Potential predictability of Southwest US rainfall : role of tropical and high-latitude variability

This study explores the potential predictability of Southwest US (SWUS) precipitation for the November-March season in a set of numerical experiments performed with the Whole Atmospheric Community Climate Model. In addition to the prescription of observed sea surface temperature and sea ice concentration, observed variability from the MERRA-2 reanalysis is prescribed in the tropics and/or the Arctic through nudging of wind and temperature. These experiments reveal how a perfect prediction of tropical and/or Arctic variability in the model would impact the prediction of seasonal rainfall over the SWUS, at various time scales. Imposing tropical variability improves the representation of the observed North Pacific atmospheric circulation, and the associated SWUS seasonal precipitation. This is also the case at the subseasonal time scale due to the inclusion of the Madden-Julian Oscillation (MJO) in the model. When additional nudging is applied in the Arctic, the model skill improves even further, suggesting that improving seasonal predictions in high latitudes may also benefit prediction of SWUS precipitation. An interesting finding of our study is that subseasonal variability represents a source of noise (i.e., limited predictability) for the seasonal time scale. This is because when prescribed in the model, subseasonal variability, mostly the MJO, weakens the El Niño Southern Oscillation (ENSO) teleconnection with SWUS precipitation. Such knowledge may benefit S2S and seasonal prediction as it shows that depending on the amount of subseasonal activity in the tropics on a given year, better skill may be achieved in predicting subseasonal rather than seasonal rainfall anomalies, and conversely.

Subseasonal prediction of the 2020 Great Barrier Reef and Coral Sea marine heatwave

The 2020 marine heatwave in the Great Barrier Reef and Coral Sea led to mass coral bleaching. Sea surface temperature anomalies reached +1.7°C for the whole of the Great Barrier Reef and Coral Sea and exceeded +2°C across broad regions (referenced to 1990-2012). The marine heatwave reached Category 2 (Strong) and warm anomalies peaked between mid-February and mid-March 2020. The marine heatwave’s peak intensity aligned with regions of reduced cloud cover and weak wind speeds. We used a marine heatwave framework to assess the ability of an operational coupled ocean-atmosphere prediction system (ACCESS-S1) to capture the marine heatwave’s severity, duration, and spatial extent. For initial week predictions, the predicted marine heatwave severity generally agreed with the magnitude and spatial extent of the observed severity for that week. The model ensemble mean did not capture the marine heatwave’s development phase at lead times beyond the first week. The model underestimated the marine heatwave’s spatial extent, which reached up to 95% of the study area with at least Moderate severity and up to 43% with at least Strong severity. However, most forecast ensemble members correctly predicted the period of Strong severity in the first week of the model forecast. The model correctly predicted marine heatwave conditions to persist from mid-February to mid-March but did not capture the end of the marine heatwave. The inability to predict the end of the event and other periods of less skilful prediction were related to subseasonal variability owing to weather systems, including the passage of tropical cyclones not simulated in the model. On subseasonal timescales, evaluating daily to weekly forecasts of ocean temperature extremes is an important step toward implementing methods for developing operational forecast extremes products for use in early warning systems.

Break events of the western North Pacific summer monsoon during 1979–2018

The monsoon break is a typical phenomenon representing the monsoon’s subseasonal variability, but its understanding is still limited for the western North Pacific (WNP) area. This study identified all break events of the WNP summer monsoon (WNPSM) from 1979 to 2018. The statistical analysis suggests that break events occur from late June to late October and peak at the end of August. The occurrence frequency of break events decreases as the duration increases, with 74% persisting 3–7 days and merely 26% lasting longer (8–15 days). During the break period, which is characterized by significant suppression of convection, there is an extensive anticyclonic anomaly in the lower troposphere, corresponding to a notable westward retreat of the monsoon trough and a southwestward shift of the subtropical high. Meanwhile, an anomalous cyclone and convergence in the upper troposphere are also conducive to inhibiting convection.The composite results indicate that both 10–25-day and 30–60-day oscillations contribute to the break, with their dry phases explaining 49.6% and 37.5% of the original suppression of convection, respectively. Around the break, the phase alternation of the 10–25-day oscillation causes convection fluctuation, while the 30–60-day oscillation maintains a stable dry phase that favors the establishment and maintenance of the break. A further case-by-case diagnosis suggests that 46 (51) out of the 61 break events occur in dry phases of the 10–25-day (30–60-day) oscillation, while only 10 (4) events occur in wet phases, indicating that the phase of the two oscillations significantly modulates the occurrence of the monsoon break.

Influence of ENSO on North American subseasonal surface air temperature variability

The wintertime influence of tropical Pacific sea surface temperature (SST) variability on subseasonal variability is revisited by identifying the dominant mode of covariability between 10–60 d band-pass-filtered surface air temperature (SAT) variability over the North American continent and winter-mean SST over the tropical Pacific. We find that the El Niño–Southern Oscillation (ENSO) explains a dominant fraction of the year-to-year changes in subseasonal SAT variability that are covarying with SST and thus likely more predictable. In agreement with previous studies, we find a tendency for La Niña conditions to enhance the subseasonal SAT variability over western North America. This modulation of subseasonal variability is achieved through interactions between subseasonal eddies and La Niña-related changes in the winter-mean circulation. Specifically, eastward-propagating quasi-stationary eddies over the North Pacific are more efficient in extracting energy from the mean flow through the baroclinic conversion during La Niña. Structural changes of these eddies are crucial to enhance the efficiency of the energy conversion via amplified downgradient heat fluxes that energize subseasonal eddy thermal anomalies. The enhanced likelihood of cold extremes over western North America is associated with both an increased subseasonal SAT variability and the cold winter-mean response to La Niña.

Tropical modulation of East Asia air pollution

Given the high population density and serious air pollution problems, understanding and predicting air pollution in East Asia are of great importance. Here, we show that the day-to-day variability of East Asia air pollution in winter is remotely controlled by the convection over the tropical Indian Ocean and western Pacific, the so-called Madden–Julian oscillation (MJO), through its extratropical teleconnections. In particular, the concentration of particulate matter with aerodynamic diameter less than 10 micron (PM10) becomes significantly high when the tropical convection is suppressed over the Indian Ocean (MJO phases 5–6). In contrast, PM10 concentration becomes significantly low when the convection is enhanced there (MJO phase 1–2). The station-averaged PM10 difference between the two MJO phases reaches up to 47% of the daily PM10 variability, indicating that the MJO is a primary source of wintertime subseasonal variability of East Asia PM10 concentration. We also show that PM10 anomaly typically lags the tropical convection by one to two weeks. This opens a new window of opportunity for subseasonal PM10 prediction in East Asia.

Characterizing quasi-biweekly variability of the Asian monsoon anticyclone using potential vorticity and large-scale geopotential height field

The spatial pattern of subseasonal variability of the Asian monsoon anticyclone is analyzed using long-term reanalysis data, focusing on the large-scale longitudinal movement. The air inside the anticyclone is quantified by a thickness-weighted low-PV (potential vorticity) area on an isentropic surface. It is shown that the longitudinal movement of the air inside the Asian monsoon anticyclone has a timescale of 1 to 2 weeks, which is shorter than the monthly dominant timescale of the variability in the anticyclone intensity. The movement of the anticyclonic air is suggested to be largely controlled by passive advection. The typical time evolution of the variability pattern, explained by two leading empirical orthogonal function (EOF) components of 100 hPa geopotential height, shows large-scale geopotential anomalies moving westward spanning from low to middle latitudes. This corresponds well with the rapid westward movement of low-PV air known as “eddy shedding” and following the eastward retreat of the anticyclonic air. The two EOF components can also explain the bimodal longitudinal distribution of geopotential maximum location.

Atmospheric Subseasonal Variability and Circulation Regimes: Spectra, Trends, and Uncertainties

The globally integrated subseasonal variability associated with the two main atmospheric circulation regimes, the balanced (or Rossby) and unbalanced (or inertia–gravity) regimes, is evaluated for the four reanalysis datasets: ERA-Interim, JRA-55, MERRA, and ERA5. The results quantify amplitudes and trends in midlatitude traveling and quasi-stationary Rossby wave patterns as well as in the equatorial wave activity across scales. A statistically significant reduction of subseasonal variability is found in Rossby waves with zonal wavenumber k = 6 along with an increase in variability in wavenumbers k = 3–5 in the summer seasons of both hemispheres. The four reanalyses also agree regarding increased variability in the large-scale Kelvin waves, mixed Rossby–gravity waves, and westward-propagating inertio-gravity waves with the lowest meridional mode. The amplitude and sign of trends in inertia–gravity modes with smaller zonal scales and greater meridional modes differ between the ERA-Interim and JRA-55 datasets on the one hand and the ERA5 and MERRA data on the other. An increased variability in the ERA-Interim and JRA-55 accounts for positive trends in their total subseasonal variability.

Identifying Subseasonal Variability Relevant to Atlantic Tropical Cyclone Activity

The primary atmospheric oscillations and variables associated with subseasonal Atlantic tropical cyclone (TC) activity are identified, based on 37 years of reanalysis data. TC activity, represented by accumulated cyclone energy (ACE), is computed for combined phases of the Madden–Julian oscillation (MJO) and El Niño–Southern Oscillation (ENSO). The MJO influence on TC activity becomes greater when the ENSO state is cooler. There is also a shift in the favorable MJO phase for TC activity with ENSO state. For strong La Niñas, MJO phases 4 and 5 (enhanced convection over the Maritime Continent) are most likely to have above-average ACE. To investigate other potential factors that influence subseasonal TC activity, two novel methods are developed: ACE by year (ABY) and seasonal and climatology removed (SNCR). Both methods isolate subseasonal signals of environmental conditions in association with a variable of interest. Vorticity, sea surface temperature, relative humidity, and genesis potential all show little signal in association with subseasonal Atlantic TC activity. The most important identifier of enhanced TC activity is negative vertical wind shear anomalies in the main development region of the Atlantic basin, and positive shear anomalies in the subtropical Atlantic. The shear pattern associated with a favorable MJO for TCs is similar to but distinct from the shear pattern associated with enhanced subseasonal TC activity. These findings demonstrate a nonlinear MJO–ENSO interaction and a pattern of wind shear anomalies that is linked to subseasonal TC activity.