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Britton B. Stephens, Matthew C. Long, Ralph F. Keeling, Eric A. Kort, Colm Sweeney, Eric C. Apel, Elliot L. Atlas, Stuart Beaton, Jonathan D. Bent, Nicola J. Blake, James F. Bresch, Joanna Casey, Bruce C. Daube, Minghui Diao, Ernesto Diaz, Heidi Dierssen, Valeria Donets, Bo-Cai Gao, Michelle Gierach, Robert Green, Justin Haag, Matthew Hayman, Alan J. Hills, Martín S. Hoecker-Martínez, Shawn B. Honomichl, Rebecca S. Hornbrook, Jorgen B. Jensen, Rong-Rong Li, Ian McCubbin, Kathryn McKain, Eric J. Morgan, Scott Nolte, Jordan G. Powers, Bryan Rainwater, Kaylan Randolph, Mike Reeves, Sue M. Schauffler, Katherine Smith, Mackenzie Smith, Jeff Stith, Gregory Stossmeister, Darin W. Toohey, and Andrew S. Watt

A recent Southern Ocean airborne campaign collected continuous, discrete, and remote sensing measurements to investigate biogeochemical and physical processes driving air–sea exchange of CO 2 , O 2 , and reactive biogenic gases. As the primary conduit for CO 2 and heat exchange between the atmosphere and the deep ocean, the Southern Ocean is an important part of the climate system. Approximately 40% of the ocean’s inventory of anthropogenic carbon entered through the air–sea interface south of

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Peter B. Wright

The Southern Oscillation (SO) is characterized by a temporal signal that dominates the variation of sea surface temperature (SST), pressure, and other fields in “core regions,” which are mostly in or near the equatorial Pacific. It involves persistence, high interannual variability, and high correlations between fields. All these characteristics vary with season, being most marked around November and weakest around April. These phenomena are best explained in terms of a positive feedback relationship between the equatorial east Pacific SST and the Walker circulation, in which the feedback varies with season. The relationship between SST anomalies and cloudiness varies with season in a sense that could account for the required variation in feedback.

The SO involves simultaneous teleconnections that can probably be explained by atmospheric dynamical processes. There are also lag teleconnections that call for different explanations. For example, tropical Indian Ocean SST tends to be low several months after high SO index. The explanation for this could involve the sequence: high index → low pressure over Indian Ocean → increased convergence → increased cloudiness → reduced net radiation at sea surface → lower SST.

Of particular interest are lag relationships in which some factor is correlated better with later SO than with simultaneous SO. There is evidence of such factors in the southeast Pacific, North Australia, and east equatorial Atlantic regions in certain seasons. Plausible physical hypotheses are available to account for the relationships. These factors represent predictors of the SO, and deserve detailed study both to improve seasonal forecasts and to shed light on the mechanisms of SO fluctuations. Some relationships seem to involve lag correlations in both senses, and thereby imply negative feedback relationships that could be the cause of the tendency for the SO to fluctuate.

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Doron Nof and Agatha M. de Boer

Since the Southern Ocean encompasses the entire circumference of the globe, the zonal integral of the pressure gradient vanishes implying that the (meridional) geostrophic mass flux is zero. Conventional wisdom has it that, in view of this, the northward Ekman flux there must somehow find its way to the northern oceans, sink to the bottom (due to cooling) and return southward either below the topography or along the western boundary. Using recent (process oriented) numerical simulations and a simple analytical model, it is shown that most of the Ekman flux in the Southern Ocean does not cross the equator, nor does it sink in the northern oceans. Rather, the water that constitutes the link between the Southern Ocean and the deep water formation in the Northern Hemisphere originates in the eastern part of the southern Sverdrup interior.

The associated path which takes the water from one hemisphere to the other resembles the letter “S”, where the top of the letter corresponds to the sinking region in the Northern Hemisphere and the bottom to the origin in the Southern Ocean. Although it is true that the amount of water that is cross crossing the equator is equal to the integrated Ekman flux in the northernmost part of the Southern Ocean, it is merely the amount (and not the origin of the water) that is equal in these two cases. The width of the transhemispheric current in the south iswhere τ is the wind stress, ∂τ/∂y the curl of the wind, β the familiar variation of the Coriolis with latitude, f0 the mean Coriolis parameter, and L is the width of the basin.

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Greg M. McFarquhar, Chris Bretherton, Roger Marchand, Alain Protat, Paul J. DeMott, Simon P. Alexander, Greg C. Roberts, Cynthia H. Twohy, Darin Toohey, Steve Siems, Yi Huang, Robert Wood, Robert M. Rauber, Sonia Lasher-Trapp, Jorgen Jensen, Jeff Stith, Jay Mace, Junshik Um, Emma Järvinen, Martin Schnaiter, Andrew Gettelman, Kevin J. Sanchez, Christina S. McCluskey, Lynn M. Russell, Isabel L. McCoy, Rachel Atlas, Charles G. Bardeen, Kathryn A. Moore, Thomas C. J. Hill, Ruhi S. Humphries, Melita D. Keywood, Zoran Ristovski, Luke Cravigan, Robyn Schofield, Chris Fairall, Marc D. Mallet, Sonia M. Kreidenweis, Bryan Rainwater, John D’Alessandro, Yang Wang, Wei Wu, Georges Saliba, Ezra J. T. Levin, Saisai Ding, Francisco Lang, Son C.H. Truong, Cory Wolff, Julie Haggerty, Mike J. Harvey, Andrew Klekociuk, and Adrian McDonald


Weather and climate models are challenged by uncertainties and biases in simulating Southern Ocean (SO) radiative fluxes that trace to a poor understanding of cloud, aerosol, precipitation and radiative processes, and their interactions. Projects between 2016 and 2018 used in-situ probes, radar, lidar and other instruments to make comprehensive measurements of thermodynamics, surface radiation, cloud, precipitation, aerosol, cloud condensation nuclei (CCN) and ice nucleating particles over the SO cold waters, and in ubiquitous liquid and mixed-phase cloudsnucleating particles over the SO cold waters, and in ubiquitous liquid and mixed-phase clouds common to this pristine environment. Data including soundings were collected from the NSF/NCAR G-V aircraft flying north-south gradients south of Tasmania, at Macquarie Island, and on the RV Investigator and RSV Aurora Australis. Synergistically these data characterize boundary layer and free troposphere environmental properties, and represent the most comprehensive data of this type available south of the oceanic polar front, in the cold sector of SO cyclones, and across seasons.

Results show a largely pristine environments with numerous small and few large aerosols above cloud, suggesting new particle formation and limited long-range transport from continents, high variability in CCN and cloud droplet concentrations, and ubiquitous supercooled water in thin, multi-layered clouds, often with small-scale generating cells near cloud top. These observations demonstrate how cloud properties depend on aerosols while highlighting the importance of confirmed low clouds were responsible for radiation biases. The combination of models and observations is examining how aerosols and meteorology couple to control SO water and energy budgets.

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E. Povl Abrahamsen, Sandra Barreira, Cecilia M. Bitz, Amy Butler, Kyle R. Clem, Steve Colwell, Lawrence Coy, Jos de Laat, Marcel D. du Plessis, Ryan L. Fogt, Helen Amanda Fricker, John Fyfe, Alex S. Gardner, Sarah T. Gille, Tessa Gorte, L. Gregor, Will Hobbs, Bryan Johnson, Eric Keenan, Linda M. Keller, Natalya A. Kramarova, Matthew A. Lazzara, Jan T. M. Lenaerts, Jan L. Lieser, Hongxing Liu, Craig S. Long, Michelle Maclennan, Robert A. Massom, François Massonnet, Matthew R. Mazloff, David Mikolajczyk, A. Narayanan, Eric R. Nash, Paul A. Newman, Irina Petropavlovskikh, Michael Pitts, Bastien Y. Queste, Phillip Reid, F. Roquet, Michelle L. Santee, Susan Strahan, Sebastiann Swart, and Lei Wang
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Seasonal rainfall for 1923–24 indicated by ocean temperature

George F. McEwen
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Patricio Aceituno
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A review of the interannual to interdecadal variability of the southern African region and its links with the Atlantic is given. Emphasis is placed on modes such as the Benguela Nĩno that develop within the Atlantic and may have some predictability. Seasonal forecasting and climate prediction efforts within the region are discussed. Most southern African countries rely on a combination of products obtained overseas and simple statistical methods. GCM based forecasts and statistical downscaling of their outputs are used operationally in South Africa and also applied to some neighboring countries. A review of these downscaling efforts and their various applications is given.

Research is also taking place into the predictability of quantities such as the onset of the rainy season (which appears to be associated with anomalous South Atlantic anticyclonic ridging) and dry spell frequencies within it. These parameters are often more useful to farmers in the region than forecasting above- or belowaverage seasonal rainfall totals. A strong link between dry spells and Nĩno-3.4 SST is evident for certain regions of southern Africa, suggesting that some predictability exists. This link is weaker for countries like Namibia and Angola that border the Atlantic than for southeastern Africa.

It is concluded that some aspects of southern African climate variability may have predictability but considerably more research is needed to better understand the influence of variability over the Atlantic. An added concern is the ongoing reduction in data collection in many parts of southern Africa. This reduction has serious implications for model development and validation, and for the accuracy of reanalysis products.

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Julia Schmale, Andrea Baccarini, Iris Thurnherr, Silvia Henning, Avichay Efraim, Leighton Regayre, Conor Bolas, Markus Hartmann, André Welti, Katrianne Lehtipalo, Franziska Aemisegger, Christian Tatzelt, Sebastian Landwehr, Robin L. Modini, Fiona Tummon, Jill S. Johnson, Neil Harris, Martin Schnaiter, Alessandro Toffoli, Marzieh Derkani, Nicolas Bukowiecki, Frank Stratmann, Josef Dommen, Urs Baltensperger, Heini Wernli, Daniel Rosenfeld, Martin Gysel-Beer, and Ken S. Carslaw

Aerosol characteristics over the Southern Ocean are surprisingly heterogeneous because of the distinct regional dynamics and marine microbial regimes, but satellite observations and model simulations underestimate the abundance of cloud condensation nuclei. Sampling aerosol and trace gases on R/V Akademik Tryoshnikov at the Antarctic coast near the Mertz Glacier (67°09'S, 144°23'E). Photo credit: Julia Schmale. The World Climate Research Programme highlights the fact that “limited understanding

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Sarah M. Kang, Matt Hawcroft, Baoqiang Xiang, Yen-Ting Hwang, Gabriel Cazes, Francis Codron, Traute Crueger, Clara Deser, Øivind Hodnebrog, Hanjun Kim, Jiyeong Kim, Yu Kosaka, Teresa Losada, Carlos R. Mechoso, Gunnar Myhre, Øyvind Seland, Bjorn Stevens, Masahiro Watanabe, and Sungduk Yu

2008 ; Zheng et al. 2011 ). In a recent paper, Song and Zhang (2018) report on a major reduction of the double ITCZ bias in their model with a revised parameterization of deep convection. On the other hand, extratropical energy biases have also been proposed as a possible cause of the double ITCZ bias. In particular, the warm bias in the Southern Hemisphere extratropics, observed in many GCMs associated with cloud biases over the Southern Ocean, is suggested to contribute to the double ITCZ bias

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