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Veit Lüschow
and
Jin-Song von Storch

Abstract

The simple scaling relation for internal-tide generation proposed by Jayne and St. Laurent is widely used for parameterizing turbulent mixing induced by breaking of internal tides. Based on the internal-tide generation derived from a 0.1° ocean general circulation model, we show that depending on which stratification is used, this relation produces different vertical distributions of internal-tide generation. When using the buoyancy frequency at the seafloor, which is a common practice, the scaling relation produces, relative to the model, too-strong internal-tide generation in the upper 2000 m and too-weak internal-tide generation in the lower 2000 m. Moreover, the different vertical distributions in the different ocean basins, characterized by a generally decreasing internal tide generation with increasing depth in the Indo-Pacific but not-decreasing or even increasing internal tide generation with increasing depth in the upper 3000 m of the Atlantic, cannot be captured when using bottom stratification. These unsatisfactory features can be easily removed by replacing the buoyancy frequency at the seafloor by a buoyancy frequency averaged over a large part of the water column. To our knowledge, this sensitivity to stratification has not been explicitly quantified for the global ocean. Because of this sensitivity, the scaling relation of Jayne and St. Laurent should be used with an averaged stratification to ensure a more adequate representation of turbulent diffusivity due to tidal mixing and water mass transformation in the deep oceans.

Open access
Hans von Storch
,
Gerd Bürger
,
Reiner Schnur
, and
Jin-Song von Storch

Abstract

The principal oscillation pattern (POP) analysis is a technique used to simultaneously infer the characteristic patterns and timescales of a vector time series. The POPs may be seen as the normal modes of a linearized system whose system matrix is estimated from data.

The concept of POP analysis is reviewed. Examples are used to illustrate the potential of the POP technique. The best defined POPs of tropospheric day-to-day variability coincide with the most unstable modes derived from linearized theory. POPs can be derived even from a space-time subset of data. POPs are successful in identifying two independent modes with similar timescales in the same dataset.

The POP method can also produce forecasts that may potentially be used as a reference for other forecast models.

The conventional POP analysis technique has been generalized in various ways. In the cyclostationary POP analysis, the estimated system matrix is allowed to vary deterministically with an externally forced cycle. In the complex POP analysis, not only the state of the system but also its “momentum” is modeled.

Associated correlation patterns are a useful tool to describe the appearance of a signal previously identified by a POP analysis in other parameters.

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Jin-Song von Storch
,
Hideharu Sasaki
, and
Jochem Marotzke

Abstract

Recent studies on the wind-generated power input to the geostrophic and nongeostrophic ocean circulation components have used expressions derived from Ekman dynamics. The present work extends and unifies previous studies by deriving an expression from the kinetic energy budget of the upper layer based on the primitive equations. Using this expression, the wind-generated power available to the deep ocean is estimated from an integration with the 1/10° ocean general circulation model of the Earth Simulator Center. The result shows that the total power generated by the wind at the sea surface is about 3.8 TW. About 30% of this power (1.1 TW) is passed through a surface layer of about 110-m thickness to the ocean beneath. Approximating the wind-generated power to the deep ocean using Ekman dynamics produces two large errors of opposite signs, which cancel each other to a large extent.

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Daniel Hernández-Deckers
and
Jin-Song von Storch

Abstract

The warming pattern due to higher greenhouse gas concentrations is expected to affect the global atmospheric energetics mainly via changes in the (i) meridional temperature gradient and (ii) mean static stability. Changes in surface meridional temperature gradients have been previously regarded as the determining feature for the energetics response, but recent studies suggest that changes in mean static stability may be more relevant than previously thought. This study aims to determine the relative importance of these two effects by comparing the energetics responses due to different warming patterns using a fully coupled atmosphere–ocean general circulation model.

By means of an additional diabatic forcing, experiments with different warming patterns are obtained: one with a 2xCO2-like pattern that validates the method, one with only the tropical upper-tropospheric warming, and one with only the high-latitude surface warming. The study’s findings suggest that the dominant aspect of the warming pattern that alters the global atmospheric energetics is not its associated meridional temperature gradient changes, but the mean static stability changes. The tropical upper warming weakens the energetics by increasing the mean static stability, whereas the surface warming strengthens it by reducing the mean static stability. The combined 2xCO2-like response is dominated by the tropical upper-tropospheric warming effect, hence the weaker energetic activity. Eddy kinetic energy changes consistently, but the two opposite responses nearly cancel each other in the 2xCO2 case. Therefore, estimates of future changes in storminess may be particularly sensitive to the relative magnitude of the main features of the simulated warming pattern.

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Jin-Song Xu
,
Hans von Storch
, and
Harryvan Loon

Abstract

Monthly mean sea level pressure (SLP) data from four low-resolution spectral GCMs–ECMWF T21, CCC, NCAR CCM and GFDL R15–are compared with observations for the Southern Hemisphere.

Characteristics of the observed Southern Hemisphere January and July mean mass distribution are:

(i) high pressure areas in the subtropics;

(ii) a steep meridional gradient at midlatitudes;

(iii) a circumpolar trough in the Antarctic;

(iv) a zonal asymmetry dominated by zonal wave 1, which has an almost complete phase reversal near 40°S;

(v) a double westerly wind maximum during the colder part of the year.

The CCC model reproduces some of these features. The ECMWF model, the NCAR CCM, and the GFDL models fail with respect to (ii) and (iii). All GCMs underestimate the intensity of the stationary eddies. None of the models considered reproduces the double westerly wind maximum.

Another marked feature of the Southern Hemisphere circulation is the semiannual wave that dominates the annual curve of SLP at mid- and polar latitudes. Regardless of the various models’ degree of success in reproducing the mean circulation, all fail in simulating the general features of the semiannual wave.

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Daniel Hernández-Deckers
and
Jin-Song von Storch

Abstract

Increasing greenhouse gas concentrations warm the troposphere. However, it is not clear whether this implies changes in the energetics. To study the energetics responses to CO2 increases, changes in the Lorenz energy cycle (LEC) are evaluated using output from the atmosphere–ocean ECHAM5/Max Planck Institute Ocean Model (MPI-OM). Equilibrium 2 × CO2 experiments and 10-yr transient experiments with 3% increase per year are analyzed.

Globally, doubling of CO2 results in a decrease in the LEC strength—defined as the total conversion of available potential energy P into kinetic energy K—but also in an increase in the zonal-mean K. These global changes are a consequence of the strengthening of the LEC in the upper troposphere and the weakening of the cycle below. The two opposite responses result from the simulated warming pattern that shows the strongest warming in the upper tropical troposphere and in the lower troposphere at high latitudes. This warming structure causes changes in the horizontal temperature variance and in mean static stability, which increase zonal-mean P in the upper troposphere and decrease it below, triggering the two opposite responses via changes in baroclinic activity. In general, the lower-region weakening is stronger in the Northern Hemisphere, while the upper-region strengthening, and the increase of zonal-mean P and K, is stronger in the Southern Hemisphere. The former is more pronounced in the transient experiments but decreases in the stabilized 2 × CO2 climate.

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Antonija Rimac
,
Jin-Song von Storch
, and
Carsten Eden

Abstract

The total energy flux leaving the ocean’s spatially and seasonally varying mixed layer is estimated using a global ⅝1/10° ocean general circulation model. From the total wind-power input of 3.33 TW into near-inertial waves (0.35 TW), subinertial fluctuations (0.87 TW), and the time-mean circulation (2.11 TW), 0.92 TW leave the mixed layer, with 0.04 TW (11.4%) due to near-inertial motions, 0.07 TW (8.04%) due to subinertial fluctuations, and 0.81 TW (38.4%) due to time-mean motions. Of the 0.81 TW from the time-mean motions, 0.5 TW result from the projection of the horizontal flux onto the sloped bottom of the mixed layer. This projection is negligible for the transient fluxes. The spatial structure of the vertical flux is determined principally by the wind stress curl. The mean and subinertial fluxes leaving the mixed layer are approximately 40%–50% smaller than the respective fluxes across the Ekman layer according to the method proposed by Stern. The fraction related to transient fluctuations tends to decrease with increasing depth of the mixed layer and with increasing strength of wind stress variability.

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Zhuhua Li
,
Jin-Song von Storch
, and
Malte Müller

Abstract

Using a concurrent simulation of the ocean general circulation and tides with the ° Max Planck Institute Ocean Model (MPI-OM), known as STORMTIDE, this study provides a near-global quantification of the low-mode M2 internal tides. The quantification is based on wavelengths and their near-global distributions obtained by applying spectral analysis to STORMTIDE velocities and on comparisons of the distributions with those derived by solving the Sturm–Liouville eigenvalue problem. The simulated wavelengths, with respect to both their magnitudes and their geographical distributions, compare well with those obtained by solving the eigenvalue problem, suggesting that the STORMTIDE internal waves are, to a first approximation, linear internal waves satisfying local dispersion relations. The simulated wavelengths of modes 1 and 2 range within 100–160 and 45–80 km, respectively. Their distributions reveal, to different degrees for both modes, a zonal asymmetry and a tendency of a poleward increase with stratification N and the Coriolis parameter f being responsible for these two features, respectively. Distributions of mode 1 wavelengths are found to be determined by both N and f, but those of mode 2 are mainly controlled by variations in N. Larger differences between the STORMTIDE wavelengths and those of the eigenvalue problem occur, particularly for mode 2, primarily in high-latitude oceans and the Kuroshio and Gulf Stream and their extensions.

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Veit Lüschow
,
Jochem Marotzke
, and
Jin-Song von Storch

Abstract

In this paper, the overturning responses to wind stress changes of an eddying ocean and a non-eddying ocean are compared. Differences are found in the deep overturning cell in the low-latitude North Atlantic Ocean with substantial implications for the deep western boundary current (DWBC). In an ocean-only twin experiment with one eddying and one non-eddying configuration of the MPI ocean model, two different forcings are being applied: the standard NCEP forcing and the NCEP forcing with 2× surface wind stress. The response to the wind stress doubling in the Atlantic meridional overturning circulation is similar in the eddying and the non-eddying configuration, showing an increase by about 4 Sv (~25%; 1 Sv ≡ 106 m3 s−1). In contrast, the DWBC responds with a speedup in the non-eddying configuration and a slowdown in the eddying configuration. This paper demonstrates that the DWBC slowdown in the eddying configuration is largely balanced by eddy vorticity fluxes. Because those fluxes are not resolved and also not captured by an eddy parameterization in the non-eddying configuration, such a DWBC slowdown is likely not to occur in non-eddying ocean models, which therefore might not capture the whole range of overturning responses. Furthermore, evidence is provided that the balancing effect of the eddies is not a passive reaction to a remotely triggered DWBC slowdown. Instead, deep eddies that are sourced from the upper ocean provide an excess input of relative vorticity that then actively forces the DWBC mean flow to slow down.

Open access
Veit Lüschow
,
Jin-Song von Storch
, and
Jochem Marotzke

Abstract

Using a 0.1° ocean model, this paper establishes a consistent picture of the interaction of mesoscale eddy density fluxes with the geostrophic deep western boundary current (DWBC) in the Atlantic between 26°N and 20°S. Above the DWBC core (the level of maximum southward flow, ~2000-m depth), the eddies flatten isopycnals and hence decrease the potential energy of the mean flow, which agrees with their interpretation and parameterization in the Gent–McWilliams framework. Below the core, even though the eddy fluxes have a weaker magnitude, they systematically steepen isopycnals and thus feed potential energy to the mean flow, which contradicts common expectations. These two vertically separated eddy regimes are found through an analysis of the eddy density flux divergence in stream-following coordinates. In addition, pathways of potential energy in terms of the Lorenz energy cycle reveal this regime shift. The twofold eddy effect on density is balanced by an overturning in the plane normal to the DWBC. Its direction is clockwise (with upwelling close to the shore and downwelling further offshore) north of the equator. In agreement with the sign change in the Coriolis parameter, the overturning changes direction to anticlockwise south of the equator. Within the domain covered in this study, except in a narrow band around the equator, this scenario is robust along the DWBC.

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