Abstract

Extreme active and break phases of the Indian summer monsoon (ISM) often bring about devastating floods and severe draughts. Here it is shown that these extreme phases exhibit distinctive precursory circulation conditions in both the tropics and extratropics over a range of antecedent periods. The extremely active monsoon over northern India is preceded by a strengthening of the upper-tropospheric central Asian high and enhancement of the tropical convection over the equatorial Indian Ocean and the South China Sea. The concurrent buildup of the anomalous high over central Asia and the arrival of tropical convection over northern India increase the likelihood of occurrence of a heavy rainy period there. Similarly, the concurrent anomalous low over central Asia and the arrival of suppressed convection originating from the equatorial Indian Ocean and the South China Sea precede extremely strong monsoon breaks over northern India. Two predictors can be used to predict the extreme active/break phases of the northern ISM: normalized 200-hPa geopotential height over central Asia and outgoing longwave radiation over southern India. Once the mean of the two predictors exceeds a threshold unit (1.0), an extreme phase is anticipated to occur over northern India after 4–5 days and reach peak intensity after an additional 2 days. In general, an event forecast by this simple scenario has a 40% probability of developing into an extreme phase, which is normally a small probability event (a less than 4% occurrence).

Abstract

This study investigated the most recurrent coupled pattern of intraseasonal variability between midlatitude circulation and the Indian summer monsoon (ISM). The leading singular vector decomposition (SVD) pattern reveals a significant, coupled intraseasonal variation between a Rossby wave train across the Eurasian continent and the summer monsoon convection in northwestern India and Pakistan (hereafter referred to as NISM). The wave train associated with an active phase of NISM rainfall displays two high pressure anomalies, one located over central Asia and the other over northeastern Asia. They are accompanied by increased rainfall over the western Siberia plain and northern China and decreased rainfall over the eastern Mediterranean Sea and southern Japan. The circulation of the wave train shows a barotropic structure everywhere except the anomalous central Asian high, located to the northwest of India, where a heat-induced baroclinic circulation structure dominates. The time-lagged SVD analysis shows that the midlatitude wave train originates from the northeastern Atlantic and traverses Europe to central Asia. The wave train enhances the upper-level high pressure and reinforces the convection over the NISM region; meanwhile, it propagates farther toward East Asia along the waveguide provided by the westerly jet. After an outbreak of NISM convection, the anomalous central Asian high retreats westward. Composite analysis suggests a coupling between the central Asian high and the convective fluctuation in the NISM. The significance of the midlatitude–ISM interaction is also revealed by the close resemblance between the individual empirical orthogonal functions and the coupled (SVD) modes of the midlatitude circulation and the ISM.

It is hypothesized that the eastward and southward propagation of the wave train originating from the northeastern Atlantic contributes to the intraseasonal variability in the NISM by changing the intensity of the monsoonal easterly vertical shear and its associated moist dynamic instability. On the other hand, the rainfall variations over the NISM reinforce the variations of the central Asian high through the “monsoon–desert” mechanism, thus reenergizing the downstream propagation of the wave train. The coupling between the Eurasian wave train and NISM may be instrumental for understanding their interaction and can provide a way to predict the intraseasonal variations of the Indian summer monsoon and East Asian summer monsoon.

Abstract

The rapid decline of summer Arctic sea ice over the past few decades has been driven by a combination of increasing greenhouse gases and internal variability of the climate system. However, uncertainties remain regarding spatial and temporal characteristics of the optimal internal atmospheric mode that most favors summer sea ice melting on low-frequency time scales. To pinpoint this mode, we conduct a suite of simulations in which atmospheric circulation is constrained by nudging tropospheric Arctic (60°–90°N) winds within the Community Earth System Model, version 1 (CESM1), to those from reanalysis. Each reanalysis year is repeated for over 10 model years using fixed greenhouse gas concentrations and the same initial conditions. Composites show the strongest September sea ice losses are closely preceded by a common June–August (JJA) barotropic anticyclonic circulation in the Arctic favoring shortwave absorption at the surface. Successive years of strong wind-driven melting also enhance declines in Arctic sea ice through enhancement of the ice–albedo feedback, reaching a quasi-equilibrium response after repeated wind forcing for over 5–6 years, as the effectiveness of the wind-driven ice–albedo feedback becomes saturated. Strong melting favored by a similar wind pattern as observations is detected in a long preindustrial simulation and 400-yr paleoclimate reanalysis, suggesting that a summer barotropic anticyclonic wind pattern represents the optimal internal atmospheric mode maximizing sea ice melting in both the model and natural world over a range of time scales. Considering strong contributions of this mode to changes in Arctic climate, a better understanding of its origin and maintenance is vital to improving future projections of Arctic sea ice.

Abstract

Analysis of the 56-yr NCEP–NCAR reanalysis data reveals a recurrent circumglobal teleconnection (CGT) pattern in the summertime midlatitude circulation of the Northern Hemisphere. This pattern represents the second leading empirical orthogonal function of interannual variability of the upper-tropospheric circulation. The CGT, having a zonal wavenumber-5 structure, is primarily positioned within a waveguide that is associated with the westerly jet stream. The spatial phases of CGT tend to lock to preferred longitudes. The geographically phase-locked patterns bear close similarity during June, August, and September, but the pattern in July shows shorter wavelengths in the North Pacific–North America sector. The CGT is accompanied by significant rainfall and surface air temperature anomalies in the continental regions of western Europe, European Russia, India, east Asia, and North America. This implies that the CGT may be a source of climate variability and predictability in the above-mentioned midlatitude regions.

The CGT has significant correlations with the Indian summer monsoon (ISM) and El Niño–Southern Oscillation (ENSO). However, in normal ISM years the CGT–ENSO correlation disappears; on the other hand, in the absence of El Niño or La Niña, the CGT–ISM correlation remains significant. It is suggested that the ISM acts as a “conductor” connecting the CGT and ENSO. When the interaction between the ISM and ENSO is active, ENSO may influence northern China via the ISM and the CGT. Additionally, the variability of the CGT has no significant association with the Arctic Oscillation and the variability of the western North Pacific summer monsoon. The circulation of the wave train shows a barotropic structure everywhere except the cell located to the northwest of India, where a baroclinic circulation structure dominates. Two possible scenarios are proposed. The abnormal ISM may excite an anomalous west-central Asian high and downstream Rossby wave train extending to the North Pacific and North America. On the other hand, a wave train that is excited in the jet exit region of the North Atlantic may affect the west-central Asian high and, thus, the intensity of the ISM. It is hypothesized that the interaction between the global wave train and the ISM heat source may be instrumental in maintaining the boreal summer CGT.

Abstract

Despite the observed high correlation between the Atlantic sea surface temperature (SST) and the Atlantic tropical cyclone (TC) activity, interpretation of this relationship remains uncertain. This study suggests that the tropical Atlantic sea surface warming induces a pair of anomalous low-level cyclones on each side of the equator, providing favorable conditions for enhancing TC formation east of 45°W, while the effect of SST warming in the tropical Indian Ocean and Pacific Ocean tends to suppress the TC formation.

Over the past 30 years (1978–2007), the TC activity in the Atlantic basin is characterized with significant enhancement of TC formation east of 45°W, where the total TC number increased significantly compared to the period 1948–77. Despite the possible undercount of TCs, this study shows that the recently enhanced TC formation may not be totally accounted for by the poor TC observing network prior to the satellite era. The Atlantic sea surface warming that occurred in recent decades might have allowed more TCs to form, to form earlier, and to take a longer track, while the effect is partially offset by the SST warming in Indian and Pacific Oceans. This study suggests that the close relationship between the Atlantic SST and TC activity over the past 30 years, including basinwide increases in the average lifetime, annual frequency, proportion of intense hurricanes, and annual accumulated power dissipation index (PDI), as reported in previous studies, is mainly a result of the SST warming in the tropical Atlantic exceeding that in the tropical Indian and Pacific Oceans. The results agree with recent argument that the relative Atlantic SST change or the SST difference between the tropical Atlantic and other oceans play an important role in controlling long-term TC activity in the Atlantic basin.

Abstract

Significant summer warming over the eastern Antarctic Peninsula in the last 50 years has been attributed to a strengthening of the circumpolar westerlies, widely believed to be anthropogenic in origin. On the western side of the peninsula, significant warming has occurred mainly in austral winter and has been attributed to the reduction of sea ice. The authors show that austral fall is the only season in which spatially extensive warming has occurred on the Antarctic Peninsula. This is accompanied by a significant reduction of sea ice cover off the west coast. In winter and spring, warming is mainly observed on the west side of the peninsula. The most important large-scale forcing of the significant widespread warming trend in fall is the extratropical Rossby wave train associated with tropical Pacific sea surface temperature anomalies. Winter and spring warming on the western peninsula reflects the persistence of sea ice anomalies arising from the tropically forced atmospheric circulation changes in austral fall.

Abstract

The impact on seasonal polar predictability from improved tropical and midlatitude forecasts is explored using a perfect-model experiment and applying a nudging approach in a GCM. We run three sets of 7-month long forecasts: a standard free-running forecast and two nudged forecasts in which atmospheric winds, temperature, and specific humidity (U, V, T, Q) are nudged toward one of the forecast runs from the free ensemble. The two nudged forecasts apply the nudging over different domains: the tropics (30°S–30°N) and the tropics and midlatitudes (55°S–55°N). We find that the tropics have modest impact on forecast skill in the Arctic or Antarctica both for sea ice and the atmosphere that is mainly confined to the North Pacific and Bellingshausen–Amundsen–Ross Seas, whereas the midlatitudes greatly improve Arctic winter and Antarctic year-round forecast skill. Arctic summer forecast skill from May initialization is not strongly improved in the nudged forecasts relative to the free forecast and is thus mostly a “local” problem. In the atmosphere, forecast skill improvement from midlatitude nudging tends to be largest in the polar stratospheres and decreases toward the surface.

Abstract

Over the past decades, Arctic climate has exhibited significant changes characterized by strong pan-Arctic warming and a large-scale wind shift trending toward an anticyclonic anomaly centered over Greenland and the Arctic Ocean. Recent work has suggested that this wind change is able to warm the Arctic atmosphere and melt sea ice through dynamically driven warming, moistening, and ice drift effects. However, previous examination of this linkage lacks a capability to fully consider the complex nature of the sea ice response to the wind change. In this study, we perform a more rigorous test of this idea by using a coupled high-resolution modeling framework with observed winds nudged over the Arctic that allows for a comparison of these wind-induced effects with observations and simulated effects forced by anthropogenic forcing. Our nudging simulation can well capture observed variability of atmospheric temperature, sea ice, and the radiation balance during the Arctic summer and appears to simulate around 30% of Arctic warming and sea ice melting over the whole period (1979–2020) and more than 50% over the period 2000–12, which is the fastest Arctic warming decade in the satellite era. In particular, in the summer of 2020, a similar wind pattern reemerged to induce the second-lowest sea ice extent since 1979, suggesting that large-scale wind changes in the Arctic are essential in shaping Arctic climate on interannual and interdecadal time scales and may be critical to determine Arctic climate variability in the coming decades.

Abstract

The onset of the Indian summer monsoon (ISM) over the southern tip of the Indian peninsula [also known as monsoon onset over Kerala (MOK)] has been considered the beginning of India’s rainy season. The Indian Meteorological Department (IMD) makes an official prediction of ISM onset every year using a subjective method. Based on an analysis of the past 60-yr (1948–2007) record, the authors show that the onset date can be objectively determined by the beginning of the sustained 850-hPa zonal wind averaged over the southern Arabian Sea (SAS) from 5° to 15°N, and from 40° to 80°E. The rapid establishment of a steady SAS westerly is in excellent agreement with the abrupt commencement of the rainy season over the southern tip of the Indian peninsula. In 90% of the years analyzed, this simple and objective index has excellent agreement with the onset dates that are subjectively defined by the IMD. There are only 3 yr of the past 60 yr during which the two onset dates differ by more than 10 days, and none of them perfectly reflects the MOK.

A prominent onset precursor on the biweekly time scale is the westward extension of the convection center from the equatorial eastern Indian Ocean toward the southeast Arabian Sea. On the intraseasonal time scale, the onset tends to be led by northeastward propagation of an intraseasonal convective anomaly from the western equatorial Indian Ocean. The objective determination of the onset based on the SAS low-level westerly is a characteristic representation of the complex process of the ISM onset. Given its objectiveness and its representation of the large-scale circulation, the proposed new onset definition provides a useful metric for verifying numerical model performance in simulating and predicting the ISM onset and for studying predictability of interannual variations of the onset.

Abstract

The global monsoon climate variability during the second half of the twentieth century simulated by 21 coupled global climate models (CGCMs) that participated in the World Climate Research Programme’s Coupled Model Intercomparison Project phase 3 (CMIP3) is evaluated. Emphasis was placed on climatology, multidecadal trend, and the response of the global monsoon precipitation to volcanic aerosols. The impact of the atmospheric model’s horizontal resolution on the group ensemble mean (GEM; obtained from the three resolution groups) simulations of global monsoon climate is also examined.

The CMIP3 CGCMs’ multimodel ensemble simulates a reasonably realistic climatology of the global monsoon precipitation and circulation. The GEMs are also able to capture the gross features of the global monsoon precipitation and westerly domains. However, the spreading among the rainfall GEMs is large, particularly at the windward side of narrow mountains (e.g., the western coast of India, the Philippines, Mexico, and the steep slope of the Tibetan Plateau). Main common biases in modeling rainfall climatology include a northeastward shift of the intertropical convergence zone (ITCZ) in the tropical North Pacific and a southward migration of the North Atlantic ITCZ during boreal winter.

The trend in the Northern Hemisphere land monsoon index (NHMI) detected in the CMIP3 models is generally consistent with the observations, albeit with much weaker magnitude. The significant decreasing NHMI trend during 1951–85 and 1951–99 occurs mainly in the models with volcanic aerosols (VOL models). This volcanic signal is detectable by comparison of the forced and free runs. It is estimated that from about one-quarter to one-third of the drying trend in the Northern Hemisphere land monsoon precipitation over the latter half of the twentieth century was likely due to the effects of the external volcanic forcings. On the other hand, the significant increasing trend in the global ocean monsoon index (GOMI) during 1980–99 appears chiefly in those models that are free of volcanic aerosols (No-VOL models). The exclusion of the volcanic aerosols is significant in simulating the positive GOMI trend against the internal variability of each model. These results suggest the climatic importance of the volcanic forcings in the global monsoon precipitation variability.