Wang, F., S. Yang, W. Higgins, Q. Li, and Z. Zuo, 2013: “Long-term changes in total and extreme precipitation over China and the United States and their links to oceanic–atmospheric features.” International Journal of Climatology, doi: 10.1002/joc.3685.
We investigate the variations of total and extreme precipitations over China and the United States, focusing on long-term changes. We also explain the features of precipitation by changes in sea surface temperature (SST) and atmospheric circulation. Features of total precipitation and the ratio of extreme to total precipitation are different between China and the United States, and differences occur in both annual and seasonal means. Both total precipitation and precipitation ratio show large seasonal and regional variations over China, but change modestly over the United States. Annually, China total precipitation changes insignificantly and the ratio shows only a slight positive trend. However, the US annual total precipitation increases significantly, although the ratio decreases moderately. In China, the ratio exhibits positive trends in all seasons and total precipitation shows small positive trends except a negative trend in fall. The US total precipitation increases remarkably in all seasons except winter when a slight decrease occurs, and the ratio decreases in winter and summer but increases in spring and fall. The change in China precipitation ratio has a strong link to SSTs around the Indian Ocean and the South and East China Seas, and the change in US total precipitation is associated with changes in the Indian Ocean and eastern Pacific SSTs. These relationships become weaker when the trend of total precipitation or precipitation ratio is removed, indicating an impact of SST on the long-term change in precipitation. The trends of US total precipitation and China precipitation ratio are also linked to the long-term changes inatmospheric circulation including the trade wind, the North Pacific anticyclone, and the circulation patternsover Asia. In most cases, the total and extreme precipitations are associated with similar SST and atmospheric patterns, except in China where the annual extreme precipitation is associated with SST and circulation features as is the precipitation ratio.
Villarini, G., J.A. Smith, R. Vitolo, and D.B. Stephenson, 2013: “On the temporal clustering of US floods and its relationship to climate teleconnection patterns.” International Journal of Climatology, v. 33, pp. 629–640, doi: 10.1002/joc.3458.
This article examines whether the temporal clustering of flood events can be explained in terms of climate variability or time-varying land-surface state variables. The point process modelling framework for flood occurrence is based on Cox processes, which can be represented as Poisson processes with randomly varying rate of occurrence. In the special case that the rate of occurrence is deterministic, the Cox process simplifies to a Poisson process. Poisson processes represent flood occurrences which are not clustered. The Cox regression model is used to examine the dependence of the rate of occurrence on covariate processes. We focus on 41 stream gauge stations in Iowa, with discharge records covering the period 1950–2009. The climate covariates used in this study are the North Atlantic Oscillation (NAO) and the Pacific/North American Teleconnection (PNA). To examine the influence of land-surface forcing on flood occurrence, the antecedent 30 d rainfall accumulation is considered. In 27 out of 41 stations, either PNA or NAO, or both are selected as significant predictors, suggesting that flood occurrence in Iowa is influenced by large-scale climate indices. Antecedent rainfall, used as a proxy for soil moisture, plays an important role in driving the occurrence of flooding in Iowa. These results point to clustering as an important element of the flood occurrence process.
Wang, B., B. Xiang, and J.-Y. Lee, 2013: “Subtropical High predictability establishes a promising way for monsoon and tropical storm predictions.” Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.1214626110.
Monsoon rainfall and tropical storms (TSs) impose great impacts on society, yet their seasonal predictions are far from successful. The western Pacific Subtropical High (WPSH) is a prime circulation system affecting East Asian summer monsoon (EASM) and western North Pacific TS activities, but the sources of its variability and predictability have not been established. Here we show that the WPSH variation faithfully represents fluctuations of EASM strength (r = –0.92), the total TS days over the subtropical western North Pacific (r = –0.81), and the total number of TSs impacting East Asian coasts (r = –0.76) during 1979–2009. Our numerical experiment results establish that the WPSH variation is primarily controlled by central Pacific cooling/warming and a positive atmosphere-ocean feedback between the WPSH and the Indo-Pacific warm pool oceans. With a physically based empirical model and the state-of-the-art dynamical models, we demonstrate that the WPSH is highly predictable; this predictability creates a promising way for prediction of monsoon and TS. The predictions using the WPSH predictability not only yields substantially improved skills in prediction of the EASM rainfall, but also enables skillful prediction of the TS activities that the current dynamical models fail. Our findings reveal that positive WPSH–ocean interaction can provide a source of climate predictability and highlight the importance of subtropical dynamics in understanding monsoon and TS predictability.
Pal, I., U. Lall, A.W. Robertson, M.A. Cane, and R. Bansal, 2013: “Diagnostics of Western Himalayan Satluj River flow: Warm season (MAM/JJAS) inflow into Bhakra dam in India.” Journal of Hydrology, v. 478, pp. 132-147, 10.1016/j.jhydrol.2012.11.053.
Here we analyze the variability of MAM (March–April–May) and JJAS (June–July–August–September) seasonal Satluj River flow into the Bhakra dam in India through Pearson anomaly correlation and composite analyses with antecedent and concurrent seasonal climatic and atmospheric circulation patterns. The MAM seasonal inflow of Bhakra dam is significantly correlated with winter (DJF/FM) precipitation and temperature of the Satluj basin while the correlation with FM was more prominent for precipitation (snow = +0.72, rainfall = +0.60), and temperature (diurnal temperature range (DTR) = −0.76 and maximum temperature (Tmax) = −0.57). The JJAS inflow was also positively correlated with DJF/FM as well as JJAS precipitation of the Satluj basin while the correlation with basin average FM was the largest (+0.54). These suggested that both MAM and JJAS inflow anomalies are linked with DJF/FM climate over the Western Himalayas and adjoining north and central Indian plains, which were also found to be linked with the fluctuation of equatorial concurrent Sea Surface Temperature anomalies over the western Indian Ocean (max anomaly correlation was > +0.70) and mean sea level pressure over western pole of the Southern Oscillation sea-saw region (max Pearson anomaly correlation was ∼ +0.60). Low (high) MAM inflow was found to be associated with negative (positive) precipitation anomalies over the basin and north India in DJF and FM while FM precipitation anomaly is more concentrated over the Western Himalayas. In addition, low (high) JJAS inflow is also associated with negative (positive) precipitation anomalies over the basin and north India in DJF and over the Western Himalaya in FM and JJAS. Negative geopotential height anomaly at 500 hPa (Z500) over Siberia and northwestern pacific in DJF, and positive Z500 anomaly over the northwest India in FM were noticed in low MAM inflow years. Whereas high inflow in MAM was linked with a negative Z500 anomaly between two positive Z500 anomaly regions – one over eastern Siberia stretched up to northern Pacific and second over the Eastern Europe in DJF, which gets stronger in FM. We also found southwesterly (northeasterly) wind vectors at 850 hPa pressure level (uv850) bringing more (less) moisture to the Western Himalayas in DJF and FM in high (low) MAM/JJAS flow years.
Tierney, J.E., J.E. Smerdon, K.J. Anchukaitis, and R. Seager, 2013: “Multidecadal variability in East African hydroclimate controlled by the Indian Ocean.” Nature, v. 493, pp. 389–392, doi: 10.1038/nature11785.
The recent decades-long decline in East African rainfall suggests that multidecadal variability is an important component of the climate of this vulnerable region. Prior work based on analysing the instrumental record implicates both Indian and Pacific ocean sea surface temperatures (SSTs) as possible drivers of East African multidecadal climate variability, but the short length of the instrumental record precludes a full elucidation of the underlying physical mechanisms. Here we show that on timescales beyond the decadal, the Indian Ocean drives East African rainfall variability by altering the local Walker circulation, whereas the influence of the Pacific Ocean is minimal. Our results, based on proxy indicators of relative moisture balance for the past millennium paired with long control simulations from coupled climate models, reveal that moist conditions in coastal East Africa are associated with cool SSTs (and related descending circulation) in the eastern Indian Ocean and ascending circulation over East Africa. The most prominent event identified in the proxy record—a coastal pluvial from 1680 to 1765—occurred when Indo-Pacific warm pool SSTs reached their minimum values of the past millennium. Taken together, the proxy and model evidence suggests that Indian Ocean SSTs are the primary influence on East African rainfall over multidecadal and perhaps longer timescales.
Ham, Y.-G., J.-S. Kug, J.-Y. Park, and F.-F. Jin, 2013: “Sea surface temperature in the north tropical Atlantic as a trigger for El Niño/Southern Oscillation events.” Nature Geoscience, doi: 10.1038/ngeo1686.
El Niño events, the warm phase of the El Niño/Southern Oscillation (ENSO), are known to affect other tropical ocean basins through teleconnections. Conversely, mounting evidence suggests that temperature variability in the Atlantic Ocean may also influence ENSO variability. Here we use reanalysis data and general circulation models to show that sea surface temperature anomalies in the north tropical Atlantic during the boreal spring can serve as a trigger for ENSO events. We identify a subtropical teleconnection in which spring warming in the north tropical Atlantic can induce a low-level cyclonic atmospheric flow over the eastern Pacific Ocean that in turn produces a low-level anticyclonic flow over the western Pacific during the following months. This flow generates easterly winds over the western equatorial Pacific that cool the equatorial Pacific and may trigger a La Niña event the following winter. In addition, El Niño events led by cold anomalies in the north tropical Atlantic tend to be warm-pool El Niño events, with a centre of action located in the central Pacific, rather than canonical El Niño events. We suggest that the identification of temperature anomalies in the north tropical Atlantic could help to forecast the development of different types of El Niño event.
Phillips, T., R.S. Nerem, B. Fox-Kemper, J.S. Famiglietti, and B. Rajagopalan, 2012: “The influence of ENSO on global terrestrial water storage using GRACE.” Geophysical Research Letters, v. 39, paper no. L16705, doi: 10.1029/2012GL052495.
The influence of the El Nino/Southern Oscillation (ENSO) on terrestrial water storage is analyzed for the time period 2003–2010 using monthly estimates of continental water storage from the Gravity Recovery and Climate Experiment (GRACE). Peak correlation between NOAA’s Multivariate ENSO Index (MEI) and the measured mass anomaly timeseries shows an R2 of 0.65 for the Amazon Basin and Borneo in Southeast Asia. By including a Hilbert transformation of the MEI to account for time lag, the R2 is improved to 0.76. Tropical regions show strong negative correlation with the MEI and arid regions are positively correlated. GRACE is able to detect all the significant known ENSO teleconnection patterns around the globe, including Alaska and Antarctica. In addition, a significant correlation suggests some of Greenland’s recent mass loss could be ENSO-related.
Lavers, D.A., G. Villarini, R.P. Allan, E.F. Wood, and A.J. Wade, 2012: “The detection of atmospheric rivers in atmospheric reanalyses and their links to British winter floods and the large-scale climatic circulation.” Journal of Geophysical Research, v. 117, paper no. D20106, doi: 10.1029/2012JD018027.
Atmospheric Rivers (ARs), narrow plumes of enhanced moisture transport in the lower troposphere, are a key synoptic feature behind winter flooding in midlatitude regions. This article develops an algorithm which uses the spatial and temporal extent of the vertically integrated horizontal water vapor transport for the detection of persistent ARs (lasting 18 h or longer) in five atmospheric reanalysis products. Applying the algorithm to the different reanalyses in the vicinity of Great Britain during the winter half-years of 1980–2010 (31 years) demonstrates generally good agreement of AR occurrence between the products. The relationship between persistent AR occurrences and winter floods is demonstrated using winter peaks-over-threshold (POT) floods (with on average one flood peak per winter). In the nine study basins, the number of winter POT-1 floods associated with persistent ARs ranged from approximately 40 to 80%. A Poisson regression model was used to describe the relationship between the number of ARs in the winter half-years and the large-scale climate variability. A significant negative dependence was found between AR totals and the Scandinavian Pattern (SCP), with a greater frequency of ARs associated with lower SCP values.
Brienen, R.J.W., G. Hellec, T.L. Ponsd, J.-L. Guyote, and M. Gloor, 2012: “Oxygen isotopes in tree rings are a good proxy for Amazon precipitation and El Niño-Southern Oscillation variability.” Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.1205977109.
We present a unique proxy for the reconstruction of variation in precipitation over the Amazon: oxygen isotope ratios in annual rings in tropical cedar (Cedrela odorata). A century-long record from northern Bolivia shows that tree rings preserve the signal of oxygen isotopes in precipitation during the wet season, with weaker influences of temperature and vapor pressure. Tree ring δ18O correlates strongly with δ18O in precipitation from distant stations in the center and west of the basin, and with Andean ice core δ18O showing that the signal is coherent over large areas. The signal correlates most strongly with basin-wide precipitation and Amazon river discharge. We attribute the strength of this (negative) correlation mainly to the cumulative rainout processes of oxygen isotopes (Rayleigh distillation) in air parcels during westward transport across the basin. We further find a clear signature of the El Niño-Southern Oscillation (ENSO) in the record, with strong ENSO influences over recent decades, but weaker influence from 1925 to 1975 indicating decadal scale variation in the controls on the hydrological cycle. The record exhibits a significant increase in δ18O over the 20th century consistent with increases in Andean δ18O ice core and lake records, which we tentatively attribute to increased water vapor transport into the basin. Taking these data together, our record reveals a fresh path to diagnose and improve our understanding of variation and trends of the hydrological cycle of the world’s largest river catchment.
Si, D., and Y. Ding, 2012: “The tropospheric biennial oscillation in the East Asian monsoon region and its influence on the precipitation in China and large-scale atmospheric circulation in East Asia.” International Journal of Climatology, v. 32, pp. 1697–1716, doi: 10.1002/joc.2386.
The precipitation in China manifests a remarkable quasi-biennial signal. For interannual variability, about 70% stations over China indicate the dominance of a quasi-2-year period. The maximum precipitation variability associated with the quasi-biennial oscillation is located over the Yangtze River valley (YRV) and Huaihe River valley (HRV) as well as South China. This paper attempts to reveal the spatial–temporal evolution of the precipitation in China and related large-scale atmospheric circulation associated with the tropospheric biennial oscillation (TBO) in East Asia by using a season-dependent empirical orthogonal function (S-EOF) analysis approach.
The leading two modes of the TBO of summer precipitation in China and associated large-scale circulations are examined by the S-EOF analysis and regression analyses based on the S-EOF time coefficients. The first TBO mode is characterized by an elongated band of positive precipitation anomalies along the YRV and negative precipitation anomalies over both North and South China. The second TBO mode is characterized by an elongated band of positive precipitation anomalies along the HRV and negative precipitation anomalies to the north of the Yellow River and to the south of the Yangtze River, respectively. Meanwhile, the leading modes of the TBO in East Asia may be determined by the meridional teleconnection Rossby wave pattern extending from the WNP to the midlatitudes of East Asia that is forced by the heating source fluctuation over the WNP during boreal summer. And the cold air activity, which is associated with the East Asian winter monsoon, may further influence the leading modes of the TBO through a modulation of the large-scale atmosphere circulation over East Asia during the following boreal summer.
Also found is that the TBO in East Aisa depends on both the large-scale air–sea coupling over the tropical Indo-Pacific Ocean regions and the tropical–midlatitude interaction in the western North Pacific (WNP)-East Asia region. A fundamental element for the TBO in East Asia is the WNP monsoon. It is not only an important component of the TBO cycle in the tropics but also serves as a major source of the TBO signal for the subtropical East Asia. The meridional teleconnection Rossby wave train over the WNP-East Asia region acts as a conveyer belt that transports the tropical TBO signal to the midlatitudes of East Asia, and produces the TBO footprints in the large-scale circulation and precipitation in East Asia. Furthermore, the cold air activity over the East Asia during boreal summer also services as an important link in the chain events of the tropical–midlatitude interaction, with enhancing the role of the TBO modes in East Asia.