Search Results
You are looking at 1 - 6 of 6 items for :
- Author or Editor: Shenfu Dong x
- Journal of Physical Oceanography x
- Refine by Access: All Content x
Abstract
Formation and the subsequent evolution of the subtropical mode water (STMW) involve various dynamic and thermodynamic processes. Proper representation of mode water variability and contributions from various processes in climate models is important in order to predict future climate change under changing forcings. The North Atlantic STMW, often referred to as Eighteen Degree Water (EDW), in three coupled models, both with data assimilation [GFDL coupled data assimilation (GFDL CDA)] and without data assimilation [GFDL Climate Model, version 2.1 (GFDL CM2.1), and NCAR Community Climate System Model, version 3 (CCSM3)], is analyzed to evaluate how well EDW processes are simulated in those models and to examine whether data assimilation alters the model response to forcing. In comparison with estimates from observations, the data-assimilating model gives a better representation of the formation rate, the spatial distribution of EDW, and its thickness, with the largest EDW variability along the Gulf Stream (GS) path. The EDW formation rate in GFDL CM2.1 is very weak because of weak heat loss from the ocean in the model. Unlike the observed dominant southward movement of the EDW, the EDW in GFDL CM2.1 and CCSM3 moves eastward after formation in the excessively wide GS in the models. However, the GFDL CDA does not capture the observed thermal response of the overlying atmosphere to the ocean. Observations show a robust anticorrelation between the upper-ocean heat content and air–sea heat flux, with upper-ocean heat content leading air–sea heat flux by a few months. This anticorrelation is well captured by GFDL CM2.1 and CCSM3 but not by GFDL CDA. Only GFDL CM2.1 captures the observed anticorrelation between the upper-ocean heat content and EDW volume. This suggests that, although data assimilation corrects the readily observed variables, it degrades the model thermodynamic response to forcing.
Abstract
Formation and the subsequent evolution of the subtropical mode water (STMW) involve various dynamic and thermodynamic processes. Proper representation of mode water variability and contributions from various processes in climate models is important in order to predict future climate change under changing forcings. The North Atlantic STMW, often referred to as Eighteen Degree Water (EDW), in three coupled models, both with data assimilation [GFDL coupled data assimilation (GFDL CDA)] and without data assimilation [GFDL Climate Model, version 2.1 (GFDL CM2.1), and NCAR Community Climate System Model, version 3 (CCSM3)], is analyzed to evaluate how well EDW processes are simulated in those models and to examine whether data assimilation alters the model response to forcing. In comparison with estimates from observations, the data-assimilating model gives a better representation of the formation rate, the spatial distribution of EDW, and its thickness, with the largest EDW variability along the Gulf Stream (GS) path. The EDW formation rate in GFDL CM2.1 is very weak because of weak heat loss from the ocean in the model. Unlike the observed dominant southward movement of the EDW, the EDW in GFDL CM2.1 and CCSM3 moves eastward after formation in the excessively wide GS in the models. However, the GFDL CDA does not capture the observed thermal response of the overlying atmosphere to the ocean. Observations show a robust anticorrelation between the upper-ocean heat content and air–sea heat flux, with upper-ocean heat content leading air–sea heat flux by a few months. This anticorrelation is well captured by GFDL CM2.1 and CCSM3 but not by GFDL CDA. Only GFDL CM2.1 captures the observed anticorrelation between the upper-ocean heat content and EDW volume. This suggests that, although data assimilation corrects the readily observed variables, it degrades the model thermodynamic response to forcing.
Abstract
The interocean exchange of water from the South Atlantic with the Pacific and Indian Oceans is examined using the output from the ocean general circulation model for the Earth Simulator (OFES) during the period 1980–2006. The main objective of this paper is to investigate the role of the interocean exchanges in the variability of the Atlantic meridional overturning circulation (AMOC) and its associated meridional heat transport (MHT) in the South Atlantic. The meridional heat transport from OFES shows a similar response to AMOC variations to that derived from observations: a 1 Sv (1 Sv ≡ 106 m3 s−1) increase in the AMOC strength would cause a 0.054 ± 0.003 PW increase in MHT at approximately 34°S. The main feature in the AMOC and MHT across 34°S is their increasing trends during the period 1980–93. Separating the transports into boundary currents and ocean interior regions indicates that the increase in transport comes from the ocean interior region, suggesting that it is important to monitor the ocean interior region to capture changes in the AMOC and MHT on decadal to longer time scales. The linear increase in the MHT from 1980 to 1993 is due to the increase in advective heat converged into the South Atlantic from the Pacific and Indian Oceans. Of the total increase in the heat convergence, about two-thirds is contributed by the Indian Ocean through the Agulhas Current system, suggesting that the warm-water route from the Indian Ocean plays a more important role in the northward-flowing water in the upper branch of the AMOC at 34°S during the study period.
Abstract
The interocean exchange of water from the South Atlantic with the Pacific and Indian Oceans is examined using the output from the ocean general circulation model for the Earth Simulator (OFES) during the period 1980–2006. The main objective of this paper is to investigate the role of the interocean exchanges in the variability of the Atlantic meridional overturning circulation (AMOC) and its associated meridional heat transport (MHT) in the South Atlantic. The meridional heat transport from OFES shows a similar response to AMOC variations to that derived from observations: a 1 Sv (1 Sv ≡ 106 m3 s−1) increase in the AMOC strength would cause a 0.054 ± 0.003 PW increase in MHT at approximately 34°S. The main feature in the AMOC and MHT across 34°S is their increasing trends during the period 1980–93. Separating the transports into boundary currents and ocean interior regions indicates that the increase in transport comes from the ocean interior region, suggesting that it is important to monitor the ocean interior region to capture changes in the AMOC and MHT on decadal to longer time scales. The linear increase in the MHT from 1980 to 1993 is due to the increase in advective heat converged into the South Atlantic from the Pacific and Indian Oceans. Of the total increase in the heat convergence, about two-thirds is contributed by the Indian Ocean through the Agulhas Current system, suggesting that the warm-water route from the Indian Ocean plays a more important role in the northward-flowing water in the upper branch of the AMOC at 34°S during the study period.
Abstract
A simple three-dimensional thermodynamic model is used to study the heat balance in the Gulf Stream region (30°–45°N, 40°–75°W) during the period from November 1992 to December 1999. The model is forced by surface heat fluxes derived from NCEP variables, with geostrophic surface velocity specified from sea surface height measurements from the TOPEX/Poseidon altimeter and Ekman transport specified from NCEP wind stress. The mixed layer temperature and mixed layer depth from the model show good agreement with the observations on seasonal and interannual time scales. Although the annual cycle of the upper-ocean heat content is underestimated, the agreement of the interannual variations in the heat content and the sea surface height are good; both are dominated by the large decrease from 1994 to 1997 and the increase afterward. As expected from previous studies, the surface heat flux dominates the seasonal and interannual variations in the mixed layer temperature. However, interannual variations in the upper-ocean heat content are dominated by the advection– diffusion term. Within the advection term itself, the largest variations are from the geostrophic advection anomaly. In the western Gulf Stream region the largest component of anomalous advection is the advection of the anomalous temperature by the mean current; elsewhere, the advection of the mean temperature by the anomalous current is also important. Other studies have shown that upper-ocean heat content is a more robust indicator of the potential contribution of the ocean to interannual heat flux anomalies than is sea surface temperature. The analysis here shows that the dominant term in interannual variations in heat content in the Gulf Stream region is anomalous advection by geostrophic currents. In fact, these ocean-forced variations in heat content appear to force air–sea fluxes: the surface heat flux anomalies in the western Gulf Stream region are negatively correlated with the anomalous upper-ocean heat content, that is, a large heat loss to the atmosphere corresponding to a positive heat content anomaly.
Abstract
A simple three-dimensional thermodynamic model is used to study the heat balance in the Gulf Stream region (30°–45°N, 40°–75°W) during the period from November 1992 to December 1999. The model is forced by surface heat fluxes derived from NCEP variables, with geostrophic surface velocity specified from sea surface height measurements from the TOPEX/Poseidon altimeter and Ekman transport specified from NCEP wind stress. The mixed layer temperature and mixed layer depth from the model show good agreement with the observations on seasonal and interannual time scales. Although the annual cycle of the upper-ocean heat content is underestimated, the agreement of the interannual variations in the heat content and the sea surface height are good; both are dominated by the large decrease from 1994 to 1997 and the increase afterward. As expected from previous studies, the surface heat flux dominates the seasonal and interannual variations in the mixed layer temperature. However, interannual variations in the upper-ocean heat content are dominated by the advection– diffusion term. Within the advection term itself, the largest variations are from the geostrophic advection anomaly. In the western Gulf Stream region the largest component of anomalous advection is the advection of the anomalous temperature by the mean current; elsewhere, the advection of the mean temperature by the anomalous current is also important. Other studies have shown that upper-ocean heat content is a more robust indicator of the potential contribution of the ocean to interannual heat flux anomalies than is sea surface temperature. The analysis here shows that the dominant term in interannual variations in heat content in the Gulf Stream region is anomalous advection by geostrophic currents. In fact, these ocean-forced variations in heat content appear to force air–sea fluxes: the surface heat flux anomalies in the western Gulf Stream region are negatively correlated with the anomalous upper-ocean heat content, that is, a large heat loss to the atmosphere corresponding to a positive heat content anomaly.
Abstract
The location of the Southern Ocean polar front (PF) is mapped from the first 3 yr of remotely sensed Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E) sea surface temperature (SST) measurements. In agreement with previous studies, the mean path of the Antarctic PF and its standard deviation are strongly influenced by bottom topography. However, the mean PF path diverges slightly from previous studies in several regions where there is high mesoscale variability. Although the SST and SST gradient at the PF show spatially coherent seasonal variations, with the highest temperature and the lowest temperature gradient during summer, the seasonal variations in the location of the PF are not spatially coherent. The temporal mean SST at the PF corresponds well to the mean PF path: the temperature is high in the Atlantic and Indian Ocean sections and is low in the Pacific Ocean section where the PF has a more southerly position. The relationship of the wind field with the Antarctic PF location and proxies for the zonal and meridional PF transports are examined statistically. Coherence analysis suggests that the zonal wind stress accelerates the zonal transport of the PF. The analysis presented herein also suggests that the meridional shifts of the Antarctic PF path correspond to the meridional shifts of the wind field.
Abstract
The location of the Southern Ocean polar front (PF) is mapped from the first 3 yr of remotely sensed Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E) sea surface temperature (SST) measurements. In agreement with previous studies, the mean path of the Antarctic PF and its standard deviation are strongly influenced by bottom topography. However, the mean PF path diverges slightly from previous studies in several regions where there is high mesoscale variability. Although the SST and SST gradient at the PF show spatially coherent seasonal variations, with the highest temperature and the lowest temperature gradient during summer, the seasonal variations in the location of the PF are not spatially coherent. The temporal mean SST at the PF corresponds well to the mean PF path: the temperature is high in the Atlantic and Indian Ocean sections and is low in the Pacific Ocean section where the PF has a more southerly position. The relationship of the wind field with the Antarctic PF location and proxies for the zonal and meridional PF transports are examined statistically. Coherence analysis suggests that the zonal wind stress accelerates the zonal transport of the PF. The analysis presented herein also suggests that the meridional shifts of the Antarctic PF path correspond to the meridional shifts of the wind field.
Abstract
Subsurface temperature data in the western North Atlantic Ocean are analyzed to study the variations in the heat content above a fixed isotherm and contributions from surface heat fluxes and oceanic processes. The study region is chosen based on the data density; its northern boundary shifts with the Gulf Stream position and its southern boundary shifts to contain constant volume. The temperature profiles are objectively mapped to a uniform grid (0.5° latitude and longitude, 10 m in depth, and 3 months in time). The interannual variations in upper-ocean heat content show good agreement with the changes in the sea surface height from the Ocean Topography Experiment (TOPEX)/Poseidon altimeter; both indicate positive anomalies in 1994 and 1998–99 and negative anomalies in 1996–97. The interannual variations in surface heat fluxes cannot explain the changes in upper-ocean heat storage rate. On the contrary, a positive anomaly in heat released to the atmosphere corresponds to a positive upper-ocean heat content anomaly. The oceanic heat transport, mainly owing to the geostrophic advection, controls the interannual variations in heat storage rate, which suggests that geostrophic advection plays an important role in the air–sea heat exchange. The 18°C isotherm depth and layer thickness also show good correspondence to the upper-ocean heat content; a deep and thin 18°C layer corresponds to a positive heat content anomaly. The oceanic transport in each isotherm layer shows an annual cycle, converging heat in winter, and diverging in summer in a warm layer; it also shows interannual variations with the largest heat convergence occurring in even warmer layers during the period of large ocean-to-atmosphere flux.
Abstract
Subsurface temperature data in the western North Atlantic Ocean are analyzed to study the variations in the heat content above a fixed isotherm and contributions from surface heat fluxes and oceanic processes. The study region is chosen based on the data density; its northern boundary shifts with the Gulf Stream position and its southern boundary shifts to contain constant volume. The temperature profiles are objectively mapped to a uniform grid (0.5° latitude and longitude, 10 m in depth, and 3 months in time). The interannual variations in upper-ocean heat content show good agreement with the changes in the sea surface height from the Ocean Topography Experiment (TOPEX)/Poseidon altimeter; both indicate positive anomalies in 1994 and 1998–99 and negative anomalies in 1996–97. The interannual variations in surface heat fluxes cannot explain the changes in upper-ocean heat storage rate. On the contrary, a positive anomaly in heat released to the atmosphere corresponds to a positive upper-ocean heat content anomaly. The oceanic heat transport, mainly owing to the geostrophic advection, controls the interannual variations in heat storage rate, which suggests that geostrophic advection plays an important role in the air–sea heat exchange. The 18°C isotherm depth and layer thickness also show good correspondence to the upper-ocean heat content; a deep and thin 18°C layer corresponds to a positive heat content anomaly. The oceanic transport in each isotherm layer shows an annual cycle, converging heat in winter, and diverging in summer in a warm layer; it also shows interannual variations with the largest heat convergence occurring in even warmer layers during the period of large ocean-to-atmosphere flux.
Abstract
Time series of Gulf Stream position derived from the TOPEX/Poseidon altimeter from October 1992 to November 1998 are used to investigate the lead and lag relation between the Gulf Stream path as it leaves the continental shelf and the changes in sea level pressure, surface wind stress, and sea surface temperature (SST), as given by the NCEP reanalysis. The dominant signal is a northward (southward) displacement of Gulf Stream axis 11 to 18 months after the North Atlantic Oscillation (NAO) reaches positive (negative) extrema. A SST warming (cooling) peaking north of the Gulf Stream is also seen to precede the latitudinal shifts, but it is a part of the large-scale SST anomaly tripole that is generated by the NAO fluctuations. There is no evidence that the Gulf Stream shifts have a direct impact onto the large-scale atmospheric circulation. A fast, passive response of the Gulf Stream to NAO forcing is also suggested by a corresponding analysis of the yearly mean Gulf Stream position estimated from XBT data at 200 m during 1954–98, where the NAO primarily leads the latitudinal Gulf Stream shifts by 1 yr. The fast Gulf Stream response seems to reflect buoyancy forcing in the recirculation gyres but, as the covariability remains significant when the NAO leads by up to 9 yr, large-scale wind stress forcing may become important after a longer delay. Because of the high NAO index of the last decades, the TOPEX/Poseidon period is one of unprecedented northward excursion of the Gulf Stream in the 45-yr record, with the Gulf Stream 50–100 km north of its climatological mean position.
Abstract
Time series of Gulf Stream position derived from the TOPEX/Poseidon altimeter from October 1992 to November 1998 are used to investigate the lead and lag relation between the Gulf Stream path as it leaves the continental shelf and the changes in sea level pressure, surface wind stress, and sea surface temperature (SST), as given by the NCEP reanalysis. The dominant signal is a northward (southward) displacement of Gulf Stream axis 11 to 18 months after the North Atlantic Oscillation (NAO) reaches positive (negative) extrema. A SST warming (cooling) peaking north of the Gulf Stream is also seen to precede the latitudinal shifts, but it is a part of the large-scale SST anomaly tripole that is generated by the NAO fluctuations. There is no evidence that the Gulf Stream shifts have a direct impact onto the large-scale atmospheric circulation. A fast, passive response of the Gulf Stream to NAO forcing is also suggested by a corresponding analysis of the yearly mean Gulf Stream position estimated from XBT data at 200 m during 1954–98, where the NAO primarily leads the latitudinal Gulf Stream shifts by 1 yr. The fast Gulf Stream response seems to reflect buoyancy forcing in the recirculation gyres but, as the covariability remains significant when the NAO leads by up to 9 yr, large-scale wind stress forcing may become important after a longer delay. Because of the high NAO index of the last decades, the TOPEX/Poseidon period is one of unprecedented northward excursion of the Gulf Stream in the 45-yr record, with the Gulf Stream 50–100 km north of its climatological mean position.
