A pilot heat and momentum flux study for the North Atlantic - base climatology

A base set of climatological heat and momentum flux fields has been calculated for the North Atlantic as part of a pilot study for a global climatology. The fields are qualitatively reasonable. Comparison with the fields of lsemer and Hasse (1987), which have been adjusted to match oceanographic constraints, indicates that the net heat loss in the base climatology is too low by between 10-30 Wfm2 over most of the North Atlantic basin if the constraints are representative of the long-term mean ocean heat transport. The deficit arises mainly as a result of differences between the latent heat flux fields, the shortwave fields are in very good agreement. The implied ocean heat transport at 24°N is 0.29 PW less that the lower limit on the hydrographic estimate (Bryden, 1993). Possible reasons for this discrepancy are discussed and at present it is not clear whether the problem lies in the calculation of the fluxes or inappropriate use of the hydrographic estimates as constraints

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

Review and assessment of latent and sensible heat flux accuracy over the global oceans

For over a decade, several research groups have been developing air-sea heat flux information over the global ocean, including latent (LHF) and sensible (SHF) heat fluxes over the global ocean. This paper aims to provide new insight into the quality and error characteristics of turbulent heat flux estimates at various spatial and temporal scales (from daily upwards). The study is performed within the European Space Agency (ESA) Ocean Heat Flux (OHF) project. One of the main objectives of the OHF project is to meet the recommendations and requirements expressed by various international programs such as the World Research Climate Program (WCRP) and Climate and Ocean Variability, Predictability, and Change (CLIVAR), recognizing the need for better characterization of existing flux errors with respect to the input bulk variables (e.g. surface wind, air and sea surface temperatures, air and surface specific humidities), and to the atmospheric and oceanic conditions (e.g. wind conditions and sea state). The analysis is based on the use of daily averaged LHF and SHF and the asso- ciated bulk variables derived from major satellite-based and atmospheric reanalysis products. Inter-comparisons of heat flux products indicate that all of them exhibit similar space and time patterns. However, they also reveal significant differences in magnitude in some specific regions such as the western ocean boundaries during the Northern Hemisphere winter season, and the high southern latitudes. The differences tend to be closely related to large differences in surface wind speed and/or specific air humidity (for LHF) and to air and sea temperature differences (for SHF). Further quality investigations are performed through comprehensive comparisons with daily-averaged LHF and SHF estimated from moorings. The resulting statistics are used to assess the error of each OHF product. Consideration of error correlation between products and observations (e.g., by their assimilation) is also given. This reveals generally high noise variance in all products and a weak signal in common with in situ observations, with some products only slightly better than others. The OHF LHF and SHF products, and their associated error characteristics, are used to compute daily OHF multiproduct-ensemble (OHF/MPE) estimates of LHF and SHF over the ice-free global ocean on a 0.25° × 0.25° grid. The accuracy of this heat multiproduct, determined from comparisons with mooring data, is greater than for any individual product. It is used as a reference for the anomaly characterization of each individual OHF product

Major variations in subtropical North Atlantic heat transport at short (5 day) timescales and their causes

Variability in the North Atlantic ocean heat transport at 26.5°N on short (5-day) timescales is identified and contrasted with different behaviour at monthly intervals using a combination of RAPID/MOCHA/WBTS measurements and the NEMO-LIM2 1/12° ocean circulation/sea ice model. Wind forcing plays the leading role in establishing the heat transport variability through the Ekman transport response of the ocean and the associated driving atmospheric conditions vary significantly with timescale. We find that at 5-day timescales the largest changes in the heat transport across 26.5°N coincide with north-westerly airflows originating over the American land mass that drive strong southward anomalies in the Ekman flow. During these events the northward heat transport reduces by 0.5-1.4 PW. In contrast, the Ekman transport response at longer monthly timescales is smaller in magnitude (up to 0.5 PW) and consistent with expected variations in the leading mode of North Atlantic atmospheric variability, the North Atlantic Oscillation. The north-westerly airflow mechanism can have a prolonged influence beyond the central 5-day timescale and on occasion can reduce the accumulated winter ocean heat transport into the North Atlantic by ∼40%

The North Atlantic Ocean is in a state of reduced overturning

The Atlantic Meridional Overturning Circulation (AMOC) is responsible for a variable and climatically important northward transport of heat. Using data from an array of instruments that span the Atlantic at 26°N, we show that the AMOC has been in a state of reduced overturning since 2008 as compared to 2004-2008. This change of AMOC state is concurrent with other changes in the North Atlantic such as a northward shift and broadening of the Gulf Stream, and altered patterns of heat content and sea-surface temperature. These changes resemble the response to a declining AMOC predicted by coupled climate models. Concurrent changes in air-sea fluxes close to the western boundary reveal that the changes in ocean heat transport and SST have altered the pattern of ocean-atmosphere heat exchange over the North Atlantic. These results provide strong observational evidence that the AMOC is a major factor in decadal scale variability of North Atlantic climate

The North Atlantic Ocean is in a state of reduced overturning

The Atlantic Meridional Overturning Circulation (AMOC) is responsible for a variable and climatically important northward transport of heat. Using data from an array of instruments that span the Atlantic at 26°N, we show that the AMOC has been in a state of reduced overturning since 2008 as compared to 2004-2008. This change of AMOC state is concurrent with other changes in the North Atlantic such as a northward shift and broadening of the Gulf Stream, and altered patterns of heat content and sea-surface temperature. These changes resemble the response to a declining AMOC predicted by coupled climate models. Concurrent changes in air-sea fluxes close to the western boundary reveal that the changes in ocean heat transport and SST have altered the pattern of ocean-atmosphere heat exchange over the North Atlantic. These results provide strong observational evidence that the AMOC is a major factor in decadal scale variability of North Atlantic climate

Developing an Observing Air–Sea Interactions Strategy (OASIS) for the global ocean

The Observing Air–Sea Interactions Strategy (OASIS) is a new United Nations Decade of Ocean Science for Sustainable Development programme working to develop a practical, integrated approach for observing air–sea interactions globally for improved Earth system (including ecosystem) forecasts, CO2 uptake assessments called for by the Paris Agreement, and invaluable surface ocean information for decision makers. Our “Theory of Change” relies upon leveraged multi-disciplinary activities, partnerships, and capacity strengthening. Recommendations from >40 OceanObs’19 community papers and a series of workshops have been consolidated into three interlinked Grand Ideas for creating #1: a globally distributed network of mobile air–sea observing platforms built around an expanded array of long-term time-series stations; #2: a satellite network, with high spatial and temporal resolution, optimized for measuring air–sea fluxes; and #3: improved representation of air–sea coupling in a hierarchy of Earth system models. OASIS activities are organized across five Theme Teams: (1) Observing Network Design & Model Improvement; (2) Partnership & Capacity Strengthening; (3) UN Decade OASIS Actions; (4) Best Practices & Interoperability Experiments; and (5) Findable–Accessible–Interoperable–Reusable (FAIR) models, data, and OASIS products. Stakeholders, including researchers, are actively recruited to participate in Theme Teams to help promote a predicted, safe, clean, healthy, resilient, and productive ocean.publishedVersio

Objective analysis of climatological fields: results of test analyses using a successive correction method

The use of objective analysis techniques in climatological studies is reviewed and the properties of a widely used scheme, namely a successive correction method (SCM), are described in detail. A sensitivity study of an SCM with a Gaussian weight function is carried out using raw 1 o x 1 o monthly mean global evaporation rates from the UWM/COADS atlas to provide the observation field. The results are used to suggest a set of parameters for the analysis of climatological fields which are being generated from ship meteorological reports in the COADS 1 a dataset. The analysed fields are found to be most sensitive to the value of the influence radius on the last pass of the analysis, with the choice of background field and number passes being of only secondary importance. A minimum value for the influence radius in the range 21 0 - 350 km, which is somewhat smaller than that used in several past analyses, is found to be appropriate in regions of high data density such as the North Atlantic The possibility of reducing the amount of noise in the analysed field by imposing a threshold on the number of observations required to generate a mean is investigated but is found to be impractical at a grid scale of 1 degree x 1 degree as too much information is lost from the observation field. Finally, the integrated characteristics of the analysed field are found to be insensitive to the level of interpolation in the SCM which suggests that this is not an important factor in the context of heat budget calculations

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

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