Abstract
The simulated Northern Hemisphere stationary wave (SW) field is investigated in 11 subseasonal-to-seasonal (S2S) models. It is shown that while most models considered can well-simulate the stationary wavenumbers 1 and 2 during the first two weeks of integration, they diverge from observations following week 3. Those models with a poor resolution in the stratosphere struggle to simulate the waves, both in the troposphere and the stratosphere, even during the first two weeks, and biases extend from the troposphere all the way up to the stratosphere. Focusing on the tropospheric regions where SWs peak in amplitude reveals that the models generally do a better job in simulating the Northwest Pacific stationary trough, while certain models struggle to simulate the stationary ridges both in Western North America and the North Atlantic. In addition, a strong relationship is found between regional biases in the stationary height field and model errors in simulated upward propagation of planetary waves into the stratosphere. In the stratosphere, biases mostly are in wave-2 in those models with high stratospheric resolution, whereas in those models with low resolution in the stratosphere, a wave-1 bias is evident, which leads to a strong bias in the stratospheric mean zonal circulation due to the predominance of wave-1 there. Finally, biases in both amplitude and location of mean tropical convection and the subsequent subtropical downwelling, are identified as possible contributors to biases in the regional SW field in the troposphere.
Highlights
The Northern Hemisphere (NH) climate is not uniform in longitude, despite the incoming solar radiation being roughly zonally symmetric on a daily average
The positive bias in ω over the Maritime continent and West Pacific Warm Pool is evident in all other models, with the National Center for Environmental Prediction (NCEP), European Centre for Medium-Range Weather Forecasts (ECMWF)-2016 United Kingdom Met. Office (UKMO)-2016, Bureau of Meteorology (BoM), Korean Meteorological Administration (KMA) and CNRM-2019 simulating a too-strong SPCZ. 265 we examine the regional biases in the tropics that possibly have an impact on the extratropical mean state
We showed that most models have like-signed biases over the North Pacific, Western North America and the North Atlantic regions, regions where the amplitude of stationary wave (SW) peaks, and most models simulate too weak a SW in these regions
Summary
The Northern Hemisphere (NH) climate is not uniform in longitude, despite the incoming solar radiation being roughly zonally symmetric on a daily average. The role played by regional diabatic heating biases in generating biases in extratropical SWs in the Coupled Model Intercomparison Project Phase 5 (CMIP5) models has been recently demonstrated by Park and Lee (2021) They show that biases in largescale tropical convection, both over the Western Pacific and Western Atlantic regions, have direct and indirect impacts on SWs biases over the North Pacific and North Atlantic sectors. Schwartz and Garfinkel (2020) investigated biases in SWs in 5 subseasonal forecast 60 models to examine their SW pattern biases and their implications for upward coupling resulting from intraseasonal tropical variability in the troposphere.
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