Zhe Wen Zheng

Transient modulation of Kuroshio upper layer flow by directly impinging typhoon Morakot in east of Taiwan in 2009

This study deals with the modulation of the Kuroshio upper layer flow (KULF) in response to the passage of Typhoon Morakot in 2009, using Regional Oceanic Modeling System (ROMS) and in situ measurements from Argos drifters and Argo floats. The analysis of the simulated current fields near the typhoon track revealed an intermittency phenomenon of the KULF, which was almost shut down for at least 6 hours. The process begun 2 days prior to the approach of typhoon center due to blockage of the KULF by the steadily northerly winds, and lasted for more than 2 days, simultaneously shifting the Kuroshio main stream (KMS) path. When the Morakot gradually moved closely to the Kuroshio, the KMS shifted vertically from the surface layer to deeper layer of 50 – 100 m depth, and the maximum current speed in the KMS decreased from more than 1.3 m s−1 to less than 1.1 m s−1. When the Morakot center approached about 100 km to the original position of the Kuroshio, the KULF spread eastward for 1.5 degrees at 24°N. When the Morakot center moved to the original position of the KMS, the Kuroshio abruptly rushed with a maximum speed near 1.4 m s−1. Meanwhile, an offshore cool jet originating from southeastern tip of Taiwan was generated and extended northward along the Kuroshio. In the cool jet, the lowest temperature reached about 5°C lower than the ambient waters. Modeled current variations and the cool jet during the Morakot passage were validated by in situ measurements.

A Long Neglected Damper in the El Niño—Typhoon Relationship: a ‘Gaia-Like’ Process

Proposed in the early 1970’s, the Gaia hypothesis suggests that our planet earth has a self-regulating ability to maintain a stable condition for life. Tropical cyclone (TC) is one of the earth’s most hazardous disasters; it is intriguing to explore whether ‘Gaia-like’ processes may exist in nature to regulate TC activities. El Niño can shift the forming position of the Western Pacific typhoons away from land. This shift enables typhoons to travel longer distances over ocean and is known to be a positive process to promote TCs to achieve higher intensity. What is neglected, however, is that there co-exists a negative process. Here we show that during El Niño, typhoons intensify over region undergoing strong ocean subsurface shoaling where upper ocean heat content can drop by 20–50%. This ‘worsen’ ocean pre-condition can effectively reduce ocean’s energy supply for typhoon intensification during typhoon-ocean interaction. We find this an elegant, ‘Gaia-like’ process demonstrating nature’s self-regulating ability. Though during El Niño, typhoons can take advantage of the longer travelling distance over ocean to achieve higher intensity, nature is also providing a damper to partially cancel this positive impact. Without the damper, the situation could be even worse.

Validation and Variation of Upper Layer Thickness in South China Sea from Satellite Altimeter Data

Satellite altimeter data from 1993 to 2005 has been used to analyze the seasonal variation and the interannual variability of upper layer thickness (ULT) in the South China Sea (SCS). Base on in-situ measurements, the ULT is defined as the thickness from the sea surface to the depth of 16°C isotherm which is used to validate the result derived from satellite altimeter data. In comparison with altimeter and in-situ derived ULTs yields a correlation coefficient of 0.92 with a slope of 0.95 and an intercept of 6 m. The basin averaged ULT derived from altimeter is 160 m in winter and 171 m in summer which is similar to the in-situ measurements of 159 m in winter and 175 m in summer. Both results also show similar spatial patterns. It suggests that the sea surface height data derived from satellite sensors are usable for study the variation of ULT in the semi-closed SCS. Furthermore, we also use satellite derived ULT to detect the development of eddy. Interannual variability of two meso-scale cyclonic eddies and one anticyclonic eddy are strongly influenced by El Niño events. In most cases, there are highly positive correlations between ULT and sea surface temperature except the periods of El Niño. During the onset of El Niño event, ULT is deeper when sea surface temperature is lower.

Numerical study on the interactions between the Kuroshio current in the Luzon Strait and a mesoscale eddy

The Luzon Strait (LS) connects the northwestern Pacific Ocean and the South China Sea (SCS) and is the western boundary gap for the Kuroshio current (KC). Satellite observations indicate that a cyclonic mesoscale eddy can trigger westward extension of the KC into the SCS and shed a smaller anticyclonic eddy to the west of the LS. We used a nonlinear reduced-gravity (primitive equation) model to study this phenomenon and analyzed the dynamic process. The location of the collision between the eddy and the KC could be critical for varying the circulation in the LS. The eddy’s deformation rate, associated with its decaying speed, is also closely related to the location of the eddy during collision. When a cyclonic eddy moved from a region to the east of the Luzon Island toward the LS, the KC intruded into the SCS with growing negative vorticity during the collision of the eddy and KC. This tendency for negative vorticity is attributed to the beta effect and squeezing of the planetary vorticity caused by the flow divergence. As the eddy dissipated, the KC in the LS recovered its original pattern. When the collision of the eddy occurred at the center of the LS, the momentum balance of the KC loop was dominated by the inertial term, and the circulation in the LS remained in a leaping state.

An Algorithm for Cold Patch Detection in the Sea off Northeast Taiwan Using Multi-Sensor Data

Multi-sensor data from different satellites are used to identify an upwelling area in the sea off northeast Taiwan. Sea surface temperature (SST) data derived from infrared and microwave, as well as sea surface height anomaly (SSHA) data derived from satellite altimeters are used for this study. An integration filtering algorithm based on SST data is developed for detecting the cold patch induced by the upwelling. The center of the cold patch is identified by the maximum negative deviation relative to the spatial mean of a SST image within the study area and its climatological mean of each pixel. The boundary of the cold patch is found by the largest SST gradient. The along track SSHA data derived from satellite altimeters are then used to verify the detected cold patch. Applying the detecting algorithm, spatial and temporal characteristics and variations of the cold patch are revealed. The cold patch has an average area of 1.92 × 104 km2. Its occurrence frequencies are high from June to October and reach a peak in July. The mean SST of the cold patch is 23.8 °C. In addition to the annual and the intraseasonal fluctuation with main peak centered at 60 days, the cold patch also has a variation period of about 4.7 years in the interannual timescale. This implies that the Kuroshio variations and long-term and large scale processes playing roles in modifying the cold patch occurrence frequency.

Unusual Warming in the Coastal Region of Northern South China Sea and Its Impact on the Sudden Intensification of Tropical Cyclone Tembin (2012)

Tropical cyclone Tembin (2012) passed twice and made landfall over south tip of Taiwan in August 2012. During its passage, an unusual sea surface warming was generated at , in the coastal region of northern South China Sea. Subsequently, Tembin passed over this extreme warming region and its intensity was enhanced drastically and suddenly from Category 1 to Category 3 within less than 1-day time interval. This unusual warming seems to largely prompt the intensification of Tembin. Next, the relationship between this extreme warming and rapid intensification of Tembin is identified by atmospheric model Weather Research and Forecast (WRF) with updated time-varying lower boundary condition. In addition, given the tight relationship between generation of unusual warming in the shore region and following possible TC intensification, a series of numerical experiments based on oceanic model Regional Oceanic Modeling System (ROMS) were designed and executed to resolve the possible generated mechanism of the extreme warming. The results indicate that a distinct positive short-wave radiation influx anomaly may dominate the generation of the unusual warming in the shore region during Tembin’s passage. This result is validated by the distributions of free cloudy coverage shown in satellite infrared images.

Modling of Upper Ocean Heat Budget Variations in Response to the Passage of Super Typhoon Sinlaku (2008) in the Western North Pacific

In 2008, Super Typhoon Sinlaku (2008) passed over a preexisting cyclonic eddy (PCE) in the western North Pacific, causing an extreme cooling response at 22.5 °N, 125 °E. This case provides a rare opportunity to explore the physical mechanisms that trigger an extreme cooling response to a typhoon underlying the influence of PCEs. In this study, cooling response to Sinlaku was observed by TMI/AMSR-E microwave SSTs and simulated using the Regional Ocean Modeling System. To elucidate the impact of a PCE, in addition to standard run (EXP_(std)), another experiment that eliminates the influence of a PCE (EXP_(nonPCE)) was designed and executed. By conducting upper ocean heat budget analysis on modeling diagnostic outputs, it is found that PCE enhances the cooling response by enhancing both the entrainment and upwelling simultaneously; but the dominant terms balancing the heat budget were not greatly altered by the PCE. Finally, the vertical thermal gradient is shown to be the essential factor boosting the enhancement of entrainment and upwelling, thus cooling the upper ocean.

Analyses of Upper Layer Thickness in the South China Sea

TOPEX/Poseidon (T/P) satellite altimeter data from 1992 to 1999 and in situ measurements from 1980 to 1999 are used to analyze the upper layer thickness (ULT) in the South China Sea (SCS). The ULT is defined as the depth of the 14degC isotherm from in situ measurements. In comparison with the ULT derived from in situ measurements and that derived from T/P altimeter data yields a correlation coefficient of 0.89 with a slope of 1.01 and an intercept of -2 m. The basin averaged ULT derived from in situ measurements is about 216 m with a seasonal variation of 200 m (winter) to 230 (summer). The ULT is highly correlated with the sea surface temperature (SST) in the study area except the periods of El Nin tildeo. During the normal and La Nin tildea years, ULT is deeper (shallower) when SST is higher (lower). However, during the onset of El Nin tildeo event, a reverse result is found, that is, ULT is deeper (shallower) when SST is lower (higher).

Satellite Observations of Thermal Variability in the South China Sea

In this study, Sea surface temperature (SST) derived from National Oceanic and Atmospheric Administration (NOAA) series satellites Advanced very high resolution radiometer (AVHRR) infrared radiometer and sea surface height (SSH) derived from TOPEX/Poseidon satellite altimeter were used to estimate the sea surface heat storage in the South China Sea (SCS). Temperature profiles from in-situ measurements are used to integrate the heat storage of water column and estimate the error of heat storage derived from satellite data. The empirical orthogonal function (EOF) method was applied to analyze the variation of heat storage of SCS. The results show that the heat storage in SCS increases from 1993 to 2001 and decreases slowly from 2001 to 2004. Furthermore, there are two dynamical instability areas appearing in SCS. The dynamical instability area west off Luzon Island has a similar heat storage variation to the whole SCS. However, at the dynamical instability area of offshore of central Vietnam coast, the heat storage increases from 1993 to 2001, but decreases rapidly from 2001 to 2004. The variation of surface heat storage is also affected by ENSO events. The surface heat storage increases during El Nino events and decreases during La Nina events.