Bradford S. Barrett

Links between Tropical Cyclone Activity and Madden–Julian Oscillation Phase in the North Atlantic and Northeast Pacific Basins

Abstract The leading intraseasonal mode of atmospheric and oceanic variability, the Madden–Julian oscillation (MJO), influences tropical and extratropical sea level pressure, temperature, divergent and rotational wind components, moisture, and deep convection. As a 40- to 50-day oscillation, the MJO is also known to influence tropical phenomena, including tropical cyclone (TC) activity in various TC basins. The links between the MJO and multiple measures of TC activity, including genesis, landfall, and an integrative accumulated cyclone energy (ACE) index, were quantified for multiple TC-formation basins across the Western Hemisphere, including the North Atlantic and northeast Pacific Ocean and subbasins, for the period 1978–2006. Using this relatively long (29 yr) TC dataset and employing an upper-tropospheric MJO diagnostic that is physically meaningful over the entire Western Hemisphere, this study extends existing research on the relationships between the MJO and TCs. The NOAA Climate Prediction Cente...

Effect of the Andes Cordillera on Precipitation from a Midlatitude Cold Front

The effects of the Andes Cordillera, the major mountain range in South America, on precipitation patterns of baroclinic systems approaching from the southeast Pacific remain largely unstudied. This study focuses on a case in late May 2008 when an upper-level trough and surface cold front produced widespread precipitation in central Chile. The primary goal was to analyze the physical mechanisms responsible for the structure and evolution of the precipitation. Weather Research and Forecasting (WRF) model simulations indicate that as an upper-level trough approached central Chile, midtropospheric flow below 700 hPa was blocked by the high topography and deflected poleward in the form of a barrier jet. This northerly jet had wind maxima in excess of 15 m s 21 , was centered around 925 hPa, and extended westward 200 km from the mountains. It intersected the cold front, which approached from the south near the coast, thereby increasing convergence along the frontal surface, slowing its equatorward progress, and enhancing rainfall over central Chile. Another separate region of heavy precipitation formed over the upwind slopes of the cordillera. A trajectory analysis confirmed that the barrier jet moved low-level parcels from their origin in the moist southeast Pacific boundary layer to the coast. When model topography was reduced to twenty percent of its original height, the cold front advanced more rapidly to the northeast, generated less precipitation in central Chile between 338 and 368S, and produced minimal orographic precipitation on the upwind Andean slopes. Based on these findings, the high topography appears responsible for not only orographic precipitation but also for substantially increasing precipitation totals over the central coast and valley.

Madden–Julian Oscillation (MJO) Modulation of Atmospheric Circulation and Chilean Winter Precipitation

AbstractThe leading intraseasonal mode of tropical atmospheric variability, the Madden–Julian oscillation (MJO), has been shown to modulate precipitation and circulation on a global and regional scale. Winter precipitation in Chile has been connected to a variety of synoptic-scale forcing mechanisms. This study explored the links between the two, first examining the intraseasonal variability of Chilean precipitation from surface gauges and the Tropical Rainfall Measuring Mission (TRMM) and then examining the variability of synoptic-scale circulation.Composites of precipitation, precipitation intensity, and lower-, middle-, and upper-tropospheric circulation were created using the Real-Time Multivariate MJO index, which divides the MJO into eight longitudinally based phases. Precipitation was found to vary across MJO phases, with positive precipitation anomalies in central and south-central Chile (30°–45°S) for MJO phases 8, 1, and 2, and negative anomalies in phases 3–7. Circulation was also found to vary...

Some Climatological Aspects of the Madden–Julian Oscillation (MJO)

AbstractOne of the most commonly used metrics for both locating the Madden–Julian oscillation (MJO) geographically and defining the intensity of MJO convective activity is the real-time multivariate MJO (RMM) index. However, a climatology of the MJO, particularly with respect to the frequency of activity levels or of consecutive days at certain activity thresholds, does not yet exist. Thus, several climatological aspects of the MJO were developed in this study: 1) annual and 2) seasonal variability in MJO intensity, quantified using four defined activity categories (inactive, active, very active, and extremely active); 3) persistence in the above-defined four categories; 4) cycle length; and 5) low-frequency (decadal) variability.On an annual basis, MJO phases 1 and 2 occurred more often, and phase 8 occurred less often, than the other phases throughout the year. Notable seasonality was also found, particularly in the frequency of extremely active MJO in March–May (8% of days) compared with June–August (o...

Arctic sea ice and the Madden-Julian Oscillation (MJO)

Arctic sea ice responds to atmospheric forcing in primarily a top-down manner, whereby near-surface air circulation and temperature govern motion, formation, melting, and accretion. As a result, concentrations of sea ice vary with phases of many of the major modes of atmospheric variability, including the North Atlantic Oscillation, the Arctic Oscillation, and the El Niño-Southern Oscillation. However, until this present study, variability of sea ice by phase of the leading mode of atmospheric intraseasonal variability, the Madden–Julian Oscillation (MJO), which has been found to modify Arctic circulation and temperature, remained largely unstudied. Anomalies in daily change in sea ice concentration were isolated for all phases of the real-time multivariate MJO index during both summer (May–July) and winter (November–January) months. The three principal findings of the current study were as follows. (1) The MJO projects onto the Arctic atmosphere, as evidenced by statistically significant wavy patterns and consistent anomaly sign changes in composites of surface and mid-tropospheric atmospheric fields. (2) The MJO modulates Arctic sea ice in both summer and winter seasons, with the region of greatest variability shifting with the migration of the ice margin poleward (equatorward) during the summer (winter) period. Active regions of coherent ice concentration variability were identified in the Atlantic sector on days when the MJO was in phases 4 and 7 and the Pacific sector on days when the MJO was in phases 2 and 6, all supported by corresponding anomalies in surface wind and temperature. During July, similar variability in sea ice concentration was found in the North Atlantic sector during MJO phases 2 and 6 and Siberian sector during MJO phases 1 and 5, also supported by corresponding anomalies in surface wind. (3) The MJO modulates Arctic sea ice regionally, often resulting in dipole-shaped patterns of variability between anomaly centers. These results provide an important first look at intraseasonal variability of sea ice in the Arctic.

Extreme temperature and precipitation events in March 2015 in central and northern Chile

From 18 to 27 March 2015, northern, central, and southern Chile experienced a series of extreme hydrometeorological events. First, the highest surface air temperature ever recorded in Santiago (with reliable records dating to 1877), 36.8°C at Quinta Normal, was measured at 15:47 local time on 20 March 2015. Immediately following this high heat event, an extreme precipitation event, with damaging streamflows from precipitation totals greater than 45 mm, occurred in the semiarid and hyperarid Atacama regions. Finally, concurrent with the heavy precipitation event, extremely warm temperatures were recorded throughout southern Chile. These events were examined from a synoptic perspective with the goal of identifying forcing mechanisms and potential interaction between each analysis which provides operational context by which to identify and predict similar events in the future. Primary findings were as follows: (1) record warm temperatures in central Chile resulted from anomalous lower troposphere ridging and easterly downslope flow, both of which developed in response to an anomalous midtroposphere ridge-trough pattern; (2) a cutoff low with anomalous heights near one standard deviation below normal slowly moved east and was steered ashore near 25°S by circulation around a very strong ridge (anomalies more than 3 standard deviations above normal) centered near 60°S; (3) anomalously high precipitable water content (20 mm above climatological norms) over the Peruvian Bight region was advected southward and eastward ahead of the cutoff low by low-level northwesterly flow, greatly enhancing observed precipitation over northern Chile.

Intraseasonal Variability of Hail in the Contiguous United States: Relationship to the Madden–Julian Oscillation

AbstractClimatologies have been developed to highlight variability of the frequency and intensity of hail in the United States. However, the intraseasonal variability of hail, including why one week might be active while the following inactive despite both having similar climatological probabilities, has not yet been explored. This paper presents relationships between spring-season (April–June) hail days and the leading mode of atmospheric intraseasonal variability, the Madden–Julian oscillation (MJO). It extends recent work on intraseasonal tornado variability to smaller spatial scales. In April, May, and June, statistically significant variability in hail days was found for different Real-time Multivariate MJO (RMM) phases of the MJO. For April, the strongest correlations between hail-day anomalies and anomalies of the product of convective available potential energy (CAPE) and 0–6-km vertical wind shear were found in RMM phase 5, with above-normal likelihood of a hail day found in the south-central Uni...

Real-Time Upper-Ocean Temperature Observations from Aircraft during Operational Hurricane Reconnaissance Missions: AXBT Demonstration Project Year One Results

Thousands of aircraft observations of upper-ocean thermal structures have been obtained during hurricane and typhoon research field experiments in recent decades. The results from these experiments suggest a strong correlation between upper-ocean thermal variability and tropical cyclone (TC) intensity change. In response to these results, during the Office of the Federal Coordinator of Meteorology (OFCM) 2011 Interdepartmental Hurricane Conference (IHC), the Working Group for Hurricane and Winter Storms Operations and Research (WG/HWSOR) approved a 3-yr project to demonstrate the usefulness of airborne expendable bathythermographs (AXBTs) in an operational setting. The goal of this project was to initialize and validate coupled TC forecast models and was extended to improve input to statistical intensity forecast models. During the first season of the demonstration project, 109 AXBTs were deployed between 28 July and 28 August 2011. Successes included AXBT deployment from WC-130J aircraft during operational reconnaissance missions tasked by the National Hurricane Center (NHC), real-time onboard and postflight dataprocessing,real-timedatatransmissiontoU.S.NavyandNOAAhurricanenumericalpredictioncenters, and near-real-time assimilation of upper-ocean temperature observations into the Naval Research Laboratory Coupled Ocean‐Atmosphere Mesoscale Prediction System-Tropical Cyclones (COAMPS-TC) forecast model. Initial results showed 1) increased model accuracy in upper-ocean temperatures, 2) minor improvements in TC track forecasts, and 3) minor improvements in TC intensity forecasts in both coupled dynamical and statistical models [COAMPS-TC and the Statistical Hurricane Intensity Prediction Scheme (SHIPS), respectively].

The Influence of the MJO on the Intraseasonal Variability of Northern Hemisphere Spring Snow Depth

AbstractIntraseasonal variability in springtime Northern Hemisphere daily snow depth change (ΔSD) by phase of the MJO was explored in this study. Principal findings of the relationship between ΔSD and the MJO included the following: 1) Statistically significant regions of lagged ΔSD anomalies for multiple phases of the MJO were found in March, April, and May in both North America and Eurasia. 2) In each month, lagged ΔSD anomalies were physically supported by corresponding lagged anomalies of 500-hPa height (Z500) and surface air temperature (SAT). Spearman rank correlation coefficients indicated a moderate to strong relationship between both Z500 and ΔSD and SAT and ΔSD in both Eurasia and North America for phases 5 and 7 in March. In April, a moderately strong relationship between Z500 and ΔSD was found over Eurasia for phase 5, but the relationship between SAT and ΔSD was weak. In May, correlations between ΔSD and both Z500 and SAT over a hemisphere-wide latitude band from 60° to 75°N were close to −0....

Relationships between Tropical Cyclone Intensity and Eyewall Structure as Determined by Radial Profiles of Inner-Core Infrared Brightness Temperature

AbstractRadial profiles of infrared brightness temperature for 2405 different satellite observations from 14 western North Pacific tropical cyclones (TCs) from the 2012 season were analyzed and compared to intensity and changes in intensity. Four critical points along the inner core of each infrared (IR) brightness temperature (BT) profile were identified: coldest cloud top (CCT), first overshooting top (FOT), and lower (L45) and upper (U45) extent of the inner eyewall. Radial movement of the mean CCT point outward with increasing TC intensity, combined with subsequent warming of the mean L45 point with intensity, highlighted structure changes that are consistent with eye and eyewall development. When stratified by latitude and vertical wind shear, the CCT point moved radially outward for all cases, notably at higher intensities for lower-latitude TCs and at lower intensities for higher-latitude TCs. The majority of the warming of the L45 point with increasing intensity occurred for low-latitude and low-s...