ALBERT

Description: The Summer East Atlantic (SEA) mode is the second dominant mode of summer low-frequency variability in the Euro-Atlantic region. Using reanalysis data, we show that SEA-related circulation anomalies significantly influence temperatures and precipitation over Europe. We present evidence that part of the interannual SEA variability is forced by diabatic heating anomalies of opposing signs in the tropical Pacific and Caribbean that induce an extratropical Rossby wave train. This precipitation dipole is related to SST anomalies characteristic of the developing ENSO phases. Seasonal hindcast experiments forced with observed sea surface temperatures (SST) exhibit skill at capturing the interannual SEA variability corroborating the proposed mechanism and highlighting the possibility for improved prediction of boreal summer variability. Our results indicate that tropical forcing of the SEA likely played a role in the dynamics of the 2015 European heat wave.

Description: In the present study, the influence of some major tropical modes of variability on northern hemisphere regional blocking frequency variability during boreal winter is investigated. Reanalysis data and an ensemble experiment with the ECMWF model using relaxation towards the ERA-Interim reanalysis data inside the tropics are used. The tropical modes under investigation are El Niño Southern Oscillation (ENSO), the Madden-Julian Oscillation (MJO) and the upper tropospheric equatorial zonal-mean zonal wind . An early (late) MJO phase refers to the part of the MJO cycle when enhanced (suppressed) precipitation occurs over the western Indian Ocean and suppressed (enhanced) precipitation occurs over the Maritime Continent and the western tropical Pacific. Over the North Pacific sector, it is found that enhanced (suppressed) high latitude blocking occurs in association with El Niño (La Niña) events, late (early) MJO phases and westerly (easterly) . Over central to southern Europe and the east Atlantic, it is found that late MJO phases, as well as a suppressed MJO are leading to enhanced blocking frequency. Furthermore, early (late) MJO phases are followed by blocking anomalies over the western North Atlantic region, similar to those associated with a positive (negative) North Atlantic Oscillation. Over northern Europe, the easterly (westerly) phase of is associated with enhanced (suppressed) blocking. These results are largely confirmed by both the reanalysis and the model experiment.

Description: The influence of some major tropical modes of variability on northern hemisphere regional blocking frequency variability during boreal winter is investigated and the most important findings will be presented here. Reanalysis data and an experiment with the ECMWF model using relaxation towards the ERA-40 reanalysis data inside the tropics are used. The tropical modes under investigation are El Nino Southern Oscillation (ENSO), the Madden-Julian Oscillation (MJO) and the upper tropospheric equatorial zonal-mean zonal wind (U150). Comparing the impact on blocking frequency of these tropical modes with the impact of two extratropical modes, namely the North Atlantic Oscillation (NAO) and the North Pacific Gyre Oscillation (NPGO), it is found that the tropical influence is of comparable amplitude. Focusing on the Euro-Atlantic sector, it is found that cold ENSO events, late MJO phases, as well as suppressed MJO are all leading to enhanced blocking frequency at lower to middle latitudes (south of 48N) on weekly to monthly timescales. At higher latitudes (north of 48N) over Europe, the blocking anomalies associated with ENSO and the MJO are less clear than at lower latitudes. Instead, at higher latitudes (north of 48N), the westerly (easterly) phase of U150 is associated with reduced (enhanced) blocking frequency.

Description: We investigate the daily variability of the East Asian summer monsoon (EASM) by projecting daily wind anomaly data onto the two major modes of an interannual multivariate Empirical Orthogonal Functions analysis. Mode 1, closely resembling the Pacific-Japan (PJ) pattern and referred to as PJ-mode, transits from positive to negative phase around mid-summer consistent with the Meiyu rains predominantly being an early summer phenomenon. Mode 2, which is influenced by the Indian summer monsoon (ISM) and referred to as ISM-mode, peaks in late July and early August and is associated with rainfall farther north over China. We then analyze the relation between the intraseasonal variation of the EASM and the Madden-Julian Oscillation (MJO) by analyzing circulation anomalies following MJO events. In the lower troposphere, the circulation anomalies associated with the MJO most strongly project on the PJ-mode. MJO phases 1-4 (5-8) favor the positive (negative) phase of the PJ-mode by favoring the anticyclonic (cyclonic) anomalies over the subtropical western North Pacific. In the upper troposphere, the circulation anomalies associated with the MJO project mainly on the ISM-mode.

Description: Influences from the Tropics, the stratosphere and the specification of observed sea surface temperature and sea-ice (SSTSI) on Northern Hemisphere winter mean circulation anomalies during the period 1960/61 to 2001/02 are studied using a relaxation technique applied to the ECMWF model. On interannual time-scales, the Tropics strongly influence the Pacific sector but also the North Atlantic sector, although weakly. The stratosphere is found to be influential on the North Atlantic Oscillation (NAO) on interannual time-scales but is less important over the Pacific sector. Adding the observed SSTSI to the tropical relaxation runs generally improves the model performance on interannual time-scales but degrades/enhances the model’s ability to capture the 42-year trend over the Pacific/Atlantic sector. While relaxing the stratosphere to the reanalysis fails to capture the trend over the whole 42-year period, the stratosphere is shown to be influential on the upward trend of the NAO index from 1965 to 1995, but with reduced amplitude compared to previous studies. Influence from the Tropics is found to be important for the trend over both time periods and over both sectors although, across all experiments, we can account for only 30% of the amplitude of the hemispheric trend. Copyright c� 2012 Royal Meteorological Society

Description: We examine the interannual variability of the seasonal mean atmospheric circulation in the Southern Hemisphere during austral winter. The three major modes are identified by rotated EOF (REOF) analysis. As expected, REOF1 is associated with the Southern Annular Mode which is dominated by internal atmospheric dynamics. REOF2 displays a wave train, linked to the western North Pacific monsoon and the Pacific-Japan pattern in East Asia in the same season; REOF3 resembles the Pacific-South American pattern. Externally-forced variability strongly projects on both REOF2 and REOF3 so that, in the ensemble mean, an atmospheric model with prescribed observed sea surface temperature (SST) captures considerable parts of the time evolution of REOF2 (50%) and REOF3 (25%), suggesting a potential predictability for the two modes.

Description: Variations in the global tropospheric zonal mean zonal wind ([U]) during boreal winter are investigated using Rotated Empirical Orthogonal Functions applied to monthly means. The first two modes correspond to the Northern and Southern Annular Mode and modes 3 and 4 represent variability in the tropics. One is related to El Nino Southern Oscillation and the other has variability that is highly correlated with the time series of [U] at 150 hPa between 5 ◦ N and 5 ◦ S ([U150]e) and is related to activity of the Madden-Julian Oscillation (MJO). High amplitude of the MJO with strong precipitation anomalies over the western tropical Pacific (late MJO phases) are associated with the westerly phase of [U150]e (and vice versa). The extratropical response to [U150]e is investigated using linear regressions of 500 hPa geopotential height onto the [U150]e time series. Use is made of reanalysis data and of the ensemble mean output from a relaxation experiment using the European Center for Medium Range Weather Forecasts (ECMWF) model in which the tropical atmosphere is relaxed towards reanalysis data. Both the 45- year ECMWF reanalysis (ERA-40) and the ERA-Interim reanalysis data sets are used for the relaxation experiment as well as for the regression analysis. Therefore the analysis is covering 52 boreal winters from 1960/61 to 2012/13. The regression analysis reveals a robust shift of the Aleutian low and a wave train across the North Atlantic associated with [U150]e. It is found that the subtropical Rossby waveguides and the link between the North Pacific and North Atlantic are stronger during the easterly phase of [U150]e. The wave train over the North Atlantic is associated with Rossby wave sources over the subtropical North Pacific and North America.