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Huijie Xue and George Mellor

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.

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Qi Quan and Huijie Xue

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By parameterizing the abyssal mixing as the exchange velocity (entrainment/detrainment) between the middle and deep layers of the South China Sea (SCS), its effects on the multilayer circulation are examined. Results indicate that the cyclonic circulation in the deep SCS appears only when the mixing induces an entrainment of at least 0.72 Sv (1 Sv ≡ 106 m3 s−1) from the deep to the middle layer, which is equivalent to a diapycnal diffusivity of 0.65 × 10−3 m2 s−1 or a net input rate of gravitational potential energy (GPE) of 6.89 GW, respectively. It is also found that tidal mixing in the SCS is stronger than the threshold for the generation of the cyclonic abyssal circulation, but the pattern and evolution of the deep circulation and meridional overturning circulation also depend on the spatiotemporal variability of the mixing. Moreover, the abyssal mixing is able to intensify the anticyclonic circulation in the middle layer but weaken the cyclonic circulation in the upper layer. Vorticity analysis suggests that the upward net flux induced by the abyssal mixing leads to vortex stretching (squeezing) and modulates the pressure gradient by redistributing the layer thickness, hence affects the pattern and strength of the circulation in the middle (deep) layer of the SCS, respectively. The depth-integrated effect of the thickness variation can modulate the pressure gradient across all layers and hence influence the upper-layer circulation.

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Huijie Xue, Fei Chai, and Neal R. Pettigrew

Abstract

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 M 2 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 (M 2) 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.

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Huijie Xue and John M. Bane Jr.

Abstract

The three-dimensional Princeton Ocean Model is used to examine the modification of the Gulf Stream and its meanders by cold air outbreaks. Two types of Gulf Stream meanders are found in the model. Meanders on the shoreward side of the Gulf Stream are baroclinically unstable. They are affected little by the atmospheric forcing because their energy source is stored at the permanent thermocline, well below the influence of the surface forcing. Meanders on the seaward side of the stream are both barotropically and baroclinically unstable. The energy feeding these meanders is stored at the surface front separating the Gulf Stream and the Sargasso Sea, which is greatly reduced in case of cold air outbreaks. Thus, meanders there reduce strength and also seem to slow their downstream propagation due to the southward Ekman flow. Heat budget calculations suggest two almost separable processes. The oceanic heat released to the atmosphere during these severe cooling episodes comes almost exclusively from the upper water column. Transport of heat by meanders from the Gulf Stream to the shelf, though it is large, does not disrupt the principal balance. It is balanced nicely with the net heat transport in the downstream direction.

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Lei Liu, Huijie Xue, and Hideharu Sasaki

Abstract

When evaluated against the 1/30°-resolution, submesoscale-resolving OFES model outputs, the previously published “interior + surface quasigeostrophic” method (from the 2013 study by Wang et al., denoted W13) for reconstructing the ocean interior from sea surface information is found to perform improperly in depicting smaller-scale oceanic motions (associated with horizontal scales smaller than about 150 km). This could be attributed to the fact that the W13 method uses only the barotropic and first baroclinic modes for the downward projection of sea surface height (SSH), while SSH at smaller scales significantly reflects other higher-order modes. To overcome this limitation of W13, an extended method (denoted L19) is proposed by employing a scale-dependent vertical projection of SSH. Specifically, the L19 method makes the projection via two gravest modes as proposed in the W13 method only for larger-scale (>150 km) signals, but for smaller scales (≤150 km) it exploits the framework of the “effective” surface quasigeostrophic (eSQG) method. Evaluation of the W13, eSQG, and L19 methods shows that the proposed L19 method can achieve the most satisfactory subsurface reconstruction in terms of both the flow and density fields in the upper 1000 m. Our encouraging results highlight the potential applicability of L19 method to the high-resolution SSH data from the upcoming Surface Water and Ocean Topography (SWOT) satellite mission.

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Huijie Xue, John M. Bane Jr., and Lauren M. Goodman

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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 to be a result of convergence/divergence of the Ekman transport due to the altered inertial frequency caused by the horizontal velocity shear of the Gulf Stream jet.

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Biao Chen, Huiling Qin, Guixing Chen, and Huijie Xue

Abstract

The sea surface salinity (SSS) varies largely as a result of the evaporation–precipitation difference, indicating the source or sink of regional/global water vapor. This study identifies a relationship between the spring SSS in the tropical northwest Pacific (TNWP) and the summer rainfall of the East Asian monsoon region (EAMR) during 1980–2017. Analysis suggests that the SSS–rainfall link involves the coupled ocean–atmosphere–land processes with a multifacet evolution. In spring, evaporation and water vapor flux divergence were enhanced in some years over the TNWP where an anomalous atmospheric anticyclone was established and a high SSS was well observed. As a result, the convergence of water vapor flux and soil moisture over the EAMR was strengthened. This ocean-to-land water vapor transport pattern was sustained from spring to summer and played a leading role in the EAMR rainfall. Moreover, the change in local spring soil moisture helped to amplify the summer rainfall by modifying surface thermal conditions and precipitation systems over the EAMR. As the multifacet evolution is closely related to the large-scale ocean-to-land water vapor transport, it can be well represented by the spring SSS in the TNWP. A random forest regression algorithm was used to further evaluate the relative importance of spring SSS in predicting summer rainfall compared to other climate indices. As the SSS is now monitored routinely by satellite and the global Argo float array, it can serve as a good metric for measuring the water cycle and as a precursor for predicting the EAMR rainfall.

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Huijie Xue, Ziqin Pan, and John M. Bane Jr.

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The two-dimensional, Advanced Regional Prediction System (ARPS) has been coupled with the Princeton Ocean Model to study air–sea interaction processes during an extreme cold air outbreak over the Gulf Stream off the southeastern United States. Emphases have been placed on the development of the mesoscale front and local winds in the lower atmosphere due to differential fluxes over the land, the cold shelf water, and the warm Gulf Stream, and on how the mesoscale front and the local winds feed back to the ocean and modify the upper-ocean temperature and current fields. Model results show that a shallow mesoscale atmospheric front is generated over the Gulf Stream and progresses eastward with the prevailing airflow. Behind the front, the wind intensifies by as much as 75% and a northerly low-level wind maximum with speeds near 5 m s−1 appears. The low-level northerly winds remain relatively strong even after the front has progressed past the Gulf Stream. The total surface heat flux in the coupled experiment is about 10% less than the total surface heat flux in the experiment with fixed SST, suggesting that the oceanic feedback to the atmosphere might not be of leading importance. On the other hand, the response of the upper-ocean velocity field to the local winds is on the order of 20 cm s−1, dominating over the response to the synoptic winds. This suggests the modification in the atmosphere by air–sea fluxes, which induces the locally enhanced winds, has considerable impact on the ocean. That is, there is significant atmospheric feedback to the ocean through the heat-flux-enhanced surface winds.

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Feng Nan, Huijie Xue, Fei Chai, Dongxiao Wang, Fei Yu, Maochong Shi, Peifang Guo, and Peng Xiu

Abstract

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 ≡ 106 m3 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.

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Gengxin Chen, Weiqing Han, Xiaolin Zhang, Linlin Liang, Huijie Xue, Ke Huang, Yunkai He, Jian Li, and Dongxiao Wang

Abstract

Using 4-yr mooring observations and ocean circulation model experiments, this study characterizes the spatial and temporal variability of the Equatorial Intermediate Current (EIC; 200–1200 m) in the Indian Ocean and investigates the causes. The EIC is dominated by seasonal and intraseasonal variability, with interannual variability being weak. The seasonal component dominates the midbasin with a predominant semiannual period of ~166 days but weakens toward east and west where the EIC generally exhibits large intraseasonal variations. The resonant second and fourth baroclinic modes at the semiannual period make the largest contribution to the EIC, determining the overall EIC structures. The higher baroclinic modes, however, modify the EIC’s vertical structures, forming multiple cores during some time periods. The EIC intensity has an abrupt change near 73°E, which is strong to the east and weak to the west. Model simulation suggests that the abrupt change is caused primarily by the Maldives, which block the propagation of equatorial waves. The Maldives impede the equatorial Rossby waves, reducing the EIC’s standard deviation associated with reflected Rossby waves by ~48% and directly forced waves by 20%. Mode decomposition further demonstrates that the semiannual resonance amplitude of the second baroclinic mode reduces by 39% because of the Maldives. However, resonance amplitude of the four baroclinic mode is less affected, because the Maldives fall in the node region of mode 4’s resonance. The research reveals the spatiotemporal variability of the poorly understood EIC, contributing to our understanding of equatorial wave–current dynamics.

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