Fronts and surface zonal geostrophic current along 115°E in the southern Indian Ocean

Altimeter and in situ data are used to estimate the mean surface zonal geostrophic current in the section along 115°E in the southern Indian Ocean,and the variation of strong currents in relation to the major fronts is studied.The results show that,in average,the flow in the core of Antarctic Circumpolar Current（ACC） along the section is composed of two parts,one corresponds to the jet of Subantarctic Front（SAF） and the other is the flow in the Polar Front Zone（PFZ）,with a westward flow between them.The mean surface zonal geostrophic current corresponding to the SAF is up to 49 cm · s^-1 at 46°S,which is the maximal velocity in the section.The eastward flow in the PFZ has a width of about 4.3 degrees in latitudes.The mean surface zonal geostrophic current corresponding to the Southern Antarctic Circumpolar Current Front（SACCF） is located at 59.7 °S with velocity less than 20 cm · s^-1.The location of zonal geostrophic jet corresponding to the SAF is quite stable during the study period.In contrast,the eastward jets in the PFZ exhibit various patterns,i.e.,the primary Polar Front（PF1） shows its strong meridional shift and the secondary Polar Front（PF2） does not always coincide with jet.The surface zonal geostrophic current corresponding to SAF has the significant periods of annual,semi-annual and four-month.The geostrophic current of the PFZ also shows significant periods of semi-annual and four-month,but is out of phase with the periods of the SAF,which results in no notable semi-annual and fourmonth periods in the surface zonal geostrophic current in the core of the ACC.In terms of annual cycle,the mean surface zonal geostrophic current in the core of the ACC shows its maximal velocity in June.

The construction of high precision geostrophic currents based on new gravity models of GOCE and satellite altimetry data

The new gravity field models of gravity field and steady-state ocean circulation explorer(GOCE),TIM_R6 and DIR_R6,were released by the European Space Agency(ESA)in June 2019.The sixth generation of gravity models have the highest possible signal and lowest error levels compared with other GOCE-only gravity models,and the accuracy is significantly improved.This is an opportunity to build high precision geostrophic currents.The mean dynamic topography and geostrophic currents have been calculated by the 5th(TIM_R5 and DIR_R5),6th(TIM_R6 and DIR_R6)release of GOCE gravity field models and ITSG-Grace2018 of GRACE gravity field model in this study.By comparison with the drifter results,the optimal filtering lengths of them have been obtained(for DIR_R5,DIR_R6,TIM_R5 and TIM_R6 models are 1°and for ITSG-Grace2018 model is 1.1°).The filtered results show that the geostrophic currents obtained by the GOCE gravity field models can better reflect detailed characteristics of ocean currents.The total geostrophic speed based on the TIM_R6 model is similar to the result of the DIR_R6 model with standard deviation(STD)of 0.320 m/s and 0.321 m/s,respectively.The STD of the total velocities are 0.333 m/s and 0.325 m/s for DIR_R5 and TIM_R5.When compared with ITSG-Grace2018 results,the STD(0.344 m/s)of total geostrophic speeds is larger than GOCE results,and the accuracy of geostrophic currents obtained by ITSG-Grace2018 is lower.And the absolute errors are mainly distributed in the areas with faster speeds,such as the Antarctic circumpolar circulation,equatorial region,Kuroshio and Gulf Stream areas.After the remove-restore technique was applied to TIM_R6 MDT,the STD of total geostrophic speeds dropped to 0.162 m/s.

Validation and error analysis of wave-modifi ed ocean surface currents in the northwestern Pacifi c Ocean

By incorporating the wave-induced Coriolis-Stokes forcing into the classical Ekman layer,the wave-modifi ed ocean surface currents in the northwestern Pacifi c Ocean were estimated.Thus,the ocean surface currents are the combination of classical Ekman current from the cross-calibrated multi-platform(CCMP)wind speed,geostrophic current from the mean absolute dynamic topography(MADT),and wave-induced current based on the European Centre for Medium-Range Weather Forecasts(ECMWF)Interim Re-Analysis(ERA-Interim)surface wave datasets.Weight functions are introduced in the Ekman current formulation as well.Comparisons with in-situ data from Lagrangian drifters in the study area and Kuroshio Extension Observatory(KEO)observations at 32.3°N,144.6°E,and 15-m depth indicate that wave-modifi ed ocean surface currents provide accurate time means of zonal and meridional currents in the northwestern Pacifi c Ocean.Result shows that the wave-modifi ed currents are quite consistent with the Lagrangian drifter observations for the period 1993-2017 in the deep ocean.The correlation(root mean square error,RMSE)is 0.96(1.45 cm/s)for the zonal component and 0.90(1.07 cm/s)for the meridional component.However,wave-modifi ed currents underestimate the Lagrangian drifter velocity in strong current and some off shore regions,especially in the regions along the Japan coast and the southeastern Mindanao.What’s more,the wave-modifi ed currents overestimate the pure Eulerian KEO current which does not consider the impact of waves,and the zonal(meridional)correlation and RMSE are 0.95(0.90)and 11.25 cm/s(12.05 cm/s)respectively.These comparisons demonstrate that our wave-modifi ed ocean surface currents have high precision and can describe the real-world ocean in the northwestern Pacifi c Ocean accurately and intuitively,which can provide important routes to calculate ocean surface currents on large spatial scales.

An assessment of the CMIP5 models in simulating the Argo geostrophic meridional transport in the North Pacifi c Ocean

Eleven climate system models that participate in the Coupled Model Intercomparison Project phase 5(CMIP5)were evaluated based on an assessment of their simulated meridional transports in comparison with the Sverdrup transports.The analyses show that the simulated North Pacifi c Ocean circulation is essentially in Sverdrup balance in most of the 11 models while the Argo geostrophic meridional transports indicate signifi cant non-Sverdrup gyre circulation in the tropical North Pacifi c Ocean.The climate models overestimated the observed tropical and subtropical volume transports signifi cantly.The non-Sverdrup gyre circulation leads to non-Sverdrup heat and salt transports,the absence of which in the CMIP5 simulations suggests defi ciencies of the CMIP5 model dynamics in simulating the realistic meridional volume,heat,and salt transports of the ocean.

Study on the current structure of the thermohaline front in the southeastern entrance of the Yellow Sea during winter

A thermohaline front is located at the southeastern entrance of the Yellow Sea in winter, and it is generated by the intrusion of warm saline water into the Yellow Sea caused by a strong northerly wind. Recently, a westward transversal current traveling away from the west coast of Korea toward the open sea area along the front was reported. The westward transversal current is dominant in the surface layer during the temperature inversion period. The formation and structure of this current are examined using a numerical vertical ocean-slice model.When two different water masses meet, a front is formed and adjusted geostrophically. In this frontal zone, a horizontal pressure gradient flow by the vertically inclined isopycnal occurs under the thermal wind process in a baroclinic effect, and the cold fresh coastal water moves westward along the front in the upper layer. The barotropic effect across the front and the bottom friction effect strengthen the westward component of the velocity. The velocity of the bottom layer decreases remarkably in the increase of the bottom drag coefficient. This means that the bottom friction with the strong background tidal current causes a reduction in the current in the bottom layer.

The Beaufort Gyre variation and its impacts on the Canada Basin in 2003–2012

The Beaufort Gyre （BG） was spun up in the last decade which is an important factor in regulating the variation of the upper ocean. The heat content and freshwater content of the upper ocean increased gradually in the Canada Basin, as did momentum input. Both the geostrophic wind curl and freshwater content could contribute to the spin-up of BG. However, even though there is no change of the wind field the increasing freshwater alone could result in the spin-up of BG. In this study we show that the Pacific Water is difficult to flow into the central basin as the BG spins up and the maximum temperature of the Pacific Summer Water （PSW） experienced a dramatic decrease inside the BG in 2005 and 2009 due to a change of flow pathway of PSW. The enhancement of Ekman Pumping （EP） contributed to the deepening of the Pacific Winter Water by piling up more freshwater. This change of water column dynamics has also contributed to the deepening ofthe Atlantic Water core after 2007. The EP decreased significantly in 2012 （indicating a spin down of BG） and the direction of Ekman transport turned to the north, which favoured the release of freshwater that had resided in the basin for years.

Ocean surface currents estimated from satellite remote sensing data based on a global hexagonal grid

Global ocean surface currents estimated from satellite derived data based on a regular global grid are affected by the grid’s shape and placement.Due to different neighbourhood relationships,the rectangular lat/lon grids lose accuracy when interpolating andfitting elevation data.Hexagonal grids have shown to be advantageous due to their isotropic,uniform neighbourhood.Considering these merits,this paper aims to estimate global ocean surface current using a global isotropic hexagonal grid from satellite remote sensing data.First,gridded satellite altimeter data and wind data with different resolutions are interpolated into the centre of the global isotropic hexagonal grid.Then,geostrophic and Ekman currents components are estimated according to the Lagerlof Ocean currents theory.Finally,the inversion results are verified.By analyzing the results,we conclude that the ocean surface currents estimated based on the global isotropic hexagonal grid have considerable accuracy,with improvement over rectangular lat/lon grids.

NON-ZONAL BAROCLINIC INSTABILITY IN THE NORTH PACIFIC SUBTROPICAL COUNTERCURRENT

Observations indicated that in the region of the Subtropical Countercurrent(STCC) over the Pacific (140°E-170°W, 19°N-28°N), the eddy kinetic energy over the western sideis much higher than that over the eastern side. The cause of such a behavior was theoreticallyinvestigated in this paper. The calculation of geostrophic current in this region indicates thatthere are relatively strong meridional geostrophic currents in this region even though the zonalcurrent is dominant in most seasons. Using a 2. 5-layer reduced-gravity model, ba-roclinicinstability of non-zonal current was discussed. It is found that at the western side of STCC thevertical shear of the meridional geostrophic current will be in favor of the ba-roclinicinstability, but at its eastern side it will suppress ba-roclinic instability, thus causing thegrowth of eddies over the western side much faster than that over the eastern side.

Sustained Decadal Warming Phase in the Southwestern Indian Ocean since the Mid-1990s

Regardless of the slowdown in global warming during the hiatus period, sea surface temperatures(SSTs) in the southwestern Indian Ocean(SWIO) have experienced sustained decadal warming for more than two decades since the mid-1990 s. The SWIO SSTs warmed steadily during 1996–2016, causing a warming hot spot of 0.4 K decade-1 in a large region east of Madagascar. An upper-layer heat budget analysis indicated that heat advection by ocean currents was the greatest contributor to the warming of the SWIO SSTs. The existence of an anticyclonic geostrophic current along the western boundary of the SWIO tended to maintain such warming by transporting warmer water from the west into the SWIO region. In addition, net positive heat transport by ocean currents also occurred at the southern boundary of the SWIO as the climatological northward transport of cold water from the Southern Ocean weakened. This reduction in northward ocean currents at the surface was caused by local wind stress changes, leading to a southward Ekman current. Below the surface, an anticyclonic geostrophic current pattern existed around the warming center near the southeastern SWIO, which reduced the transport of cold waters from the Southern Ocean and warmed the SWIO. These processes near the two boundaries formed a self-sustaining positive feedback mechanism and favored the maintenance of sustained warming in the SWIO. More attention is needed to analyze the sustained long-lasting warming in the SWIO, as it is a unique phenomenon occurring under the background of the ongoing global warming.