Jian Lan

Japan Sea Thermohaline Structure and Circulation. Part I: Climatology

Abstract In this study, the U.S. Navy’s Generalized Digital Environmental Model (GDEM) climatological temperature and salinity data on a 0.5° × 0.5° grid is used to investigate the seasonal variabilities of the Japan/East Sea (JES) thermohaline structure and circulations. The GDEM for the JES was built up on historical (1930–97) 136 509 temperature and 52 572 salinity profiles. A three-dimensional estimate of the absolute geostrophic velocity field was obtained from the GDEM temperature and salinity fields using the P-vector method. The seasonal variabilities of the thermohaline structure and the inverted currents such as the Subpolar Front, the salinity minimum and maximum in the Japan Sea Intermediate Water, and the Tsushima Warm Current and its bifurcation are identified.

Japan Sea Thermohaline Structure and Circulation. Part II: A Variational P-Vector Method

Abstract The second part of this work investigates the seasonal variabilities of the Japan/East Sea (JES) circulation using the U.S. Navy Generalized Digital Environmental Model (GDEM) climatological temperature and salinity dataset (public domain) on a 0.5° × 0.5° grid. A variational P-vector method was developed to invert the velocity field. The GDEM for the JES was built up on historical (1930–97) 136 509 temperature and 52 572 salinity profiles. The climatological mean and seasonal variability of the current systems are well inverted, especially the Tsushima Warm Current and its bifurcation, the East Korean Warm Current (EKWC), the Japan nearshore branch, the confluence of the EKWC, and the North Korean Cold Current near the Korean coast and flows northeastward along the subpolar front, and a mesoscale anticyclonic eddy in the Ulleng/Tsushima Basin. Furthermore, this method has the capability to invert flow reasonably well across the shallow straits such as the Tsushima/Korea, Tsugaru, and Soya Strait...

Importance of Ocean Dynamics for the Skewness of the Indian Ocean Dipole Mode

AbstractInterannual anomalies of sea surface temperature (SST), wind, and cloudiness in the southeastern tropical Indian Ocean (SE-TIO) show negative skewness. In this research, asymmetry between warm and cold episodes in the SE-TIO and the importance of ocean dynamics are investigated. A coupled model simulation and observations show an asymmetric relationship between SST and the thermocline depth in the SE-TIO where SST is more sensitive to an anomalous shoaling than to deepening of the thermocline. This asymmetric thermocline feedback on SST is a result of a deep mean thermocline. Sensitivity experiments with an ocean general circulation model (OGCM) show that a negative SST skewness arises in response to sinusoidal zonal wind variations that are symmetric between the westerly and easterly phases. Heat budget analysis with an OGCM hindcast also supports the importance of ocean dynamics for SST skewness off Sumatra and Java.

Process modeling studies of physical mechanisms of the formation of an anticyclonic eddy in the central Red Sea

Surface drifters released in the central Red Sea during April 2010 detected a well-defined anticyclonic eddy around 23°N. This eddy was ∼45–60 km in radius, with a swirl speed up to ∼0.5 m/s. The eddy feature was also evident in monthly averaged sea surface height fields and in current profiles measured on a cross-isobath, shipboard CTD/ADCP survey around that region. The unstructured-grid, Finite-Volume Community Ocean Model (FVCOM) was configured for the Red Sea and process studies were conducted to establish the conditions necessary for the eddy to form and to establish its robustness. The model was capable of reproducing the observed anticyclonic eddy with the same location and size. Diagnosis of model results suggests that the eddy can be formed in a Red Sea that is subject to seasonally varying buoyancy forcing, with no wind, but that its location and structure are significantly altered by wind forcing, initial distribution of water stratification and southward coastal flow from the upstream area. Momentum analysis indicates that the flow field of the eddy was in geostrophic balance, with the baroclinic pressure gradient forcing about the same order of magnitude as the surface pressure gradient forcing.

Seasonal variation in the South China Sea deep circulation

The previous studies show that the SCS deep circulation is featured by a basin-scale cyclonic gyre. On the basis of the Hybrid Coordinate Ocean Model (HYCOM) and the Simple Ocean Data Assimilation (SODA), this study attempts to examine its seasonal variability and to investigate the driving mechanism. During summer season, the basin-scale cyclonic gyre is dominant and strong, corresponding to higher value of the deepwater overflow transport. During winter season, the basin-scale cyclonic gyre can hardly be identified, corresponding to lower value of the deepwater overflow transport. The control run and the SODA show the similar results. Two sensitivity experiments are designed to investigate what could be possible responsible for the seasonal variation in the SCS deep circulation. The results reveal that the deepwater overflow through the Luzon Strait contributes to the seasonal variability of the SCS deep circulation, and the seasonal variation of the surface forcings have less influence on that. The mechanism is related to the potential vorticity flux by the deepwater overflow.

Seasonal and interannual variations of mixed layer salinity in the southeast tropical Indian Ocean

In this study, seasonal and interannual variations of the mixed layer salinity (MLS) in the southeast tropical Indian Ocean (SETIO) are analyzed using satellite observations, historical data sets, and data-assimilating ocean model outputs. On the seasonal cycle, the MLS in the SETIO becomes fresher in austral winter and saltier in austral summer: between the Java-Lesser Sunda coast and the South Equatorial Current (SEC, 12 degrees S), where positive entrainment and fresh advections counterbalance each other, the annual cycle of the MLS closely follows the variation of the air-sea freshwater forcing; off the northwest and west Australian coasts, the MLS variations are influenced by the annual cycles of the Indonesian Throughflow (ITF) and Leeuwin Current (LC) transports as well as the air-sea freshwater forcing, with eddy fluxes acting to freshen the MLS along the SEC, the Eastern Gyral Current, and the LC. On the interannual-scale, El Nino (La Nina) events are typically associated with saltier (fresher) MLS in the SETIO. Composite and budget analyses reveal that interannual variations in precipitations drive the MLS anomalies off the Java-Lesser Sunda coast; between 12 degrees S and the northwest Australian coast, the MLS variations are influenced by both advection anomalies and local precipitation anomalies; whereas anomalous meridional currents contribute to the MLS variations off the west Australian coast. Both enhanced local precipitations and the ITF transport anomalies have substantial contributions to the drastic freshening of the Indonesian-Australian Basin between the Java-Lesser Sunda coast and the northwest Australian coast during the extended La Nina events in 1999-2001 and 2010-2012.

A Triggering Mechanism for the Indian Ocean Dipoles Independent of ENSO

AbstractAlthough the Indian Ocean dipole (IOD) and ENSO are significantly correlated, there are indeed some IODs independent of ENSO. In this research, the characteristics of independent IOD are investigated and a new triggering mechanism is proposed based on case study and statistical analysis. Results show that the independent IODs peak in an earlier season and have a weaker intensity compared with the IODs associated with ENSO. The wind anomaly associated with the independent IOD is very unique and shows a monsoonlike pattern, in addition to the equatorial easterly wind anomaly (EEWA) common to all IODs. The evolution of the EEWA associated with the independent IOD is well captured by the second EOF mode of the equatorial zonal wind interannual variability, suggesting that the independent IOD is an important climate mode inherent to the tropical Indian Ocean. The EEWA associated with the independent IOD is tightly linked to Indian summer monsoon activities in spring, and the convection anomalies associ...

Extremely strong thermohaline sources/sinks generated by diagnostic initialization

[1] One difficulty for ocean modeling is the lack of velocity data for specifying the initial condition. Diagnostic initialization is widely used; it integrates the model from known temperature (Tc) and salinity (Sc) and zero velocity fields while holding (Tc, Sc) unchanged. After a period (around 30 days) of the diagnostic run, the velocity field (Vc) is established, and (Tc, Sc, Vc) fields are then treated as the initial conditions for the prognostic numerical modeling. During the diagnostic initialization period, the heat and salt ‘source/sink’ terms are generated at each time step. Maximum time rates of absolute change of the monthly mean T, S (0.1� /day, 0.1 ppt/day) are taken as the standard measures to identify the strength of the thermohaline ‘sources/sinks’. Twenty four times of the standard measures (0.1� /hr, 0.1 ppt/hr) represent strong ‘sources/sinks’. Ten times of the strong ‘sources/sinks’ (1� /hr, 1 ppt/hr) represent extremely strong ‘sources/sinks’. The Princeton Ocean Model implemented for the Japan/East Sea is used to demonstrate the existence of extremely strong thermohaline sources and sinks generated by the diagnostic initialization with the annual mean Tc, Sc from the Navy’s Global Digital Environmental Model. The effects of extremely strong and spatially nonuniform initial heating/cooling (salting/ freshening) rates on thermohaline and velocity fields need to be further investigated. INDEX TERMS: 4263 Oceanography: General: Ocean prediction; 4255 Numerical modeling; 4243 Marginal and semienclosed seas; 4520 Oceanography: Physical: Eddies and mesoscale processes; 4528 Fronts and jets. Citation: Chu, P., and J. Lan, Extremely strong thermohaline sources/sinks generated by diagnostic initialization, Geophys. Res. Lett., 30 (6), 1341, doi:10.1029/2002GL016525, 2003.

A study of Indian Ocean Subtropical Mode Water: subduction rate and water characteristics

The annual subduction rate in the South Indian Ocean was calculated by analyzing Simple Ocean Data Assimilation (SODA) outputs in the period of 1950–2008. The subduction rate census for potential density classes showed a peak corresponding to Indian Ocean subtropical mode water (IOSTMW) in the southwestern part of the South Indian Ocean subtropical gyre. The deeper mixed layer depth, the sharper mixed-layer fronts and the associated relatively faster circulation in the present climatology resulted in a larger lateral induction, which primarily dominants the IOSTMW subduction rate, while with only minor contribution from vertical pumping. Without loss of generality, through careful analysis of the water characteristics in the layer of minimum vertical temperature gradient (LMVTG), the authors suggest that the IOSTMW was identified as a thermostad, with a lateral minimum of low potential vorticity (PV, less than 200×10–12 m–1·s–1) and a low dT⁄dz (less than 1.5°C/(100 m)). The IOSTMW within the South Indian Ocean subtropical gyre distributed in the region approximately from 25° to 50° E and from 30° to 39°S. Additionally, the average characteristics (temperature, salinity, potential density) of the mode water were estimated about (16.38 ± 0.29)°C, (35.46 ± 0.04), (26.02 ± 0.04) σθ over the past 60 years.

Hydrodynamical characteristics of falling cylinder in water column

The hydrodynamic features of a falling cylinder into the water column is investigated experimentally. The experiment consisted of dropping three cylinders of various lengths into a pool where the trajectories were filmed from two angles. The controlled parameters were, cylinder parameters (length to diameter ratio, center of mass location), and initial conditions (initial velocity, and drop angle). Results indicate that center of mass position has the largest influence on the cylinder’s trajectory and that accurate trajectory modeling requires the inclusion of both momentum and moment equations. A statistical-dynamic model has been established to predict the trajectories of the falling cylinders.