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Akira Kuwano-Yoshida and Shoshiro Minobe

Abstract

The storm-track response to sea surface temperature (SST) fronts in the northwestern Pacific region is investigated using an atmospheric general circulation model with a 50-km horizontal resolution. The following two experiments are conducted: one with 0.25° daily SST data (CNTL) and the other with smoothed SSTs over an area covering SST fronts associated with the Kuroshio, the Kuroshio Extension, the Oyashio, and the subpolar front (SMTHK). The storm track estimated from the local deepening rate of surface pressure (LDR) exhibits a prominent peak in this region in CNTL in January, whereas the storm-track peak weakens and moves eastward in SMTHK. Storm-track differences between CNTL and SMTHK are only found in explosive deepening events with LDR larger than 1 hPa h−1. A diagnostic equation of LDR suggests that latent heat release associated with large-scale condensation contributes to the storm-track enhancement. The SST fronts also affect the large-scale atmospheric circulation over the northeastern Pacific Ocean. The jet stream in the upper troposphere tends to meander northward, which is associated with positive sea level pressure (SLP) anomalies in CNTL, whereas the jet stream flows zonally in SMTHK. A composite analysis for the northwestern Pacific SLP anomaly suggests that frequent explosive cyclone development in the northwestern Pacific in CNTL causes downstream positive SLP anomalies over the Gulf of Alaska. Cyclones in SMTHK developing over the northeastern Pacific enhance the moisture flux along the west coast of North America, increasing precipitation in that region.

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Ryusuke Masunaga, Hisashi Nakamura, Takafumi Miyasaka, Kazuaki Nishii, and Bo Qiu

Abstract

The Kuroshio Extension (KE) fluctuates between its different dynamic regimes on (quasi) decadal time scales. In its stable (unstable) regime, the KE jet is strengthened (weakened) and less (more) meandering. The present study investigates wintertime mesoscale atmospheric structures modulated under the changing KE regimes, as revealed in high-resolution satellite data and data from a particular atmospheric reanalysis (ERA-Interim). In the unstable KE regime, a positive anomaly in sea surface temperature (SST) to the north of the climatological KE jet accompanies positive anomalies in upward heat fluxes from the ocean, surface wind convergence, and cloudiness. As revealed in the atmospheric reanalysis, these positive anomalies coincide with local lowering of sea level pressure, weaker vertical wind shear, warming and thickening of the marine atmospheric boundary layer (MABL), anomalous ascent, and convective precipitation. In the stable KE regime, by contrast, the corresponding imprints of sharp SST gradients across the KE and Oyashio fronts on the wintertime MABL are separated more distinctly, and so are the surface baroclinic zones along those two SST fronts. In the ERA-Interim data, such mesoscale imprints of the KE variability as above are not well represented in a period during which the resolution of SST data prescribed is relatively low. The present study thus elucidates the importance of high-resolution SST data prescribed for atmospheric reanalysis in representing modulations of the MABL structure and air–sea fluxes by the variability of oceanic fronts and/or jets, including the modulations occurring with the changing KE regimes through the hydrostatic pressure adjustment and vertical mixing mechanisms.

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Adèle Révelard, Claude Frankignoul, Nathalie Sennéchael, Young-Oh Kwon, and Bo Qiu

Abstract

The atmospheric response to the Kuroshio Extension (KE) variability during 1979–2012 is investigated using a KE index derived from sea surface height measurements and an eddy-resolving ocean general circulation model hindcast. When the index is positive, the KE is in the stable state, strengthened and shifted northward, with lower eddy kinetic energy, and the Kuroshio–Oyashio Extension (KOE) region is anomalously warm. The reverse holds when the index is negative. Regression analysis shows that there is a coherent atmospheric response to the decadal KE fluctuations between October and January. The KOE warming generates an upward surface heat flux that leads to local ascending motions and a northeastward shift of the zones of maximum baroclinicity, eddy heat and moisture fluxes, and the storm track. The atmospheric response consists of an equivalent barotropic large-scale signal, with a downstream high and a low over the Arctic. The heating and transient eddy anomalies excite stationary Rossby waves that propagate the signal poleward and eastward. There is a warming typically exceeding 0.6 K at 900 hPa over eastern Asia and western United States, which reduces the snow cover by 4%–6%. One month later, in November–February, a high appears over northwestern Europe, and the hemispheric teleconnection bears some similarity with the Arctic Oscillation. Composite analysis shows that the atmospheric response primarily occurs during the stable state of the KE, while no evidence of a significant large-scale atmospheric response is found in the unstable state. Arguments are given to explain this strong asymmetry.

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Kazutoshi Sato, Atsuyoshi Manda, Qoosaku Moteki, Kensuke K. Komatsu, Koto Ogata, Hatsumi Nishikawa, Miki Oshika, Yuriko Otomi, Shiori Kunoki, Hisao Kanehara, Takashi Aoshima, Kenichi Shimizu, Jun Uchida, Masako Shimoda, Mitsuharu Yagi, Shoshiro Minobe, and Yoshihiro Tachibana

Abstract

Two mesoscale convective events in the baiu frontal zone (BFZ) were documented, based on intensive atmospheric soundings and oceanic castings in the East China Sea during May 2011, in addition to continuous surface meteorological observations, satellite products, and objective analyses. These events occurred while the BFZ was nearly stagnant and a mesolow was deepening in the zone. Near-surface southerlies associated with the low-level jet transported a warm, humid air mass from south of the BFZ. Enhanced evaporation, which was mainly attributable to the high sea surface temperature of the Kuroshio, augmented the moisture content of the air mass and helped maintain a convectively unstable stratification in the lower troposphere around the BFZ.

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Kotaro Katsube and Masaru Inatsu

Abstract

A set of short-term experiments using a regional atmospheric model (RAM) were carried out to investigate the response of tropical cyclone (TC) tracks to sea surface temperature (SST) in the western North Pacific. For 10 selected TC cases occurring during 2002–07, a warm and a cold run are performed with 2 and −2 K added to the SSTs uniformly over the model domain, respectively. The cases can be classified into three groups in terms of recurvature: recurved tracks in the warm and cold runs, a recurved track in the warm run and a nonrecurved track in the cold run, and nonrecurved tracks in both runs. Commonly the warm run produced northward movement of the TC faster than the cold run. The rapid northward migration can be mainly explained by the result that cyclonic circulation to the west of the TC is found in the steering flow in the warm run and it is not in the cold run. The beta effect is also activated under the warm SST environment. For the typical TC cases, a linear baroclinic model experiment is performed to examine how the cyclonic circulation is intensified in the warm run. The stationary linear response to diabatic heating obtained from the RAM experiment reveals that the intensified TC by the warm SST excites the cyclonic circulation in the lower troposphere to the west of the forcing position. The vorticity and thermodynamic equation analysis shows the detailed mechanism. The time scale of the linear response and the teleconnection are also discussed.

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Hyodae Seo, Arthur J. Miller, and Joel R. Norris

Abstract

The summertime California Current System (CCS) is characterized by energetic mesoscale eddies, whose sea surface temperature (SST) and surface current can significantly modify the wind stress and Ekman pumping. Relative importance of the eddy–wind interactions via SST and surface current in the CCS is examined using a high-resolution (7 km) regional coupled model with a novel coupling approach to isolate the small-scale air–sea coupling by SST and surface current. Results show that when the eddy-induced surface current is allowed to modify the wind stress, the spatially averaged surface eddy kinetic energy (EKE) is reduced by 42%, and this is primarily due to enhanced surface eddy drag and reduced wind energy transfer. In contrast, the eddy-induced SST–wind coupling has no significant impact on the EKE. Furthermore, eddy-induced SST and surface current modify the Ekman pumping via their crosswind SST gradient and surface vorticity gradient, respectively. The resultant magnitudes of the Ekman pumping velocity are comparable, but the implied feedback effects on the eddy statistics are different. The surface current-induced Ekman pumping mainly attenuates the amplitude of cyclonic and anticyclonic eddies, acting to reduce the eddy activity, while the SST-induced Ekman pumping primarily affects the propagation. Time mean–rectified change in SST is determined by the altered offshore temperature advection by the mean and eddy currents, but the magnitude of the mean SST change is greater with the eddy-induced current effect. The demonstrated remarkably strong dynamical response in the CCS system to the eddy-induced current–wind coupling indicates that eddy-induced current should play an important role in the regional coupled ocean–atmosphere system.

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Ryu Saiki and Humio Mitsudera

Abstract

Ice bands are frequently observed over marginal ice zones in polar seas. A typical ice-band pattern has a regular spacing of about 10 km and extends over 100 km in the marginal ice zone. Further, the long axis of an ice band lies to the left (right) with respect to the wind direction in the Northern (Southern) Hemisphere. Here, the study shows that the resonance between ice-band pattern propagation and internal inertia–gravity waves below the sea ice well explains the ice-band pattern formation. Internal waves are generated by the difference between the stress on the open water and the stress on ice-covered water. This in turn reinforces the formation of an ice-band pattern with a regular band spacing. Specifically, the authors have found the following: 1) A band spacing on the order of 10 km is selected by the resonance condition in which the ice-band pattern propagation speed coincides with the phase speed of internal inertia–gravity waves. 2) The ice bands tend to develop favorably when the wind direction and the band propagation direction are nearly parallel. The velocity acceleration caused by the periodic differential stress associated with the ice bands, driven by the wind parallel to the band propagation direction, is important. The wind direction may turn to the left (right) slightly in the Northern (Southern) Hemisphere as a result of the Coriolis force acting on ice. Satellite images confirmed that the band spacing of the ice-band pattern in the polar seas is consistent with this theory.

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Shinichiro Kida, Bo Qiu, Jiayan Yang, and Xiaopei Lin

Abstract

The mechanism responsible for the annual cycle of the flow through the straits of the Japan Sea is investigated using a two-layer model. Observations show maximum throughflow from summer to fall and minimum in winter, occurring synchronously at the three major straits: Tsushima, Tsugaru, and Soya Straits. This study finds the subpolar winds located to the north of Japan as the leading forcing agent, which first affects the Soya Strait rather than the Tsushima or Tsugaru Straits. The subpolar winds generate baroclinic Kelvin waves along the coastlines of the subpolar gyre, affect the sea surface height at the Soya Strait, and modify the flow through the strait. This causes barotropic adjustment to occur inside the Japan Sea and thus affect the flow at the Tsugaru and Tsushima Straits almost synchronously. The barotropic adjustment mechanism explains well why the observations show a similar annual cycle at the three straits. The annual cycle at the Tsugaru Strait is further shown to be weaker than that in the other two straits based on frictional balance around islands, that is, frictional stresses exerted around an island integrate to zero. In the Tsugaru Strait, the flows induced by the frictional integrals around the northern (Hokkaido) and southern (Honshu) islands are in opposite directions and tend to cancel out. Frictional balance also suggests that the annual cycle at the Tsugaru Strait is likely in phase with that at the Soya Strait because the length scale of the northern island is much shorter than that of the southern island.

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Yi-Hui Wang and W. Timothy Liu

Abstract

This study investigates the regional atmospheric response to the Kuroshio Extension (KE) using a combination of multiple satellite observations and reanalysis data from boreal winter over a period of at least a decade. The goal is to understand the relationship between KE variations and atmospheric responses at low frequencies. A climate index is used to measure the interannual to decadal KE variability, which leaves remarkable imprints on the mesoscale sea surface temperature (SST). Clear spatial coherence between the SST signals and frontal-scale atmospheric variables, including surface wind convergence, vertical velocity, precipitation, and clouds, is identified by linear regression analysis. Consistent with previous studies, the penetrating effect of the KE variability on the free atmosphere is found. The westward tilt of the atmospheric response above the KE near 500 hPa is revealed. The difference in the associations of frontal-scale air temperature and geopotential height with the KE variability between the satellite observations and the reanalysis data suggests an imperfect interpretation of frontal-scale oceanic forcing on the overlying atmosphere in the reanalysis assimilation system.

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Larry W. O’Neill, Tracy Haack, and Theodore Durland

Abstract

Two methods of computing the time-mean divergence and vorticity from satellite vector winds in rain-free (RF) and all-weather (AW) conditions are investigated. Consequences of removing rain-contaminated winds on the mean divergence and vorticity depend strongly on the order in which the time-average and spatial derivative operations are applied. Taking derivatives first and averages second (DFAS_RF) incorporates only those RF winds measured at the same time into the spatial derivatives. While preferable mathematically, this produces mean fields biased relative to their AW counterparts because of the exclusion of convergence and cyclonic vorticity often associated with rain. Conversely, taking averages first and derivatives second (AFDS_RF) incorporates all RF winds into the time-mean spatial derivatives, even those not measured coincidentally. While questionable, the AFDS_RF divergence and vorticity surprisingly appears qualitatively consistent with the AW means, despite using only RF winds. The analysis addresses whether the AFDS_RF method accurately estimates the AW mean divergence and vorticity.

Model simulations indicate that the critical distinction between these two methods is the inclusion of typically convergent and cyclonic winds bordering rain patches in the AFDS_RF method. While this additional information removes some of the sampling bias in the DFAS_RF method, it is shown that the AFDS_RF method nonetheless provides only marginal estimates of the mean AW divergence and vorticity given sufficient time averaging and spatial smoothing. Use of the AFDS_RF method is thus not recommended.

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