OS22A-01
Multidecadal variability of the Atlantic MOC and its link to the North Pacific
The 60-80 years multidecadal variability (MDV) of the Atlantic MOC in the coupled ECHAM5/MPIOM is presented as an atmosphere-ocean coupled mode. Strengthening of the MOC is associated with cooling in the subpolar basin and warming south of the North Atlantic Current (NAC) region. In response to this anomalous positive across-NAC temperature gradient, the polar-front jet shifts northward. The resulting wind anomalies further enhance the cooling in the subpolar basin by speeding up the subpolar gyre and extracting more heat from the deep-water formation region. The MOC reaches its maximum when the big body of the newly formed deep-water "flees" from the stronger-than-normal subpolar gyre entering the subtropical basin. Near the surface, the warm anomalies south to the NAC are advected into the subpolar basin weakening the deep-water formation. The opposite half of the cycle starts. Multidecadal signals are also present in the North Pacific; its possible relation to the MDV of the Atlantic MOC is discussed.
OS22A-02
Decadal Sea Level Variability in the South Pacific in a Global Eddy-Resolving Ocean Model Hindcast
Sea level variability and related oceanic changes in the South Pacific from 1970 to 2003 are investigated using a hindcast simulation of an eddy-resolving Ocean general circulation model For the Earth Simulator (OFES) along with sea level data from tide-gauges since 1970 and satellite altimeter since 1992. The first empirical orthogonal function mode of Sea Level Anomalies (SLAs) of OFES exhibits broad positive SLAs over the central and western South Pacific. The corresponding principal component indicates roughly stable high, low and high SLAs, separated by rapid sea level fall in the late 1970s and sea level rise in the late 1990s, consistent with tide-gauge and satellite observations. These decadal changes are accompanied by circulation changes of the subtropical gyre at 1000-m depth, and changes of upper-ocean zonal current and eddy activity around the Tasman Front. In general agreement with previous related studies, it is found that sea level variations in the Tasman Sea can be explained by propagation of long baroclinic Rossby waves forced by wind stress curl anomalies, if the impact of New Zealand is taken into account. The corresponding atmospheric variations are associated with decadal variability of the El Nino-Southern Oscillation (ENSO). Hence, decadal sea level variability in the western and central South Pacific in the past three and half decades and decadal ENSO variability are likely to be connected. The sea level rise in the 1990s, which attracted much attention in relation to the global warming, is likely associated with the decadal cooling in the tropical Pacific.
OS22A-03
The Impact of Ocean Tides on a Climate Model Simulation.
We explicitly include the forcing of ocean tides in a global ocean general circulation model (OGCM). The tidal forcing is deduced from lunisolar ephemerides according to the instantaneous positions of moon and sun. In this real-time approach we consider the complete lunisolar tides of second degree. The OGCM is part of a state-of-the-art climate model which was used for the fourth assessment report simulations of the Intergovernmental Panel on Climate Change (IPCC). An ensemble of five IPCC A1B climate scenarios covering the period 1860 to 2059 has been computed. The induced tidal currents affect the ocean circulation by nonlinear interaction and through vertical mixing. The latter is described in the model by a Richardson number dependent mixing term. Thus, mixing depends on the density stratification and the vertical velocity shear. In regions of high tidal velocities the vertical velocity shear is enhanced in the deepest layers induced by bottom friction. Our study focuses on the North Atlantic region, where the highest tidal velocities occur. There, the representation of the present state of the ocean is improved significantly. The tides adjust the pathway of the North Atlantic Current, which leads to improved sea surface temperatures of up to 3 degree in the North Atlantic. Further, the simulation of the deep convection in the Labrador Sea, one of the driving mechanisms of the meridional overturning circulation, becomes more realistic when forcing ocean tides. The modified oceanic dynamics in the North Atlantic have implications for the simulation of the European climate and for the future projection of the sea surface temperature of the North Atlantic. This study reveals that ocean tides are an important component in the simulation of ocean dynamics and are essential for an appropriate simulation of a changing ocean under climate warming conditions.
OS22A-04
Meso- and Submesoscale Mechanisms of Vertical Exchange of Fluid associated with Mesoscale Eddies around Hawaii
Meso- and submesoscale surface temperature fronts are ubiquitous in the surface ocean around the Hawaiian Islands and tend to be regions where the surface chlorophyll concentration is larger. These surface fronts are shown to be coincident with regions of large straining by calculating the Finite-Size Lyapunov Exponents. Elongated submesoscale filaments, generated by the straining of the eddying flow are associated with large vertical velocities and can act as an important pathway for the vertical exchange of fluid between the nutrient-rich waters below the euphotic zone and the sun-lit upper ocean. Using high-resolution model simulations we focus on surface frontogenesis and nonlinear Ekman pumping as the main mechanisms in generating submesoscale vertical velocities of the order of 70m/day around Hawaii. Flow conditions are shown flow to be favorable to the development of Ageostrophic Anticyclonic Instabilities, which are thought to be responsible for the onset of submesoscale motions. Given that the Hawaiian region is characterized by the robust northeasterly trade winds and by strong mesoscale eddies, both nonlinear Ekman pumping and surface frontogenesis are relevant for the region and provide a possible explanation for the observed patterns in surface chlorophyll with potential impact on primary and export production.
OS22A-05
Global hydrographic variability patterns during 2003-2007
Global temperature and salinity fields from the near surface layer down to 2000m depth based on ARGO measurements are used to analyze large-scale variability patterns on annual to interannual time scales as they are derived from the monthly mean values during the years 2003-2007. Previous estimates of global hydrographic fluctuations have been created using different data sets, partly based on scarce sampling. The substantial advantage of this study includes a detailed summary of annual to interannual variability patterns of the global ocean based on a single and more uniform data base. The dominant signal of upper ocean variability is the annual cycle characterized by a clear hemispheric asymmetry with strongest amplitudes in the northern hemisphere where changes of the annual signal from year to year are strong. Annual amplitudes of the salinity field play an important role in the near surface tropical and subpolar parts of the global ocean. The dominant harmonic of the global temperature field increases at mid-latitudes from the surface down to more than 300m depth and amplitudes are subsurface intensified in the tropical basin. At mid-latitudes in the area of the subtropical and subpolar fronts, interannual anomalies show a deep baroclinic component down to more than 1500m depth in all three ocean basins. In the tropical basins, dominant patterns of baroclinic variability are mostly confined to the upper 500m depth. However, the dominant signatures of global interannual variability occur in the equatorial band and between 5-10°N during the years 2003-2007. A global estimation of long-term changes of heat and salt content as well as of steric height are derived from the gridded field. Heat content and steric height changes are clearly associated with a positive trend during the 5 years of measurements whereas changes of salinity content are low, indicating a weak freshening tendency from the years 2003 to 2007. Altough strong discrepancies remain, steric height changes are in better agreement with satellite derived quantities than it was reported in previous results.
OS22A-06
A new estimate of the North Atlantic 3D circulation from altimetry and in-situ measurements.
A new estimate of the North Atlantic 3D circulation has been computed for the 1993-2002 period combining altimetry to temperature and salinity data through the thermal wind equation. The 3D monthly current field compares well to the Mera-11 reanalysis from the Mercator operational system. Also, the regression coefficients computed between the new field and in-situ subsurface velocities measured by RAFOS and P- ALACE floats reach 0.66 and 0.65 for the zonal and meridional components respectively. This new 3D observed currents product represents a unique tool to monitor the North Atlantic circulation over the past decade. First analysis show that the weakening over the 1993-2002 period of the subpolar gyre already known at the surface extends down to 1500m. Associated to this weakening, we also observe a decrease of the MOC strength over the same period (-0.2 Sv per year) although the interannual signal is large.
OS22A-07
Glider-based Measurements of Kuroshio Seasonal Variation
The Kuroshio, the strongest western boundary current in the Pacific, carries significant heat and momentum northward from the tropics. Along its path, it interacts with mesoscale eddies and topography and modulates internal waves formation and propagation. The Kuroshio flows northward offshore of Luzon, and often intrudes through the Luzon Strait into the South China Sea. After leaving the Luzon Strait, it continues northward, strengthening as it flows along the east coast of Taiwan. The processes that govern the Kuroshio's growth as it passes from Luzon to Taiwan are poorly understood, perhaps due to the limited availability of data collected at the relevant temporal and spatial scales. This study employed autonomous long-endurance gliders to collect one year of repeated hydrographic sections extending from the northern half of Luzon to Taiwan, the region that marks of the beginning of the Kuroshio. The Kuroshio is characterized by a salty layer (maximum ~35) at the depth of about 300 m and a fresher layer at about 700 m. The upper high-salinity layer extends deeper after passing through the Luzon Strait, as does the 18 °C isotherm, which could act as an indicator of the main axis of the Kuroshio. Kuroshio transport increases northward from the Luzon to Taiwan suggesting the possible contribution of water from the Luzon Strait/South China Sea or the eastern side of the Kuroshio. Seasonal variation of the Kuroshio under the distinct northeast and southwest monsoons are shown from the changes in temperature and salinity and the intrusion positions of the Kuroshio front. These results are compared with historical observations to further our understanding of the Kuroshio dynamics.
OS22A-08
Processes influencing the modification of the East Siberian Arctic boundary current
Microstructure and hydrographic observations, during September 2007 in the boundary current on the East Siberian continental slope, document upper ocean stratification and along-stream water mass changes. A thin warm surface layer overrides a shallow halocline characterized by a ~40-m thick temperature minimum layer beginning at ~30 m depth. Below the halocline, well-defined thermohaline diffusive staircases extended downwards to warm Atlantic Water intrusions found at 200-800 m depth. Observed turbulent eddy kinetic energy dissipations are extremely low (ε < 10-6 W m-3), thus allowing molecular diffusion to control vertical mixing in the upper-ocean. The consequent doubly-diffusive heat fluxes FdcH ~1 W m-2, are at least an order of magnitude too small to account for the observed along-stream cooling of the boundary current. Our results implicate circulation patterns and the influence of shelf waters in causing the observed differences in the boundary current waters.