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Niklas Schneider

Abstract

The ocean–atmosphere response to the surfacing of temperature anomalies from the oceanic thermocline is a key process in climate variability with decadal time scales. Using a coupled general circulation model, it is shown how density-compensating temperature and salinity (spiciness) anomalies emerging in the upwelling region of the equatorial Pacific modulate tropical climate.

Upon reaching the surface in the central equatorial Pacific, warm and salty spiciness anomalies increase sea surface temperature and salinity, and vent their heat anomaly to the atmosphere, primarily by the latent heat flux. The associated surface buoyancy flux increases vertical mixing, and thereby dampens surface temperature anomalies. The moisture added to the atmosphere increases precipitation in the western Pacific and intertropical convergence zone, and strengthens the trade winds east, and weakens them west of the date line. Central equatorial Pacific surface temperatures are slightly warmed by the resulting deepened thermocline, and additional warm spiciness anomalies due to a northward displacement of the climatological spiciness front on the equator, recycling salt anomalies in the shallow equatorial circulation and subduction from the Southern Hemisphere. From the Northern Hemisphere source regions of equatorial thermocline waters, cool and fresh anomalies result from the increased air–sea freshwater fluxes and wind-driven changes of the flow paths in the thermocline. The amplitudes of the model's El Niño–La Niña are diminished by warm spiciness anomalies due to a reduction of the temperature gradient in density coordinates that controls the thermocline feedback.

The coupled response is qualitatively consistent with a coupled climate mode that results from a positive feedback between the equatorial emergence of spiciness anomalies and the equatorial pycnocline and Southern Hemisphere responses, and a delayed, negative feedback due to Northern Hemisphere subduction. However, feedbacks are weak, and, at best, slightly enhance a decadal modulation of the Tropics due to spiciness anomalies generated by stochastic atmospheric forcing.

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Niklas Schneider

Abstract

The role of the Indonesian Throughflow in the global climate system is investigated with a coupled ocean–atmosphere model by contrasting simulations with realistic throughflow and closed Indonesian passages.

The Indonesian Throughflow affects the oceanic circulation and thermocline depth around Australia and in the Indian Ocean as described in previous studies and explained by Sverdrup transports. An open throughflow thereby increases surface temperatures in the eastern Indian ocean, reduces temperatures in the equatorial Pacific, and shifts the warm pool and centers of deep convection in the atmosphere to the west. This control on sea surface temperature and deep convection affects atmospheric pressure in the entire Tropics and, via atmospheric teleconnections, in the midlatitudes. As a result, surface wind stress in the entire Tropics changes and meridional and zonal gradients of the tropical thermocline and associated currents increase in the Pacific and decrease in the Indian Ocean. The response includes an acceleration of the equatorial undercurrent in the Pacific, and a deceleration in the Indian Ocean. Thus the Indonesian Throughflow exerts significant control over the global climate in general and the tropical climate in particular.

Changes of surface fluxes in the Pacific warm pool region are consistent with the notion that shading by clouds, rather than increases of evaporation, limit highest surface temperatures in the open ocean of the western Pacific. In the marginal seas of the Pacific and in the Indian Ocean no such relationship is found. The feedback of the throughflow transport and its wind forcing is negative and suggests that this interplay cannot excite growing solution or lead to self-sustained oscillations of the ocean–atmosphere system.

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Niklas Schneider

Abstract

The horizontal scale dependences of in-phase and lagged imprints of ocean-mesoscale sea surface temperatures on surface winds are investigated using daily AMSR-E radiometer and QuikSCAT scatterometer observations in the Southern Ocean. Spectral transfer functions separate underlying processes dependent on large-scale winds, horizontal wavenumbers, and corresponding Rossby numbers. For Rossby numbers smaller than 1, winds reflect modulations of the Ekman layer by sea surface temperature–induced changes of hydrostatic pressure. Rossby numbers large compared to 1 suggest a balance of advection and modulations of vertical mixing. Impulse response functions reveal Southern Hemisphere, Doppler-shifted, near-inertial lee waves excited by warm ocean-mesoscale sea surface temperatures. On the right (left) flank of the downwind wake of warm air and low atmospheric pressure, winds are enhanced (diminished) due to constructive (destructive) interference of inertial turning, pressure gradient forces, and vertical mixing. Wind convergence over the warm wake is stronger compared to the upwind divergence. Time averaging smooths the response, and degrades the lee wave.

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Niklas Schneider and Peter Müller

Abstract

This study investigates the sensitivity of the dynamics of the surface equatorial ocean to the parameterization of vertical mixing. A new high-resolution, numerical model of a zonally independent equatorial channel helps to explore this question and includes three parameterizations, all of which increase mixing for decreasing Richardson numbers. It compares the smooth increase of eddy coefficients traditionally used in general circulation models, the dramatic increase of the eddy coefficients for small Richardson numbers recently observed in the equatorial Pacific, and the combination of a mixing mechanism based on the diagnostic adjustment of the water column to noncritical Richardson numbers and of a bulk mixed layer model.

The meridional and vertical velocity fields in the surface layer are very sensitive to the strength of mixing implied by the different parameterizations. For the smooth Richardson number dependence of the eddy coefficients, equatorial upwelling due to easterly winds reaches the surface. The dramatically increasing eddy coefficients for small Richardson numbers yield reduced equatorial upwelling rates in the surface layer. The diagnostic adjustment of the Richardson number shows in the surface layer close to the equator reversed meridional shear and downwelling in response to easterly winds!

A simple model for the low-latitude wind current in the presence of horizontal density gradients reproduces this reversal of the meridional and vertical flows. If the equatorial Ekman number is large, there is a latitude range where within the upper layer the vertically averaged flow and density are dominated by rotation, while the vertical shear of horizontal velocities is strongly influenced by vertical friction. In this region vertical shears point downstream of the wind stress and of the pressure forces due to gradients in density. For an easterly wind the pressure gradient forces surface waters toward the equator and can reverse the vertical shear of meridional velocity and the equatorial vertical velocity. The critical value of the vertical eddy coefficient for this reversal to occur is of the order of 5 × 10−3 m2 s−1. This value is of the same order as measured in the surface equatorial Pacific and used in general circulation models. The physics of this reversal are so basic it is likely they are active in the ocean and three-dimensional circulation models.

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Bunmei Taguchi and Niklas Schneider

Abstract

Upper ocean heat content (OHC) is at the heart of natural climate variability on interannual-to-decadal time scales, providing climate memory and the source of decadal prediction skill. In the midlatitude North Pacific Ocean, OHC signals are often found to propagate eastward as opposed to the frequently observed westward propagation of sea surface height, another variable that represents the ocean subsurface state. This dichotomy is investigated using a 150-yr coupled GCM integration. Simulated OHC signals are distinguished in terms of two processes that can support eastward propagation: higher baroclinic Rossby wave (RW) modes that are associated with density perturbation, and spiciness anomalies due to density-compensated temperature and salinity anomalies. The analysis herein suggests a unique role of the Kuroshio–Oyashio Extension (KOE) region as an origin of the spiciness and higher mode RW signals. Wind-forced, westward-propagating equivalent barotropic RWs cause meridional shifts of the subarctic front in the KOE region. The associated anomalous circulation crosses mean temperature and salinity gradients and thereby generates spiciness anomalies. These anomalies are advected eastward by the mean currents, while the associated surface temperature anomalies are damped by air–sea heat exchange. The accompanying surface buoyancy flux generates higher baroclinic, eastward-propagating RWs. The results suggest that the large OHC variability in the western boundary currents and their extensions is associated with the spiciness gradients and axial variability of oceanic fronts.

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Kohei Takatama and Niklas Schneider

Abstract

The effect of ocean current drag on the atmosphere is of interest as a test case for the role of back pressure, because the response is independent of the thermally induced modulation of the boundary layer stability and hydrostatic pressure. The authors use a regional atmospheric model to investigate the impact of drag induced by the Kuroshio in the East China Sea on the overlying winter atmosphere. Ocean currents dominate the wind stress curl compared to the impacts of sea surface temperature (SST) fronts. Wind stress convergences and divergences are weakly enhanced even though the ocean current is almost geostrophic. These modifications change the linear relationships (coupling coefficients) between the wind stress curl/divergence and the SST Laplacian, crosswind, and downwind gradients. Clear signatures of the ocean current impacts are found beyond the sea surface: sea surface pressure (back pressure) decreases near the current axis, and precipitation increases over the downwind region. However, these responses are very small despite strong Ekman pumping due to the current. A linear reduced gravity model is used to explain the boundary layer dynamics. The linear vorticity equation shows that the oceanic influence on wind stress curl is balanced by horizontal advection decoupling the boundary layer from the interior atmosphere. Spectral transfer functions are used to explain the general response of back pressure to geostrophic ocean currents and sea surface height.

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Niklas Schneider and Bo Qiu

Abstract

The response of the atmospheric boundary layer to fronts of sea surface temperature (SST) is characterized by correlations between wind stress divergence and the downwind component of the SST gradient and between the wind stress curl and the crosswind component of the SST gradient. The associated regression (or coupling) coefficients for the wind stress divergence are consistently larger than those for the wind stress curl. To explore the underlying physics, the authors introduce a linearized model of the atmospheric boundary layer response to SST-induced modulations of boundary layer hydrostatic pressure and vertical mixing in the presence of advection by a background Ekman spiral. Model solutions are a strong function of the SST scale and background advection and recover observed characteristics. The coupling coefficients for wind stress divergence and curl are governed by distinct physics. Wind stress divergence results from either large-scale winds crossing the front or from a thermally direct, cross-frontal circulation. Wind stress curl, expected to be largest when winds are parallel to SST fronts, is reduced through geostrophic spindown and thereby yields weaker coupling coefficients.

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Niklas Schneider and Peter Müller

Abstract

We describe the meridional and seasonal structures of daily mean mixed-layer depth and its diurnal amplitude and their relation to atmospheric fluxes by compositing mixed-layer depth estimates derived from density observations. The diurnal mean mixed-layer depth shows a ridge at the equator, troughs, which vary seasonally in intensity, at 10° to 15°N and 5° to 10°S, and a trough appearing just north of the equator in the second half of the year. This is in contrast to the ridge-trough structure of the top of the main thermocline, which reflects the dynamic topography associated with the equatorial current system. The diurnal amplitude is significantly different from zero for most latitudes year-round, indicating that the diurnal cycle of mixed-layer depth is a widespread phenomenon. For sufficiently strong heating, both the mixed-layer depth and its diurnal amplitude are significantly correlated with Monin-Obukhov length scales based on the mean net heat flux, mean wind stress, and mean shortwave radiation. This suggests a possible parameterization of the mixed-layer depth and diurnal amplitude in terms of the mean atmospheric fluxes for meridional scales of a few degrees and seasonal time scales.

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James T. Potemra and Niklas Schneider

Abstract

The relationship between 3- and 10-yr variability in Indian Ocean temperatures and Indonesian throughflow (ITF) volume transport is examined using results from a 300-yr integration of the coupled NCAR Parallel Climate Model (PCM). Correlation and regression analyses are used with physical reasoning to estimate the relative contributions of changes in ITF volume transport and Indian Ocean surface atmospheric forcing in determining low-frequency temperature variations in the Indian Ocean. In the PCM, low-frequency variations in ITF transport are small, 2 Sv (1 Sv ≡ 106 m3 s−1), and have a minimal impact on sea surface temperatures (SSTs). Most of the low-frequency variance in Indian Ocean temperature (rms > 0.5°C) occurs in the upper thermocline (75–100 m). These variations largely reflect concurrent atmospheric forcing; ITF-induced temperature variability at this depth is limited to the outflow region between Java and Australia extending westward along a band between 10° and 15°S.

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Michael A. Spall and Niklas Schneider

Abstract

A simple analytic model is developed to represent the offshore decay of cold sea surface temperature (SST) signals that originate from wind-driven upwelling at a coastal boundary. The model couples an oceanic mixed layer to an atmospheric boundary layer through wind stress and air–sea heat exchange. The primary mechanism that controls SST is a balance between Ekman advection and air–sea exchange. The offshore penetration of the cold SST signal decays exponentially with a length scale that is the product of the ocean Ekman velocity and a time scale derived from the air–sea heat flux and the radiative balance in the atmospheric boundary layer. This cold SST signal imprints on the atmosphere in terms of both the boundary layer temperature and surface wind. Nonlinearities due to the feedback between SST and atmospheric wind, baroclinic instability, and thermal wind in the atmospheric boundary layer all slightly modify this linear theory. The decay scales diagnosed from two-dimensional and three-dimensional eddy-resolving numerical ocean models are in close agreement with the theory, demonstrating that the basic physics represented by the theory remain dominant even in these more complete systems. Analysis of climatological SST off the west coast of the United States also shows a decay of the cold SST anomaly with scale roughly in agreement with the theory.

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