Shuiming Chen

Interannual-to-Decadal Variability in the Bifurcation of the North Equatorial Current off the Philippines

Abstract Satellite altimeter sea surface height (SSH) data from the past 17 yr are used to investigate the interannual-to-decadal changes in the bifurcation of the North Equatorial Current (NEC) along the Philippine coast. The NEC bifurcation latitude migrated quasi decadally between 10° and 15°N with northerly bifurcations observed in late 1992, 1997–98, and 2003–04 and southerly bifurcations in 1999–2000 and 2008–09. The observed NEC bifurcation latitude can be approximated well by the SSH anomalies in the 12°–14°N and 127°–130°E box east of the mean NEC bifurcation point. Using a 1 ½-layer reduced-gravity model forced by the ECMWF reanalysis wind stress data, the authors find that the SSH anomalies in this box can be simulated favorably to serve as a proxy for the observed NEC bifurcation. With the availability of the long-term reanalysis wind stress data, this helps to lengthen the NEC bifurcation time series back to 1962. Although quasi-decadal variability was prominent in the last two decades, the N...

Seasonal Modulations in the Eddy Field of the South Pacific Ocean

Abstract Decade-long satellite altimeter data from the Ocean Topography Experiment (TOPEX)/Poseidon and European Remote Sensing Satellite-1 and-2 (ERS-1/2) missions are analyzed to investigate the eddy signals in the South Pacific Ocean. High–eddy kinetic energy (EKE) bands with well-defined annual cycles are detected along the eastward-flowing surface currents of the South Tropical Countercurrent (STCC) between 21°–29°S and the South Equatorial Countercurrent (SECC) centered near 9°S. Overriding the westward-flowing South Equatorial Current (SEC), the STCC layer has the sign of its mean potential vorticity gradient opposite to that in the SEC layer, subjecting the vertically sheared STCC–SEC system to baroclinic instability. In austral winter, the STCC–SEC system is baroclinically more unstable than in other seasons because of the large vertical shear and weak stratification. This seasonal variation in the intensity of baroclinic instability is responsible for the seasonal modulation of the STCCs EKE fi...

Interannual Variability of the North Pacific Subtropical Countercurrent and Its Associated Mesoscale Eddy Field

Interannual changes in the mesoscale eddy field along the Subtropical Countercurrent (STCC) band of 188‐258N in the western North Pacific Ocean are investigated with 16 yr of satellite altimeter data. Enhanced eddy activities were observed in 1996‐98 and 2003‐08, whereas the eddy activities were below average in 1993‐95 and 1999‐2002. Analysis of repeat hydrographic data along 1378E reveals that the vertical shear between the surface eastward-flowing STCC and the subsurface westward-flowing North Equatorial Current (NEC) was larger in the eddy-rich years than in the eddy-weak years. By adopting a 2Ω-layer reduced-gravity model, it is shown that the increased eddy kinetic energy level in 1996‐98 and 2003‐08 is because of enhanced baroclinic instability resulting from the larger vertical shear in the STCC‐NEC’s background flow. The cause for the STCC‐NEC’s interannually varying vertical shear can be sought in the forcing by surface Ekman

Decadal Variability in the Formation of the North Pacific Subtropical Mode Water: Oceanic versus Atmospheric Control

In situ temperature and altimetrically derived sea surface height data are used to investigate the lowfrequency variations in the formation of the North Pacific Ocean Subtropical Mode Water (STMW) over the past 12 yr. Inside the Kuroshio Extension (KE) recirculation gyre where STMW forms, the dominant signal is characterized by a gradual thinning in the late winter mixed layer depth and in the 16°–18°C thermostad layer from 1993 to 1999 and a subsequent steady thickening of these features after 2000. This same decadal signal is also seen in the low-potential-vorticity (PV) STMW layer in the interior subtropical gyre south of the recirculation gyre. By analyzing the air–sea flux data from the NCEP–NCAR reanalysis project, little correlation is found between the decadal STMW signal and the year-to-year changes in the cumulative wintertime surface cooling. In contrast, the decadal signal is found to be closely related to variability in the dynamic state of the KE system. Specifically, STMW formation is reduced when the KE path is in a variable state, during which time high regional eddy variability infuses high-PV KE water into the recirculation gyre, increasing the upper-ocean stratification and hindering the development of a deep winter mixed layer. A stable KE path, on the other hand, favors the maintenance of a weak stratification, leading to a deep winter mixed layer and formation of a thick STMW layer. The relative importance of the surface air–sea flux forcing versus the preconditioning stratification in controlling the variations in the late winter mixed layer depth is quantified using both a simple upper-ocean heat conservation model and a bulk mixed layer model. The majority of the variance (80%) is found to be due to the stratification changes controlled by the dynamic state of the KE system.

Eddy-Induced Heat Transport in the Subtropical North Pacific from Argo, TMI, and Altimetry Measurements

Basin-scale heat transport induced by mesoscale oceanic eddies is estimated by combining satellite-derived sea surface height and temperature [temperature data are from the TRMM Microwave Imager (TMI)] data with Argo float temperature-salinity data. In the North Pacific Ocean subtropical gyre, warm (cold) temperature anomalies of mesoscale eddies are found to be consistently located to the west of high (low) SSH anomalies. The phase misalignment between the temperature and velocity anomalies, however, is largely confined to the seasonal thermocline, causing most of the eddy-induced heat transport to be carried in the surface 200-m layer. By establishing a statistical relationship between the surface and depth-integrated values of the eddy heat transport, the basin-scale eddy heat transport is derived from the concurrent satellite SSH/SST data of the past six years. In the Kuroshio Extension region, the meandering zonal jet is found to generate oppositely signed eddy heat fluxes. As a result, the zonally integrated poleward heat transport associated with the Kuroshio Extension is at a level O(0.1 PW), smaller than the previous estimates based on turbulent closure schemes. Large poleward eddy heat transport is also found in the subtropical North Pacific along a southwest-northeast-tilting band between Taiwan and the Midway Islands. This band corresponds to the region of the subtropical front, and it is argued that the relevant temperature field for identifying this band in the turbulent closure scheme models should be that averaged over the seasonal thermocline.

Decadal Variability in the Large-Scale Sea Surface Height Field of the South Pacific Ocean: Observations and Causes

Large-scale sea surface height (SSH) changes in the extraequatorial South Pacific Ocean are investigated using satellite altimetry data of the past 12 yr. The decadal SSH signals in the 1990s were dominated by an increasing trend in the 30°–50°S band and a decreasing trend in the central South Pacific Ocean poleward of 50°S. In recent years since 2002 there has been a reversal in both of these trends. Spatially varying low-frequency SSH signals are also found in the tropical region of 10°–25°S where the decadal SSH trend is negative in the eastern basin, but positive in the western basin. To clarify the causes for these observed spatially varying SSH signals, a 11U2-layer reduced-gravity model that includes the wind-driven baroclinic Rossby wave dynamics and the responses forced by SSH changes along the South American coast was adopted. The model hindcasts the spatially varying decadal trends in the midlatitude and the eastern tropical regions well. Accumulation of the wind-forced SSH anomalies along Rossby wave characteristics is found to be important for both previously reported long-term trends and their reversals in recent years. The boundary forcing associated with the time-varying SSH signals along the South American coast is crucial for understanding the observed SSH signals of all time scales in the eastern tropical South Pacific basin, but it has little impact upon the midlatitude interior SSH signals.

Effect of Mesoscale Eddies on Subtropical Mode Water Variability from the Kuroshio Extension System Study (KESS)

Abstract Forty-eight profiling floats have been deployed in the Kuroshio Extension (KE) region since May 2004 as part of the Kuroshio Extension System Study (KESS) project. By combining the float temperature–salinity measurements with satellite altimetry data, this study investigates the role played by mesoscale eddies in controlling the property changes in North Pacific Subtropical Mode Water (STMW). Following a 3-yr period of low eddy activity in 2002–04, the KE showed a transition to a high eddy kinetic energy state in 2005. This transition is the result of delayed oceanic response to the 2002 shift in the basin-scale surface wind forcing in connection with the Pacific decadal oscillation. The high eddy kinetic energy state of the KE is characterized by successive shedding of strong cold-core rings into the recirculation gyre, resulting from the interaction of the KE jet with the Shatsky Rise or the preexisting cutoff rings. By transporting northern-origin, high-potential-vorticity (PV) KE water into t...

Coupled decadal variability in the north pacific : An observationally constrained idealized model

Abstract Air–sea coupled variability is investigated in this study by focusing on the observed sea surface temperature signals in the Kuroshio Extension (KE) region of 32°–38°N and 142°E–180°. In this region, both the oceanic circulation variability and the heat exchange variability across the air–sea interface are the largest in the midlatitude North Pacific. SST variability in the KE region has a dominant time scale of ∼10 yr and this decadal variation is caused largely by the regional, wind-induced sea surface height changes that represent the lateral migration and strengthening/weakening of the KE jet. The importance of the air–sea coupling in influencing KE jet is explored by dividing the large-scale wind forcing into those associated with the intrinsic atmospheric variability and those induced by the SST changes in the KE region. The latter signals are extracted from the NCEP–NCAR reanalysis data using the lagged correlation analysis. In the absence of the SST feedback, the intrinsic atmospheric for...

Observations of the Subtropical Mode Water Evolution from the Kuroshio Extension System Study

Abstract Properties and seasonal evolution of North Pacific Ocean subtropical mode water (STMW) within and south of the Kuroshio Extension recirculation gyre are analyzed from profiling float data and additional hydrographic and shipboard ADCP measurements taken during 2004. The presence of an enhanced recirculation gyre and relatively low mesoscale eddy variability rendered this year favorable for the formation of STMW. Within the recirculation gyre, STMW formed from late-winter convection that reached depths greater than 450 m near the center of the gyre. The lower boundary of STMW, corresponding to σθ ≃ 25.5 kg m−3, was set by the maximum depth of the late-winter mixed layer. Properties within the deep portions of the STMW layer remained largely unchanged as the season progressed. In contrast, the upper boundary of the STMW layer eroded steadily as the seasonal thermocline deepened from late April to August. Vertical eddy diffusivity responsible for this erosion was estimated from a budget analysis of ...

Length Scales of Eddy Generation and Nonlinear Evolution of the Seasonally Modulated South Pacific Subtropical Countercurrent

The dynamical processes behind the seasonal modulation of the two-dimensional eddy kinetic energy (EKE) wavenumber spectrum in the Subtropical Countercurrent region of the South Pacific are investigated with 14 yr of satellite altimeter data and climatological hydrographic data. The authors find a seasonally modulated generation of EKE via baroclinic instability in modes with larger meridional length scales. Subsequent nonlinear eddy–eddy interactions redistribute the EKE to larger total horizontal length scales, and larger zonal scales in particular. This is confirmed by diagnosing the spectral transfer of EKE in the surface geostrophic flow, which is found to drive an anisotropic inverse cascade, being redirected in the sense consistent with the effect, as predicted by geostrophic turbulence theory on the plane. Because of the seasonal renewal of meridionally elongated anomalies by baroclinic instability and possibly because of the barotropization process, however, the net outcome for the formation of surface zonal flows is observed to be limited.