Deep Eastern Boundary Currents: idealized models and dynamics

Concentrated poleward flows along eastern boundaries between two and four kilometers depth in the southeast Pacific, Atlantic, and Indian Oceans have been observed, and appear in data-assimilation products and regional model simulations at sufficiently high horizontal resolution, but their dynamics are still not well understood. We study the local dynamics of these Deep Eastern Boundary Currents (DEBCs) using idealized GCM simulations and use a conceptual vorticity model for the DEBCs, to gain additional insights into the dynamics. Over most of the zonal width of the DEBCs, the vorticity balance is between meridional advection of planetary vorticity and vortex stretching, which is an interior-like vorticity balance. Over a thinner layer very close to the eastern boundary, a balance between vorticity tendencies due to friction and stretching that rapidly decay away from the boundary is found. Over the part of the DEBC that is governed by an interior-like vorticity balance, vertical stretching is driven by both the topography and temperature diffusion, while in the thinner boundary layer, it is driven instead by parameterized horizontal temperature mixing. The topographic driving acts via a cross-isobath flow that leads to stretching and thus to vorticity forcing for the concentrated DEBCs.

Deep Eastern Boundary Currents: Realistic Simulations and Vorticity Budgets

Concentrated poleward flows near the eastern boundaries between 2- and 4-km depth have been observed repeatedly, particularly in the Southern Hemisphere. These deep eastern boundary currents (DEBCs) play an important role in setting the large-scale tracer distribution and have nonnegligible contribution to global transports of mass, heat, and tracers, but their dynamics are not well understood. In this paper, we first demonstrate the significant role of DEBCs in the southeastern Atlantic, Indian, and Pacific Oceans, using the Southern Ocean State Estimate (SOSE) data assimilating product, and using high-resolution regional general circulation model configurations. The vorticity balances of these DEBCs reveal that, over most of the width of such currents, they are in an interior-like vorticity budget, with the meridional advection of planetary vorticity βυ and vortex stretching fwz being the largest two terms, and with contributions of nonlinearity and friction that are of smaller spatial scale. The stretching is shown, using a temperature budget, to be largely forced by resolved or parameterized eddy temperature transport. Strongly decaying signals from the eastern boundary in friction and stretching form the dominant balance in a sublayer close to the eastern boundary. The temporal variability of DEBCs is then examined, to help to interpret observations that tend to be sporadic in both time and space. The probability distribution functions of northward velocity in DEBC regions are broad, implying that flow reversals are common. Although the regions of the simulated DEBCs are generally local minima of eddy kinetic energy, they are still constantly releasing westward-propagating Rossby waves.

Impacts of Shifting Subtropical Highs on North American Climate

<p>Numerous observational studies have found that the Hadley cells have expanded poleward in both the Northern and Southern Hemispheres, and model results suggest that such expansion is likely to continue throughout the 21st century as a result of global warming.  This has led to concerns about future impacts of Hadley cell expansion, including a poleward shift of the subtropical dry zone.  However, climatic changes associated with Hadley cell expansion are zonally asymmetric—especially in the Northern Hemisphere—suggesting that a more regional focus may be necessary.  In this study, we consider the influence of the Northern Hemisphere subtropical highs, and contrast this with the influence of Hadley cell expansion. </p><p>Specifically, we consider the North Pacific and North Atlantic subtropical highs and define, for each high, three indices representing longitude, latitude, and strength.  We find that 21st century trends in variables as diverse as precipitation, sea-level pressure, winds, and ocean upwelling in eastern boundary currents are all driven more by the trends of these subtropical high indices than by the expansion of the Hadley cell.  We conclude that 21st century trends in subtropical high positions and strengths are crucial to understanding the future of Northern Hemisphere climate.  Further work will be needed to determine the dynamical drivers of these subtropical high trends.  </p>

The effect of mountain uplift on eastern boundary currents and upwelling systems

All major mountain ranges are assumed to have been subject to increased uplifting processes during the late Miocene and Pliocene. Previous work has demonstrated that African uplift is an important element to explain Benguela upper-ocean cooling in the late Miocene–Pliocene. According to proxy records, a surface ocean cooling also occurred in other eastern boundary upwelling regions during the late Neogene. Here we investigate a set of sensitivity experiments altering topography in major mountain regions (Andes, North American Cordillera, and southern and East African mountains) separately with regard to the potential impact on the intensity of near-coastal low-level winds, Ekman transport and Ekman pumping, and upper-ocean cooling. The simulations show that mountain uplift is important for upper-ocean temperature evolution in the area of eastern boundary currents. The impact is primarily on the atmospheric circulation which is then acting on upper-ocean temperatures through changes in strengths of upwelling, horizontal heat advection and surface heat fluxes. Different atmosphere–ocean feedbacks additionally alter the sea surface temperature response to uplift. The relative importance of the different feedback mechanisms depends on the region, but it is most likely also influenced by model and model resolution.

The effect of mountain uplift on eastern boundary currents and upwelling systems

All major mountain ranges are assumed to have been subject to increased uplifting processes during the late Miocene and Pliocene. Previous work has demonstrated that African uplift is an important element to explain Benguela upper-ocean cooling in the late Miocene/Pliocene. According to proxy records, a surface ocean cooling also occurred in other Eastern Boundary upwelling regions during the late Neogene. Here we investigate a set of sensitivity experiments altering topography in major mountain regions (Andes, North American Cordillera and South/East African mountains) separately with regard to the potential impact on the intensity of near-coastal low-level winds, Ekman transport and Ekman pumping as well as upper-ocean cooling. The simulations show that mountain uplift is important for upper-ocean temperature evolution in the area of Eastern Boundary Currents. The impact is primarily on the atmospheric circulation which is then acting on upper-ocean temperatures through changes in strengths of upwelling, horizontal heat advection and surface heat fluxes. Different atmosphere-ocean feedbacks additionally alter the sea surface temperature response to uplift. The relative importance of the different feedback mechanisms depends on the region, but is most likely also influenced by model and model resolution.