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Bo Qiu

Abstract

A forcing mechanism is sought for the large-scale circulation changes in the Kuroshio Extension region of the western North Pacific Ocean as inferred by TOPEX/Poseidon sea surface height (SSH) data. The low-frequency signal of the Kuroshio Extension over the last decade was characterized by a modulation in its zonal mean flow intensity: the mean Kuroshio Extension jet weakened progressively from 1993 to 1996 and this trend reversed after 1997. The ability to simulate the major trends in the observed SSH signals with linear vorticity dynamics leads the authors to conclude that the modulation in the zonal mean jet was remotely forced by wind stress curl anomalies in the eastern North Pacific Ocean related to the Pacific decadal oscillations (PDOs). To be specific, the weakening (strengthening) trend in 1993–96 (1997–2001) was caused by westward expansions of negative (positive) SSH anomalies south of the Kuroshio Extension and positive (negative) SSH anomalies north of the Kuroshio Extension. Emergence of oppositely signed SSH anomalies on the two sides of the Kuroshio Extension jet is due to the different propagating speeds of the baroclinic Rossby waves, which carry the wind-induced SSH anomalies generated in the eastern North Pacific at different phases of the PDOs. Hindcasting the Kuroshio Extension jet strength over the last 45 years reveals that the jet modulation has a dominant timescale of ∼12 yr. Given the location of the Kuroshio Extension jet relative to the maximum atmospheric forcing, it is found that this dominant timescale is consistent with the preferred timescale under a stochastic white-noise atmospheric forcing. It is hypothesized that this connection between the Kuroshio Extension strength and the latitudinally dependent baroclinic adjustment contributes to an increase in variance and persistence of the North Pacific midlatitude coupled system on the decadal timescale.

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Bo Qiu

Abstract

The North Pacific Intermediate Water, characterized by a salinity minimum confined to density surfaces of σθ = 26.7–26.9, exists throughout the subtropical gyre and has been observed to originate in the subarctic North Pacific. The physical processes that determine the density range on which the NPIW resides are not yet well understood. This study attempts to clarify these processes by combining observational data and a simple advection–diffusion isopycnal model. Due to the regional excessive precipitation over evaporation, the salinity in the upper-layer subarctic North Pacific generally decreases with decreasing water depth. Both alongisopycnal advection and diffusion work to carry this salinity/depth characteristic into the subtropical circulation. For the isopycnal surfaces overlying the NPIW, however, this transport mechanism is hindered by the seasonal outcropping. The outcropping not only blocks the fresh subarctic water from advecting and diffusing along these isopycnals into the subtropical gyre, but also results in shoaling of the isopycnals in the Kuroshio–Oyashio mixed water region, where the turbulent mixing in the deep winter mixed layer is able to conduit the surface salt flux into these outcropping isopycnal surfaces. This seasonal forcing creates a high-salinity overlying layer, leaving σθ = 26.7–26.9 the lightest density surfaces that are free to transport the uppermost (i.e., the freshest) subarctic water into the subtropical North Pacific. This model result is consistent with high-resolution CTD observations that showed that σθa = 26.7–26.9 are the least dense isopycnal surfaces on which the alongisopycnal potential vorticity is homogenized. The NPIW surfaces contrast with the shallower isopycnal surfaces where strong potential vorticity gradients exist.

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Bo Qiu

Abstract

Altimetric data from the 8-yr TOPEX/Poseidon (T/P) mission (Oct 1992–Jul 2000) are used to investigate large-scale circulation changes in the three current systems of the midlatitude North Pacific Ocean: the North Pacific Current (NPC), the Alaska gyre, and the western subarctic gyre (WSG). To facilitate the understanding of the observed changes, a two-layer ocean model was adopted that includes first-mode baroclinic Rossby wave dynamics and barotropic Sverdrup dynamics. The NPC intensified steadily over the T/P period from 1992 to 1998. Much of this intensification is due to the persistent sea surface height (SSH) drop on the northern side of the NPC. A similar SSH trend is also found in the interior of the Alaska gyre. Both of these SSH changes are shown to be the result of surface wind stress curl forcing accumulated along the baroclinic Rossby wave characteristics initiated from the eastern boundary. In addition to the interior SSH signals, the intensity of the Alaska gyre is shown to depend also on the SSH anomalies along the Canada/Alaska coast, and these anomalies are shown to be jointly determined by the signals propagating from lower latitudes and those forced locally by the alongshore surface winds. The WSG changed interannually from a zonally elongated gyre in 1993–95 to a zonally more contracted gyre in 1997–99. This structural change is due to the interannual SSH anomalies within the WSG as a result of the baroclinic Rossby wave adjustment attenuated by eddy dissipation. Along the western boundary of the subpolar North Pacific, variability of the East Kamchatka Current (EKC) and Oyashio is in balance with that of the interior Sverdrup flow on the annual and year-to-year timescales. On the multiyear timescales, the EKC/Oyashio variability is shown to be determined by the baroclinic SSH signals.

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Bo Qiu

Abstract

Altimetry data from the first 21-month TOPEX mission (September 1992–June 1994) are analysed to investigate the sea surface height (SSH) fluctuations in the Kuroshio Extension and its southern recirculation gyre regions (25°–40°N, 136°E–180°). To separate the time-dependent (SSH) signals associated with the Kuroshio Extension from those associated with the westward recirculating flows, and to study the energetics of these currents, the author first estimated the mean SSH profiles along individual ground tracks by assuming the velocity profile of the Kuroshio Extension to be Gaussian shaped and by successively fitting the synthetic current's height profile to the time-dependent SSH data. The mean SSH field, including the influence from the recirculating flows, is then derived through the constraint from climatological hydrographic data.

During the 21-month period of the TOPEX mission, the eddy kinetic energy (EKE) of the Kuroshio Extension had relatively uniform values during three separate stages, each of which lasted longer than 6 months. A significant drop in the EKE level is found in the Kuroshio Extension after the end of 1993. In contrast, the EKE level in the southern recirculation region increased steadily over the two-year period. The energetics analysis shows that this EKE increase is due to the energy transfers from the mean flow field to the eddy field through barotropic instabilities. These barotropic eddy processes are found to be less important for the EKE changes in the Kuroshio Extension and its northern areas. On both seasonal and interannual timescales, the large-scale path fluctuations of the Kuroshio Extension are found to correlate significantly with the surface transport fluctuations: a more northerly Kuroshio Extension tends to correspond to a larger surface transport. Over the two-year period, both the eastward-flowing Kuroshio Extension and the westward recirculating flows weakened steadily. This decline in the intensity of the recirculation gyre is caused by the energy transfers from the mean flow field to the eddy field occurring in the region south of the Kuroshio Extension.

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Bo Qiu

Abstract

Altimetry data from the first 5¼-yr TOPEX/Poseidon mission (October 1992–December 1997) are analyzed focusing on the North Pacific Subtropical Countercurrent (STCC) near the center of the Pacific’s western subtropical gyre. The multiyear altimetry data reveal that the eastward-flowing STCC is a highly variable zonal current, whose area-averaged eddy kinetic energy level (338 cm2 s−2) reaches half the eddy kinetic energy level of the Kuroshio Extension. The eddy kinetic energy of the STCC has a well-defined annual cycle with a maximum in April/May and a minimum in December/January. The peak-to-peak amplitude of this seasonal eddy kinetic energy modulation exceeds 200 cm2 s−2. No such distinct annual cycle of the eddy kinetic energy is found in any other zonal current of the North Pacific Ocean. Using a 2½-layer reduced-gravity model representing the vertically sheared STCC–North Equatorial Current (NEC) system, it is shown that the seasonal modulation of the STCC’s eddy field is a manifestation in the intensity of baroclinic instability. In spring the STCC–NEC system has a large vertical velocity shear and a weak vertical stratification, subjecting it to strong baroclinic instability. In fall, reduction in the vertical velocity shear between the STCC and its underlying NEC, and intensification of the upper-layer stratification weakens the baroclinic instability. In comparison with the STCC of 19°–25°N, the altimetry data reveal that the westward-flowing NEC existing between 10° and 15°N has a relatively low eddy kinetic energy level, despite being a stronger vertically sheared zonal current than the STCC. That the NEC is less eddy energetic is shown to be due to both its presence in a low-latitude band and its unidirectional flow. Both of these factors make it more difficult to reverse the potential vorticity gradient of the mean state (i.e., satisfying the necessary condition for the baroclinic instability) in the NEC than in the STCC–NEC system.

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Bo Qiu

Abstract

A forcing mechanism is sought for the large-scale circulation changes in the Kuroshio Extension region of the western North Pacific Ocean as inferred by TOPEX/Poseidon sea surface height (SSH) data. The low-frequency signal of the Kuroshio Extension over the last decade was characterized by a modulation in its zonal mean flow intensity: the mean Kuroshio Extension jet weakened progressively from 1993 to 1996 and this trend reversed after 1997. The ability to simulate the major trends in the observed SSH signals with linear vorticity dynamics leads the authors to conclude that the modulation in the zonal mean jet was remotely forced by wind stress curl anomalies in the eastern North Pacific Ocean related to the Pacific decadal oscillations (PDOs). To be specific, the weakening (strengthening) trend in 1993–96 (1997–2001) was caused by westward expansions of negative (positive) SSH anomalies south of the Kuroshio Extension and positive (negative) SSH anomalies north of the Kuroshio Extension. Emergence of oppositely signed SSH anomalies on the two sides of the Kuroshio Extension jet is due to the different propagating speeds of the baroclinic Rossby waves, which carry the wind-induced SSH anomalies generated in the eastern North Pacific at different phases of the PDOs. Hindcasting the Kuroshio Extension jet strength over the last 45 years reveals that the jet modulation has a dominant timescale of ∼12 yr. Given the location of the Kuroshio Extension jet relative to the maximum atmospheric forcing, it is found that this dominant timescale is consistent with the preferred timescale under a stochastic white-noise atmospheric forcing. It is hypothesized that this connection between the Kuroshio Extension strength and the latitudinally dependent baroclinic adjustment contributes to an increase in variance and persistence of the North Pacific midlatitude coupled system on the decadal timescale.

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Bo Qiu

Abstract

Altimetry data from the first 7-yr TOPEX/Poseidon (T/P) mission (October 1992–September 1999) are analyzed to investigate the interannual variability in regions of the Kuroshio Extension and its southern recirculation gyre. Large-scale, interannual changes of the Kuroshio Extension system are characterized by the oscillation between an elongated state and a contracted state. In the elongated state, the Kuroshio Extension has a larger eastward surface transport, a greater zonal penetration, and a more northerly zonal-mean path. All these characteristics are closely connected to the presence of an intense, zonally elongated southern recirculation gyre. In its contracted state, the Kuroshio Extension has a smaller eastward surface transport, a more southerly mean path, and is accompanied by a weaker southern recirculation gyre. During the T/P period, the Kuroshio Extension changed from the elongated state in 1992–93 to the contracted state in 1996–97, and back to the elongated state since late 1997.

In addition to these large-scale changes, the mesoscale eddy field also exhibited prominent interannual changes in the Kuroshio Extension region. In the upstream region between the Izu Ridge and the Shatsky Rise, the eddy kinetic energy level was generally low while the Kuroshio Extension was in its elongated state, and high while in the contracted state. Downstream of the Shatsky Rise, on the other hand, the EKE level was high (low) when the Kuroshio Extension was in the elongated (contracted) state. The large-scale, interannual changes in the Kuroshio Extension system have a significant impact on the regional wintertime SST anomaly field: the warm (cold) wintertime SST anomalies tend to persist in years when the Kuroshio Extension is in its elongated (contracted) state. A diagnostic analysis of the surface ocean heat balance indicates that the nonseasonal geostrophic advection by the ocean circulation works to reduce (increase) the wintertime SST anomalies when the Kuroshio Extension changes from an elongated (contracted) state to a contracted (elongated) state. The SST anomalies associated with the large-scale changes of the Kuroshio Extension have an area-averaged, peak-to-peak amplitude of >1°C and appear independent of the interannual SST changes in the tropical Pacific Ocean.

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Bo Qiu and Shuiming Chen

Abstract

Twelve years of sea surface height (SSH) data from multiple satellite altimeters are used to investigate the low-frequency changes and the interconnections of the Kuroshio Extension (KE) jet, its southern recirculation gyre, and their mesoscale eddy field. The dominant signal is characterized by the steady weakening of the KE jet/recirculation gyre from 1993 to 1996, followed by a gradual strengthening after 1997. During the weakening period of 1993–96, the KE path migrated southward in general, and this path migration reversed in direction during the strengthening period of the KE jet and recirculation gyre after 1997. By hindcasting the SSH signals using linear vorticity dynamics, it was found that weakening (strengthening) in the KE jet and recirculation gyre is consistent with westward propagation of negative (positive) SSH anomalies generating in the eastern North Pacific and strengthening during their westward propagation. When the KE jet and recirculation gyre were in a weak mode during 1996–2001, the regional eddy kinetic energy level was observed to be higher than when the jet and recirculation gyre were in a strong mode. This negative correlation between the mean flow intensity and the level of regional eddy kinetic energy is found in both the SSH data and the linear vorticity model to result from the migration of the KE jet inflow over the Izu–Ogasawara Ridge. When it is forced southward by the impinging negative SSH anomalies, the KE jet inflow rides over the ridge through a shallow segment, leading to large-amplitude downstream meanders. Impinging of positive SSH anomalies, on the other hand, strengthens the recirculation gyre and forces the inflow northward where it passes through a deep channel, minimizing the path perturbations in the downstream region.

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Bo Qiu and Weifeng Miao

Abstract

In the two decades following 1975, the Kuroshio path south of Japan was observed to oscillate interannually between a straight path state and a meandering state. This bimodal condition is in sharp contrast to the decade prior to 1975 when the Kuroshio remained consistently in the straight path state. In situ observations in the upstream East China Sea indicate no obvious correspondence between the magnitude (or the temporal change) of the inflow transport and the Kuroshio path, suggesting the Kuroshio’s path alternations in the recent two decades may not be externally determined by the upstream inflow. Using a two-layer primitive-equation model of the North Pacific driven by observed climatological surface wind data, the authors propose that the Kuroshio path oscillation since 1975 can be explained by a self-sustained internal mechanism of the Kuroshio current system. The proposed self-sustained oscillation is maintained by the accumulation of the low potential vorticity (PV) water carried northward by the upstream Kuroshio. This accumulation of the low-PV anomalies strengthens the Kuroshio’s southern recirculation gyre and presses the Kuroshio to flow along the coast. The strengthening of the southern recirculation gyre increases the velocity shear of the straight-pathed Kuroshio and leads eventually to the meander path development due to baroclinic/barotropic instability of the system. As the meander of the Kuroshio path grows, detachment of cyclonic eddies mixes coastal-origin, high-PV water offshoreward, weakening, as a result, the recirculation gyre. The above cycle will restart after the low-PV water is replenished from the south and the recirculation gyre spins up again. For the past two decades, this internal oscillation has been operative because the upstream inflow is relatively large due to the decadal increase in the wind-driven Sverdrup transport. In the decade before 1975 when the upstream inflow was small, the internal oscillation would cease because the dissipative forces are able to remove the southern-origin low-PV anomalies without resorting to instability and the meander development.

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Niklas Schneider and Bo Qiu

Abstract

The response of the atmospheric boundary layer to fronts of sea surface temperature (SST) is characterized by correlations between wind stress divergence and the downwind component of the SST gradient and between the wind stress curl and the crosswind component of the SST gradient. The associated regression (or coupling) coefficients for the wind stress divergence are consistently larger than those for the wind stress curl. To explore the underlying physics, the authors introduce a linearized model of the atmospheric boundary layer response to SST-induced modulations of boundary layer hydrostatic pressure and vertical mixing in the presence of advection by a background Ekman spiral. Model solutions are a strong function of the SST scale and background advection and recover observed characteristics. The coupling coefficients for wind stress divergence and curl are governed by distinct physics. Wind stress divergence results from either large-scale winds crossing the front or from a thermally direct, cross-frontal circulation. Wind stress curl, expected to be largest when winds are parallel to SST fronts, is reduced through geostrophic spindown and thereby yields weaker coupling coefficients.

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