Huijie Xue

Observations of the Eastern Maine Coastal Current and its offshore extensions in 1994

Cold surface temperatures, reflecting Scotian Shelf origins and local tidal mixing, serve as a tracer of the Eastern Maine Coastal Current and its offshore extensions, which appear episodically as cold plumes erupting from the eastern Maine shelf. A cold water plume emanating from the Eastern Maine Coastal Current in May 1994 was investigated using advanced very high resolution radiometer (AVHRR) imagery, shipboard surveys of physical and biochemical properties, and satellite-tracked drifters. Evidence is presented that suggests that some of the plume waters were entrained within the cyclonic circulation over Jordan Basin, while the major portion participated in an anticyclonic eddy at the distal end of the plume. Calculations of the nitrate transported offshore by the plume show that this feature can episodically export significant quantities of nutrients from the Eastern Maine Coastal Current to offshore regions that are generally nutrient depleted during spring-summer. A series of AVHRR images is used to document the seasonal along-shelf progression of the coastal plume separation point. We speculate on potential causes and consequences of plume separation from the coastal current and suggest that this feature may be an important factor influencing the patterns and overall biological productivity of the eastern Gulf of Maine.

A Model Study of the Seasonal Circulation in the Gulf of Maine

The Princeton Ocean Model is used to study the circulation in the Gulf of Maine and its seasonal transition in response to wind, surface heat flux, river discharge, and the M2 tide. The model has an orthogonal-curvature linear grid in the horizontal with variable spacing from 3 km nearshore to 7 km offshore and 19 levels in the vertical. It is initialized and forced at the open boundary with model results from the East Coast Forecast System. The first experiment is forced by monthly climatological wind and heat flux from the Comprehensive Ocean Atmosphere Data Set; discharges from the Saint John, Penobscot, Kennebec, and Merrimack Rivers are added in the second experiment; the semidiurnal lunar tide (M2) is included as part of the open boundary forcing in the third experiment. It is found that the surface heat flux plays an important role in regulating the annual cycle of the circulation in the Gulf of Maine. The spinup of the cyclonic circulation between April and June is likely caused by the differential heating between the interior gulf and the exterior shelf/slope region. From June to December, the cyclonic circulation continues to strengthen, but gradually shrinks in size. When winter cooling erodes the stratification, the cyclonic circulation penetrates deeper into the water column. The circulation quickly spins down from December to February as most of the energy is consumed by bottom friction. While inclusion of river discharge changes details of the circulation pattern, the annual evolution of the circulation is largely unaffected. On the other hand, inclusion of the tide results in not only the anticyclonic circulation on Georges Bank but also modifications to the seasonal circulation.

An exceptional anticyclonic eddy in the South China Sea in 2010

The highest sea level near the Xisha Islands in recent 20 years occurred during August 2010. Satellite altimeter data indicated that the extreme event was largely due to an anticyclonic eddy, whose amplitude exceeded 20 cm and size exceeded 400 km on 11 August 2010. Cruise observations showed the eddy raised the center temperature by 7.7 degrees C at 75 m and vertically extended to 500 m. Eddy tracking showed it had a life span of more than 8 months and propagated far from the south of Xisha Islands. Such strong and long-lasting eddy that moved northward for such a long distance was observed for the first time in the South China Sea (SCS). Observational data from CTD/XBT and the reconstructed three-dimensional temperature and salinity were used to explore the eddys features and vertical structure. Our analyses show the 2010 summer monsoon and current in the western boundary of the SCS were largely altered after the 09/10 El Nino event. From May onward, the wind blew northward and strengthened over the northwestern SCS. Such wind drove a strong northward current along the western boundary, which carried the eddy northward by advection from May to July. Energy budget showed, during the eddy northward propagation, the boundary current passed energy to the eddy, which led to the continuing growth of the eddy in both strength and size.

Instability of the Gulf Stream Front in the South Atlantic Bight

Abstract To understand Gulf Stream meanders in the South Atlantic Bight, the growth of three-dimensional perturbations along two-dimensional frontal zones is examined by using linearized primitive equations. The Fourier–Galerkin method and the orthogonal collocation method are combined to formulate the spectral model. Emphasis is placed on the effects of cross-frontal topographic slope on the stability of the front, and on the characteristics of the most unstable modes. Attention is directed to the cross sections upstream and downstream of the Charleston Bump, which is a topographic feature near 31°N. The major results obtained from this linear study are that 1) the growth rate of the most unstable mode decreases and the associated phase speed increases after incorporating cross-front topographic gradients; 2) the most unstable solution found in the region downstream of the Charleston Bump has a slightly longer wavelength and slower phase speed than those found in the region upstream of the Bump.

Meridional overturning circulation in the South China Sea envisioned from the high-resolution global reanalysis data GLBa0.08

The pattern of meridional overturning circulation (MOC) in the South China Sea (SCS) is studied using a numerical Lagrangian tracing method with the HYCOM+NCODA Global 1/12 degrees Analysis (GLBa0.08) data. The SCS MOC has a sandwich structure, which consists of a layer of stronger clockwise circulation above 500 m depth, a counterclockwise layer in the mid layer between 500 and 1000 m depth, and a weaker clockwise layer below 1000 m. The deep (below 1000 m depth) clockwise layer is divided into three cells, namely, the deep southern MOC cell, DSMOC; the deep middle MOC cell, DMMOC; and the unclosed deep northern MOC cell, DNMOC. The inflow through the Luzon Strait is the main source for the SCS MOCs. The upper layer Luzon Strait inflow dominates the upper SCS MOC structure but has relatively less contribution to the DNMOC, whereas the deep layer Luzon Strait inflow mainly influences the DNMOC and it mostly rises near 18 degrees N. The inflow through the Taiwan Strait mainly contributes to the upper layer MOC. Moreover, inflows from the Mindoro and Karimata straits contribute negatively to the upper MOC but play a significant role on the DSMOC. The backward integration of Lagrangian trajectories further validates that the SCS deep water comes not only from the deep inflow but also from the entrainment of the middle and upper layer inflow through the Luzon Strait. In the SCS basin, there are three northwest-southeast tilted zones where tracers upwell, which correspond to the three deep MOC cells. One possible mechanism for these upwelling zones is the interaction between the continental slope-trapped waves and the westward planetary Rossby waves.

Cross‐shelf exchange in the shelf of the East China Sea

A high-resolution, 3-dimensional primitive equation model is used to investigate the cross-shelf exchange in the East China Sea (ECS). Favorable comparisons between field data and model simulations from both climatological run and hindcast run for 2006 indicate that the model has essential skills in capturing the key physics of the ECS. Temporal and spatial variations of the cross-shelf exchanges are further analyzed. It was demonstrated from both observations and simulations that in 2006 high saline water could be delivered to the north of the Changjiang River mouth (near 32 degrees N) as a result of stronger than typical cross-shelf exchanges at the shelf break and flows through the Taiwan Strait with an annual mean rate of 2.59 and 1.83 Sv, respectively. A few new places at the shelf break were also identified where persistent and vigorous onshore or offshore exchanges occur throughout the year. Cross-shelf exchange is largely determined by the along-shelf geostrophic balance with weak seasonality, which is modulated in upper layers by northeasterly monsoon from early-fall to late-spring and at seabed by bottom friction during December-January, May, and August-September. Nonlinear effect, with strong spatial variations and intraseasonal variability, is a secondary but persistent contributor to the net seaward transport, except for northeast of Taiwan where the nonlinear effect becomes significant but more varied.

Modification of the Gulf Stream through Strong Air–Sea Interactions in Winter: Observations and Numerical Simulations

Abstract The greatest fluxes of heat and moisture from the ocean to the atmosphere occur off the east coast of North America during winter when the Gulf Stream is vigorously cooled by strong cold air outbreaks that move off the continent. In this paper observational and numerical modeling methods are employed to investigate the response of the Gulf Stream to such strong cooling events. Both methods show that the surface mixed layer can deepen several tens of meters during a single strong outbreak and that the heat decrease within the upper layer of the Gulf Stream, 2.9 × 1013 J in the model and 3.2(±0.7) × 1013 J in observations (per meter alongstream) for one case study, is balanced closely by the amount of oceanic heat released to the atmosphere. Computations also show that the cross-stream circulation is dominated by Ekman-like, wind-driven motion with velocities on the order of 20 cm s−1. A vertical circulation cell within the Gulf Stream, with vertical velocities on the order of 0.1 cm s−1, is found ...

Freshening in the South China Sea during 2012 revealed by Aquarius and in situ data

Newly available sea surface salinity (SSS) data from the Aquarius together with in situ hydrographic data are used to explore the spatial and temporal characteristics of SSS in the South China Sea (SCS). Using in situ observations as the reference, an evaluation of daily Aquarius data indicates that there exists a negative bias of 0.45 psu for the version 3.0 data set. The root-mean-square difference for daily Aquarius SSS is about 0.53 psu after correcting the systematic bias, and those for weekly and monthly Aquarius SSSs are 0.45 and 0.29 psu, respectively. Nevertheless, the Aquarius SSS shows a reliable freshening in the SCS in 2012, which is larger than the Aquarius uncertainty. The freshening of up to 0.4 psu in the upper-ocean of the northern SCS was confirmed by in situ observations. This freshening in 2012 was caused by a combined effect of abundant local freshwater flux and limited Kuroshio intrusion. By comparing the Kuroshio intrusion in 2012 with that in 2011, we found the reduction as a relatively important cause for the freshening over the northern SCS. In contrast to the northern SCS, reduced river discharge in 2012 played the leading role to the saltier surface in the region near the Mekong River mouth with respect to 2011.

Weakening of the Kuroshio Intrusion into the South China Sea over the Past Two Decades

Inferred from the satellite and in situ hydrographic data from the 1990s and 2000s, the Kuroshio intrusion into the South China Sea (SCS) had a weakening trend over the past two decades. Associated with the weakened Kuroshio intrusion, the Kuroshio loop and eddy activity southwest of Taiwan became weaker, whereas the water above the salinity minimum became less saline in the northern SCS. The sea surface height southwest of Taiwan increased at a slower rate compared to other regions of the SCS because of the weakened Kuroshio intrusion. Simulations using the Regional Ocean Modeling System (ROMS) Pacific model show that the strength of the Kuroshio intrusion into the SCS decreased from 1993 to 2010 with a negative trend, -0.24 sverdrups (Sv) yr(-1) (1 Sv 10(6) m(3) s(-1)), in the total Luzon Strait transport (LST). Although wind-induced Ekman transport through the Luzon Strait became weaker, the magnitude at 0.001 Sv yr(-1) was too small to compensate for the negative trend of the LST. On the other hand, the piling up of the water induced by monsoon winds was an important mechanism for changing the pressure gradient across the Luzon Strait and eventually affecting the LST. The sea level gradient between the western Pacific and the SCS had a negative trend, -0.10 cm yr(-1), corresponding to a negative trend in the geostrophic transport at -0.20 Sv yr(-1). The Kuroshio transport east of Luzon Island also had a negative trend, which might also be linked to the weakening Kuroshio intrusion.

Quantification of diffusion distance within the spongy myocardium of hearts from antarctic fishes

We developed a stereological method for quantifying diffusion distance within spongy myocardium. Using this method we compared the hearts of three species of Antarctic fishes that vary in expression of oxygen-binding proteins. We examined hearts from Gobionotothen gibberifrons, a red-blooded species whose ventricle has myoglobin (Mb), and hearts of two species of icefish that lack hemoglobin (Hb) and vary in expression of cardiac Mb; Chionodraco rastrospinosus expresses Mb, Chaenocephalus aceratus does not. Average diffusion distance within ventricular tissue is greater in red-blooded Antarctic teleosts (9.82 + or - 1.37 microm) compared with icefish (C. rastrospinosus, 6.20 microm + or - 0.86; C. aceratus, 6.23 + or - 0.41 microm). Average diffusion distance to a mitochondrion parallels this trend because mitochondria are uniformly distributed within cardiac muscle. Results show that loss of Hb is correlated with increased trabeculation of heart ventricle. Loss of Mb however, is not correlated with an increase in trabeculation of ventricular tissue, despite significant differences in cellular ultrastructure compared with species that express the protein.