Xuhua Cheng

Abnormal upwelling and chlorophyll‐a concentration off South Vietnam in summer 2007

The present study investigates a strong upwelling and concurrent phytoplankton bloom off the South Vietnam coast in August 2007. Analysis of the alongshore wind, offshore Ekman transport, wind stress curl and other parameters indicates the southwesterly summer monsoon plays an important role in this peculiar case. The change of monsoonal wind involves variations of atmospheric circulation on multiple time scales. On the interannual time scale, a positive Indian Ocean Dipole (IOD) in 2007 accompanying with a La Nina event acts to enhance the southwesterly monsoon over the South China Sea (SCS). The enhanced southwesterly wind is regarded as a major factor in promoting the upwelling and the phytoplankton bloom off the South Vietnam coast. On the intraseasonal time scale, variations of the sea surface temperature (SST) and sea surface wind at 30-60 day periods reveal that the SST cooling develops with the evolution of the southwesterly wind anomalies with nearly one week delay, implying the great significance of the Madden-Julian Oscillation (MJO). Moreover, a tropical storm formed in the western SCS in early August reinforces the westerly wind and SST cooling. Among multiple factors on different time scales, the MJO event is considered as the major cause, since it induces the maximum velocity (4 m/s) of the southwesterly wind anomalies. As a result, the high chlorophyll-a concentration (>0.56 mg/m(3)) and low SST center (<27 degrees C) develops in the region off the South Vietnam coast and advects to the central SCS, riding on the northern rim of the southern anticyclonic gyre.

Interannual Variability of High-Wind Occurrence over the North Atlantic*

AbstractInterannual variability of high-wind occurrence over the North Atlantic is investigated based on observations from the satellite-borne Special Sensor Microwave Imager (SSM/I). Despite no wind direction being included, SSM/I data capture major features of high-wind frequency (HWF) quite well. Climatology maps show that HWF is highest in winter and is close to zero in summer. Remarkable interannual variability of HWF is found in the vicinity of the Gulf Stream, over open sea south of Iceland, and off Cape Farewell, Greenland. On interannual scales, HWF south of Iceland has a significant positive correlation with the North Atlantic Oscillation (NAO). An increase in the mean westerlies and storm-track intensity during a positive NAO event cause HWF to increase in this region. In the vicinity of the Gulf Stream, HWF is significantly correlated with the difference between sea surface temperature and surface air temperature (SST − SAT), indicative of the importance of atmospheric instability. Cross-front...

Intra-seasonal variability of Pacific-origin sea level anomalies around the Philippine Archipelago

Intra-seasonal variability of sea level anomalies (SLAs) originated in the Pacific Ocean around the Philippine Archipelago was investigated using merged altimetry SLA measurements and eddy-resolving ocean model outputs. The results suggest the SLA signals from the tropical North Pacific propagate westward as baroclinic Rossby waves on an intra-seasonal time scale. Upon impinging the east coast of the Philippines, these Rossby wave signals transform into coastal trapped waves (CTWs), propagate clockwise along the coast of the Philippine Archipelago and enter into the eastern South China Sea (SCS) through the Sibutu Passage and Mindoro Strait. The SLA signals, however, cannot propagate anticlockwise and enter into the eastern SCS through the Luzon Strait. The intra-seasonal oceanic wave processes are clearly identified by the eddy-resolving model. The effect of along-shore wind forcing on the SLA signals appears insignificant when compared with the remote signals coming from the interior Pacific. While identified in the model simulation, future observations are needed to verify the intra-seasonal CTWs encircling the Philippine Archipelago.

Annual and Interannual Variability of the Tropical Instability Vortices in the Equatorial Eastern Pacific Observed from Lagrangian Surface Drifters

AbstractThis study documents the spatial distributions and temporal variations of anticyclonic eddies with identified radii ≥100 km in the equatorial eastern tropical Pacific Ocean [viz., tropical instability vortices (TIVs)] using Lagrangian surface drifters. The TIVs identified from Lagrangian surface drifters are distributed in a band along 5°N and are closely associated with latitudinal barotropically unstable shear between the westward South Equatorial Current (SEC) and the eastward North Equatorial Countercurrent (NECC). Fewer TIVs are identified from February to June when the shear between the SEC and NECC is weak, whereas more TIVs are found from July to January when the shear is enhanced. The number of identified TIVs also exhibits substantial interannual variability, with fewer TIVs identified during El Nino events and more TIVs found during La Nina events. This relationship is likely associated with the interannual variations of the zonal circulation in the equatorial Pacific modulated by El Ni...

Heat budget of the western Pacific warm pool and the contribution of eddy heat transport diagnosed from HYCOM assimilation

Using the high-resolution Hybrid Coordinate Ocean Model and the Navy Coupled Ocean Data Assimilation Global 1/12° Analysis (GLBa0.08), and the Objectively Analyzed Air–Sea Fluxes and the International Satellite Climatology Cloud Project products, we investigated the seasonal and interannual evolutions of heat budget, including the pseudo-heat content change, the net air–sea heat flux and the eddy heat transport (EHT), based on the time-dependent heat budget analysis in the western Pacific warm pool (WPWP). The results show that the pseudo-heat content change has significant semi-annual variation, which peaks in April–May and September. There is strong positive feedback between EHT and the net air–sea heat flux. EHT is important in balancing the sea surface heat flux into the WPWP. The seasonal EHT variability is dominated by its meridional component. On the interannual time scale, the zonal and vertical components of EHT show comparable amplitudes with the meridional one. The observed net air–sea heat flux in the WPWP is highly correlated with EHT and the pseudo-heat content change on the interannual time scale. The net air–sea heat flux leads the pseudo-heat content change by about half a month and leads EHT by about one month. The variations of the air–sea heat flux and EHT are connected to the El Niño Southern Oscillation events: during the development of El Niño (La Niña) events, the warm pool expanded eastward (retreated westward), the net air–sea surface flux into the WPWP increased (decreased) and EHT enhanced (weakened) significantly.

Performances of Seven Datasets in Presenting the Upper Ocean Heat Content in the South China Sea

In this study, the upper ocean heat content (OHC) variations in the South China Sea (SCS) during 1993–2006 were investigated by examining ocean temperatures in seven datasets, including World Ocean Atlas 2009 (WOA09) (climatology), Ishii datasets, Ocean General Circulation Model for the Earth Simulator (OFES), Simple Ocean Data Assimilation system (SODA), Global Ocean Data Assimilation System (GODAS), China Oceanic ReAnalysis system (CORA), and an ocean reanalysis dataset for the joining area of Asia and Indian-Pacific Ocean (AIPO1.0). Among these datasets, two were independent of any numerical model, four relied on data assimilation, and one was generated without any data assimilation. The annual cycles revealed by the seven datasets were similar, but the interannual variations were different. Vertical structures of temperatures along the 18°N, 12.75°N, and 120°E sections were compared with data collected during open cruises in 1998 and 2005–08. The results indicated that Ishii, OFES, CORA, and AIPO1.0 were more consistent with the observations. Through systematic comparisons, we found that each dataset had its own shortcomings and advantages in presenting the upper OHC in the SCS.

Mechanism of seasonal eddy kinetic energy variability in the eastern equatorial Pacific Ocean

Enhanced mesoscale eddy activities or tropical instability waves (TIWs) exist along the northern front of the cold tongue in the eastern equatorial Pacific Ocean. In this study, we investigate seasonal variability of eddy kinetic energy (EKE) over this region and its associated dynamic mechanism using a global, eddy-resolving ocean general circulation model (OGCM) simulation, the equatorial mooring data and satellite altimeter observations. The seasonal-varying enhanced EKE signals are found to expand westward from 100°W in June to 180°W in December between 0°−6°N. This westward expansion in EKE is closely connected to the barotropically-baroclinically unstable zonal flows that are in thermal-wind balance with the seasonal-varying thermocline trough along 4°N. By adopting an 1½-layer reduced-gravity model, we confirm that the seasonal perturbation of the thermocline trough is dominated by the anti-cyclonic wind stress curl forcing, which develops due to southerly winds along 4°N from June to December.