Abstract

Forecast characteristics of Northern Hemisphere atmospheric blocking and the Madden–Julian oscillation (MJO) were diagnosed using an extensive time series (December–February 1985–2012) of daily medium-range ensemble reforecasts based on a version of the NCEP Global Ensemble Forecast System (GEFS).

For blocking, (i) interannual variability of analyzed blocking frequency was quite large, (ii) the GEFS slightly underforecasted blocking frequency at longer leads in the Euro-Atlantic sector, (iii) predictive skill of actual blocking was substantially smaller than its perfect-model skill, (iv) block onset and cessation were forecast less well than overall blocking frequency, (v) there was substantial variability of blocking skill between half-decadal periods, and (vi) the reliability of probabilistic blocking forecasts degraded with increasing lead time.

For the MJO, (i) forecasts of strong Indian Ocean MJOs propagated too slowly, especially the component associated with outgoing longwave radiation (OLR), that is, convection; (ii) tropical precipitation was greatly overforecast at early lead times; (iii) the ensemble predictions were biased and/or underdispersive, manifested in U-shaped rank histograms of MJO indices (magnitude forecasts were especially U shaped); (iv) MJO correlation skill was larger for its wind than for its OLR component, and was larger for the higher-amplitude MJO events; (v) there was some half-decadal variability in skill; and (vi) probabilistic skill of the MJO forecast was modest, and skill was larger when measured relative to climatology than when measured relative to a lagged persistence forecast.

For longer-lead forecasts, the GEFS demonstrated little ability to replicate the changes in blocking frequency due to a strong MJO that were noted in analyzed data.

Abstract

The relationship between deep tropical convection and large-scale atmospheric circulation in the 6–30-day period range is examined. Regression relationships between filtered outgoing longwave radiation at various locations in the Tropics and 200- and 850-mb circulation are mapped for the standard seasons, and the spatial structure and seasonal dependence of the results are interpreted in view of the basic-state circulation.

In regions where the convection is embedded in upper-level easterlies, anomalous equatorial easterly flow is typically present at 200 mb within and to the west of the convective signal, along with patterns of meridional outflow into subtropical anticyclonic perturbations. Lagged relationships suggest that the convection is forcing the circulation in many of these cases. The outflow and subtropical circulations are strongest into the winter hemisphere during the solstitial seasons, with more symmetric signals about the equator seen in the equinoctial seasons. The longitudinal positioning of the subtropical features with respect to the convection varies but is generally located due poleward or just to the east of the convection. There tends to be a first baroclinic mode vertical structure to these circulations, such that equatorial westerlies are present at 850 mb within the convection, with closed circulations on either side of the equator resembling equatorial Rossby modes especially common over the Atlantic and Pacific sectors.

As a contrast, in regions located within upper-level westerlies or along the margin of influence of upper westerly disturbances, convection appears to be forced by upper-level wave energy propagating into the deep Tropics, with the heating located in the upward motion region ahead of upper-level troughs. This occurs over the Atlantic and eastern Pacific sectors during northern winter and spring, and over Australia, the South Pacific, and South America during southern summer, when upper westerlies are at relatively low latitudes where they can interact with deep tropical convection. The results confirm theoretical and modeling ideas that suggest that Rossby wave energy is able to propagate into the deep Tropics in regions where upper-level westerlies exist in the Tropics.

Abstract

The interaction between high-frequency transient disturbances and convection, and the Madden–Julian Oscillation (MJO), is investigated using NCEP–NCAR reanalysis and satellite outgoing longwave radiation data for 15 northern winters. During the phase of the MJO with enhanced convection over the East Indian Ocean and Indonesia, and suppressed convection over the South Pacific convergence zone, both the Asian–Pacific jet and the region of upper-tropospheric tropical easterlies over the warm pool are displaced westward. These changes in the basic state lead to a weaker or “leakier” waveguide in the Asian–Pacific jet, with a westward-displaced “forbidden” region of tropical easterlies, such that high-frequency transient waves propagate equatorward into the deep Tropics over the central Pacific near the date line. As these waves induce convection in the region of ascent and reduced static stability ahead of the upper-level cyclonic disturbances, there is an enhancement of high-frequency convective variability over the central Pacific intertropical convergence zone during this phase of the MJO. This enhanced high-frequency convective variability appears to project back onto intraseasonal timescales and forms an integral part of the slowly varying diabatic heating field of the MJO. In the opposite phase of the MJO, the Asian–Pacific jet is extended eastward and there is an almost continuous waveguide across the Pacific. Together with the expanded forbidden region of tropical easterlies over the warm pool, this leads to a more zonal propagation of high-frequency transients along the waveguide with less equatorward propagation, and hence reduced high-frequency convective variability over the tropical central Pacific. There is also evidence of high-frequency waves propagating into the Indian Ocean region at the beginning of the MJO cycle, which may be important in the initiation of intraseasonal convective anomalies there.

Abstract

Interactions between the convection and circulation fields of the boreal summer intraseasonal oscillation (ISO) and two types of higher-frequency tropical wave activity are examined through a statistical analysis of 22 yr of data. During the convectively active phase of the ISO, westward-propagating mixed Rossby–gravity (MRG)–tropical depression (TD)-type wave activity is enhanced within the low-frequency ISO convective envelope, and is strongly correlated with low-frequency 850-hPa westerly anomalies. At the same time, eastward-propagating convectively coupled Kelvin wave activity is enhanced well to the east of the active ISO convection, in the central Pacific.

A case study of an ISO event during July–September 1987 illustrates these statistically derived relationships. The enhanced phase of the ISO is shown to consist primarily of westward-propagating higher-frequency variability, including seven named tropical cyclones in the western Pacific, two of which project onto MRG–TD-type modes as they propagate westward across Southeast Asia into the Bay of Bengal. Successive eastward development of three tropical storms is suggested to be associated with an eastward dispersion of energy in the MRG–TD mode. Several Kelvin waves propagate across the Pacific to the east of the active ISO convective envelope.

Based on the statistical results and the 1987 case study, it is suggested that the high-frequency, westward-propagating MRG–TD disturbances and tropical cyclones may compose a significant portion of the low-frequency ISO signal. Eastward-propagating Kelvin wave variability, on the other hand, is more active outside the ISO convective envelope, to its east.

Abstract

Lagged cross correlations between outgoing longwave radiation (OLR) and National Meteorological Center global analyses are utilized to isolate the preferred upper-level and surface circulation anomalies associated with tropical convection during northern winter. Three intraseasonal time scales are studied: 30–70, 14–30, and 6–14 days. In the 30–70-day band, the upper-level circulation signals are zonally elongated, with zonal wavenumbers 0–2 dominant. Higher-frequency signals are dominated by zonal wavenumbers 5 and 6. In the 14–30-day band, convection over the eastern hemisphere is associated with upper-level anticyclones in the subtropics and appears to be linked in some cases to midlatitude wave trains. The strongest signals are for convection over Africa, Australia, and the eastern Indian Ocean. Only weak signals are seen for convection over Indonesia. In these regions of upper-level easterlies, OLR anomalies peak prior to the maximum anomalies in wind, suggesting forcing of the circulation by tropical heating.

In contrast, 14–30-day and 6–14-day convection over the eastern tropical Pacific, eastern South America, and central South Pacific is primarily associated with the intrusion of troughs in the westerlies originating in the extratropics. These are regions of mean upper level westerly flow, or where upper-westerlies lie adjacent to tropical convergence zones overlain by only weak easterly flow aloft. The large amplitude of these troughs prior to the OLR anomaly is indicative of the forcing of the convection by these disturbances.

Abstract

Statistical evidence is presented to support the notion that tropical convection in the eastern Pacific and Atlantic intertropical convergence zone (ITCZ) during northern winter can be forced by disturbances originating in the extratropics. The synoptic-scale transients in these regions are characterized at upper levels by strong positive tilts in the horizontal and appear to induce vertical motions ahead of troughs as in midlatitude baroclinic systems. Two case studies of such interactions are examined, one for the eastern North Pacific ITCZ and another somewhat different type of interaction for the South Pacific convergence zone (SPCZ) over the western South Pacific.

Both cases are associated with upper-level troughs, strong cold advection deep into the tropics, and the formation of a frontal boundary at low levels. The ITCZ case is characterized by the advection of anomalously high isentropic potential vorticity air southward, a strong poleward flux of heat and westerly momentum, and the development of a subtropical jet downstream of the disturbance. The SPCZ disturbance is not strongly tilted, but is still accompanied by a strong poleward flux of heat and momentum. Evidence for the occurrence of cross-equatorial wave dispersion in the eastern Pacific during northern winter is also presented. These observations are consistent with theory and modeling of Rossby waves in a westerly basic state extending from the midlatitudes into the tropics.

Abstract

Composite surface pressure, temperature, and precipitation anomalies are mapped over the Indian and Pacific sectors during the various stages of Warm and Cold Events in the Southern Oscillation. In the year before the development of positive sea surface temperature anomalies in the central and eastern equatorial Pacific (Year–1 of a Warm Event), a strong South Pacific High is associated with below normal surface pressure over Australia and the Indian Ocean. This occurs concurrently with a poleward displacement of the Pacific convergence zones, with above normal air temperature and precipitation over the subtropical Pacific, and opposite conditions along the equator. By the next year (Year 0) of the Warm Event, thew anomalies have the opposite sign. The sequence of anomalies during a Cold Event is inverse to that during a Warm Event but otherwise the anomaly patterns are remarkably similar.

It appears that enhanced convection and low surface pressure within the Pacific convergence zones contribute to the observed westerly wind anomalies in the western equatorial Pacific at the end of Year–1, which are in turn tied to the onset of above normal equatorial SST in the following year. The observed reversal in atmospheric anomalies over the Indian and Pacific oceans daring Warm Events is an extreme manifestation of a general biennial tendency in these anomalies, with Cold Events occupying the opposite extreme.

Abstract

A comparison of the 1877–78 and 1982–83 El Niño/Southern Oscillation (ENSO) events was made using monthly and seasonal values of sea surface temperature (SST) and station pressure in the tropics, sea level pressure (SLP) in North America and the North Atlantic, temperature in North America and precipitation in several key areas around the globe.

SST anomalies in the eastern tropical Pacific, heavy rains in coastal Peru and extreme pressure anomalies across the Pacific and Indian Oceans during 1877–78 indicate an ENSO event of comparable magnitude to that during 1982–83. Both events were also associated with drought conditions in the Indonesian region, India, South Africa, northeastern Brazil and Hawaii. Wintertime teleconnections in the midlatitudes of the Northern Hemisphere were similar in terms of SLP from the North Pacific to Europe, resulting in significantly higher than normal temperatures over most of the United States and extreme rains in California.

Abstract

Within the tropical Pacific intertropical convergence zone (ITCZ), organized cloud systems that evolve over synoptic time scales frequently propagate eastward and contribute significantly to the clouds and precipitation in that region. This study analyzes eastward-propagating disturbances (EPDs) in the tropical Pacific during boreal winter (DJF) and spring (MAM) and their connection to Northern Hemisphere (NH) extratropical Rossby wave activity using cloud and precipitation fields from satellite and dynamical fields from reanalysis. During DJF, EPDs are located north of the ITCZ (around 15°N), propagate eastward at 10 m s⁻¹ within the central Pacific, and exhibit high cloudiness associated with upper-level divergence on the east side of NH Rossby waves propagating into the tropics. During MAM, EPDs initiate in the west Pacific and propagate along the ITCZ axis (around 7°N) into the east Pacific at 15 m s⁻¹ where NH Rossby waves induce upper-level divergence, enhancing their convective activity. The MAM EPDs are decidedly associated with Kelvin wave characteristics, while the DJF EPDs are not. The shallow meridional circulation (SMC) in the east Pacific is also studied in the context of EPDs. During DJF, EPDs do not impact the SMC, but the deep meridional circulation in the northern part of the ITCZ strengthens. During MAM, the shallow convection ahead of the EPDs enhances the SMC in the southern part of the ITCZ. These results distinguish between two types of EPDs during DJF and MAM that have different physical characteristics, forcing mechanisms, and regional impacts.

Abstract

A set of 30-day reforecast experiments, focused on the Northern Hemisphere (NH) cool season (November–March), is performed to quantify the remote impacts of tropical forecast errors on the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS). The approach is to nudge the model toward reanalyses in the tropics and then measure the change in skill at higher latitudes as a function of lead time. In agreement with previous analogous studies, results show that midlatitude predictions tend to be improved in association with reducing tropical forecast errors during weeks 2–4, particularly over the North Pacific and western North America, where gains in subseasonal precipitation anomaly pattern correlations are substantial. It is also found that tropical nudging is more effective at improving NH subseasonal predictions in cases where skill is relatively low in the control reforecast, whereas it tends to improve fewer cases that are already relatively skillful. By testing various tropical nudging configurations, it is shown that tropical circulation errors play a primary role in the remote modulation of predictive skill. A time-dependent analysis suggests a roughly 1-week lag between a decrease in tropical errors and an increase in NH predictive skill. A combined tropical nudging and conditional skill analysis indicates that improved Madden–Julian oscillation (MJO) predictions throughout its life cycle could improve weeks 3–4 NH precipitation predictions.