Air-sea heat and freshwater water fluxes in the Mediterranean Sea play a crucial role in dense water formation. Here, we compare estimates of Mediterranean Sea heat and water budgets from a range of ...observational datasets and discuss the main differences between them. Taking into account the closure hypothesis at the Gibraltar Strait, we have built several observational estimates of water and heat budgets by combination of their different observational components. We provide then three estimates for water budget and one for heat budget that satisfy the closure hypothesis. We then use these observational estimates to assess the ability of an ensemble of ERA40-driven high resolution (25 km) Regional Climate Models (RCMs) from the FP6-EU ENSEMBLES database, to simulate the various components, and net values, of the water and heat budgets. Most of the RCM Mediterranean basin means are within the range spanned by the observational estimates of the different budget components, though in some cases the RCMs have a tendency to overestimate the latent heat flux (or evaporation) with respect to observations. The RCMs do not show significant improvements of the total water budget estimates comparing to ERA40. Moreover, given the large spread found in observational estimates of precipitation over the sea, it is difficult to draw conclusions on the performance of RCM for the freshwater budget and this underlines the need for better precipitation observations. The original ERA40 value for the basin mean net heat flux is −15 W/m
2
which is 10 W/m
2
less than the value of −5 W/m
2
inferred from the transport measurements at Gibraltar Strait. The ensemble of heat budget values estimated from the models show that most of RCMs do not achieve heat budget closure. However, the ensemble mean value for the net heat flux is −7 ± 21 W/m
2
, which is close to the Gibraltar value, although the spread between the RCMs is large. Since the RCMs are forced by the same boundary conditions (ERA40 and sea surface temperatures) and have the same horizontal resolution and spatial domain, the reason for the large spread must reside in the physical parameterizations. To conclude, improvements are urgently required to physical parameterizations in state-of-the-art regional climate models, to reduce the large spread found in our analysis and to obtain better water and heat budget estimates over the Mediterranean Sea.
The first decade of the 21st century was characterized by a hiatus in global surface warming. Using ocean model hindcasts and reanalyses we show that heat uptake between the 1990s and 2000s increased ...by 0.7 ± 0.3W m−2. Approximately 30% of the increase is associated with colder sea surface temperatures in the eastern Pacific. Other basins contribute via reduced heat loss to the atmosphere, in particular, the Southern and subtropical Indian Oceans (30%) and the subpolar North Atlantic (40%). A different mechanism is important at longer timescales (1960s–present) over which the Southern Annular Mode trended upward. In this period, increased ocean heat uptake has largely arisen from reduced heat loss associated with reduced winds over the Agulhas Return Current and southward displacement of Southern Ocean westerlies.
Key PointsHeat uptake increased in the Southern, Atlantic, and Indian OceansThe increase of 0.5–1 W/m2 in ocean heat uptake is enough to explain the hiatusTropical Pacific SST is not the only way to change global ocean heat uptake
The Gulf Stream plays an important role in North Atlantic climate variability on a range of timescales. The North Atlantic is notable for large decadal variability in sea surface temperatures (SST). ...Whether this variability is driven by atmospheric or oceanic influences is a disputed point. Long time series of atmospheric and ocean variables, in particular long time series of Gulf Stream position, reveal differing sources of SST variability on quasi‐decadal and multidecadal timescales. On quasi‐decadal timescales, an oscillatory signal identified in the North Atlantic Oscillation (NAO) controls SST evolution directly via air‐sea heat fluxes. However, on multidecadal timescales, this relationship between the NAO and SST changes, while the relationship between the NAO and Gulf Stream position remains consistent in phase and resonant in amplitude. Recent changes in the Gulf Stream Extension show a weakening and broadening of the current, consistent with increased instability. We consider these changes in the context of a weakening Atlantic overturning circulation.
Plain Language Summary
The North Atlantic Ocean is a region of remarkable variability in surface temperatures on timescales of decades and longer. Much debate surrounds whether this variability is driven by the atmosphere or by ocean currents, such as the Gulf Stream, moving heat around. In this study, we show that on timescales around 10 years, the atmosphere is the likely cause of Atlantic temperature variability but that this changes when multidecadal variability is considered. Changes ongoing in the Gulf Stream coincide with changes in the broader Atlantic—changes that imply a relatively cooler Atlantic in the coming decades.
Key Points
On quasi‐decadal timescales, NAO varies in phase with GSNW and in antiphase with Atlantic SSTs
On multidecadal timescales, NAO continues to vary in phase with GSNW, but the relationship to Atlantic SSTs has changed
The weakening and broadening of the Gulf Stream is consistent with increased instability since 2005 and not with a northward shift
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 sea surface temperature 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.
Key Points
New data from the RAPID 26°N array show that the AMOC has been in a state of reduced overturning since mid‐2008
Observations of heat content and SSH indicate that the impact of the reduction in the AMOC is similar to that predicted by climate models
The results indicate that changes in ocean heat transport have altered ocean‐atmosphere heat exchange over the North Atlantic
The Mediterranean Sea is a mid-latitude marginal sea, particularly responsive to climate change as reported by recent studies. The Sicily Channel is a choke point separating the sea in two main ...basins, the Eastern Mediterranean Sea and the Western Mediterranean Sea. Here, we report and analyse a long-term record (1993-2016) of the thermohaline properties of the Intermediate Water that crosses the Sicily Channel, showing increasing temperature and salinity trends much stronger than those observed at intermediate depths in the global ocean. We investigate the causes of the observed trends and in particular determine the role of a changing climate over the Eastern Mediterranean, where the Intermediate Water is formed. The long-term Sicily record reveals how fast the response to climate change can be in a marginal sea like the Mediterranean Sea compared to the global ocean, and demonstrates the essential role of long time series in the ocean.
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 associated 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.
•Establishing reference input dataset maximizing the use of remotely sensed data•Performing a cross-comparison of different heat flux algorithms and approaches•Generating an ensemble of turbulent fluxes, including multiple approaches•Evaluating the quality and consistency of ensemble realizations•Exploiting integral heat constraints at local, regional and global scales
Changes in the Mediterranean water cycle since 1950 are investigated using salinity and reanalysis based air–sea freshwater flux datasets. Salinity observations indicate a strong basin-scale ...multi-decadal salinification, particularly in the intermediate and deep layers. Evaporation, precipitation and river runoff variations are all shown to contribute to a very strong increase in net evaporation of order 20–30%. While large temporal uncertainties and discrepancies are found between
E–P
multi-decadal trend patterns in the reanalysis datasets, a more robust and spatially coherent structure of multi-decadal change is obtained for the salinity field. Salinity change implies an increase in net evaporation of ~ 8 to 12% over 1950–2010, which is considerably lower than that suggested by air–sea freshwater flux products, but still largely exceeding estimates of global water cycle amplification. A new method based on water mass transformation theory is used to link changes in net evaporation over the Mediterranean Sea with changes in the volumetric distribution of salinity. The water mass transformation distribution in salinity coordinates suggests that the Mediterranean basin salinification is driven by changes in the regional water cycle rather than changes in salt transports at the straits.
The Southern Ocean is a key component of the global climate system: insulating the Antarctic polar region from the subtropics, transferring climate signals throughout the world's oceans and forming ...the southern component of the global overturning circulation. However, the air‐sea fluxes that drive these processes are severely under‐observed due to the harsh and remote location. This paucity of reference observations has resulted in large uncertainties in ship‐based, numerical weather prediction, satellite and derived flux products. Here, we report observations from the Southern Ocean Flux Station (SOFS); the first successful air‐sea flux mooring deployment in this ocean. The mooring was deployed at 47°S, 142°E for March 2010 to March 2011 and returned measurements of near surface meteorological variables and radiative components of the heat exchange. These observations enable the first accurate quantification of the annual cycle of net air‐sea heat exchange and wind stress from a Southern Ocean location. They reveal a high degree of variability in the net heat flux with extreme turbulent heat loss events, reaching −470 Wm−2in the daily mean, associated with cold air flowing from higher southern latitudes. The observed annual mean net air‐sea heat flux is a small net ocean heat loss of −10 Wm−2, with seasonal extrema of 139 Wm−2 in January and −79 Wm−2in July. The novel observations made with the SOFS mooring provide a key point of reference for addressing the high level of uncertainty that currently exists in Southern Ocean air‐sea flux datasets.
Key Points
Southern Ocean air‐sea fluxes are under‐observed, leading to large uncertainty
The first year‐long air‐sea flux observations quantify an annual cycle
Shows seasonal cycle, small annual net ocean heat loss and extreme events
The Tropical Pacific mooring array has been a key component of the climate observing system since the early 1990s. We identify a pattern of strong near surface humidity anomalies, colocated with the ...array, in the widely used European Center for Medium Range Weather Forecasting Interim atmospheric reanalysis. The pattern generates large, previously unrecognized latent and net air‐sea heat flux anomalies, up to 50 Wm−2 in the annual mean, in reanalysis derived data sets employed for climate studies (TropFlux) and ocean model forcing (the Drakkar Forcing Set). As a consequence, uncertainty in Tropical Pacific ocean heat uptake between the 1990s and early 2000s at the mooring sites is significant with mooring colocated differences in decadally averaged ocean heat uptake as large as 20 Wm−2. Furthermore, these results have major implications for the dual use of air‐sea flux buoys as reference sites and sources of assimilation data that are discussed.
Key Points
Unphysical features in leading reanalysis linked to Tropical Pacific mooringsLarge, previously unrecognized annual air‐sea heat flux anomalies, up to 50 Wm−2Uncertainty in Tropical Pacific heat uptake pattern between 1990s and 2000s
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