Emanuele Di Lorenzo

Understanding ENSO Diversity

El Nino–Southern Oscillation (ENSO) is a naturally occurring mode of tropical Pacific variability, with global impacts on society and natural ecosystems. While it has long been known that El Nino events display a diverse range of amplitudes, triggers, spatial patterns, and life cycles, the realization that ENSO’s impacts can be highly sensitive to this event-to-event diversity is driving a renewed interest in the subject. This paper surveys our current state of knowledge of ENSO diversity, identifies key gaps in understanding, and outlines some promising future research directions.

Highly Variable El Niño–Southern Oscillation Throughout the Holocene

ENSO Variability The El Niño–Southern Oscillation (ENSO) is the most energetic, quasiperiodic climate oscillation in the world—every few years warming large expanses of the surface equatorial Pacific Ocean surface and impacting temperatures and rainfall patterns across the globe. A pressing question, in the context of global warming, is whether ENSO might be affected by the rising atmospheric temperatures caused by anthropogenic greenhouse gas emissions. Climate models do not agree on the answer to this question, but one place to look for data about how global temperatures might influence ENSO is the record of past ENSO variability. Cobb et al. (p. 67) present a record of ENSO variability spanning the past 7000 years, in an attempt better to define its response to insolation forcing over this same period. The findings reveal high variability in ENSO behavior that has no clear dependence on insolation, which implies that a link to warming, if it exists, may be difficult to detect. Coral records show that the El Niño–Southern Oscillation may be less sensitive to past climate forcing than previously thought. The El Niño–Southern Oscillation (ENSO) drives large changes in global climate patterns from year to year, yet its sensitivity to continued anthropogenic greenhouse forcing is uncertain. We analyzed fossil coral reconstructions of ENSO spanning the past 7000 years from the Northern Line Islands, located in the center of action for ENSO. The corals document highly variable ENSO activity, with no evidence for a systematic trend in ENSO variance, which is contrary to some models that exhibit a response to insolation forcing over this same period. Twentieth-century ENSO variance is significantly higher than average fossil coral ENSO variance but is not unprecedented. Our results suggest that forced changes in ENSO, whether natural or anthropogenic, may be difficult to detect against a background of large internal variability.

The Pacific Decadal Oscillation, Revisited

AbstractThe Pacific decadal oscillation (PDO), the dominant year-round pattern of monthly North Pacific sea surface temperature (SST) variability, is an important target of ongoing research within the meteorological and climate dynamics communities and is central to the work of many geologists, ecologists, natural resource managers, and social scientists. Research over the last 15 years has led to an emerging consensus: the PDO is not a single phenomenon, but is instead the result of a combination of different physical processes, including both remote tropical forcing and local North Pacific atmosphere–ocean interactions, which operate on different time scales to drive similar PDO-like SST anomaly patterns. How these processes combine to generate the observed PDO evolution, including apparent regime shifts, is shown using simple autoregressive models of increasing spatial complexity. Simulations of recent climate in coupled GCMs are able to capture many aspects of the PDO, but do so based on a balance of ...

The Warming of the California Current System: Dynamics and Ecosystem Implications

Abstract Long-term changes in the observed temperature and salinity along the southern California coast are studied using a four-dimensional space–time analysis of the 52-yr (1949–2000) California Cooperative Oceanic Fisheries Investigations (CalCOFI) hydrography combined with a sensitivity analysis of an eddy-permitting primitive equation ocean model under various forcing scenarios. An overall warming trend of 1.3°C in the ocean surface, a deepening in the depth of the mean thermocline (18 m), and increased stratification between 1950 and 1999 are found to be primarily forced by large-scale decadal fluctuations in surface heat fluxes combined with horizontal advection by the mean currents. After 1998 the surface heat fluxes suggest the beginning of a period of cooling, consistent with colder observed ocean temperatures. Salinity changes are decoupled from temperature and appear to be controlled locally in the coastal ocean by horizontal advection by anomalous currents. A cooling trend of –0.5°C in SST is...

Seasonal dynamics of the surface circulation in the Southern California Current System

Abstract The seasonal dynamics of the Southern California Current (SCC) is investigated using a primitive equation ocean model with real coastlines and topography. The model is tested with different wind forcing, and the resulting flow fields are compared to the mean and seasonal circulation inferred from long-term in situ observations (California Cooperative Oceanic Fisheries Investigation (CalCOFI)). The model integration forced with the output winds of a regional atmospheric model (RSM) best captures the statistics of the observed circulation, with a 0.9 correlation coefficient for the streamlines and 0.5 for the velocity fields. The model integrations reveal a pronounced linear response of the flow field to changes in winds on the shelf region. A dynamical feature inferred from CalCOFI hydrography, also suggested in TOPEX/ERS maps, is an annually recurrent westward propagation of SSH anomalies originated in the Southern California Bight (SCB) during the upwelling season. The RSM integration is the only one to capture the correct timing and spatial evolution of this process. We therefore use this model integration for guidance in constructing a dynamical framework to interpret the observed circulation and its variability. During the upwelling season in spring, there is an upward tilt of the isopycnals along the coast directly forced by the winds in the Bight. As the spring transitions to the summer the upwelling winds relax in the Bight but are still strong in the region offshore, approximately in correspondence of the continental slope (positive wind-stress curl condition). Anomalous denser waters in the location of the Southern California Eddy are maintained and reinforced by the combined interaction of the coastal/islands geometry and the wind-stress curl (through Ekman dynamics). The adjustment process to the denser water initiates a westward propagation of ocean density anomaly through Rossby waves, and reinforces the cyclonic gyre-like circulation of the SCE (increasing positive vorticity). Surface poleward flow, maintained by the positive wind-stress curl, is also reinforced in proximity of Point Conception as a consequence of the adjustment. During the summer the cyclonic gyre becomes increasingly unstable as the core of the ocean anomalies crosses the continental slope. Instability processes within the cyclonic region, characterized by a sharp increase in EKE, shed eddies that leave the region either drifting to the west or interacting with existing eddies in the region offshore. The EKE reaches a seasonal maximum at the end the summer in the cyclonic region, and in late fall further offshore where the eddies are fully developed. The shedding of eddies cannot be directly seen in the CalCOFI observations because of the sampling aliasing. For this point we rely on the strong suggestion of the model, which we assume is able to capture the leading order dynamics. Additional integrations with a linearized version of the model are also presented to reinforce our interpretation of the westward propagation of the isopycnal anomalous displacement associated with Rossby wave dynamics.

Forcing of Low-Frequency Ocean Variability in the Northeast Pacific*

Abstract An ocean model is used to examine and compare the forcing mechanisms and underlying ocean dynamics of two dominant modes of ocean variability in the northeast Pacific (NEP). The first mode is identified with the Pacific decadal oscillation (PDO) and accounts for the most variance in model sea surface temperatures (SSTs) and sea surface heights (SSHs). It is characterized by a monopole structure with a strong coherent signature along the coast. The second mode of variability is termed the North Pacific Gyre Oscillation (NPGO). This mode accounts for the most variance in sea surface salinities (SSSs) in the model and in long-term observations. While the NPGO is related to the second EOF of the North Pacific SST anomalies (the Victoria mode), it is defined here in terms of SSH anomalies. The NPGO is characterized by a pronounced dipole structure corresponding to variations in the strengths of the eastern and central branches of the subpolar and subtropical gyres in the North Pacific. It is found tha...

North Pacific Gyre Oscillation Synchronizes Climate Fluctuations in the Eastern and Western Boundary Systems

Abstract Recent studies have identified the North Pacific Gyre Oscillation (NPGO) as a mode of climate variability that is linked to previously unexplained fluctuations of salinity, nutrient, and chlorophyll in the northeast Pacific. The NPGO reflects changes in strength of the central and eastern branches of the subtropical gyre and is driven by the atmosphere through the North Pacific Oscillation (NPO), the second dominant mode of sea level pressure variability in the North Pacific. It is shown that Rossby wave dynamics excited by the NPO propagate the NPGO signature in the sea surface height (SSH) field from the central North Pacific into the Kuroshio–Oyashio Extension (KOE), and trigger changes in the strength of the KOE with a lag of 2–3 yr. This suggests that the NPGO index can be used to track changes in the entire northern branch of the North Pacific subtropical gyre. These results also provide a physical mechanism to explain coherent decadal climate variations and ecosystem changes between the No...

North Pacific Decadal Variability and Climate Change in the IPCC AR4 Models

AbstractThe two leading modes of North Pacific sea surface temperature (SST) and sea level pressure (SLP), as well as their connections to tropical variability, are explored in the 24 coupled climate models used in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) to evaluate North Pacific decadal variability (NPDV) in the past [twentieth century; climate of the twentieth century (20C3M) scenario] and future [twenty-first century; Special Report on Emissions Scenarios (SRES) A1B scenario] climate. Results indicate that the two dominant modes of North Pacific oceanic variability, the Pacific decadal oscillation (PDO) and the North Pacific Gyre Oscillation (NPGO), do not exhibit significant changes in their spatial and temporal characteristics under greenhouse warming. However, the ability of the models to capture the dynamics associated with the leading North Pacific oceanic modes, including their link to the corresponding atmospheric forcing patterns and to tropical varia...

Decadal-Scale SST and Salinity Variations in the Central Tropical Pacific: Signatures of Natural and Anthropogenic Climate Change

Accurate projections of future temperature and precipitation patterns in many regions of the world depend on quantifying anthropogenic signatures in tropical Pacific climate against its rich background of natural variability. However, the detection of anthropogenic signatures in the region is hampered by the lack of continuous, century-long instrumental climate records. This study presents coral-based sea surface temperature (SST) and salinity proxy records from Palmyra Island in the central tropical Pacific over the twentieth century, based on coral strontium/calcium and the oxygen isotopic composition of seawater (d 18 OSW), respectively. On interannual time scales, the Sr/Ca-based SST record captures both eastern and central Pacific

Biological communities in San Francisco Bay track large-scale climate forcing over the North Pacific.

Long-term observations show that fish and plankton populations in the ocean fluctuate in synchrony with large-scale climate patterns, but similar evidence is lacking for estuaries because of shorter observational records. Marine fish and invertebrates have been sampled in San Francisco Bay since 1980 and exhibit large, unexplained population changes including record-high abundances of common species after 1999. Our analysis shows that populations of demersal fish, crabs and shrimp covary with the Pacific Decadal Oscillation (PDO) and North Pacific Gyre Oscillation (NPGO), both of which reversed signs in 1999. A time series model forced by the atmospheric driver of NPGO accounts for two-thirds of the variability in the first principal component of species abundances, and generalized linear models forced by PDO and NPGO account for most of the annual variability of individual species. We infer that synchronous shifts in climate patterns and community variability in San Francisco Bay are related to changes in oceanic wind forcing that modify coastal currents, upwelling intensity, surface temperature, and their influence on recruitment of marine species that utilize estuaries as nursery habitat. Ecological forecasts of estuarine responses to climate change must therefore consider how altered patterns of atmospheric forcing across ocean basins influence coastal oceanography as well as watershed hydrology.