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Haiming Xu
,
Hiroki Tokinaga
, and
Shang-Ping Xie

Abstract

In the summer of 2004, the Kuroshio took a large meander path south of Japan for the first time since 1991, and this large meander event persisted until the next summer. Satellite observations and numerical model simulations are used to study the effect of this large meander event on the atmosphere. The large meander leaves a cool water pool between the Kuroshio and Japanese coast. Sea surface temperature (SST) in the cool water pool is about 2°–3°C colder than the surroundings during winter and spring, whereas the SST signature substantially weakens in summer. A local reduction of wind speed is found over the cool water pool, and the positive SST–wind speed correlation is indicative of ocean forcing of the atmosphere. Cloud liquid water (CLW) content and precipitation also decrease over the cool SST pool.

A regional atmospheric model successfully simulates atmospheric response to the Kuroshio large meander. The model experiments suggest that the reduced surface wind speed and precipitation are due to the large meander-induced SST cooling. Analysis of the surface perturbation momentum budgets shows the importance of the pressure adjustment mechanism in surface wind response to the cold SST anomalies.

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Akira Kuwano-Yoshida
,
Shoshiro Minobe
, and
Shang-Ping Xie

Abstract

The precipitation response to sea surface temperature (SST) gradients associated with the Gulf Stream is investigated using an atmospheric general circulation model. Forced by observed SST, the model simulates a narrow band of precipitation, surface convergence, and evaporation that closely follows the Gulf Stream, much like satellite observations. Such a Gulf Stream rainband disappears in the model when the SST front is removed by horizontally smoothing SST. The analysis herein shows that it is convective precipitation that is sensitive to SST gradients. The Gulf Stream anchors a convective rainband by creating surface wind convergence and intensifying surface evaporation on the warmer flank. Deep convection develops near the Gulf Stream in summer when the atmosphere is conditionally unstable. As a result, a narrow band of upward velocity develops above the Gulf Stream throughout the troposphere in summer, while it is limited to the lower troposphere in other seasons.

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Bunmei Taguchi
,
Hisashi Nakamura
,
Masami Nonaka
, and
Shang-Ping Xie

Abstract

Influences of oceanic fronts in the Kuroshio and Oyashio Extension (KOE) region on the overlying atmosphere are investigated by comparing a pair of atmospheric regional model hindcast experiments for the 2003/04 cold season, one with the observed finescale frontal structures in sea surface temperature (SST) prescribed at the model lower boundary and the other with an artificially smoothed SST distribution. The comparison reveals the locally enhanced meridional gradient of turbulent fluxes of heat and moisture and surface air temperature (SAT) across the oceanic frontal zone, which favors the storm-track development both in winter and spring. Distinct seasonal dependency is found, however, in how dominantly the storm-track activity influences the time-mean distribution of the heat and moisture supply from the ocean.

In spring the mean surface sensible heat flux (SHF) is upward (downward) on the warmer (cooler) side of the subarctic SST front. This sharp cross-frontal contrast is a manifestation of intermittent heat release (cooling) induced by cool northerlies (warm southerlies) on the warmer (cooler) side of the front in association with migratory cyclones and anticyclones. The oceanic frontal zone is thus marked as both the largest variability in SHF and the cross-frontal sign reversal of the SHF skewness. The cross-frontal SHF contrasts in air–sea heat exchanges counteract poleward heat transport by those atmospheric eddies, to restore the sharp meridional gradient of SAT effectively for the recurrent development of atmospheric disturbances. Lacking this oceanic baroclinic adjustment associated with the SST front, the experiment with the smoothed SST distribution underestimates storm-track activity in the KOE region.

In winter the prevailing cold, dry continental airflow associated with the Asian winter monsoon induces a large amount of heat and moisture release even from the cooler ocean to the north of the frontal zone. The persistent advective effects of the monsoonal wind weaken the SAT gradient and smear out the sign reversal of the SHF skewness, leading to weaker influences of the oceanic fronts on the atmosphere in winter than in spring.

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Shoshiro Minobe
,
Masato Miyashita
,
Akira Kuwano-Yoshida
,
Hiroki Tokinaga
, and
Shang-Ping Xie

Abstract

The atmospheric response to the Gulf Stream front in sea surface temperature is investigated using high-resolution data from satellite observations and operational analysis and forecast. Two types of atmospheric response are observed with different seasonality and spatial distribution.

In winter, surface wind convergence is strong over the Gulf Stream proper between Cape Hatteras and the Great Banks, consistent with atmospheric pressure adjustments to sea surface temperature gradients. The surface convergence is accompanied by enhanced precipitation and the frequent occurrence of midlevel clouds. Local evaporation and precipitation are roughly in balance over the Florida Current and the western Gulf Stream proper. In summer, strong precipitation, enhanced high clouds, and increased lightning flash rate are observed over the Florida Current and the western Gulf Stream proper, without seasonal surface convergence enhancement. For the precipitation maximum over the Florida Current, local evaporation supplies about half of the water vapor, and additional moisture is transported from the south on the west flank of the North Atlantic subtropical high.

Atmospheric heating estimated by a Japanese reanalysis reveals distinct seasonal variations. In winter, a shallow-heating mode dominates the Gulf Stream proper, with strong sensible heating in the marine atmospheric boundary layer and latent heating in the lower troposphere. In summer, a deep-heating mode is pronounced over the Florida Current and the western Gulf Stream proper, characterized by latent heating in the middle and upper troposphere due to deep convection. Possible occurrences of these heating modes in other regions are discussed.

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