Role of solar activity and Pacific decadal oscillation in the hydroclimatic patterns of eastern China over the past millennium

Highlights

• Hydroclimatic patterns in eastern China (EC) are related to solar activity and PDO.

• A consistent pattern in EC for strong solar period (SS) and cold PDO initial phase.

• The tripole-like patterns for SS and the neutral or warm PDO initial phase.

• A tripole-like pattern for weak solar period (WS) and neutral PDO initial phase.

• The dipole-like patterns for WS and the cold or warm PDO initial phase.

Abstract

The hydroclimatic patterns in eastern China (EC) over the past millennium, both influenced by solar activity and Pacific decadal oscillation (PDO), are presented based on the climatic reconstructions and simulations. For strong solar activity, a consistent pattern exists where drought appears in the majority of EC initialized from the cold PDO phase, which is related to less moisture caused by the negative East Asia/Pacific-like (EAP-like) pattern and the enhanced western Pacific subtropical high (WPSH). When the initial condition with a neutral or warm PDO phase is set up, there is a tripole-like pattern from north to south, i.e., drought–flood–drought for the neutral PDO phase and flood–drought–flood for the warm PDO phase, which is related to the positive EAP-like pattern with various moisture transportation paths. For weak solar activity, there are dipole-like patterns where drought occurs in the northern part of the Yangtze River and flood appears in the southern part initialized from the cold or warm PDO phase, which is possibly related to the influence of the weakened WPSH. When the initial condition is the neutral PDO phase, there is a tripole-like pattern from north to south (flood–drought–flood) in EC affected by the weakened WPSH and the intensive India–Burma trough. Notably, the vertical velocity anomalies of airflow, representing the vertical moisture transportations, correspond well with the hydroclimatic patterns in EC whenever the various initialization strategies of PDO phases are employed.

Introduction

Multiscale variations and differences in regional drought/flood patterns have drawn increasing attention in recent years, since they help explain the spatial–temporal characteristics of global monsoons and hydroclimate under climate change (Trenberth et al., 2014; Freychet et al., 2015; Kamizawa and Takahashi, 2018; Zhang and Zhou, 2019). The Sixth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR6) has revealed that climate change may be closely related to anomalous hydroclimatic patterns in many parts of the world, profoundly influencing the global climate extremes and water resources (IPCC, 2021). Hence, important scientific questions have been raised concerning the key factors that affect global hydroclimatic patterns. On long-term time scales, solar activity determines the balance of the energy budget in the atmosphere and is a major natural external forcing factor influencing the global dry–wet status (Kiehl and Trenberth, 1997; Gray et al., 2010; Weng, 2012; Zhang et al., 2020).

Solar activity has important influences on hydroclimatic pattern variations through the modulation of large-scale tropospheric circulation systems throughout mid–low latitudes (Gleisner and Thejll, 2003); it causes anomalous variations in convective activity, monsoons, and other climate systems in regions under the influence of the Pacific and Indian oceans (Xu and Yang, 1993; Kodera, 2004; Gupta et al., 2005; van Loon et al., 2007; Camp and Tung, 2007). To inform collective efforts to mitigate the global effects of climate extremes and water resource shortages under future climate change scenarios, an improved understanding of hydroclimatic responses to solar activity is needed (McGowan et al., 2010; Nichols and Huang, 2012; Czymzik et al., 2016).

Located in the East Asian monsoon region, eastern China (EC) is an ideal area for investigating the response of hydroclimatic patterns to solar activity (Peng et al., 2009; Li et al., 2011; Jiang et al., 2016; Freychet et al., 2017). For example, variations in the latitude of the rain belt in central–eastern China during the onset of the East Asian summer monsoon (EASM) depend on the periodic phases and number of sunspots (Zhao et al., 2012; Zhao and Wang, 2014). Additionally, solar activity has remarkable effects on the drought/flood patterns in EC; it causes high-frequency variations in the EASM and influences the intensity of the Asian monsoon, particularly on the decadal–centennial scale (Wu et al., 2006; Tan et al., 2007). However, the specific contributions of solar activity to hydroclimatic patterns are still partly obscure, due to the complexity of the climate system (Haarsma et al., 2000; Bauer et al., 2003). For instance, although tripole-like patterns of the ensemble mean have been observed in relation to flood in the middle and lower reaches of the Yangtze River, and to droughts in southern China (SC) and northern China (NC), the hydroclimatic patterns observed during different periods of solar maxima are different from each other (Ge et al., 2016).

Modern climatologists have focused on the pronounced effects of internal variabilities (e.g., the El Niño–Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), the Atlantic Multidecadal Oscillation (AMO), and the Pacific Decadal Oscillation (PDO), and other factors) on the climate system (Dong and Zhou, 2014; van Loon and Meehl, 2014; Dai et al., 2015; Roy and Kripalani, 2019; Dong et al., 2021). Of these, the PDO plays an indispensable role in modulating the global atmospheric circulation, especially on the decadal–centennial scale (Mochizuki et al., 2010; Chylek et al., 2014; McAfee, 2014; Zhang and Delworth, 2016; Geng et al., 2019). Numerous studies have demonstrated that the PDO has deeply influenced the variations in drought/flood patterns in China over the last 50 years, especially in EC (Deng et al., 2009; Zhu et al., 2011, Zhu et al., 2015). In addition to its direct interaction with climate, the PDO is closely associated with solar activity and affects the regional climatic response to radiation (Chiacchio et al., 2010; Maruyama et al., 2017). It has been suggested that solar forcing, fluctuating on a quasi-centennial time scale, acted as a pace-maker of the PDO before 1850 (Shen et al., 2006).

Previous studies have mainly emphasized the dominant role of external forcings or internal variabilities in modulating the hydroclimate in EC (Peng et al., 2010; Wang et al., 2013; Peng et al., 2015; Ge et al., 2016); meanwhile, few works have focused on both of their effects and the underlying mechanisms. Ning et al. (2020) found that decadal megadroughts in EC are associated with volcanic eruptions and sea surface temperature anomalies (SSTA) in the western Pacific, suggesting the possibility of both effects of internal variabilities and external forcings on the regional hydroclimate. In this paper, we study the mechanism linking solar activity and the PDO to hydroclimatic patterns in EC based on the climatic reconstructions and simulations. Our findings can be used as a basis for future assessments of climate extremes and water resources in EC and Asia in general.

The remainder of this paper is organized as follows. The datasets, methods, and experimental design used in this study are described in Section 2. The reconstructions and simulations of hydroclimatic patterns in EC, accompanied by anomalous solar activity and various PDO phases, are introduced in Section 3. The circulation responses to the effects of solar activity and PDO phases, as well as the possible mechanism, are revealed in Section 4. The responses of the SSTA patterns over the North Pacific (NP) to initial PDO phases and solar activity are discussed in Section 5. Finally, conclusions are provided in Section 6.