Matt NewmanUniversity of Colorado/CIRES and NOAA/ESRL/PSD

Diagnosing the mean atmospheric moisture transport budget

Newman, M., G. N. Kiladis, K. M. Weickmann, F. M. Ralph, and P. D. Sardeshmukh 2012: Relative contributions of synoptic and low-frequency eddies to time-mean atmospheric moisture transport, including the role of atmospheric rivers. J. Climate, 25, 7341-7361. doi: http://dx.doi.org/10.1175/JCLI-D-11-00665.1

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How does atmospheric variability on different time scales affect total moisture

transport?• Synoptic scale Extreme precipitation events (atmospheric rivers)

• Short-term climate Droughts

• Long-term trends in hydrological cycle Interaction with secular trends in tropical SST

(e.g., Compo and Sardeshmukh 2009)

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Atmospheric moisture budgetw = precipitable water (<q>)Q = vertically integrated moisture flux = <vq><> = mass-weighted vertical integralq = specific humidityE = surface evapotranspirationP = precipitation

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Atmospheric moisture budgetw = precipitable water (<q>)Q = vertically integrated moisture flux = <vq><> = mass-weighted vertical integralq = specific humidityE = surface evapotranspirationP = precipitation

To explore moisture transport by different time scales, define

total = longterm mean + climate anomaly + synoptic anomaly(1968-2007) (periods>10 days) (periods <10 days)

When , the mean moisture budget is a balance between moisture flux divergence and the water source/sink:

where

Moisture transport (DJF), 1968-2007

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Mean moisture transport is mostly due to transport by the mean flow (flux in left panels is 6x right panels)

Moisture transport (DJF), 1968-2007

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Transport by synoptic anomalies is generally greater than transport by climate anomalies, except along west coast

Moisture transport (JJA), 1968-2007

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Moisture transport (JJA), 1968-2007

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Transport by low-frequency anomalies (e.g., summertime blocks) dominates moisture transport into Arctic

Seasonal cycle of meridional moisture transport

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Tropics and subtropics: Meridional moisture flux dominated by transport by mean flow

Midlatitudes:Meridional flux by synoptic anomalies equivalent to (or greater than)meridional flux by mean flow

Moisture transport into selected regions

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Left: moisture transport fromocean to land for selected regions

Right: moisture transport into the specified regions (from outside their boundaries)

Arctic is defined as the region north of 70N

The sign of all terms is chosen so that their sum is equal to P – E (blue bars)

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Meridional moisture transport by synoptic anomalies is importantNote that over this 2-month period (JF 2005), most flux occurs in fairly narrow meridional bands, termed atmospheric rivers.

This is a general result.

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Moisture transport is dominated by atmospheric river (AR) conditionsTotal moisture fluxcompared to moisture flux composited by atmospheric river criterion

Top: AR is region of narrow and intense moisture transportMiddle: AR is region of high IWVBottom: AR is region of low-level anomalous poleward wind

Shading is frequency of AR conditions

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Moisture and meridional wind DJF climatology

High-frequency: driven by lateral mixingLow-frequency: driven by moisture-wind covariance

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Understanding synoptic vs. low-frequency moisture transport

High-frequency: driven by lateral mixingLow-frequency: driven by moisture-wind covariance

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How do low-frequency anomalies act to transport moisture in the climatological mean? Top: regression of low-frequency low-level wind and precipitable water anomalies on leading Pacific or leading Atlantic 850 mb zonal wind PCBottom: composite of moisture transport over both+/-2 standard deviation PC1

Summary and Conclusions

• Mean moisture budget is primarily a balance between moisture transport by the mean flow and mean moisture source/sinks, especially within ocean basins

• However, synoptic and low-frequency variability drive about half of the extratropical meridional mean moisture transport Transport is focused within “atmospheric rivers” Low-frequency anomalies extract moisture from sub/extratropical

oceans and transport it to land Focus not just on transport but flux divergence/convergence

• Arctic transport dominated by low-frequency anomalies Can changes in variability drive changes in Arctic rather than the

other way around?16

Moisture flux products• 4x daily zonal/meridional vertically integrated flux and flux divergence• 4x daily precipitable water• 10-day Lanczos filtered (high/low pass) pw, shum, uwnd, vwnd, all levels

Data coverage• 20CR 1970-2011• ERA 1979-2011• NCEP/R1 1948-2011