Recirculation gyres adjacent to western boundary currents (WBCs) in the ocean enhance the
poleward transport of these currents. While it is well-established that the WBC in a barotropic ocean strengthens
with increase in basin’s aspect ratio (the meridional-to-zonal extent ratio), how intensity of the recirculation
through the western boundary layer varies with this parameter remains unexplored. I address this using the
non-dimensional form of the nonlinear, wind-driven Stommel-Munk model of westward intensification that comprises
three parameters—the aspect ratio (δ), the damping coefficient (ε), and the β-Rossby number
(Rβ). Here, εis set by the ratio of Rayleigh friction coefficient (or eddy viscosity) to the
meridional gradient of the Coriolis frequency and the basin’s zonal dimension, while is Rβ is proportional
to wind stress amplitude and quantifies the strength of nonlinearity. In the weak-to-moderate nonlinearity limit,
perturbation analysis reveals that recirculation varies concavely with aspect ratio, suggesting existence of an
optimal aspect ratio (δopt) for which the recirculation is maximum and for typical values of ε(10-3-10-2), δopt follows the power-law relation δopt=4.3ε1/2. Numerical
simulations further validate the existence of δopt. For large ε(>5x10-3), the power-law predictsδopt$ for the numerical solutions rather
accurately, but does not hold for smaller ε(2x10-3) due to increased importance of
nonlinear terms. Nevertheless, the nonlinear variation in recirculation through the western boundary layer with
aspect ratio is observed for all values and may contribute to the heterogeneous increase in the WBC’s transport
across different ocean basins in a warming climate.
@article{gianchandani2024recirculation,author={Gianchandani, Kaushal},title={Recirculation through western boundary currents varies nonlinearly with the ocean basin's aspect ratio},journal={Physics of Fluids},volume={36},number={9},pages={096608},year={2024},month=sep,issn={1089-7666},url={https://doi.org/10.1063/5.0226883},doi={10.1063/5.0226883},}
Production of Neoproterozoic banded iron formations in a partially ice-covered ocean
Kaushal Gianchandani, Itay Halevy, Hezi Gildor, and
2 more authors
The meridional extent of marine ice during the Neoproterozoic snowball Earth events is debated. Banded iron formations associated with the Sturtian glaciation are considered evidence for a completely ice-covered, ferruginous ocean (hard snowball). Here, using an ocean general circulation model with thick sea glaciers and Neoproterozoic biogeochemistry, we find that circulation in a partially ice-covered ocean (soft snowball) yields iron deposition patterns similar to the observed distribution of Sturtian banded iron formations.
@article{gianchandani2024production,author={Gianchandani, Kaushal and Halevy, Itay and Gildor, Hezi and Ashkenazy, Yosef and Tziperman, Eli},title={Production of Neoproterozoic banded iron formations in a partially ice-covered ocean},journal={Nature Geoscience},volume={17},number={4},pages={298--301},year={2024},month=apr,issn={1752-0908},url={https://doi.org/10.1038/s41561-024-01377-6},doi={10.1038/s41561-024-01406-4},}
The convergence of a cluster of water columns at the ocean surface subject to a zonal wind stress on the β-plane is studied analytically by substituting the pseudo angular momentum for the zonal velocity in the Lagrangian dynamical equations. The horizontal convergence at the surface is a primary driver of Ekman pumping that connects the surface of the ocean with its deeper layers. The derived analytical expressions are verified by numerical simulations of the nonlinear equations. Both direct simulations and analysis show that, in contrast to the f-plane, for a uniform wind stress, water columns on the β-plane in the northern hemisphere always converge (diverge) when the overlying wind is directed westward (eastward). For a zonal wind stress that is westward directed at low latitudes and eastward directed at high latitudes, the β-effect mitigates (enhances) the f-plane convergence toward the latitude of vanishing wind stress for water columns located north (south) of the latitude at which the stress vanishes. For the same zonal wind stress field, the β induced convergence is of the order of 25% of that on the f-plane.
@article{gianchandani2024ekman,author={Gianchandani, Kaushal and Paldor, Nathan},title={Ekman pumping on the $\beta$-plane},journal={Physics of Fluids},volume={36},number={2},pages={026617},year={2024},month=feb,issn={1070-6631},url={https://doi.org/10.1063/5.0194042},doi={10.1063/5.0194042},}
2023
Effects of paleogeographic changes and CO2 variability on northern mid-latitudinal temperature gradients in the Cretaceous
Kaushal Gianchandani, Sagi Maor, Ori Adam, and
4 more authors
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 CO2 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 CO2 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.
@article{gianchandani2023cretaceous,author={Gianchandani, Kaushal and Maor, Sagi and Adam, Ori and Farnsworth, Alexander and Gildor, Hezi and Lunt, Daniel J. and Paldor, Nathan},title={Effects of paleogeographic changes and CO<sub>2</sub> variability on northern mid-latitudinal temperature gradients in the Cretaceous},journal={Nature Communications},year={2023},month=aug,volume={14},pages={5193},issn={2041-1723},url={https://doi.org/10.1038/s41467-023-40905-7},doi={10.1038/s41467-023-40905-7},}
2021
On the role of domain aspect ratio in the westward intensification of wind-driven surface ocean circulation
Kaushal Gianchandani, Hezi Gildor, and Nathan Paldor
The two seminal studies on westward intensification, carried out by Stommel and Munk over 70 years ago, are revisited to elucidate the role of the domain aspect ratio (i.e., meridional to zonal extents of the basin) in determining the transport of the western boundary current (WBC). We examine the general mathematical properties of the two models by transforming them to differential problems that contain only two parameters – the domain aspect ratio and the non-dimensional damping viscous coefficient. Explicit analytical expressions are obtained from solutions of the non-dimensional vorticity equations and verified by long-term numerical simulations of the corresponding time-dependent equations. The analytical expressions as well as the simulations imply that in Stommel’s model both the domain aspect ratio and the damping parameter contribute to the non-dimensional transport of the WBC. However, the transport increases as a cubic power of the aspect ratio and decreases linearly with the damping coefficient. On the other hand, in Munk’s model the WBC’s transport increases linearly with the domain aspect ratio, while the damping coefficient plays a minor role only. This finding is employed to explain the weak WBC in the South Pacific. The decrease in transport of the WBC for small-domain aspect ratio results from the decrease in Sverdrup transport in the basin’s interior because the meridional shear of the zonal velocity cannot be neglected as an additional vorticity term.
@article{gianchandani2021role,title={On the role of domain aspect ratio in the westward intensification of wind-driven surface ocean circulation},author={Gianchandani, Kaushal and Gildor, Hezi and Paldor, Nathan},journal={Ocean Science},volume={17},pages={351--363},year={2021},month=feb,publisher={Copernicus GmbH},url={https://doi.org/10.5194/os-17-351-2021},doi={10.5194/os-17-351-2021},}
2020
Statistical tests for the distribution of surface wind and current speeds across the globe
Salvatore Campisi-Pinto, Kaushal Gianchandani, and Yosef Ashkenazy
The distribution of surface winds and currents is important from climatic and energy production aspects. It is commonly assumed that the distribution of surface winds and currents speed is Weibull, yet, previous studies indicated that this assumption is not always valid. An inaccurate probability distribution function (PDF) of wind (current) statistic can lead to erroneous power estimation; thus, it is necessary to examine the accuracy of the PDFs employed. We propose statistical tests to check the validity of an assumed distribution of wind and current speeds. The main statistical test can be applied to any distribution and is based on surrogate data where the different moments of the data are compared with the moments of the surrogate data. We applied this and other tests to global surface wind and current speeds and found that the generalized gamma distribution fits the data distributions better than the Weibull distribution. The percentage of locations that fall within the confidence interval of the assumed distribution varies with the moment. The third moment is used to estimate the potential power of winds and currents — we find that 89% (95%) of the wind (current) grid points fall within the 95% confidence interval of the generalized gamma distribution.
@article{campisi2020statistical,title={Statistical tests for the distribution of surface wind and current speeds across the globe},author={Campisi-Pinto, Salvatore and Gianchandani, Kaushal and Ashkenazy, Yosef},journal={Renewable Energy},volume={149},pages={861--876},year={2020},month=apr,publisher={Elsevier},url={https://doi.org/10.1016/j.renene.2019.12.041},doi={10.1016/j.renene.2019.12.041},}
