Search Results
Abstract
The existence of a cool and salty sea surface skin under evaporation was first proposed by Saunders in 1967, but few efforts have since been made to perceive the salt component of the skin layer. With two salinity missions scheduled to launch in the coming years, this study attempted to revisit the Saunders concept and to utilize presently available air–sea forcing datasets to analyze, understand, and interpret the effect of the salty skin and its implication for remote sensing of ocean salinity.
Similar to surface cooling, the skin salinification would occur primarily at low and midlatitudes in regions that are characterized by low winds or high evaporation. On average, the skin is saltier than the interior water by 0.05–0.15 psu and cooler by 0.2°–0.5°C. The cooler and saltier skin at the top is always statically unstable, and the tendency to overturn is controlled by cooling. Once the skin layer overturns, the time to reestablish the full increase of skin salinity was reported to be on the order of 15 min, which is approximately 90 times slower than that for skin temperature. Because the radiation received from a footprint is averaged over an area to give a single pixel value, the slow recovery by the salt diffusion process might cause a slight reduction in area-averaged skin salinity and thus obscure the salty skin effect on radiometer retrievals. In the presence of many geophysical error sources in remote sensing of ocean salinity, the salt enrichment at the surface skin does not appear to be a concern.
Abstract
The existence of a cool and salty sea surface skin under evaporation was first proposed by Saunders in 1967, but few efforts have since been made to perceive the salt component of the skin layer. With two salinity missions scheduled to launch in the coming years, this study attempted to revisit the Saunders concept and to utilize presently available air–sea forcing datasets to analyze, understand, and interpret the effect of the salty skin and its implication for remote sensing of ocean salinity.
Similar to surface cooling, the skin salinification would occur primarily at low and midlatitudes in regions that are characterized by low winds or high evaporation. On average, the skin is saltier than the interior water by 0.05–0.15 psu and cooler by 0.2°–0.5°C. The cooler and saltier skin at the top is always statically unstable, and the tendency to overturn is controlled by cooling. Once the skin layer overturns, the time to reestablish the full increase of skin salinity was reported to be on the order of 15 min, which is approximately 90 times slower than that for skin temperature. Because the radiation received from a footprint is averaged over an area to give a single pixel value, the slow recovery by the salt diffusion process might cause a slight reduction in area-averaged skin salinity and thus obscure the salty skin effect on radiometer retrievals. In the presence of many geophysical error sources in remote sensing of ocean salinity, the salt enrichment at the surface skin does not appear to be a concern.
Abstract
The WKBJ method and a multiple-scale expansion technique are used to study equatorially trapped waves propagating on a zonally sloping themocline. Assuming that variations of the main thermocline depth (MTD) are slow (the change of the MTD over one wavelength is smaller than the wave amplitude), wave reflections can be neglected and the amplitudes of equatorially trapped waves can be derived by using the energy conservation law. It is found that the wavelengths and amplitudes of free waves are significantly modified by the MTD variations. While propagating eastward in an ocean basin (where the MTD is shallower), Kelvin waves shrink meridionally and zonally but their amplitudes increase to preserve wave energy; short Rossby waves behave in the opposite way. The wavelength of westward-propagating long Rossby waves becomes longer when they propagate into the deeper western ocean. The response of a Yanai wave to the changing thermocline depends on the sign of phase speed.
A simple numerical method is designed to verify the WKBJ results and also to study the cast of a relatively steep thermocline profile where the WKBJ method breaks down. Reflection of a Kelvin wave impinging on a thermocline front is also investigated in this work.
Abstract
The WKBJ method and a multiple-scale expansion technique are used to study equatorially trapped waves propagating on a zonally sloping themocline. Assuming that variations of the main thermocline depth (MTD) are slow (the change of the MTD over one wavelength is smaller than the wave amplitude), wave reflections can be neglected and the amplitudes of equatorially trapped waves can be derived by using the energy conservation law. It is found that the wavelengths and amplitudes of free waves are significantly modified by the MTD variations. While propagating eastward in an ocean basin (where the MTD is shallower), Kelvin waves shrink meridionally and zonally but their amplitudes increase to preserve wave energy; short Rossby waves behave in the opposite way. The wavelength of westward-propagating long Rossby waves becomes longer when they propagate into the deeper western ocean. The response of a Yanai wave to the changing thermocline depends on the sign of phase speed.
A simple numerical method is designed to verify the WKBJ results and also to study the cast of a relatively steep thermocline profile where the WKBJ method breaks down. Reflection of a Kelvin wave impinging on a thermocline front is also investigated in this work.
Abstract
A variational optimal control technique is used to assimilate both meteorological and oceanographic observations into an oceanic Ekman layer model. An identical twin experiment is discussed first in which the “observations” are created by the dynamic model. The field measurements from the LOTUS-3 (Long-Term Upper Ocean Study-3) buoy are then analysed. By fitting the model results to the data, the unknown boundary condition (the wind stress drag coefficient) and the unknown vertical eddy viscosity distribution are deduced simultaneously from the data, and an optimal estimate of the current field is obtained.
Though the model is simple, the results show that the variational assimilation technique is capable of extracting from the available observations a reasonable wind stress drag coefficient and vertical eddy viscosity distribution.
Abstract
A variational optimal control technique is used to assimilate both meteorological and oceanographic observations into an oceanic Ekman layer model. An identical twin experiment is discussed first in which the “observations” are created by the dynamic model. The field measurements from the LOTUS-3 (Long-Term Upper Ocean Study-3) buoy are then analysed. By fitting the model results to the data, the unknown boundary condition (the wind stress drag coefficient) and the unknown vertical eddy viscosity distribution are deduced simultaneously from the data, and an optimal estimate of the current field is obtained.
Though the model is simple, the results show that the variational assimilation technique is capable of extracting from the available observations a reasonable wind stress drag coefficient and vertical eddy viscosity distribution.
Abstract
An ocean general circulation model (OGCM) of the North Atlantic Ocean is fitted to the monthly averaged climatological temperatures and salinities of Levitus using the adjoint method, representing a significant step forward with respect to previous steady OGCM assimilations. The inverse approach has two important advantages over purely prognostic calculations: (i) it provides an estimate of the North Atlantic circulation and of its seasonal variability, which is optimally consistent with the OGCM dynamics and with the assimilated hydrography; (ii) it provides optimal estimates of the monthly surface heat and freshwater fluxes consistent with the used climatology, which are the most poorly known surface forcing functions.
Seasonality is ensured by penalizing field differences between month 13 and month 1 of the forward time integration within each iteration of the adjoint procedure. The primary goal of this work is to estimate large-scale oceanic properties important for climate issues and how they are affected by the inclusion of the seasonal cycle. The resultant meridional overturning displays significant seasonal variations. The surface Ekman cell centered at 35°N reaches a maximum intensity of ∼7 Sv (Sv ≡ 106 m3 s−1) in wintertime, while the North Atlantic Deep Water cell reaches a maximum strength of ∼19 Sv in summertime. Its annual average is of ∼17 Sv, in good agreement with the recent estimate of Schmitz and McCartney. The poleward heat transport exhibits the strongest seasonal variations, reaching its maximum value of 0.85 × 1015 W at ∼25°N in summertime or 0.85 PW (1 PW = 1015 W). The annual average at 25°N is ∼0.7 PW, weaker than observational estimates. The dynamical analysis indicates that the wind forcing is the controlling factor for these variations by controlling the time-varying Ekman cell.
Comparison with previous steady-state optimizations of Yu and Malanotte-Rizzoli shows that the optimization with seasonal forcing produces three major improvements in the inverse results. First, the inclusion of the seasonal cycle greatly improves the estimated hydrography (temperature and salinity fields) by eliminating the basinwide cold bias in the upper ocean and the warm bias in the deep ocean found in the steady-state inversions. As a consequence, the velocity fields are also significantly improved, with a tight and strong Gulf Stream jet.
Second, the monthly optimal estimates of surface heat and freshwater fluxes provide an annual average resembling closely the observational climatological means, a striking contrast to the fluxes estimated in the steady assimilation.
Finally, the most important improvement is in the estimate of the poleward heat transport. The annual mean meridional heat transport shows an increase of ∼0.2 PW at all latitudes with respect to the steady-state heat transport, thus demonstrating the importance of rectification effects of the seasonal cycle.
Abstract
An ocean general circulation model (OGCM) of the North Atlantic Ocean is fitted to the monthly averaged climatological temperatures and salinities of Levitus using the adjoint method, representing a significant step forward with respect to previous steady OGCM assimilations. The inverse approach has two important advantages over purely prognostic calculations: (i) it provides an estimate of the North Atlantic circulation and of its seasonal variability, which is optimally consistent with the OGCM dynamics and with the assimilated hydrography; (ii) it provides optimal estimates of the monthly surface heat and freshwater fluxes consistent with the used climatology, which are the most poorly known surface forcing functions.
Seasonality is ensured by penalizing field differences between month 13 and month 1 of the forward time integration within each iteration of the adjoint procedure. The primary goal of this work is to estimate large-scale oceanic properties important for climate issues and how they are affected by the inclusion of the seasonal cycle. The resultant meridional overturning displays significant seasonal variations. The surface Ekman cell centered at 35°N reaches a maximum intensity of ∼7 Sv (Sv ≡ 106 m3 s−1) in wintertime, while the North Atlantic Deep Water cell reaches a maximum strength of ∼19 Sv in summertime. Its annual average is of ∼17 Sv, in good agreement with the recent estimate of Schmitz and McCartney. The poleward heat transport exhibits the strongest seasonal variations, reaching its maximum value of 0.85 × 1015 W at ∼25°N in summertime or 0.85 PW (1 PW = 1015 W). The annual average at 25°N is ∼0.7 PW, weaker than observational estimates. The dynamical analysis indicates that the wind forcing is the controlling factor for these variations by controlling the time-varying Ekman cell.
Comparison with previous steady-state optimizations of Yu and Malanotte-Rizzoli shows that the optimization with seasonal forcing produces three major improvements in the inverse results. First, the inclusion of the seasonal cycle greatly improves the estimated hydrography (temperature and salinity fields) by eliminating the basinwide cold bias in the upper ocean and the warm bias in the deep ocean found in the steady-state inversions. As a consequence, the velocity fields are also significantly improved, with a tight and strong Gulf Stream jet.
Second, the monthly optimal estimates of surface heat and freshwater fluxes provide an annual average resembling closely the observational climatological means, a striking contrast to the fluxes estimated in the steady assimilation.
Finally, the most important improvement is in the estimate of the poleward heat transport. The annual mean meridional heat transport shows an increase of ∼0.2 PW at all latitudes with respect to the steady-state heat transport, thus demonstrating the importance of rectification effects of the seasonal cycle.
Abstract
The meridional shift of the Kuroshio Extension (KE) front and changes in the formation of the North Pacific Subtropical Mode Water (STMW) during 1979–2018 are reported. The surface-to-subsurface structure of the KE front averaged over 142°–165°E has shifted poleward at a rate of ~0.23° ± 0.16° decade−1. The shift was caused mainly by the poleward shift of the downstream KE front (153°–165°E, ~0.41° ± 0.29° decade−1) and barely by the upstream KE front (142°–153°E). The long-term shift trend of the KE front showed two distinct behaviors before and after 2002. Before 2002, the surface KE front moved northward with a faster rate than the subsurface. After 2002, the surface KE front showed no obvious trend, but the subsurface KE front continued to move northward. The ventilation zone of the STMW, defined by the area between the 16° and 18°C isotherms or between the 25 and 25.5 kg m−3 isopycnals, contracted and displaced northward with a shoaling of the mixed layer depth h m before 2002 when the KE front moved northward. The STMW subduction rate was reduced by 0.76 Sv (63%; 1 Sv ≡ = 106 m3 s−1) during 1979–2018, most of which occurred before 2002. Of the three components affecting the total subduction rate, the temporal induction (−∂h m /∂t) was dominant accounting for 91% of the rate reduction, while the vertical pumping (−w mb) amounted to 8% and the lateral induction (−u mb ⋅ ∇h m ) was insignificant. The reduced temporal induction was attributed to both the contracted ventilation zone and the shallowed h m that were incurred by the poleward shift of KE front.
Abstract
The meridional shift of the Kuroshio Extension (KE) front and changes in the formation of the North Pacific Subtropical Mode Water (STMW) during 1979–2018 are reported. The surface-to-subsurface structure of the KE front averaged over 142°–165°E has shifted poleward at a rate of ~0.23° ± 0.16° decade−1. The shift was caused mainly by the poleward shift of the downstream KE front (153°–165°E, ~0.41° ± 0.29° decade−1) and barely by the upstream KE front (142°–153°E). The long-term shift trend of the KE front showed two distinct behaviors before and after 2002. Before 2002, the surface KE front moved northward with a faster rate than the subsurface. After 2002, the surface KE front showed no obvious trend, but the subsurface KE front continued to move northward. The ventilation zone of the STMW, defined by the area between the 16° and 18°C isotherms or between the 25 and 25.5 kg m−3 isopycnals, contracted and displaced northward with a shoaling of the mixed layer depth h m before 2002 when the KE front moved northward. The STMW subduction rate was reduced by 0.76 Sv (63%; 1 Sv ≡ = 106 m3 s−1) during 1979–2018, most of which occurred before 2002. Of the three components affecting the total subduction rate, the temporal induction (−∂h m /∂t) was dominant accounting for 91% of the rate reduction, while the vertical pumping (−w mb) amounted to 8% and the lateral induction (−u mb ⋅ ∇h m ) was insignificant. The reduced temporal induction was attributed to both the contracted ventilation zone and the shallowed h m that were incurred by the poleward shift of KE front.
Abstract
The authors present a study for determining the seasonal net surface heat flux over the tropical Pacific Ocean using an adjoint technique. A simple tropical ocean model with thermodynamics is chosen and the seasonal sea surface temperature (SST) observations are assimilated. A least-squares fitting of the model state to data is used. The cost function has a misfit term that measures the difference between the modeled and observed SST and two additional terms that penalize the departure of the estimated parameters from their prior information.
The adjoint method ensures that the flux pattern obtained is consistent with the model's dynamics and thermodynamics and is also in agreement with observations. Comparisons with heat flux atlases of Oberhuber and Fu et al. show that our adjoint calculations have successfully captured the main seasonal signals of the surface heat flux distribution over the tropical Pacific Ocean although, not surprisingly, some differences exist. The differences are examined from both thermodynamic and air-sea interaction viewpoints. Two experiments are conducted to study the effects of the prior information on the optimal solution.
Abstract
The authors present a study for determining the seasonal net surface heat flux over the tropical Pacific Ocean using an adjoint technique. A simple tropical ocean model with thermodynamics is chosen and the seasonal sea surface temperature (SST) observations are assimilated. A least-squares fitting of the model state to data is used. The cost function has a misfit term that measures the difference between the modeled and observed SST and two additional terms that penalize the departure of the estimated parameters from their prior information.
The adjoint method ensures that the flux pattern obtained is consistent with the model's dynamics and thermodynamics and is also in agreement with observations. Comparisons with heat flux atlases of Oberhuber and Fu et al. show that our adjoint calculations have successfully captured the main seasonal signals of the surface heat flux distribution over the tropical Pacific Ocean although, not surprisingly, some differences exist. The differences are examined from both thermodynamic and air-sea interaction viewpoints. Two experiments are conducted to study the effects of the prior information on the optimal solution.
Abstract
An in-depth data analysis was conducted to understand the occurrence of a strong sea surface temperature (SST) front in the central Bay of Bengal before the formation of Cyclone Nargis in April 2008. Nargis changed its course after encountering the front and tracked along the front until making landfall. One unique feature of this SST front was its coupling with high sea surface height anomalies (SSHAs), which is unusual for a basin where SST is normally uncorrelated with SSHA. The high SSHAs were associated with downwelling Rossby waves, and the interaction between downwelling and surface fresh waters was a key mechanism to account for the observed SST–SSHA coupling.
The near-surface salinity field in the bay is characterized by strong stratification and a pronounced horizontal gradient, with low salinity in the northeast. During the passage of downwelling Rossby waves, freshening of the surface layer was observed when surface velocities were southwestward. Horizontal convergence of freshwater associated with downwelling Rossby waves increased the buoyancy of the upper layer and caused the mixed layer to shoal to within a few meters of the surface. Surface heating trapped in the thin mixed layer caused the fresh layer to warm, whereas the increase in buoyancy from low-salinity waters enhanced the high SSHA associated with Rossby waves. Thus, high SST coincided with high SSHA.
The dominant role of salinity in controlling high SSHA suggests that caution should be exercised when computing hurricane heat potential in the bay from SSHA. This situation is different from most tropical oceans, where temperature has the dominant effect on SSHA.
Abstract
An in-depth data analysis was conducted to understand the occurrence of a strong sea surface temperature (SST) front in the central Bay of Bengal before the formation of Cyclone Nargis in April 2008. Nargis changed its course after encountering the front and tracked along the front until making landfall. One unique feature of this SST front was its coupling with high sea surface height anomalies (SSHAs), which is unusual for a basin where SST is normally uncorrelated with SSHA. The high SSHAs were associated with downwelling Rossby waves, and the interaction between downwelling and surface fresh waters was a key mechanism to account for the observed SST–SSHA coupling.
The near-surface salinity field in the bay is characterized by strong stratification and a pronounced horizontal gradient, with low salinity in the northeast. During the passage of downwelling Rossby waves, freshening of the surface layer was observed when surface velocities were southwestward. Horizontal convergence of freshwater associated with downwelling Rossby waves increased the buoyancy of the upper layer and caused the mixed layer to shoal to within a few meters of the surface. Surface heating trapped in the thin mixed layer caused the fresh layer to warm, whereas the increase in buoyancy from low-salinity waters enhanced the high SSHA associated with Rossby waves. Thus, high SST coincided with high SSHA.
The dominant role of salinity in controlling high SSHA suggests that caution should be exercised when computing hurricane heat potential in the bay from SSHA. This situation is different from most tropical oceans, where temperature has the dominant effect on SSHA.
Abstract
Interannual variability in the volumetric water mass distribution within the North Atlantic Subtropical Gyre is described in relation to variability in the Atlantic meridional overturning circulation. The relative roles of diabatic and adiabatic processes in the volume and heat budgets of the subtropical gyre are investigated by projecting data into temperature coordinates as volumes of water using an Argo-based climatology and an ocean state estimate (ECCO version 4). This highlights that variations in the subtropical gyre volume budget are predominantly set by transport divergence in the gyre. A strong correlation between the volume anomaly due to transport divergence and the variability of both thermocline depth and Ekman pumping over the gyre suggests that wind-driven heave drives transport anomalies at the gyre boundaries. This wind-driven heaving contributes significantly to variations in the heat content of the gyre, as do anomalies in the air–sea fluxes. The analysis presented suggests that wind forcing plays an important role in driving interannual variability in the Atlantic meridional overturning circulation and that this variability can be unraveled from spatially distributed hydrographic observations using the framework presented here.
Abstract
Interannual variability in the volumetric water mass distribution within the North Atlantic Subtropical Gyre is described in relation to variability in the Atlantic meridional overturning circulation. The relative roles of diabatic and adiabatic processes in the volume and heat budgets of the subtropical gyre are investigated by projecting data into temperature coordinates as volumes of water using an Argo-based climatology and an ocean state estimate (ECCO version 4). This highlights that variations in the subtropical gyre volume budget are predominantly set by transport divergence in the gyre. A strong correlation between the volume anomaly due to transport divergence and the variability of both thermocline depth and Ekman pumping over the gyre suggests that wind-driven heave drives transport anomalies at the gyre boundaries. This wind-driven heaving contributes significantly to variations in the heat content of the gyre, as do anomalies in the air–sea fluxes. The analysis presented suggests that wind forcing plays an important role in driving interannual variability in the Atlantic meridional overturning circulation and that this variability can be unraveled from spatially distributed hydrographic observations using the framework presented here.
