January 18, 2022 9:44 pm

Impact of Gulf Stream Errors on Forecasts

Francisco Martin Leon 6 min
Satellite image of the ocean surface temperature, SST, estimated in the North Atlantic. NASA Wikipedia

The ECMWF Integrated Forecasting System (IFS) includes a representation of the state of the ocean in 3D that evolves over time. East coupling to a dynamic ocean model improves representation of air-sea interactions and it has been shown that improves forecasts in the sub-seasonal to seasonal time range (S2S, from about 2 weeks to about 2 months). However, the inclusion of additional ocean processes also introduces the possibility of systematic errors in the sea-surface temperature (SST), which can have a negative impact on the quality of the forecast.

Ocean models with a grid spacing of approximately 25 km, as used in the IFS, struggle to accurately simulate the location and structure of the Gulf Stream and its associated strong gradients in SST.

Here we describe the Recent work at ECMWF to assess the impact of such SST errors on S2S forecasts using a methodology of SST bias correction online. The baseline S2S experiment (CTRL) is a 15-member set initialized on the 1st and 15th of each month of an extended winter period (November-March) from 1989 to 2015 (that is, 270 start dates). The bias correction experiment (BCFC) is the same as the CTRL, but the observed SSTs for the atmosphere in the North Atlantic region are adjusted for the SST bias of the CTRL-derived model, which varies with location, the calendar start date and expected delivery time.

Impact of SST bias correction on southerly wind forecast ability at 200 hPa. Changes in Continuous Classified Probability Skill Score (CRPSS; shaded) and Anomaly Correlation (0.1 gray outline spacing) are shown for weekly mean anomalies derived from forecast days 26 to 32. The contours Yellow highlights the position of the northern hemisphere waveguide diagnosed from the southern gradient of absolute vorticity. (See Figure 3 of the article in Geophysical Research Letters.

Experiment results

The applied bias correction effectively reduces SST biases in the North Atlantic region. The resulting southward shift of the Gulf Stream drives changes in convective precipitation and vertical motion (not shown), which has consequences for atmospheric predictability beyond the North Atlantic. In particular, we found that reducing North Atlantic SST biases leads to forecasts S2S significantly enhanced from atmospheric circulation anomalies in Europe. Furthermore, the impacts extend beyond the North Atlantic and Europe and circumnavigate the globe along the subtropical waveguide of the northern hemisphere (the figure shows an example). This response is typical of the propagation of stationary Rossby wave activity along the jet stream..

Interestingly, this impact of SST biases on forecasting ability is modulated by the Madden-Julian Oscillation (MJO). Forecasts with an active MJO in the initial conditions exhibit a stronger impact over Europe and along the Northern Hemisphere waveguide, while forecasts without an active MJO have a stronger response over the Gulf Stream and the Atlantic. North. This sensitivity is not related to changes in the forecasting ability of the OMJ, which is not sensitive to North Atlantic SST biases. Rather, we speculate that this effect is a consequence of the OMJ’s impact on the background state and its associated teleconnections that direct or obstruct planetary wave activity that begins in the North Atlantic region.

In conclusion, the results of these sensitivity experiments provide important evidence of the potential benefits for ECMWF forecasts from higher resolution ocean models (that is, with a grid spacing of less than 10 km) that can better resolve the position of the Gulf Stream. Such models will be investigated at ECMWF over the next several years as part of the recently funded Horizon 2020 NextGEMS project.

More information on the impact of SST biases on ECMWF forecasts can be found in an article by the authors titled:

Hemispheric impact of North Atlantic SSTs in sub‐seasonal forecasts’ in Geophysical Research Letters, https://doi.org/10.1029/2020GL091446.

Christopher D. Roberts, Frédéric Vitart, Magdalena Balmaseda


This entry was published in News on Nov 28, 2021 by Francisco Martín León


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