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Yuki Kanno and Toshiki Iwasaki

Abstract

The present study develops a diagnostic framework for investigating the three-dimensional (3D) structure of mass-weighted isentropic time-mean (T-MIM) meridional circulations and conducts a preliminary analysis of the winter hemispheres. The T-MIM meridional velocity can unfold, in the zonal direction, time-averaged two-dimensional (2D) mass-weighted isentropic zonal means. Furthermore, the T-MIM velocity can be decomposed into the unweighted isentropic time-mean (uTM) velocity and the temporal eddy-correlated transport velocity, the so-called bolus velocity. The bolus velocity greatly contributes to the 2D extratropical direct circulation in the troposphere and to the Brewer–Dobson circulation in the stratosphere. The 3D bolus velocity seems to reflect the geographical distributions of baroclinic instability wave activity. In the boreal winter, both low-level equatorward flows and upper-level poleward flows are located around the North Pacific and North Atlantic storm tracks. In the austral winter, low-level equatorward flows extend zonally across the midlatitudes. In the subtropics, the 3D bolus velocity is found to be significant in the upper branch of the Hadley circulation. A zonal momentum equation is formulated to examine the 3D momentum balance of the meridional circulation in the T-MIM framework. In the extratropics, the uTM and bolus meridional velocities are in geostrophic balance with the stationary and transient components of the 3D Eliassen–Palm (EP) flux divergence, respectively. The pressure gradient force of transient baroclinic instability waves balances with the low-level equatorward flows of the bolus velocity in the storm tracks.

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Yuki Kanno, John E. Walsh, and Toshiki Iwasaki

Abstract

In boreal winter, the cold air mass (CAM) flux of air with a potential temperature below 280 K forms climatological mean CAM streams in East Asia and North America (NA). This study diagnoses the interannual variability of the NA stream by an analysis of the CAM flux across 60°N between Greenland and the Rocky Mountains. The first empirical orthogonal function (EOF) represents the variations in intensity of the NA stream. When the first principal component (PC1) is highly positive, the central part of the NA stream is intensified, with cold anomalies east of the Rocky Mountains. At the same time, a stratospheric polar vortex tends to split or displace toward NA. PC1 is highly correlated with the tropical Northern Hemisphere pattern, implying that this pattern is associated with the intensity of the NA stream. The second EOF shows a longitudinal shift of the NA stream toward Greenland or the Rocky Mountains. A highly negative PC2 results in a cold anomaly from western Canada to the Midwestern United States and anomalous heavy snowfall in the northeastern United States. PC2 is positively correlated with the Arctic Oscillation, which suggests that the longitudinal position of the NA stream varies with the Arctic Oscillation. These results illustrate how the intensity and location of cold air outbreaks vary with large-scale modes of atmospheric variability, with corresponding implications for the predictability of winter severity in NA.

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Muhammad Rais Abdillah, Yuki Kanno, and Toshiki Iwasaki

Abstract

Intraseasonal variations of East Asian cold air outbreaks (CAOs) in relation to the tropical atmosphere during 34 winters (DJF) are investigated. This study is a continuation of Part I, which discussed the interannual variability of East Asian CAOs. Two types of quantitative East Asian CAOs, western and eastern CAOs, are examined. Their variations are identified by the zonal integration of equatorward flux of cold air mass (CAM) below 280 K at 45°N over 90°–135°E and 135°E–180°. A day-lagged regression analysis reveals that peaks of intraseasonal western and eastern CAO events are preconditioned by large-scale tropical convection anomalies resembling particular phases of the Madden–Julian oscillation (MJO). Western CAO events tend to occur when the convective phase of the MJO crosses over the Maritime Continent. In contrast, eastern CAO events are triggered by the MJO over the western Pacific. Observations of MJO-related atmospheric anomalies indicate the important roles of poleward Rossby wave trains in affecting extratropical East Asian CAOs. The barotropic Rossby waves develop negative geopotential height anomalies in midlatitude East Asia, which then induce a low-level equatorward cold airflow. Several experiments in an atmospheric model using prescribed MJO-like heating anomalies demonstrate that the Maritime Continent MJO and the western Pacific MJO clearly affect the equatorward CAM flux over the western and eastern CAO regions, respectively. Compared with the western CAO, the eastern CAO shows a more robust response to the MJO because of stronger wave activity during the western Pacific MJO.

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Muhammad Rais Abdillah, Yuki Kanno, and Toshiki Iwasaki

Abstract

Interannual variability of winter-mean East Asian cold air outbreaks (CAOs) and its relationship with the tropical climate system during 56 boreal winters (DJF) are investigated. The magnitude of CAO is quantified as winter-mean equatorward cold airmass (CAM) flux below 280-K potential temperature across the 45°N latitude. EOF analysis shows that the interannual variation of East Asian CAOs is attributed mainly to the contributions from western and eastern CAOs. In particular, the western and eastern CAOs tend to be remotely forced by La Niña and El Niño events, respectively.

The western and eastern CAOs have distinct climate variability. The western CAO, which is enhanced under the climatic anomalies of high pressure over northern Eurasia and low pressure over the western North Pacific, causes negative CAM anomalies over northern Eurasia and positive ones over midlatitude East Asia. In the tropical region, the western CAO negatively correlates with the eastern Pacific and Indian Ocean SST, both of which enhance precipitation over the Maritime Continent. On the other hand, the eastern CAO is enhanced by the strong Aleutian low and results in positive CAM anomalies in the western North Pacific and substantial negative anomalies in western North America. The eastern CAO positively correlates with the tropical SST anomalies and accordingly precipitation anomalies over the central Pacific. ENSO influences western and eastern CAOs through upper and poleward Rossby wave trains excited by convective anomalies over the Maritime Continent and central Pacific, respectively.

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Teruhisa Shimada, Yuki Kanno, and Toshiki Iwasaki

Abstract

The climatology of low-level cool air over the midlatitude oceans in summer is presented based on an isentropic analysis. This study focuses on isentropic surfaces of 296 K to analyze an adiabatic invariant referred to as the negative heat content representing the coldness of the air layer below the threshold isentropic surface. This approach allows a systematic analysis and a quantitative comparison of the cool air distribution and a diagnosis of diabatic heating of the air mass. The cool air covers most of the subarctic oceans and extends equatorward over the coastal upwelling regions in the east of the ocean basins. In these regions, the genesis of the cool air is diagnosed. The loss of the cool air occurs over land and the subtropical oceans, particularly on the offshore side of the coastal upwelling regions. In the Pacific sector and the Indian Ocean sector of the Southern Ocean, another large loss of the cool air occurs along the oceanic frontal zone including the Agulhas Return Current. Over the zonally extended region where the cool air is generated in the Southern Hemisphere and the coastal upwelling regions, it is suggested that diabatic cooling associated with low-level clouds overcome heating by turbulent surface heat fluxes. The genesis of the cool air over the subarctic oceans in the Northern Hemisphere in the warm season switches into the loss of the cold air in the cool season on a basin scale. Meanwhile, over the oceans in the Southern Hemisphere, there is no basin-scale seasonal switch.

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Qian Liu, Guixing Chen, Lin Wang, Yuki Kanno, and Toshiki Iwasaki

Abstract

The winter monsoon has strong impacts on East Asia via latitude-crossing southward cold airmass fluxes called cold air outbreaks (CAOs). CAOs have a high diversity in terms of meridional extent and induced weather. Using the daily cold airmass flux normalized at 50°N and 30°N during 1958–2016, we categorize the CAOs into three groups: high–middle (H–M), high–low (H–L) and middle–low (M–L) latitude events. The H–L type is found to have the longest duration, and the M–L type is prone to the strong CAOs regarding normalized intensity. The H–L and H–M events feature a large-scale dipole pattern of cold airmass flux over high-latitude Eurasia, and the former (latter) events feature relatively strong anticyclonic circulation over Siberia (cyclonic circulation over northeastern Asia). In contrast, the M–L events are characterized by a cyclonic anomaly over northeastern Asia but no obvious high-latitude precursor. The H–L events have the greatest coldness anomaly in airmasses near the surface, and the M–L events mainly feature a strong northerly wind. As a result, the H–L events induce widespread long-lasting low temperatures over East Asia, while the M–L events induce a sharp temperature drop at mainly low latitudes. Both H–L and M–L events coupling with the MJO enhance rainfall over the South China Sea, while H–M events increase rainfall over southern China. Moreover, the occurrences of H–L and M–L events experience a long-term decrease since the 1980s, which induce a stronger warming trend in the cold extremes than in the winter mean temperature at mid-low latitudes over East Asia.

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Takamichi Shoji, Yuki Kanno, Toshiki Iwasaki, and Koutarou Takaya

Abstract

The equatorward cold airmass flux below potential temperature θ T = 280 K across 45°N integrated from 90°E to 180° is used as an index to quantitatively measure cold air outbreaks (CAOs) in the East Asian winter monsoon. Intermittent CAOs over East Asia significantly contribute to the global equatorward cold airmass flux. An autocorrelation analysis indicates that CAO events persist for approximately 5 days. The geographical distributions of lagged correlations/regressions with the CAO index (CAOI) clarify the temporal evolution of synoptic conditions associated with CAOs. The developing Siberian high located northwest of Lake Baikal (65°N, 100°E) on day −4 slowly moves southeastward, reaches maximum intensity over Siberia (50°N, 110°E) on day 0, and then decays while moving rapidly southward. By contrast, the Aleutian low is almost stagnant and maintains a strong intensity. The eastward pressure gradient geostrophically induces the equatorward cold airmass flux. After day −2, the cold air mass significantly decreases over Siberia, but increases over East Asia and the western North Pacific Ocean. The cold air mass continues to migrate southward while spreading eastward, and disappears mainly over the ocean. The leading edge of the high pressure anomaly moves southward at 13 m s−1 and reaches the equator simultaneously with the equatorward wind anomaly on about day +4. An additional analysis of separating the equatorward flux into 90°–135°E and 135°E–180° suggests that CAOs are, to some extent, caused by the Siberian high and the Aleutian low acting separately.

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Muhammad Rais Abdillah, Yuki Kanno, Toshiki Iwasaki, and Jun Matsumoto

Abstract

Cold surge occurrences are one of the robust features of winter monsoon in East Asia and are characterized by equatorward outbreaks of cold air from the high latitudes. Beside greatly affecting weather variability across the Far East, cold surges are of importance for Southeast Asian countries because they can propagate far to the tropics and excite convective activities. However, the tropical responses highly depend on the downstream pathways of the surges. To better understand how cold surges influence tropical weather, we investigate 160 cold surges identified using a quantitative approach during 40 winters from 1979/80 to 2018/19, and then classify them into several groups based on their prominent pathways. At the midlatitudes, we find two groups: one for surges that show clear equatorward propagation of cold air to lower latitudes and the other for surges that turn eastward and bring cold air to the North Pacific. These groups arise due to the strength difference of the Siberian high expansion controlled by cold air blocking near the Tibetan Plateau. The tropical impact is evident in the former group. We perform further classification on this group and find four types of surges based on their pathways in the low latitudes: 1) South China Sea (SCS) surges, 2) Philippines Sea (PHS) surges, 3) both SCS and PHS surges, and 4) blocked surges. They exhibit distinct precipitation signatures over the Maritime Continent, which are driven by interactions between the surges and the pre-existing synoptic conditions over the tropics, particularly the Madden–Julian oscillation (MJO).

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Toshiki Iwasaki, Takamichi Shoji, Yuki Kanno, Masahiro Sawada, Masashi Ujiie, and Koutarou Takaya

Abstract

An analysis method is proposed for polar cold airmass streams from generation to disappearance. It designates a threshold potential temperature θ T at around the turning point of the extratropical direct (ETD) meridional circulation from downward to equatorward in the mass-weighted isentropic zonal mean (MIM) and clarifies the geographical distributions of the cold air mass, the negative heat content (NHC), their horizontal fluxes, and their diabatic change rates on the basis of conservation relations of the air mass and thermodynamic energy. In the Northern Hemispheric winter, the polar cold air mass below θ T = 280 K has two main streams: the East Asian stream and the North American stream. The former grows over the northern part of the Eurasian continent, flows eastward, turns down southeastward toward East Asia via Siberia, and disappears over the western North Pacific Ocean. The latter grows over the Arctic Ocean, flows toward the eastern coast of North America via Hudson Bay, and disappears over the western North Atlantic Ocean. In their exit regions, wave–mean flow interactions are considered to transfer the angular momentum from the cold airstreams to the upward Eliassen–Palm flux and convert the available potential energy to wave energy.

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Takahiro Toyoda, Hideyuki Nakano, Hidenori Aiki, Tomomiti Ogata, Yoshiki Fukutomi, Yuki Kanno, L. Shogo Urakawa, Kei Sakamoto, Goro Yamanaka, and Motoki Nagura

Abstract

A method is introduced for diagnosing the time evolution of wave energy associated with ENSO from an ocean reanalysis. In the diagnosis, time changes of kinetic and available potential energy are mainly represented by energy inputs caused by surface wind stress and horizontal energy fluxes for each vertically decomposed normal mode. The resulting time evolutions of the wave energy and vertical thermocline displacements in the 1997–1998 and 2014–2016 El Niño events are consistent with our previous knowledge of these events. Further, our result indicated that representation of several vertical modes is necessary to reproduce the broadly distributed downward thermocline displacements in the central to eastern equatorial Pacific, generated by a westerly wind event in the western equatorial Pacific (e.g., in March 1997), that are preconditioning for El Niño development. In addition, we investigated the wave energy budget, including the influence of data assimilation, on the complicated time evolution of equatorial thermocline displacements caused by repeated westerly and easterly wind events during the 2014–2016 El Niño event. Our result suggests that noise from a momentum imbalance near the equator associated with data assimilation, which possibly affected the El Niño prediction failure in 2014, was much reduced by our developed ocean data assimilation system and reanalysis. This study, which provides a new connection between the theoretical works and reanalysis products that use sophisticated systems for synthesizing OGCMs and observations, should be useful for climate research and operational communities interested in ENSO.

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