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Alan Robock and Jianping Mao

Abstract

Climate records of the past 140 years are examined for the impact of major volcanic eruptions on surface temperature. After the low-frequency variations and El Niño/Southern Oscillation signal are removed, it is shown that for 2 years following great volcanic eruptions, the surface cools significantly by 0.1°–0.2°C in the global mean, in each hemisphere, and in the summer in the latitude bands 0°–30°S and 0°–30°N and by 0.3°C in the summer in the latitude band 30°–30°60°N. By contrast, in the first winter after major tropical eruptions and in the second winter after major high-latitude eruptions, North America and Eurasia warm by several degrees, while northern Africa and southwestern Asia cool by more than 0.5°C.

Because several large eruptions occurred at the same time as ENSO events, the warming produced by the ENSO masked the volcanic cooling during the first year after the eruption. The timescale of the ENSO response is only 1 year while the volcanic response timescale is 2 years, so the cooling in the second year is evident whether the ENSO signal is removed or not.

These results, both the global cooling and Northern Hemisphere continental winter warming, agree with general circulation model calculations.

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Jianping Mao and Alan Robock

Abstract

Thirty surface air temperature simulations for 1979–88 by 29 atmospheric general circulation models are analyzed and compared with the observations over land. These models were run as part of the Atmospheric Model Intercomparison Project (AMIP). Several simulations showed serious systematic errors, up to 4°–5°C, in globally averaged land air temperature. The 16 best simulations gave rather realistic reproductions of the mean climate and seasonal cycle of global land air temperature, with an average error of −0.9°C for the 10-yr period. The general coldness of the model simulations is consistent with previous intercomparison studies. The regional systematic errors showed very large cold biases in areas with topography and permanent ice, which implies a common deficiency in the representation of snow-ice albedo in the diverse models. The SST and sea ice specification of climatology rather than observations at high latitudes for the first three years (1979–81) caused a noticeable drift in the neighboring land air temperature simulations, compared to the rest of the years (1982–88). Unsuccessful simulation of the extreme warm (1981) and cold (1984–85) periods implies that some variations are chaotic or unpredictable, produced by internal atmospheric dynamics and not forced by global SST patterns.

Among the 16 best simulations, 8 reproduced the dominant El Niño–Southern Oscillation (ENSO) mode in the 10-yr period, which includes the 1982–83 and 1986–87 warm episodes and the 1988 cold episode. On the average, the ENSO mode explains about 30% of the total variance in surface air temperature fluctuation and has a 2-month lag from the Southern Oscillation index. In this mode, North America displays a Pacific–North American–like anomaly pattern, but Eurasia gave little response to warm SSTs in the eastern equatorial Pacific, in good agreement with results based on historical data.

The special design of the AMIP experiment provides a unique opportunity to estimate the effects of the El Chichón volcanic eruption in spring 1982, which was not included in the model forcing. Comparison of the simulations with data delineated a visible global cooling in the first months following the El Chichón eruption, in addition to the cooling from the volcanic eruption of Nyamuragira in December 1981, due to the reduction of incoming solar radiation by volcanic aerosols. However, the mean climate shift in the AMIP experiment due to the forcing data discontinuity at the end of 1981 made the quantitative estimate of El Chichón global cooling influence impossible. The contrast between the simulated ENSO signal and observations shows that the major warming over the northern continents during the 1982/83 winter (DJF) is not an ENSO-like signal. Instead it is most likely a pattern resulting from the enhanced polar vortex produced by a larger pole-to-equator temperature gradient. This gradient was due to the larger absorption of radiation in low latitudes by the El Chichón volcanic sulfate aerosols in the stratosphere. These results suggest that during the Northern Hemisphere wintertime, the stratospheric polar vortex has substantial influence on surface air temperature fluctuations through its effects on vertically propagating planetary waves of the troposphere, and imply that current GCMs are deficient in simulation of stratospheric processes and their coupling with the troposphere.

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Jiaqing Xue, Cheng Sun, Jianping Li, Jiangyu Mao, Hisashi Nakamura, Takafumi Miyasaka, and Yidan Xu

Abstract

Global sea surface temperature (SST) evolution exhibits an antiphase variation between the two hemispheres that is referred to as the SST interhemispheric dipole (SSTID) mode. The impacts of the SSTID on extratropical atmospheric circulation in boreal winter are explored by both regression analysis and SST-forced numerical simulations. The responses of extratropical circulation to SSTID thermal forcing bear an equivalent barotropic structure. For the Southern Hemisphere (SH), positive SSTID events lead to a meridional dipolar perturbation in sea level pressure (SLP), similar in pattern to the positive southern annular mode (SAM). Although SSTID-forced SLP anomalies over the Northern Hemisphere (NH) do not exhibit a zonally symmetric pattern as is the case over the SH, they still show signs of a meridional dipole opposite to the SH over the oceans. Divergent circulation responses to SSTID forcing between the two hemispheres are suggested to be associated with contrasting storm-track variations. Positive SSTID events weaken oceanic fronts in both the North Atlantic and North Pacific, and thus lead to the decline of NH storm-track activity by decreasing atmospheric baroclinicity. In the SH, positive SSTID events correspond to the enhancement of SH transients by intensifying the Antarctic polar-frontal zone. Additionally, local baroclinic energy conversions are diagnosed to explain the SSTID-related storm-track variations over both hemispheres. Finally, an investigation of transient eddy feedback indicates that the SSTID mode modulates extratropical atmospheric circulation, primarily by regulating storm tracks and changing the corresponding eddy feedback.

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Jilan Jiang, Yimin Liu, Jiangyu Mao, Jianping Li, Shuwen Zhao, and Yongqiang Yu

Abstract

The relationship between the Indian Ocean dipole (IOD) and the South Asian summer monsoon (SASM), which remains a subject of controversy, was investigated using data analyses and numerical experiments. We categorized IOD events according to their sea surface temperature anomaly (SSTA) pattern: type W and type E are associated with stronger SSTA amplitudes in the western and eastern poles of the IOD, respectively, while type C has comparable SSTA amplitudes in both poles during boreal autumn. Type W is associated with a weak SASM from May to summer, which contributes to substantial warming of the western pole in autumn; the east–west SST gradient linked to the warming of the western pole causes weak southeasterly wind anomalies off Sumatra and feeble and cold SSTAs in the eastern pole during the mature phase. Type E is associated with a strong SASM and feeble warming of the western pole; interaction between the strong SASM and cold SSTAs in the eastern pole in summer results in strong southeasterly wind anomalies off Sumatra and substantial cooling of the eastern pole during the mature phase. For type C, warming of the western pole and cooling of the eastern pole develop synchronously without apparent SASM anomalies and reach comparable intensities during the mature phase. Observations and numerical simulation results both indicate the role of disparate SASM anomalies in modulating SSTA patterns during the development of positive IODs. Warming of the tropical Indian Ocean becomes established in the winter and spring following type W and type C IODs but not following type E events.

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