Atmospheric Signature of the Agulhas Current

Njouodo, Arielle Stela Nkwinkwa; Koseki, Shunya; Keenlyside, Noel; Rouault, Mathieu

doi:10.1029/2018gl077042

Cited by 28 publications

(22 citation statements)

References 42 publications

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“…For instance, a weaker Gulf Stream SST front leads to a decrease in cold and dry spells over Europe (O'Reilly et al, 2016), while a sharper SST front in the Kuroshio Extension increases cyclogenesis and shifts the storm track northward, causing warming over eastern Asia and the western United States that can reduce snow cover by 4-6% (O'Reilly and Czaja, 2015; Révelard et al, 2016). Variability in the warm waters of the Agulhas influences summer rainfall over parts of South Africa (Jury et al, 1993;Nkwinkwa Njouodo et al, 2018). In EBC systems, SST minima are collocated with maxima in sea level pressure that are in turn associated with alongshore wind stress, wind stress curl, and cloud cover along the boundary (Sun et al, 2018), suggesting coupling with the full Hadley-Walker tropical atmospheric circulation, though the details of such coupling remain an open question.…”

Section: Climate and Weathermentioning

confidence: 99%

Global Perspectives on Observing Ocean Boundary Current Systems

et al. 2019

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Ocean boundary current systems are key components of the climate system, are home to highly productive ecosystems, and have numerous societal impacts. Establishment of a global network of boundary current observing systems is a critical part of ongoing development of the Global Ocean Observing System. The characteristics of boundary current systems are reviewed, focusing on scientific and societal motivations for sustained observing. Techniques currently used to observe boundary current systems are reviewed, followed by a census of the current state of boundary current observing systems globally. The next steps in the development of boundary current observing systems are considered, leading to several specific recommendations.

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Section: Climate and Weathermentioning

confidence: 99%

Global Perspectives on Observing Ocean Boundary Current Systems

et al. 2019

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“…Mechanistically, this oceanic imprint has traditionally been associated with a time-mean atmospheric response associated with either a boundary layer pressure adjustment mechanism (Lindzen and Nigam 1987;Minobe et al 2008;Nkwinkwa Njouodo et al 2018) in which fluxdriven spatial pressure gradients drive secondary circulations (Wai and Stage 1989) or the momentum mixing anomalies induced down through the boundary layer by differential SST values (Hayes et al 1989;Wallace et al 1989). However, recent studies have shown that this band of time-mean wind convergence is dominated by synoptic storms (Parfitt 2014;Parfitt and Czaja 2016;O'Neill et al 2017) and is in fact set by atmospheric fronts embedded within them (Parfitt and Seo 2018).…”

Section: Introductionmentioning

confidence: 99%

The Modulation of Gulf Stream Influence on the Troposphere by the Eddy-Driven Jet

Parfitt

Kwon

2020

Journal of Climate

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This study suggests that the Gulf Stream influence on the wintertime North Atlantic troposphere is most pronounced when the eddy-driven jet (EDJ) is farthest south and better collocated with the Gulf Stream. Using the reanalysis dataset NCEP-CFSR for December–February 1979–2009, the daily EDJ latitude is separated into three regimes (northern, central, and southern). It is found that the average trajectory of atmospheric fronts covaries with EDJ latitude. In the southern EDJ regime (~19% of the time), the frequency of near-surface atmospheric fronts that pass across the Gulf Stream is maximized. Analysis suggests that this leads to significant strengthening in near-surface atmospheric frontal convergence resulting from strong air–sea sensible heat flux gradients (due to strong temperature gradients in the atmosphere and ocean). In recent studies, it was shown that the pronounced band of time-mean near-surface wind convergence across the Gulf Stream is set by atmospheric fronts. Here, it is shown that an even smaller subset of atmospheric fronts—those associated with a southern EDJ—primarily sets the time mean, due to enhanced Gulf Stream air–sea interaction. Furthermore, statistically significant anomalies in vertical velocity extending well above the boundary layer are identified in association with changes in EDJ latitude. These anomalies are particularly strong for a southern EDJ and are spatially consistent with increases in near-surface atmospheric frontal convergence over the Gulf Stream. These results imply that much of the Gulf Stream influence on the time-mean atmosphere is modulated on synoptic time scales, and enhanced when the EDJ is farthest south.

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“…In particular, a positive correlation between sea surface temperature (SST) and near‐surface wind speed over western boundary currents (WBCs) suggest an ocean‐to‐atmosphere forcing through turbulent heat fluxes (Nonaka & Xie, ), whose variability on monthly and longer timescales is largely driven by internal ocean processes (Bishop et al, ). Studies considering the atmospheric impact of WBCs typically fall into one of two broad categories—the oceanic impact on the mean state (e.g., Shimada & Minobe, ) and on synoptic storms (e.g., Hirata et al, , ), with community efforts still very much ongoing (e.g., Nkwinkwa Njouodo et al, ; Small et al, ). One major outstanding issue, however, is a lack of mechanistic understanding concerning how these two distinct research avenues coalesce.…”

Section: Introductionmentioning

confidence: 99%

A New Framework for Near‐Surface Wind Convergence Over the Kuroshio Extension and Gulf Stream in Wintertime: The Role of Atmospheric Fronts

Parfitt

Seo

2018

Geophysical Research Letters

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It is well known that the wintertime time-mean surface wind convergence patterns over the Kuroshio Extension and Gulf Stream show significant imprints of the underlying oceanic fronts. Previous studies have suggested that this collocation results from a time-mean response to sea level pressure forcing from sea surface temperature gradients. However, more recent work has illustrated this phenomenon is heavily influenced by extratropical cyclones, although exact mechanisms are still debated. The purpose of this study is to introduce a new framework that explicitly distinguishes between two separate components in their contribution to the time-mean surface wind convergence, that associated with and without atmospheric fronts. It is then argued that this distinction can help better explain the mechanisms driving the Kuroshio Extension and Gulf Stream influence on the atmosphere. Plain Language Summary This paper presents a new framework for understanding the mean wintertime atmospheric state over the Kuroshio Extension and Gulf Stream regions, in the context of advancing our understanding of how these strong ocean currents can impact on the seasonal atmosphere. In recent years, many studies have attributed the oceanic imprint on the atmosphere in these regions to mechanisms based upon the time-mean response. However, observational analysis here illustrates that the regional wintertime atmosphere is, in fact, dominated by continuous extratropical storm systems. It is suggested this contribution to the mean atmospheric state can be further decomposed into situations when atmospheric fronts embedded within these storms are and are not present. By studying each of these distinct atmospheric scenarios, it is then argued that the oceanic imprint on the mean wintertime atmospheric state should instead be considered as an accumulation of processes driven by mechanisms acting on a synoptic timescale, not on the timescale of the time-mean. This framework presents a new paradigm for understanding extratropical frontal-scale air-sea interactions and is expected to have an immediate impact on atmospheric, oceanic, and climate communities.PARFITT AND SEO 9909

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Atmospheric Signature of the Agulhas Current

Cited by 28 publications

References 42 publications

Global Perspectives on Observing Ocean Boundary Current Systems

Global Perspectives on Observing Ocean Boundary Current Systems

The Modulation of Gulf Stream Influence on the Troposphere by the Eddy-Driven Jet

A New Framework for Near‐Surface Wind Convergence Over the Kuroshio Extension and Gulf Stream in Wintertime: The Role of Atmospheric Fronts

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