A very likely weakening of Pacific Walker Circulation in constrained near-future projections

The observational records have shown a strengthening of the Pacific Walker circulation (PWC) since 1979. However, whether the observed change is forced by external forcing or internal variability remains inconclusive, a solid answer to more societal relevantly question of how the PWC will change in the near future is still a challenge. Here we perform a quantitative estimation on the contributions of external forcing and internal variability to the recent observed PWC strengthening using large ensemble simulations from six state-of-the-art Earth system models. We find the phase transition of the Interdecadal Pacific Oscillation (IPO), which is an internal variability mode related to the Pacific, accounts for approximately 63% (~51–72%) of the observed PWC strengthening. Models with sufficient ensemble members can reasonably capture the observed PWC and IPO changes. We further constrain the projection of PWC change by using climate models’ credit in reproducing the historical phase of IPO. The result shows a high probability of a weakened PWC in the near future.

Uncertainty in El Niño-like warming and California precipitation changes linked by the Interdecadal Pacific Oscillation

Marked uncertainty in California (CA) precipitation projections challenges their use in adaptation planning in the region already experiencing severe water stress. Under global warming, a westerly jet extension in the North Pacific analogous to the El Niño-like teleconnection has been suggested as a key mechanism for CA winter precipitation changes. However, this teleconnection has not been reconciled with the well-known El Niño-like warming response or the controversial role of internal variability in the precipitation uncertainty. Here we find that internal variability contributes > 70% and > 50% of uncertainty in the CA precipitation changes and the El Niño-like warming, respectively, based on analysis of 318 climate simulations from several multi-model and large ensembles. The Interdecadal Pacific Oscillation plays a key role in each contribution and in connecting the two via the westerly jet extension. This unifying understanding of the role of internal variability in CA precipitation provides critical guidance for reducing and communicating uncertainty to inform adaptation planning.

Reconstructing an Interdecadal Pacific Oscillation Index from a Pacific Basin–Wide Collection of Ice Core Records

Using an assemblage of four ice cores collected around the Pacific basin, one of the first basinwide histories of Pacific climate variability has been created. This ice core–derived index of the interdecadal Pacific oscillation (IPO) incorporates ice core records from South America, the Himalayas, the Antarctic Peninsula, and northwestern North America. The reconstructed IPO is annually resolved and dates to 1450 CE. The IPO index compares well with observations during the instrumental period and with paleo-proxy assimilated datasets throughout the entire record, which indicates a robust and temporally stationary IPO signal for the last ~550 years. Paleoclimate reconstructions from the tropical Pacific region vary greatly during the Little Ice Age (LIA), although the reconstructed IPO index in this study suggests that the LIA was primarily defined by a weak, negative IPO phase and hence more La Niña–like conditions. Although the mean state of the tropical Pacific Ocean during the LIA remains uncertain, the reconstructed IPO reveals some interesting dynamical relationships with the intertropical convergence zone (ITCZ). In the current warm period, a positive (negative) IPO coincides with an expansion (contraction) of the seasonal latitudinal range of the ITCZ. This relationship is not stationary, however, and is virtually absent throughout the LIA, suggesting that external forcing, such as that from volcanoes and/or reduced solar irradiance, could be driving either the ITCZ shifts or the climate dominating the ice core sites used in the IPO reconstruction.

Change in the occurrence frequency of landfalling and non-landfalling tropical cyclones over the Northwest Pacific

Understanding the tropical cyclone (TC) activity changes in response to climate change is of great importance for disaster mitigation and climate change adaptation. Change in the annual occurrence frequency of landfalling and non-landfalling weak, strong, and super TCs during 1980–2018 was analyzed. Results indicate that the super TCs are more likely to make landfall in the Northwest Pacific since 1980. With an empirical orthogonal function-based method was proposed to decompose the space-time field of TC occurrence into different patterns, the anthropogenic influence on the change in super TC occurrence was detected when the impacts of El Niño-Southern Oscillation (ENSO), Pacific Meridional Mode (PMM), and Interdecadal Pacific Oscillation (IPO) were separated. Results further show that TCs forms in the sea surface near land (130°–137°E, 6°–21°N) are more likely to intensify to super TCs in recent years. These intensified TCs tend to favor subsequent landfall, which may be the reason for the increase in landfalling super TCs. The intensification of TC is mainly due to the increase in the intensification rate, which increases with increased sea surface temperature (SST), especially during the stronger wind periods. Along with the change in the occurrence of landfalling super TCs, the landfalling locations of super TCs also changed. For example, western South China, Southeast China, and Japan are facing an increase in landfalling super TCs. The destructiveness of super TCs to these economically developed and highly populated regions is great, more attention therefore should be paid to mitigate TC disasters.

Decadal Modulation of the ENSO-Indian Ocean Basin Warming relationship during the decaying summer by the Interdecadal Pacific Oscillation

Many previous studies have shown that an Indian Ocean basin warming (IOBW) occurs usually during El Niño-Southern Oscillation (ENSO) decaying spring to summer seasons through modifying the equatorial zonal circulation. Decadal modulation associated with the Interdecadal Pacific Oscillation (IPO) is further investigated here to understand the nonstationary ENSO-IOBW relationship during ENSO decaying summer (July-August-September, JAS). During the positive IPO phase, significant warm sea surface temperature (SST) anomalies are observed over the tropical Indian Ocean in El Niño decaying summers and vice versa for La Niña events, while these patterns are not well detected in the negative IPO phase. Different decaying speeds of ENSO associated with the IPO phase, largely controlled by both zonal advective and thermocline feedbacks, are suggested to be mainly responsible for these different ENSO-IOBW relationships. In contrast to ENSO events in the negative IPO phase, the ones in the positive IPO phase display a slower decaying speed and delay their transitions both from a warm to a cold state and a cold to a warm state. The slower decay of El Niño and La Niña thereby helps to sustain the teleconnection forcing over the equatorial Indian Ocean and corresponding SST anomalies there can persist into summer. This IPO modulation of the ENSO-IOBW relationship carries important implications for the seasonal prediction of the Indian Ocean SST anomalies and associated summer climate anomalies.