The South Pacific convergence zone (SPCZ) is the Southern Hemisphere's most expansive and persistent rain band, extending from the equatorial western Pacific Ocean southeastward towards French Polynesia. Owing to its strong rainfall gradient, a small displacement in the position of the SPCZ causes drastic changes to hydroclimatic conditions and the frequency of extreme weather events--such as droughts, floods and tropical cyclones--experienced by vulnerable island countries in the region. The SPCZ position varies from its climatological mean location with the El Niño/Southern Oscillation (ENSO), moving a few degrees northward during moderate El Niño events and southward during La Niña events. During strong El Niño events, however, the SPCZ undergoes an extreme swing--by up to ten degrees of latitude toward the Equator--and collapses to a more zonally oriented structure with commensurately severe weather impacts. Understanding changes in the characteristics of the SPCZ in a changing climate is therefore of broad scientific and socioeconomic interest. Here we present climate modelling evidence for a near doubling in the occurrences of zonal SPCZ events between the periods 1891-1990 and 1991-2090 in response to greenhouse warming, even in the absence of a consensus on how ENSO will change. We estimate the increase in zonal SPCZ events from an aggregation of the climate models in the Coupled Model Intercomparison Project phases 3 and 5 (CMIP3 and CMIP5) multi-model database that are able to simulate such events. The change is caused by a projected enhanced equatorial warming in the Pacific and may lead to more frequent occurrences of extreme events across the Pacific island nations most affected by zonal SPCZ events.

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The Southwest Pacific Ocean Circulation and Climate Experiment (SPICE) is an international research program under the auspices of CLIVAR. The key objectives are to understand the Southwest Pacific Ocean circulation and the South Pacific Convergence Zone (SPCZ) dynamics, as well as their influence on regional and basin-scale climate patterns. South Pacific thermocline waters are transported in the westward flowing South Equatorial Current (SEC) toward Australia and Papua-New Guinea. On its way, the SEC encounters the numerous islands and straits of the Southwest Pacific and forms boundary currents and jets that eventually redistribute water to the equator and high latitudes. The transit in the Coral, Solomon, and Tasman Seas is of great importance to the climate system because changes in either the temperature or the amount of water arriving at the equator have the capability to modulate the El Niño-Southern Oscillation, while the southward transports influence the climate and biodiversity in the Tasman Sea. After 7 years of substantial in situ oceanic observational and modeling efforts, our understanding of the region has much improved. We have a refined description of the SPCZ behavior, boundary currents, pathways, and water mass transformation, including the previously undocumented Solomon Sea. The transports are large and vary substantially in a counter-intuitive way, with asymmetries and gating effects that depend on time scales. This paper provides a review of recent advancements and discusses our current knowledge gaps and important emerging research directions.

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Coastal locations around the United States (US), particularly along the Atlantic coast, are experiencing recurrent flooding at high tide. Continued sea-level rise (SLR) will exacerbate the issue where present, and many more locations will begin to experience recurrent high-tide flooding (HTF) in coming decades. Here we use established SLR scenarios and flooding thresholds to demonstrate how the combined effects of SLR and nodal cycle modulations of tidal amplitude lead to acute inflections in projections of future HTF. The mid-2030s, in particular, may see the onset of rapid increases in the frequency of HTF in multiple US coastal regions. We also show how annual cycles and sea-level anomalies lead to extreme seasons or months during which many days of HTF cluster together. Clustering can lead to critical frequencies of HTF occurring during monthly or seasonal periods 1-2 decades prior to being expected on an annual basis.The impact of HTF accumulates over numerous seemingly minor occurrences, which can exceed the impact of rare extremes over time [1][2][3] . These impacts are subtle-e.g., loss of revenue due to recurrent road and business closures 4 -compared to the physical damage of property and infrastructure associated with extreme storm-driven events. As SLR increases the frequency of HTF in the US [5][6][7][8][9][10][11] , coastal communities will need to adapt. However, developing adaptation pathways for recurrent coastal flooding is challenging and requires knowledge of environmental and social tipping points for which current actions and policies become ineffective [12][13][14] .Here we characterize projected increases in US HTF-including the impact of the 18.6-year nodal cycle in tidal amplitude [15][16][17] -in a way that can be used to establish planning horizons and develop adaptation pathways. First, we focus on the rate of flooding-frequency increase, which is not well understood despite being critical to establishing SLR impact timelines 18 .More specifically, we examine acute inflections, or tipping points, in the rate of increase that mark transitions from periods of gradual (and potentially imperceptible) change to rapid increase in HTF frequency. Second, we focus on the tendency for HTF episodes to cluster in time 19 . Scientists, engineers, and decision-makers are accustomed to the statistics and impacts of isolated extreme events [20][21][22][23] , but given the cumulative nature of HTF impacts 1-3 , we describe extreme months or seasons during which the number of flooding episodes, rather than the magnitude, is exceptional. Projections of high-tide flooding frequencyEnsemble projections of twenty-first century HTF frequency (Methods) are generated for 89 tide-gauge locations across the contiguous United States (US) and US-affiliated Pacific and Caribbean islands (Supplementary Data). HTF frequencies are represented as counts of days in monthly and annual windows for which at least one hourly sea-level value exceeds the flooding threshold of interest. NOAA SLR Scenarios 24 and derived HTF t...