Comparing surface wind stress and sea surface temperature biases over the tropical and subtropical oceans in subsets of CMIP6 models categorized by frozen hydrometeors-radiation interactions

Abstract

We evaluate the simulations of surface wind stress (TAU) and sea surface temperature (SST) over subtropical and tropical Pacific and Atlantic oceans in subsets of CMIP6 models that are categorized by frozen hydrometeors-radiation interactions. The CMIP6 models are divided into two subsets with combined (SON1) and separated (SON2) radiative properties of cloud ice and falling ice (snow) and compared to the set with cloud ice radiative effects only (NOS). There is evidence that these hydrometeors-radiation interaction treatments induce different atmospheric dynamic responses that influence the surface properties. Excessive westerly TAU and meridional TAU divergence away from convective zones are reduced significantly in SON1 and SON2 relative to NOS against QuikSCAT observations; while the differences between SON2 and SON1 are small. SON2 reduces cold SST biases over north oceans and equatorial zones drastically (1 to 2 K), and warm biases (up to 1K) off the coasts of America and zonal TAU biases are reduced relative to NOS. Unlike SON2, SON1 improves SSTs mainly over south of Pacific Ocean and limited areas over the tropical belts relative to NOS although TAU is reduced drastically as in SON2, implying that other factors play a role in degrading the SST simulations in SON1 relative to SON2. SON2 outperforms NOS and SON1 in the seasonal cycles of SST mean biases and mean absolute biases averaged over the equatorial area, north ocean, and South Pacific against ERSST observations. Despite the significant improvements in TAU and SST simulations, SON2 models still exhibit non-trivial biases over south and north flanks of equatorial zones. These results suggest that there are direct linkages of TAU with SST changes resulting from the hydrometeors-radiation interactions in SON2, but not in SON1, relative to NOS, implying that a separated treatment of cloud ice and falling ice radiative properties in climate models is preferred.

Comparisons of simulated radiation, surface wind stress and SST fields over tropical pacific by the GISS CMIP6 versions of global climate models with observations

“…In particular, convective snow and stratiform + convective forms of rain are not included in our previous experiences (Li et al 2015(Li et al , 2016. We argue that including rain radiative effects might not have large impacts on the Pacific climates, but adding stratiform snow can have substantially changes the simulation of Pacific climate found by, for example, Li et al (2015Li et al ( , 2016Li et al ( , 2020bLi et al ( , 2022aLi et al ( , 2022b and Michibata et al (2019). It might be worth to separate the roles played by the stratiform and convective snow radiative effects and their impacts on the circulation-radiation coupling in GISS-E3.…”

“…Li et al (2015Li et al ( , 2016 examined this mechanism by turning on and off falling ice radiative effects (FIREs) using fully-coupled historical runs of the CESM1-CAM5 following CMIP5 protocol. The FIREs-off experiment, which is the same experiment as most of the GCMs performed in the CMIP5/CMIP6, do not consider the radiative impacts of larger size (∼100 to 120 um) of snow that often result in further biases in the radiation fields (Waliser et al 2011, surface wind stress, sea surface temperatures and precipitation (Li et al 2014a(Li et al , 2014b(Li et al , 2016(Li et al , 2020a(Li et al , 2022a. For example, it has been reported that current coupled GCMs (e.g., CMIP5/CMIP6) show biases of underestimated TOA reflected shortwave radiation, excessive downward shortwave radiation at the surface and overestimated outgoing longwave radiation over precipitation active regions such as the ITCZ/SPCZ and TWP (e.g., Li et al 2013Li et al , 2014aLi et al , 2014bLi et al , 2020a.…”

Possible linkage of sea surface height anomaly, surface wind stress and sea surface temperature with the falling ice radiative effects under a gradual warming scenario

“…This leads to a weaker surface wind stress and decreasing upper ocean mixing, resulting in warmer SSTs over the trade-wind regions (Li et al 2014(Li et al , 2016(Li et al , 2018. The inclusion of FIREs is shown to improve the simulated tropical climate states of radiation fields, TAU and SST in CMIP5 and CMIP6 (Li et al 2014(Li et al , 2015(Li et al , 2022b and SSHA (Li et al 2020(Li et al , 2022a for the present-day climate simulation. Chen et al (2018) further illustrated that the radiation-circulation response to global warming would present similar patterns to those of the present-day climate simulations.…”

“…Several studies have investigated the present-day SSH in climate models and their ensembles, such as Community Earth System Model (CESM) (Fasullo and Nerem 2018), Coupled Model Intercomparison Project phase 5 (CMIP5) (Hay et al 2015, Widlansky et al 2015, Li et al 2022a, and CMIP6 (Dhage and Widlansky 2022). They all pointed out in common that the rising sea level is associated with warmer SST over certain regions, for example, the tropical Pacific is expected to have a higher rate of sea level rise compared to the global average changes of SSH.…”

“…Chen et al (2018) further illustrated that the radiation-circulation response to global warming would present similar patterns to those of the present-day climate simulations. Li et al (2022a) linked sea surface height anomaly (SSHA), surface wind stress, and sea surface temperature with the falling ice (snow) radiative effects over the tropical and subtropical Pacific Ocean in the present-day CMIP5 historical runs using sensitivity experiments with FIREs-off and FIREs-on using CEMS1-CAM5 and obs4MIPs monthly SSH satellite measurements as a reference. The results showed that the performance of SSHA with FIREs-on was improved, which was associated with improved SST and TAU over the trade-wind areas and that the mean absolute bias of SSHA was reduced up to 30% against NOS relative to observations.…”