Impact of Gulf Stream SST biases on the global atmospheric circulation

Abstract

The UK Met Office Unified Model in the Global Coupled 2 (GC2) configuration has a warm bias of up to almost 7 K in the Gulf Stream SSTs in the winter season, which is associated with surface heat flux biases and potentially related to biases in the atmospheric circulation. The role of this SST bias is examined with a focus on the tropospheric response by performing three sensitivity experiments. The SST biases are imposed on the atmosphere-only configuration of the model over a small and medium section of the Gulf Stream, and also the wider North Atlantic. Here we show that the dynamical response to this anomalous Gulf Stream heating (and associated shifting and changing SST gradients) is to enhance vertical motion in the transient eddies over the Gulf Stream, rather than balance the heating with a linear dynamical meridional wind or meridional eddy heat transport. Together with the imposed Gulf Stream heating bias, the response affects the troposphere not only locally but also in remote regions of the Northern Hemisphere via a planetary Rossby wave response. The sensitivity experiments partially reproduce some of the differences in the coupled configuration of the model relative to the atmosphere-only configuration and to the ERA-Interim reanalysis. These biases may have implications for the ability of the model to respond correctly to variability or changes in the Gulf Stream. Better global prediction therefore requires particular focus on reducing any large western boundary current SST biases in these regions of high ocean-atmosphere interaction.

Mechanisms for a remote response to Asian anthropogenic aerosol in boreal winter

“…Used alongside HadGEM3-GC2, it provides additional insight into the origins and pathways of the large-scale atmospheric response to Asian anthropogenic aerosols. Such an approach has been successfully used by Teng et al (2012) and to demonstrate the potential of Asian emissions of black carbon to influence temperatures in the United States and Northern Hemisphere extratropics respectively and by Lewinschal et al (2013) to explain the Northern Hemisphere response to changes in global aerosol emissions.…”

“…LUMA is a linearized version of the Portable University Model of the Atmosphere (PUMA) (Fraedrich et al, 1998), which solves the steady-state primitive equations. The equations are linearized around a zonally symmetric basic state, and include tendencies for surface pressure, temperature, divergence, and vorticity (Liakka et al, 2012;Lewinschal et al, 2013). LUMA has a horizontal resolution of T21 and 10 levels in the vertical.…”

“…The basic state in LUMA is specified from the 1980-2005-mean zonal-mean zonal, meridional, and vertical wind, temperature, and sea level pressure from HadGEM3-GC2. Lewinschal et al (2013) showed that the anthropogenic aerosol-induced precipitation changes, and the associated diabatic heating, are the primary sources of Rossby waves in response to global aerosol emissions, and that the hemispheric-scale temperature pattern is the result of a wave response. Here, we will examine the role of precipitation anomalies in response to Asian anthropogenic aerosol in driving far-field responses.…”

“…As such, aerosols can induce substantial changes in atmospheric circulation and extend an influence over long distances (Rotstayn and Lohmann, 2002;Ramanathan et al, 2005;Allen et al, 2015). Indeed, the influence of remote forcing can outweigh that of local forcing (Shindell et al, 2012;Lewinschal et al, 2013). This adds additional complexity and complications in identifying the climate imprint of aerosols due to the large uncertainty in the atmospheric circulation response, especially at sub-continental scales, yet this is key to understanding drivers and projections of regional climate change.…”

“…Liu et al (2018) and Menon et al (2002) both identify remote responses to Asian aerosols in JJA. In addition, Chung and Seinfeld (2005), Allen and Sherwood (2011), and Lewinschal et al (2013) all identify far-field responses to global aerosol emissions in DJF, particularly at high latitudes in the Northern Hemisphere.…”