Impacts of Multi-Timescale Circulations on Meridional Moisture Transport

Relative impacts of the climatological annual mean, the climatological annual variation, the synoptic, the intra-seasonal and the inter-annual flows on meridional moisture transport were investigated based on reanalysis data. Due to an in-phase relationship between the poleward wind and specific humidity, the synoptic and intra-seasonal motions contribute about 50% and 30% of the maximum zonal and annual mean poleward moisture transport in the middle latitudes, respectively. The preferred latitudinal location (40°N or S) of the maximum zonal mean moisture transport by the synoptic motion is attributed to the combined effect of the maximum wind variability poleward of 40°N or S in association with atmospheric baroclinic instability and the maximum moisture variability equatorward of 40°N or S in association with the anomalous advection of the mean moisture. While the MJO and ENSO have a small contribution to the long-term mean transport, they may strongly affect regional moisture transport through interaction with the mean moisture and through the modulation to higher-frequency modes. A statistical relationship between tropical cyclone (TC) moisture and intensity was constructed based on a large number of high-resolution Weather Research and Forecasting (WRF) model simulations, and the so-derived relationship was further applied to estimate TC moisture transport. It is found that TC transport accounts for about 30% (53%) of the climatological seasonal mean total moisture transport over key northern (southern) hemispheric TC track regions in the northern (southern) hemispheric TC season.

Role of the South China Sea in Southern China rainfall: meridional moisture flux transport

The South China Sea (SCS) serves as the main source of moisture for rainfall in Southern China (SC) and the meridional moisture transport to SC is dominated by wind changes during the first rainy season (April–June). El Niño-Southern Oscillation (ENSO) and Tropical Northwestern Pacific (TNWP) variability modulate the SC rainfall through anomalous anticyclonic circulation over the western North Pacific by strengthening the SCS meridional moisture transport to SC. However, our study indicates that the SCS is not only the intermediary in which ENSO or the TNWP affects the SC rainfall but also plays an independent role in the modulation of the SC rainfall. Notably, the SCS meridional moisture transport has a lower impact on the SC rainfall during the second rainy season (July–September), especially in July. At that time, the main cause of the SC rainfall is the southward moisture flux anomaly across its northern boundary with the anomalous cyclone over SC. This cyclone suppresses the moisture flux out of SC and leads to moisture convergence in SC. Moreover, we present a new concept by analyzing internal differences of moisture circulation during the second rainy season. Either strengthening the meridional moisture flux into SC across its southern boundary or suppressing the moisture flux out of SC across its northern boundary is important depending on whether or not the moisture from the SCS can converge in SC, which is mainly determined by the amplitude of moisture transport fluxes in SC.

Atmospheric circulation controls on the inter-annual variability in precipitation isotope ratio in Japan

This study explored the primary driver of variations of precipitation isotopes at multiple temporal scales (event, seasonal and inter-annual scales) to provide a greater depth of interpretation for isotope proxy records in Japan. A one-year record of the isotopic composition of event-based precipitation at Nagoya in central Japan showed less seasonal variation, but there is large isotopic variability on a storm-to-storm basis. In the summer, southerly flows transport isotopically enriched moisture from subtropical marine regions with the result that the rainfall produced by the subtropical air, or warm rainfall, was relatively enriched in heavy isotopes in comparison with the other rainfall events. In the winter, storm tracks are the dominant driver of storm-to-storm isotopic variation, and relatively lower isotopic values occurred when northerly winds in association with extratropical cyclones passing off the south coast of Japan (Nangan cyclone) brings cold precipitation. Using the historical 17 year record of monthly isotopes in precipitation at Tokyo station, we explored if the factors controlling event-scale isotopic variability can account for inter-annual isotopic variability. The relatively higher isotopes in summer precipitation were attributed to the higher contribution of the warm rainfall to the total summer precipitation. On the other hand, year-to-year variation of isotopic values in winter precipitation was negatively correlated with the relative ratio of the Nangan cyclone rainfall to the total winter precipitation. The 17 year precipitation history demonstrates that event-scale isotopic variability related to changes in meridional moisture transport is the primary driver of inter-annual isotopic variability in winter and summer precipitation. The meridional moisture transport to central Japan is likely linked to the activity of the western North Pacific subtropical high in summer and the intensity of the East Asian winter monsoon in winter. Therefore, isotope-based proxy records archived in central Japan may enable us to examine past atmospheric circulation changes in East Asia in response to climate variability.

Atmospheric Meridional Moisture Flux over the Southern Ocean: A Story of the Amundsen Sea

The Antarctic ice sheet constitutes the largest reservoir of freshwater on earth, representing tens of meters of sea level rise if it were to melt completely. However, because of the remote location of the continent and the concomitant sparse data coverage, much remains unknown regarding the climate variability in Antarctica and the surrounding Southern Ocean. This study uses the high-resolution ECMWF Interim Re-Analysis (ERA-Interim) data during 1979–2010 to calculate the meridional moisture transport associated with the mean circulation, planetary waves, and synoptic-scale systems. The resulting moisture flux, which is dominated by the synoptic scales, is largely consistent with results from theoretical assumptions and previous studies. Here, high interannual and regional variability in the total meridional moisture flux is found, with no significant trend over the last 30 years. Further, the variability of the meridional moisture flux cannot be explained by the southern annular mode or El Niño–Southern Oscillation, even in the Pacific sector. In addition, the Amundsen Sea sector experiences the highest variability in meridional moisture transport and reveals a statistically significant decrease in the moisture flux at synoptic scales along the coastal zone. These results suggest that the Amundsen Sea provides a window on the complex nature of atmospheric moisture transport in the high southern latitudes.