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Andrew M. Chiodi and D. E. Harrison

Abstract

The processes responsible for the onset of La Niña events have not received the same attention as those responsible for the onset of El Niño events, for which westerly wind events (WWEs) in the tropical Pacific have been identified as important contributors. Results here show that synoptic-scale surface easterly wind surges (EWSs) play an important role in the onset of La Niña events, akin to the role of WWEs in the onset of El Niño events. It is found that EWSs are a substantial component of zonal wind stress variance along the equatorial Pacific. Using reanalysis wind stress fields, validated against buoy measurements, 340 EWS events are identified between 1986 and 2012. Their distributions in space, time, and El Niño–Southern Oscillation (ENSO) state are described. About 150 EWSs occur during ENSO-neutral conditions, during the months associated with La Niña initiation and growth (April–December). Composites of changes in sea surface temperature anomaly (SSTA) following these ~150 events show statistically significant cooling (0.1°–0.4°C) along the oceanic waveguide that persists for 2–3 months following the EWSs. Experiments with EWS forcing of an ocean general circulation model show SSTA patterns like those in the observations. It is suggested that EWSs play an important role in the onset of La Niña waveguide surface cooling and deserve additional study.

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A. M. Chiodi and D. E. Harrison

Abstract

It is well known that some austral summertime subtropical Indian Ocean sea surface temperature (SST) variability correlates with rainfall over certain regions of Africa that depend on rainfall for their economic well-being. Recent studies have determined that this SST variability is at least partially driven by latent heat flux variability, but the mechanism has not been fully described. Here, the mechanism that drives this SST variability is reexamined using analyses of operational air–sea fluxes, ocean mixed layer modeling, and simple atmospheric boundary layer physics. The SST variability of interest is confirmed to be mainly driven by latent heat flux variability, which is shown, for the first time, to be mainly caused by near-surface humidity variability. This humidity variability is then shown to be fundamentally driven by the anomalous meridional advection of water vapor. The meridional wind anomalies of interest are subsequently found to occur when the subtropical atmospheric anticyclone is preferentially located toward one of the sides (east/west) of the basin.

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A. M. Chiodi and D. E. Harrison

Abstract

It is shown that space–time smoothed outgoing longwave radiation (OLR) indices of equatorial Pacific seasonal variability can give an interestingly different perspective on El Niño than is obtained from sea surface temperature (SST) indices or the Southern Oscillation index (SOI). In particular, the index defined by averaging over an eastern-central region exhibits strong event like character—more so than in any other El Niño–Southern Oscillation (ENSO) warm-phase index known to the authors. Although the historical record for OLR is much shorter than for SST or SOI, OLR offers a direct connection to anomalous atmospheric heating. It is suggested that the years identified as events by this OLR index deserve particular recognition; and it is noteworthy that they all meet the criteria for “El Niño” years. Other years, whose warm-ENSO status differs depending upon the index favored, are not particularly distinctive from an OLR perspective, and a case could be made that either the other years do not deserve special classification or that they should be identified as different from the OLR-distinguished El Niño years.

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Gabriel A. Vecchi and D. E. Harrison

Abstract

The Indian southwest monsoon directly affects the lives of over one billion people, providing almost 90% of the annual precipitation to the Indian subcontinent. An important characteristic of the southwest monsoon is variability on subseasonal timescales, with “active” periods of heavy rain interrupted by drier “break” periods. Both the number of monsoon breaks in a season and the timing of these breaks profoundly impact agricultural output from the Indian subcontinent. Most research on monsoon breaks has emphasized possible atmospheric mechanisms. However, new satellite data reveal large-amplitude basin-scale subseasonal sea surface temperature (SST) variability in the Bay of Bengal (BoB), in which northern BoB cooling precedes monsoon breaks by about 1 week. The relationship is statistically significant at the 95% level over the 3 yr examined, and so offers a potential statistical predictor for short-term monsoon variability. The basinwide averaged amplitude of SST changes is 1°–2°C and local changes can exceed 3°C over 2 weeks; these changes are as large as those seen in the local climatological seasonal cycle. This raises the possibility that air–sea interaction may be a significant factor in monsoon variability; the SST variability is coherent with monsoon variability with a phase relationship consistent with a coupled oscillation. A schematic coupled air–sea oscillator mechanism is offered for further study, in which oceanic changes play a dynamical role in monsoon variability.

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Andrew M. Chiodi and D. E. Harrison

Abstract

The fundamental importance of near-equatorial zonal wind stress in the evolution of the tropical Pacific Ocean’s seasonal cycle and El Niño–Southern Oscillation (ENSO) events is well known. It has been two decades since the TAO/TRITON buoy array was deployed, in part to provide accurate surface wind observations across the Pacific waveguide. It is timely to revisit the impact of TAO/TRITON winds on our ability to simulate and thereby understand the evolution of sea surface temperature (SST) in this region. This work shows that forced ocean model simulations of SST anomalies (SSTAs) during the periods with a reasonably high buoy data return rate can reproduce the major elements of SSTA variability during ENSO events using a wind stress field computed from TAO/TRITON observations only. This demonstrates that the buoy array usefully fulfills its waveguide-wind-measurement purpose. Comparison of several reanalysis wind fields commonly used in recent ENSO studies with the TAO/TRITON observations reveals substantial biases in the reanalyses that cause substantial errors in the variability and trends of the reanalysis-forced SST simulations. In particular, the negative trend in ERA-Interim is much larger and the NCEP–NCAR Reanalysis-1 and NCEP–DOE Reanalysis-2 variability much less than seen in the TAO/TRITON wind observations. There are also mean biases. Thus, even with the TAO/TRITON observations available for assimilation into these wind products, there remain oceanically important differences. The reanalyses would be much more useful for ENSO and tropical Pacific climate change study if they would more effectively assimilate the TAO/TRITON observations.

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Narasimhan K. Larkin and D. E. Harrison

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Previous studies by the authors have described the composite global marine surface anomalies of ENSO warm (El Niño) events and cold (La Niña) events. Here the similarities and differences in these life cycles are examined. Qualitatively different behavior between warm events and cold events exists in the tropical Indian and Atlantic Oceans and in the extratropical Pacific. Even in the tropical Pacific statistically significantly different behavior is found in some variables for particular regions and phases of the life cycles. A single-mode regression analysis of the ENSO signal is done; the patterns are very similar to those of previously published ENSO EOF and regression analyses. The authors describe how the regression patterns obscure many of the interesting life cycles and life cycle differences of cold events and warm events. Most of the regression structures outside of the tropical Pacific are not statistically significant because of such differences. ENSO models should be evaluated against their ability to reproduce the observed cold event and warm event life cycles and not just single EOF or regression mode patterns.

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Andrew M. Chiodi and D. E. Harrison

Abstract

The unexpected halt of warm sea surface temperature anomaly (SSTA) growth in 2014 and development of a major El Niño in 2015 has drawn attention to our ability to understand and predict El Niño development. Wind stress–forced ocean model studies have satisfactorily reproduced observed equatorial Pacific SSTAs during periods when data return from the TAO/TRITON buoy network was high. Unfortunately, TAO/TRITON data return in 2014 was poor. To study 2014 SSTA development, the observed wind gaps must be filled. The hypothesis that subseasonal wind events provided the dominant driver of observed waveguide SSTA development in 2014 and 2015 is used along with the available buoy winds to construct an oceanic waveguide-wide surface stress field of westerly wind events (WWEs) and easterly wind surges (EWSs). It is found that the observed Niño-3.4 SSTA development in 2014 and 2015 can thereby be reproduced satisfactorily. Previous 2014 studies used other wind fields and reached differing conclusions about the importance of WWEs and EWSs. Experiment results herein help explain these inconsistencies, and clarify the relative importance of WWEs and EWSs. It is found that the springtime surplus of WWEs and summertime balance between WWEs and EWSs (yielding small net wind stress anomaly) accounts for the early development and midyear reversal of El Niño–like SSTA development in 2014. A strong abundance of WWEs in 2015 accounts for the rapid SSTA warming observed then. Accurately forecasting equatorial Pacific SSTA in years like 2014 and 2015 may require learning to predict WWE and EWS occurrence characteristics.

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D. E. Harrison and Gabriel A. Vecchi

Abstract

Based on examination of 10 yr of 10-m winds and wind anomalies from European Centre for Medium-Range Weather Forecasts (ECWMF) analysis, definitions for westerly wind events (WWEs) of eight different types are proposed. The authors construct a composite for each type of event, show that a simple propagating Gaussian model satisfactorily describes the evolution of zonal wind anomaly for each type of event, and determine the scales of each composite event by fitting the model to each composite. The authors discuss the WWEs that occurred during the Tropical Oceans Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) intensive observing period (IOP) and show the extent to which these composite events are able to reproduce the major westerly wind features of the IOP. The frequency of occurrence of each type of WWE for each year of this record and by calendar month are described; the authors find several types of events are negatively correlated with the annual mean troup Southern Oscillation index (SOI), and that the stronger WWEs often have a statistically significant seasonality. Several instances of widespread westerly wind anomaly are identified and described, but these “mega”-WWEs have few features in common. Although the authors’ composites underestimate the peak amplitude of many WWEs and cannot always accurately represent the time evolution of each WWE, the authors believe that they offer a useful framework for representing the sort of westerly wind variability that occurs in the western and central tropical Pacific and can provide a basis for further study of the importance of such winds in the climatological and interannual variability of this part of the World Ocean.

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Narasimhan K. Larkin and D. E. Harrison

Abstract

ENSO cold (La Niña) events are shown to exhibit a distinctive life cycle. The first near-global description of ENSO cold (La Niña)–event anomaly features is described using ocean surface data. It is found that cold-event anomalies are not simply the mirror image of warm (El Niño) events. The Comprehensive Ocean–Atmosphere Data Set marine surface record [SST, sea level pressure (SLP), and wind] is used to identify the statistically significant features of the nine cold-event periods during 1946–95 and to focus on the large-scale elements that are typical of most events. By examining time series, the most robust features of the composite that have occurred during nearly all of the post–World War II cold events are identified.

These robust cold-event features are more numerous and cover more of the globe than their warm-event counterparts. Of the 90 composite features examined, 57 (63%) are found to be robust. Most of these are located in the Tropics (70%) and in the Pacific (65%). However, robust elements are found in all the ocean basins (Indian—14%; Atlantic—21%) and in both hemispheres (Northern—18%; Southern—12%), making cold events truly global. In addition, a true life cycle for the cold event is found, with different anomalies occurring at different phases of the evolution of the event and not just during the peak (largest amplitude) phase. The evolution and simulation of these characteristic features of cold events offer as important a challenge to coupled models as the more familiar warm-event anomalies.

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Andrew M. Chiodi and D. E. Harrison

Abstract

El Niño–Southern Oscillation (ENSO) events are associated with particular seasonal weather anomalies in many regions around the planet. When the statistical links are sufficiently strong, ENSO state information can provide useful seasonal forecasts with varying lead times. However, using conventional sea surface temperature or sea level pressure indices to characterize ENSO state leads to many instances of limited forecast skill (e.g., years identified as El Niño or La Niña with weather anomalies unlike the average), even in regions where there is considerable ENSO-associated anomaly, on average. Using outgoing longwave radiation (OLR) conditions to characterize ENSO state identifies a subset of the conventional ENSO years, called OLR El Niño and OLR La Niña years herein. Treating the OLR-identified subset of years differently can both usefully strengthen the level of statistical significance in the average (composite) and also greatly reduce the year-to-year deviations in the composite precipitation anomalies. On average, over most of the planet, the non-OLR El Niño and non-OLR La Niña years have much more limited statistical utility for precipitation. The OLR El Niño and OLR La Niña indices typically identify years in time to be of use to boreal wintertime and later seasonal forecasting efforts, meaning that paying attention to tropical Pacific OLR conditions may offer more than just a diagnostic tool. Understanding better how large-scale environmental conditions during ENSO events determine OLR behavior (and deep atmospheric convection) will lead to improved seasonal precipitation forecasts for many areas.

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