[1] In tropical regions, the empirical negative relationship between the isotopic content of precipitation and rainfall amount, known as the "amount effect", has been used as a rationale for paleohydroclimate reconstruction from isotope records. However, there is still no comprehensive physical explanation for this empirical effect. Here we reconsider the well-known amount effect using newly available isotope data for both surface water vapor and precipitation obtained from shipboard observations. In this study, we hypothesized that stratiform rainfall associated with mesoscale convective systems (MCSs) is a key process in reducing tropical water isotopic concentrations and tested this hypothesis with an idealized MCS model. Our conceptual model reasonably accounted for several observed features and indicated that isotopic reductions in tropical oceanic regions reflect a precipitating system's change. Relatively high isotope ratios corresponded to disorganized convection. On the other hand, MCSs were characterized by lower isotope ratios with increasing stratiform area. In addition, the amplitude of this isotopic depletion was related to the scale of the precipitation system. The lowest isotopic ratios were observed during the passage of large-scale disturbances-corresponding to the Madden-Julian oscillation convective envelope-in which MCSs are embedded. This means that the frequent appearance of MCSs results in further decreases in the isotopic ratio of surface vapor and precipitation. From this, we conclude that the amount effect can be interpreted as the development of a precipitation system from an isolated convection cell into a large-scale system containing several MCSs and that past large-scale convective activity can be reconstructed from isotope records. Kurita, N. (2013), Water isotopic variability in response to mesoscale convective system over the tropical ocean, Citation:

[1] The extent of summer Arctic sea ice has reduced dramatically in recent years and, simultaneously, we have observed surface freshening over the Canada Basin in 2006 and 2007. In order to identify the source of this fresh water, either meteoric or sea ice meltwater, salinity, d 18 O, and alkalinity were analyzed. Results show that sea ice meltwater increased in the surface water over the central part of the basin in 2006 and 2007, corresponding to the melting of an additional 2.7 m (1.3 m a À1 ) of sea ice. Anomalously fresh surface water observed in the southern part in 2007, however, was mostly attributed to Mackenzie River water extending into the basin interior, a source that was mainly absent in the early 2000s. Comparison with previous data shows that the meltwater component of surface water in the southern part of the Canada Basin has progressively increased at a mean rate of 0.27 m a À1 since 1987. This can be explained by a reduction of winter sea ice formation rate by 0.45 m or more during the past two decades. The runoff component showed larger variability in the southern basin but no obvious temporal trend. In the central basin, the river runoff component showed an increasing trend of 0.7 m a À1 .

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[1] The Madden-Julian Oscillation (MJO) is the dominant mode of intraseasonal variability in the tropical atmosphere. This study examines the evolution of the hydrologic regime from before the onset of the MJO (pre-onset period) to the MJO onset period, using deuterated water vapor (HDO) measurements from the Tropospheric Emission Spectrometer (TES) and from ground-based stations. Ground-based observations reveal a clear transition between high HDO/H 2 O isotope ratios during the pre-onset period to a period of repeated abrupt decreases in the HDO/H 2 O isotope ratio associated with intense convection. Each observed minimum in the HDO/H 2 O ratio corresponded to a maximum in stratiform rainfall fraction, which was derived independently from radar precipitation coverage area. The ground-based observations are consistent with the satellite observations of the HDO/H 2 O ratio. In order to attribute the mechanisms that bring about the isotopic changes within the MJO convection, an isotope-enabled general circulation model (GCM) constrained by observed meteorological fields was used to simulate this MJO period. The GCM reproduced many of the observed isotopic features that accompanied the onset of an MJO. After the development of deep convection, large-scale stratiform cloud cover appears, and isotope ratios respond, as a consequence of diffusive exchange between stratiform raindrops and the surrounding vapor. In this diffusive exchange process, heavy isotopes tend to become enriched in precipitation and depleted in the surrounding vapor, and thus successive stratiform rainfall results in decreasing isotope values in the middle and lower troposphere. On the basis of these characteristics, isotope tracers can be used to partition stratiform and convective rainfall from observed isotope data and to validate the simulated proportions of convective/stratiform rainfall.

Recent extreme minima in Arctic summer sea ice extent have led to enhanced heat flux from the ocean to the atmosphere. This change may increase the humidity in Arctic air masses during the ice‐growth season. Humidity increases may also be sustained by enhanced moisture transport into the Arctic and the relative influence of local‐ versus distant‐moisture sources remains uncertain. Here we examined the predominant origin of Arctic water vapor during the ice‐growth period, using water isotopologues (HDO, H218O) as tracers. An exploration of the isotopic evolution of surface water vapor in the Arctic Ocean found that isotopic values of moisture originating from the Arctic Ocean were characterized by higher d‐excess values, a second‐order isotopic parameter, than those of moisture originating from lower latitudes. These high d‐excess values (>20‰) in Arctic‐origin air masses were observed in mid‐autumn. Subsequently, high d‐excess values gradually decreased to the global average (d = 10) and disappeared in early winter, when sea ice covered a large part of the Arctic Ocean. This change suggests that the humidity source of Arctic air masses switches in early winter from locally driven to moisture transport from lower latitudes.