Simon A. Josey

New Insights into the Ocean Heat Budget Closure Problem from Analysis of the SOC Air–Sea Flux Climatology

Abstract Results from an analysis of the Southampton Oceanography Centre (SOC) global air–sea heat flux climatology, which has been calculated using in situ weather reports from voluntary observing ships covering the period 1980–93, are presented. Systematic errors in the fluxes arising from differences in observing procedure have been quantified and corrected; the magnitude of these errors is up to 15 W m−2 with strong seasonal and regional variations. Despite these corrections, closure of the ocean heat budget is not obtained as the global mean net heat flux is an oceanic gain of 30 W m−2. The validity of closing the heat budget by global scaling of the flux components is assessed by comparison of the SOC flux fields with Woods Hole Oceanographic Institute research buoy measurements. The level of agreement between the two is found to vary from one site to another. Thus, closure of the ocean heat budget requires regional adjustments to the flux components in order to avoid significant biases in the adjus...

Atmospheric forcing in the Arabian Sea during 1994–1995: observations and comparisons with climatology and models

Accurate, year-long time series of winds, incoming shortwave and longwave radiation, air and sea temperatures, relative humidity, barometric pressure, and precipitation were collected from a surface mooring deployed off the coast of Oman along the climatological axis of the Findlater Jet from October 1994 to October 1995. Wind stress, heat flux, and freshwater flux were computed using bulk formulae. The Northeast Monsoon was characterized by steady but moderate winds, clear skies, relatively dry air, and two months, December and January, in which the ocean, on average, lost 45 W m-2 to the atmosphere. The Southwest Monsoon had strong winds, cloudy skies, and moist air. Because of reduced latent and longwave heat loss, it was accompanied by sustained oceanic heat gain, with the strongest monthly mean warming, 147 W m-2, in August. Large differences are found between the observations and older climatologies. Recent climatologies agree better with the observations. The means of the Southampton Oceanography Center climatology for 1980–1995 are close to the buoy monthly means. Monthly means from that climatology show that 1994–1995 was in general a typical year, with surface meteorology and air–sea fluxes within one standard deviation of the long term means. Concurrent data from the NCEP, ECMWF, and FNMOC show that the models provide realistic surface winds. FNMOC winds show that the timing and character of the onset of the Southwest Monsoon in 1995 differed from 1994 and 1996 when variability within one month is resolved. The models fail to replicate other observed surface meteorology and to produce realistic heat fluxes. Annual and monsoonal mean net heat fluxes from the models differed from those of the buoy by 50 to 80 W m-2. Because of these differences, some care is warranted in selecting and using air-sea flux fields in studies of the Arabian Sea.

Estimating air-sea fluxes of heat, freshwater, and momentum through global ocean data assimilation

Spectral properties of whitecaps are of importance for color ocean remote sensing and aerosol optical thickness probing from satellite-based instruments. They also influence planetary albedo and climate. In particular, whitecaps may affect the response of the climate system to changes in greenhouse gases and other atmospheric constituents. Several experimental measurements of whitecap spectral reflectance have been performed both in the surf zone and in the open ocean, which indicate that oceanic foam cannot be considered as a gray body (e.g., for satellite remote sensing techniques). This paper is devoted to the interpretation of experiments performed in terms of the radiative transfer theory. Only the case of a semi-infinite foam is studied in detail. However, results can be easily extended to the case of finite foamed media having large optical thickness. The model introduced is capable of explaining main features observed, like a sharp decrease of the foam spectral reflectance in the infrared as compared with the visible part of the electromagnetic spectrum and a high correlation of the foam reflectance R and the water absorption coefficient a. A simple method to retrieve the spectral dependence of a from the spectral foam reflectance R is proposed.

Forcing of the Mediterranean Sea by atmospheric oscillations over the North Atlantic

Measurements in the Mediterranean Sea made over the last century reveal a link between sea level variability and the North Atlantic Oscillation (NAO). The link arises from the combined effects of atmospheric pressure anomalies and changes in evaporation and precipitation. The strengthening of the NAO from the 1960s to the 1990s explains a significant proportion of the reduction in Mediterranean Sea level over this period. This finding highlights the need to take atmospheric variability into account when looking for the signature of anthropogenic climate change in the ocean. The change in the freshwater flux in the basin, caused by the consistently higher NAO during the 1990s is linked to the appearance of the Eastern Mediterranean Transient.

Inverse Analysis Adjustment of the SOC Air-Sea Flux Climatology Using Ocean Heat Transport Constraints

Abstract Results are presented from a linear inverse analysis of the Southampton Oceanography Centre (SOC) air–sea flux climatology using 10 hydrographic ocean heat transport constraints distributed throughout the Atlantic and North Pacific Oceans. A solution is found that results in an adjusted set of fluxes that is consistent with all of the available constraints within their estimated error bounds. The global mean net ocean heat loss to the atmosphere with these adjustments is –5 W m–2, compared with a gain of 30 W m–2 for the original climatology. The primary changes to the net heat flux arise from an increase of 15% to the latent heat and reduction of 9% to the shortwave flux. The analysis has been extended to include the additional constraint that the global mean net heat flux lies in the range 0 ± 2 W m–2. In the latter case, the solution is modified such that the adjustment of the latent heat increases to 19%, the reduction of the shortwave decreases to 6%, and the global mean net heat flux is –2 ...

Changes in the heat and freshwater forcing of the eastern Mediterranean and their influence on deep water formation

Anomalies in the air-sea heat and freshwater forcing of the eastern Mediterranean are related to observations of deep water formation in the Aegean Sea between 1987 and 1995. Fields of the buoyancy exchange (expressed as a density flux) are determined from the Southampton Oceanography Centre (SOC) flux climatology and National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis. A coherent picture emerges from the two data sets, in both cases the strongest losses occur in winters 1991–1992 and 1992–1993, when Aegean Sea net heat loss and net evaporation anomalies of 60 W m-2 and 0.05 m month-1, respectively, are found. These correspond to thermal and haline density fluxes of 3.3 × 10-6 and 0.6 × 10-6 kg m-2 s-1. Thus the thermal term makes the major contribution to the increase in density of the surface waters over these winters and the haline term plays only a minor role. Similar results are obtained when annual mean fields are considered as the winter forcing dominates. The NCEP/NCAR reanalysis is used to place the winters of the early 1990s in the context of variability over the longer period 1949–2002. Anomalous winter losses of similar magnitude occurred in the mid-1970s and the possibility of a related previous episode of deep water formation in the Aegean is discussed. The relationship between Aegean Sea heat loss and large-scale atmospheric pressure patterns is also investigated. Heat loss anomalies are found to be uncorrelated with the North Atlantic Oscillation. However, a strong correlation (r2 = 0.52) is found with a pattern whose main feature is anomalously high pressure over western Europe.

A Comparison of ECMWF, NCEP–NCAR, and SOC Surface Heat Fluxes with Moored Buoy Measurements in the Subduction Region of the Northeast Atlantic

The accuracy of surface heat flux estimates from the NCEP/NCAR and ECMWF atmospheric model reanalyses is assessed by comparison with WHOI research buoy measurements made during the Subduction Experiment in the North-East Atlantic. Each of the reanalyses persistently underestimates the ocean heat gain in this region, the array averaged net heat gain being less than the corresponding buoy value by 32±9 Wm-2 for ECMWF and 35±12 Wm-2 for NCEP/NCAR. The model biases are primarily due to a combination of underestimated shortwave gain and overestimated latent heat loss. They are similar in sign and magnitude but show a greater spread between the various buoys than was found in an analysis of operational model output by Moyer and Weller (1997). The tendency for the reanalyses to overestimate the latent heat loss in this region is consistent with the results of other studies which show that a bias of this sort is to be expected given the choice of bulk flux algorithm in the models. The poor performance of the reanalyses contrasts with estimates based on ship meteorological reports in the SOC flux dataset. The array averaged net heat flux from the SOC dataset agrees with the buoy value to within 10 Wm-2. Similar results are obtained when the comparison is restricted to winter, which is the period most relevant to studies of subduction. The December - February array averaged net heat flux is -52 Wm-2 from the buoys, -57 Wm-2 for SOC, -78 Wm-2 for NCEP/NCAR and -93 Wm-2 for ECMWF. The results from the buoy comparisons reinforce the need for basin scale evaluations of surface fluxes to be supplemented by local comparisons against high quality flux measurements.

Wind Stress Forcing of the Ocean in the SOC Climatology: Comparisons with the NCEP–NCAR, ECMWF, UWM/COADS, and Hellerman and Rosenstein Datasets

Results from an analysis of the Southampton Oceanography Centre (SOC) global wind stress climatology, which is based on in situ reports for the period 1980-93, are presented. The accuracy of the SOC stresses has been assessed at several locations by comparison of individual monthly means with measurements from Woods Hole Oceanographic Institution research buoy deployments. For the subduction buoy array, situated in the subtropical North Atlantic, the random error in the SOC individual monthly mean wind stress ranges from 0.004 to 0.008 N m-2, which corresponds to between 5% and 10% of the mean stress depending on which buoy is considered. The large-scale characteristics of the SOC fields are compared with those of the NCEP-NCAR and ECMWF atmospheric model reanalyses, and the in situ observation based on the University of Wisconsin-Milwaukee/Comprehensive Ocean–Atmosphere Dataset (UWM/COADS) and Hellerman and Rosenstein (HR) climatologies. The NCEP-NCAR fields show noticeably weaker wind stress forcing in the Tropics than SOC, while ECMWF and UWM/COADS are in good agreement. From the Tropics to the midlatitudes, the HR stresses tend to be stronger than SOC and the other recent climatologies. At higher latitudes, differences in the spatial structure of the Northern Hemisphere subpolar gyres in SOC and HR are found that are consistent with variations in the state of the North Atlantic and North Pacific Oscillations within the periods on which the climatologies are based. A detailed comparison of the wind-driven response of the ocean is presented for SOC and HR. The North Atlantic subpolar gyre is more intense in SOC than HR and this leads to a doubling in the strength of the Ekman suction. January mean upwelling velocities in this region deduced from the two datasets are 18.9 and 8.6 m month-1, respectively. In the North Pacific a single large-scale subpolar gyre is evident in SOC compared with two smaller gyres in HR. Seasonal to interannual variability in the wind-driven ocean response is quantified using an extended version of the SOC dataset covering the period 1980–97. Significant variability in the Ekman transport across several latitudes that correspond to WOCE hydrographic sections is observed and related to the major atmospheric pressure oscillations

Impacts of atmospheric modes of variability on Mediterranean Sea surface heat exchange

The impacts of variations in the state of the first four modes of atmospheric variability in the North Atlantic/Europe region on air-sea heat exchange in the Mediterranean Sea are considered. Observation-based indices of these modes from the NOAA Climate Prediction Centre are used together with two reanalysis (NCEP/NCAR and ARPERA) surface flux data sets for the period 1958–2006 to determine their relative influence on the mean heat budget of the full Mediterranean basin and the eastern and western subbasins. The modes considered are the North Atlantic Oscillation (NAO), East Atlantic pattern (EA), Scandinavian pattern (SCAN), and East Atlantic/West Russian pattern (EA/WR). Similar results are obtained with both NCEP/NCAR and ARPERA. In each case, winter anomalies dominate the annual mean heat budget and the leading mode, the NAO, has a surprisingly small impact on the full basin winter mean heat budget, <5 Wm?2. In contrast, the EA mode has a major effect, of order 25 Wm?2, with similar impacts on both the eastern and western Mediterranean. The SCAN mode has the weakest influence of those considered. The EA/WR mode plays a significant role but, in contrast to the EA mode, it generates a dipole in the heat exchange with an approximately equal and opposite signal of about 15 Wm?2 on the eastern and western subbasins. A particularly strong impact in the Aegean Sea is observed for the EA/WR mode and this is discussed in the context of episodic deep water formation in this region.

Observations of seasonal exchange through the Straits of Hormuz and the inferred heat and freshwater budgets of the Persian Gulf

The exchange between the Persian (Arabian) Gulf and the Indian Ocean is investigated using hydrographic and moored acoustic Doppler current profiler data from the Straits of Hormuz during the period December 1996 to March 1998. The moored time series records show a relatively steady deep outflow through the strait from 40 m to the bottom with a mean speed of approximately 20 cm/s. A variable flow is found in the upper layer with frequent reversals on timescales of several days to weeks. The annual mean flow in the near-surface layer is found to be northeastward (out of the Persian Gulf) in the southern part of the strait, suggesting a mean horizontal exchange with the Indian Ocean that is superimposed on the vertical overturning exchange driven by evaporation over the gulf. The salinity of the deep outflow varies from 39.3 to 40.8 psu with highest outflow salinities occurring in the winter months (December–March). The annual mean deep outflow through the strait is estimated to be 0.15 ± 0.03 Sv. Calculation of the associated heat and freshwater fluxes through the strait yields estimates for the annual heat loss over the surface of the gulf of ?7 ± 4 W/m2 and an annual water loss (E-P-R) of 1.68 ± 0.39 m/yr. These values are shown to be in relatively good agreement with climatological surface fluxes derived from the Southampton Oceanography Centre global flux climatology after known regional biases in the radiative budget are taken into account.