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  • Author or Editor: Bradford S. Barrett x
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Bradford S. Barrett
and
Sultan Hameed

Abstract

Monthly precipitation in Chile (30°–55°S) was found to vary by intensity, latitude, and longitude of the South Pacific high (SPH). In austral winter, precipitation was higher when the SPH was weaker and when it was centered farther west. In austral spring, precipitation was higher when the SPH was weaker, similar to winter. However, spring precipitation was not found to be related to SPH longitude, and higher precipitation was found when the SPH was centered farther north. In austral summer, no relationship was found between precipitation and either SPH intensity or longitude, but positive correlations were found between precipitation and latitude of the SPH. In austral autumn, correlation patterns between precipitation and all three SPH metrics more closely resembled those seen in winter. The results of a multiple linear regression confirmed the importance of two SPH metrics (intensity and longitude) and the unimportance of a third SPH metric (latitude) in understanding variability in winter, summer, and autumn precipitation in central and southern Chile. In spring, regression results confirmed a relationship between precipitation and SPH intensity and latitude. Furthermore, the SPH intensity and longitude in winter combined to hindcast monthly precipitation with a better goodness of fit than five El Niño–Southern Oscillation metrics traditionally related to Chilean precipitation. Anomalies of lower-tropospheric circulation and vertical velocities were found to support the observed relationships between SPH and precipitation. Based on these results, a physical mechanism is proposed that employs the SPH as a metric to aid in understanding variability in precipitation in central and south-central Chile in all seasons.

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Bradford S. Barrett
,
Gina R. Henderson
, and
Joshua S. Werling

Abstract

Intraseasonal 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 −0.5 and −0.4, respectively. Given the strength of these statistical relationships, the following physical pathway is proposed for intraseasonal variability of spring snow depth changes: poleward-propagating Rossby waves in response to tropical MJO convection interact with Northern Hemisphere background flow, leading to anomalous troughing and ridging. These anomalous circulation centers then impact daily snow depth change via precipitation processes and anomalies in surface air temperature.

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Julio C. Marín
,
Deniz Bozkurt
, and
Bradford S. Barrett

Abstract

We analyze the seasonal evolution and trends of atmospheric blocking from 1979 to 2018 using a geopotential-height-based method over two domains, one located to the west (150°–90°W, 50°–70°S) and the other over and to the east (90°–30°W, 50°–70°S) of the Antarctic Peninsula. Spatial patterns of geopotential heights on days with blocking feature well-defined ridge axes over and west of much of South America, and days with the most extreme blocking (above the 99th percentile) showed upper-tropospheric ridge and cutoff low features that have been associated with extreme weather patterns. Blocking days were found to be more frequent in the first half of the period (1979–98) than the second (1999–2018) in all seasons in the west domain, whereas they seem to be more common over the eastern (peninsula) domain in 1999–2018 for austral winter, spring, and autumn, although these differences were not statistically significant. West of the Antarctic Peninsula, blocking days occur most frequently when the Antarctic Oscillation (AAO) is negative, whereas they are more frequent over the peninsula when the AAO is positive. We propose that our blocking index can be used to indicate atmospheric blocking affecting the Antarctic Peninsula, similar to how the Greenland blocking index has been used to diagnose blocking, its trends, and impacts over the Arctic.

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Juliet Perdigón-Morales
,
Rosario Romero-Centeno
,
Bradford S. Barrett
, and
Paulina Ordoñez

Abstract

The aim of this study is to understand how the Madden–Julian oscillation (MJO) modulates the bimodal seasonal rainfall distribution across the regions in Mexico where the midsummer drought (MSD) occurs. The MSD is characterized by a precipitation decrease in the middle of the rainy season. Relative frequencies of each active phase of the Real-time Multivariate MJO index were calculated at each grid point in the high-resolution Climate Hazards Group Infrared Precipitation with Stations (CHIRPS) rainfall dataset for the first (MAX1) and second (MAX2) rainfall peaks and the MSD minimum (MIN). In addition, standardized anomalies of precipitation (from the CHIRPS dataset) and 300-hPa omega, 500-hPa geopotential height, and 850-hPa u- and υ-wind components (from the Climate Forecast System Reanalysis) were calculated for each MJO phase and each month in the rainy season. Results show that the MIN (MAX2) occurs more frequently during the dry (wet) MJO phases, while the MJO seems not to influence MAX1 significantly. Anomalous anticyclonic (cyclonic) circulations at 850 hPa, positive (negative) 500-hPa geopotential height anomalies, northeast (southwest) 850-hPa wind anomalies over southern Mexico, and a low-level westward (eastward) flow in the northeastern tropical Pacific support the MIN (MAX2) pattern under the influence of the dry (wet) MJO phases. These features are more clearly observed in the MSDs of 1- and 2-month duration and over the southern half of Mexico. The results suggest that the bimodal distribution is less influenced by the MJO in regions of northeastern Mexico.

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Donald M. Lafleur
,
Bradford S. Barrett
, and
Gina R. Henderson

Abstract

One 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 (only 1% of days). The MJO was persistent in time and across intensity categories, and all activity categories the following day had at least an 80% chance of maintaining their amplitudes. Implications of this climatology are discussed, including length of complete MJO cycles (the shortest of which was 17 days) and correlations between MJO amplitude and atmospheric response.

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Bradford S. Barrett
,
Jorge F. Carrasco
, and
Anthony P. Testino

Abstract

The 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 across phase, in good agreement with precipitation: low geopotential height and negative omega (corresponding to upward vertical motion) anomalies were found over and upstream of Chile during the rainier phases, and the anomalies reversed during the drier phases. Surface pressure and middle- and upper-tropospheric geopotential height anomalies showed a classic equivalent barotropic wave train, indicating a teleconnection response to deep convective activity in the Maritime Continent in agreement with numerous earlier observational, modeling, and theoretical studies.

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