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Ewan Crosbie and Yolande Serra

Abstract

The influence of the Madden–Julian oscillation (MJO) on synoptic-scale waves, important precursors to tropical cyclones and tropical cyclogenesis, is investigated using a regional model of the eastern North Pacific basin. Cyclogenesis frequency is evaluated with respect to the MJO using a combination of the regional model and the archive best-track data. The statistics of the regional model compare well to the data and suggest that the MJO predominantly deters cyclogenesis during the suppressed phase rather than enhances it during the active phase when compared to neutral conditions. Synoptic-scale variability of cloudiness and eddy kinetic energy and analysis of vortex track statistics show a consistent enhancement of wave strength during the active phases of the MJO supported by both barotropic energetics and tropospheric moisture availability. Vertical wind shear, of critical importance to cyclone development, also exhibits strong variability associated with the phase of the MJO but, contrary to synoptic activity and moisture, results in unfavorable conditions for cyclogenesis during the active phase of the MJO in the region. The result is for the MJO to enhance cyclogenesis frequency relative to neutral conditions during the early active phase but otherwise to result in suppressed activity. In addition to the basinwide longitudinal structure, the local MJO exhibits a distinct north–south structure not apparent in the global pattern but in agreement with recent studies of tropical eastern North Pacific subseasonal variability.

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Yolande L. Serra and Kerrie Geil

Abstract

The tracks of westward-propagating synoptic disturbances across the Intra-Americas Sea (IAS) and far-eastern Pacific, known as easterly waves or tropical depression (TD) waves, are an important feature of the region’s climate. They are associated with heavy rainfall events, seed the majority of tropical cyclones, and contribute to the mean rainfall across the region. This study examines the ability of current climate models (CMIP5) to simulate TD-wave activity and associated environmental factors across the IAS and far-eastern Pacific as compared to reanalysis. Model projections for the future are then compared with the historical model experiment to investigate the southward shift in CMIP5 track density and the environmental factors that may contribute to it. While historical biases in TD-wave track-density patterns are well correlated with model biases in sea surface temperature and midlevel moisture, the projected southward shift of the TD track density by the end of the twenty-first century in CMIP5 models is best correlated with changes in deep wind shear and midlevel moisture. In addition, the genesis potential index is found to be a good indicator of both present and future regions of high TD-wave track density for the models in this region. This last result may be useful for understanding the more complex relationship between tropical cyclones and this index in models found in other studies.

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Yolande L. Serra and Robert A. Houze Jr.

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Data obtained in the eastern Pacific intertropical convergence zone (ITCZ) during the Tropical Eastern Pacific Process Study (TEPPS) show a 3–6-day variability. The NOAA ship Ronald H. Brown collected surface meteorological observations, C-band Doppler radar volumes, atmospheric soundings, and rainfall data while on station at 7.8°N, 125°W from 8–23 August 1997. The 3–6-day variability was a prominent timescale in the meridional wind and humidity data. The maxima of the surface-to-700-mb meridional wind anomalies were 5–12 m s–1. Maxima of the moisture anomalies at the same levels were ∼1–3 g kg–1, in phase with the low-level southerlies, while upper-level moisture lagged the southerlies by less than a day. The radar indicated a close connection between precipitation in the vicinity of the ship and the meridional wind, with the precipitation occurring when the surface southerlies were strongest. The meridional and zonal wind components had a maximum of 850–450-mb wind shear during the southerly events. The vertical profiles of wind, humidity, and temperature in each of the events at 3–6-day intervals were generally consistent with theoretical studies of easterly waves.

Satellite infrared (IR) data indicated significant variance at 3–6-day periods within the region 0°–20°N, 75°–150°W for July–August 1997. The variability seen by satellite was most pronounced off the coast of South America and over the warm waters north of the equator, which mark the latitude of the ITCZ. Satellite-observed cloudiness exhibited wavelike organization with disturbed areas moving westward at ∼8 m s–1 (6°–7° day–1), with wavelengths of 2000–3000 km. Consistent with previous descriptions of easterly waves, a chain of low clouds and moisture formed inverted V patterns in visible and water-vapor channels of satellite imagery. Mesoscale convective systems formed along this chain. Some of these systems appeared to have been associated with low-pressure regions that eventually led to hurricanes. However, others did not spin off tropical storms, thus implying that their existence did not necessarily depend on these events. One set of mesoscale convective systems moved eastward through the 3–6-day westward-moving waves in conjunction with a Kelvin wave.

Surface meteorological data from the Woods Hole Oceanographic Institution's Improved Meteorological Instrumentation (IMET) buoy moored at 10°N, 125°W and the NOAA Tropical Atmosphere–Ocean (TOA) moored buoys in the eastern Pacific also indicated significant 3–6-day variability in the meridional wind component during August 1997. Spectral analysis of the buoy data were consistent with spectrally analyzed satellite IR data in showing a seasonal spatial pattern of 3–6-day variability with easterly waves most active during the July–September season. Together the ship, satellite, and buoy data show that the TEPPS ship data were collected in a large-scale environment that was characterized by an interference pattern of two wave types. A regular progression of easterly waves produced the dominant 3–6-day variability, which was modified during one time period by the passage of a Kelvin wave approaching the region from the west.

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Yolande L. Serra and Michael J. McPhaden

Abstract

In this study, the diurnal cycles in rain accumulation, intensity, and frequency are investigated for the 1997– 2001 time period using measurements from self-siphoning rain gauges on moored buoys within the tropical Pacific and Atlantic Oceans. These measurements are unique in that they provide in situ, quantitative information on both the amplitude and phase of diurnal variability in tropical oceanic precipitation over an extended period of time at selected locations. Results indicate that the diurnal and semidiurnal harmonics explain a significant portion of the diurnal variance for all three rainfall parameters at the buoys. The diurnal harmonic in particular dominates the composite diurnal cycle in hourly rain accumulation, with a maximum from 0400 to 0700 local time (LT) and a minimum around 1800 LT. An early morning maximum and evening minimum are also observed in the composite diurnal cycles of rain intensity and frequency, indicating that both are contributing to the diurnal cycle in accumulation. Afternoon maxima in accumulation are also observed at several locations and are generally associated with maxima in rain intensity. While there is considerable variation in the estimates of the diurnal cycle both seasonally and regionally (especially for intensity), the results are overall consistent with previous studies of the diurnal cycle in rainfall and tropical cloudiness.

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Yolande L. Serra and Michael J. McPhaden

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This study compares the Tropical Rainfall Measuring Mission (TRMM) microwave imager (TMI) and precipitation radar (PR) rainfall measurements to self-siphoning rain gauge data from 14 open-ocean buoys located in heavy-rain areas of the tropical Pacific and Atlantic Oceans. These 14 buoys are part of the Tropical Atmosphere–Ocean (TAO) array and Pilot Research Moored Array in the Tropical Atlantic (PIRATA). Differences between buoy and TRMM monthly and seasonal rainfall accumulations are calculated from satellite data within 0.1° × 0.1°–5.0° × 5.0° square areas centered on the buoys. Taking into account current best estimates of sampling and instrumental errors, mean differences between the buoy and TMI rainfall are not significant at the 95% confidence level, assuming no wind-induced undercatch by the buoy gauges. Mean differences between the buoy and PR monthly and seasonal accumulations for these spatial scales suggest that the PR underestimates these accumulations by about 30% in comparison with the buoys. If the buoy rain rates are corrected for wind-induced undercatch, TMI accumulations fall systematically and significantly below buoy values, with underestimates of up to 22% for both monthly and seasonal data. Also the PR underestimates, relative to wind-corrected buoy values, increase to up to 40% for both monthly and seasonal data. Regional and rain-rate dependencies of these comparisons are also investigated.

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Kerrie L. Geil, Yolande L. Serra, and Xubin Zeng

Abstract

Precipitation, geopotential height, and wind fields from 21 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are examined to determine how well this generation of general circulation models represents the North American monsoon system (NAMS). Results show no improvement since CMIP3 in the magnitude (root-mean-square error and bias) of the mean annual cycle of monthly precipitation over a core monsoon domain, but improvement in the phasing of the seasonal cycle in precipitation is notable. Monsoon onset is early for most models but is clearly visible in daily climatological precipitation, whereas monsoon retreat is highly variable and unclear in daily climatological precipitation. Models that best capture large-scale circulation patterns at a low level usually have realistic representations of the NAMS, but even the best models poorly represent monsoon retreat. Difficulty in reproducing monsoon retreat results from an inaccurate representation of gradients in low-level geopotential height across the larger region, which causes an unrealistic flux of low-level moisture from the tropics into the NAMS region that extends well into the postmonsoon season. Composites of the models with the best and worst representations of the NAMS indicate that adequate representation of the monsoon during the early to midseason can be achieved even with a large-scale circulation pattern bias, as long as the bias is spatially consistent over the larger region influencing monsoon development; in other words, as with monsoon retreat, it is the inaccuracy of the spatial gradients in geopotential height across the larger region that prevents some models from realistic representation of the early and midseason monsoon system.

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Yolande L. Serra, George N. Kiladis, and Meghan F. Cronin

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Outgoing longwave radiation (OLR) and low-level wind fields in the Atlantic and Pacific intertropical convergence zone (ITCZ) are dominated by variability on synoptic time scales primarily associated with easterly waves during boreal summer and fall. This study uses spectral filtering of observed OLR data to capture the convective variability coupled to Pacific easterly waves. Filtered OLR is then used as an independent variable to isolate easterly wave structure in wind, temperature, and humidity fields from open-ocean buoys, radiosondes, and gridded reanalysis products. The analysis shows that while some Pacific easterly waves originate in the Atlantic, most of the waves appear to form and strengthen within the Pacific. Pacific easterly waves have wavelengths of 4200–5900 km, westward phase speeds of 11.3–13.6 m s−1, and maximum meridional wind anomalies at about 600 hPa. A warm, moist boundary layer is observed ahead of the waves, with moisture lofted quickly through the troposphere by deep convection, followed by a cold, dry signal behind the wave. The waves are accompanied by substantial cloud forcing and surface latent heat flux fluctuations in buoy observations. In the central Pacific the horizontal structure of the waves appears as meridionally oriented inverted troughs, while in the east Pacific the waves are oriented southwest–northeast. Both are tilted slightly eastward with height. Although these tilts are consistent with adiabatic barotropic and baroclinic conversions to eddy energy, energetics calculations imply that Pacific easterly waves are driven primarily by convective heating. This differs from African easterly waves, where the barotropic and baroclinic conversions dominate.

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Sandra E. Yuter, Yolande L. Serra, and Robert A. Houze Jr.

An ad hoc experiment in the marine stratocumulus region to the west of Mexico was conducted from 29 August to 6 September 1997 as part of the Pan American Climate Studies Tropical Eastern Pacific Process Study cruise on the National Oceanic and Atmospheric Administration ship Ronald H. Brown after a medical emergency cut short the planned time in the eastern Pacific ITCZ. The joint variation of cloud structure, drizzle, and tropospheric stratification was documented by a combination of three hourly upper air soundings, scanning C-band radar, hourly cloud photography, and visual observation. The sensitive C-band Doppler radar mounted on the ship was able to obtain observations of drizzle cells with regions of greater than 10 dBZ of 2–3-km scale in the horizontal and peak reflectivities of greater than 25 dBZ.

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Yolande L. Serra, George N. Kiladis, and Kevin I. Hodges

Abstract

Easterly waves (EWs) are prominent features of the intertropical convergence zone (ITCZ), found in both the Atlantic and Pacific during the Northern Hemisphere summer and fall, where they commonly serve as precursors to hurricanes over both basins. A large proportion of Atlantic EWs are known to form over Africa, but the origin of EWs over the Caribbean and east Pacific in particular has not been established in detail. In this study reanalyses are used to examine the coherence of the large-scale wave signatures and to obtain track statistics and energy conversion terms for EWs across this region. Regression analysis demonstrates that some EW kinematic structures readily propagate between the Atlantic and east Pacific, with the highest correlations observed across Costa Rica and Panama. Track statistics are consistent with this analysis and suggest that some individual waves are maintained as they pass from the Atlantic into the east Pacific, whereas others are generated locally in the Caribbean and east Pacific. Vortex anomalies associated with the waves are observed on the leeward side of the Sierra Madre, propagating northwestward along the coast, consistent with previous modeling studies of the interactions between zonal flow and EWs with model topography similar to the Sierra Madre. An energetics analysis additionally indicates that the Caribbean low-level jet and its extension into the east Pacific—known as the Papagayo jet—are a source of energy for EWs in the region. Two case studies support these statistics, as well as demonstrate the modulation of EW track and storm development location by the MJO.

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John B. Mickett, Yolande L. Serra, Meghan F. Cronin, and Matthew H. Alford

Abstract

Westward-propagating atmospheric easterly waves contribute to much of the variability of the low-level wind fields within the northeast tropical Pacific. With the dominant period of these waves (3–5 days) close to the local inertial period (2.4 days at 12°N to 5.7 days at 5°N), there is the expectation that the associated winds may resonantly force mixed layer inertial motions in this region. The authors test this hypothesis using a simple slab model and roughly 4½ yr of wind data from four NOAA Tropical Atmosphere Ocean/Eastern Pacific Investigation of Climate Processes (TAO/EPIC) buoys along 95°W at 12°, 10°, 8°, and 5°N. The degree of resonance is determined by comparing model simulations using observed wind stress with simulations forced with reversed-rotation wind stress. Results strongly indicate that Pacific easterly waves (PEWs) resonantly force inertial motions in the region. This resonance shows both significant seasonality and latitudinal dependence that appears to be related to the meridional position and intensity of the PEWs. North of the zonal axis of the mean track of the PEWs, the low-level winds associated with the waves rotate predominantly clockwise with time and resonantly force mixed layer inertial motions. South of this axis, the winds rotate counterclockwise, resulting in dissonant (antiresonant) forcing. As this axis migrates annually from about 4°N during the boreal winter/spring to a maximum northerly position of about 8°–10°N in the late boreal summer/early fall, the region of strongest resonance follows, consistently remaining to its north. Model output suggests that resonant forcing results in roughly 10%–25% greater net annual flux of kinetic energy from the wind to mixed layer inertial motions than in neutral or nonresonant conditions. This finding has strong implications for mixed layer properties, air–sea coupling, and the generation of near-inertial internal waves.

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