Mark Wimbush

Wind-Induced Kuroshio Intrusion into the South China Sea

The Kuroshio flows north along the east coasts of the Philippines and Taiwan. Between these two land masses lies the Luzon Strait which connects the Pacific Ocean to the South China Sea. The Kuroshio usually flows north past this strait, but at times part or all of it flows west through the strait into the South China Sea forming a loop current. It has been suggested that the loop current forms when the northeast monsoon deflects the Kuroshio through the Luzon Strait. In this study, satellite-derived sea-surface temperature images are used to observe the Kuroshio in the Luzon Strait region. Together with wind data from the region, these observations indicate a loop-current development process which is largely determined by an integrated supercritical wind stress parameter. The loop current grows when a four-day average of the local wind-stress component directed to the south exceeds 0.08 Nm−2. When this average wind-stress component drops below the critical value, the Kuroshio returns to its northward path.

Seasonal Variability of the South Equatorial Current Bifurcation in the Atlantic Ocean: A Numerical Study

Abstract In this study, a reduced-gravity, primitive equation OGCM is used to investigate the seasonal variability of the bifurcation of the South Equatorial Current (SEC) into the Brazil Current (BC) to the south and the North Brazil Undercurrent/Current (NBUC/NBC) system to the north. Annual mean meridional velocity averaged within a 2° longitude band off the South American coast shows that the SEC bifurcation occurs at about 10°–14°S near the surface, shifting poleward with increasing depth, reaching 27°S at 1000 m, in both observations and model. The bifurcation latitude reaches its southernmost position in July (∼17°S in the top 200 m) and its northernmost position in November (∼13°S in the top 200 m). The model results show that most of the seasonal variability of the bifurcation latitude in the upper thermocline is associated with changes in the local wind stress curl due to the annual north–south excursion of the marine ITCZ complex. As the SEC bifurcation latitude moves south (north) the NBUC tra...

Oscillations of Dynamic Topography in the Eastern Equatorial Pacific

Abstract For 14 months in 1980–81, surface dynamic height was monitored with inverted echo sounders at five sites from 0 to 9°N along 110°W. These records show that the SEC/NECC equatorial current system was well-developed during the boreal summer and fall, but weak and irregular during winter and spring when westward flow associated with the NEC extended as far south as 6°N. Superimposed on the mean dynamic topography of this region are energetic 20-to-80-day oscillations, longer periods being associated with higher latitudes. Near the equatorial ridge (∼5°N), these oscillations have predominantly monthly periods, and amplitude of ∼10 dyn cm comparable to the mean dynamic-height difference across the NECC. The broad in-phase meridional extent of these monthly oscillations implies that the principal mode of ridge variation is vertical undulation rather than meridional meandering, producing large in-phase monthly modulations in transport of the SEC and NECC. Oscillations or the equatorial ridge are correla...

Program Studies the Kuroshio Extension

The Kuroshio Extension system links to North Pacific climate through its role in subtropical-subpolar exchange, the formation and distribution of mode waters, and the intensification of the extratropical storm track across the North Pacific. The Kuroshio Extension System Study (KESS) offers a window into these processes through integrated measurements of the ocean and atmosphere and through modeling efforts (Figure 1). The northward flowing waters of the Kuroshio western boundary current leave the Japanese coast to flow eastward as a free jet—the Kuroshio Extension. The Extension forms a vigorously meandering boundary between the warm subtropical and cold northern waters.

The Residual GEM Technique and Its Application to the Southwestern Japan/East Sea

Abstract The standard gravest empirical mode (GEM) technique for utilizing hydrography in concert with integral ocean measurements performs poorly in the southwestern Japan/East Sea (JES) because of a spatially variable seasonal signal and a shallow thermocline. This paper presents a new method that combines the U.S. Navys Modular Ocean Data Assimilation System (MODAS) static climatology (which implicitly contains the mean seasonal signal) with historical hydrography to construct a “residual GEM” from which profiles of such parameters as temperature (T) and specific volume anomaly (δ) can be estimated from measurements of an integral quantity such as geopotential height or acoustic echo time (τ). This is called the residual GEM technique. In a further refinement, sea surface temperature (SST) measurements are included in the profile determinations. In the southwestern JES, profiles determined by the standard GEM technique capture 70% of the T variance and 64% of the δ variance, while the residual GEM tec...

The Dok Cold Eddy

Current and temperature patterns in the Ulleung Basin of the Japan/East Sea are examined using acoustic travel-time measurements from an array of pressure-gauge-equipped inverted echo sounders moored between June 1999 and July 2001. The focus here is the formation and behavior of a persistent cold eddy observed south of Dok Island, referred to as the Dok Cold Eddy (DCE), and meandering of the Subpolar Front. The DCE is typically about 60 km in diameter and originates from the pinching off of a Subpolar Front meander between Ulleung and Dok Islands. After formation, the DCE dwells southwest of Dok Island for 1–6 months before propagating westward toward Korea, where it deflects the path of the East Korean Warm Current (EKWC). Four such DCE propagation events between January and June 2000 each deflected the EKWC, and after the fourth deflection the EKWC changed paths and flowed westward along the Japanese shelf as the “Offshore Branch” from June through November 2000. Beginning in March 2001, a deep, persistent meander of the Subpolar Front developed and oscillated with a period near 60 days, resulting in the deformation and northwestward displacement of the Ulleung Eddy. Satellite-altimeter data suggest that the Ulleung Eddy may have entered the northern Japan/East Sea. The evolution of this meander is compared with thin-jet nonlinear dynamics described by the modified Korteweg–deVries equation.

Using inverted echo sounders to measure dynamic height in the eastern equatorial Pacific during the 1982–1983 El Niño

Abstract We investigate the relationship between the dynamic height anomaly ( ΔD ) and roundtrip acoustic travel time (τ) measured by inverted echo sounders (IES) in the eastern equatorial Pacific. Salinity variations in the upper 50 m have little effect on τ but do affect surface dynamic height, ΔD 0–2500 . Consequently, in the eastern equatorial Pacific it is better to interpret τ as a record of dynamic height below 50 dbar, ΔD 50–2500 . The slope o f the calibration, derived from a regression of ΔD 50–2500 with τ, is −60 dyn m s -1 . This slopeis controlled mainly by the highly energetic El Nino signal, and is approximately 17% steeper than the simulated IES response to the first baroclinic mode. This difference may be explained by the significant second-vertical-mode component of El Nino, but is small since τ is relatively insensitive to the second vertical mode. As a result, purely first-mode signals (which are likely to dominate IES records because of the low second-mode sensitivity) will still be estimated will with the same calibration.

Second-Mode Internal Tides in the East China Sea Deduced from Historical Hydrocasts and a Model

during the two-year deployment period. To obtain temperature (T) and specific volume anomaly (d) profiles from t and sea surface temperature (SST), observed respectively by PIES and satellite, we constructed multi-index gravest empirical mode (MI-GEM) fields [Park et al., 2005] using historical hydrocasts. However, comparison between the MI-GEM fields and the historical hydrocasts reveals that the MI-GEM fields fail to capture 44% and 52% of the T and d variance, respectively, in the p = 100–200-dbar layer. This may be caused by 2nd- or higher-mode variations of vertical structure, since t is insensitive to all but the lowest baroclinic mode [Watts and Rossby, 1977]. [4] In this study, we demonstrate that 2nd-mode ITs in the ECS are responsible for failure of the MI-GEM fields to captureadequatelyTand dvariationsinthep=100–200-dbar layer as seen in historical hydrographic measurements. We verify our observations with a numerical simulation of ITs.

Model results of flow instabilities in the tropical Pacific Ocean

A two-and-a-half-layer model of the tropical Pacific Ocean is used to investigate the energy source for the intraseasonal dynamic-height variability observed near 6°N. A simulation of equatorial circulation is produced by forcing the model with mean-monthly wind-stress climatology. Two westward-propagating waves appear in the upper layer in the central and eastern portion of the model basin. These two waves are distinguished by period and meridional structure. An off-equatorial wave with period of 30 days and wavelength of 1100 km has a meridional sea-level maximum near 6°N similar to that of the 30–50 day intraseasonal wave observed in the ocean. The meridional velocity signal also is asymmetric with respect to the equator, with maximum near 4°N. The second wave with period of 15 days has a strong meridional velocity signal centered on the equator. The sea-level and zonal velocity signals associated with this equatorial wave have maxima near 1.5°N and 1.5°S. The eddy-energy budget reveals strong conversions from the mean-flow to eddy field through baroclinic and upper-layer barotropic conversion terms. Conversion terms north of the equator exhibit a bimodal structure: one maximum between the equator and 3°N is dominated by upper-layer barotropic conversion spatially coincident with the cyclonic shear along the equatorward edge of the South Equatorial Current (SEC), and a second smaller maximum between 3°N and 5°N is a combination of upper-layer barotropic conversion along the poleward edge of the SEC (anticyclonic shear) and baroclinic conversion near the core of the SEC. The two peaks in the conversion terms, combined with similar structure in the flux-divergence terms in the model eddy-energy budget, provide evidence that two wave processes are generated at the different source regions: one near the equator and a second between 2°N and 5°N.

Comment on ''Current system east of the Ryukyu Islands'' by A. Nagano et al.

[1] In a recent paper, Nagano et al. [2007] (hereafter N2007) discussed the variation of current structure and volume transport of the Ryukyu Current System (RCS) east of the Ryukyu Islands using an inverse technique with hydrographic-section data collected from three cruises. Their paper appears to be important for understanding this western boundary current, since it is based on repeated hydrographic surveys in the region east of the Ryukyu Islands. But acquiring meaningful results from inverse calculations requires great care. Fiadeiro and Veronis [1982, p. 160] say ‘‘Because inverse analysis always supplies a solution, it appeared that the assumed reference level matters less in inverse theory than in the hydrographer’s [level-of-no-motion] approach. Actual computations have shown that is not true. A bad assumption leads to bad results. The ‘advantage’ of inverse theory is that one sees how bad the results are.’’ The inverse technique minimizes the velocity at the reference level, and that point is indeed raised in the final paragraph of N2007, which used the reference level of 100 dbar. To determine whether this shallow reference level is an appropriate condition or not, one should investigate the effect of choosing other reference levels, in particular deeper levels. In this note, we perform two inverse calculations to calculate the current east of the Ryukyu Islands from the same data as one of the N2007 cruises but using a reference level of 2000 dbar. The resulting RCS circulation field is, we believe, more realistic than that of N2007. [2] Three hydrographic surveys were carried out during May–June, September and October 2002 along three lines (AE, E, OS) east of the Ryukyu Islands (Figure 1). May– June and September cruises used XCTD/XBT casts along the E line, but the October cruise collected CTD casts along all three lines. Surprisingly, absolute geostrophic velocity sections, determined by N2007 with a 100-dbar reference level, revealed large values up to 30–40 cm s 1 at 1000 dbar, the deepest level shown (Figures 2, 3b and 4c in N2007). We have calculated full-depth absolute geostrophic velocity sections using the inverse results of N2007 and plotted them to 3000 dbar (Figures 2a, 2b, 3a, 3b, 4a and 4b). Figures 2b, 3b and 4b show strong deep currents with maximum speeds of 20–50 cm s 1 on each line. In each case, note the contour lines of velocity are nearly vertical and strong currents extend to the bottom. For example, a current velocity of 57 cm s 1 touches the bottom near 3100 dbar in the AE section. Is the computed strong deepcurrent field real, or is it an artifact of the shallow reference level? We address this question by carrying out the inverse calculation using a reference level of 2000 dbar instead of 100 dbar, and compare results using the two reference levels. We present results for the October cruise data only, because the XBT/XCTD casts reached only 1000 dbar; also the CTD measurements provide more accuracy in the inverse calculations.