Upper troposphere circulation anomalies over Asia-Pacific region associated with the interannual variation of Indian summer monsoon

Monthly mean vector wind and geopotential heights at 200 hPa of 67 radiosonde stations from Asia Pacific regions for the period 1963-1988 are used to examine the composite circulation anomaly patterns for the month of May and the monsoon season (June- September) with respect to good monsoon years and bad monsoon years (both associated with ENSO and not associated with ENSO). There are significant differences in the anomalous circulation features between good and bad monsoon years. During the month of May an anomalous anticyclonic (cyclonic) circulation over-central Asia and an anomalous cyclonic (anticyclonic) circulation over Pacific ocean were observed during good (bad) monsoon years. These anomalies persist in the subsequent monsoon season. The key mechanisms of the development of these anomalous circulation patterns and their consequences are discussed.

Simulation of daily variation of precipitation during anomalously-wet dry season event over the western Maritime Continent

Local seas play a significant role in causing anomalously wet of the dry season over the Indonesia Maritime Continent (10°S-8°N, 95-145°E). As a result, modeling the anomalously-wet dry season over Indonesia lead challenges due to several subregional processes over local seas could not be captured well in the regional climate model. This study explores subregions processes of sea-air interaction over the western Maritime Continent by simulating diurnal precipitation using Cubic Conformal Atmospheric Model (CCAM) with a spatial resolution of 32 km during the anomalously-wet dry season periods during May-to-September (MJJAS) 2020. The simulated results were confirmed by precipitation data from Tropical Rainfall Measuring Mission (TRMM) satellite observation. The results show anomalous circulation patterns induce anomalous regional precipitation over western MC is induced by anomalous circulation patterns over four keys of seas subregion, i.e., Indian Ocean, South China Sea, southern Sumatra (Lampung and Sunda strait), and the Java Sea. Furthermore, the anomalous circulation also modulates anomalous local circulation and enhances surface water vapor by an increased surface latent heat flux.

Factors Regulating the Multidecadal Changes in MJO Amplitude over the Twentieth Century

This study examined multidecadal changes in the amplitude of the boreal-winter Madden–Julian oscillation (MJO) over the twentieth century using two century-long reanalysis datasets (20CR and ERA-20C). Both revealed reasonable MJO variability compared to other state-of-the-art reanalysis datasets. We detected pronounced multidecadal variations along with an increasing trend in MJO amplitude during the period 1900–2009 in both datasets, although this linear trend was less significant in the reconstructed MJO index proposed by Oliver and Thompson. The two twentieth-century reanalysis datasets and the Oliver–Thompson MJO index consistently showed the intensified amplitude of MJO precipitation and circulation in the later decades (1970–99) compared to the earlier decades (1920–49). The most significant enhancement of MJO precipitation in the later decades appeared over the western Pacific warm pool. To understand the mechanisms controlling the changes in western Pacific MJO precipitation amplitude over the twentieth century, we diagnosed the moisture budget equation. The enhanced MJO precipitation variability in the later decades mainly came from increased moisture associated with a strengthened low-level convergence anomaly working on background mean moisture []. Further diagnosis showed that the effect of anomalous circulation (∇ ⋅ V′) change on the MJO precipitation amplitude change over the twentieth century was about an order larger than that of mean moisture () change, different from the mechanisms (i.e., increased gradient of ) responsible for the intensified MJO precipitation amplitude under future warmer climate. The enhanced MJO circulation anomalies during 1970–99 may be caused by an enhanced diabatic heating anomaly, offset partly by the increased mean static stability.

Anomalous Upper-Ocean Circulation of the Western Equatorial Pacific following El Niño Events

Mooring measurements at ~140°E in the western equatorial Pacific Ocean documented greatly intensified eastward subsurface currents, which largely represent the nascent Equatorial Undercurrent, to ~67 cm s−1 in boreal summer of 2016. The eastward currents occupied the entire upper 500 m while the westward surface currents nearly disappeared. Historical in situ data observed similar variations after most El Niño events. Further analysis combining satellite and reanalysis data reveals that the eastward currents observed at ~140°E are a component of an anomalous counterclockwise circulation straddling the equator, with westward current anomalies retroflecting near the western boundary and feeding southeastward current anomalies along the New Guinea coast. A 1.5-layer reduced-gravity ocean model is able to crudely reproduce these variations, and a hierarchy of sensitivity experiments is performed to understand the underlying dynamics. The anomalous circulation is largely the delayed ocean response to equatorial wind anomalies over the central-to-eastern Pacific basin emerging in the mature stage of El Niño. Downwelling Rossby waves are generated by the reflection of equatorial Kelvin waves and easterly winds in the eastern Pacific. Upon reaching the western Pacific, the southern lobes of Rossby waves encounter the slanted New Guinea island and deflect to the equator, establishing a local sea surface height maximum and leading to the detour of westward currents flowing from the Pacific interior. Additional experiments with edited western boundary geometry confirm the importance of topography in regulating the structure of this cross-equatorial anomalous circulation.

Relationship between Thermodynamic Anomalies in Arabian Sea–Bay of Bangle on Rainy-season Precipitation in Yunnan

<p>This study examines how the thermodynamic anomaly in Arabian Sea (AS)–Bay of Bangle (BOB) relates to Yunnan precipitation in the rainy season. The observational diagnosis basing on data sets of atmospheric circulation reanalysis, precipitation from 124 stations in Yunnan and outgoing longwave radiation indicates that, when the thermodynamic anomaly in the AS–BOB is weaker during rainy-season, an anomalous anticyclone will control the AS–BOB. An anomalous cyclone in Yunnan resulted from the anomalous anticyclone in the AS–BOB induces anomalous water vapor converging with anomalous cold air in the same region. As a result, heavier-than-normal precipitation occurs in Yunnan in rainy-season. When the thermodynamic anomaly in the AS–BOB is stronger, the opposite configuration of anomalous circulation will cause less-than-normal precipitation in Yunnan. The results of several numerical experiments obtained from a linear baroclinic model support the key physical processes revealed in the observational diagnosis.</p>

Why do Benguela Niños lead Atlantic Niños?

<p>We investigate the lag between warm interannual Sea Surface Temperature (SST) events in the eastern equatorial Atlantic, the Atlantic Niños, and the occurrence of Benguela Niños along the southwestern Angolan coast. It is commonly agreed that both events are associated with equatorial and subsequent coastal-trapped wave propagations driven remotely by a relaxation of the trade-winds. Yet, we observe that coastal SST anomalies off Angola tend to precede the ones in the equatorial cold tongue region by ~1 month.</p><p>We explain this counter-intuitive behavior using experimentation with a tropical Atlantic Ocean model. Using idealized wind-stress perturbations from a composite analysis, we simulate warm equatorial and coastal events over a stationary and then, seasonally-varying ocean mean-state. Results show that when wind-stress perturbations are confined to the western central equatorial Atlantic, the model yields equatorial events leading the coastal variability, consistent with the propagation path of the waves. This implies that neither the differences in the ocean stratification between the two regions (thermocline depths or modal wave contributions) nor its seasonal variability controls the timing between events. Only if wind-stress anomalies are prescribed in the coastal fringe, the coastal warming precedes the eastern equatorial SST anomaly peak, emphasizing the role of the local forcing in the phenology of Benguela Niños.</p><p>Both warmings originate from a reduction in the strength of the South-Atlantic Anticyclone. Nevertheless, local processes initiate the coastal warming before the remotely-forced equatorial waves impact the eastern equatorial SST. Then, equatorward coastal wind anomalies, driven by a convergent anomalous circulation located on the warm Atlantic Niño, stop the remotely-forced coastal warming prematurely.</p><p>In conclusion, this study shows evidence that Atlantic and Benguela Niños are connected via an ocean teleconnection associated with equatorial and coastal wave propagations, but they are also tied by a large-scale atmospheric circulation and ocean-atmosphere interactions.</p>