Gary Shaffer

Circulation and Low-Frequency Variability near the Chilean Coast: Remotely Forced Fluctuations during the 1991–92 El Niño

Results are reported from the first long, recording current meter observations over the slope off Chile. These observations, at 308S during the 1991-92 El Nino event, are analyzed together with observations of currents at a local deep sea site; local wind and sea level; sea level from the Peru and Chile coasts; and wind, temperature, and currents from the equatorial Pacific. Mean poleward flow of 12 cm s21 was observed within the Peru-Chile Undercurrent over the slope. Mean flow in the depth range of Antarctic Intermediate Water was not distinguishable from zero in the presence of strong, low-frequency (LF) variability, which dominated slope currents at all depths. The strongest LF fluctuations had periods of about 50 days, but periods of 10 and about 5 days were also observed. Significant, local wind forcing of slope currents was only found in the period band 6-10 days and may be related to coastal-trapped waves in the atmosphere. Our analysis shows that free, coastal-trapped waves in the ocean, arriving from the north, dominated the LF variability over the shelf and slope off northern and central Chile during the 1991-92 El Nino event. Strong 50-day period fluctuations there started their journey about two months earlier—and 15 000 km farther up the coastal-equatorial waveguide—near the date line in the equatorial Pacific as equatorial Kelvin waves forced by westerly wind events of similar period. Upon reaching the South American coast, these waves forced coastal- trapped waves, which propagated along the Peru coast into the study region. Likewise, a scenario of equatorial- trapped waves forcing coastal-trapped waves may explain 10-day as well as 6-day and 4.5-day period coastal- trapped waves off Chile stemming from mixed Rossby-gravity and inertia-gravity waves trapped at the equator. Since the large, 50-day period, coastal-trapped waves may strongly modify coastal upwelling source water, such remotely forced waves may have a significant influence on the pelagic ecosystem off Chile, at least during El Nino events.

Long-Term Climate Change Commitment and Reversibility: An EMIC Intercomparison

AbstractThis paper summarizes the results of an intercomparison project with Earth System Models of Intermediate Complexity (EMICs) undertaken in support of the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5). The focus is on long-term climate projections designed to 1) quantify the climate change commitment of different radiative forcing trajectories and 2) explore the extent to which climate change is reversible on human time scales. All commitment simulations follow the four representative concentration pathways (RCPs) and their extensions to year 2300. Most EMICs simulate substantial surface air temperature and thermosteric sea level rise commitment following stabilization of the atmospheric composition at year-2300 levels. The meridional overturning circulation (MOC) is weakened temporarily and recovers to near-preindustrial values in most models for RCPs 2.6–6.0. The MOC weakening is more persistent for RCP8.5. Elimination of anthropogenic CO2 emissions after 2300 resu...

Seasonal and interannual variability of currents and temperature off central Chile

Results are reported for continuous, 6 year records of current and temperature from a continental slope station at 308S off Chile. These observations for November 1991 to November 1997 span the moderate, weak, and strong El Nino events of 1991-1992, 1994, and 1997 and the La Nina event of 1995-1996. Results for current and temperature are analyzed together with observations of wind along the South American coast from the ERS-1 and -2 satellite scatterometers, wind from a local coastal site, sea level from northern and central Chile, sea surface temperature anomalies from the South Pacific region, and traditional El Nino indices. Mean poleward flow of 12.8 cm s 21 was observed for the 6 year period in the core of the Peru-Chile Undercurrent (220 m). Mean flow in the depth range of Antarctic Intermediate Water (750 m) was equatorward at 1.1 cm s 21 . Intraseasonal-scale variability associated with coastal trapped waves was weaker during the austral winter: greatest during El Nino events and weakest during the La Nina event. Winds adjacent to the coast showed a strong annual cycle with a southward progression of maximum equatorward (upwelling favorable) winds. Alongshore flow over the slope exhibited strong semiannual and weaker annual variations. Poleward flow in the undercurrent was strongest in the austral spring and fall and weakest in the winter. Near the bottom (750 m), seasonal-scale variability was less intense but led that higher up in the water column (220 m) by ;1 month. Strong interannual variability of alongshore current and temperature was found at all depths over the slope, whereby changes at depth led changes higher up. A rapid warming event was observed at the end of 1996 - beginning of 1997 over the slope. This event, also seen in sea level and sea surface temperature off Chile, led El Nino indices by several months and may be best explained by local wind anomalies coupled to atmospheric teleconnections from the western tropical Pacific at the onset of El Nino. Some aspects of our current and temperature observations are consistent with results from eastern boundary current models. Our results indicate that both ocean and atmosphere pathways from the equatorial Pacific contribute significantly to interannual-scale variability of alongshore flow and thermocline depth off Chile.

Currents in the deep ocean off Chile (30°S)

Results are reported from the first recording current meter observations in the eastern boundary current system off Chile. Currents at 100 m and 3400 m were observed for a 4–6 month period during the austral spring-summer-fall period of 1991–1992 at a deep ocean site 150 km off the Chilean coast and 70 km seaward of the axis of the Peru-Chile Trench. Results indicate energetic inertial oscillations, low eddy kinetic energy at 3400 m, southward flow, decreasing from austral spring to summer, at 100 m and 3400 m, and quite strong eastward flow, increasing from spring to summer, at 100 m. The latter result may help to explain satellite images, showing the zone of high biological production near the coast in the study region to be very narrow during the austral summer despite strong coastal upwelling in that season. The authors current observations, local geostrophic current profiles and abyssal potential temperature distributions indicate a five-layer structure for the alongshore flow in the eastern boundary current system off Chile: northward flow above 100 m, between about 400 m and 1700 m (maximum around 600–700 m, northward transport of Antarctic Intermediate Water) and below 3400 m; southward flow from above 100 m to about 400 m (maximum at 180 m, Peru-Chile Undercurrent) and from about 1700 m to below 3400. This proposed vertical structure is consistent with temperature, salinity, dissolved oxygen and nutrient distributions of the study area and bears resemblance to eastern boundary current simulations (McCreary et al., 1987).

Local and remote forcing of sea surface temperature in the coastal upwelling system off Chile

An analysis is presented of records of sea surface temperature (SST), sea level and winds collected during the period 1991–1995 along the coast of northern and central Chile (sea level also from the Peru coast in 1991–1994), and of SST from remote sensing during the austral summers of 1991 and 1992 off northern Chile. Large, interannual fluctuations of SST and sea level off northern Chile are linked to vertical displacements of the equatorial thermocline during El Nino-La Nina events. In the intraseasonal, 50-day band, SST and sea level are intimately related all along the coast, whereby SST anomalies lag sea level perturbations by about 12 days. Such sea level fluctuations are due to free, coastal-trapped waves, ultimately forced via equatorial Kelvin waves by 50-day wind events in the western and central tropical Pacific. These results and those of the remote sensing analysis suggest significant remote forcing of SST fluctuations in the coastal zone off Chile by the action of the coastal-trapped waves. Off central Chile, SST is also related to local wind stress in the intraseasonal band. Simulations with a simple, local, wind-driven upwelling/entrainment model capture rather well observed synoptic and intraseasonal oscillations of SST there. Off northern Chile, however, this simple model is not successful in simulating observed SST. An attempt to include effects of coastal-trapped waves into this simple, one-dimensional model did not improve model fit to data at either location. An explanation for the apparent strong influence of these waves on SST along this coast should therefore be sought in other effects not considered by this model such as cross-shelf and alongshore advection.

Ocean subsurface warming as a mechanism for coupling Dansgaard‐Oeschger climate cycles and ice‐rafting events

[1] Heinrich events have been attributed to surging of the Laurentide ice sheet every 7 thousand years or so during glacial time. These massive ice-rafting events only occurred during cold phases of millennial-scale, Dansgaard-Oeschger climate cycles. Other observed ice-rafting events, sourced from ice streams at various locations around the northern North Atlantic, occurred during all Dansgaard-Oeschger cold phases and led Heinrich events when the latter took place. Here it is suggested that ocean subsurface warming in the northern North Atlantic during the cold phases may provide the key to explaining these climate - ice rafting phasings. Such warming would lead to ice shelf melting and breakup. Without buttressing by ice shelves, ice streams may surge, leading to increased iceberg production. This interpretation is supported by results of a simplified, coupled climate model and by available sediment and ice sheet data.

The international at-sea intercomparison of fCO2 systems during the R/V Meteor Cruise 36/1 in the North Atlantic Ocean

The ‘International Intercomparison Exercise of fCO2 Systems’ was carried out in 1996 during the R/V Meteor Cruise 36/1 from Bermuda/UK to Gran Canaria/Spain. Nine groups from six countries (Australia, Denmark, France, Germany, Japan, USA) participated in this exercise, bringing together 15 participants with seven underway fugacity of carbon dioxide (fCO2) systems, one discrete fCO2 system, and two underway pH systems, as well as systems for discrete measurement of total alkalinity and total dissolved inorganic carbon. Here, we compare surface seawater fCO2 measured synchronously by all participating instruments. A common infrastructure (seawater and calibration gas supply), different quality checks (performance of calibration procedures for CO2, temperature measurements) and a common procedure for calculation of final fCO2 were provided to reduce the largest possible amount of controllable sources of error. The results show that under such conditions underway measurements of the fCO2 in surface seawater and overlying air can be made to a high degree of agreement (±1 μatm) with a variety of possible equilibrator and system designs. Also, discrete fCO2 measurements can be made in good agreement (±3 μatm) with underway fCO2 data sets. However, even well-designed systems, which are operated without any obvious sign of malfunction, can show significant differences of the order of 10 μatm. Based on our results, no “best choice” for the type of the equilibrator nor specifics on its dimensions and flow rates of seawater and air can be made in regard to the achievable accuracy of the fCO2 system. Measurements of equilibrator temperature do not seem to be made with the required accuracy resulting in significant errors in fCO2 results. Calculation of fCO2 from high-quality total dissolved inorganic carbon (CT) and total alkalinity (AT) measurements does not yield results comparable in accuracy and precision to fCO2 measurements.

A numerical study of the seasonal variability in the circulation off central Chile

The Princeton Ocean Model is used to study the coastal ocean circulation off central Chile (between 30° and 45°S) and its seasonal transitions in response to monthly mean climatological wind stress and surface heat flux. The model reproduces many of the dominant circulation features, including an equatorward coastal current, coastal upwelling, and a poleward undercurrent. To get the correct seasonal variability in the undercurrent, it was necessary to prescribe the inflow of the undercurrent on the northern model boundary. Meanders and eddies form throughout the simulation period seaward of the coastal current in response to baroclinic instability of the flow. The flow associated with the meanders shows upwelling in the shoreward flow and downwelling in the seaward flow. This upwelling/downwelling pattern is explained by conservation of relative vorticity in the flow. The upwelling velocity associated with the meanders corresponds to a vertical migration of 15–30 m over a typical timescale of the meanders of 1–2 months. It is argued that this upwelling may be important for maintaining observed high phytoplankton biomass and primary production seaward of the coastal zone directly affected by wind-driven upwelling.

Biogeochemical cycling in the global ocean: 1. A new, analytical model with continuous vertical resolution and high-latitude dynamics

A new, simple analytical model of ocean chemistry is presented which includes continuous vertical resolution, high-latitude dynamics, air-sea exchange and sea ice cover. In this high-latitude exchange/interior diffusion-advection (HILDA) model, ocean physics are represented by four parameters: k and w, an eddy diffusion coefficient and a deep upwelling velocity in the stratified interior; q, a rate of lateral exchange between the interior and a well-mixed, deep polar ocean; and u, an exchange velocity between surface and deep layers in the polar ocean. First, estimates are made of ice-free and ice-covered areas at high latitudes, surface temperatures, and air-sea exchange velocities from available data. Then values of the physical parameters are estimated from simultaneous, least mean square fits of model solutions for temperature (T) and “abiotic” carbon 14 (Δ14C) to interior profiles of T and Δ14C and surface layer Δ14C values all derived from available data. Best fit values for k, w, q, and u are 3.2×10−5 m2 s−1, 2.0×10−8 m s−1, 7.5×10−11 s−1 and 1.9×10−6 m s−1 respectively. These results are interpreted in terms of modes of ocean circulation and mixing and compared with results from other simplier and more complex models. In parts 2 and 3 of this series, these values for k, w, q and u are taken as inputs for studying phosphorus, oxygen, and carbon cycling in the global ocean with the HILDA model.

Effects of the Marine Biota on Global Carbon Cycling

Natural carbon cycling in the global ocean and its influence on atmospheric CO2 concentrations can be examined conveniently in terms of a solubility pump, an organic carbon pump and a calcium carbonate pump (Volk and Hoffert 1985). Here a review is presented of the physical, chemical and biological processes which determine the strength of these pumps. Ocean new (export) production and biogenic calcium carbonate production lie in the ranges 5–10 GtCyr-1 and 1–2 GtCyr-1 respectively. A new analysis using the High Latitude exchange/ interior Diffusion-Advection (HILDA) model (Shaffer and Sarmiento 1992, Shaffer 1992) shows that the organic carbon pump is about twice as strong as the solubility pump in terms of atmospheric CO2 drawdown. Model results for the modem, pre-industrial ocean imply that atmospheric transport of CO2 between high and low-mid latitudes associated with the solubility pump (0.7 GtCyr-1 poleward) and the two “biological” pumps (0.8 GtCyr-1 equatorward) tended to balance. This indicates that CO2 outgassing from equatorial upwelling was balanced by CO2 uptake at mid latitudes. Model results are also used to study the sensitivity of atmospheric CO2 levels to changes in ocean physics and biology. It is concluded that even very large changes in ocean biology would probably not make a large impact on ocean uptake of CO2 in the near future. However over century time scales such changes could be important for atmospheric CO2 evolution.