A Gallery of Tropical Influences: MJO, CCKW, TIW, and La Niña

By: Bob Henson , 3:38PM,GMT on June 28,2016



 

Given the quick start we’ve seen to the 2016 Atlantic hurricane season--with Tropical Storms Danielle and Colin the earliest third and fourth named storms on record--the Atlantic may not need much help working its way well through the alphabet. Even so, this moment of relative quiet is a good time to look at some factors that could help move the process along. One player now approaching is a strong convectively coupled Kelvin wave (CCKW) located near the International Date Line. CCKWs are large but subtle atmospheric impulses, centered on the equator, that roll eastward at 30-40 mph, with showers and thunderstorms typically along their forward flank. When an eastward-moving CCKW encounters a tropical wave in the Atlantic, the enhanced moisture and upward motion may give it a boost and help it consolidate into a tropical cyclone. For more background on CCKWs, see our post from last July, “Danny’s Leg Up: A Convectively Coupled Kelvin Wave.”


Figure 1. Schematic cross section through a convectively coupled Kelvin wave (CCKW). Image credit: Michael Ventrice.

A CCKW worth watching
The CCKW now in the central Pacific is the strongest that expert Michael Ventrice (The Weather Company) has seen in almost a decade of researching these waves. While passing through the Indian Ocean in mid-June, this CCKW produced near-equatorial westerlies of up to 30 knots (35 mph). This CCKW will be moving into the far eastern Pacific over the next few days. Although Ventrice doubts that this CCKW will maintain its strength as it moves into this area, it still could enhance the odds of tropical cyclone development in the eastern Pacific into the first week of July, as noted in a NOAA discussion on Monday. WU member Levi Cowan (tropicaltidbits.com) observed on Monday that long-range GFS ensemble runs have been unusually consistent in developing a strong tropical cyclone during the first week of July southwest of Mexico. The 8 AM EDT Tuesday tropical weather outlook from the National Hurricane Center gives an area of disturbed weather now along the west coast of Costa Rica a 30% chance of gradual development by next Sunday, July 3, as it moves into the eastern North Pacific well south of Mexico.

The CCKW’s influence may also extend northward to favor development in the Bay of Campeche and/or western Gulf of Mexico. Last week, a few ensemble members from long-range GEFS runs flagged this area for possible development in early July, but the location and strength of the potential tropical cyclone varied greatly from run to run and across ensemble members. The “ghost storm” has since disappeared from more recent GFS runs. (In a Facebook post last Thursday, the NWS office in Tallahassee pointed out the hazard of fixating on a single long-range solution from an ensemble.)


Figure 2. Predicted state of the Madden-Julian Oscillation through mid-July (top to bottom), with the tropical Indian Ocean at left and the eastern tropical Atlantic at right. Bluish colors denote an active phase, favoring showers and thunderstorms (convection) and tropical cyclone formation; red colors show a tendency for convection to be suppressed. The graphics are based on recent conditions (top panel), ECMWF ensemble forecasts (second and third panels), and extrapolation (fourth panel). Image credit: Michael Ventrice, The Weather Company.


Another traveling feature that can influence the Atlantic is the Madden-Julian Oscillation. Typically stronger and much slower-moving than a CCKW, an active MJO phase can favor upward motion and tropical cyclone development for a week or two as its forward flank approaches an ocean basin. MJOs can reinforce or dampen the effects of a CCKW. Ventrice notes: “Usually, the MJO will act to enhance the higher-frequency waves traveling through it. So you will get stronger CCKWs within the active envelope of the MJO and vice versa.” As of last week, the nearest active MJO phase was located over the Maritime Continent, a location that typically works against tropical cyclone development in the western part of the Atlantic basin. It will be a couple of weeks until this MJO makes it far enough east to boost conditions in the Gulf of Mexico, and its strength at that point is still uncertain (see Figure 2). Hurricanes in the Gulf of Mexico and western Caribbean are several times more likely to form during an active MJO phase as opposed to a suppressed phase.

Looking further ahead, the smart money remains on La Niña arriving by autumn, just in time to serve as a favorable influence for tropical cyclones in the Atlantic. Sea surface temperatures in the eastern equatorial Pacific, as tracked by the Niño3.4 index, plummeted from strong El Niño territory in March (departures from normal of greater than +1.5°C) to slightly below-average values in early June, which prompted NOAA to declare an end to the 2015-16 El Niño event. The shift toward La Niña should lead to reduced vertical wind shear over the Atlantic, thus favoring a greater amount of tropical cyclone development.


Figure 3. Departures from the seasonal average of sea surface height (SSH) as measured by NASA satellites on June 8, 1998, and June 9, 2016. Red and white areas denote higher-than-average SSH, which corresponds to warmer-than-average water in the uppermost part of the ocean. Although the El Niño events of 1997-98 and 2015-16 were roughly comparable in timing and strength, the transition toward La Niña was much more advanced at this point in 1998 in terms of cooler-than-average water along the equatorial Pacific. Image credit: NASA/JPL/Caltech.


Figure 4. Departures from the seasonal norm for sea surface temperatures in the Niño3.4 region of the eastern tropical Pacific for April-June 2016. The decrease in SSTs this spring has been marked by large variability, especially in June. Image credit: tropicaltidbits.com.

Is La Niña having second thoughts?
The ongoing transition toward an expected La Niña has been a bit quirky this month. In response to a Western Hemisphere MJO passage, trade winds in early June were unusually weak over much of the eastern tropical Pacific, which runs counter to the La Niña mold. Moreover, the region of cooler-than-average SSTs along the equator is very weak and narrow compared to this point in 1998, when the last “super” El Niño transitioned to La Niña (see Figure 3). The latest weekly Niño3.4 value is -0.4°C, compared to -1.1°C at the same point in June 1998. Another oddity: the daily values of Niño3.4 spiked back above +0.5°C for nearly a week in mid-June, then sank back into negative territory in a matter of days (see Figure 4).

This last quirk may be related to a picturesque feature called tropical instability waves, or TIW. These waves often develop across the eastern equatorial Pacific in northern summer and fall, especially during the onset of La Niña, when SST contrasts are heightened between the equatorial and subtropical regions. The sharp contrast, together with shear produced by contrasting ocean currents, can lead to a line of eddy-like features straddling the equator and marching westward, with a typical separation of about 700 miles (1100 km) between each wave (see Figure 5). Overall, TIW can have a dampening effect on La Niña events, as confirmed in a high-resolution modeling study led by Yukiko Imade (University of Tokyo). In addition, Ventrice notes that the TAO/TRITON buoys that monitor SST across the tropical Pacific have an east-to-west spacing of 15° of longitude, or about 1000 miles (1600 km). According to Ventrice, the wavelength of the TIW and the buoy spacing can sometimes be similar enough to allow regional SST reports to rise and fall in unison over very short periods when the TIW are especially active, as was the case this month (see Figure 6).


Figure 5. Departures from the seasonal average in SSTs across the eastern Pacific at 18Z Friday, June 24, 2016, reveal a sequence of tropical instability waves (TIW) straddling the equator. Image credit: earth.nullschool.net.


Figure 6. For the 7-day period from 12Z June 17, 2016, to 12Z June 24, sea surface temperatures rose and fell in a distinct wavelike pattern across the eastern tropical Pacific, showing the influence of tropical instability waves. Image credit: tropicaltidbits.com.

The bottom line
Despite its unorthodox entrance cues, La Niña is very likely still on its way. One strong sign: just below the surface of the equatorial Pacific, there is still a large area of cooler-than-normal water dominating the topmost 200 meters (660 feet). Any downwelling at the surface would more likely postpone or weaken an impending La Niña rather than quash it. Moreover, an impressive surge of trade winds now across the central/eastern Pacific is expected to force a period of enhanced upwelling in the eastern Pacific that’s likely to boost the evolving La Nina event, according to Ventrice.

Climatology also supports the switch away from El Niño. Going back to 1950, the longest continuous El Niño episode was the 15 overlapping three-month periods from March-May 1982 to May-July 1983. As of March-May 2016, we’re already up to 14 overlapping three-month periods, so if El Niño were to hang on another few months, we would be in record territory.

Meanwhile, the Pacific Decadal Oscillation continues to hum along in a strongly positive mode, with last month’s value of +2.35 the highest for any May in records going back to 1900. Strongly positive PDOs are correlated with more/stronger El Niño events and fewer/weaker La Niña events (although there is a chicken-or-egg factor here, as El Niño and La Niña events themselves feed into the PDO).

We’ll be back with a new post on Thursday.

Bob Henson