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.
Twenty‐nine years of tide‐gauge data are analyzed in conjunction with wind and satellite‐derived sea‐surface height and ocean velocity data to study the interannual and seasonal variations of the ...Kuroshio transport off the northeastern coast of Taiwan. The data reveals an interannual variation of ±0.1 m (transport‐variation of approximately ±3.5 Sv; 1 Sv = 106 m3 s−1), and a much weaker (5–10 times weaker) seasonal fluctuation that is minimum in May and maximum in November. The interannual fluctuations are not directly wind‐driven by linear dynamics; rather, the Kuroshio strengthens in years of abundant eddies of the Subtropical Counter Current, which is related to the current's instability state driven by the slow fluctuations of the large‐scale wind stress curl in the western Pacific. The seasonal transport fluctuation is also eddy‐forced, but has weaker amplitude because the seasonal time scale is of the same order as the eddy‐propagation time scale, and transport‐producing eddy signals tend to overlap east of Taiwan.
Key Points
Interannual Kuroshio variations
Eddy‐driven dynamics
Far‐reaching impacts on western Pacific
The observed seasonal preferences of Loop Current eddy shedding, more in summer and winter and less in fall and spring, are shown for the first time to be due to a curious combination of forcing by ...the seasonal winds in the Caribbean Sea and the Gulf of Mexico. The conditions are favorable for the Loop to shed eddies in summer and winter when strong trade winds in the Caribbean produce large Yucatan transport and Loop's intrusion, and concurrently when weak easterlies in the Gulf offer little impediment to eddy shedding. The conditions are less favorable in fall and spring as the trade winds and Yucatan transport weaken, and the strengthening of the Gulf's easterlies impedes shedding.
Key Points
The observed seasonal preferences of Loop Current eddy shedding, more in summer
The preference is forced, instead of the natural shedding
It is due to the be due to a curious combination of forcing by the seasonal wind
Loop Current and Deep Eddies OEY, L.-Y
Journal of physical oceanography,
07/2008, Letnik:
38, Številka:
7
Journal Article
Recenzirano
Odprti dostop
Abstract
In contrast to the Loop Current and rings, much less is known about deep eddies (deeper than 1000 m) of the Gulf of Mexico. In this paper, results from a high-resolution numerical model of ...the Gulf are analyzed to explain their origin and how they excite topographic Rossby waves (TRWs) that disperse energy to the northern slopes of the Gulf. It is shown that north of Campeche Bank is a fertile ground for the growth of deep cyclones by baroclinic instability of the Loop Current. The cyclones have horizontal (vertical) scales of about 100 km (1000∼2000 m) and swirl speeds ∼0.3 m s−1. The subsequent development of these cyclones consists of two modes, A and B. Mode-A cyclones evolve into the relatively well-known frontal eddies that propagate around the Loop Current. Mode-A cyclone can amplify off the west Florida slope and cause the Loop Current to develop a “neck” that sometimes leads to shedding of a ring; this process is shown to be the Loop Current’s dominant mode of upper-to-deep variability. Mode-B cyclones are “shed” and propagate west-northwestward at speeds of about 2–6 km day−1, often in concert with an expanding loop or a migrating ring. TRWs are produced through wave–eddy coupling originating primarily from the cyclone birthplace as well as from the mode-B cyclones, and second, but for longer periods of 20∼30 days only, also from the mode-A frontal eddies. The waves are “channeled” onto the northern slope by a deep ridge located over the lower slope. For very short periods (≲10 days), the forcing is a short distance to the south, which suggests that the TRWs are locally forced by features that have intruded upslope and that most likely have accompanied the Loop Current or a ring.
Abstract
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.
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.
Abstract
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.
In recent decades, wintertime sea surface temperatures off the eastern coast of China have steadily increased. The warming is accompanied by on‐coast wind convergence across East China Sea and by ...stronger northeasterly wind which is spatially inhomogeneous being greatest in the Taiwan Strait. Strong winds favor more frequent cross‐shelf currents and vigorous spreading of heat from the Kuroshio, which warms the coastal sea in a positive feedback loop. The process also weakens the East Asian winter monsoon over eastern China, contributing to its decoupling from the recent rebound of the Siberian High.
Key Points
Recent warming off coastal China is accompanied by stronger northeasterly wind
Stronger winter monsoon spreads heat from Kuroshio producing air‐sea response
The increased wind is detached from the recent rebound of the Siberian High
Loop Current warming by Hurricane Wilma Oey, L.-Y.; Ezer, T.; Wang, D.-P. ...
Geophysical research letters,
April 2006, Letnik:
33, Številka:
8
Journal Article
Recenzirano
Odprti dostop
Hurricanes mix and cool the upper ocean, as shown here in observations and modeling of the Caribbean Sea and the Gulf of Mexico during the passage of hurricane Wilma. Curiously, the upper ocean ...around the Loop Current warmed prior to Wilma's entrance into the Gulf. The major cause was increased volume and heat transports through the Yucatan Channel produced by storm‐induced convergences in the northwestern Caribbean Sea. Such oceanic variability may have important impacts on hurricane predictions.
Abstract
Recent studies suggest that as the trade wind in the Caribbean Sea weakens from summer to fall, conditions become more favorable for the Loop Current in the Gulf of Mexico to shed an ...anticyclonic ring. This idea originated with observations showing a preference for more eddies from summer through fall, and it was confirmed using multidecadal model experiments. Here, the hypothesis is further tested by studying the dynamics of a specific eddy-shedding event in summer 2011 using a model experiment initialized with observation-assimilated reanalysis and forced by reanalysis wind from NCEP. Eddy shedding in July 2011 is shown to follow the weakening of the trade wind and Yucatan transport in late June. The shedding time is significantly earlier than can be explained based on reduced-gravity Rossby wave dynamics. Altimetry and model data are analyzed to show that empirical orthogonal function modes 1 + 2 dominate the reduced-gravity process, while higher modes contain the coupling of the Loop Current with deep layer underneath. The Loop’s westward expansion at incipient shedding induces a deep cyclonic gyre in the eastern Gulf, embedded within which are small cyclones caused by the baroclinic instability of the strongly sheared current north of the Campeche Bank. The associated deep upwelling and upper-layer divergence from these cyclonic circulations accelerate eddy shedding.