Search Results

You are looking at 1 - 10 of 26 items for

  • Author or Editor: David W. Pierce x
  • Refine by Access: All Content x
Clear All Modify Search
David W. Pierce

Abstract

The North Pacific Oscillation (NPO) is a decadal to interdecadal fluctuation of sea surface temperatures (SSTs) in the North Pacific. Previous works have shown that during individual El Niño and La Niña winters, atmospheric circulation anomalies over North America are characteristically different for different phases of the NPO. Two physical mechanisms could account for this observed link between North Pacific SSTs and ENSO's effects over North America: 1) NPO SSTs could force changes in the overlying atmosphere that modulate ENSO's effects, and 2) the atmosphere could undergo internal variability that both modulates ENSO's effects and imprints a characteristic pattern of North Pacific SSTs. The first mechanism suggests methods for increasing the skill of seasonal climate predictions by incorporating the state of the North Pacific, using simple persistence of SSTs if nothing else. The second mechanism implies that North Pacific SSTs add no information that could be used to improve seasonal climate predictions of ENSO's effects. Analysis of a 300-yr run of a coupled ocean–atmosphere model shows that it exhibits links between NPO and ENSO similar to those observed. It is found that specifying NPO SSTs does not force these links. This suggests that the association found between NPO SSTs and ENSO's effects is primarily due to the fact that both are responding to the same internal atmospheric variability. In such a case, incorporating accurate predictions of NPO SSTs into ENSO prediction schemes would have little ability to improve forecasts of ENSO's effects.

Full access
David W. Pierce

Abstract

Restoring boundary conditions are often used to drive ocean general circulation models. As typically such conditions impose time lags and amplitude errors in the seasonal cycle of the model surface tracer fields. Restoring boundary conditions also damp out the high-frequency components of the forcing with more damping for higher frequencies; thus, models using such conditions systematically underrepresent high-frequency variability in the surface tracer fields. A solution to these problems is presented for use when the forcing field is known beforehand. It is shown that this new formulation significantly reduces the time lags associated with the traditional form of restoring boundary conditions and improves the model's representation of surface variability. The new condition has no run-time overhead and does not impose any additional restrictions on the ability of the model to deviate from observations. The results of using the new boundary condition in an oceanic general circulation model are shown for cases with both monthly and weekly forcing.

Full access
Jochem Marotzke and David W. Pierce

Abstract

The authors identify spatial and temporal scales in a one-dimensional linear, diffusive atmospheric energy balance model coupled everywhere to a slab mixed layer of fixed depth. Mathematically, the model is identical to a heat conducting rod, which over its entire length both radiates and is in contact with a large but finite“reservoir.” Three characteristic timescales mark, respectively, the atmosphere’s adjustment to a sea surface temperature (SST) anomaly, the decay of a pointwise SST anomaly, and the radiative decay of a large-scale SST anomaly. The first and the third of these timescales are associated with diffusive length scales characterizing, respectively, the distance over which heat is diffused in the atmosphere before being lost to the ocean beneath, and the distance over which heat is diffused in the coupled system before being radiated to space. For spatial scales between the two diffusive lengths, the SST anomaly does not decay exponentially but with the square root of time; this regime has not previously been identified. Apparent discrepancies between published discussions of diffusive length scales are reconciled.

Full access
David W. Pierce and Peter B. Rhines

Abstract

The convective building of a pycnocline is examined using a two-dimensional nonhydrostatic numerical model forced by a balanced salinity dipole (source and sink). Although the forcing fields are steady, the model develops oscillations that renew the model’s analog of “deep waters” only intermittently. The oscillation cycle consists of a freshwater layer that advects along the surface, capping off the water column under the dense source and preventing sinking; after a time, continuing densification forms a plume that breaks through the salinity barrier and convects beneath the dense source, ventilating the deep water. Increasing the viscosity reduces but does not eliminate this cycle. When the hydrostatic assumption is added, the model evolves systematically different salinity distributions than the nonhydrostatic model due to the isolation of part of the tank by a persistent convective column. The deep flow is also different in this case because of differences between the entrainment/detrainment profile of a hydrostatic plume and one modeled explicitly. The model evolves a characteristically skewed distribution of densities that is similar to the distribution of temperature in the World Ocean. Rotation increases the range of this distribution due to the inhibition of meridional flow.

Full access
David W. Pierce and Peter B. Rhines

Abstract

The convective building of a pycnocline is examined using a laboratory model forced by surface fluxes of saline water at one end and fresh water at the other. A deep recirculation evolves in the tank, which homogenizes the interior fluid by repeated passes through the dense, descending plume. A thin, fresh surface layer develops and modifies the effective buoyancy flux into the dense plume, causing the interior velocities to fall to an intermediate-time minimum. Adding bottom topography under the dense source greatly reduces the amount of entrainment that the descending plume undergoes. In this case, the tank fills with a deep, heavy layer, which causes the plume to “lift off” the bottom of the tank and detrain at successively higher depths in the water column. A simple numerical “plume” model shows that this cannot be a steady state, as it is not in diffusive balance; the plume must eventually return to the bottom of the tank and ventilate the interior waters. Adding rotation increases the surface mixing, thickens the halocline, and increases the observed variability in the salinity field.

Full access
David W. Pierce and Daniel R. Cayan

Abstract

The effect of human-induced climate warming on different snow measures in the western United States is compared by calculating the time required to achieve a statistically significant linear trend in the different measures, using time series derived from regionally downscaled global climate models. The measures examined include the water content of the spring snowpack, total cold-season snowfall, fraction of winter precipitation that falls as snow, length of the snow season, and fraction of cold-season precipitation retained in the spring snowpack, as well as temperature and precipitation. Various stakeholders may be interested in different sets of these variables. It is found that temperature and the fraction of winter precipitation that falls as snow exhibit significant trends first, followed in 5–10 years by the fraction of cold-season precipitation retained in the spring snowpack, and later still by the water content of the spring snowpack. Change in total cold-season snowfall is least detectable of all the measures, since it is strongly linked to precipitation, which has large natural variability and only a weak anthropogenic trend in the western United States. Averaging over increasingly wider areas monotonically increases the signal-to-noise ratio of the 1950–2025 linear trend from 0.15 to 0.37, depending on the snow measure.

Full access
David W. Pierce, Daniel R. Cayan, and Bridget L. Thrasher

Abstract

A new technique for statistically downscaling climate model simulations of daily temperature and precipitation is introduced and demonstrated over the western United States. The localized constructed analogs (LOCA) method produces downscaled estimates suitable for hydrological simulations using a multiscale spatial matching scheme to pick appropriate analog days from observations. First, a pool of candidate observed analog days is chosen by matching the model field to be downscaled to observed days over the region that is positively correlated with the point being downscaled, which leads to a natural independence of the downscaling results to the extent of the domain being downscaled. Then, the one candidate analog day that best matches in the local area around the grid cell being downscaled is the single analog day used there. Most grid cells are downscaled using only the single locally selected analog day, but locations whose neighboring cells identify a different analog day use a weighted combination of the center and adjacent analog days to reduce edge discontinuities. By contrast, existing constructed analog methods typically use a weighted average of the same 30 analog days for the entire domain. By greatly reducing this averaging, LOCA produces better estimates of extreme days, constructs a more realistic depiction of the spatial coherence of the downscaled field, and reduces the problem of producing too many light-precipitation days. The LOCA method is more computationally expensive than existing constructed analog techniques, but it is still practical for downscaling numerous climate model simulations with limited computational resources.

Full access
David W. Pierce, Tim P. Barnett, and Uwe Mikolajewicz

Abstract

The physical mechanisms causing century-scale Southern Ocean thermohaline oscillations in a primitive equation oceanic general circulation model are described. The oscillations have been shown to occur on a 320-year timescale when random fluctuations am added to the freshwater flux field that forces the model; this result is extended to show that they occur in a variety of situations, including ones without added noise. The oscillations involve movement between two model states: one characterized by strong convection and an active thermohaline circulation. and the other with a holocline around Antarctica capping off the water column, thus preventing convection. The physical mechanism that forces the model from the quiescent state to an actively convecting one is subsurface (300 m) heating around Antarctica, which destabilizes the water column; the ultimate source of this heat is advected North Atlantic Deep Water. This leads to a teleconnection between forcing conditions in the North Atlantic and the thermohaline structure of the Southern Ocean. The mechanism that shuts off convection is surface freshening, primarily by precipitation, in the region poleward of the Antarctic Circumpolar Current. The oscillations are analyzed in terms of a simple “flip-flop” model, which indicates that nonlinearities in the seawater equation of state are necessary for the oscillations to occur. The spatial pattern of convection around Antarctica affects the time evolution of the Southern Ocean's thermohaline overturning and the way in which different surface forcings cause the model to oscillate.

Full access
Zhenhai Zhang, David W. Pierce, and Daniel R. Cayan

Abstract

This study investigates the forecast skill of seasonal-mean near-surface (2 m) air temperature in the North American Multimodel Ensemble (NMME) Phase 2, with a focus on the West Coast of the United States. Overall, 1-month lead time NMME forecasts exhibit skill superior or similar to persistence forecasts over many continental regions, and skill is generally higher over the ocean than the continent. However, forecast skill along most West Coast regions is markedly lower than in the adjacent ocean and interior, especially during the warm seasons. Results indicate that the poor forecast skill along the West Coast of the United States reflects deficiencies in their representation of multiple relevant physical processes. Analyses focusing on California find that summer forecast errors are spatially coherent over the coastal region and the inland region individually, but the correlation of forecast errors between the two regions is low. Variation in forecast performance over the coastal California region is associated with anomalous geopotential height over the lower middle latitudes and subtropics of the eastern Pacific, North America, and the western Atlantic. In contrast, variation in forecast performance over the inland California region is associated with the atmospheric circulation over the western United States. Further, it is found that forecast errors along the California coast are linked to anomalies of low cloudiness (stratus clouds) along the coastal region.

Full access
Elena Yulaeva, Niklas Schneider, David W. Pierce, and Tim P. Barnett

Abstract

Potential predictability of low-frequency climate changes in the North Pacific depends on two main factors. The first is the sensitivity of the atmosphere to ocean-induced anomalies at the sea surface in midlatitudes. The second is the degree of teleconnectivity of the tropical low-frequency variability to midlatitudes. In contrast to the traditional approach of prescribing sea surface temperature (SST) anomalies, the response of a coupled atmospheric general circulation (CCM3)–mixed layer ocean model to oceanic perturbations of the mixed layer heat budget is examined. Since positive oceanic heat flux perturbations partially increase SST anomalies (locally), and partially are vented directly into the atmosphere, expressing boundary forcing on the atmosphere by prescribing upper-ocean heat flux anomalies allows for better understanding of the physical mechanism of low-frequency variability in midlatitudes. In the framework of this approach SST is considered to be a part of the adjustment of the coupled system rather than an external forcing. Wintertime model responses to mixed layer heat budget perturbations of up to 40 W m−2 in the Kuroshio extension region and in the tropical central Pacific show statistically significant anomalies of 500-mb geopotential height (Z500) in the midlatitudes. The response to the tropical forcing resembles the well-known Pacific–North American pattern, one of the leading modes of internal variability of the control run. The amplitude of the Z500 geopotential height reaches 40 m in the region of the Aleutian low. The response of Z500 to forcing in the Kuroshio Current extension region resembles the mixture of western Pacific and Pacific–North American patterns, the first two modes of the internal variability of the atmosphere. In midlatitudes this response is equivalent barotropic, with the maximum of 80 m at (60°N, 160°W). Examination of the vorticity and thermodynamic budgets reveals the crucial role of submonthly transient eddies in maintaining the anomalous circulation in the free atmosphere.

At the surface the response manifests itself in changes of surface temperature and the wind stress. The amplitude of response to the tropical forcing in the SST field at the Kuroshio Current extension region is up to 0.3 K (in absolute value) that is 2 times weaker than SST anomalies induced by midlatitude forcing of the same amplitude. In addition, the spatial structures of the responses in the SST field over the North Pacific are different. While tropical forcing induces SST anomalies in the central North Pacific, the midlatitude forcing causes SST anomalies off the east coast of Japan, in the Kuroshio–Oyashio extension region. Overall, remote tropical forcing appears to be effective in driving anomalies over the central North Pacific. This signal can be transported westward by the oceanic processes. Thus tropical forcing anomalies can serve as a precursor of the changes over the western North Pacific.

In the case of midlatitude forcing, the response in the wind stress field alters Ekman pumping in such a way that the expected change of the oceanic gyre, as measured by the Sverdrup transport, would counteract the prescribed forcing in the Kuroshio extension region, thus causing a negative feedback. This response is consistent with the hypothesis that quasi-oscillatory decadal climate variations in the North Pacific result from midlatitude ocean–atmosphere interaction.

Full access