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  • Author or Editor: Shang-Ping Xie x
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Shang-Ping Xie

Abstract

The interaction between the annual and interannual variations is investigated by contrasting a pair of experiments with a general circulation model of the tropical Pacific Ocean. The atmospheric forcing applied to the model includes both annual and interannual components The phase of the annual forcing is shifted one-half year in the two runs, which are otherwise identical. Significant differences are found in the sea surface temperature (SST) evolution between the two runs that have the same interannual forcing function. SST anomalies tend to be phase-locked to the solar calendar and to appear in the cold season. A SST variance function in response to interannual forcings with a random phase distribution is constructed, which has an annual cycle and reaches its maximum in the cold season as is observed. It is suggested that this seasonality of an ocean origin is amplified by the interaction with the atmosphere, leading to the observed phase-locking.

The phase-locking of the interannual cycle and the interannual variation of the annual cycle in SST are two manifestations of the interaction between the annual and interannual cycles. A simple conceptual model is proposed to explain these two features, in which strong interaction between the annual and interannual cycles occurs through the nonlinearity associated with the thermocline depth change and upwelling.

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Tangdong Qu
,
Shang-Ping Xie
,
Humio Mitsudera
, and
Akio Ishida

Abstract

The annual subduction rate in the North Pacific is estimated using five-day outputs from a high-resolution general circulation model (GCM). Two maxima (>200 m yr−1) are found in the western North Pacific: one is responsible for the formation of the subtropical mode water (STMW) and the other for the formation of the central mode water (CMW). A local maximum (>75 m yr−1) is also found in the formation region of the eastern subtropical mode water (ESMW). These results are compared with a calculation using the winter mixed layer depth and annual mean velocity fields to examine the effect of mesoscale eddies. Although the mesoscale eddies do not markedly affect the general subduction pattern, they enhance the annual subduction rate by up to 100 m yr−1 in the formation region of the STMW/CMW, a 34% increase in a regional average (30°–44°N, 140°E–170°W). Further analysis shows that the effects of the mean seasonal cycle and smaller-scale (<30 days) eddies are generally small. The authors suggest that the two peaks in the subduction rate are related to a double-front structure on the intergyre boundary in the western North Pacific.

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Takahiro Endoh
,
Humio Mitsudera
,
Shang-Ping Xie
, and
Bo Qiu

Abstract

The Miami Isopycnic Coordinate Ocean Model configured with 1° horizontal resolution and 23 layers is used to examine processes that maintain the mesothermal structure, a subsurface temperature inversion, in the subarctic North Pacific. The model successfully reproduces the mesothermal structure consisting of a shallow temperature minimum and an underlying temperature maximum that are called the dichothermal and mesothermal waters, respectively. The mesothermal water is formed through cross-gyre exchange between the subtropical and subarctic gyres, whereas the dichothermal water originates from cold and low-salinity waters formed in the winter mixed layer. The horizontal distribution of the passive tracer injected into the subsurface layers south of Japan shows that warm and saline water of the Kuroshio in the density range of 26.8–27.0 σ θ is the source of the mesothermal water. There are three pathways through which the Kuroshio waters enter the subarctic region. First, the Kuroshio waters that cross the gyre boundary in the western boundary region are carried to the Alaskan gyre by the northern part of the North Pacific Current. Second, the Kuroshio waters carried by the southern part of the North Pacific Current enter the Alaskan gyre through a cross-gyre window in the eastern basin. Third, the Kuroshio waters that diffuse along the isopycnal in the Kuroshio–Oyashio Extension enter the western subarctic gyre. The mesothermal water thus formed in the subarctic region is entrained into the winter mixed layer and returns to the subtropics as surface water by the southward Ekman drift, forming the subpolar cell.

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Masami Nonaka
,
Julian P. McCreary Jr.
, and
Shang-Ping Xie

Abstract

The stratification of the equatorial thermocline is a key variable for tropical climate dynamics, through its influence on the temperature of the water that upwells in the eastern equatorial ocean. In this study, two types of ocean models are used, an ocean general circulation model (GCM) and a 1½-layer model, to investigate processes by which changes in the midlatitude winds affect the equatorial stratification. Specifically, the influences of anomalous mode-water formation, Ekman pumping, and entrainment in the subpolar ocean are examined. The effects of a “sponge layer” adjacent to the northern boundary of the basin are also assessed. Solutions are forced by idealized zonal winds with strong or weak midlatitude westerlies, and they are found in rectangular basins that extend from the equator to 36°N (small basin) or to 60°N (large basin). In the GCM solutions, a prominent response to reduced winds is the thinning of the mixed layer in the northwestern region of the subtropical gyre, leading to less subduction of low-potential-vorticity mode water and hence thinning of the upper thermocline in the central-to-eastern subtropics. Almost all of this thinning signal, however, recirculates within the subtropics, and does not extend to the equator. Another midlatitude response is shallowing (deepening) of the thermocline in the subtropical (subpolar) ocean in response to Ekman pumping. This, primarily, first-baroclinic-mode (n = 1) response has the most influence on the equatorial thermocline. First-baroclinic-mode Rossby waves propagate to the western boundary of the basin where they reflect as packets of coastal Kelvin and short-wavelength Rossby waves that carry the midlatitude signal to the equator. Subsequently, equatorial Kelvin waves spread it along the equator, leading to a shoaling and thinning of the equatorial thermocline. The layer-thickness field h in the 1½-layer model corresponds to thermocline depth in the GCM. Both the sponge layer and subpolar Ekman suction are important factors for the 1½-layer model solutions, requiring water upwelled in the interior ocean to be transported into the sponge layer via the western boundary layer. In the small basin, equatorial h thins in response to weakened westerlies when there is a sponge layer, but it thickens when there is not. In the large basin, equatorial h is unaffected by weakened westerlies when there is a sponge layer, but it thins when water is allowed to entrain into the layer in the subpolar gyre. It is concluded that the thinning of the equatorial thermocline in the GCM solutions is caused by the sponge layer in the small basin and by entrainment in the subpolar ocean in the large one.

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Minyang Wang
,
Yan Du
,
Bo Qiu
,
Shang-Ping Xie
, and
Ming Feng

Abstract

Energetic mesoscale eddies (vortices) associated with tropical instability waves (TIWs) exist in the eastern equatorial Pacific Ocean between 0° and 8°N. This study examines the seasonal variations in eddy kinetic energy (EKE) of TIWs using in situ and satellite observations and elucidates the underlying dynamical mechanisms. The results reveal that the cross-equatorial southerly winds are key to sustaining the high-level EKE (up to ~600 cm2 s−2) from boreal summer to winter in 0°–6°N and 155°–110°W. Because of the β effect and the surface wind divergence, the southerly winds generate anticyclonic wind curls north of the equator that intensify the sea surface temperature (SST) fronts and force the downwelling annual Rossby waves. The resultant sea surface height ridge induces strong horizontal current shears between 0° and 5°N. The intensified current shears and SST fronts generate EKE via barotropic and baroclinic instabilities, respectively. To the extent that the seasonal migration of a northward-displaced intertropical convergence zone intensifies the southerly winds north of, but not south of, the equator, our study suggests that the climatic asymmetry is important for the oceanic eddy generations in the eastern equatorial Pacific Ocean—a result with important implications for coupled climate simulation/prediction.

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Shang-Ping Xie
,
Tatsuga Kunitani
,
Atsushi Kubokawa
,
Masami Nonaka
, and
Shigeki Hosoda

Abstract

An ocean general circulation model is forced with the NCEP reanalysis wind stress for 1958–97 to understand mechanisms of ocean subsurface variability. With relatively high horizontal (1° × 1°) and vertical (41 levels) resolutions, the model produces mode waters on a range of density surfaces in the western, central, and eastern North Pacific, in qualitative agreement with observations.

These mode waters appear as a thermostad or a region of weak stratification in the upper thermocline as they flow southward from their formation regions in the Kuroshio and its extension. In the model, subsurface temperature variability in the central subtropical gyre reaches a maximum within the thermostad, in contrast to what might be expected from the linear baroclinic Rossby wave theory. This variance maximum is associated with the longitudinal shift in the path of mode waters. In particular, deepened mixed layer and accelerated eastward currents in the Kuroshio Extension by wind changes in the mid-1970s act cooperatively to move the central mode waters toward the east, causing large subsurface temperature anomalies.

Besides the local maximum in the central North Pacific subtropical gyre, two additional maxima of the subsurface anomaly are identified in the northwestern and southern parts of the gyre, respectively. Among these subsurface anomaly centers, the one in the northwestern North Pacific has a strong effect on the model sea surface temperature, suggesting that the Kuroshio and its extension are a key region to decadal/interdecadal ocean–atmosphere interaction. Finally, possible effects of atmospheric thermal forcing are discussed.

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Minyang Wang
,
Shang-Ping Xie
,
Samuel S. P. Shen
, and
Yan Du

Abstract

Mesoscale activities over the equatorial Pacific Ocean are dominated by the Rossby and Yanai modes of tropical instability waves (TIWs). The TIW-induced surface velocity has not been accurately estimated in previous diagnostic models, especially for the meridional component across the equator. This study develops a diagnostic model that retains the acceleration terms to estimate the TIW surface velocity from the satellite-observed sea surface height. Validated against moored observations, the velocity across the equator is accurately estimated for the first time, much improved from existing products. The results identify the Rossby- and Yanai-mode TIWs as the northwest–southeastward (NW–SE) velocity oscillations north of the equator and the northeast–southwestward (NE–SW) velocity oscillations on the equator, respectively. Barotropic instability is the dominant energy source of the two TIW modes. The NE–SW velocity oscillation of the Yanai mode is associated with the counterclockwise shear of the South Equatorial Current on the equator. The two TIW modes induce different sea surface temperature patterns and vertical motions. Accurate estimates of TIW velocity are important for studying equatorial ocean dynamics and climate variability in the tropical Pacific Ocean.

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