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- Author or Editor: Kensuke Takeuchi x
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Abstract
Response of the Subtropical Countercurrent and the Subtropical Front in the North Pacific Ocean to seasonally changing wind stress and thermal condition are examined using the same idealized numerical model that the author used in 1984 for steady state modeling of the Subtropical Countercurrent and the Subtropical Front. The model reproduces the main features of the observed seasonal variations reasonably well, especially that the Subtropical Countercurrent is strong in spring and weak in fall. It is also shown that the seasonal variation of wind stress and thermal condition intensifies the annual mean strength of the Subtropical Countercurrent.
The relative importance of the seasonal variations of wind stress and thermal condition is examined using models in which only one of these changes and the other is fixed. The results indicate that the seasonal variation of the Subtropical Countercurrent is mainly due to the seasonal change of wind stress, while the seasonal change of thermal condition is mostly responsible for the intensification of the annual mean of the Subtropical Countercurrent and the Subtropical Front.
Abstract
Response of the Subtropical Countercurrent and the Subtropical Front in the North Pacific Ocean to seasonally changing wind stress and thermal condition are examined using the same idealized numerical model that the author used in 1984 for steady state modeling of the Subtropical Countercurrent and the Subtropical Front. The model reproduces the main features of the observed seasonal variations reasonably well, especially that the Subtropical Countercurrent is strong in spring and weak in fall. It is also shown that the seasonal variation of wind stress and thermal condition intensifies the annual mean strength of the Subtropical Countercurrent.
The relative importance of the seasonal variations of wind stress and thermal condition is examined using models in which only one of these changes and the other is fixed. The results indicate that the seasonal variation of the Subtropical Countercurrent is mainly due to the seasonal change of wind stress, while the seasonal change of thermal condition is mostly responsible for the intensification of the annual mean of the Subtropical Countercurrent and the Subtropical Front.
Abstract
While high salinity water extends to the equator in the upper thermocline of the Pacific in the Southern Hemisphere (SH), it hits the western boundary (WB) farther north of the equator in the Northern Hemisphere (NH), suggesting that no interior pathway exists to the equatorial region. By contrast, high tritium water appears on the equator in the central Pacific, apparently through a NH interior pathway within the thermocline. The mechanisms of forming these salinity and tritium distributions and the causes of their difference are investigated using a realistic ocean general circulation model (OGCM).
The OGCM reproduces the properties of tropical salinity distribution quite well and displays interior pathways in the NH. Analysis indicates that the observed salinity distribution is compatible with the existence of a NH interior pathway. Key to the hemispheric difference in thermocline salinity is the sea surface salinity (SSS) distribution in relation to the so-called WB (interior) exchange window, from which subducted water goes to the equatorial region through the WB region (interior ocean). In the NH, high SSSs are found only in the WB exchange window, and high salinity water thus appears to turn onto the WB before reaching the equator. In the SH, on the other hand, high SSSs are found in both the WB and interior exchange windows, and, as a result, high salinity water extends to the equatorial region through both the WB region and interior ocean.
The sea surface tritium field has high values near the eastern boundary within the interior exchange window in the midlatitude North Pacific. Thus, high tritium water takes the NH interior pathway to the equatorial region after the subduction. This is demonstrated by a passive tracer experiment with a sea surface distribution resembling that of tritium. This result suggests that the apparent differences between the isopycnal salinity and tritium distributions are largely due to differences in surface distribution, raising caution about interpreting ocean circulation with tracer fields alone.
Abstract
While high salinity water extends to the equator in the upper thermocline of the Pacific in the Southern Hemisphere (SH), it hits the western boundary (WB) farther north of the equator in the Northern Hemisphere (NH), suggesting that no interior pathway exists to the equatorial region. By contrast, high tritium water appears on the equator in the central Pacific, apparently through a NH interior pathway within the thermocline. The mechanisms of forming these salinity and tritium distributions and the causes of their difference are investigated using a realistic ocean general circulation model (OGCM).
The OGCM reproduces the properties of tropical salinity distribution quite well and displays interior pathways in the NH. Analysis indicates that the observed salinity distribution is compatible with the existence of a NH interior pathway. Key to the hemispheric difference in thermocline salinity is the sea surface salinity (SSS) distribution in relation to the so-called WB (interior) exchange window, from which subducted water goes to the equatorial region through the WB region (interior ocean). In the NH, high SSSs are found only in the WB exchange window, and high salinity water thus appears to turn onto the WB before reaching the equator. In the SH, on the other hand, high SSSs are found in both the WB and interior exchange windows, and, as a result, high salinity water extends to the equatorial region through both the WB region and interior ocean.
The sea surface tritium field has high values near the eastern boundary within the interior exchange window in the midlatitude North Pacific. Thus, high tritium water takes the NH interior pathway to the equatorial region after the subduction. This is demonstrated by a passive tracer experiment with a sea surface distribution resembling that of tritium. This result suggests that the apparent differences between the isopycnal salinity and tritium distributions are largely due to differences in surface distribution, raising caution about interpreting ocean circulation with tracer fields alone.
Abstract
A three-dimensional ocean general circulation model, forced by idealized zonal winds, is used to investigate the effect of an abrupt intensification of westerly winds on the subduction process. Four experiments are carried out: 1) a control experiment with standard wind stress forcing, 2) an intensified winds experiment with wind stress that is larger in the region of the westerlies than the control, 3) an increased surface cooling experiment, and 4) an experiment with both intensified wind stress and surface cooling. Experiments 2 through 4, which contain surface anomalous forcing, are run from the steady state obtained in experiment 1, the control experiment. The results obtained for each of these runs are compared to the control experiment. A subarctic tracer injection experiment is also carried out to verify the differences in the subduction process of each of these experiments.
In the wind stress intensified experiment, an examination of the subsurface temperature field shows that negative temperature anomalies occupy the western portion of the southern part of the subtropical gyre, whereas in the surface cooling experiment, negative temperature anomalies occupy the eastern portion of the basin. The source of these negative temperature anomalies is not local since the forcing in the southern part of the subtropical gyre does not change from the control. A close analysis of the evolution of a subarctic surface tracer field indicates that the intensification of the wind stress increases the tracer concentrations, whereas surface cooling decreases the temperature in the region that contains the maximum tracer concentration.
In the standard case, the mixed layer is deep (shallow) in the northern (southern) part of the subtropical gyre. Between these two regions a mixed layer front, where the mixed layer depth changes drastically from north to south, exists. A water column with low potential vorticity that originates in the mixed layer penetrates into a subsurface layer from the point where an outcrop line and the mixed layer front intersect. This point is called the penetration point.
Intensified westerly winds bring about a deeper thermocline and shoaling subsurface isopycnals. These shoaling subsurface isopycnals are not predicted in classical models such as that of Luyten et al. The model experiment with intensified westerlies demonstrates that the penetration point shifts to the west. As a result, low potential vorticity water penetrates southwestward from the shifted penetration point and takes a more westward path. Therefore, the negative temperature anomalies appear in the southwestern part of the subtropical gyre. This study shows that the westward shift of the path of low potential vorticity water could cause the shoaling of subsurface isopycnal surfaces.
The intensification of the westerlies increases Ekman pumping and cools the ocean surface by enhancing sensible and latent heat flux. In the surface cooling experiment, the position of the outcrop lines moves southward significantly. This southward shift makes the subducted water colder and distributes it throughout the ventilated region of the southern part of the subtropical gyre.
The combined effect of wind intensification and surface cooling is approximately a linear combination of both experiments.
Abstract
A three-dimensional ocean general circulation model, forced by idealized zonal winds, is used to investigate the effect of an abrupt intensification of westerly winds on the subduction process. Four experiments are carried out: 1) a control experiment with standard wind stress forcing, 2) an intensified winds experiment with wind stress that is larger in the region of the westerlies than the control, 3) an increased surface cooling experiment, and 4) an experiment with both intensified wind stress and surface cooling. Experiments 2 through 4, which contain surface anomalous forcing, are run from the steady state obtained in experiment 1, the control experiment. The results obtained for each of these runs are compared to the control experiment. A subarctic tracer injection experiment is also carried out to verify the differences in the subduction process of each of these experiments.
In the wind stress intensified experiment, an examination of the subsurface temperature field shows that negative temperature anomalies occupy the western portion of the southern part of the subtropical gyre, whereas in the surface cooling experiment, negative temperature anomalies occupy the eastern portion of the basin. The source of these negative temperature anomalies is not local since the forcing in the southern part of the subtropical gyre does not change from the control. A close analysis of the evolution of a subarctic surface tracer field indicates that the intensification of the wind stress increases the tracer concentrations, whereas surface cooling decreases the temperature in the region that contains the maximum tracer concentration.
In the standard case, the mixed layer is deep (shallow) in the northern (southern) part of the subtropical gyre. Between these two regions a mixed layer front, where the mixed layer depth changes drastically from north to south, exists. A water column with low potential vorticity that originates in the mixed layer penetrates into a subsurface layer from the point where an outcrop line and the mixed layer front intersect. This point is called the penetration point.
Intensified westerly winds bring about a deeper thermocline and shoaling subsurface isopycnals. These shoaling subsurface isopycnals are not predicted in classical models such as that of Luyten et al. The model experiment with intensified westerlies demonstrates that the penetration point shifts to the west. As a result, low potential vorticity water penetrates southwestward from the shifted penetration point and takes a more westward path. Therefore, the negative temperature anomalies appear in the southwestern part of the subtropical gyre. This study shows that the westward shift of the path of low potential vorticity water could cause the shoaling of subsurface isopycnal surfaces.
The intensification of the westerlies increases Ekman pumping and cools the ocean surface by enhancing sensible and latent heat flux. In the surface cooling experiment, the position of the outcrop lines moves southward significantly. This southward shift makes the subducted water colder and distributes it throughout the ventilated region of the southern part of the subtropical gyre.
The combined effect of wind intensification and surface cooling is approximately a linear combination of both experiments.
Abstract
Influence of sea-ice extent anomalies within the Sea of Okhotsk on the large-scale atmospheric circulation is investigated through an analysis of the dynamic and thermodynamic characteristics of the response in an atmospheric general circulation model to specified anomalous sea-ice cover. Significant response appears not only around the Sea of Okhotsk, but also downstream over the Bering Sea, Alaska, and North America in the form of a stationary wave train in the troposphere. This remote response, associated with wave activity flux emanating from the Okhotsk area to the downstream, is regarded as a stationary Rossby wave generated thermally by the anomalous turbulent heat fluxes from the ocean surface as a result of the anomalous sea-ice cover. The Pacific storm track in the model that extends zonally at 35°N is located too far south of the Sea of Okhotsk to exert substantial feedback forcing on the local and remote response. Since a similar stationary wave train is identified in the composite difference fields of the observed data between heavy and light ice years, it is believed that the model appropriately reproduces the real atmospheric response to the Okhotsk sea-ice extent anomalies. Simulated seesaws in the meridional surface wind and surface air temperature anomalies between the eastern Sea of Okhotsk and eastern Bering Sea associated with the local and remote response, respectively, to the Okhotsk sea-ice anomalies seem to be consistent with the observed seesaw in the anomalous sea-ice cover between these maritime regions. There is a hint of reinforcement of the remote response around the Alaskan Pacific coast through destabilization of barotropic Rossby waves due to the thermal damping effect associated with the anomalous atmosphere–ocean heat exchange both in the model and real atmosphere.
Abstract
Influence of sea-ice extent anomalies within the Sea of Okhotsk on the large-scale atmospheric circulation is investigated through an analysis of the dynamic and thermodynamic characteristics of the response in an atmospheric general circulation model to specified anomalous sea-ice cover. Significant response appears not only around the Sea of Okhotsk, but also downstream over the Bering Sea, Alaska, and North America in the form of a stationary wave train in the troposphere. This remote response, associated with wave activity flux emanating from the Okhotsk area to the downstream, is regarded as a stationary Rossby wave generated thermally by the anomalous turbulent heat fluxes from the ocean surface as a result of the anomalous sea-ice cover. The Pacific storm track in the model that extends zonally at 35°N is located too far south of the Sea of Okhotsk to exert substantial feedback forcing on the local and remote response. Since a similar stationary wave train is identified in the composite difference fields of the observed data between heavy and light ice years, it is believed that the model appropriately reproduces the real atmospheric response to the Okhotsk sea-ice extent anomalies. Simulated seesaws in the meridional surface wind and surface air temperature anomalies between the eastern Sea of Okhotsk and eastern Bering Sea associated with the local and remote response, respectively, to the Okhotsk sea-ice anomalies seem to be consistent with the observed seesaw in the anomalous sea-ice cover between these maritime regions. There is a hint of reinforcement of the remote response around the Alaskan Pacific coast through destabilization of barotropic Rossby waves due to the thermal damping effect associated with the anomalous atmosphere–ocean heat exchange both in the model and real atmosphere.
Abstract
The seasonal dependence and life cycle of the well-known interannual seesawlike oscillation between the intensities of the surface Aleutian and Icelandic lows (AL and IL, respectively) are investigated, based on the National Meteorological Center operational analyses for the period from 1973 to 1994. It is found that the correlation between the AL and IL intensities is significantly negative only from February to mid-March. It is also found that the seesaw exhibits an equivalent barotropic structure within the troposphere. For this late-winter period an index is defined that measures the intensity difference between the two lows. A linear lag regression analysis between this index and circulation anomalies averaged in each of the nine 45-day periods from early winter to midspring reveals that the stationary AL and IL anomalies constituting the seesaw do not start developing simultaneously over the respective ocean basins in the course of a particular winter season. Rather, the seesaw formation is initiated by the amplification of the AL anomalies with wave-activity accumulation in early through midwinter. In midwinter, part of the wave activity accumulated over the North Pacific propagates across North America in the form of a stationary Rossby wave train, which appears to trigger the formation of stationary anomalies over the North Atlantic. The IL anomalies thus initiated amplify and then become matured by late winter through the persistent feedback forcing from migratory eddies around the Atlantic storm track, while the AL anomalies remain strong until late winter through the continual feedback forcing from the Pacific storm track. It is suggested that interannual variability in the IL intensity for late winter tends to be strongly influenced by the AL anomalies that develop over the North Pacific in early through midwinter. The AL–IL seesaw is robust in a sense that it is apparent even after the influence of El Niño–Southern Oscillation is statistically removed from the data, suggestive of the importance of midlatitude processes in the seesaw formation.
Abstract
The seasonal dependence and life cycle of the well-known interannual seesawlike oscillation between the intensities of the surface Aleutian and Icelandic lows (AL and IL, respectively) are investigated, based on the National Meteorological Center operational analyses for the period from 1973 to 1994. It is found that the correlation between the AL and IL intensities is significantly negative only from February to mid-March. It is also found that the seesaw exhibits an equivalent barotropic structure within the troposphere. For this late-winter period an index is defined that measures the intensity difference between the two lows. A linear lag regression analysis between this index and circulation anomalies averaged in each of the nine 45-day periods from early winter to midspring reveals that the stationary AL and IL anomalies constituting the seesaw do not start developing simultaneously over the respective ocean basins in the course of a particular winter season. Rather, the seesaw formation is initiated by the amplification of the AL anomalies with wave-activity accumulation in early through midwinter. In midwinter, part of the wave activity accumulated over the North Pacific propagates across North America in the form of a stationary Rossby wave train, which appears to trigger the formation of stationary anomalies over the North Atlantic. The IL anomalies thus initiated amplify and then become matured by late winter through the persistent feedback forcing from migratory eddies around the Atlantic storm track, while the AL anomalies remain strong until late winter through the continual feedback forcing from the Pacific storm track. It is suggested that interannual variability in the IL intensity for late winter tends to be strongly influenced by the AL anomalies that develop over the North Pacific in early through midwinter. The AL–IL seesaw is robust in a sense that it is apparent even after the influence of El Niño–Southern Oscillation is statistically removed from the data, suggestive of the importance of midlatitude processes in the seesaw formation.
Abstract
In this study, the authors focused on the seasonal variations of precipitation properties over the western Pacific, particularly those associated with the wind direction of the monsoon. An observational project over Peleliu Island in the Republic of Palau was carried out, and data on precipitation, equivalent cloud amount, and precipitable water were collected from 28 June 2001 to 30 April 2002. First, the monsoon season over Palau was defined as a period with 850-hPa zonal-wind sounding data with sustained winds exceeding 5 m s−1. The westerly wind regime continued until 25 November 2001, and the next westerly wind regime began on 18 May 2002. The equivalent cloud amount increased during the period when the westerly wind intensified. The precipitation had a diurnal variation in the active phase of the westerly wind regime, increasing from nighttime to early morning and decreasing in the afternoon. The diurnal variation was weak in the inactive phase and had a lesser afternoon maximum. Precipitation intensity was high and its duration was short during the westerly wind regime.
The precipitable water decreased during the easterly wind regime when a dry period appeared, and precipitation was also suppressed during those days. However, there was little difference between the precipitation amounts of the westerly and easterly wind regimes. The equivalent cloud amount did not decrease as the zonal-wind direction changed to easterlies during the easterly wind regime. The authors noticed no diurnal variation of precipitation during the easterly wind regime. These differences in the precipitation properties during westerlies and easterlies may be related to the seasonal variation of humidity in the environment.
Abstract
In this study, the authors focused on the seasonal variations of precipitation properties over the western Pacific, particularly those associated with the wind direction of the monsoon. An observational project over Peleliu Island in the Republic of Palau was carried out, and data on precipitation, equivalent cloud amount, and precipitable water were collected from 28 June 2001 to 30 April 2002. First, the monsoon season over Palau was defined as a period with 850-hPa zonal-wind sounding data with sustained winds exceeding 5 m s−1. The westerly wind regime continued until 25 November 2001, and the next westerly wind regime began on 18 May 2002. The equivalent cloud amount increased during the period when the westerly wind intensified. The precipitation had a diurnal variation in the active phase of the westerly wind regime, increasing from nighttime to early morning and decreasing in the afternoon. The diurnal variation was weak in the inactive phase and had a lesser afternoon maximum. Precipitation intensity was high and its duration was short during the westerly wind regime.
The precipitable water decreased during the easterly wind regime when a dry period appeared, and precipitation was also suppressed during those days. However, there was little difference between the precipitation amounts of the westerly and easterly wind regimes. The equivalent cloud amount did not decrease as the zonal-wind direction changed to easterlies during the easterly wind regime. The authors noticed no diurnal variation of precipitation during the easterly wind regime. These differences in the precipitation properties during westerlies and easterlies may be related to the seasonal variation of humidity in the environment.
Abstract
Tropical instability waves (TIWs), with a typical wavelength of 1000 km and period of 30 days, cause the equatorial front to meander and result in SST variations on the order of 1°–2°C. Vertical soundings of temperature, humidity, and wind velocity were obtained on board a Japanese research vessel, which sailed through three fully developed SST waves from 140° to 110°W along 2°N during 21–28 September 1999. A strong temperature inversion is observed throughout the cruise along 2°N, capping the planetary boundary layer (PBL) that is 1–1.5 km deep. Temperature response to TIW-induced SST changes penetrates the whole depth of the PBL. In response to an SST increase, air temperature rises in the lowest kilometer and shows a strong cooling at the mean inversion height. As a result, this temperature dipole is associated with little TIW signal in the observed sea level pressure (SLP).
The cruise mean vertical profiles show a speed maximum at 400–500 m for both zonal and meridional velocities. SST-based composite profiles of zonal wind velocity show weakened (intensified) vertical shear within the PBL that is consistent with enhanced (reduced) vertical mixing, causing surface wind to accelerate (decelerate) over warm (cold) SSTs. Taken together, the temperature and wind soundings indicate the dominance of the vertical mixing over the SLP-driving mechanism. Based on the authors' measurements, a physical interpretation of the widely used PBL model proposed by Lindzen and Nigam is presented.
Abstract
Tropical instability waves (TIWs), with a typical wavelength of 1000 km and period of 30 days, cause the equatorial front to meander and result in SST variations on the order of 1°–2°C. Vertical soundings of temperature, humidity, and wind velocity were obtained on board a Japanese research vessel, which sailed through three fully developed SST waves from 140° to 110°W along 2°N during 21–28 September 1999. A strong temperature inversion is observed throughout the cruise along 2°N, capping the planetary boundary layer (PBL) that is 1–1.5 km deep. Temperature response to TIW-induced SST changes penetrates the whole depth of the PBL. In response to an SST increase, air temperature rises in the lowest kilometer and shows a strong cooling at the mean inversion height. As a result, this temperature dipole is associated with little TIW signal in the observed sea level pressure (SLP).
The cruise mean vertical profiles show a speed maximum at 400–500 m for both zonal and meridional velocities. SST-based composite profiles of zonal wind velocity show weakened (intensified) vertical shear within the PBL that is consistent with enhanced (reduced) vertical mixing, causing surface wind to accelerate (decelerate) over warm (cold) SSTs. Taken together, the temperature and wind soundings indicate the dominance of the vertical mixing over the SLP-driving mechanism. Based on the authors' measurements, a physical interpretation of the widely used PBL model proposed by Lindzen and Nigam is presented.
