Anticyclonic Warm-core Eddies Research Articles

Comparative analysis of four types of mesoscale eddies in the North Pacific Subtropical Countercurrent region - part II seasonal variation

The North Pacific Subtropical Countercurrent area (STCC) is high in mesoscale eddy activities. According to the rotation direction of the eddy flow field and the sign of temperature anomaly within the eddy, they can be divided into four categories: cyclonic cold-core eddy (CCE), anticyclonic warm-core eddy (AWE), cyclonic warm-core eddy (CWE) and anticyclonic cold-core eddy (ACE). CCE and AWE are called normal eddies, and CWE and ACE are named abnormal eddies. Based on the OFES data and vector geometry automatic detection method, we find that at the sea surface, the maximum monthly number of the CCE, AWE, CWE, and ACE occurs in December (765.70 ± 52.05), January (688.20 ± 82.53), August (373.40 ± 43.09) and August (533.00 ± 56.92), respectively. The number of normal eddies is more in winter and spring, and less in summer and autumn, while abnormal eddies have the opposite distribution. The maximum rotation velocity of the four types of eddies appears in June (11.71 ± 0.75 cm/s), June (12.24 ± 0.86 cm/s), May (10.63 ± 0.99 cm/s) and June (9.97 ± 0.91 cm/s), which is fast in winter and spring. The moving speed of the four types of eddies is almost similar (about 10 ~ 11 cm/s). The amplitude of normal and abnormal eddies is both high in summer and autumn, and low in winter and spring, with larger amplitudes in normal than abnormal eddies. The eccentricity (defined as the eccentricity of the ellipse obtained by fitting the eddy boundary) of the four types of eddies is also close to each other, and their variation ranges from 0.7 to 0.8, with no apparent seasonal variation. The vertical penetration depth, which has no significant seasonal difference, is 675.13 ± 67.50 m in cyclonic eddies (CCE and CWE), which is deeper than that 622.32 ± 81.85 m in anticyclonic eddies (ACE and AWE). In addition, increasing the defined temperature threshold for abnormal eddies can significantly reduce their numbers but does not change their seasonal variation trend.

Comparative analysis of four types of mesoscale eddies in the north pacific subtropical countercurrent region – part I spatial characteristics

The North Pacific Subtropical Countercurrent (STCC) region has high mesoscale eddy activities due to its complex circulation structure. This study divides these mesoscale eddies into four types: cyclonic cold-core eddy (CCE), anticyclonic warm-core eddy (AWE), cyclonic warm-core eddy (CWE), and anticyclonic cold-core eddy (ACE) according to the rotation direction of the eddy flow field and the sign of average temperature anomaly within the eddy after spatial high-pass filtering. CCE and ACE are called normal eddies, while CWE and ACE are named abnormal eddies. Using eddy-resolving model data (OFES), this work finds that the abnormal eddy phenomenon mainly occurs in the ocean’s upper layer. The eddy number proportion for CCEs, AWEs, CWEs, and ACEs at the sea surface is 35.60, 32.08, 12.95, and 19.37%. The corresponding average radius is 79.14 ± 3.7, 83.34 ± 3.75, 73.74 ± 4.14, and 79.46 ± 3.89 km, respectively. Each type of eddy’s average amplitude is about 3 cm. Regarding the eddy average eccentricity, the four types of eddies have very close eccentricities, with a range of 0.73 ~ 0.76. If the types of eddies are not distinguished, the eddies generated north of 21°N tend to move southward, while eddies generated south of that latitude tend to move northward. The depth of CCEs, AWEs, CWEs, and ACEs with average eddy nonlinearity larger than one is concentrated in the ocean’s upper layer at 109.0, 116.0, 159.0, and 52.0 m, respectively. This study deepens the understanding of the spatial distribution characteristics of mesoscale eddies in the STCC region.

Comparative analysis of four types of mesoscale eddies in the Kuroshio-Oyashio extension region

Oceanic mesoscale cyclonic (anticyclonic) eddies usually have cold (warm) cores and counterclockwise (clockwise) flow fields in the Northern Hemisphere. However, “abnormal” cyclonic (anticyclonic) eddies with warm (cold) cores and counterclockwise (clockwise) flow fields have recently been identified in the Kuroshio-Oyashio Extension (KOE) region. Here, traditional cyclonic cold-core eddies (CCEs) and anticyclonic warm-core eddies (AWEs) are termed normal eddies, and cyclonic warm-core eddies (CWEs) and anticyclonic cold-core eddies (ACEs) are called abnormal eddies. Applying a vector geometry-based automatic eddy detection method to the Ocean General Circulation Model for the Earth Simulator reanalysis data (OFES), a three-dimensional eddy dataset is obtained and used to quantify the statistical characteristics of these eddies. Results illustrate that the number of CCEs, AWEs, CWEs, and ACEs accounted for 38.46, 36.15, 13.40, and 11.99%, respectively. In the vertical direction, normal eddies are concentrated in the upper 2,000 m, while abnormal eddies are mainly found in the upper 600 m of the ocean. On seasonal scales, normal eddies are more abundant in winter and spring than in summer and autumn, with the opposite trend found for abnormal eddies. Potential density changes modulated by normal eddies are dominated by eddies-induced temperature anomalies, while salinity anomalies dominate the changes modulated by abnormal eddies. This study expands the types of eddies and enriches their understanding in the KOE region.

Cyclone Ockhi Induced Upwelling and Associated Changes in Biological Productivity in Arabian Sea

In this study, the effect of severe tropical cyclone Ockhi on the physical and biological changes in ocean properties has been investigated. Winds from SCATSAT-1 was used for estimating the cyclone induced Ekman pumping. The role of ocean stratification, cyclonic wind stress, eddies and cyclone translation speed on the post cyclone oceanic response has been investigated. 8-day composite data of winds, sea surface temperature (SST), chlorophyll and sea level anomaly (SLA) have been analyzed for pre-cyclone, during cyclone and after cyclone period to understand the effect of wind forcing on the ocean variables. It was observed that there was a significant phytoplankton bloom like concentration along with 2-3 °C drop in SST after the passage of Ockhi. Thermocline displacements of up to 56 m were observed along the cyclone track. SLA data analyzed for the period shows formation of a cyclonic cold core eddy and weakening of anticyclonic warm core eddies. The entire cyclone track was divided into 12 track points based on the 12 hourly position and the change in SST and chlorophyll analyzed. The maximum change in chlorophyll and SST was observed where the cyclone was slowest and mixed layer shallow.

Mesoscale Warm-Core Eddies Drive Interannual Modulations of Swordfish Catch in the Kuroshio Extension System

Recent observational and numerical studies have suggested that the decadal modulation of the Kuroshio Extension system, driven by mesoscale variabilities, profoundly affect the basin scale physical and biogeochemical oceanography. However, it remains unclear how these decadal changes affect distribution and abundance of fish species in this region. In this study, 26,964 swordfish catch data obtained by longliners during 2004 and 2010 in the western North Pacific are analyzed with an eddy-resolving ocean reanalysis by using mesoscale dynamic parameters and an eddy detection technique, to clarify the effects of mesoscale eddies and their variabilities on the swordfish catch and distribution. During this period, the Kuroshio Extension underwent two different dynamic phases: stable path state in 2004, 2005 and 2010; and unstable path state during 2006-2009. Based on our analyses, we show here that swordfish are more concentrated in and near the anticyclonic warm-core eddies in the northern site, 36 -45 • N, of the Kuroshio Extension system, especially during the unstable path phase. This is found to be caused by the interannual modulation of mesoscale eddy activities due to more warm-core rings generated from the unstable Kuroshio Extension, making it easier for fishermen to target swordfish in this region.

Adjoint sensitivity studies of loop current and eddy shedding in the Gulf of Mexico

[1] Adjoint model sensitivity analyses were applied for the loop current (LC) and its eddy shedding in the Gulf of Mexico (GoM) using the MIT general circulation model (MITgcm). The circulation in the GoM is mainly driven by the energetic LC and subsequent LC eddy separation. In order to understand which ocean regions and features control the evolution of the LC, including anticyclonic warm-core eddy shedding in the GoM, forward and adjoint sensitivities with respect to previous model state and atmospheric forcing were computed using the MITgcm and its adjoint. Since the validity of the adjoint model sensitivities depends on the capability of the forward model to simulate the real LC system and the eddy shedding processes, a 5 year (2004–2008) forward model simulation was performed for the GoM using realistic atmospheric forcing, initial, and boundary conditions. This forward model simulation was compared to satellite measurements of sea-surface height (SSH) and sea-surface temperature (SST), and observed transport variability. Despite realistic mean state, standard deviations, and LC eddy shedding period, the simulated LC extension shows less variability and more regularity than the observations. However, the model is suitable for studying the LC system and can be utilized for examining the ocean influences leading to a simple, and hopefully generic LC eddy separation in the GoM. The adjoint sensitivities of the LC show influences from the Yucatan Channel (YC) flow and Loop Current Frontal Eddy (LCFE) on both LC extension and eddy separation, as suggested by earlier work. Some of the processes that control LC extension after eddy separation differ from those controlling eddy shedding, but include YC through-flow. The sensitivity remains stable for more than 30 days and moves generally upstream, entering the Caribbean Sea. The sensitivities of the LC for SST generally remain closer to the surface and move at speeds consistent with advection by the high-speed core of the current, while sensitivities to SSH generally extend to deeper layers and propagate more slowly. The adjoint sensitivity to relative vorticity deduced from the sensitivities to velocity fields suggests that advection of cyclonic (positive) relative vorticity anomalies from the YC or the LCFEs accelerate the LC eddy separation. Forward model perturbation experiments were performed to complement and check the adjoint sensitivity analysis as well as sampling the predictability and nonlinearity of the LC evolution. The model and its adjoint can be used in four-dimensional variational assimilation (4D-VAR) to produce dynamically consistent ocean state estimates for analysis and forecasts of the circulation of the GoM.

Anticyclonic eddies in the northeastern South China Sea during winter 2003/2004

The origins and evolutions of two anticyclonic eddies in the northeastern South China Sea (SCS) were examined using multi-satellite remote sensing data, trajectory data of surface drifting buoys, and in-situ hydrographic data during winter 2003/2004. The results showed that buoy 22918 tracked an anti-cyclonic warm-core eddy (AE1) for about 20 days (December 4–23, 2003) in the northeastern SCS, and then escaped from AE1 eventually. Subsequently to that, buoy 22517 remained within a different anti-cyclonic warm-core eddy (AE2) for about 78 days (from January 28 to April 14, 2004) in the same area. It drifted southwestward for about 540 km, and finally entered into the so-called “Luzon Gyre”. Using inference from sea level anomaly (SLA), sea surface temperature (SST), geostrophic currents and the buoys’ trajectories, it is shown that both eddies propagated southwestward along the continental slope of the northern SCS. The mean speeds of AE1 and AE2 movements were 9.7 cm/s and 10.5 cm/s, respectively, which are similar to the phase speed of Rossby waves in the northern SCS. The variation of instantaneous speeds of the eddy movement and intensity of anticyclonic eddy may suggest complex interactions between an anticyclonic eddy and its ambient fluids in the northern SCS, where the eddy propagated southwestward with Rossby waves. Furthermore, SLA and SST images in combination with the temperature and salinity profiles obtained during a cruise suggested that AE1 was generated in the interior SCS and AE2 was shed from the “Kuroshio meander”.

Eddy formation through the interaction between the Leeuwin Current, Leeuwin Undercurrent and topography

Abstract The surface and subsurface circulation off south-west Australia was simulated using the Regional Ocean Modelling System (ROMS), a primitive equation ocean model with terrain-following s-coordinates. The major currents in this region—the Leeuwin Current and Leeuwin Undercurrent—were reproduced by specifying only temperature and salinity distributions from climatology. The application of wind stress subsequent to the model reaching a quasi-steady state, resulted in the generation of the seasonal Capes Current, a northward-flowing current on the continental shelf associated with coastal upwelling. The simulated currents compared well with the observed current patterns including the location and maximum current speeds. Shelf topography variations and bottom shear, which generated vorticity, influenced eddy formation. Eddies separated from the Leeuwin Current and the Leeuwin Undercurrent, migrated westward, and exited the model domain through merging, dissipating or through an open boundary. In the model simulations, the Undercurrent produced mostly cyclonic eddies due to strong negative vorticity where the current flowed against the continental slope. The Leeuwin Current produced anticyclonic warm-core eddies initiated from the formation of meanders, which were strongest at the surface. The eddy field was dominated by anticyclonic eddies at the surface and cyclonic eddies at 500 m. The interaction between the Leeuwin Current and Leeuwin Undercurrent led to the formation of eddy pairs.