Frontolysis by surface heat flux in the Agulhas Return Current region with a focus on mixed layer processes: observation and a high-resolution CGCM

Ohishi, Shun; Tozuka, Tomoki; Komori, Nobumasa

doi:10.1007/s00382-016-3056-0

Cited by 10 publications

(5 citation statements)

References 74 publications

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“…The fact that the surface heat flux actually strengthens the SST front in the upstream KE region, particularly from autumn to early winter, is a remarkable result that has not been seen in other frontal regions. For instance, surface heat fluxes tend to weaken the SST front in the Agulhas Return Current region from 40°E to 55°E 25 , 26 , although the meridional variations in MLD do affect the sensitivity of the ocean to the surface heat flux. The frontolysis is amplified (damped) in austral summer (winter), because warming (cooling) by the surface heat flux is amplified south of the front where the MLD is shallower, and is reduced north of the front where the MLD is deeper.…”

Section: Discussionmentioning

confidence: 99%

“…Taking the meridional derivative of Eq. ( 1 ), we may obtain the rate of frontogenesis/frontolysis 25 , 26 , 40 : Here, the monthly mean climatology from the MIMOC data is used for the mixed layer temperature and depth, that from the J-OFURO2 data is used for the surface heat flux, and the second term on the right hand side is computed as a residual.…”

Section: Methodsmentioning

confidence: 99%

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Surface frontogenesis by surface heat fluxes in the upstream Kuroshio Extension region

Tozuka

Cronin

Tomita

2017

Sci Rep

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Western boundary currents bring warm tropical water poleward and eastward and are characterized by a sharp sea surface temperature (SST) front on the poleward edge of the current as it extends into the interior basin. One of the most prominent such front is associated with the Kuroshio Extension (KE) as it extends east of Japan (“upstream KE”). Large latent and sensible heat fluxes that warm the atmosphere and cool the ocean project this front into the atmosphere, thereby affecting weather and climate both locally and remotely. While one might assume that these larger surface heat fluxes on the equatorward side would tend to damp the SST front, here we present observational evidence that the surface heat loss actually strengthens the front during October-April in monthly climatology and about 87% of months from October to January during the 2004/05–2014/15 period, although the percentage lowers to about 38% for February-April of the same period, suggesting some temporal/data dependency in the analysis. The key to understanding this counterintuitive result for frontogenesis is that the effective heat capacity of the surface water depends on mixed layer thickness. SSTs are more (less) sensitive to surface heat fluxes in regions with shallow (deep) mixed layer.

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Section: Discussionmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Surface frontogenesis by surface heat fluxes in the upstream Kuroshio Extension region

Tozuka

Cronin

Tomita

2017

Sci Rep

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“…In the case that 3° latitude north of the GS at any given longitude is over land, the coastline is used as the border at that longitude. It is emphasized that the northern and southern regions are not fixed here due to the large monthly variability in GS path (Andres, 2016), although fixed regions may be sufficient for other current systems (e.g., Ohishi et al, 2016Ohishi et al, , 2017. Within each of these time-varying areas, the spatially averaged SST is calculated and the respective climatological monthly means for 1993-2019 are removed from each to obtain a northern and a southern SST anomaly.…”

Section: Methodsmentioning

confidence: 99%

A Monthly Index for the Large‐Scale Sea Surface Temperature Gradient Across the Separated Gulf Stream

Parfitt

Kwon

Andres

2022

Geophysical Research Letters

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The strong sea‐surface temperature (SST) gradient associated with the Gulf Stream (GS) is widely acknowledged to play an important role in shaping mid‐latitude weather and climate. Despite this, an index for the GS SST gradient has not yet been standardized in the literature. This paper introduces a monthly index for the large‐scale SST gradient across the separated GS based on the time‐varying GS position detected from sea‐surface height. Analysis suggests that the variations in the monthly average SST gradient throughout the year result primarily from SST variability to the north of the GS, with little contribution from SST to the south. The index exhibits a weak periodicity at ∼2 years. Sea level pressure and turbulent heat flux patterns suggest that variability in the large‐scale SST gradient is related to atmospheric (rather than oceanic) forcing. Ocean‐to‐atmosphere feedback does not persist throughout the year, but there is some evidence of wintertime feedback.

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“…As the J1 jet position is quasi-stationary due to topographic effects (Isoguchi et al 2006;Mitsudera et al 2018), the ensemble spread in the horizontal flow along the J1 jet is not as large as that for the KE. A huge amount of turbulent heat is released around the SST fronts because of strong wind speed and large air-sea humidity and temperature differences (e.g., Ohishi et al 2016Ohishi et al , 2019aTozuka et al 2017); therefore, the perturbed atmospheric forcing may result in the large temperature and salinity spread around the J1 jet. It is beyond the scope of this study to quantitatively investigate the causes of the formation of the large ensemble spread.…”

Section: Lora-wnpmentioning

confidence: 99%

LORA: a local ensemble transform Kalman filter-based ocean research analysis

2023

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We have produced an eddy-resolving local ensemble transform Kalman filter (LETKF)-based ocean research analysis (LORA) for the western North Pacific (WNP) and Maritime Continent (MC) regions (LORA-WNP and LORA-MC, respectively). This paper describes the system configuration and validation comparisons with Japan Coastal Ocean Predictability Experiment 2M (JCOPE2M) reanalysis and Archiving, Validation, and Interpretation of Satellite Oceanographic Data (AVISO) observational datasets. The results show that the surface horizontal velocity in the LORA-WNP is closer to independent drifter buoy observations in the mid-latitude region, especially along the Kuroshio Extension (KE), and is less close in the subtropical region than the JCOPE2M, although the AVISO is the closest over the whole domain. The sea surface temperatures (SSTs) in the LORA-WNP correspond better to assimilated satellite observations than the JCOPE2M over most of the domain except for coastal regions. The results using an independent buoy south of the KE indicate that better fit of temperature in the LORA-WNP may be limited to the upper 300 m depth, probably because of the prescribed vertical localization cutoff length of 370 m. In the MC region, the surface velocity in the LORA-MC is closer to the independent drifter buoys in the equatorial coastal region and is less close in the offshore region than the AVISO. The SSTs in the LORA-MC correspond better to the assimilated satellite observations in the offshore region than the nearshore region. Therefore, the LORA-WNP and LORA-MC have sufficient accuracy for geoscience research applications as well as for fisheries, marine transport, and environment consultants.

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Frontolysis by surface heat flux in the Agulhas Return Current region with a focus on mixed layer processes: observation and a high-resolution CGCM

Cited by 10 publications

References 74 publications

Surface frontogenesis by surface heat fluxes in the upstream Kuroshio Extension region

Surface frontogenesis by surface heat fluxes in the upstream Kuroshio Extension region

A Monthly Index for the Large‐Scale Sea Surface Temperature Gradient Across the Separated Gulf Stream

LORA: a local ensemble transform Kalman filter-based ocean research analysis

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