Summer teleconnections

ENSO Summertime Teleconnections & Impacts on U.S. during a Multiyear La Niña Life Cycle

Previous studies have shown that maize and soybean yields over major crop-producing area of the United States were reduced in developing La Niña summers (link), caused by an anomalous anticyclone typically sits over North America and thereby anomalously warm temperature over the central U.S. We examined the physical processes that lead to these strong extratropical responses in developing La Niña summers (June-August).

It is well-known that a La Niña tends to persist through the following summer and often reintensifies in the subsequent winter, leading to a multiyear La Niña event. We further found out that all the first-year La Niñas since 1950 transitioned from El Niño winters (with the only exception of the ongoing 2020-2023 La Niña). Therefore, developing La Niña summers can be when an El Niño is transitioning to a La Niña (Fig.1 a-c) or when a La Niña is persisting from one year to the next (Fig.1 d-f).

Our results show that during the transitioning summer, both the previous El Niño and the developing La Niña induce tropical forcings over the tropical Pacific and the corresponding Rossby waves propagating toward North America (Fig.1c), resulting in a significant anomalous ridge over Northeast North America and therefore robust warming signal over the Midwest US (Fig.2c). These features are unique to the developing La Niña transitioning from El Niño, but not the persisting La Niña (Fig.1f and 2f).

Fig.1 Composites of precipitation anomalies (shaded; mmday21) and 200-hPa geopotential height anomalies with the zonal-mean removed (contours; interval: 5m) for the (left) transitioning and (right) persisting La Niña summers from (a),(d) the preceding winters D(-1)JF(0) and (b),(e) the preceding springs MAM(0) to (c),(f) the developing La Niña summers JJA(0). The differences in the composites between the transitioning and persisting La Niña summers are shown in (g). (From Fig. 3 in Jong et al. 2020)

Fig. 2 As in Fig. 1, but for composites of CRU detrended surface temperature. Stippling denotes the 90% significance for detrended surface temperature anomalies using a two-tailed Student’s t test. (From Fig. 4 in Jong et al. 2020)

For more details and stationary wave model experiments, please see our publication:
Jong, B.-T., M. Ting, R. Seager, and W. B. Anderson (2020): ENSO teleconnections and impacts on US summertime temperature during multi-year La Niña life-cycle. J. Clim., 33, 6009-6024. http://doi.org/10.1175/JCLI-D-19-0701.1

Assess ENSO summer teleconnections in climate models

We further assessed some state-of-the-art climate models in capturing the prominent observed extratropical responses over North America during the transitioning La Niña summer. We focused on the three crucial components that link the La Niña tropical SST anomalies to the U.S. surface temperature responses: tropical precipitation, teleconnection patterns triggered by the tropical forcing, the extratropical surface responses due to the teleconnections.

We found that while models with active air-sea interaction can simulate anomalous precipitation in the tropical Pacific quite well, the downstream wave train pattern and the resulting extratropical responses over North America exhibit large disagreement across the models and are consistently weaker than in observations. Furthermore, in these climate models, an anomalous anticyclone does not robustly translate into a warm anomaly over the Midwest, in disagreement with observations. These suggest that our current climate models have limited skill to simulate ENSO extratropical responses in the summer season. Improving representations of the Rossby wave propagation and land–atmosphere processes might be helpful.

Composites of precipitation anomalies (shaded; mm/day) and 200-hPa geopotential height anomalies with the zonal mean removed (contours; interval: 5m) for the transitioning La Niña summers during 1982–2018 from (a) NMME multimodel ensemble mean and (b)–(h) the ensemble means of the individual models.

Composites of detrended Ts for the transitioning La Niña summers during 1982–2018 from (a) NMME multimodel ensemble mean and (b)–(h) the ensemble means of the individual models.

For more details about this work, please see:

Jong, B.-T., M. Ting, and R. Seager (2021): Assessing ENSO summer teleconnections, impacts, and predictability in North America. J. Clim., 34, 3629-3643. http://doi.org/10.1175/JCLI-D-20-0761.1

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