Yuki Kanno

Isentropic Analysis of Polar Cold Airmass Streams in the Northern Hemispheric Winter

AbstractAn analysis method is proposed for polar cold airmass streams from generation to disappearance. It designates a threshold potential temperature θT 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 θT = 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 ov...

An Isentropic Analysis of the Temporal Evolution of East Asian Cold Air Outbreaks

AbstractThe equatorward cold airmass flux below potential temperature θT = 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 sig...

Charge and discharge of polar cold air mass in northern hemispheric winter

This study shows the variability of polar cold air mass amount below potential temperature of 280 K, and north of 45°N can be understood with a concept of charge and discharge, where anomalously large daily discharge indicates an intermittent occurrence of cold air outbreak. The polar cold air mass amount north of 45°N gradually charges up due to diabatic cooling but dramatically discharges due to cold air outbreak with a pulse width of about 5 days. Cold air outbreaks tend to bring colder winter in East Asia and the east coast of North America, while warmer winter prevails on the northern side of these regions. The cold air mass amount south of 45°N increases just after a cold air outbreak but returns to the normal level soon because of its life time of about 3 days. Therefore, monthly mean of total cold air mass amount in the Northern Hemisphere is negatively correlated with the monthly mean discharge.

Tropical–Extratropical Interactions Associated with East Asian Cold Air Outbreaks. Part I: Interannual Variability

AbstractInterannual 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 Nina and El Nino 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...

Tropical–Extratropical Interactions Associated with East Asian Cold Air Outbreaks. Part II: Intraseasonal Variation

AbstractIntraseasonal 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 tr...

Long‐term trend of cold air mass amount below a designated potential temperature in Northern and Southern Hemispheric winters using reanalysis data sets

This study shows that the total hemispheric cold air mass (CAM) amount below a designated potential temperature of 280 K serves as a good indicator of long-term climate change trends in the polar region. We conduct quantitative analyses of the warming trends of Northern Hemisphere (NH) and Southern Hemisphere (SH) winters using five reanalysis datasets (CFSR, ERA-interim, JRA-55, JRA-25, and NCEP-NCAR) and JRA-55-related datasets (JRA-55C and JRA-55AMIP). Total hemispheric CAM amount for the NH winter shows a significant decreasing trend for most of the reanalyses at a rate of approximately -1.37 to -0.77% decade-1 from 1959 to 2012 and at a rate of approximately -1.57 to -0.84% decade-1 from 1980 to 2012. The CAM amount trends show very different geographic patterns during the two periods, which suggests that geographical patterns of the trends estimated for these periods are largely controlled by internal low-frequency dynamics. The CAM flux across 45°N, which serves as an indicator of hemispheric scale cold air outbreaks, has so large interannual variability that its trend becomes insignificant. In the SH winter, trends of total hemispheric CAM amount and its equatorward flux across 50°S are not consistent among the new reanalyses for 1980-2012. Although total hemispheric CAM amount decreases for 1959-2012, sparse observations and changes in systematic bias, due to the implementation of assimilation of satellite observation data in 1979, compromise the reliability of the trends. Improvements to the reanalyses are desirable as a means of monitoring historical changes in CAM for the SH winter.

Interannual Variability of the North American Cold Air Stream and Associated Synoptic Circulations

AbstractIn 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 towards 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 Unite...

Three-Dimensional Structure of Mass-Weighted Isentropic Time-Mean Meridional Circulations

AbstractThe present study develops a diagnostic framework for investigating the three-dimensional (3-D) 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 (2-D) 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 2-D extratropical direct circulation in the troposphere and to the Brewer-Dobson circulation in the stratosphere. The 3-D 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 w...

Low-Level Cool Air over the Midlatitude Oceans in Summer

AbstractThe 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. Ove...