Abstract
The Cretaceous ‘greenhouse’ period (~145 to ~66 million years ago, Ma) in Earth’s history is relatively well documented by multiple paleoproxy records, which indicate that the meridional sea ...surface temperature (SST) gradient increased (non-monotonically) from the Valanginian (~135 Ma) to the Maastrichtian (~68 Ma). Changes in atmospheric CO
2
concentration, solar constant, and paleogeography are the primary drivers of variations in the spatiotemporal distribution of SST. However, the particular contribution of each of these drivers (and their underlying mechanisms) to changes in the SST distribution remains poorly understood. Here we use data from a suite of paleoclimate simulations to compare the relative effects of atmospheric CO
2
variability and paleogeographic changes on mid-latitudinal SST gradient through the Cretaceous. Further, we use a fundamental model of wind-driven ocean gyres to quantify how changes in the Northern Hemisphere paleogeography weaken the circulation in subtropical ocean gyres, leading to an increase in extratropical SSTs.
Simple analytic models developed in this study are applied to long‐term averages of reanalysis surface salinity data to quantify two fundamental properties of ocean currents. The first model is based ...on the new Freshening Length schema and its application to the Irminger Current yields a ratio of about 5 between the turbulent entrainment rates of surrounding fresher surface waters west and east of Greenland. The second model is based on the steady solution of the advection‐diffusion equation subject to suitable boundary conditions. The application of this model to the spreading of fresh, snow‐melt, water from the delta of the Po river in the northwest Adriatic Sea into the rest of the Sea yields a ratio of 8 × 104 m between the eddy exchange coefficient and the speed of advection in the Sea.
Plain Language Summary
Differences in ocean water salinity were used for over a century to quantify the horizontal fluxes in and out of evaporative, motionless, basins such as the Mediterranean Sea. In the present study we develop simple expressions based on analytic models that extend the century‐old approach to ocean currents where the water is constantly moving rather than remaining stagnant. The models developed here are combined with long‐term data of sea surface salinity along two currents—the salty Irminger Current that flows around the southern tip of Greenland and the flow of fresh snow‐melt water from the Po river into the Adriatic Sea. The models and climatological data used here yield quantitative estimates of two basic parameters: (a) the rate at which a high‐salinity current detrains salt to the surrounding ocean. (b) The balance between the slow downstream propagation and eddy (turbulent) exchange coefficient. The models developed in this study can be applied to other currents and regions of the world ocean.
Key Points
In some oceanic circumstances, changes in Sea Surface Salinity (SSS) gradient provide a simple, reliable and robust diagnostic of ocean currents
Changes in the entrainment rate of surrounding water into a current, correspond to observable changes in SSS gradient
Reanalysis SSS data quantify the ratio between the speed of advection and the eddy exchange coefficient in slow currents
An intermediate complexity moist general circulation model is used to investigate the sensitivity of the quasi‐biennial oscillation (QBO) to resolution, diffusion, tropical tropospheric waves, and ...parameterized gravity waves. Finer horizontal resolution is shown to lead to a shorter period, while finer vertical resolution is shown to lead to a longer period and to a larger amplitude in the lowermost stratosphere. More scale‐selective diffusion leads to a faster and stronger QBO, while enhancing the sources of tropospheric stationary wave activity leads to a weaker QBO. In terms of parameterized gravity waves, broadening the spectral width of the source function leads to a longer period and a stronger amplitude although the amplitude effect saturates in the mid‐stratosphere when the half‐width exceeds ∼25m/s. A stronger gravity wave source stress leads to a faster and stronger QBO, and a higher gravity wave launch level leads to a stronger QBO. All of these sensitivities are shown to result from their impact on the resultant wave‐driven momentum torque in the tropical stratosphere. Atmospheric models have struggled to accurately represent the QBO, particularly at moderate resolutions ideal for long climate integrations. In particular, capturing the amplitude and penetration of QBO anomalies into the lower stratosphere (which has been shown to be critical for the tropospheric impacts) has proven a challenge. The results provide a recipe to generate and/or improve the simulation of the QBO in an atmospheric model.
Plain Language Summary
The most prominent mode of variability in the tropical stratosphere is the quasi‐biennial oscillation (QBO), characterized by easterly and westerly winds alternating sign every ∼14 months. Only relatively recently have comprehensive models begun to simulate a QBO spontaneously, and even in these models the representation of the QBO typically suffers from biases. Here we elucidate the sensitivities of the QBO to a wide range of model parameters, and explore how these parameters affect the QBO behavior. We expect that these results will be helpful for improving the QBO in more comprehensive models.
Key Points
Sensitivity of the quasi‐biennial oscillation (QBO) to resolution, dissipation, wave forcing, and parameterized gravity waves is explored in a single framework
The influence of these factors on the QBO can be related to their impact on wave‐induced momentum fluxes in the deep tropics
The QBO period can be tuned independently of its amplitude, but the vertical structure (particularly at lower levels) is harder to capture
The mixed Rossby–gravity wave on the spherical Earth Paldor, Nathan; Fouxon, Itzhak; Shamir, Ofer ...
Quarterly journal of the Royal Meteorological Society,
July 2018 Part B, 2018-Jul, 2018-07-00, 20180701, Letnik:
144, Številka:
715
Journal Article
Recenzirano
Odprti dostop
This work revisits the theory of the mixed Rossby–gravity (MRG) wave on a sphere. Three analytic methods are employed in this study: (a) derivation of a simple ad hoc solution corresponding to the ...MRG wave that reproduces the solutions of Longuet‐Higgins and Matsuno in the limits of zero and infinite Lamb's parameter, respectively, while remaining accurate for moderate values of Lamb's parameter, (b) demonstration that westward‐propagating waves with phase speed equalling the negative of the gravity‐wave speed exist, unlike the equatorial β‐plane, where the zonal velocity associated with such waves is infinite, and (c) approximation of the governing second‐order system by Schrödinger eigenvalue equations, which show that the MRG wave corresponds to the branch of the ground‐state solutions that connects Rossby waves with zonally symmetric waves. The analytic conclusions are confirmed by comparing them with numerical solutions of the associated second‐order equation for zonally propagating waves of the shallow‐water equations. We find that the asymptotic solutions obtained by Longuet‐Higgins in the limit of infinite Lamb's parameter are not suitable for describing the MRG wave even when Lamb's parameter equals 104. On the other hand, the dispersion relation obtained by Matsuno for the MRG wave on the equatorial β‐plane is accurate for values of Lamb's parameter as small as 16, even though the equatorial β‐plane formally provides an asymptotic limit of the equations on the sphere only in the limit of infinite Lamb's parameter.
This article provides explicit expressions for the dispersion relation and meridional structure of the mixed Rossby–gravity (MRG) wave on a sphere and the accuracy of these analytic expressions is confirmed numerically. Unlike the equatorial β‐plane, on a sphere the westward‐propagating gravity wave is not excluded from the MRG spectrum. The approximation of the governing equations by a Schrödinger equation confirms that the MRG wave connects the n = 0 Rossby mode at large zonal wavenumber with the n = 0 zonally symmetric mode.
PDF
