Zuojun Yu

Meridional Asymmetry and Energetics of Tropical Instability Waves

Abstract One of the striking features of tropical instability waves (TIWs) is that they appear to be more prominent north of the equator. A linearized, 2½-layer ocean model is used to investigate effects of various asymmetric background states on structures of equatorial, unstable waves. Our results suggest that the meridional asymmetry of TIWs is due to asymmetries of the two branches of the South Equatorial Current (SEC) and of the equatorial, sea surface temperature front; it is not due to the presence of the North Equatorial Countercurrent. Energetics analyses indicate that frontal instability associated with the equatorial, SST front, as well as barotropic instability due to shear associated with the SEC, are energy sources for the model TIWS.

Dynamics of the Pacific Subsurface Countercurrents

A hierarchy of models, varying from 2 -layer to 4 -layer systems, is used to explore the dynamics of the 11 22 Pacific Subsurface Countercurrents, commonly referred to as ‘‘Tsuchiya Jets’’ (TJs). The TJs are eastward currents located on either side of the equator at depths from 200 to 500 m and at latitudes varying from about 28 to 78 north and south of the equator, and they carry about 14 Sv of lower-thermocline (upper-intermediate) water throughout the tropical Pacific. Solutions are found in idealized and realistic basins and are obtained both analytically and numerically. They are forced by winds and by a prescribed Pacific interocean circulation (IOC) with transport M (usually 10 Sv), representing the outflow of water in the Indonesian passages and a compensating inflow from the Antarctic Circumpolar Current. Analytic solutions to the 2 -layer model suggest that the TJs are geostrophic currents along arrested fronts. 1 2 Such fronts are generated when Rossby wave characteristics, carrying information about oceanic density structure away from boundaries, converge or intersect in the interior ocean. They indicate that the southern and northern TJs are driven by upwelling along the South American coast and in the ITCZ band, respectively, that the northern TJ is strengthened by a recirculation gyre that extends across the basin, and that TJ pathways are sensitive to stratification parameters. Numerical solutions to the 2 -layer and 4 -layer models confirm the analytic results, 11 22 demonstrate that the northern TJ is strengthened considerably by unstable waves along the eastward branch of the recirculation gyre, show that the TJs are an important branch of the Pacific IOC, and illustrate the sensitivity of TJ pathways to vertical-mixing parameterizations and the structure of the driving wind. In a solution to the 2 -layer model with M 5 0, the southern TJ vanishes but the northern one remains, being 1 2 maintained by the unstable waves. In contrast, both TJs vanish in the M 5 0 solution to the 4 -layer model,

The South China Sea Throughflow Retrieved from Climatological Data

The salinity distribution in the South China Sea (SCS) has a pronounced subsurface maximum from 150‐ 220 m throughout the year. This feature can only be maintained by the existence of a mean flow through the SCS, consisting of a net inflow of salty North Pacific tropical water through the Luzon Strait and outflow through the Mindoro, Karimata, and Taiwan Straits. Using an inverse modeling approach, the authors show that the magnitude and space‐time variations of the SCS thermohaline structure, particularly for the salinity maximum, allow a quantitative estimate of the SCS throughflow and its distribution among the three outflow straits. Results from the inversion are compared with available observations and output from a 50-yr simulation of a highly resolved ocean general circulation model. The annual-mean Luzon Strait transport is found to be 2.4 6 0.6 Sv (Sv [ 10 6 m 3 s 21 ). This inflow is balanced by the outflows from the Karimata (0.3 6 0.5 Sv), Mindoro (1.5 6 0.4), and Taiwan (0.6 6 0.5 Sv) Straits. Results of the inversion suggest that the Karimata transport tends to be overestimated in numerical models. The Mindoro Strait provides the only passage from the SCS deeper than 100 m, and half of the SCS throughflow (1.2 6 0.3 Sv) exits the basin below 100 m in the Mindoro Strait, a result that is consistent with a climatological run of a 0.18 global ocean general circulation model.

Vertical Eddy Mixing in the Tropical Upper Ocean: Its Influence on Zonal Currents

Abstract In this study, the authors explore how vertical-mixing parameterizations influence the structure of zonal currents in the eastern equatorial Pacific using an isopycnal ocean model that contains an explicit surface mixed layer. The mixing parameterizations considered are the schemes that depend on the Richardson number (Ri). One of the schemes (the Step scheme) consists of high (νc) and low (νb) values of mixing coefficients, depending on whether Ri is less than or greater than a critical value. In simulations using the Step scheme, there is a region of large vertical shear just beneath the mixed layer where Ri is low and the mixing coefficient is νc; this high mixing controls the depth and strength of the westward surface drift. Near the undercurrent core, Ri is high and the mixing coefficient is νb; this low mixing is nevertheless dynamically important in that it affects the strength of the undercurrent. For the Ri-dependent schemes investigated, it is demonstrated that the extrema attained by m...

NSCAT tropical wind stress maps: Implications for improving ocean modeling

Using wind vectors from the NASA scatterometer (NSCAT), daily maps of pseudostress have been constructed for the tropical Pacific Ocean and compared with pseudostress maps derived from the European Centre for Medium-Range Weather Forecasts (ECMWF) 10-m wind product. The map resolution for the NSCAT pseudostress maps was selected using both a statistical measure of the expected mapping errors and tests on realistic wind fields. The selected map resolution is 5 days and 2°, which minimizes residual effects from the NSCAT sampling pattern, while maximizing temporal and spatial resolution. Comparisons with the ECMWF maps showed significant differences in most regions, corresponding to mean wind speeds of 2–3 m s−1, particularly in the Intertropical Convergence Zone (ITCZ) and at 20°S and 20°N. A canonical correlation analysis between NSCAT and ECMWF fields showed a high degree of correlation of temporal variations and systematic differences in spatial structure. In the NSCAT fields the ITCZ is narrower, stronger, and is located 1–2° latitude farther south than in the ECMWF fields. The high degree of correlation between the two fields suggests that “hybrid” fields can be generated. The dynamical implications of the differences in wind forcing are illustrated using estimates of the Sverdrup stream function and the Ekman pumping. A simple reduced-gravity, linear vorticity model, forced by both the ECMWF and hybrid winds to examine predicted differences in ocean response, showed higher skill for the NSCAT winds.

Influence of Equatorial Dynamics on the Pacific North Equatorial Countercurrent

The Pacific North Equatorial Countercurrent (NECC) is generally not well simulated in numerical models. In this study, the causes of this problem are investigated by comparing model solutions to observed NECC estimates. The ocean model is a general circulation model of intermediate complexity. Solutions are forced by climatological and interannual wind stresses, t 5 (t x, t y), from Florida State University and the European Centre for MediumRange Weather Forecasts. Estimates of the observed NECC structure and transport are prepared from expendable bathythermograph data and from the ocean analysis product of NOAA/National Centers for Environmental Prediction. In solutions forced by climatological winds, the NECC develops a discontinuity in the central Pacific that is not present in the observations. The character of the error suggests that it arises from the near-equatorial (5 8S‐ 58N) zonal wind stress, t x, being relatively too strong compared to the y derivative of the wind stress curl term, (curlt )y, associated with the intertropical convergence zone. This is confirmed in solutions forced by interannual winds, which exhibit a wide range of responses from being very similar to the observed NECC to being extremely poor, the latter occurring when near-equatorial t x is relatively too strong. Results show further that the model NECC transport is determined mainly by the strength of (curlt )y, but that its structure depends on near-equatorial t x; thus, NECC physics involves equatorial as well as Sverdrup dynamics. Only when the two forcing features are properly prescribed do solutions develop a NECC with both realistic spatial structure and transport.

Subsurface Salinity Balance in the South China Sea

Abstract The South China Sea (SCS) is often treated as a semienclosed water body, with the Luzon Strait as its only connection to the Pacific Ocean. A branch of the Kuroshio flows northwestward across the Luzon Strait to enter the SCS, carrying North Pacific Tropical Water (NPTW) into the basin. Using the subsurface salinity maximum as a tracer for NPTW, the authors show how important three secondary straits—the Taiwan Strait to the north and the Karimata and Mindoro Straits to the south—are to the NPTW intrusion at the Luzon Strait. The authors demonstrate that the SCS cannot reach an equilibrium state that is consistent with the observed subsurface salinity distribution unless all of the following components are in place: the Kuroshio, transports through the three secondary straits, downward mixing of freshwater, horizontal mixing induced by mesoscale eddies, and forcing by the local monsoonal winds.

Dynamics of the Southern Tsuchiya Jet

Abstract The Tsuchiya jets (TJs) are narrow eastward currents, located a few degrees on either side of the equator at depths from 200 to 500 m in the Pacific Ocean. In this study, non-eddy-resolving, oceanic general circulation models (OGCMs) are used to investigate the dynamics of the southern TJ. Most solutions are found in a rectangular basin extending 100° zonally and from 40°S to 10°N. They are forced by idealized zonal and meridional winds representing the trades and the southerly winds near the South American coast, by a prescribed interocean circulation (IOC) that enters the basin through the southern boundary and exits through the western boundary from 2° to 6°N (the model’s Indonesian passages), and by surface heating that warms the ocean in the Tropics. A suite of solutions is presented to isolate effects of each forcing and mixing process. A few solutions are also found to a global OGCM driven by realistic forcings. Solutions forced by all of the aforementioned processes and with minimal diffu...

On the Annual Cycle of Upper-Ocean Circulation in the Eastern Equatorial Pacific

Abstract An oceanic general circulation model is used to investigate the annual cycle of the near-surface currents in the eastern equatorial Pacific Ocean; in particular, the causes of the springtime increase of eastward momentum that reverses the westward surface flow and intensifies the Equatorial Undercurrent are examined. A set of process experiments are carried out that isolates effects due to three forcing mechanisms: local zonal and meridional winds, and remote zonal winds. It is demonstrated that the springtime weakening of the local easterly trades is the primary cause of the eastward-momentum increase. In addition, due to meridional advection, the local southerly wind drives a westward current on the equator throughout the year; this flow is weakest in the spring, and therefore this process also contributes to the anomalous eastward flow. On the other hand, remote forcing tends to weaken the springtime momentum increase: Anomalous easterlies in the far-western and central Pacific during the wint...

Validating the NSCAT winds in the vicinity of the Pacific intertropical convergence zone

It is known that the NSCAT winds are influenced by precipitation, and so are potentially in error in regions of high rainfall, such as the Pacific ITCZ. We assess this potential error by comparing NSCAT winds to other products and by determining the possible impact on modeling the Pacific NECC. In the latitude band of the ITCZ, there are large differences between wind products from the ECMWF and NSCAT. A comparison with TAO buoy winds shows a bias of NSCAT data towards a higher zonal wind stress at 8°N. The spatial distribution of this bias distorts the Ekman pumping velocity across the ITCZ, and results in a NECC that is 0.1-0.2 m/s weaker than that forced by the ECMWF winds. This is not a trivial amount during the spring, when the strength of the NECC in the eastern Pacific is of the same magnitude.