The Pacific Decadal Oscillation (PDO) and North Pacific Gyre Oscillation (NPGO) are the two dominant low-frequency modes in the North Pacific. This study focused on the simulation capability of the two leading low-frequency modes in current coupled models, based on 24 coupled model outputs from the Coupled Model Intercomparison Project Phase 5 (CMIP5). Results showed that most of these models captured the two low-frequency modes, but the air-sea coupling relationship (covariability of the ocean low-frequency modes with the atmospheric forcing modes) captured by CMIP5 models had drastic differences. Four models (CCSM4, CESM-WACCM, MIROC5 and NorESM1-M) not only captured the spatial and temporal characteristics of PDO and NPGO modes but also simulated their air-sea coupling relationships. Therefore, we selected these four models to examine changes in PDO and NPGO modes under different global warming scenarios using RCP4.5 and RCP8.5 forcing (RCP: Representative Concentration Pathway). In future RCP scenarios, the spatial patterns of PDO and NPGO showed no obvious changes. However, the dominant periods of PDO and NPGO modes were shorter, which is consistent with faster oceanic Rossby waves induced by enhanced upper oceanic stratification in the warming scenarios.
Based on the ERA-20C, climatic variations of the boundary layer height (BLH) over arid and semiarid areas in East Asia and North Africa spanning 1900-2010 were analyzed. In East Asia, the BLH expressed a descending trend from arid region centers to the periphery. Over the past 111 years, the BLH had a rising trend of 14.0m/10yr in the representative region (EA1) of the eastern areas with annual mean of 725m, and that in the representative region (EA2) of the western areas with annual mean of 792m had a decreasing trend of -1.6m/10yr. From the mid-1960s to 1970s, EA1’s BLH had a sharp rise causing the average to increase by 93m after 1980s. In North Africa, the BLH expressed a high spatial distribution in the western and southern areas, and a relatively low spatial distribution in the eastern and northern areas. Over the past 111 years, the BLH had a rising trend of 9.7m/10yr in the representative region (NA1) of the southwestern region with annual mean of 915m, and that in the representative region (NA2) of other regions with annual mean of 882m had a decreasing trend of -6.3m/10yr. In 1940s and 1970s, NA1’s BLH had two obvious increases causing the average to increase by 51m and 22m respectively, while NA2’s BLH had two obvious declines causing the average to decrease by 48m and 7m respectively. On the spatial distribution, the BLH, sensible heat flux, latent heat flux and volumetric soil water had a good corresponding relationship. On the temporal change, in East Asia, the BLH had a stronger correlation with the thermodynamic factors than the dynamic factors, while in the North Africa, the BLH had a stronger correlation with the dynamic factors. Besides, the upper level stratification also has some influence on the BLH’s change.
This paper proposes a new simple method of multivariable maximum covariance analysis (MMCA), for extracting common variability from multiple (more than two) datasets, that expands the singular value decomposition analysis method. The method is based on iteration of a recurrence equation derived by a dual relationship between pattern vectors and time coefficients. Two approaches of the method are proposed, one using the extreme of a summation of covariances (sum MMCA) and the other using the product of covariances (product MMCA). Both approaches are demonstrated by successfully extracting the variability related to the Arctic Oscillation from three monthly-mean meteorological datasets. The method is useful because it is easily programmed and is computationally inexpensive. The method can be applied to an arbitrary number of datasets, although a complete set of the product MMCA method cannot apply to an even number of datasets.
It has been argued that the Coupled Model Intercomparison Project phase 5 (CMIP5) models underestimate the frequency of atmospheric blocking, while project a decreasing trend of blocking in the 21st century in the Northern Hemisphere. This average trend may not be true for regional blockings. Focusing on three key regions in Eurasia (the Urals, Baikal, and Okhotsk regions) where blocking significantly influences the weather and climate of East Asia, this study first evaluates the performance of the CMIP5 models by comparing historical simulations with NCEP/NCAR reanalysis (NNR). Possible changes in the first half of the 21st century are then analyzed using the RCP4.5 and RCP8.5 experiments.
It is found that instantaneous blocking frequencies are underestimated in the Urals and Baikal regions for the whole year and in the Okhotsk region in summertime, but are overestimated in Okhotsk in wintertime. Blocking episode frequency in the Urals and Baikal regions is underestimated by most of the 13 CMIP5 models, especially the short-duration blocking episodes (4–5 days), and the simulations are better in wintertime than in summertime. However, in the Okhotsk region the modeled frequency of blocking episodes is close to the value from NNR in summertime but overestimated in wintertime. Model projections of instantaneous blocking frequency for the first half of the 21st century (2016–2065) show that both RCP4.5 and RCP8.5 runs yield an increasing frequency except during June–August in the Eurasia. The multi-model ensemble mean frequency of blocking episodes clearly decreases in the whole year in the Urals and Baikal regions (especially blocking episodes with short duration) and increases a little in summertime of the Okhotsk region in the first half of the 21st century. The model ensemble-mean frequency of blocking episodes with long duration (more than 9 days) decreases by ~40% in the Urals region but increases by no more than 5% in Okhotsk.
Polar mesoscale cyclones (PMCs) frequently developed over the Japan Sea. Genesis of PMCs over the East China Sea is rare, but could occur under the certain synoptic-scale conditions. In this observational case study, the feature of a PMC generated over the eastern East China Sea on 20 February 1975 is studied by using observation data including those obtained during Air-mass Transformation Experiment, satellite cloud images and objective-reanalysis data.
The PMC with comma-cloud formed within cyclonic polar-air streams induced by an upper cold trough and a synoptic-scale parent cyclone which developed near Japan. Within 3-hour after generation of the PMC, its central pressure deepened from 1016 hPa to 1012 hPa. Strong surface winds occurred in the trail of the comma-cloud. The large-scale conditions for the generation stage were characterized by the southward intruding of the cold core in the upper cold trough beyond 34°N to the East China Sea, positive vorticity advection at 500 hPa, and the moist-neutral layer formed over the warm Tsushima Current in the eastern East China Sea.
The PMC, after passing over Kyushu, developed as it moved eastward along the Pacific coast of Japan. It developed further in the low-level baroclinic zone over the Northwestern Pacific, into the secondary cyclone comparable to the parent cyclone. The large-scale conditions for the development were characterized by the upper cold trough and the low-level baroclinic zone formed over the zone of maximum sea-surface temperature gradient along north of the Kuroshio extension.