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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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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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Masami Nonaka
,
Hisashi Nakamura
,
Bunmei Taguchi
,
Nobumasa Komori
,
Akira Kuwano-Yoshida
, and
Koutarou Takaya

Abstract

An integration of a high-resolution coupled general circulation model whose ocean component is eddy permitting and thus able to reproduce a sharp gradient in sea surface temperature (SST) is analyzed to investigate air–sea heat exchanges characteristic of the midlatitude oceanic frontal zone. The focus of this paper is placed on a prominent SST front in the south Indian Ocean, which is collocated with the core of the Southern Hemisphere storm track. Time-mean distribution of sensible heat flux is characterized by a distinct cross-frontal contrast. It is upward and downward on the warmer and cooler flanks, respectively, of the SST front, acting to maintain the sharp gradient of surface air temperature (SAT) that is important for preconditioning the environment for the recurrent development of storms and thereby anchoring the storm track. Induced by cross-frontal advection of cold (warm) air associated with migratory atmospheric disturbances, the surface flux is highly variable with intermittent enhancement of the upward (downward) flux predominantly on the warmer (cooler) flank of the front. Indeed, several intermittent events of cold (warm) air advection, whose total duration accounts for only 21% (19%) of the entire analysis period, contribute to as much as 60% (44%) of the total amount of sensible heat flux during the analysis period on the warmer (cooler) flank. This antisymmetric behavior yields the sharp cross-frontal gradient in the time-mean flux. Since the flux intensity is strongly influenced by local magnitude of the SST–SAT difference that tends to increase with the SST gradient, the concentration of the flux variance to the frontal zone and cross-frontal contrasts in the mean and skewness of the flux all become stronger during the spinup of the SST front. Synoptically, the enhanced sensible heat flux near the SST front can restore SAT toward the underlying SST effectively with a time scale of a day, to maintain a frontal SAT gradient against the relaxing effect of atmospheric disturbances. The restoration effect of the differential surface heating at the SST front, augmented by the surface latent heating concentrated on the warm side of the front, represents a key process through which the atmospheric baroclinicity and ultimately the storm track are linked to the underlying ocean.

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