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R. J. Curran and M-L. C. Wu

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Benjamin M. Herman, Samuel R. Browning, and Robert J. Curran

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Theoretical computations of the intensity and polarization of diffusively transmitted sunlight are presented for two wavelengths, λ = 4290 Å and λ = 5000 Å. The computations are for atmospheres containing various distributions of aerosols, as well as normal molecular constituents, and allow for all significant orders of scattering. The theoretical computations are compared with observations, and it is shown that inclusion of aerosols in the theoretical models results in considerably better agreement between observation and theory than can be achieved by assuming a pure molecular atmosphere for the theoretical computations.

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C. Prabhakara, R. S. Fraser, G. Dalu, Man-Li C. Wu, R. J. Curran, and T. Styles

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Spectral differences in the extinction between the 10.8 and 12.6 μm bands of the infrared window region, due to optically thin clouds, are observed in the measurements made by a broad-band infrared aircraft radiometer. Similar spectral properties are also revealed by the measurements made by the high-resolution infrared inter-ferometer spectrometer (IRIS) aboard the Nimbus-4 satellite, which had a field of view of ∼ 95 km. These observations show that the extinction due to cloud particles at 12.6 μm is appreciably larger than that at 10.8 μm. Both water or ice particles in the clouds can account for such spectral difference in extinction provided that the particles are smaller than the wavelength of radiation. This spectral effect is demonstrated with the help of multiple scattering radiative transfer calculations. As the IRIS data reveal this spectral feature, about 100 to 200 km away from the center of high altitude cold clouds (∼ 230 K), it is inferred that this feature is related to the spreading of cirrus clouds. Based on this hypothesis, we have deduced mean seasonal maps of the distribution of thin cirrus clouds over the oceans from 50°N to 50°S from the IRIS data. These maps reveal that, over the oceans, such clouds are often present over the convectively active areas such as ITCZ, SPCZ, and the Bay of Bengal. These results have application to studies of earth radiation balance and climate.

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Danielle G. Udy, Tessa R. Vance, Anthony S. Kiem, Neil J. Holbrook, and Mark A. J. Curran

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Weather systems in the southern Indian Ocean (SIO) drive synoptic-scale precipitation variability in East Antarctica and southern Australia. Improved understanding of these dynamical linkages is beneficial to diagnose long-term climate changes from climate proxy records as well as informing regional weather and climate forecasts. Self-organizing maps (SOMs) are used to group daily 500-hPa geopotential height (z500; ERA-Interim) anomalies into nine regional synoptic types based on their dominant patterns over the SIO (30°–75°S, 40°–180°E) from January 1979 to October 2018. The pattern anomalies represented include four meridional, three mixed meridional–zonal, one zonal, and one transitional node. The frequency of the meridional nodes shows limited association with the phase of the southern annular mode (SAM), especially during September–November. The zonal and mixed patterns were nevertheless strongly and significantly correlated with SAM, although the regional synoptic representation of SAM+ conditions was not zonally symmetric and was represented by three separate nodes. We recommend consideration of how different synoptic conditions vary the atmospheric representation of SAM+ in any given season in the SIO. These different types of SAM+ mean a hemispheric index fails to capture the regional variability in surface weather conditions that is primarily driven by the synoptic variability rather than the absolute polarity of the SAM.

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Tessa R. Vance, Tas D. van Ommen, Mark A. J. Curran, Chris T. Plummer, and Andrew D. Moy

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ENSO causes climate extremes across and beyond the Pacific basin; however, evidence of ENSO at high southern latitudes is generally restricted to the South Pacific and West Antarctica. Here, the authors report a statistically significant link between ENSO and sea salt deposition during summer from the Law Dome (LD) ice core in East Antarctica. ENSO-related atmospheric anomalies from the central-western equatorial Pacific (CWEP) propagate to the South Pacific and the circumpolar high latitudes. These anomalies modulate high-latitude zonal winds, with El Niño (La Niña) conditions causing reduced (enhanced) zonal wind speeds and subsequent reduced (enhanced) summer sea salt deposition at LD. Over the last 1010 yr, the LD summer sea salt (LDSSS) record has exhibited two below-average (El Niño–like) epochs, 1000–1260 ad and 1920–2009 ad, and a longer above-average (La Niña–like) epoch from 1260 to 1860 ad. Spectral analysis shows the below-average epochs are associated with enhanced ENSO-like variability around 2–5 yr, while the above-average epoch is associated more with variability around 6–7 yr. The LDSSS record is also significantly correlated with annual rainfall in eastern mainland Australia. While the correlation displays decadal-scale variability similar to changes in the interdecadal Pacific oscillation (IPO), the LDSSS record suggests rainfall in the modern instrumental era (1910–2009 ad) is below the long-term average. In addition, recent rainfall declines in some regions of eastern and southeastern Australia appear to be mirrored by a downward trend in the LDSSS record, suggesting current rainfall regimes are unusual though not unknown over the last millennium.

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