Water Cycle Changes

This chapter assesses multiple lines of evidence to evaluate past, present and future changes in the global water cycle. It complements material in Chapters 2, 3 and 4 on observed and projected changes in the water cycle, and Chapters 10 and 11 on regional climate change and extreme events. The assessment includes the physical basis for water cycle changes, observed changes in the water cycle and attribution of their causes, future projections and related key uncertainties, and the potential for abrupt change. Paleoclimate evidence, observations, reanalyses and global and regional model simulations are considered. The assessment shows widespread, nonuniform human-caused alterations of the water cycle, which have been obscured by a competition between different drivers across the 20th century and that will be increasingly dominated by greenhouse gas forcing at the global scale

Portrayal of the Indian summer monsoon in the land-ocean-atmosphere system of a coupled GCM

A 150 year-long numerical simulation of present-day climate using the Max-Planck Institute's coupled ocean-atmosphere model ECHAM4-T42-OPYC3 is analysed with regard to the interannual variability of the strength of the Indian summer monsoon and its relation to land- surface and ocean interactions. Individual years are categorised into three classes of monsoons: normal, strong and weak (greater or less than one standard deviation of precipitation over India). The ensembles of anomalous monsoons are then sub-divided into a composite of cases coinciding with sea surface temperature (SST) anomalies in the Pacific related to the El Nifio- Southern Oscillation (ENSO) phenomenon and a second composite of anomalous monsoons occurring, when no SST anomalies are found in the Pacific. The coupled model shows variations of the SST in the Pacific which are as large as and occur at a similar frequency as in observations. Thus it provides the basis for a realistic simulation of the interannual monsoon variability in ENSO-related conditions, but it overemphasizes the biennial component of occurrence. As in observations, about a third of all cases of weak monsoons occur in the summer when an El Nico begins to develop in the Pacific, while strong monsoons are often associated with La NiNa events. This is an improvement from earlier coupled model simulations. In the model simulation, a modulation of the strength of the monsoon is due to a change of the large-scale land/ocean temperature gradient in the Indian Ocean sector in the mid- troposphere. The two composites show different developments during the annual cycle. In ENSO-related strong monsoon cases the atmosphere over land warms up during the spring in association with generally warmer tropics as a remnant from a warm event in the previous winter. During the summer months the warming in the Indian Ocean region is replaced by a cooling in association with the developing La NiNa, while the land remains significantly warmer than normal. Therefore, in the coupled simulation the Indian Ocean only shows very small SST anomalies, while observed SSTs may vary in connection with ENSO events at a time lag of four months. Also for non-ENSO related monsoons a warming occurs over land in the summer, but then neither the Indian Ocean nor the tropical west Pacific exhibit any significant anomalies during the spring. Simulated temperature anomalies responsible for these modulations are relatively small. Independent of the origin of the monsoon anomaly, strong monsoons differ from weak monsoons by a significant precipitation pattern over India and a modification of the 850 hPa zonal wind field over the maritime continent and the West Pacific. Similar to observations, the anomalous monsoons in the coupled model are related to precursors in the 200 hPa zonal wind field in the spring, but no evidence could be found for a significant influence from the Eurasian snow pack in the spring on the subsequent Indian summer monsoon. While the model shows many realistic features, the variation of the monsoon occurs against a background of a deficient regional rainfall pattern in India and too small a range of Indian Ocean SST variations in conjunction with ENSO events

Sustaining the Pearl River: A Critical Review of Changes in Fluvial Geomorphological Processes and the Driving Forces in the Pearl River Basin

The Pearl River is one of China’s large rivers, the second-largest river and the fourthlongest river in China. Its unique geography, landform, and climate conditions create unique fluvial geomorphological processes. Affected by human activities and climate change, the fluvial geomorphological processes in the Pearl River Basin have undergone significant changes in recent decades, seriously affecting the river’s sustainable development. This paper critically reviews changes in fluvial geomorphological processes and analyzes influencing factors in the Pearl River Basin with a focus on possibilities for policy overhaul and strategic adjustments

The global monsoon system: research and forecast

The main objective of this workshop was to provide a forum for discussion between researchers and forecasters on the current status of monsoon forecasting and on priorities and opportunities for monsoon research. WMO hopes that through this series of quadrennial workshops, the following goals can be accomplished: (a) to update forecasters on the latest reseach findings and forecasting technology; (b) to update researchers on monsoon analysis and forecasting; (c) to identify basic and applied research priorities and opportunities; (d) to identify opportunities and priorities for acquiring observations; (e) to discuss the approach of a web-based training document in order to update forecasters on developments of direct relevance to monsoon forecasting

A Reconstruction of Subtropical Western North Pacific SST Variability Back to 1578, Based on a Porites Coral Sr/Ca Record from the Northern Ryukyus, Japan

We present a seasonal reconstruction of sea surface temperature (SST) from 1578 to 2008, based on a Porites coral Sr/Ca record from the northern Ryukyus, within the Kuroshio southern recirculation gyre. Interannual SST anomalies are generally similar to 0.5 degrees C, making Sr/Ca-derived SST reconstructions a challenging task. Replicate measurements along adjacent coral growth axes, enabled by the laser ablation inductively coupled plasma mass spectrometry technique used here, give evidence of rather large uncertainties. Nonetheless, derived winter SST anomalies are significantly correlated with the Western Pacific atmospheric pattern which has a dominant influence on winter temperature in East Asia. Annual mean SSTs show interdecadal variations, notably cold intervals between 1670 and 1700 during the Maunder Minimum (MM) and between 1766 and 1788 characterized by a negative phase of the North Atlantic Oscillation. Cold summers in 1783 and 1784 coincide with the long-lasting Laki eruption that had a profound impact on the Northern Hemisphere climate, including the severe Tenmei famine in Japan. The decades between 1855 and 1900 are significantly cooler than the first half of the twentieth century, while those between 1700 and 1765, following the MM, are warmer than average. SST variability in the Ryukyus is only marginally influenced by the Pacific Decadal Oscillation, so that external forcing remains the main driver of low-frequency temperature changes. However, the close connection between the Kuroshio extension (KE) and its recirculation gyre suggests that decadal SST anomalies associated with the KE front also impact the Ryukyus, and there is a possible additional role for feedback of the Kuroshio-Oyashio variability to the large-scale atmosphere at decadal timescale.Part of this research was supported from a grant by JSPS KAKENHI (JP15KK0151 and JP17H01168) to Y. Y. Data used in this paper are provided as supporting information (Table S2)