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Y.-L. Chang and L.-Y. Oey
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Y.-L. Chang and L.-Y. Oey

Abstract

The North Pacific Subtropical Countercurrent (STCC) has a weak eastward velocity near the surface, but the region is populated with eddies. Studies have shown that the STCC is baroclinically unstable with a peak growth rate of 0.015 day−1 in March, and the ~60-day growth time has been used to explain the peak eddy kinetic energy (EKE) in May observed from satellites. It is argued here that this growth time from previously published normal-mode instability analyses is too slow. Growth rates calculated from an initial-value problem without the normal-mode assumption are found to be 1.5 to 2 times faster and at shorter wavelengths, due to the existence of (i) nonmodal solutions and (ii) sea surface temperature front in the mixed layer in winter. At interannual time scales it is shown that because of rapid surface adjustments, the STCC geostrophic shear, hence also the instability growth, is approximately in phase with surface forcing, leading to EKE modulation that peaks approximately 10 months later. However, the EKE can only be partially explained by this mechanism of modulation by baroclinic instability. It is suggested that the unexplained variance may be caused additionally by modulation of the EKE by dissipation.

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Y.-L. Chang and L.-Y. Oey

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It is first shown that wind in the Gulf of Mexico can delay the shedding of Loop Current eddies. A time-dependent, three-dimensional numerical experiment forced by a spatially and temporally constant westward wind stress within the Gulf is analyzed and then is compared with an otherwise identical no-wind run, and the result is confirmed with reduced-gravity experiments. It is shown that the wind produces westward transports over the northern and southern shelves of the Gulf, convergence in the west, and a returned (i.e., eastward) upper-layer flow over the deep central basin toward the Loop Current. The theory from T. Pichevin and D. Nof is then used to explain that the returned flow constitutes a zonal momentum flux that delays eddy shedding. Mass-balance analysis shows that wind also forces larger Loop Current and rings (because the delayed shedding allows more mass to be accumulated in them) and produces more efficient mass exchange between the Gulf and the Caribbean Sea. It is shown that eddies alone (without wind stress curl) can force a boundary current and downward flow in the western Gulf and a corresponding deep flow from the western Gulf to the eastern Gulf.

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Y.-L. Chang and L.-Y. Oey

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Tide gauge and satellite data reveal an interannual oscillation of the ocean’s thermoclines east of the Philippines and Taiwan, forced by a corresponding oscillation in the wind stress curl. This so-called Philippines–Taiwan Oscillation (PTO) is shown to control the interannual variability of the circulation of the subtropical and tropical western North Pacific. The PTO shares some characteristics of known Pacific indices, for example, Niño-3.4. However, unlike PTO, these other indices explain only portions of the western North Pacific circulation. The reason is because of the nonlinear nature of the forcing in which mesoscale (ocean) eddies play a crucial role. In years of positive PTO, the thermocline east of the Philippines rises while east of Taiwan it deepens. This results in a northward shift of the North Equatorial Current (NEC), increased vertical shear of the Subtropical Countercurrent (STCC)/NEC system, increased eddy activity dominated by warm eddies in the STCC, increased Kuroshio transport off the northeastern coast of Taiwan into the East China Sea, increased westward inflow through Luzon Strait into the South China Sea, and cyclonic circulation and low sea surface height anomalies in the South China Sea. The reverse applies in years of negative PTO.

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Y-L. Chang and L-Y. Oey

Abstract

The Gulf of Mexico (GOM) receives heat from the Caribbean Sea via the Yucatan–Loop Current (LC) system, and the corresponding ocean heat content (OHC) is important to weather and climate of the continental United States. However, the mechanisms that affect this heat influx and how it is distributed in the Gulf have not been studied. Using the Princeton Ocean Model, the authors show that a steady, uniform westward wind in the Gulf increases (∼100 KJ cm−2) the upper OHC (temperature T > 18°C) of the Gulf. This is because wind increases the water exchange between the Gulf and the Caribbean Sea, and the heat input into the Gulf is also increased, by about 50 TW. The westward heat transport to the western Gulf is ∼30 TW, and a substantial portion of this is due to wind-induced shelf currents, which converge to produce downwelling near the western coast. Finally, eddies are effective transporters of heat across the central Gulf. Wind forces larger LC and rings with deeper isotherms. This and downfront-wind mixing on the southern side of anticyclonic rings, northward spread of near-zero potential vorticity waters, and downwelling on the northern shelf break result in wide and deep eddies that transport large OHCs across the Gulf.

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Y.-L. Chang and L.-Y. Oey

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Air–sea coupling in the IntraAmerican seas (IAS; Caribbean Sea and Gulf of Mexico) is studied through analyses of observational data from satellite, reanalysis products, and in situ measurements. A strong coupling is found between the easterly trade wind −U and meridional SST gradient ∂T/∂y across a localized region of the southern-central Caribbean Sea from seasonal and interannual to decadal time scales. The ∂T/∂y anomaly is caused by a variation in the strength of coastal upwelling off the Venezuelan coast by the wind, which in turn strengthens (weakens) for stronger (weaker) ∂T/∂y. Wind speeds and seasonal fluctuations in IAS have increased in the past two decades with a transition near 1994 coinciding approximately with when the Atlantic multidecadal oscillation (AMO) turned from cold to warm phases. In particular, the seasonal swing from summer's strong to fall's weak trade wind has become larger. The ocean's upper-layer depth has also deepened, by as much as 50% on average in the eastern Gulf of Mexico. These conditions favor the shedding of eddies from the Loop Current, making it more likely to shed at a biannual frequency, as has been observed from altimetry data.

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Y.-L. Chang and L.-Y. Oey

Abstract

Although the upper-layer dynamics of the Loop Current and eddies in the Gulf of Mexico are well studied, the understanding of how they are coupled to the deep flows is limited. In this work, results from a numerical model are analyzed to classify the expansion, shedding, retraction, and deep-coupling cycle (the Loop Current cycle) according to the vertical mass flux across the base of the Loop. Stage A is the “Loop reforming” period, with downward flux and deep divergence under the Loop Current. Stage B is the “incipient shedding,” with strong upward flux and deep convergence. Stage C is the “eddy migration,” with waning upward flux and deep throughflow from the western Gulf into the Yucatan Channel. Because of the strong deep coupling between the eastern and western Gulf, the Loop’s expansion is poorly correlated with deep flows through the Yucatan Channel. Stage A is longest and the mean vertical flux under the Loop Current is downward. Therefore, because the net circulation around the abyssal basin is zero, the abyssal gyre in the western Gulf is cyclonic. The gyre’s strength is strongest when the Loop Current is reforming and weakest after an eddy is shed. The result suggests that the Loop Current cycle can force a low-frequency [time scales ∼ shedding periods; O(months)] abyssal oscillation in the Gulf of Mexico.

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L.-Y. Oey, Y.-L. Chang, Y.-C. Lin, M.-C. Chang, S. Varlamov, and Y. Miyazawa

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In winter, a branch of the China Coastal Current can turn in the Taiwan Strait to join the poleward-flowing Taiwan Coastal Current. The associated cross-strait flows have been inferred from hydrographic and satellite data, from observed abundances off northwestern Taiwan of cold-water copepod species Calanus sinicus and, in late March of 2012, also from debris found along the northwestern shore of Taiwan of a ship that broke two weeks earlier off the coast of China. The dynamics related to such cross flows have not been previously explained and are the focus of this study using analytical and numerical models. It is shown that the strait’s currents can be classified into three regimes depending on the strength of the winter monsoon: equatorward (poleward) for northeasterly winds stronger (weaker) than an upper (lower) bound and cross-strait flows for relaxing northeasterly winds between the two bounds. These regimes are related to the formation of the stationary Rossby wave over the Changyun Ridge off midwestern Taiwan. In the weak (strong) northeasterly wind regime, a weak (no) wave is produced. In the relaxing wind regime, cross-strait currents are triggered by an imbalance between the pressure gradient and wind and are amplified by the finite-amplitude meander downstream of the ridge where a strong cyclone develops.

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Y. J. Lin and P. T. Chang

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A three-dimensional severe thunderstorm model is employed to study some effects that shearing and veering environmental winds exert on the structure and internal dynamics of a typical supercell rotating storm during its quasi-steady mature stage. These environmental winds are analytically formulated to conform with observations deduced from seven well-documented supercell storms. Horizontal relative winds are generated using the Rankine vortex concept for the inner core region (radii 0–4 km), the potential flow concept for the outer portion (radii 8–25 km) and the transition zone in between (radii 4–8 km). The temperature field is considered to conform with the observed warm-core structured storm. Using these semi-realistic data as input, six numerical experiments are conducted allowing the environmental wind to veer and to shear systematically from one case to another. Vertical velocities are obtained by solving the scaled mass continuity equation. Values of total pressure and perturbation pressure are computed from the diagnostic pressure equation obtained from the horizontal momentum equations using the sequential relaxation method. Results show that fields of perturbation pressure and vertical velocity are quite sensitive to the veering and shearing environmental wind in the region surrounding the central updraft core. Specifically, pronounced upward and downward motions are found on the right and left flank of a storm's updraft core, respectively. The magnitude of these induced vertical velocities increases in proportion to the vertical wind shear and is found to be closely related to perturbation pressure gradients. These findings are in good qualitative agreement with observational evidence reported in the literature. The role of these perturbation pressure forces in protecting the storm's main updraft is emphasized.

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Y. K. Sasaki and L. P. Chang

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In a diagnostic study by expanding global data in normal mode functions, Kasahara and Puri found that for zonal wavenumber one, even the seventh vertical mode (the highest mode they presented) contains about 50% of the energy of the external mode. The vertical normal modes are eigensolutions of the vertical structure equation, and each mode is associated with well‐defined physical significance. Consequently, it is of interest to look into the accuracy of representation of, say, the first ten vertical modes in a discretized model because seriously misrepresented normal mode functions may not be able to honestly express the physics embedded in the data to be expanded. Along this line, a systematic method of obtaining matching eigensolutions of the vertical structure equation of a multilayered stratified atmosphere was developed. The resultant eigensolutions were used to investigate the influence of the upper boundary condition, the judicious method of the vertical grid levels and the relative accuracy of a finite‐difference and a finite‐element method in obtaining the discretized vertical normal mode functions. An important conclusion of this study is that in a discretized model, an inadequate grid resolution in the upper domain may result in considerable misrepresentation of the vertical structure functions even in the lower part of the domain for vertical modes higher than mode 5.

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