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The simulation and prediction of the climatology and interannual variability of the East Asia winter monsoon (EAWM), as well as the associated atmospheric circulation, was investigated using the hindcast data from Global Seasonal Forecast System version 5 (GloSea5), with a focus on the evolution of model bias among different forecast lead times. While GloSea5 reproduces the climatological means of large-scale circulation systems related to the EAWM well, systematic biases exist, including a cold bias for most of China's mainland, especially for North and Northeast China. GloSea5 shows robust skill in predicting the EAWM intensity index two months ahead, which can be attributed to the performance in representing the leading modes of surface air temperature and associated background circulation. GloSea5 realistically reproduces the synergistic effect of El Niño—Southern Oscillation (ENSO) and the Arctic Oscillation (AO) on the EAWM, especially for the western North Pacific anticyclone (WNPAC). Compared with the North Pacific and North America, the representation of circulation anomalies over Eurasia is poor, especially for sea level pressure (SLP), which limits the prediction skill for surface air temperature over East Asia. The representation of SLP anomalies might be associated with the model performance in simulating the interaction between atmospheric circulations and underlying surface conditions. [ABSTRACT FROM AUTHOR]

This paper studies the spatiotemporal variability of the IRt index (defined as the difference between the percentage of precipitation corresponding to warm days and cold days in year t) calculated at 35 stations in the Iberian Peninsula during 1951–2019. Spatial variability is estimated using cluster analysis (Euclidean distance, Ward's method), which allows regionalization of the data set for each year's season. For each cluster, the < IR > index is defined as the average of the indexes corresponding to each cluster element. The temporal evolution of the < IR > indices is analyzed by studying the appearance of trends using the Mann–Kendall test and the possible existence of various subperiods using the cumulative sum of deviations and the t-test for the difference between the means. The role of EA, EA/WR, NAO, SCAN and WeMO teleconnection patterns in modulating the relationship between temperature and precipitation is analyzed by applying multiple regression analysis. The results show a trend towards positive (or less negative) values in the relationship between temperatures and precipitation (of varying magnitude depending on the region and season) and the contribution of atmospheric dynamics to these changes. [ABSTRACT FROM AUTHOR]

Using a high-resolution atmospheric general circulation model simulation of unprecedented ensemble size, we examine potential predictability of monthly anomalies under El Niño–Southern Oscillation (ENSO) forcing and background internal variability. This study reveals the pronounced month-to-month evolution of both the ENSO forcing signal and internal variability. Internal variance in upper-level geopotential height decreases (~10%) over the North Pacific during El Niño as the westerly jet extends eastward, allowing forced signals to account for a greater fraction of the total variability, and leading to increased potential predictability. We identify February and March of El Niño years as the most predictable months using a signal-to-noise analysis. In contrast, December, a month typically included in teleconnection studies, shows little to no potential predictability. We show that the seasonal evolution of SST forcing and variability leads to significant signal-to-noise relationships that can be directly linked to both upper-level and surface variable predictability for a given month. The stark changes in forced response, internal variability, and thus signal-to-noise across an ENSO season indicate that subseasonal fields should be used to diagnose potential predictability over North America associated with ENSO teleconnections. Using surface air temperature and precipitation as examples, this study provides motivation to pursue "windows of forecast opportunity" in which statistical skill can be developed, tested, and leveraged to determine times and regions in which this skill may be elevated. [ABSTRACT FROM AUTHOR]

The Arctic polar vortex (APV) system plays an important role in controlling winter and spring atmospheric circulation pattern in China. Here, we evaluate the anomalous APV-induced spring vegetation variability and lagged productivity responses in China. We found that both strong and weak APV conditions have almost equally negative impacts on spring vegetation growth in China, e.g., negative NDVI anomalies occurred in 48.6 and 53.2% of China's vegetated areas under strong and weak APV conditions, respectively. However, large seasonal compensation effects were associated with weak APV conditions, and beneficial lagged vegetation productivity responses occurred in 67.2% of China's vegetated areas, whereas adverse responses occurred in only 32.8% of vegetated areas. Under a strong APV, adverse lagged vegetation productivity responses occurred in 54.5% of China's vegetated areas, whereas beneficial responses occurred in only 45.5% of vegetated areas. The temperature, precipitation, and solar radiation changes caused by anomalous APV-induced changes in circulation patterns are the main reasons for spring vegetation variability. The lagged vegetation productivity responses were attributed to anomalous APV-related precipitation and air temperature anomalies in the following summer and autumn. This improved understanding of the strong links between APV anomalies and vegetation dynamics in China should facilitate early warning of vegetation productivity reductions under anomalous APV conditions. [ABSTRACT FROM AUTHOR]

The persistence of atmospheric circulation anomalies over East Asia shows a winter to winter recurrence (WTWR) phenomenon. Seasonal variations in sea level pressure anomalies and surface wind anomalies display significantly different characteristics between WTWR and non-WTWR years. The WTWR years are characterized by the recurrence of both a strong (weak) anomalous Siberian High and an East Asian winter monsoon over two successive winters without persistence through the intervening summer. However, anomalies during the non-WTWR years have the opposite sign between the current and ensuing winters. The WTWR of circulation anomalies contributes to that of surface air temperature anomalies (SATAs), which is useful information for improving seasonal and interannual climate predictions over East Asia and China. In the positive (negative) WTWR years, SATAs are cooler (warmer) over East Asia in two successive winters, but the signs of the SATAs are opposite in the preceding and subsequent winters during the non-WTWR years. [ABSTRACT FROM AUTHOR]

The Intertropical Front (ITF) is a fundamental feature of the atmospheric circulation over West Africa. It separates the wedge of warm moist southwesterly monsoon flow off the tropical Atlantic from much hotter and very dry northeasterly wind from the Sahara Desert. Here, the daily temperature, humidity, and rainfall data for 1974–2003 are analyzed to document the northward advance and southward retreat of the ITF between boreal spring and autumn, and assess its role in Sudan–Sahel (10°–20°N) rainfall patterns. Using largely dekadal (10 day) and monthly resolutions, analyses are performed for the 30-yr-average seasonal time scale and sets of extreme years, with a major focus on concurrent monthly ITF–rainfall relations. The seasonal rainfall predictive potential of the early season ITF latitude is also investigated, as is the secular variation of ITF latitude–weather system–rainfall associations during 1974–2003. The northward advance of the ITF across the Sudan–Sahel from April to early August is relatively slow, averaging 0.8° latitude dekad−1 (8.8 km day−1). The southward ITF retreat between mid-August and mid-November is almost twice as fast, averaging 1.4° latitude dekad−1 (15.5 km day−1). Coupled with the ITF advance, the monsoon rainbelt migrates northward and intensifies. However, its northern boundary (1 mm day−1 monthly average isohyet) lags 100–250 km south of the ITF, while the most useful rainfall for society (>3 or 4 mm day−1 monthly average) generally occurs more than 400 km south of the ITF. There, the monsoon wedge is thickest and the horizontal velocity and moisture convergence are maximized in a regional ITCZ. The rapid ITF retreat during September–October is preceded by a similar rainbelt displacement. During both ITF advance and retreat, rainfall over the Sudan–Sahel region is positively related to the ITF’s latitude. The association is strongest during the early (April–June) and late (October) rainy season months (linear correlation, r = +0.74 to +0.81), when the ITF is located to the south and rainfall is low. It is weaker during the July–September rainy season core when the ITF is farthest north ( r = +0.50 to +0.58). This concurrent rainfall dependence on ITF latitude is established further by contingency analyses for the 30-yr study period and by investigation of several extremely dry and wet individual seasons. The April ITF latitude anomaly is a moderately consistent indicator of the subsequent ITF latitude and associated rainfall anomaly through the first core rainy season month (July). This seasonal prediction potential does not persist into the rainy season peak (August), when the concurrent ITF–rainfall relationship is weakest ( r = +0.50), the monsoon wedge is thickest, and rain-producing mesoscale dynamical processes are developed fully. However, because the ITF tends to retreat early (late) in seasons when it advanced early (late), the April ITF latitude specification of the September–October ITF latitude and rainfall (negative) is almost as consistent as that for July (positive). The secular variation of ITF latitude during 1974–2003 strongly influenced mesoscale weather systems and rainfall variability on decadal time scales. [ABSTRACT FROM AUTHOR]

Wavelet and cross-wavelet analysis are used to identify and describe spatial and temporal variability in Canadian seasonal streamflows, and to gain insights into the dynamical relationship between the seasonal streamflows and the dominant modes of climate variability in the Northern Hemisphere, Results from applying continuous wavelet transform to mean seasonal streamflows from 79 rivers selected from the Canadian Reference Hydrometric Basin Network (RHBN) reveal striking climate-related features before and after the 1950s. The span of available observations, 1911–99, allows for depicting variance and covariance for periods up to 12 yr. Scale-averaged wavelet power spectra are used to simultaneously assess the temporal and spatial variability in each set of 79 seasonal streamflow time series. The most striking feature, in the 2–3-yr period and in the 3—6-yr period—the 6–12-yr period is dominated by white noise and is not considered further–is a net distinction between the timing and intensity of the temporal variability in autumn, winter, and spring-summer streamflows. It is found that the autumn season exhibits the most intense activity (or variance) in both the 2–3- and the 3–6-yr periods. The spring-summer season corresponds to the least intense activity for the 2–3-yr period, but it exhibits more activity than winter for the 3–6-yr period. Cross-wavelet analysis is provided between the seasonal streamflows and three selected climatic indices: the Pacific-North America (PNA), the North Atlantic Oscillation (NAO), and the sea surface temperature series over the Niño-3 region (ENSO3), The wavelet cross-spectra reveal strong climate-streamflow activity (or covariance) in the 2–6-yr period starting after 1950 whatever the climatic index and the season. Prior to 1950, local and weaker 2–6-yr activity is revealed in central and western Canada essentially in winter and autumn, but overall a non-significant streamflow-climate relationship is observed prior to 1950. Correlation analysis in the 2–6-yr band between the seasonal streamflow and the selected climatic indices revealed strong positive correlations with the ENSO in the spring-summer and winter seasons for the post-1950 period for both eastern and western Canada. A similar correlation pattern is revealed in the west with the NAO, while in the east moderate negative NAO correlations are observed only in the autumn season prior to 1950, After 1950 strong NAO correlations emerge for all the seasons. The cross-wavelet spectra and the correlation analysis in the 2–6-yr band suggest the presence of a change point around 1950 in the east and west seasonal streamflows. [ABSTRACT FROM AUTHOR]

Subseasonal variability of the stratus/stratocumulus cloud deck over the subtropical southeast Pacific is studied using satellite and buoy observations as well as the NCEP-NCAR reanalysis. It is found that subseasonal variability in the stratus cloud deck is closely related to variations in surface wind velocity, water vapor, sea level pressure, and 500-hPa geopotential height. An increase in cloud liquid water (CLW) over the subtropical southeast Pacific is found to be associated with the development of an anomalous anticyclonic circulation to the south off the west coast of Chile, The enhanced southerly to southeasterly winds advect cold and dry air into the stratus region against the mean sea surface temperature (SST) gradient. This cold and dry advection, together with increased wind speed, intensifies surface latent and sensible heat fluxes and destabilizes the boundary layer. Anomalous offshore easterlies north of the anomalous anticyclone cause a tow-level divergence. The associated subsidence warming, together with the cold advection in the surface layer, strengthens the temperature inversion, conducive to the development of stratus clouds. Buoy observations confirm this subseasonal cloud variability and its relationship with surface meteorological variables. A lead/lag composite analysis indicates that circulation variables such as sea level pressure and surface wind lead cloud liquid water by 1–2 days while SST tags CLW by 1–2 days, suggesting that low-cloud variability is caused by atmospheric circulation changes rather than by the underlying ocean. The dynamic adjustment that leads to cloud fluctuations and possible orographic effects of the Andes are also discussed. [ABSTRACT FROM AUTHOR]

An atmospheric GCM coupled to a slab ocean model is used lo investigate how temperature and precipitation over South America and Africa affect the annual cycle of the Atlantic ITCZ. The main conclusion of this study is that variations in precipitation and temperature forced by the annual cycle of insolation over the continents are as important as variations in insolation over the ocean and in ocean heal transport convergence in forcing the annual march of the Atlantic ITCZ observed in the control simulation. The processes involved are as follows. The intensity of precipitation over land affects the stability of the atmosphere over the tropical Atlantic Ocean, and thus modulates the intensity of deep convection and convergence in the ITCZ. Both the imposed changes in land precipitation and the subsequent changes in the strength of the ITCZ drive surface wind anomalies, thereby changing the meridional gradient of SST in proximity of the basic-stale ITCZ. Finally, atmosphere-ocean feedbacks cause the ITCZ to be displaced meridionally. Seasonal changes in surface temperature in the Sahara also have a strong influence on the position of the Atlantic ITCZ. Cold wintertime temperatures produce high surface pressure anomalies over Africa and into the tropical North Atlantic and drive stronger trade winds, which cool the North Atlantic by evaporation. The coupled interactions between the SST. the wind, and the ITCZ intensify the anomalies in the equatorial region, causing the southward displacement of the ITCZ in boreal spring. [ABSTRACT FROM AUTHOR]

The Aleutian low is examined as a primary determinant of surface air temperature (SAT) variability in the Bering Sea during the winter [December-January-February-March (DJFM)] months. The Classification and Regression Tree (CART) method is used to classify five types of atmospheric circulation for anomalously warm months (W1–W5) and cold months (C1–C5). For the Bering Sea, changes in the position of the Aleutian low are shown to be more important than changes in its central pressure. The first two types, W1 and C1, account for 51% of the ‘warm’ and 37% of the ‘cold’ months. The W1-type pattern is characterized by the anomalously deep Aleutian low shifted west and north of its mean position. In this situation, an increased cyclonic activity occurs in the western Bering Sea. The C1-type pattern represents a split Aleutian low with one center in the northwestern Pacific and the other in the Gulf of Alaska. The relative frequency of the W1 to C1 types of atmospheric circulation varies on decadal time scales, which helps to explain the predominance of fluctuations on these time scales in the weather of the Bering Sea. Previous work has noted the prominence of multidecadal variability in the North Pacific. The present study finds multidecadal variations in frequencies of the W3 and C3 patterns, both of which are characterized by increased cyclonic activity south of 51°N. In general, the CART method is found to be a suitable means for characterizing the wintertime atmospheric circulation of the North Pacific in terms of its impact on the Bering Sea. The results show that similar pressure anomaly patterns for the North Pacific as a whole can actually result in different conditions for the Bering Sea, and that similar weather conditions in the Bering Sea can arise from decidedly different large-scale pressure patterns. [ABSTRACT FROM AUTHOR]

Tropical and extratropical interactions on the intraseasonal time scale are studied in the context of the Arctic Oscillation (AO) and the Madden-Julian oscillation (MJO). To simplify the discussion, a high (low) MJO phase is defined as strong (suppressed) convective activity over the Indian Ocean, In the Northern Hemisphere (NH) winter season, a high (low) AO phase is found more likely coupled with a high (low) MJO phase. Based on the regressed patterns and composites of various dynamical fields and quantities, possible mechanisms linking the AO and the MJO are examined. The analysis indicates that the MJO influence on extratropical circulations seems more evident than the AO influence on tropical circulations. The MJO interacts with the AO through meridional dispersion of Rossby waves in the Pacific sector. The geopotential height anomaly center over the North Pacific associated with the MJO can either reinforce or offset the AO Pacific action center. As a result, the AO pattern can be greatly affected by the MJO. When the AO and the MJO are in the same (opposite) phase, the Pacific action center becomes much stronger (weaker) than the Atlantic action center. The eddy momentum transports associated with the MJO in the Pacific sector are closely related to the retraction and extension of tropical Pacific easterlies and the subtropical Asian-Pacific jet. Because of its large scale, this regional effect is also reflected in the zonal mean stale of wave transport and wave forcing on zonal wind, which in turn affects the phase of the AO. [ABSTRACT FROM AUTHOR]

The article discusses a study conducted which explores the effects of hydroclimatic changes at the North America Monsoon System (NAMS) region. It reveals that warming occurred during dry season while minimum temperatures happened during wet season. Moreover, remote-sensing data and regional modeling experiments are needed to determine the changes in atmospheric circulation.