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D. E. Harrison, B. S. Giese, and E. S. Sarachik

Abstract

Four different datasets of monthly mean new-equatorial Pacific sea surface temperature for 1982–83 are compared, and the space-time regions for which there was consensus that cooling or warming took place, are determined. There was consensus that warming took place east of the date line, averaged over the period July-December 1982, and that the warming progressed eastward from the central Pacific. There was also consensus that weak cooling took place in April 1983, and that substantial cooling occurred in June-July 1983, generally over the central and eastern Pacific. However, the analyses tend to agree on the sign of SST change only in periods of cooling or warming in excess of 1°C/month; quantitative agreement at the level of 0.5°C/month or better is almost never found.

SST changes in five ocean-circulation model hindcasts of the 1982–83 period (differing only in that each used a different analyzed monthly mean surface wind stress field to drive the ocean), are compared with the observations and with each other. There is agreement that net warming occurred in the July-December 1982 period and cooling in mid-1983. The heat budgets of these experiments indicate that the major model central Pacific warmings occurred primarily from anomalous eastward surface advection of warm water. Further, east zonal advection remains significant but a diminished cooling tendency from meridional advection can also be important; different hindcasts differ on the relative importance of these terms. Surface heat flux changes do not contribute to the warmings. The reduced cooling tendency from meridional advection is consistent with diminished surface Ekman divergence, suggesting that southward transport of warm north equatorial counter current water was not a major factor in the model warmings. The hindcasts do not agree on the relative importance of local or remote forcing of the eastward surface currents; while there is clear evidence of remote forcing in some hindcasts in particular regions, local forcing is also often significant. The main 1983 midocean cooling began because of increased vertical advection of cool water; but once cooling began horizontal advection often contributed. Further east, where the easterlies generally return later than they do in midocean, upwelling and horizontal advection all can be important. Again no model consensus exists concerning the details of SST evolution.

Because the observations do not agree on the sign of SST change during much of the 1982–83 period, improved SST data is needed in order to document the behavior of the ocean through future ENSO periods. Better forcing data will be needed to carry out improved ocean-model validation studies, and to explore the mechanisms likely responsible for SST change through entire ENSO cycles.

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D. E. Harrison, William S. Kessler, and Benjamin S. Giese

Abstract

Five different analyses of 1982–83 monthly average surface wind stress fields have been used to force an ocean general circulation model of the tropical Pacific, in a series of El Niño hindcast experiments, like the one reported by Philander and Seigel. Although there were prominent common departures from climatology in the surface wind stress field during 1982–83 according to each wind analysis, there are also very substantial differences between analyses. This study was done to investigate the sensitivity of such hindcasts to our uncertain knowledge of the surface wind stress field. We concentrate here on the behavior along the Pacific ship-of-opportunity tracks.

According to the ship-of-opportunity XBT data, the ocean underwent major changes during this period. The vertical temperature gradients and mixed layer temperatures, as well as the depth of the thermocline, underwent substantial changes. There were also major changes in the geostrophic flow of the major current systems, as revealed by upper ocean dynamic height differences. Comparing the hindcasts with observations, we find that the gross large-scale changes of the ENSO event—surface warming in the second half of 1982, continued warmth into 1983 and cooling in mid-1983, together with major thermocline depth changes—are found in each hindcast. However, major quantitative differences exist between each hindcast and the observations in at least some region for some time and some variable.

Within the waveguide, dynamic height changes generally are hindcast with quantitative skill using each wind stress field and the best hindcasts differ from the observations by only a few dyn-cm more than the estimated uncertainty in the observations. Such hindcast skill is unlikely to be fortuitous: evidently the major elements of the waveguide variability are forced by the 1982–83 surface wind stress field rather than evolving out of some aspect of the state of the ocean during late 1981. Sea surface temperature changes are generally hindcast with qualitative skill, but rms errors of 2–3°C are frequent. Subsurface temperature variability skill varies with hindcast, location and depth; skill is greatest in the thermocline.

Outside the waveguide, hindcast skill tends to be reduced, and varies greatly with location and hindcast. Quantitative hindcast skill is found near 10°S and 10°N in some hindcasts in the WP, and near 10°S in most hindcasts in the CP, but there is never quantitative skill in the NECC region. The most striking inconsistency found involves the behavior of the NMC hindcast in the region of the North Equatorial Counter Current. Wind stress curl-forced Ekman pumping appears to be a significant factor in the variations in the more successful hindcasts.

In almost every comparison, the range of hindcast results brackets the observations, suggesting that the model physics is plausible. Overall, the special research effort wind fields produced better dynamic height results than did the operational wind product fields, but the operational fields produced generally better waveguide SST results. Improved knowledge of the surface wind stress field (and its curl) is a minimum requirement if we are to assess more critically model performance, and to identify needed model improvements.

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Andrew M. Chiodi, D. E. Harrison, and Gabriel A. Vecchi

Abstract

Westerly wind events (WWEs) have previously been shown to initiate equatorial Pacific waveguide warming. The relationship between WWEs and Madden–Julian oscillation (MJO) activity, as well as the role of MJO events in initiating waveguide warming, is reconsidered here over the 1986–2010 period. WWEs are identified in observations of near-surface zonal winds using an objective scheme. MJO events are defined using a widely used index, and 64 are identified that occur when the El Niño–Southern Oscillation (ENSO) is in its neutral state. Of these MJO events, 43 have one or more embedded WWEs and 21 do not. The evolution of sea surface temperature anomaly over the equatorial Pacific waveguide following the westerly surface wind phase of the MJO over the western equatorial Pacific is examined. Waveguide warming is found for the MJO with WWE events in similar magnitudes as following the WWEs not embedded in an MJO. There is very little statistically significant waveguide warming following MJO events that do not contain an embedded WWE. The observed SST anomaly changes are well reproduced in an ocean general circulation model forced with the respective composite wind stress anomalies. Further, it is found that the occurrence of an MJO event does not significantly affect the likelihood that a WWE will occur. These results extend and confirm the earlier results of Vecchi with a near doubling of the period of study. It is suggested that understanding the sources and predictability of tropical Pacific westerly wind events remains essential to improving predictions of the onset of El Niño events.

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D. E. Harrison, W. J. Emergy, J. P. Dugan, and Bo-Cheng Li

Abstract

We discuss the spatial character of mesoscale temperature variability as observed in six midlatitude multiship expendable bathythermograph (XBT) surveys, two from the North Pacific and four from the North Atlantic. These surveys sample mid-ocean regions as well as regions adjacent to the western boundary currents of both subtropical gyres. Mesoscale characteristics exhibit substantial changes between surveys, between ship tracks within each survey and between different portions of individual ship tracks. These results suggest that a single XBT section across a region is unlikely to represent the range of variability within that region satisfactorily.

Fluctuations below the mixed layer are strongly vertically correlated, but surface temperatures are not well correlated with those below the mixed layer. We concentrate on the horizontal variability of the temperature at 450 m, and present rms thermal variability values and zonal autocorrelation functions as conventional measures of the variability. We also discuss the utility of analysis perspectives that focus on the “event-like” character of much of the variability and describe the properties of the different features in the data. Using climatology rather than a linear trend to remove the large scale can result in markedly different feature characteristics and survey statistics. In particular, warm and cold features (defined relative to climatology) often have quite different scales. Assuming that the mesoscale temperature variability is a Guassian process with 150 km decorrelation length is acceptable at the 95% level in several surveys using linearly detrended data, but is generally unacceptable when climatology is removed.

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G. L. Potter, R. D. Cess, P. Minnis, E. F. Harrison, and V. Ramanathan

Abstract

This study addresses two aspects of the planetary albedo's diurnal cycle, the first of which refers to directional models of the planetary albodo. It is found that even for clear regions there appear to be deficiencies in our knowledge of how to model this quantity. Over land surfaces, for example, Nimbus-7 data for the directional planetary albedo compare best with model calculations for which a Lambertian surface is assumed, despite ample evidence that the albedo of land surfaces is dependent upon solar zenith angle. Similarly, over ocean surfaces both GOES and Nimbus-7 data produce a weaker dependence of the planetary albedo upon solar zenith angle than would be suggested by model calculations.

The second aspect of the study concerns a comparison of the diurnal amplitude factor, defined as the ratio of the diurnally averaged planetary albedo to that at noon, between two general circulation models (GCMs) and measurements made from a geostationary satellite (GOES). While these comparisons indicate reasonable consistency between the GCMs and the satellite measurements, this is due in part to compensating differences, such as an underestimate in cloud amount by a GCM being compensated for by a corresponding underestimate of the diurnal amplitude factor for overcast regions. The comparisons further underscore difficulties associated with converting local-time albedo measurements, as made from sun-synchronous satellites, to diurnally averaged albedos.

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William S. Kessler, M. C. Spillane, Michael J. McPhaden, and D. E. Harrison

Abstract

The highly temporally resolved time series from the Tropical Atmosphere-Ocean moored buoy array are used to evaluate the scales of thermal variability in the upper equatorial Pacific. The TAO array consists of nearly 70 deep-ocean moorings arranged nominally 15° longitude and 2°–3° latitude apart across the equatorial Pacific. The bulk of the data from the array consists of daily averages telemetered in real time, with some records up to 15 years long. However, at several sites more finely resolved data exist, in some cases with resolution of 1 minute. These data form the basis for spectral decomposition spanning virtually all scales of variability from the Brunt-Väiälä frequency to the El Niño-Southern Oscillation timescale. The spectra are used to define the signal to noise ratio as a function of sample rate and frequency, and to investigate the effects of aliasing that results from sparser sampling, such as ship-based observational techniques. The results show that the signal to noise ratio is larger in the east, mostly because the low-frequency signals are larger there. The noise level for SST varies by as much as a factor of 10 among the locations studied, while noise in thermocline depth is relatively more homogeneous over the region. In general, noise due to aliased high-frequency variability increases by roughly a factor of 10 as the sample rate decreases from daily to 100-day sampling. The highly resolved spectra suggest a somewhat more optimistic estimate of overall signal-to-noise ratios for typical ship of opportunity (VOS) XBT sampling (generally about 2) than had been found in previous studies using sparser data. Time scales were estimated for various filtered versions of the time series by integration of the autocorrelation functions. For high-passed data (periods longer than about 150 days removed), the timescale is about 5 days for both surface and subsurface temperatures everywhere in the region. Conversely, for low-passed data (the annual cycle and periods shorter than 150 days removed), the timescale is roughly 100 days. Horizontal space scales were estimated from cross-correlations among the buoys. Zonal scales of low-frequency SST variations along the equator were half the width of the Pacific, larger than those of thermocline depth (about 30°–40° longitude). In the cast, meridional scales of low-frequency SST were large (greater than about 15° latitude), associated with the coherent waxing and waning of the equatorial cold tongue, whereas in the west these scales were shorter. Thermocline depth variations had meridional scales associated with the equatorial waves, particularly in the east. Spatial scale estimates reported here are generally consistent with those found from the VOS datasets when the ENSO signals in the records of each dataset are taken into account. However, if signals with periods of 1 to 2 months are to be properly sampled, then sampling scales of 1°–2° latitude by 8°–10° longitude, with a 5-day timescale, are needed.

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A. R. Robinson, D. E. Harrison, Y. Mintz, and A. J. Semtner

Abstract

We present the results of a multi-level, constant depth, primitive equation general ocean numerical circulation simulation with mesoscale resolution. A single mid-latitude model gyre is driven by wind and heating. After 30 years of spin-up with a relatively coarse grid and large diffusion coefficients, the grid size and diffusion coefficients are reduced. The circulation then adjusts into a nonlinear and time-dependent flow with periods of tens of days and space scales of hundreds of kilometers. After a quasi-equilibrium state is achieved, two years of data are obtained which are separated into time-mean and time-dependent fluctuations, and analyzed. Dynamically distinct regions are intensified, momentum, heat and vorticity balances examined, and energy integrals calculated. Statistical measures of significance and of uncertainty are computed where possible. Eddy energy is produced primarily by Reynolds stress work (barotropic instability) on the mean circulation shear in the recirculation and near-field region of the northern current system. Mean fluctuation correlation terms are presented in some regions at order 1 in the mean heat and vorticity balance and can be the leading ageostrophic effect in the mean momentum balance. The flow is non-quasigeostrophic in some parts of the intense boundary currents.

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S. P. Harrison, D. Jolly, F. Laarif, A. Abe-Ouchi, B. Dong, K. Herterich, C. Hewitt, S. Joussaume, J. E. Kutzbach, J. Mitchell, N. de Noblet, and P. Valdes

Abstract

The response of ten atmospheric general circulation models to orbital forcing at 6 kyr BP has been investigated using the BIOME model, which predicts equilibrium vegetation distribution, as a diagnostic. Several common features emerge: (a) reduced tropical rain forest as a consequence of increased aridity in the equatorial zone, (b) expansion of moisture-demanding vegetation in the Old World subtropics as a consequence of the expansion of the Afro–Asian monsoon, (c) an increase in warm grass/shrub in the Northern Hemisphere continental interiors in response to warming and enhanced aridity, and (d) a northward shift in the tundra–forest boundary in response to a warmer growing season at high northern latitudes. These broadscale features are consistent from model to model, but there are differences in their expression at a regional scale. Vegetation changes associated with monsoon enhancement and high-latitude summer warming are consistent with palaeoenvironmental observations, but the simulated shifts in vegetation belts are too small in both cases. Vegetation changes due to warmer and more arid conditions in the midcontinents of the Northern Hemisphere are consistent with palaeoenvironmental data from North America, but data from Eurasia suggests conditions were wetter at 6 kyr BP than today. The models show quantitatively similar vegetation changes in the intertropical zone, and in the northern and southern extratropics. The small differences among models in the magnitude of the global vegetation response are not related to differences in global or zonal climate averages, but reflect differences in simulated regional features. Regional-scale analyses will therefore be necessary to identify the underlying causes of such differences among models.

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Christopher D. Karstens, James Correia Jr., Daphne S. LaDue, Jonathan Wolfe, Tiffany C. Meyer, David R. Harrison, John L. Cintineo, Kristin M. Calhoun, Travis M. Smith, Alan E. Gerard, and Lans P. Rothfusz

Abstract

Providing advance warning for impending severe convective weather events (i.e., tornadoes, hail, wind) fundamentally requires an ability to predict and/or detect these hazards and subsequently communicate their potential threat in real time. The National Weather Service (NWS) provides advance warning for severe convective weather through the issuance of tornado and severe thunderstorm warnings, a system that has remained relatively unchanged for approximately the past 65 years. Forecasting a Continuum of Environmental Threats (FACETs) proposes a reinvention of this system, transitioning from a deterministic product-centric paradigm to one based on probabilistic hazard information (PHI) for hazardous weather events. Four years of iterative development and rapid prototyping in the National Oceanic and Atmospheric Administration (NOAA) Hazardous Weather Testbed (HWT) with NWS forecasters and partners has yielded insights into this new paradigm by discovering efficient ways to generate, inform, and utilize a continuous flow of information through the development of a human–machine mix. Forecasters conditionally used automated object-based guidance within four levels of automation to issue deterministic products containing PHI. Forecasters accomplished this task in a timely manner while focusing on communication and conveying forecast confidence, elements considered necessary by emergency managers. Observed annual increases in the usage of first-guess probabilistic guidance by forecasters were related to improvements made to the prototyped software, guidance, and techniques. However, increasing usage of automation requires improvements in guidance, data integration, and data visualization to garner trust more effectively. Additional opportunities exist to address limitations in procedures for motion derivation and geospatial mapping of subjective probability.

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S. I. Bohnenstengel, S. E. Belcher, A. Aiken, J. D. Allan, G. Allen, A. Bacak, T. J. Bannan, J. F. Barlow, D. C. S. Beddows, W. J. Bloss, A. M. Booth, C. Chemel, O. Coceal, C. F. Di Marco, M. K. Dubey, K. H. Faloon, Z. L. Fleming, M. Furger, J. K. Gietl, R. R. Graves, D. C. Green, C. S. B. Grimmond, C. H. Halios, J. F. Hamilton, R. M. Harrison, M. R. Heal, D. E. Heard, C. Helfter, S. C. Herndon, R. E. Holmes, J. R. Hopkins, A. M. Jones, F. J. Kelly, S. Kotthaus, B. Langford, J. D. Lee, R. J. Leigh, A. C. Lewis, R. T. Lidster, F. D. Lopez-Hilfiker, J. B. McQuaid, C. Mohr, P. S. Monks, E. Nemitz, N. L. Ng, C. J. Percival, A. S. H. Prévôt, H. M. A. Ricketts, R. Sokhi, D. Stone, J. A. Thornton, A. H. Tremper, A. C. Valach, S. Visser, L. K. Whalley, L. R. Williams, L. Xu, D. E. Young, and P. Zotter

Abstract

Air quality and heat are strong health drivers, and their accurate assessment and forecast are important in densely populated urban areas. However, the sources and processes leading to high concentrations of main pollutants, such as ozone, nitrogen dioxide, and fine and coarse particulate matter, in complex urban areas are not fully understood, limiting our ability to forecast air quality accurately. This paper introduces the Clean Air for London (ClearfLo; www.clearflo.ac.uk) project’s interdisciplinary approach to investigate the processes leading to poor air quality and elevated temperatures.

Within ClearfLo, a large multi-institutional project funded by the U.K. Natural Environment Research Council (NERC), integrated measurements of meteorology and gaseous, and particulate composition/loading within the atmosphere of London, United Kingdom, were undertaken to understand the processes underlying poor air quality. Long-term measurement infrastructure installed at multiple levels (street and elevated), and at urban background, curbside, and rural locations were complemented with high-resolution numerical atmospheric simulations. Combining these (measurement–modeling) enhances understanding of seasonal variations in meteorology and composition together with the controlling processes. Two intensive observation periods (winter 2012 and the Summer Olympics of 2012) focus upon the vertical structure and evolution of the urban boundary layer; chemical controls on nitrogen dioxide and ozone production—in particular, the role of volatile organic compounds; and processes controlling the evolution, size, distribution, and composition of particulate matter. The paper shows that mixing heights are deeper over London than in the rural surroundings and that the seasonality of the urban boundary layer evolution controls when concentrations peak. The composition also reflects the seasonality of sources such as domestic burning and biogenic emissions.

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