Storm tracks in the Southern Hemisphere subtropical oceans

Abstract

Ocean storm tracks have previously been associated with the midlatitude western boundary currents (WBCs) and the Antarctic Circumpolar Current (ACC). Here we identify and examine large-scale baroclinically unstable waves occurring within waveguides associated with potential density gradients in the subtropical regions of the Southern Hemisphere (SH) oceans where the trade winds and westerlies meet and at depths associated with mode water formation. In contrast to the Northern Hemisphere subtropics, the SH pathways are more extensive allowing large-scale coherent disturbances to communicate information westward from the midlatitudes to the subtropics (South Pacific Ocean) and from the subtropics to the tropics (Indian Ocean). Particular consideration is given to the subtropical South Pacific Ocean as this is a region where resonant interactions between large-scale Rossby waves and significant topographic features have been reported to occur. Using an ocean general circulation model and a simple potential energy transfer diagnostic, we identify the relevant nonlinearly modified structures comparing their propagation characteristics to planetary Rossby waves calculated using a shallow water model. Although at first appearance baroclinic disturbances resemble planetary Rossby waves, we show they are inherently nonlinear, multiscale and are amplified where topography occurs. The location of the disturbances coincides with regions of high variability in sea surface height observed in satellite altimetry and their speeds closely match the large-scale coherent westward propagating structures described in the observational literature. Our study provides evidence that, in addition to the midlatitude WBCs and the ACC, significant ocean storm tracks are also manifest in the SH subtropics.

The Dynamics of Southern Ocean Storm Tracks

“…However, some recent work has focused on the observed inhomogeneity of the Southern Ocean's eddy field. This result is supported by O'Kane et al (2014), who demonstrated in an eddy permitting GCM that baroclinic disturbances generated in the subtropics were able to propagate and influence the eddy field long distances from their genesis regions. They concluded that the eddy field is, at least partially, set by nonlocal effects, such as eddies or energy propagating into a region from afar.…”

“…Unlike in the midlatitude atmosphere (Danielson et al 2006) and the subtropical oceans (O'Kane et al 2014), the propagation of coherent eddies does not significantly contribute to the downstream development of the storm track. In contrast to the commonly assumed mechanism (described in WWH07) of individual eddies forming in the regions of high baroclinicity and then propagating downstream into the regions of low baroclinicity, our mechanism posits that both the growth and decay of eddies occurs in the low baroclinicity regions, which, if true, would have implications for the parameterization of eddy tracer flux.…”

Drivers of decadal variability in the Tasman Sea

“…O'Kane et al . [, section 4.2.3 and associated supporting information], applying complex empirical Orthogonal Functions (CEOFs) and other methods to baroclinic disturbances in the South Pacific Ocean, found that CEOF modes 2–4 explain approximately 25% of the variance and that modes 3 and 4 propagate westward from the southeast Pacific to the eastern boundary of the Tasman Sea (PX06). They further show that once these baroclinic disturbances reach the eastern boundary of the Tasman Sea they enter the EAC either directly or via the South Equatorial Current.…”

“…Here our focus is on the stability of the EAC and its connection to the South Pacific subtropical ocean storm track described recently by O'Kane et al . [].…”

Influence of Global Warming on The Trajectories and Intensities of Extratropical Cyclones in the Southern Hemisphere

“…Dessa forma, os ciclones extratropicais exercem grande influência em regiões de latitudes médias e altas (Hoskins & Hodges, 2005;O'Kane et al, 2014). Os ciclones extratropicais apresentam sua faixa de preferência de ocorrência entre os paralelos de 35° e 65° em ambos os hemisférios, e por isso são importantes componentes da circulação geral atmosférica, através do forte impacto que exercem nos processos de troca de calor, momentum e vapor d'água (Tings & Held, 1990;Justino, 2004).…”