In the following analysis, the variability of the SALLJ is analyzed with respect to its occurrences in the northern and central Andes. The central Andes is defined here by the region including locations in southern Peru (CE1: 12.42S, 70.133W, 294 m), Bolivia (CE2: 17.81S, 63.16W, 400 m), and Paraguay (CE3: 22.04S, 60.62W, 43 m). Likewise, the northern Andes is defined by northern Peru (NO3: 3.63S, 75.62W, 173 m), Colombia (NO2: 2.93N, 72.37W, 199 m), and Venezuela (NO1: 8.18N, 67.45W, 54 m) (indicated by open circles in Fig. S1 and the following figures). These regions were selected to geographically separate the northern and central Andes and, more importantly, based on trends in wind shear (see the section “Methods” and Fig. S6). The identification of the SALLJ shows that the low-level jet can occur only in the northern, only in the central, and simultaneously in both regions of the Andes. These variations of the SALLJ are defined as northern branch, central branch, and simultaneous SALLJ events. Because locations NO1 and NO3 are in the northern hemisphere, seasonal variations are referred by 3 months calendar (DJF, December, February, March; MAM, March, April, May; JJA, June, July, August; and SON, September, October, November).

Figure 1 shows seasonal vertical profiles of wind speeds during SALLJ days. As discussed in previous studies, SALLJ wind speeds exceed 10 m s-1 in all three locations in the central Andes (Fig. 1, right column), especially over Bolivia (CE2) and Paraguay (CE3); the low-level jet is maximum at 850 hPa. The jet is maximum in MAM and JJA in Bolivia (CE2) and Paraguay (CE3) and, interestingly, maximum in SON and DJF in southern Peru (CE1). A jet vertical structure is also observed over the northern Andes (Fig. 1 left column); wind speeds over Venezuela (NO1) and Colombia (NO2) exceed 10 m s−1 during DJF and SON, while the low-level jet is considerably weaker in MAM and JJA. Except over Venezuela (NO1) where the jet is maximum at 900 hPa, the SALLJ is also maximum at 850 hPa over Colombia (CE2) and northern Peru (CE3).

Fig. 1 Vertical wind profiles during SALLJ occurrences over the northern (left column) and central (right) Andes. Locations NO1, NO2, NO3, CE1, CE2, and CE3 are indicated by open circles in Fig. 2 (from north to south). Profiles are separated by each season Full size image

During SALLJ central branch events (Supplementary Fig. S2), the seasonal climatology of winds (850 hPa) shows the typical pattern of trade winds from the equatorial Atlantic flowing over South America and systematically deflected by the Andes. Winds on the eastern slopes in the northern Andes near Venezuela and Colombia are on the order of 4–8 m s−1. In contrast, as expected, wind speeds higher than 10 m s−1 are observed around the central Andes. While wind speeds in the central Andes are generally largest during JJA, moisture transport is indeed largest during SON and DJF7,12 (not shown). During SALLJ central branch days (Supplementary Fig. S3), precipitation anomalies southward of the jet exit region over southeast South America (SESA) exceed 4 mm day−1 in all seasons except during JJA.

Distinct climatological wind (850 hPa) features are found during the SALLJ northern branch (Fig. 2). During DJF (Fig. 2a), easterly winds exceeding 8 m s−1 are observed over a large area in the western tropical Atlantic between [5N–17.5N; 100W–40W]; this is the region where the Caribbean low-level jet (CLLJ) is active.26,27 At the core of the trade winds extending over the Caribbean, Central America, and eastern Pacific, wind speeds exceed 10 m s−1. The trade winds in the Caribbean split into another branch which, influenced by the presence of the continent and the mountain barrier, flows southwestward to Venezuela, Colombia, and northern Peru reaching speeds higher than 10 m s−1 over a narrow extent known as Llanos.26 This jet is referred here as the northern branch of the SALLJ. During other seasons, when the SALLJ northern branch is active, the extent and intensity of the CLLJ is more pronounced than during SALLJ occurrences limited to the central Andes. Interestingly, when the SALLJ northern branch is active, wind speeds are very low from Bolivia towards SESA during all seasons (compare Fig. 2 and Supplementary Fig. S2).

Fig. 2 Seasonal climatology of winds (850 hPa) during SALLJ northern branch occurrences. a December–February, b March–May, c June–August, and d September–November, 1979–2017. Colors indicate wind speed. Open circles along the eastern Andes indicate reference locations used to identify SALLJ events in the northern and central regions. Gray shading indicates elevations >1500 m Full size image

The seasonal distributions of northern, central, and simultaneous SALLJ events are shown in Fig. 3. The frequency of SALLJ central branch shows weak seasonal variations with minimum in March–April and September. In contrast, the northern branch shows high frequency during October–April and minimum during JJA. Simultaneous SALLJ events show a similar seasonal pattern to the northern branch. These climatological differences in the northern, central, and simultaneous SALLJ have not been discussed in previous studies, and suggest that different mechanisms may control the activity of the low-level jet. It is interesting to note that there are more simultaneous SALLJ than separated low-level jets (northern/central branches) only in November–February. This is likely related to the fact that the SALLJ northern branch is strongest in DJF and is perhaps a limiting factor for the occurrence of simultaneous SALLJ throughout the year.

Fig. 3 Seasonal frequency distribution of low-level jet occurrences in the northern branch (red), central branch (black), and simultaneous occurrences (blue) Full size image

The influence of the northern branch SALLJ on precipitation is seen in Fig. 4, which shows precipitation anomalies for each season. When the northern branch is active, precipitation over SESA decreases by 1–4 mm day−1 during all seasons except in JJA, when anomalies are much weaker. In contrast, precipitation increases by 2–4 mm day−1 on the eastern slopes of the central Andes near southeast Peru and northern Bolivia during DJF, MAM, and SON. These results provide additional support that the SALLJ northern branch is important even during MAM, when wind speeds are relatively weak (Fig. 1).

Fig. 4 Daily precipitation anomalies (mm yr-1) during SALLJ northern branch events: a December–February, b March–May, c June–August, and d September–November, 1979–2017. Colors indicate statistically significant (5% level) differences in precipitation anomalies. Open circles indicate reference locations used to identify the SALLJ Full size image

Differences in the large-scale atmospheric circulation patterns associated with northern and central low-level jet types are remarkable. Figure 5 shows the difference between SALLJ northern branch days and days with no low-level jet occurrences in the northern region. Figure 5a shows differences in OLR, winds (200 hPa) and SST anomalies; likewise, Fig. 5b shows differences in wind anomalies at 850 hPa. A broad region with positive OLR anomalies extends from the west coast of South America towards the western Pacific coinciding with a cold tongue of negative SST anomalies (although not statistically significant). The largest OLR anomalies (+3–5 W m−2) are located over the dateline. A wave-train pattern emanating from the tropical eastern Pacific to the extratropics of both hemispheres is observed in the wind circulation anomalies at 200 and 850 hPa. The enhancement of the North Atlantic Subtropical High-pressure (NASH) (Fig. 5b) is associated with northeasterly wind anomalies over the northern parts of South America and, therefore, contribute to the formation of the SALLJ northern branch. Interestingly, positive OLR anomalies exceeding +7 W m−2 are observed over Colombia and Venezuela indicating suppressed convective activity. In contrast, the cyclonic circulation (850 hPa) anomaly (Fig. 5b) over South America and South Atlantic shows southeasterly wind anomalies inhibiting the formation of the SALLJ central branch. The circulation anomalies at 850 hPa is associated with large negative OLR anomalies over South America, South Atlantic Convergence Zone (SACZ), and tropical Atlantic and indicative of enhanced precipitation. Furthermore, positive OLR anomalies over SESA indicate decreased precipitation.

Fig. 5 Composite differences between samples with and without SALLJ northern branch events (i.e., low-level jet sample minus low-level jet free sample). Top: Colors indicate OLR anomalies and contours show SST anomalies; vectors indicate anomalies in winds at 200 hPa. Bottom: as on top panel, but for wind anomalies at 850 hPa. Colored regions and vectors indicate anomalies in OLR and winds (in blue) that are statistically significant at 5% level. SST anomalies are not statistically significant, but are plotted for comparison with OLR anomalies. Gray shading indicates elevations >1500 m. Differences are computed for the entire 1 January 1979–31 December 2017 period for wind and SST anomalies and 1 January 1979–31 December 2016 for OLR Full size image

An additional set of composites were constructed by taking the difference between northern and central branches SALLJ days (Supplementary Fig. S4). While the patterns in OLR, SST and wind (200, 850 hPa) anomalies are similar to Fig. 5, the magnitudes of the differences are larger especially in SST anomalies in the tropical Pacific Ocean. While the patterns of OLR and SST anomalies in Fig. 5 and Supplementary Fig. S4 suggest influences of cold El Niño/Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) phases, the occurrence of SALLJ northern branch events does not correspond to a particular phasing of ENSO and PDO (Supplementary Fig. S5).

In addition to the important role in the climate of South America, the SALLJ northern branch shows considerable positive long-term trends in the last 39 years. Figure 6 displays the monthly frequency (left column) and intensity (right column) of northern branch events (see the section “Methods”). Statistically significant positive trends are observed in frequency and intensity of the low-level jet in all three location in the northern Andes, although the rates of increase are larger over Venezuela (NO1) and Colombia (NO2) than in northern Peru (NO3). These trends in the frequency and intensity at NO1–3 can be placed in a broader context by investigating the 850–700 hPa wind shear field. Supplementary Fig. S6 shows the Sen’s slope estimator of trends in monthly wind shear anomalies (850−700 hPa) during SALLJ northern branch events for each season separately. In all seasons, a large region of positive trends extends from the eastern tropical Atlantic towards the northern parts of South America and eastern Pacific. The increase in wind shear is especially large over northern Amazon and the SALLJ northern branch. In contrast, negative trends are observed over the central Andes indicating decreasing wind shear and, therefore, less favorable conditions for SALLJ occurrences in that region. This is consistent with Montini et al.,7 who found negative trends in the frequency of SALLJ days in Santa Cruz de La Sierra and Mariscal Estigarribia during MAM. Interestingly, the largest negative trends in wind shear anomalies are found over southeast Brazil when the SALLJ is active in the northern Andes.