Creating a La Nina to El Nino Transition Climatology
Summary: An interesting discussion this month focuses on climatology for La Nina to El Nino ENSO regimes. There are many operational examples of the increasing evidence these ENSO phase changes of causing dramatic climate. The most recent was an El Nino to La Nina transition late last summer when a record accumulated cyclone energy (ACE) index was observed in September across the North Atlantic including devastating influences on the U.S. The opposite condition may occur the next 3-6 month: Weak La Nina to weak El Nino. This report is focused on a climatology for these ENSO phase transitions.
Fig. 1: Current global SSTA observations.
Current conditions: La Nina 2017-18 is fading with both the SSTA signature in the equatorial East Pacific (Fig. 1) and influence on the atmosphere based on southern oscillation index (SOI) and multivariate ENSO index (MEI) trend.
The sub-tropical North Pacific remains very warm and may become a warming factor regarding ENSO phase during the summer season. The Pacific decadal oscillation (PDO) is weak after 2 strong events, cool phase 2011-2013 and warm phase 2014-2016. In the North Atlantic the tropics have cooled sharply in 2018 after a robust anomalous warm 2017. The waters to the north are marginally warm. The cool/warm mix leads to a neutral phase of the Atlantic multi-decadal oscillation (+AMO). The Indian Ocean tropics have fluctuated dramatically in recent weeks and are averaging warmer than normal although Indian Ocean Dipole (IOD) is neutral.
Fig. 2: Bureau of Meteorology/Australia subsurface equatorial Pacific temperature anomaly analysis identifies warming shifting east.
Key to the ENSO phase forecast is the current subsurface temperature anomalies in the equatorial central and east portions of the Pacific Ocean. Based on the Bureau of Meteorology/Australia analysis (NOAA is similar) a warm Kelvin Wave is pushing eastward into the East Pacific (Fig. 2). Normally, this set of circumstances not only defeats La Nina but increases the risk of El Nino.
Forecast conditions: Nino34 SSTA forecasts clearly erode La Nina followed by neutral ENSO phase for northern hemisphere summer (Fig. 3). However, recent global SSTA forecast revisions now indicate that El Nino forms during AUG/SEP/OCT according to both the NMME and ECMWF models (Fig. 4-5). Interestingly, the warming seems to expand south from the North Pacific subtropics into the equatorial region.
Fig. 3: The Bureau of Meteorology/Australia ESO forecast using Nino34 SSTA reveals neutral ENSO ahead approaching weak EL Nino later this year. Most operational models favor this approaching but not arriving El Nino scenario.
The idea of a neutral ENSO in 2018 is currently the “safe” forecast. However, given the warmer changes generated in the eastern equatorial Pacific Ocean by the NMME, ECMWF, CFS V2 and GFDL models of the past couple days the weak La Nina climate fading now transitioning to an El Nino climate after northern hemisphere summer concludes is the (ENSO) climate requiring strong consideration as to U.S. (and global) climate influences through the middle and final third of 2018.
Fig. 4: Global SSTA forecast by the NMME model indicates developing El Nino by AUG/SEP/OCT 2018.
Fig. 5: Global SSTA forecast by the ECMWF model indicates developing El Nino by AUG/SEP/OCT 2018.
Historically, 1950-2107 May-September La Nina to El Nino transitions: Very few La Nina to El Nino transitions have occurred during the northern hemisphere warm season based on the operational Nino index (ONI). Since 1950 there have been 5: 2009, 2006, 1976, 1972 and 1965 (Fig. 6). On average the summertime pattern was cooler than normal (Fig. 7) during this ENSO transition with dryness in the Upper Midwest and Gulf region while the West-Central U.S. and New England were wetter than normal (Fig. 8).
Fig. 6: La Nina to El Nino transitions taking place during the middle third of the year since 1950.
Fig. 7-8: Middle third of the year temperature and precipitation anomalies given a La Nina to El Nino transition since 1950.
May-September La Nina to El Nino transitions in the modern era: Interesting are the implications over a long period of time. Unfortunately, due to changing climate cycles identified by ENSO/PDO and AMO each changing their long-term cycles in the mid-to-late 1990’s and the rise in global CO2 levels since the 1980’s climate analogs can only be selected from the past 2-3 decades. Therefore 2009 and 2006 combine for the La Nina to El Nino transition for May to September simply because there no other years in which this scenario occurred.
Results for JUN/JUL/AUG are not a surprise due to their similarity to many summer 2018 operational forecasts. The Midwest is cool while a hot summer affects the South and West with hottest anomalies in Texas and the West Coast (Fig. 9). The precipitation pattern is very wet in the Northeast Corridor while the Gulf States are dry (no tropical cyclones) and Upper Midwest is also dry (Fig. 10).
The circumstances implied are becoming increasingly confident. Adjustments taking into consideration the AMO and PDO are possible although the PDO and AMO are currently neutral and forecast to return to the positive phase. If so, a drier/warmer scenario would generate.
Fig. 9-10: Middle third of the year temperature and precipitation anomalies given a La Nina to El Nino transition since 1950.
Where would extremes align: The upper air pattern for JUN/JUL and AUG/SEP (Fig. 11-12) identifies where extremes may take place. Based on the analog early summer produces anomalous heat and dryness over the western half of the nation while an upper trough suppresses heat in the East and may soak the Northeast. The late summer upper air pattern indicates the upper trough is likely to linger although shifting west and weaker while the ridge pattern affects the Northwest only. The late summer pattern would defeat late season heat in the Great Plains while the East could become quite warm.
Fig. 11-12: Based on the 2009 and 2006 analogs the upper air projections for early and late summer are indicated.
Summary: Most would agree that 2 analog years is not enough to base a climate forecast. However, given the uniqueness of the current and projected ENSO climate through the middle third of 2018 the 2009/2006 analogs do provide clues as to what to expect with the general U.S. pattern in 2018.
First, is the La Nina to El Nino transition expectation correct? Most forecasters are relying on neutral ENSO for the warm season mostly due to lack of confidence. Climate Impact Company leans in the direction of EL Nino ahead give the warm tendencies of 4 models: NMME, ECMWF, CFS V2 and GFDL.
Second, do the climate implications make sense with the current climate pattern? The prevailing climate NOW is a persistent central North America trough sustaining cold and snow well into springtime. This has been an extreme scenario. It is likely the upper trough will not dissipate but linger into summer gravitating toward wet soil regions expected to evolve farther East while the Southwest U.S. drought is poised to trigger a hot upper ridge.
Normally, an intensifying drought and the attendant heat especially over the Southwest U.S. extends eastward mid-to-late summer. The upper air pattern for AUG/SEP using the analog places a trough in the Central U.S. and this feature is so dominant that subtropical ridging fades from the U.S. So the drought would likely not expand associated with anomalous heat. The upper pattern points toward very warm and humid conditions late summer in the East and likely limited tropical cyclone activity in the Gulf of Mexico.