A mechanism for the latitudinal dependence of peak-spectrum sea surface height variability

Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 119 (2014): 1431–1444, doi:10.1002/2013JC009642.Previous studies have shown that the power spectrum of satellite-observed sea surface height (SSH) variability peaks at a certain frequency (or a wave number) band at a given latitude. Lin et al. (2008) attributed this latitudinal dependence to the critical frequency of the first baroclinic mode Rossby waves in the tropical and subtropical oceans. Their study was based on the linear Rossby wave theory and focused on SSH variability in the tropical and subtropical oceans since the altimetry data do not adequately resolve lengths of baroclinic Rossby waves at and near the critical frequency in high latitudes. In this study, we expand their analysis to high-latitude oceanic basins and to include nonlinear eddy effects, by using a linear wave model and a high-resolution model output from the OGCM for the Earth Simulator (OFES). It is found that the linear wave mechanism by and large remains valid in the tropical and subtropical oceans. In higher latitudes as well as in some regions in the western tropical and subtropical oceans, other mechanisms, like nonlinear eddy, play more important role in determining the SSH variability.This work was supported by the China’s National Basic Research Priorities Programmer (2013CB956202), Strategic Priority Research Program of the Chinese Academy of Sciences (XDA11010103), the Natural Science Foundation of China (41222037 and 41221063), the project of Global Change and Air-Sea interaction (GASI-03-01-01–02), the Ministry of Education’s 111 Project (B07036), the National Natural Science Foundation of Shandong (JQ201111), and the National Special Research Fund for Non-Profit Marine Sector (201205018). J. Y. is supported by US NSF (OCE 0927017 and OCE 1028739).2014-08-2

On the dynamics of the seasonal variation in the South China Sea throughflow transport

Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 118 (2013): 6854–6866, doi:10.1002/2013JC009367.The Luzon Strait transport (LST) of water mass from the Pacific Ocean to the South China Sea (SCS) varies significantly with seasons. The mechanisms for this large variability are still not well understood. The steady-state island rule, which is derived from a steady-state model, is not applicable to seasonal time scale variations in a large basin like the Pacific Ocean. In this paper, we will use a theoretical model that is based on the circulation integral around the Philippines. The model relates the LST variability to changes in the boundary currents along the east coast of the Philippines, including the North Equatorial Current (NEC) Bifurcation Latitude (NECBL), the transports of Kuroshio and Mindanao Currents (KC and MC), and to the local wind-stress forcing. Our result shows that a northward shift of the NECBL, a weakening of the KC or a strengthening of the MC would enhance the LST into the SCS. This relationship between the LST and the NEC-KC-MC is consistent with observations. The analytical result is tested by a set of idealized numerical simulations.This study has been supported by the National Science Foundation Grants (OCE 1028739, 0927017) (JY), and by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA11010103), the project of Global Change and Air-Sea interaction (GASI-03-01-01-02), the Natural Science Foundation of China (40930844, 41222037), the National Basic Research Program of China (2013CB956202), Ministry of Education’s 111 Project (B07036) of China, Yong Science Foundation of Shandong (JQ201111) and Public science and technology research funds projects of ocean (201205018) (XL and DW).2014-06-1

Poleward shift of the Kuroshio Extension front and its impact on the North Pacific Subtropical Mode Water in the recent decades

Author Posting. © American Meteorological Society, 2021. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 51(2), (2021): 457–474, https://doi.org/10.1175/JPO-D-20-0088.1.The meridional shift of the Kuroshio Extension (KE) front and changes in the formation of the North Pacific Subtropical Mode Water (STMW) during 1979–2018 are reported. The surface-to-subsurface structure of the KE front averaged over 142°–165°E has shifted poleward at a rate of ~0.23° ± 0.16° decade−1. The shift was caused mainly by the poleward shift of the downstream KE front (153°–165°E, ~0.41° ± 0.29° decade−1) and barely by the upstream KE front (142°–153°E). The long-term shift trend of the KE front showed two distinct behaviors before and after 2002. Before 2002, the surface KE front moved northward with a faster rate than the subsurface. After 2002, the surface KE front showed no obvious trend, but the subsurface KE front continued to move northward. The ventilation zone of the STMW, defined by the area between the 16° and 18°C isotherms or between the 25 and 25.5 kg m−3 isopycnals, contracted and displaced northward with a shoaling of the mixed layer depth hm before 2002 when the KE front moved northward. The STMW subduction rate was reduced by 0.76 Sv (63%; 1 Sv ≡ = 106 m3 s−1) during 1979–2018, most of which occurred before 2002. Of the three components affecting the total subduction rate, the temporal induction (−∂hm/∂t) was dominant accounting for 91% of the rate reduction, while the vertical pumping (−wmb) amounted to 8% and the lateral induction (−umb ⋅ ∇hm) was insignificant. The reduced temporal induction was attributed to both the contracted ventilation zone and the shallowed hm that were incurred by the poleward shift of KE front.Xiaopei Lin is supported by the National Natural Science Foundation of China (41925025 and 92058203) and China’s national key research and development projects (2016YFA0601803). Baolan Wu is supported by the China Scholarship Council (201806330010). Lisan Yu thanks NOAA for support for her study on climate change and variability

Wind-driven exchanges between two basins : some topographic and latitudinal effects

Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 118 (2013): 4585–4599, doi:10.1002/jgrc.20333.This study examines some topographic effects on the island rule. We use an idealized and barotropic model to investigate the throughflow between a semienclosed marginal sea and a larger oceanic basin that are connected to each other by two channels. Two sets of experiments are conducted in parallel, one with a flat bottom and the other with a ridge between two basins. The model results show that the ridge affects the island rule considerably in several ways. First, the ridge blocks geostrophic contours and restricts a free exchange between two basins. The bottom pressure torque (or the form drag) is a dominant term in the balance of the depth-integrated vorticity budget and always results in a significant reduction of the throughflow transport. Second, horizontal friction promotes cross-isobathic flows and enhances the throughflow transport over the ridge. This is the opposite of what friction does in the original island rule in which a friction tends to reduce the throughflow transport. Third, the forcing region in the open ocean for the marginal-sea throughflow is shifted meridionally. Last, the topographic effect becomes small near the equator due to its dependence on f. This may explain why the PV barrier effect is smaller in the South China Sea than in the Japan/East Sea. The limitation of the barotropic model and some baroclinic effects will be discussed.This study has been supported by the National Science Foundation grants OCE 1028739, OCE 0927017, ARC 1107412, and ARC 0902090 (J.Y.), the WHOI Coastal Institute, and by the Ministry of Education’s 111 Project (B07036), National Basic Research Priorities Programmer (2013CB956202), Natural Science Foundation (41222037, 41221063), Natural Science Foundation of Shandong (JQ201111), and Public Welfare Scientific Research Project (201205018) of China (X.L. and D.W.).2014-03-1

Dynamics of an idealized Beaufort Gyre : 1. The effect of a small beta and lack of western boundaries

Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 121 (2016): 1249–1261, doi:10.1002/2015JC011296.The Beaufort Gyre in the Arctic Ocean differs from a typical moderate-latitude gyre in some major aspects of its dynamics. First, it is located in a basin without a western boundary, which is essential for closing midlatitude circulations. Second, the gradient in Coriolis parameter, β, is small and so the validity of the Sverdrup balance is uncertain. In this paper, we use an idealized two-layer model to examine several processes that are related to these two issues. In a circular basin with closed geostrophic contours in interior, the variability of vorticity in the upper layer is dominated by eddies. But in the time-mean circulation, the main dynamical balance in the basin's interior is between the curl of wind stress and the eddy vorticity fluxes. The torque of friction becomes important along the boundary where the rim current is strong. It is found that the smallness of β has only a relatively small impact in a circular basin without a meridional boundary. The gyre is considerably more sensitive to the existence of a meridional boundary. The time-mean circulation weakens considerably when a peninsula is inserted between the model's center and the rim. (One side of the peninsula is dynamically equivalent to a midlatitude western boundary.) The gyre's sensitivity to β has also increased significantly when a meridional boundary is present. Subsurface ridges have similar effects on the gyre as a boundary, indicating that such topographic features may substitute, to some extents, the dynamical role of a western boundary.This study has been supported by the National Science Foundation's Arctic Natural Science Program for J.Y. and A.P. via grant PRL-1107412, and for AP via grants PRL-1313614, PRL-1302884, and PRL-1107277.2016-08-1

Decadal to multidecadal variability of the mixed layer to the south of the Kuroshio Extension region

Author Posting. © American Meteorological Society, 2020. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 33(17), (2020): 7697-7714, https://doi.org/10.1175/JCLI-D-20-0115.1.The decadal to multidecadal mixed layer variability is investigated in a region south of the Kuroshio Extension (130°E–180°, 25°–35°N), an area where the North Pacific subtropical mode water forms, during 1948–2012. By analyzing the mixed layer heat budget with different observational and reanalysis data, here we show that the decadal to multidecadal variability of the mixed layer temperature and mixed layer depth is covaried with the Atlantic multidecadal oscillation (AMO), instead of the Pacific decadal oscillation (PDO). The mixed layer temperature has strong decadal to multidecadal variability, being warm before 1970 and after 1990 (AMO positive phase) and cold during 1970–90 (AMO negative phase), and so does the mixed layer depth. The dominant process for the mixed layer temperature decadal to multidecadal variability is the Ekman advection, which is controlled by the zonal wind changes related to the AMO. The net heat flux into the ocean surface Qnet acts as a damping term and it is mainly from the effect of latent heat flux and partially from sensible heat flux. While the wind as well as mixed layer temperature decadal changes related to the PDO are weak in the western Pacific Ocean. Our finding proposes the possible influence of the AMO on the northwestern Pacific Ocean mixed layer variability, and could be a potential predictor for the decadal to multidecadal climate variability in the western Pacific Ocean.Xiaopei Lin is supported by the China’s national key research and development projects (2016YFA0601803) and the National Natural Science Foundation of China (41925025 and U1606402). Baolan Wu is supported by the China Scholarship Council (201806330010). Lisan Yu thanks NOAA for support for her study on climate change and variability

Recent decadal change in the North Atlantic subtropical underwater associated with the poleward expansion of the surface salinity maximum

Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research-Oceans 124(7), (2019): 4433-4448, doi: 10.1029/2018JC014508.Yu et al. (2017, https://doi.org/10.1002/2017GL075772) reported that the annual mean sea surface salinity maximum (SSS‐max) in the North Atlantic expanded northward by 0.35 ± 0.11° per decade over the 34‐year data record (1979–2012). The expansion shifted and expanded the ventilation zone northward and increased the production of the Subtropical Underwater (STUW). As a result, the STUW became deeper, thicker, and saltier. In this study, the seasonal characteristics of the poleward expansion of the North Atlantic SSS‐max and their effects on the STUW are examined. The results show that the SSS‐max expansion occurred primarily during boreal spring (April, May, and June) and expanded northward by 0.43 ± 0.21° per decade over the 34‐year period. The annual volume of the STUW increased by 0.21 ± 0.09 1014 m3 per decade over the same period, and the spring (April, May, and June) volume increased by 0.31 ± 0.02 1014 m3 per decade (a relative increase of 48 ± 1%). The characteristics of the decadal changes in STUW were attributable to the increased subduction rate associated with the northward expansion of the SSS‐max. The annual subduction rate increased by 0.29 ± 0.07 Sv per decade over the 34 years, and the greatest increase of 1.73 ± 0.61 Sv per decade occurred in April. The change in subduction associated with the expansion of the SSS‐max appeared to be consistent with the Atlantic Multidecadal Oscillation.Most of the work was conducted at the Woods Hole Oceanographic Institution, while H. Liu was a guest student sponsored by the China Scholarship Council (201506330001). H. Liu thanks Drs. Ruixin Huang and Xiangze Jin for discussions on the computation of the STUW formation and subduction rates. The Ishii subsurface salinity and temperature analysis data sets were downloaded from https://rda.ucar.edu/datasets/ds285.3/. The EN4 data set is available at https://www.metoffice.gov.uk/hadobs/en4/download‐en4‐2‐1.html. The LEGOS SSS is accessible from http://www.legos.obs‐mip.fr/observations/sss/datadelivery/products.The OAFlux vector wind analysis is available at http://oaflux.whoi.edu. The NAO index was downloaded from https://www.ncdc.noaa.gov/teleconnections/nao/. The AMO index is available at https://www.esrl.noaa.gov/psd/data/timeseries/AMO/. X. Lin is supported by China's National Key Research and Development Projects (2016YFA0601803) in addition to the National Natural Science Foundation of China (41521091 and U1606402) and the Qingdao National Laboratory for Marine Science and Technology (2017ASKJ01).2019-12-1

On the Wave Energy Assessment in the South China Sea

This paper presents a thirty year (1976-2005) assessment of wave energy resource within the South China Sea (SCS) by simulation. Significant wave height (SWH) between simulation and observation shows good agreement. This shows the reliability of an along-side simulated wave period in estimating wave energy in the SCS. Results show that estimates of wave power density are more reliable in the north-central SCS and most sufficient during winter. The annual mean wave power density peaked at 12.7kW/m and 12.9kW/m during years 1986 and 1999 respectively while the highest seasonal mean of 29kW/m occurred in year 1999 during winter. The wave power density is most stable in winter and is generally more stable in offshore regions of SCS. Wave power density is most stable in years 1976, 1997 and 2004 with stability values of 1.96, 1.98 and 1.9 respectively. The stability value of 0.9 in year 1980 is the greatest in the winter of all years. Relative-rich energy regions occupy the largest area during winter. The relatively richest energy is generally concentrated in the central and north-central SCS. No area is identified as a relative-rich energy region during spring. Winter 1999 has the highest relative-rich energy with value of 37kW/m

The annual cycle of the Japan Sea throughflow

Author Posting. © American Meteorological Society, 2016. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 46 (2016): 23–39, doi:10.1175/JPO-D-15-0075.1.The mechanism responsible for the annual cycle of the flow through the straits of the Japan Sea is investigated using a two-layer model. Observations show maximum throughflow from summer to fall and minimum in winter, occurring synchronously at the three major straits: Tsushima, Tsugaru, and Soya Straits. This study finds the subpolar winds located to the north of Japan as the leading forcing agent, which first affects the Soya Strait rather than the Tsushima or Tsugaru Straits. The subpolar winds generate baroclinic Kelvin waves along the coastlines of the subpolar gyre, affect the sea surface height at the Soya Strait, and modify the flow through the strait. This causes barotropic adjustment to occur inside the Japan Sea and thus affect the flow at the Tsugaru and Tsushima Straits almost synchronously. The barotropic adjustment mechanism explains well why the observations show a similar annual cycle at the three straits. The annual cycle at the Tsugaru Strait is further shown to be weaker than that in the other two straits based on frictional balance around islands, that is, frictional stresses exerted around an island integrate to zero. In the Tsugaru Strait, the flows induced by the frictional integrals around the northern (Hokkaido) and southern (Honshu) islands are in opposite directions and tend to cancel out. Frictional balance also suggests that the annual cycle at the Tsugaru Strait is likely in phase with that at the Soya Strait because the length scale of the northern island is much shorter than that of the southern island.S. Kida is supported by KAKENHI (22106002). B. Qiu is supported by NASA (NNX13AE15G). J. Yang is supported by the U.S. National Science Foundation. X. Lin is supported by the Natural Science Foundation of China (41222037 and U1406401), China’s National Basic Research Priorities Programme (2013CB956202), and the Global Air-Sea Interaction Project (GASI-03-01-01-02).2016-07-0

Meridional heat transport variability induced by mesoscale processes in the subpolar North Atlantic

© The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Nature Communications 9 (2018): 1124, doi:10.1038/s41467-018-03134-x.The ocean’s role in global climate change largely depends on its heat transport. Therefore, understanding the oceanic meridional heat transport (MHT) variability is a fundamental issue. Prevailing observational and modeling evidence suggests that MHT variability is primarily determined by the large-scale ocean circulation. Here, using new in situ observations in the eastern subpolar North Atlantic Ocean and an eddy-resolving numerical model, we show that energetic mesoscale eddies with horizontal scales of about 10–100 km profoundly modulate MHT variability on time scales from intra-seasonal to interannual. Our results reveal that the velocity changes due to mesoscale processes produce substantial variability for the MHT regionally (within sub-basins) and the subpolar North Atlantic as a whole. The findings have important implications for understanding the mechanisms for poleward heat transport variability in the subpolar North Atlantic Ocean, a key region for heat and carbon sequestration, ice–ocean interaction, and biological productivity.J.Z. was financially supported by the Postdoctoral Scholar Program at WHOI, with funding provided by the Ocean and Climate Change Institute. This work was also supported by the US National Science Foundation (OCE-1258823 and OCE-1634886), as well as by China’s national key research and development projects (2016YFA0601803), the National Natural Science Foundation of China (41521091 and U1606402), the Qingdao National Laboratory for Marine Science and Technology (2015ASKJ01), and the Fundamental Research Funds for the Central Universities (201424001 and 201362048)