Chau Ron Wu

The Interaction of Supertyphoon Maemi (2003) with a Warm Ocean Eddy

Understanding the interaction of ocean eddies with tropical cyclones is critical for improving the understanding and prediction of the tropical cyclone intensity change. Here an investigation is presented of the interaction between Supertyphoon Maemi, the most intense tropical cyclone in 2003, and a warm ocean eddy in the western North Pacific. In September 2003, Maemi passed directly over a prominent (700 km 500 km) warm ocean eddy when passing over the 22°N eddy-rich zone in the northwest Pacific Ocean. Analyses of satellite altimetry and the best-track data from the Joint Typhoon Warning Center show that during the 36 h of the Maemi–eddy encounter, Maemi’s intensity (in 1-min sustained wind) shot up from 41 ms 1 to its peak of 77 m s 1 . Maemi subsequently devastated the southern Korean peninsula. Based on results from the Coupled Hurricane Intensity Prediction System and satellite microwave sea surface temperature observations, it is suggested that the warm eddies act as an effective insulator between typhoons and the deeper ocean cold water. The typhoon’s self-induced sea surface temperature cooling is suppressed owing to the presence of the thicker upper-ocean mixed layer in the warm eddy, which prevents the deeper cold water from being entrained into the upper-ocean mixed layer. As simulated using the Coupled Hurricane Intensity Prediction System, the incorporation of the eddy information yields an evident improvement on Maemi’s intensity evolution, with its peak intensity increased by one category and maintained at category-5 strength for a longer period (36 h) of time. Without the presence of the warm ocean eddy, the intensification is less rapid. This study can serve as a starting point in the largely speculative and unexplored field of typhoon–warm ocean eddy interaction in the western North Pacific. Given the abundance of ocean eddies and intense typhoons in the western North Pacific, these results highlight the importance of a systematic and in-depth investigation of the interaction between typhoons and western North Pacific eddies.

A unique seasonal pattern in phytoplankton biomass in low‐latitude waters in the South China Sea

elevated to 0.3 mg/m 3 , 35 mg/m 2 and 300 mg-C/m 2 /d, respectively, in the winter but stayed low, at 0.1 mg/m 3 , 15 mg/m 2 and 110 mg-C/m 2 /d as commonly found in other low latitude waters, in the rest of the year. Concomitantly, soluble reactive phosphate and nitrate+nitrite in the mixed layer also became readily detectable in the winter. The elevationofphytoplanktonbiomasscoincidedapproximately with the lowest sea surface temperature and the highest wind speed in the year. Only the combined effect of convective overturn by surface cooling and wind-induced mixing could have enhanced vertical mixing sufficiently to make the nutrients in the upper nutricline available for photosynthetic activities and accounted for the higher biomass in the winter. Citation: Tseng, C.-M., G. T. F. Wong, I.-I. Lin, C.-R. Wu, and K.-K. Liu (2005), A unique seasonal pattern in phytoplankton biomass in low-latitude waters in the South China Sea, Geophys. Res. Lett., 32, L08608, doi:10.1029/2004GL022111.

Seasonal and Interannual Variations in the Velocity Field of the South China Sea

A three-dimensional numerical model is used to simulate sea level and velocity variations in the South China Sea for 1992–1995. The model is driven by daily wind and daily sea surface temperature fields derived from the NCEP/NCAR 40-year reanalysis project. The four-year model outputs are analyzed using time-domain Empirical Orthogonal Functions (EOF). Spatial and temporal variations of the first two modes from the simulation compare favorably with those derived from satellite altimetry. Mode 1, which is associated with a southern gyre, shows symmetric seasonal reversal. Mode 2, which contributes to a northern gyre, is responsible for the asymmetric seasonal and interannual variations. In winter, the southern and northern cyclonic gyres combine into a strong basin-wide cyclonic gyre. In summer, a cyclonic northern gyre and an anticyclonic southern gyre form a dipole with a jet leaving the coast of Vietnam. Interannual variations are particularly noticeable during El Niño. The winter gyre is generally weakened and confined to the southern basin, and the summer dipole structure does not form. Vertical motions weaken accordingly with the basin-wide circulation. Variations of the wind stress curl in the first two EOF modes coincide with those of the model-derived sea level and horizontal velocities. The mode 1 wind stress curl, significant in the southern basin, coincides with the reversal of the southern gyre. The mode 2 curl, large in the central basin, is responsible for the asymmetry in the winter and summer gyres. Lack of the mode 2 contribution during El Niño events weakens the circulation. The agreement indicates that changes in the wind stress curl contribute to the seasonal and interannual variations in the South China Sea.

TOPEX/Poseidon observations of mesoscale eddies over the Subtropical Countercurrent: Kinematic characteristics of an anticyclonic eddy and a cyclonic eddy

[1] Relative dynamic heights and geostrophic fields were derived from TOPEX/Poseidon altimetry data and then used to track mesoscale eddies over the Subtropical Countercurrent (STCC). The radii, centers, vorticities, shearing deformation rates, stretching deformation rates, divergences, and center velocities of all identified eddies over the STCC were determined using a model that assumes constant velocity gradients. Most eddies are concentrated in a zonal band near 22� N, and there is an interannual variation in the number of eddies. A case study was made for a cyclonic eddy and an anticyclonic eddy, with time series of eddy kinematic parameters computed. Both eddies survive for � 220 days and propagate westward along over 22� N–24� N to reach the Kuroshio Current east coast of Taiwan, where the eddies were dissipated and in turn affected the Kuroshio Current in many ways. Sea surface temperature data and drifter data confirm the existence of these two eddies. The radii of both eddies vary and their shapes are mostly elliptical during propagation. The anticyclonic eddy propagated almost westward with oscillating north-south components, and the mean speed is 8.3 km/day. The cyclonic eddy moved southwestward before reaching 130� E and then moved northwestward, with a mean speed of 7.6 km/day. The propagations of these two eddies are basically consistent with the standard theory of eddy propagation but with larger speeds. The propagating direction could be altered while passing steep bottom topography or merging with the other eddies. INDEX TERMS: 4520 Oceanography: Physical: Eddies and mesoscale processes; 4576 Oceanography: Physical: Western boundary currents; 4512 Oceanography: Physical: Currents; 4594 Oceanography: Physical: Instruments and techniques; 4556 Oceanography: Physical: Sea level variations;

Assimilating Altimetric Data into a South China Sea Model

Sea surface heights from the TOPEX/Poseidon altimeter are assimilated into a three-dimensional primitive equation model to derive the circulation in the South China Sea. With data assimilation the model resolves not only the basin-wide circulation but also a dipole off Vietnam and a low/high feature near the Luzon Strait. Mesoscale features are missing in the simulation without data assimilation because of poor resolution in the wind field and inadequate knowledge of the transport through the Luzon Strait. Compared to the case without data assimilation, data assimilation reduces the root mean square error between the simulated and observed sea surface heights by a factor of 2–3. Circulation derived from data assimilation under climatological conditions is contrasted with that during El Nino. In the normal winter of 1993–1994, flow at 50 m depth is strong and cyclonic. Flow at 900 m depth is cyclonic as well. The deep cyclone persists into the following summer. During the 1994–1995 El Nino winter, features in the flow field at 50 m depth either weaken or disappear, and circulation at 900 m depth is anticyclonic. In the summer of 1995 the dipole and the eastward jet off Vietnam at 50 m depth are missing, and the anticyclonic circulation at 900 m depth persists. Temperature at 65 m shows significant warming from fall 1994 to summer 1995. A weakened flow field and warming in the upper ocean are consistent with findings from earlier El Nino events.

High resolution modeling and data assimilation in the Monterey Bay area

Abstract A high resolution, data assimilating ocean model of the Monterey Bay area (ICON model) is under development within the framework of the project “An Innovative Coastal-Ocean Observing Network” (ICON) sponsored by the National Oceanographic Partnership Program. The main objective of the ICON model development is demonstration of the capability of a high resolution model to track the major mesoscale ocean features in the Monterey Bay area when constrained by the measurements and nested within a regional larger-scale model. This paper focuses on the development of the major ICON model components, including grid generation and open boundary conditions, coupling with a larger scale, Pacific West Coast (PWC) model, atmospheric forcing etc. Impact of these components on the Models predictive skills in reproducing major hydrographic conditions in the Monterey Bay area are analyzed. Comparisons between observations and the ICON model predictions with and without coupling to the PWC model, show that coupling with the regional model improves significantly both the correlation between the ICON model and observed ADCP currents, and the ICON models skill in predicting the location and intensity of observed upwelling events. Analysis of the ICON model mixed layer depth predictions show that the ICON model tends to develop a thicker than observed mixed layer during the summer time, and while assimilation of sea surface temperature data is enough for development of observed thin mixed layer in the regional larger-scale model, the fine-resolution ICON model needs variable heat fluxes as surface boundary conditions for the accurate prediction of the vertical thermal structure. The paper targets researchers involved in high-resolution numerical modeling of coastal areas in which the dynamics are determined by the complex geometry of a coastline, variable bathymetry and by the influence of complex water masses from a complicated hydrographic system (such as the California Current system).

Validation and Application of Altimetry-Derived Upper Ocean Thermal Structure in the Western North Pacific Ocean for Typhoon-Intensity Forecast

This paper uses more than 5000 colocated and near-coincident in-situ profiles from the National Oceanic and Atmospheric Administration/Global Temperature and Salinity Profile Program database spanning over the period from 2002 to 2005 to systematically validate the satellite-altimetry-derived upper ocean thermal structure in the western North Pacific ocean as such ocean thermal structure information is critical in typhoon-intensity change. It is found that this satellite-derived information is applicable in the central and the southwestern North Pacific (covering 122-170degE, 9-25degN) but not in the northern part (130-170degE, 25-40degN). However, since > 80% of the typhoons are found to intensify in the central and southern part, this regional dependence should not pose a serious constraint in studying typhoon intensification. Further comparison with the U.S. Naval Research Laboratorys North Pacific Ocean Nowcast/Forecast System (NPACNFS) hydrodynamic ocean model shows similar regional applicability, but NPACNFS is found to have a general underestimation in the upper ocean thermal structure and causes a large underestimation of the tropical cyclone heat potential (TCHP) by up to 60 kJ/cm2. After validation, the derived upper ocean thermal profiles are used to study the intensity change of supertyphoon Dianmu (2004). It is found that two upper ocean parameters, i.e., a typhoons self-induced cooling and the during-typhoon TCHP, are the most sensitive parameters (with R 2~0.7) to the 6-h intensity change of Dianmu during the study period covering Dianmus rapid intensification to category 5 and its subsequent decay to category 4. This paper suggests the usefulness of satellite-based upper ocean thermal information in future research and operation that is related to typhoon-intensity change in the western North Pacific

Enhanced primary production in the oligotrophic South China Sea by eddy injection in spring

concentration typically at ∼0.05–0.08 mgm −3 ), it is intriguing to explore this unusual happening. Based on six different remote sensing data and numerical modelling, the results suggest that the injection of an ocean eddy is the most likely cause of the bloom. Due to long‐range transport of a large (700 × 500 km) anti‐cyclonic ocean eddy, coastal nutrients and plankton could be brought across hundreds of kilometres to the centre of northern SCS and impact the biogeochemistry. The open ocean part of the northern SCS basin has long been considered generally free from coastal influences. This work provides new evidence that proves otherwise. Moreover, from the perspective of physical oceanography, it is interesting to observe that, outside the monsoon seasons, there can be well‐defined anti‐ cyclonic ocean circulation existing in the SCS without the prevailing monsoonal wind. Citation: Lin, I‐I, C.‐C. Lien, C.‐R. Wu, G. T. F. Wong, C.‐W. Huang, and T.‐L. Chiang (2010), Enhanced primary production in the oligotrophic South China Sea by eddy injection in spring, Geophys. Res. Lett., 37, L16602, doi:10.1029/2010GL043872.

Typhoon Kai-Tak: An Ocean’s Perfect Storm

An unusually intense sea surface temperature drop (DSST) of about 10.88C induced by the Typhoon KaiTak is observed in the northern South China Sea (SCS) in July 2000. Observational and high-resolution SCS model analyses were carried out to study the favorable conditions and relevant physical processes that cause the intense surface cooling by Kai-Tak. Upwelling and entrainment induced by Kai-Tak account for 62% and 31% of the DSST, respectively, so that upwelling dominates vertical entrainment in producing the surface cooling for a subcritical storm such as Kai-Tak. However, wind intensity and propagation speed alone cannot account for the large DSST. Prior to Kai-Tak, the sea surface was anomalously warm and the main thermocline was anomalously shallow. The cause was a delayed transition of winter to summer monsoon in the northern SCS in May 2000. This produced an anomalously strong wind stress curl and a cold eddy capped by a thin layer of very warm surface water west of Luzon. Kai-Tak was the ocean’s perfect storm in passing over the eddy at the ‘‘right time,’’ producing the record SST drop and high chlorophyll-a concentration.

Mindanao Current/Undercurrent in an eddy-resolving GCM

Analysis of results from an eddy-resolving general circulation model showed two subsurface velocity cores in the mean within the depth range between 400 and 1000 m below the Mindanao Current (MC). One is confined to the inshore edge at about 126.8 degrees E and connected with the Sulawesi Sea. The other takes place somewhat offshore around 127.7 degrees E, being closely related to the intrusion of South Pacific water. Both cores are referred to as the Mindanao Undercurrent (MUC). The MC/MUC is approximately a geostrophic flow, except on the inshore edge of the MUC where up to 50% of the mean flow can be explained by ageostrophic dynamics. In contrast with the well-defined southward flowing MC, the MUC is of high velocity variance relative to the mean. Empirical orthogonal function (EOF) analysis shows that approximately 60% of the total velocity variance is associated with two meandering modes, with their major signatures in the subthermocline. The dominant time scale of variability is 50-100 days. An ensemble of these meso-scale fluctuations provides a northward freshwater flux on the offshore edge of the Philippine coast, which to a certain extent explains why water of South Pacific origin appears to extend farther northward than the mean MUC. In the offshore velocity core of the MUC, for example, eddy induced freshwater flux is equivalent to a mean flow of about 0.3 m s(-1) in the density range between 26.9 and 27.3 kg m(-3), which is greater than the mean current by a factor of 6.