mb) level, wet and dry years were less consistently different. We also found the differences between wet and dry regions to be consistent with differences in the basic flow pattern, or state, of the atmosphere during a given year. It is proposed that the wet basic state, and in particular the northward displaced AEJ led to longer of wave seasons and that the enhanced shear, led to enhanced wave activity that likely served to enhance rainfall over West Africa.ÑJeremy Grist. "Easterly Waves over Africa. Part I: The Seasonal Cycle and Contrasts between Wet and Dry Years" and Jeremy Grist et al. "Easterly Waves over Africa. Part II: Observed and Modeled Contrasts between Wet and Dry Years.