Search Results

You are looking at 1 - 10 of 52 items for

  • Author or Editor: Andrew T. Wittenberg x
  • Refine by Access: All Content x
Clear All Modify Search
Andrew T. Wittenberg

Abstract

Surface wind stresses are fundamental to understanding El Niño, yet most observational stress products are too short to permit multidecadal ENSO studies. Two exceptions are the Florida State University subjective analysis (FSU1) and the NCEP–NCAR reanalysis (NCEP1), which are widely used in climate research. Here, the focus is on the aspects of the stress most relevant to ENSO—namely, the climatological background, anomaly spectrum, response to SST changes, subannual “noise” forcing, and seasonal phase locking—and how these differ between FSU1 and NCEP1 over the tropical Pacific for 1961–99.

The NCEP1 stress climatology is distinguished from FSU1 by weaker equatorial easterlies, stronger off-equatorial cyclonic curl, stronger southerlies along the Peruvian coast, and weaker convergence zones with weaker seasonality. Compared to FSU1, the NCEP1 zonal stress anomalies ( τ x ) are weaker, less noisy, and show less persistent westerly peaks during El Niño events. NCEP1 also shows a more stationary spectrum that more closely resembles that of equatorial east Pacific SST anomalies. After the 1970s, the equatorial trade winds and stress variability shift east and strengthen in FSU1, while the opposite occurs in NCEP1. Both products show increased mean convergence in the equatorial far west Pacific in recent decades, with weaker mean easterlies near the date line, an increased stress response to SST anomalies, and stronger interannual and subannual τ x in the central equatorial Pacific (Niño-4; 5°N–5°S, 160°E–150°W). The variance of Niño-4 τ x is highly seasonal in both datasets, with an interannual peak in October–November and a subannual peak in November–February; yet apart from interannual Niño-4 τ x after 1980, stress anomalies are not well correlated between the products. Newer and more reliable stress estimates generally fall between NCEP1 and FSU1, with most closer to FSU1.

Full access
Jonghun Kam
,
Thomas R. Knutson
,
Fanrong Zeng
, and
Andrew T. Wittenberg
Full access
Thomas R. Knutson
,
Jonghun Kam
,
Fanrong Zeng
, and
Andrew T. Wittenberg
Full access
Jonghun Kam
,
Thomas R. Knutson
,
Fanrong Zeng
, and
Andrew T. Wittenberg
Full access
Jonghun Kam
,
Thomas R. Knutson
,
Fanrong Zeng
, and
Andrew T. Wittenberg
Full access
Kit-Yan Choi
,
Gabriel A. Vecchi
, and
Andrew T. Wittenberg

Abstract

The El Niño–Southern Oscillation (ENSO) exhibits well-known asymmetries: 1) warm events are stronger than cold events, 2) strong warm events are more likely to be followed by cold events than vice versa, and 3) cold events are more persistent than warm events. Coupled GCM simulations, however, continue to underestimate many of these observed features.

To shed light on these asymmetries, the authors begin with a widely used delayed-oscillator conceptual model for ENSO and modify it so that wind stress anomalies depend more strongly on SST anomalies (SSTAs) during warm conditions, as is observed. Then the impact of this nonlinearity on ENSO is explored for three dynamical regimes: self-sustained oscillations, stochastically driven oscillations, and self-sustained oscillations disrupted by stochastic forcings. In all three regimes, the nonlinear air–sea coupling preferentially strengthens the feedbacks (both positive and delayed negative) during the ENSO warm phase—producing El Niños that grow to a larger amplitude and overshoot more rapidly and consistently into the opposite phase, than do the La Niñas. Finally, the modified oscillator is applied to observational records and to control simulations from two global coupled ocean–atmosphere–land–ice models [Geophysical Fluid Dynamics Laboratory Climate Model version 2.1 (GFDL CM2.1) and version 2.5 (GFDL CM2.5)] to elucidate the causes of their differing asymmetries.

Full access
Thomas R. Knutson
,
Fanrong Zeng
, and
Andrew T. Wittenberg

Abstract

Regional surface temperature trends from phase 3 of the Coupled Model Intercomparison Project (CMIP3) and CMIP5 twentieth-century runs are compared with observations—at spatial scales ranging from global averages to individual grid points—using simulated intrinsic climate variability from preindustrial control runs to assess whether observed trends are detectable and/or consistent with the models' historical run trends. The CMIP5 models are also used to detect anthropogenic components of the observed trends, by assessing alternative hypotheses based on scenarios driven with either anthropogenic plus natural forcings combined, or with natural forcings only. Modeled variability is assessed via inspection of control run time series, standard deviation maps, spectral analyses, and low-frequency variance consistency tests. The models are found to provide plausible representations of internal climate variability, although there is room for improvement. The influence of observational uncertainty on the trends is assessed and is found to be generally small in comparison with intrinsic climate variability. Observed temperature trends over 1901–2010 are found to contain detectable anthropogenic warming components over a large fraction (about 80%) of the analyzed global area. In about 70% of the analyzed area, the modeled warming is consistent with the observed trends; in about 10% it is significantly greater than simulated. Regions without detectable warming include the high-latitude North Atlantic Ocean, the eastern United States, and parts of the eastern and northern Pacific Ocean. For 1981–2010, the observed warming trends over only about 30% of the globe are found to contain a detectable anthropogenic warming: this includes a number of regions within about 40°–45° of the equator, particularly in the Indian Ocean, western Pacific, South Asia, and tropical Atlantic.

Full access
Hui Ding
,
Matthew Newman
,
Michael A. Alexander
, and
Andrew T. Wittenberg

Abstract

Seasonal forecasts made by coupled atmosphere–ocean general circulation models (CGCMs) undergo strong climate drift and initialization shock, driving the model state away from its long-term attractor. Here we explore initializing directly on a model’s own attractor, using an analog approach in which model states close to the observed initial state are drawn from a “library” obtained from prior uninitialized CGCM simulations. The subsequent evolution of those “model-analogs” yields a forecast ensemble, without additional model integration. This technique is applied to four of the eight CGCMs comprising the North American Multimodel Ensemble (NMME) by selecting from prior long control runs those model states whose monthly tropical Indo-Pacific SST and SSH anomalies best resemble the observations at initialization time. Hindcasts are then made for leads of 1–12 months during 1982–2015. Deterministic and probabilistic skill measures of these model-analog hindcast ensembles are comparable to those of the initialized NMME hindcast ensembles, for both the individual models and the multimodel ensemble. In the eastern equatorial Pacific, model-analog hindcast skill exceeds that of the NMME. Despite initializing with a relatively large ensemble spread, model-analogs also reproduce each CGCM’s perfect-model skill, consistent with a coarse-grained view of tropical Indo-Pacific predictability. This study suggests that with little additional effort, sufficiently realistic and long CGCM simulations provide the basis for skillful seasonal forecasts of tropical Indo-Pacific SST anomalies, even without sophisticated data assimilation or additional ensemble forecast integrations. The model-analog method could provide a baseline for forecast skill when developing future models and forecast systems.

Full access
Sulagna Ray
,
Andrew T. Wittenberg
,
Stephen M. Griffies
, and
Fanrong Zeng

Abstract

The heat budget of the Pacific equatorial cold tongue (ECT) is explored using the GFDL-FLOR coupled GCM (the forecast-oriented low ocean resolution version of CM2.5) and ocean reanalyses, leveraging the two-layer framework developed in Part I. Despite FLOR’s relatively weak meridional stirring by tropical instability waves (TIWs), the model maintains a reasonable SST and thermocline depth in the ECT via two compensating biases: 1) enhanced monthly-scale vertical advective cooling below the surface mixed layer (SML), due to overly cyclonic off-equatorial wind stress that acts to cool the equatorial source waters; and 2) an excessive SST contrast between the ECT and off-equator areas, which boosts the equatorward heat transport by TIWs. FLOR’s strong advective cooling at the SML base is compensated by strong downward diffusion of heat out of the SML, which then allows FLOR’s ECT to take up a realistic heat flux from the atmosphere. Correcting FLOR’s climatological SST and wind stress biases via flux adjustment (FA) leads to weaker deep advective cooling of the ECT, which then erodes the upper-ocean thermal stratification, enhances vertical mixing, and excessively deepens the thermocline. FA does strengthen FLOR’s meridional shear of the zonal currents in the east Pacific, but this does not amplify either the simulated TIWs or their equatorward heat transport, likely due to FLOR’s coarse zonal ocean resolution. The analysis suggests that to advance coupled simulations of the ECT, improved winds and surface heat fluxes must go hand in hand with improved subseasonal and parameterized ocean processes. Implications for model development and the tropical Pacific observing system are discussed.

Full access
Sulagna Ray
,
Andrew T. Wittenberg
,
Stephen M. Griffies
, and
Fanrong Zeng

Abstract

The Pacific equatorial cold tongue plays a leading role in Earth’s strongest and most predictable climate signals. To illuminate the processes governing cold tongue temperatures, the upper-ocean heat budget is explored using the GFDL-FLOR coupled GCM (the forecast-oriented low ocean resolution version of CM2.5). Starting from the exact temperature budget for layers of time-varying thickness, the layer temperature tendency terms are studied using hourly-, daily-, and monthly-mean output from a 30-yr simulation driven by present-day radiative forcings. The budget is then applied to 1) a surface mixed layer whose temperature is highly correlated with SST, in which the air–sea heat flux is balanced mainly by downward diffusion of heat across the layer base, and 2) a thicker advective layer that subsumes most of the vertical mixing, in which the air–sea heat flux is balanced mainly by monthly-scale advection. The surface warming from shortwave fluxes and submonthly meridional advection and the subsurface cooling from monthly vertical advection are both shown to be essential to maintain the cold tongue thermal stratification against the destratifying effects of vertical mixing. Although layer undulations strongly mediate the tendency terms on diurnal-to-interannual scales, the 30-yr-mean tendencies are found to be well summarized by analogous budgets developed for stationary but spatially varying layers. The results are used to derive practical simplifications of the exact budget, to support the analyses in Part II of this paper, and to facilitate broader application of heat budget analyses when evaluating and comparing climate simulations.

Full access