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D. E. Harrison

Abstract

Multidecadel time series of surface winds from central tropical Pacific islands are used to compute trends in the trade winds between the end of WWII and 1985. Over this period, averaged over the whole region, there is no statistically significant trend in speed or zonal or meridional wind (or pseudostress). However, there is some tendency, within a few degrees of the equator, toward weakening of the easterlies and increased meridional flow toward the equator. Anomalous conditions subsequent to the 1972–73 ENSO event make a considerable contribution to the long-term trends. The period 1974–80 has been noted previously to have been anomalous, and trends over that period are sharply greater than those over the longer records.

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D. E. Harrison

Abstract

Using a version of the global surface marine observation historical data set, a new 1° spatial resolution global ocean surface wind stress climatology has been evaluated using the Large and Pond surface drag coefficient formulation. These new results are compared, after spatial smoothing, with those of Hellerman and Rosenstein, who used a different drag coefficient form. It is found that the new stresses are almost everywhere smaller than those of Hellerman and Rosenstein, often by 20%–30%, which is greater than the formal error estimates from their calculations. The stress differences show large-scale spatial structure, as would he expected given the spatial variation of the surface stability parameter and the known different wind variability regions. Basin zonally averaged Ekman transports are computed to provide perspective on the significance of the stress differences; annual mean differences can exceed 10 Sv (Sv = 106 m3 s−1) equatorward of 20° lat, but are smaller poleward. Wind stress curl and Sverdrup transport calculations provide a different perspective on the differences; particularly noticeable differences are found in the regions of the Gulf Stream and Kuroshio separation. Large annual variations in midlatitude wind stress curl suggest that study of the forced response at annual periods should be of interest.

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D. E. Harrison

Abstract

Several primitive-equation ocean general circulation model experiments have been carried out in order to explore the sensitivity of equatorial sea surface temperature (SST) results to uncertainty in the net surface heal flux (Q) imposed at the surface. Both climatological seasonal cycle experiments and hindcasts of the 1982/33 ENSO event are considered. It is found that regions of light winds, which typically reach values of SST in excess of 31°C using this ocean model and past Q parameterizations, attain more realistic SST values of 29°–30°C when Q is reduced by as little as 10 W m−2. Sensitivity in this regime is about 0.1–0.2°C (W m−2)−1 for low-frequency SST changes. In regions of easterly winds with their associated upwelling, horizontal advection, and stronger mixing, changes of Q in excess of 50 W m−2 produce SST changes typically of 0.7°C, for a sensitivity of about 0.02°C (W m−2)−1. These results apply equally well to the ENSO hindcasts and the seasonal cycle studies. The reasons for the large variation in sensitivity and the very large sensitivity under light winds are described. To the extent that these results are representative of oceanic conditions, very accurate Q information will be required for studies of the low-frequency variability of SST in light wind regions like the western Pacific; much less accurate fluxes appear needed for studies of comparable variability in upwelling regions.

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Gabriel A. Vecchi
and
D. E. Harrison

Abstract

The 1997–98 El Niño was both unusually strong and terminated unusually. Warm eastern equatorial Pacific (EEqP) sea surface temperature anomalies (SSTAs) exceeded 4°C at the event peak and lasted well into boreal spring of 1998, even though subsurface temperatures began cooling in December 1997. The oceanic processes that controlled this unusual termination are explored here and can be characterized by three features: (i) eastward propagating equatorial Pacific thermocline (Z tc) shoaling beginning in the central Pacific in November 1997; (ii) persistent warm EEqP SSTA between December 1997 and May 1998, despite strong EEqP Z tc shoaling (and subsurface cooling); and (iii) an abrupt cooling of EEqP SSTA in early May 1998 that exceeded 4°C within two weeks.

It is shown here that these changes can be understood in terms of the oceanic response to changes to the meridional structure of the near-equatorial zonal wind field. Equatorial near-date-line westerly wind anomalies greatly decreased in late 1997, associated with a southward shift of convective and wind anomalies. In the EEqP, equatorial easterlies disappeared (reappeared) in late January (early May) 1998, driving the springtime extension (abrupt termination) of this El Niño event. The authors suggest that the wind changes arise from fundamentally meridional processes and are tied to the annual cycle of insolation.

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D. E. Harrison
and
Narasimhan K. Larkin

Abstract

Using COADS data for the period 1946–1993, the near-global sea level pressure (SLP) patterns associated with interannual variability and the climatological seasonal march are discussed. A particular focus concerns the patterns associated with the two years before and after the South American sea surface temperatures rise (El Niño). The ten El Niño events in this record are composited, and the robustness of the features of this composite is tested.

Many features of the composite are quite robust; they occur during most El Niño events and are infrequent during non-El Niño periods. The most robust feature is an area of negative SLP anomaly (SLPA) in the eastern equatorial Pacific during Year(0) of the composite. This feature exceeds significance thresholds during every El Niño year and never during non-El Niño years; it correlates better with central Pacific SST variability than does the SOI. A west-central North Pacific positive SLPA, occurring late in Year(0) and lasting into the spring of year (+1) is the second most robust feature. Strong SLPA signals occur in the eastern South Pacific and around Australia in many events, but the behavior varies greatly from event to event. Some events show interesting signals in the Indian and Atlantic Oceans, but the behavior is not sufficiently general to be a statistically meaningful element of the composite.

The largest signals in the composite occur in the eastern equatorial and west-central North Pacific and not in the Southern Hemisphere. Thus, the large-scale SLP variations associated with El Niño periods are not dominated by the classical Southern Oscillation. Little evidence is found for phase propagation of the signal in El Niflo years. Although several features of the composite occur during the same season in each El Niño period, so that the main signals are “phase locked” to the seasonal cycle, the patterns of variability have little in common with the patterns of the seasonal march of SLP.

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Andrew M. Chiodi
and
D. E. Harrison

Abstract

The tropical Pacific moored-buoy array spacing was based on wind coherence scales observed from low-lying islands in the western-central tropical Pacific. Since the array was deployed across the full basin in the mid-1990s, winds from the array have proven critical to accurately monitoring for decadal-scale changes in tropical Pacific winds and identifying spurious trends in wind analysis products used to monitor for long-term change. The array observations have also greatly advanced our ability to diagnostically model (hindcast) and thereby better understand the observed development of central Pacific sea surface temperature anomaly development associated with El Niño and La Niña events, although the eastern equatorial Pacific is not yet accurately hindcast. The original array-design assumptions that the statistics calculated from the western-central Pacific island records are representative of open-ocean conditions and other regions of the tropical Pacific have not been thoroughly reexamined. We revisit these assumptions using the basinwide wind observations provided by the array and find that key wind statistics change across the tropical Pacific basin in ways that could not be determined from the original island wind study. The island results provided a best-case answer for mooring zonal spacing with minimally redundant coherence between adjacent buoys. Buoy-observed meridional coherence scales are longer than determined from the islands. Enhanced zonal sampling east of 140°W and west of 180° is needed to obtain minimal redundancy (optimal spacing). Reduced meridional sampling could still yield minimal redundancy for wind and wind stress fields over the ocean waveguide.

Open access
Mark Carson
and
D. E. Harrison

Abstract

Regional interdecadal variability, on subbasin to basin scales, is shown to be a robust feature of the post–World War II (WWII) historical temperature record, even after a recently proposed bias correction to XBT fall rates is applied. This study shows that the previously reported strong regional variability is generally unaffected by this correction, even though the interdecadal variability in the most recently published estimates of global ocean heat content is much reduced after a correction is applied. Following methods used in previous trend analysis work, estimates of interdecadal trends are calculated for individual regions of the global ocean where there are sufficient data. Spatial maps of temperature trends for the surface and three subsurface depths (50, 100, and 300 m) are presented, with both bias-corrected and uncorrected data trends at 100 and 300 m shown for comparison. In the upper two depths and at the surface, interdecadal variability is shown to be present and strong in most of the analysis regions. At 100 m, the differences between trends based on bias-corrected versus uncorrected data are small, and barely distinguishable for much of the ocean analyzed. There are more differences at 300 m between the two data treatments, but large-scale patterns are still present in the bias-corrected trends, especially where the trends are stronger.

Given the sampling issues discussed in previous works, the presence of strong interdecadal variability on smaller scales raises concerns that global interdecadal variability in the historical record still may not be properly resolved.

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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
Andrew M. Chiodi

Abstract

El Niño and La Niña seasonal weather anomaly associations provide a useful basis for winter forecasting over the North American regions where they are sufficiently strong in amplitude and consistent in character from one event to another. When the associations during La Niña are different than El Niño, however, the obvious quasi-linear-statistical approach to modeling them has serious shortcomings. The linear approach of L’Heureux et al. is critiqued here based on observed land surface temperature and tropospheric circulation associations over North America. The La Niña associations are quite different in pattern from their El Niño counterparts. The El Niño associations dominate the statistics. This causes the linear approach to produce results that are inconsistent with the observed La Niña–averaged associations. Further, nearly all the useful North American associations have been contributed by the subset of El Niño and La Niña years that are identifiable by an outgoing longwave radiation (OLR) El Niño index and a distinct OLR La Niña index. The remaining “non-OLR events” exhibit winter weather anomalies with large event-to-event variability and contribute very little statistical utility to the composites. The result is that the linear analysis framework is sufficiently unable to fit the observations as to question its utility for studying La Niña and El Niño seasonal temperature and atmospheric circulation relationships. An OLR-event based approach that treats La Niña and El Niño separately is significantly more consistent with, and offers an improved statistical model for, the observed relationships.

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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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