Impact of projected sea surface temperature biases on tropical cyclones projections in the South Pacific

Abstract

too far eastward 11,12 . Forcing a high-resolution atmospheric model with those biased boundary conditions induces systematic biases on the simulated TC climatology, and probably hampers the future projections 13,14 . A commonly-used strategy to alleviate this shortcoming is to prescribe "climate change anomalies", i.e. atmospheric simulations are forced by observed present-day SSTs, onto which the anomalous ensemble mean sea surface warming pattern from CMIP5 projections is added 15 . This approach is sometimes referred to as pseudo-global warming (PGW) downscaling, although this term was also used to describe the application of more ad-hoc anomalies 16 . Strong uncertainties in regional projections of future TC activity also arise from the various projected SST warming patterns for a given scenario in the CMIP database 17-21 . For instance, Murakami et al. 18 showed a significant variation in future TC number per basin depending on the CMIP3 SST pattern used. This raises the issue of the sensitivity of TC projections to the projected SST change does not correct.Using a multi-model ensemble mean projected SST change as the one displayed in Fig. 1e reduces the projected SST errors as it largely cancels the model-dependent part of the errors 22 . Applying the PGW approach with the multi-model ensemble mean projected SST change therefore improves the reliability of projections. This method however does not correct for the systematic biases (i.e. common to all models) in the SST response to anthropogenic forcing. A way to circumvent this issue is to correct the projected warming using a statistical Scientific RepoRtS | (2020) 10:4838 | https://doi.

Fine-scale rainfall over New Caledonia under climate change

“…These results indicate that correcting for the projected SST change bias does not qualitatively modify the precipitation projections over New Caledonia. In contrast, Dutheil et al (2020) show that South Pacific projected Tropical Cyclone activity changes area strongly sensitive to the SST changes, with a clear reduction when the projected SST change is corrected that does not materialize without correction. In that case, the drastic changes described in Dutheil et al (2019Dutheil et al ( , 2020 on the SPCZ drying and cyclone activity is not as striking and that may be due to the remote situation of NC which lies on the edges of the SPCZ influence.…”

“…In contrast, Dutheil et al (2020) show that South Pacific projected Tropical Cyclone activity changes area strongly sensitive to the SST changes, with a clear reduction when the projected SST change is corrected that does not materialize without correction. In that case, the drastic changes described in Dutheil et al (2019Dutheil et al ( , 2020 on the SPCZ drying and cyclone activity is not as striking and that may be due to the remote situation of NC which lies on the edges of the SPCZ influence. However, we hypothesize that for other islands that lie nearer the SPCZ centre of action, implementing such downscaling strategy with corrections will be important.…”

“…1a). To simulate the NC climate, we further include a 21-km resolution two-way nested domain located over the Southwest Pacific [145°E-130°W; 32°S-2°S] as in Dutheil et al (2020). A third level of nesting, a 1-way nesting, allows to reach a 4.2 km horizontal resolution over the (Fig.…”

“…Li et al (2016) applied this methodology to reduce biases in the SST change projected by CMIP models in the tropical Pacific, resulting in a reduction of the ensemble-mean western Pacific warming and of the spread in projected SST changes across models. Dutheil et al (2019Dutheil et al ( , 2020 have recently applied this emergent constraint method within the PGW framework to force a regional atmospheric model over the western Pacific. Their results demonstrate that simulations forced with bias-corrected CMIP SST projections yield a considerably larger (~ 25%) rainfall reduction under the SPCZ than in those forced with uncorrected SST CMIP projections (~ 7% rainfall reduction).…”

“…To that end, we use a South Pacific regional configuration of the Weather Research and Forecasting (WRF) model similar to that in Dutheil et al (2019) by using a PGW downscaling method where the projected SST anomalies are corrected following Li et al (2016)'s methodology. As in Dutheil et al (2019Dutheil et al ( , 2020, we also employ a nesting strategy to increase the horizontal resolution up to ~ 4 km around the NC archipelago in order to improve the NC topography and the reliability of climatic projections at the island scale. A similar strategy had been applied by Zhang et al (2016ab) to downscale the Hawaiian climate, although that study did not attempt correcting the projected SST change as we do in the present work.…”