Exploring the Tropically Excited Arctic Warming Mechanism with Station Data: Links between Tropical Convection and Arctic Downward Infrared Radiation Steven Feldstein, Matthew Flournoy, Sukyoung Lee, and Eugene Clothiaux The Pennsylvania State University 2015 AGU Fall Meeting, San Francisco, December 16, 2015

(2) eastward acceleration at the wave source; if strong enough, generates equatorial superrotation pole height equator (1) Zonally localized convective heating (3) Poleward and upward propagating waves transport heat poleward (a) Poleward eddy heat flux (4) Storm track eddies may respond, and transport heat farther poleward Tropically Excited Arctic warMing (TEAM) Mechanism

pole height equator (b) Downward IR (7) Poleward heat flux is accompanied by moisture flux (8) Moisture condenses, clouds form, and emit downward IR Tropically Excited Arctic warMing (TEAM) Mechanism

Questions: 1. Previous studies of the TEAM mechanism have used ERA- 40 and ERA-Interim data (Lee et al. 2011; Lee 2012, 2014). Is the veracity of the TEAM mechanism borne out in BSRN station downward infrared radiation (IR) data? Methods: Regress key variables, e.g., 2-m temperature, OLR, moisture flux and its convergence, cloud liquid and frozen water, σT 4 and cloud fraction, against BSRN Barrow and Ny- Ålesund downward IR. Data: ERA-Interim Reanalysis, NOAA OLR, BSRN downward IR data at Barrow and Ny-Ålesund.

Regressed 2-m Temperature (against Barrow downward IR) Barrow

Regressed 300-hPa Geopotential Height (against Barrow downward IR) Barrow Signature of tropically forced wave train

Regressed NOAA/CPC PNA Index

Regressed Outgoing Longwave Radiation Lag day -14 Lag day -10 Lag day -6 Enhanced convection

Regressed Multivariate MJO Indices (Wheeler and Hendon 2004)

Regressed water vapor flux vector & convergence multiplied by latent heat of vaporization Regressed Downward IR Poleward moisture flux

Cloud Liquid Water Frozen Water

Low clouds 1.0 > σ > 0.8 Mid clouds 0.8 > σ > 0.45 High clouds 0.45 > σ σT 4 cloud fraction

CONCLUSIONS An increase in BSRN downward IR at Barrow (and Ny-Ålesund) is consistent with the TEAM mechanism: Enhanced downward IR at this Arctic stations occurs following stronger convection in the western tropical Pacific & the MJO, poleward Rossby wave propagation, and an increased moisture flux into the Arctic. Arctic surface warming arises from (1) latent heat release, (2) warm advection throughout troposphere, an increase in (3) water vapor and (4) clouds, which result in greater downward IR.

Implications The TEAM mechanism operates on intraseasonal, interannual, and interdecadal time scales. Interdecadal Arctic warming is associated with an increase in the frequency of intraseasonal TEAM events (Feldstein and Lee 2014). The results of this study suggest that Arctic Amplification in climate models may more accurate with improvement in the parameterization of tropical convection.