Abstract: The Pacific Decadal Oscillation (PDO) and North Pacific Gyre Oscillation (NPGO) are generally referred to as the first two EOF modes of Sea Surface Temperature (SST) in the North Pacific (20°-60°N, 120°E-120°W). In the present study, the authors compared the first two EOF modes of winter SST anomalies in North Pacific for the period before and after 1990. Based on the impacts of key regional SST changes, the North Pacific Oscillation (NPO), Central Pacific Warming (CPW), as well as Artic Oscillation (AO), the authors discuss the possible causes of the first mode of the NPGO after 1990. Our results suggest that before 1990, the first two EOF modes of SST exhibit the PDO and NPGO pattern, respectively. However, after 1990 the center of the maximal load of the first EOF tilts to the dateline, and gives rises to positive SST anomalies north of 40°N, resulting in a negative phase of NPGO; meanwhile, the previous NPGO changes from a dipole pattern to the triple mode. Analyses suggest that the negative correlation between the changes of SST in the northern (44°-49°N, 151°-177°W) and central (28°-36°N, 152°-178°W) North Pacific could have resulted in the internal shift of the dominant SST modes after 1990. Evidence shows that the NPO played an important external forcing in enhancing the NPGO after 1990. Based on wind-driven oceanic current theory, the increased amplitude of NPO in Sea Level Pressure (SLP) may intensify the polar gradient of the SLP along 45°N, enhancing the zonal sea surface winds, and causing the NPGO mode to prevail after 1990. The increase in CPW and AO may have enhanced the NPO by intensifying the southern and northern branch of the NPO in Hawaii and Alaska after 1990, but there is no evidence to show that both these factors were closely related to the NPO before 1990.