Using the reanalysis data and the numerical experiments of a coupled general circulation model (CGCM), we illustrated that perturbations in the second dominant mode (EOF2) of springtime North Pacific sea surface temperature (SST) variability, referred to as the Victoria mode (VM), are closely linked to variations in the intensity of the South China Sea summer monsoon (SCSSM). The underlying physical mechanism through which the VM affects the SCSSM is similar to the seasonal footprinting mechanism (SFM). Thermodynamic ocean-atmosphere coupling helps the springtime SST anomalies in the subtropics associated with the VM to persist into summer and to develop gradually toward the equator, leading to a weakened zonal SST gradient across the western North Pacific (WNP) to central equatorial Pacific, which in turn induces an anomalous cyclonic flow over the WNP and westerly anomalies in the western equatorial Pacific that tend to strengthen the WNP summer monsoon (WNPSM) as well as the SCSSM. The VM influence on both the WNPSM and SCSSM is intimately tied to its influence on ENSO through westerly anomalies in the western equatorial Pacific.Atmosphere 2018, 9, 229 2 of 15 due to the El Niño warming can lead to a strong SCSSM. In contrast, during the summer following El Niño, an anomalous WNP anticyclone, which develops rapidly during the El Niño developing fall and persists until the following summer possibly because of a local positive thermodynamic feedback [15], Indian Ocean capacitor effect [16], or nonlinear interactions between ENSO and the Western Pacific warm pool annual cycle [17], can affect the SCS's climate and hence SCSSM. In addition to ENSO, other factors have also been shown to play a role in the year-to-year variations in the SCSSM intensity or onset, including the Indian Ocean sea surface temperature (SST) forcing [18][19][20][21], local air-sea interaction [22,23], and the extratropical-tropical interaction [24,25].Recent studies reported that the spring Victoria mode (VM), defined as the second empirical orthogonal function mode (EOF2) of North Pacific SST anomalies poleward of 20 • N [26], has a significant impact on the Pacific Intertropical Convergence Zone (ITCZ) precipitation during the following summer [27] and ENSO during the following winter [28]. Given that the developing ENSO exerts a forcing effect on the SCSSM, it is very likely that the spring VM could also have an impact on the SCSSM. However, the influences of the VM on the SCSSM have not been documented in the literature. The focus of this paper is to establish the delayed influence of the VM on the SCSSM. Specific questions of interest: Is there a close connection between the spring VM and the subsequent SCSSM? If so, how does the VM affect the SCSSM? Is there any relationship between influences of the VM on the SCSSM and ENSO?
Data, Model, and Indices
Observed DatasetsWe used atmospheric fields from the National Centers for Environmental Prediction-Department of Energy (NCEP-DOE) reanalysis version 2 (NCEP2) [29]. We used SST from t...