Reynolds Sea Surface Temperature Research Articles

Intraseasonal signals in the daily high resolution blended Reynolds sea surface temperature product over the tropical Indian Ocean and their validation

National Oceanic and Atmospheric Administration daily sea surface temperature (SST) products based on Advanced Microwave Scanning Radiometer (AMSR) and Advanced Very High Resolution Radiometer (AVHRR) have been used to understand the variability in the tropical Indian Ocean SST. These products are comparable with the deep sea moored buoy observations and the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) SST in the tropical Indian Ocean. However considerable difference is noticed between these satellite SST products and deep sea buoys, especially at the intraseasonal time scale. Further the first Complex Empirical Orthogonal Function (CEOF) mode of TMI and AVHRR SST explains respectively 46.49% and 46.19% of the total variance. The second CEOF mode of TMI and AVHRR SST explains respectively 23.19% and 18.94% of the total SST variance in the tropical Indian Ocean. The AVHRR SST product is important because this daily product has been available since 1985. The analysis shows that AMSR measurements are contributing considerably to the understanding of the tropical Indian Ocean SST variability. Though satellite SST products are able to capture the observed intraseasonal variability reasonably well, more accurate satellite SST products are therefore necessary to understand the climatologically important Indian Ocean region and its air–sea interaction processes.

Coastal oceanic climate change and variability from 1982 to 2009 around South Africa

Changes and fluctuations in sea surface temperature (SST) around the South African coast are analysed at a monthly scale from 1982 to 2009. There is a statistically significant negative trend of up to 0.5 °C per decade in the southern Benguela from January to August, and a cooling trend of lesser magnitude along the South Coast and in the Port Elizabeth/Port Alfred region from May to August. The cooling is due to an increase in upwelling-favourable south-easterly and easterly winds. There is a positive trend in SST of up to 0.55 °C per decade in most parts of the Agulhas Current system during all months of the year, except for KwaZulu-Natal where warming is in summer. The warming was attributed to an intensification of the Agulhas Current in response to a poleward shift of westerly winds and an increase in trade winds in the South Indian Ocean at relevant latitudes. This intensification of the Agulhas Current could also have contributed to the coastal cooling in the Port Alfred dynamic upwelling region. The El Niño Southern Oscillation (ENSO) is significantly positively correlated at a 95% level with the southern Benguela and South Coast from February to May, and negatively correlated with the Agulhas Current system south of 36° S. The correlation with the Antarctic Annular Oscillation is weaker and less coherent. El Niño suppresses upwelling along the coast, whereas La Niña increases it. Although there does not seem to be a linear relationship between the strength of the ENSO and the magnitude of coastal SST perturbation, El Niño and La Niña appear to be linked to major warm and cool events, respectively, at a seasonal scale in summer in the southern Benguela and along the South Coast. However, care must be taken in interpreting low-resolution reanalysed climate data (ERA40 and NCEP) and optimally interpolated Reynolds SST, such as used here.

Sensitivity of the simulated African monsoon of summers 1993 and 1999 to convective parameterization schemes in RegCM3

In this study, the International Center for Theoretical Physics Regional Climate Model version 3 (RegCM3) was used to investigate the sensitivity of the simulation of the West African monsoon using four different cumulus and closures parameterization schemes of Anthes Kuo (AK), Grell and Fristish Chappell (GFC), Grell and Arakawa Schubert (GAS), and MIT-Emmanuel (EM) while maintaining other physical packages unchanged. The contrasting monsoon years of 1993 and 1999, which were dry and wet years, respectively, were simulated. The model was integrated from a period of 5 months, starting from May 1 to September 30 of each year using the European Centre for Medium-Range-Weather Forecast (ECMWF) Reanalysis data (ERA40) as input boundary conditions. The 6-hourly reanalysis data were used to provide the lateral boundary conditions, and the observed weekly Reynolds Sea Surface Temperature interpolated to 6 h was used as the lower boundary forcing. The results show that in West Africa, monsoon precipitations are sensitive to the choice of cumulus parameterization and closure schemes. None of the schemes is able to simulate the monsoon rainfall accurately, and furthermore, there is little difference in behavior among schemes between dry and wet years. The spatial features of precipitation are not identical among schemes, although they all show a northward shift of the rain bands, giving a very wet Sahel and dry Guinean Coast. The GFC and EM schemes are able to capture the diurnal cycle of precipitation and the zonal averages of stratiform rain fractions as observed in the Tropical Rainfall Measuring Mission (TRMM), although they overestimated rainfall amounts. The most important deficiencies, however, cannot be attributed to the schemes. In particular, the northward shift of both the rain band and the AEJ in RegCM3 is the result of unrealistic soil moisture resulting from the way albedo is parameterized, leading to an excessive northward penetration of monsoon flow. A sensitivity study showed that an adjustment of initial albedo values over the Sahel improved the simulation, and in particular the position of rain bands and of the AEJ.

Coupling between Sea Surface Temperature and Low-Level Winds in Mesoscale Numerical Models

Abstract This study evaluates the impacts of sea surface temperature (SST) specification and grid resolution on numerical simulations of air–sea coupling near oceanic fronts through analyses of surface winds from the European Centre for Medium-Range Weather Forecasts (ECMWF) model. The 9 May 2001 change of the boundary condition from the Reynolds SST analyses to the NOAA Real-Time Global (RTG) SST in the ECMWF model resulted in an abrupt increase in mesoscale variance of the model surface winds over the ocean. In contrast, the 21 November 2000 change of the grid resolution resulted in an abrupt increase in mesoscale variability of surface winds over mountainous regions on land but had no significant effect on winds over the ocean. To further investigate model sensitivity to the SST boundary condition and grid resolution, a series of simulations were made with the Weather Research and Forecasting (WRF) model over a domain encompassing the Agulhas return current (ARC: also called “retroflection”) region in the south Indian Ocean. Results from three WRF simulations with SST measured by the Advanced Microwave Scanning Radiometer on the Earth Observing System Aqua satellite (AMSR-E) and the Reynolds and RTG SST analyses indicate the vital importance of the resolution of the SST boundary condition for accurate simulation of the air–sea coupling between SST and surface wind speed. WRF simulations with grid spacings of 40 and 25 km show that the latter increased energy only on scales shorter than 250 km. In contrast, improved resolution of SST significantly increased the mesoscale variability for scales up to 1000 km. Further sensitivity studies with the WRF model conclude that the weak coupling of surface wind speeds from the ECMWF model to SST is likely attributable primarily to the weak response of vertical turbulent mixing to SST-induced stability in the parameterization of boundary layer turbulence, with an overestimation of vertical diffusion by about 60% on average in stable conditions and an underestimation by about 40% in unstable conditions.

Interannual horizontal heat advection in the surface mixed layer over the equatorial Pacific Ocean: assimilation versus TAO analysis

The interannual anomalies of horizontal heat advection in the surface mixed layer over the equatorial Pacific Ocean in an assimilation experiment are studied and compared with existing observational analyses. The assimilation builds upon a hindcast study that has produced a good simulation of the observed equatorial currents and optimizes the simulation of the Reynolds sea surface temperature (SST) data. The comparison suggests that the assimilation has improved the simulation of the interannual horizontal heat advection of the surface mixed layer significantly. During periods of interrupted current measurements, the assimilation is shown to produce more meaningful anomalies of the heat advection than the interpolation of the observational data does. The assimilation also shows that the eddy heat flux due to the correlation between high-frequency current and SST variations, which is largely overlooked by the existing observational analyses, is important for the interannual SST balance over the equatorial Pacific. The interannual horizontal heat advection anomalies are found to be sensitive to SST errors where oceanic currents are strong, which is a challenge for ENSO prediction. The study further suggests that the observational analyses of the tropical SST balance based on the TAO and the Reynolds SST data contain significant errors due to the large gradient errors in the Reynolds SST data, which are amplified into the advection anomalies by the large equatorial currents.

Atmospheric response to Atlantic tropical instability waves in Community Atmosphere Model version 3

Tropical instability waves (TIWs) are 20‐ to 40‐day waves that appear along the sea surface temperature (SST) front of the equatorial cold tongues. In this study, we investigate the atmospheric planetary boundary layer response to TIW‐induced SST variations in the tropical Atlantic Ocean using Community Atmosphere Model version 3 (CAM3) developed at the National Center for Atmospheric Research and compare the model simulation with available satellite measurements. CAM3 is used to simulate the response to Atlantic TIWs with T85 and T42 horizontal resolutions forced with daily Tropical Rainfall Measuring Mission microwave imager (TMI) SSTs and T85 horizontal resolution forced with weekly TMI and Reynolds SSTs. All four runs exhibit an atmospheric response to the SST anomalies, but the amplitude of the response is different in each case. The model simulates winds, convergence, column‐integrated water vapor, and precipitation perturbations associated with TIWs that are similar in many respects to the satellite observations. However, the wind convergence and divergence anomalies in the satellite observations are located west of the SST anomalies. In the CAM3 simulations the wind convergence is more in phase with SST anomalies. Differences between the observations and the model simulations appear to be due to both differences in the boundary layer response and differences in the horizontal advection process.

Data assimilation into a Princeton Ocean Model of the Mediterranean Sea using advanced Kalman filters

This study investigates the effectiveness of the Singular Evolutive Extended Kalman filter (SEEK) and its variants (SEIK and SFEK filters) for data assimilation into a Princeton Ocean Model (POM) of the Mediterranean Sea. The SEEK filters are sub-optimal Kalman filters based on the approximation of the filter's error covariance matrices by singular low-rank matrices, reducing in this way extensive computational burden. At the initialization, the filters error covariance matrix is parameterized by a set of multivariate empirical orthogonal functions (EOFs) which describe the dominant modes of the system's variability. The Mediterranean model is implemented on a 1/4° × 1/4° horizontal grid with 25 sigma levels and is forced with 6-hour ECMWF re-analysis atmospheric data. Several twin experiments, in which pseudo-observations of altimetric data and/or data profiles were assimilated, were first performed to evaluate the filters performances and to study their sensitivities to different parameters and setups. The results of these experiments were very encouraging and helped in setting up an effective configuration for the assimilation of real data in near-real time situation. In the hindcast experiments, Topex/Poseidon and ERS weekly sea level anomaly data were first assimilated during 1993 and the filters solution was evaluated against independent Reynolds sea surface temperature (SST) analysis. The assimilation system was able to significantly enhance the consistency between the model and the assimilated data, although the improvement with respect to independent SST data was significantly less pronounced. The model SST was only improved after including SST data in the assimilation system.

TRMM derived sea surface temperature in the wake of a cyclonic storm over the central Bay of Bengal

Sea surface cooling associated with a cyclone in the Bay of Bengal was investigated using the data derived from TRMM Microwave Imager (TMI) onboard Tropical Rainfall Measuring Mission (TRMM) spacecraft. Though the TRMM/TMI sensor has all weather capabilities, sea surface temperature (SSTs) can not be obtained during heavy rain conditions. Hence, to overcome the problem of having no observations during the cyclone day, weekly analysis was carried out during the cyclone week (27 March–2 April 2000) and pre‐cyclone week (20–26 March 2000). To compute the magnitude of SST cooling in the cyclone track, weekly SSTs of the cyclone period were subtracted from the pre‐cyclone period. Similar analysis was carried out during non‐cyclonic periods of 20–26 March and 27 March–2 April of 2001, 2002. The analysis indicated that the TMI SST was reduced by maximum of 1.57°C along the cyclone track during the passage of cyclonic storm. Such an activity was not observed during 27 March–2 April 2001 and 2002, indicating that the cooling observed in 27 March–2 April 2000 was due to the cyclonic storm. On the other hand, SST anomalies are positive during 27 March–2 April of 2001, 2002 over these regions. TRMM observations shows higher wind speed and precipitation rate associated with the storm and are responsible for decrease in SST. Analysis of Pathfinder Advanced Very High Resolution Radiometer (AVHRR) SST showed the cyclone induced cooling but the SSTs measurement was blocked by clouds during the cyclone period (27 March–2 April 2000). In the same time, Reynolds SSTs was unable to detect the cooled sea surface. In these circumstances, the cyclone induced sea surface cooling was well captured by TRMM/TMI and had distinct advantage of providing SSTs in presence of cloud as compared to infrared SSTs measurement like those from pathfinder SSTs.

Sensitivity of North American Monsoon Rainfall to Multisource Sea Surface Temperatures in MM5

Abstract In this article, four continually processed sea surface temperature (SST) datasets, including the Reynolds SST (RYD), the global final analysis of skin temperature at oceans (FNL), and two Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua SSTs retrieved from thermal infrared imagery (TIR) and midinfrared imagery (MIR), were compared. The results show variations from each other. In comparison with the RYD SST, the FNL data have −0.5° ∼ 0.5°C perturbations, while the TIR and MIR SSTs possess larger deviations of −2° ∼ 1°C, mainly due to algorithm and/or sensor differences in these SST datasets. A regional model, the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (Penn State–NCAR) Mesoscale Model (MM5), was used to investigate whether model atmospheric predictions, especially those concerning precipitation during the North American monsoon season, are sensitive to these SST variations. A comparison of rainfall, atmospheric height, temperature, and wind fields produced by model results, reanalysis data, and observations indicates that, at monthly scale, the model shows changes in the simulations for three consecutive years; in particular, rainfall amounts, timing, and even patterns vary at some specific regions. Forced by the MODIS Aqua midinfrared SST (MIR), which includes large regions with SST values lower than the conventional Reynolds SST, the MM5 rain field predictions show reduced errors over land and oceans compared to when the model is forced by other SST data. Specifically, rainfall estimates are improved over the offshore of southern Mexico, the Gulf of Mexico, the coastal regions of southern and eastern Mexico, and the southwestern U.S. monsoon active region, but only slightly improved over the monsoon core and the high-elevated Great Plains. Using MIR SST data, one is also capable of improving geopotential height and temperature fields in comparison with the reanalysis data.

Treating strong adjoint sensitivities in tropical eddy‐permitting variational data assimilation

AbstractA variational data assimilation system has been implemented for the tropical Pacific Ocean using an eddy‐permitting regional implementation of the MITgcm. The adjoint assimilation system was developed by the Estimation of the Circulation and the Climate of the Ocean consortium, and has been extended to deal with open boundaries. This system is used to adjust the model to match observations in the tropical Pacific region using control parameters which include initial conditions, open boundaries and time‐dependent surface fluxes. This paper focuses on problems related to strong adjoint sensitivities that may impede the model fit to the observations. A decomposition of the velocities at the open boundaries into barotropic and baroclinic modes is introduced to deal with very strong sensitivities of the model sea surface height to the barotropic component of the inflow. Increased viscosity and diffusivity terms are used in the adjoint model to reduce exponentially growing sensitivities in the backward run associated with nonlinearity of the forward model. Simplified experiments in which the model was constrained with Levitus temperature and salinity data, Reynolds sea surface temperature data and TOPEX/POSEIDON altimeter data were performed to demonstrate the controllability of this assimilation system and to study its sensitivity to the starting guesses for forcing and initial conditions. Copyright © 2005 Royal Meteorological Society

Sensitivity of the Simulated Monsoons of 1987 and 1988 to Convective Parameterization Schemes in MM5

AbstractIn this study the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) version 3.5.2 was used to simulate the Indian summer monsoon during the two contrasting years of 1987 and 1988, a dry year and a wet year, respectively. Three different convection parameterization schemes of Betts–Miller–Janjic, Kain–Fritsch, and Grell were used to study the sensitivity of monsoon to cumulus effects. The model was integrated for a period of 6 months, starting from three different initial conditions of 0000 UTC on 1, 2, and 3 May of each year using the NCEP–NCAR reanalysis data as input. The 6-hourly reanalysis data were used to provide the lateral boundary conditions, and the observed weekly Reynolds sea surface temperature, linearly interpolated to 6 h, was used as the lower boundary forcing. The results show that all three cumulus schemes were able to simulate the interannual and intraseasonal variabilities in the monsoon with reasonable accuracy. However, the spatial distribution of the rainfall and its quantity were different in all the schemes. The Grell scheme underestimated the rainfall in both the years. The Kain–Fritsch scheme simulated the observed rainfall well during July and August, the peak monsoon months, of the year 1988 but overestimated the rainfall in June and September of 1988 and throughout the monsoon season of 1987. The Betts–Miller–Janjic scheme simulated less rainfall in the drought year of 1987 and overestimated the rainfall in June and July of 1988. The circulation patterns simulated by the Betts–Miller–Janjic and Kain–Fritsch schemes are comparable to the observed patterns.

High-Resolution Satellite Measurements of the Atmospheric Boundary Layer Response to SST Variations along the Agulhas Return Current

Abstract The marine atmospheric boundary layer (MABL) response to sea surface temperature (SST) perturbations with wavelengths shorter than 30° longitude by 10° latitude along the Agulhas Return Current (ARC) is described from the first year of SST and cloud liquid water (CLW) measurements from the Advanced Microwave Scanning Radiometer (AMSR) on the Earth Observing System (EOS) Aqua satellite and surface wind stress measurements from the QuikSCAT scatterometer. AMSR measurements of SST at a resolution of 58 km considerably improves upon a previous analysis that used the Reynolds SST analyses, which underestimate the short-scale SST gradient magnitude over the ARC region by more than a factor of 5. The AMSR SST data thus provide the first quantitatively accurate depiction of the SST-induced MABL response along the ARC. Warm (cold) SST perturbations produce positive (negative) wind stress magnitude perturbations, leading to short-scale perturbations in the wind stress curl and divergence fields that are linearly related to the crosswind and downwind components of the SST gradient, respectively. The magnitudes of the curl and divergence responses vary seasonally and spatially with a response nearly twice as strong during the winter than during the summer along a zonal band between 40° and 50°S. These seasonal variations closely correspond to seasonal and spatial variability of large-scale MABL stability and surface sensible heat flux estimated from NCEP reanalysis fields. SST-induced deepening of the MABL over warm water is evident in AMSR measurements of CLW. Typical annual mean differences in cloud thickness between cold and warm SST perturbations are estimated to be about 300 m.

The Relationship between the Interannual Variation of the North Indian Ocean SST Induced by Surface Wind and ENSO during Boreal Summer

Abstract The relationship between the interannual variation of the surface wind in the north Indian Ocean (0°–30°N, 30°–100°E) and El Niño–Southern Oscillation (ENSO) during boreal summer is investigated. The association of the surface wind with the sea surface temperature (SST) in the north Indian Ocean is evaluated. The NCEP–NCAR reanalysis, NOAA outgoing longwave radiation (OLR), and Reynolds SST data are used. The June–August mean of the surface wind anomaly over the north Indian Ocean is decomposed by EOF analysis, and two dominant modes are extracted. The first (second) mode shows the corresponding variation with the ENSO events maturing in the subsequent (previous) winter. The first mode has a large amplitude during the 1990s, while the amplitude of the second mode is large mainly during the 1980s. Such contrast of the amplitude of the two modes results in the temporal change of the surface wind–ENSO relationships between the two decades. The temporal characteristics of the first and second modes are consistent with those of convective variability in the eastern Indian Ocean and the Philippine Sea, respectively. The local thermal forcings associated with these two contrastive modes are compared with the time change of the SST anomaly. The thermal forcings are evaluated in terms of the latent heat flux and the Ekman heat transport. The thermal forcing of the first mode is consistent with a meridionally antisymmetric pattern of the SST anomaly during the 1990s, while that of the second mode is correlated with the basinwide SST anomaly during the 1980s. This result suggests that the temporal change is also found in the north Indian Ocean SST anomaly.

The role of SST on the South American atmospheric circulation during January, February and March 2001

Precipitation deficits were observed over southeastern, northeastern and Central Brazil during the 2001 Austral Summer. They contributed to the worsening of the energy crisis that was occurring in the country. A low-level anomalous anticyclonic circulation observed over eastern Brazil enhanced the deviation of moisture transport that usually occurs from the Amazon Basin to southeastern Brazil and inhibited the occurrence of South Atlantic Convergence Zone events in that period. However, an anomalous low-level northerly moisture flux was observed over the La Plata Basin, and positive precipitation anomalies occurred over Bolivia, Paraguay, northeastern Argentina and southern Brazil. Using the ensemble technique, a numerical study was carried out to investigate the role of different sea surface temperature (SST) forcings observed over this anomalous South American atmospheric circulation. Reynolds SST monthly means were used as boundary conditions to study the influence of South Atlantic, South Indian, South Pacific and Equatorial Pacific oceans. The simulations were run from September 2000 to April 2001 using the Community Climate Model version 3.6 General Circulation Model. Ten integrations using different initial conditions were done to each experiment. Numerical experiments suggested that the combined influence of South Pacific and Equatorial Pacific oceans could be responsible for the drought observed over Central Brazil. These experiments simulated the low-level anticyclonic anomaly observed over eastern Brazil. However, both experiments have poorly reproduced the intensity of the anomalous low-level northerly moisture flux observed over the La Plata Basin. Therefore, the intensity of the simulated precipitation anomalies over the subtropical regions was much weaker than observed.

Mixed layer transformation for the North Atlantic for 1990–2000

The buoyancy balance between two outcropping isopycnals leads to diagnostics for quantifying water mass formation rates between them due to air‐sea buoyancy fluxes [Walin, 1982]. The surface air‐sea transformation gets modified by mixed layer processes so that the net formation rate below the winter mixed layer depth is different from that given by surface air‐sea fluxes alone. Here we estimate the role of time dependence and mixed layer deepening to quantify the water mass transformation due to mixed layer entrainment fluxes. We focus on the mixed layer transformation in the North Atlantic for 1990–2000 during the World Ocean Circulation Experiment period, using both entrainment parameters and isopycnal geometry. The water mass transformation due to mixed layer entrainment is calculated using a large number of gridded one‐dimensional mixed layer models forced by National Centers for Environmental Prediction (NCEP) reanalysis air‐sea fluxes (daily/6 hourly) to calculate the local entrainment parameters; Reynolds sea surface temperature and Levitus monthly salinity data determine the isopycnal geometry. To get a closed annual cycle in the mixed layer depth, any net annual heat flux is ascribed to advective processes. These are included in the annual mean, leaving any synoptic forcing in the NCEP forcing unperturbed. In general, the mixed layer transformation opposes air‐sea interaction, with amplitudes of (1.3 Sv) in the equatorial region (without equatorial upwelling contribution) and (0.5 Sv) in the overflow region. Mean cross‐isopycnal volume fluxes are O(1 Sv), with considerable interannual variability. These estimates of water mass formation due to mixed layer processes are sensitive to synoptic frequencies, but not to climatological mean air‐sea fluxes, and are within the imposed noise levels for inverse box models of the North Atlantic.

A cloud‐free, satellite‐derived, sea surface temperature analysis for the West Florida Shelf

Clouds are problematic in using Advanced Very High Resolution Radiometer (AVHRR) imagery for describing sea surface temperature (SST). The Tropical Rainfall Measuring Mission Microwave Imager (TMI) observes SST through clouds, providing daily, 1/4° maps under all weather conditions excepting rain. A TMI limitation, however, is coarse resolution. Optimal interpolation (OI) is used to generate a cloud‐free, 5‐km, daily SST analysis for the West Florida Shelf (WFS) by merging the high‐resolution (cloud‐covered) AVHRR with the coarse‐resolution (cloud‐free) TMI SST products. Comparisons with in‐situ data show good agreements. Given large spatial gradients by coastal ocean processes, this regional analysis has advantage over the global, weekly, 1° Reynolds SST. A 5‐year (1998–2002) OI SST analysis is diagnosed using Empirical Orthogonal Functions. The first two modes represent annual cycles, one by surface heat flux and another by shelf circulation dynamics.

Algal biomass and sea surface temperature in the Mediterranean Basin: Intercomparison of data from various satellite sensors, and implications for primary production estimates

The Mediterranean Basin, subject both to climate changes and to increasing anthropogenic inputs, is an appropriate test site for observing the evolution of algal biomass and primary production on a long-term basis. With this aim, it is first necessary to study the consistency of the various sets of satellite data as provided by the space agencies, and to compare them to in situ available data. Pixel-by-pixel comparisons of the Level 3 chlorophyll products derived from the ocean color and temperature scanner (OCTS; Version 4, August 1999), polarization and directionality of earth reflectances (POLDER; reprocessing no. 2, July 2000), and the sea-viewing wide field-of-view sensor (SeaWiFS; reprocessing no. 3, May 2000) reveal a reasonably good agreement. Discrepancies, however, appear particularly in oligotrophic areas: weekly (or 10-day) means for OCTS and POLDER (which were operating simultaneously) differ in these areas by 30–70% on average, and OCTS and SeaWiFS weekly means, at 1-year distance, reveal differences by up to a factor of 2. Comparisons with measurements at sea, performed during various cruises, show that all these sensors tend to overestimate chlorophyll concentrations in oligotrophic waters. A “regional algorithm” is proposed to reduce this bias. The impact of using the various datasets for chlorophyll concentration, and for seawater temperature (Reynolds sea surface temperature [SST] analyses, Levitus climatological profiles) for primary production computations is shown. Because they are simultaneous to ocean color data, Reynolds analyses appear to be the most appropriate inputs to such computations. They have, however, to be combined with climatological vertical profiles of seawater temperature, so as to provide representative values for the productive layer.

Large-scale sea-level variations and associated atmospheric forcing in the subtropical north-east Atlantic Ocean

Abstract The large-scale sea-level variations in the subtropical north-eastern Atlantic Ocean are characterised by a predominant seasonal fluctuation. Analysis of 6 years (1993–98) of combined TOPEX/POSEIDON and ERS-1/2 altimetric data together with data of concurrent oceanic surface geophysical fields (ECMWF net heat fluxes, ECMWF winds, and Reynolds sea-surface temperature) reveals that the dominant physical forcing of sea-level variations on an annual timescale is the air–sea net heat fluxes. This local forcing generates a seasonal ocean steric response, whose amplitude (of the order of 4–5 cm) varies spatially within the studied area and temporally over the 6-year period. The residual variability is mainly characterised by a non-seasonal signal, which is observed northwards of the Azores Current (∼34°N), and its time variation resembles that of the North Atlantic Oscillation (NAO). A combination of steric effects (wind-induced heat fluxes) and wind effects (barotropic Sverdrup response) appear to explain this signal. Some evidence of a local Ekman pumping response is also found regionally, both in the seasonal and non-seasonal variations of the observed sea level. At the interannual timescale, an overall sea-level trend with a mean rate of 0.5 cm/year is detected. However, the magnitude of the trend varies locally, so that the entire sea surface appears to undergo a general tilt, which extends beyond the studied area. Superimposed on this long-term trend, a sharp temporal sea-level rise occurs in 1995, mainly between the Azores and Madeira Islands. All together, these low-frequency sea-level variations seem to co-vary with the large-scale sea-surface temperature (SST) variations. This implies a comparable evolution of the sea level with the upper-ocean thermal content, which suggests a contribution from steric effects. A weaker situation is observed in the northern area, indicating a possible low-frequency wind-forcing-enhanced contribution.

The nontidal flow in the Providence River of Narragansett Bay: a stochastic approach to estuarine circulation : WEISBERG R. H., 1976 J. phys. Oceanogr., 6 (5): 721–734

The large-scale sea-level variations in the subtropical north-eastern Atlantic Ocean are characterised by a predominant seasonal fluctuation. Analysis of 6 years (1993–98) of combined TOPEX/POSEIDON and ERS-1/2 altimetric data together with data of concurrent oceanic surface geophysical fields (ECMWF net heat fluxes, ECMWF winds, and Reynolds sea-surface temperature) reveals that the dominant physical forcing of sea-level variations on an annual timescale is the air–sea net heat fluxes. This local forcing generates a seasonal ocean steric response, whose amplitude (of the order of 4–5 cm) varies spatially within the studied area and temporally over the 6-year period. The residual variability is mainly characterised by a non-seasonal signal, which is observed northwards of the Azores Current (∼34°N), and its time variation resembles that of the North Atlantic Oscillation (NAO). A combination of steric effects (wind-induced heat fluxes) and wind effects (barotropic Sverdrup response) appear to explain this signal. Some evidence of a local Ekman pumping response is also found regionally, both in the seasonal and non-seasonal variations of the observed sea level. At the interannual timescale, an overall sea-level trend with a mean rate of 0.5 cm/year is detected. However, the magnitude of the trend varies locally, so that the entire sea surface appears to undergo a general tilt, which extends beyond the studied area. Superimposed on this long-term trend, a sharp temporal sea-level rise occurs in 1995, mainly between the Azores and Madeira Islands. All together, these low-frequency sea-level variations seem to co-vary with the large-scale sea-surface temperature (SST) variations. This implies a comparable evolution of the sea level with the upper-ocean thermal content, which suggests a contribution from steric effects. A weaker situation is observed in the northern area, indicating a possible low-frequency wind-forcing-enhanced contribution.