The Northern Hemisphere eastern equatorial Pacific is an interesting area from a climatological point of view. It is affected by two different low-level jets whose interaction determines the moisture transport towards wide regions of Central America and northern South America, generating huge amounts of precipitation and affecting the living of millions of people (Arias et al., 2015). In the eastern equatorial Pacific, the Intertropical Convergence Zone (ITCZ) is predominantly located in the Northern Hemisphere (Wodzicki and Rapp, 2016). In this region, the Southern Hemisphere trade winds cross the Equator and the change in the sign of the Coriolis term, facilitated by the coast orientation and the land–sea temperature gradients, deflects the trades to the east, entering northern South America at 5∘ N as a low-level westerly jet, whose core is located at the 925 hPa level (Fig. 1a) introducing huge amounts of moisture into the continent (Poveda and Mesa, 1999, 2000). This jet was first described by Poveda and Mesa (1999), who named it as the Chocó Jet, the name functioning both as an acronym for “CHorro del Occidente COlombiano” (Western Colombian Jet) and as the place name Chocó, one of Colombia's departments most affected by the moisture advection from the Pacific related to this jet (Poveda and Mesa, 1999, 2000). In some localities such as Lloró (5∘30′ N, 76∘32′ W), the large amounts of moisture transported from the Pacific into the Colombian coast result in average rainfall ranging from 8000 to 13 000 mm, making this region one of the rainiest in the world (Murphy, 1939; Trojer, 1958; Arnett and Steadman, 1970; Meisner and Arkin, 1987; Janowiak et al., 1994; Poveda and Mesa, 2000; Poveda et al., 2011; Jaramillo et al., 2017; King et al., 2017). At 80∘ W, the cross section of the zonal wind from 5∘ S to 20∘ N (Fig. 1b) shows the distinctive characteristics of this system, a jet core located at 925 hPa and 5∘ N confined to the lower troposphere, with westerlies restricted to altitudes below 850 hPa. Although the location of the core is almost constant throughout the year (Poveda and Mesa, 2000; Sakamoto et al., 2011; Sierra et al., 2018), its intensity has a strong seasonal cycle (Fig. 1c, black line). Minimum velocities at this jet core are found in February–March (below 1 m s-1), and maximum ones occur in October–November (6 to 7 m s-1). From May up to December, the jet is quite active and its relation with the ITCZ seasonal migration is evidenced by the presence of two relative maxima in June and October (Fig. 1c).

In Fig. 1b a maximum of easterly zonal winds around 10 m s-1 is observed at 15∘ N and 925 hPa. It corresponds to the Caribbean Low-Level Jet (CLLJ) (Amador, 1998; Poveda and Mesa, 1999; Poveda et al., 2006; Wang, 2007). This jet also shows a marked seasonal variation. The absolute value of the zonal wind at 15∘ N, 75∘ W and 925 hPa constitutes a good measure of the intensity of this jet (Fig. 1c, dashed blue line). At the core of the CLLJ the absolute value of the wind speed varies between a minimum around 7 m s-1 (October) and a maximum of 14 m s-1 in July, with a secondary maximum in December and January (12 m s-1).

While the location of the core of both jets is rather constant at inter-annual scales, their relative strength is quite variable and strongly dependent upon the SST (sea surface temperature) in the tropical Pacific. This dependence is different in each case. Ultimately, the Chocó jet originates in the Southern Hemisphere trades; so, the weakening of the trades associated with positive SST anomalies in the eastern tropical Pacific (El Niño conditions) results in a weaker Chocó jet (Poveda et al., 2001). On the other hand, the dependence of the CLLJ with the SST anomalies is season dependent. As demonstrated by Wang (2007), during the boreal winter, a weak (strong) CLLJ corresponds to warm (cold) SST anomalies in the tropical Pacific. By contrast, during the boreal summer, a strong (weak) CLLJ is associated with warm (cold) SST anomalies. In this way, during the boreal winter, warm (cold) SST anomalies are associated both with weaker (stronger) Chocó jet and the CLLJ. In summer, the modulation of the SST over both jets is the opposite. Warm (cold) SST anomalies are associated with weak (strong) Chocó jets but strong (weak) CLLJs. This divergent behaviour results in a major signature on the distribution of the westerlies or easterlies noticeable at the surface level and over a large area of the eastern tropical North Pacific. As an example, Fig. 2 shows the average winds during August 1997, a month with high positive SST anomalies over the tropical Pacific. The pressure–latitude cross section of the zonal wind at 80∘ W (Fig. 2b) shows the consequent weak Chocó jet (although still existing and evidenced by positive zonal wind at 5∘ N) and an enhanced CLLJ. It is clearly evidenced that during these episodes, easterlies dominate a large area in the tropical North Pacific (outlined area in Fig. 2a). Westward of 90∘ W the prevalence of the easterly component is absolute, even at the surface. By contrast, in August 2010 (Fig. 3), a month of strong SST negative anomalies in the eastern equatorial Pacific, the enhanced Chocó jet is quite evident (Fig. 3b) and, most interestingly, in these cases, the westerlies (predominantly southwesterlies) dominate a wide area of the tropical Pacific (outlined area in Fig. 3a). Remarkably, the cross section of the zonal wind at 90∘ W (Fig. 3c) shows that at the surface the westerlies can reach 14∘ N.

From the previous discussion, it is clear that, although the Chocó jet always blows from the west at its core (925 hPa, 5∘ N and 80∘ W), for the boreal warm season the changes in the strength of the Chocó jet are concurrent with significant changes in the distribution of the wind direction over a broad area at the surface. In a recent review paper, Garcia-Herrera et al. (2018) demonstrated that this kind of signature in the surface wind field can be exploited to design indices relying only on wind direction measurements. They also showed that these indices are highly representative of the moisture advection toward continental areas. The main advantage of these so-called “directional indices” is that by design, they only require the knowledge of the wind direction, a variable that has been routinely measured aboard ships since the end of the 17th century and avoid the uncertainties associated with the measurement of wind speed for early times (Prieto et al., 2005; Gallego et al., 2007). As a result of several data recovery projects (García-Herrera et al., 2005; Allan et al., 2011; Wilkinson et al., 2011, among others), millions of these early wind observations are today incorporated into the International Comprehensive Ocean-Atmosphere Data Set (ICOADS) database (Freeman et al., 2017). In its most recent release ICOADS holds over 456 million individual marine reports, covering the period 1662–2014. The objective of this paper is to develop a new index representative of the Chocó jet strength by exclusively using the raw wind direction measurement currently incorporated in ICOADS. As we will show below, this method provides an extension of almost one century to the current available indices for this jet.

Traditionally, it has been considered that the Caribbean Sea was the main moisture supplier for Central America and northern South America through the CLLJ (Wang et al., 2006). However, the importance of the transport from the Pacific source by the Chocó jet has been recently highlighted, and it is estimated that the moisture advected from this ocean can contribute up to 30 % of the total precipitation in areas of the western coast of Central America (Duran-Quesada et al., 2010, 2017; Hoyos, 2018). The relevance of the Pacific source is captured by CHOCO-D. Figure 7 shows the difference between precipitation composites for months with CHOCO-D above and below 1 standard deviation of its average value for the 1901–2013 period (“positive” and “negative” Chocó jet phases in succession) covered by the Global Precipitation Climatology Centre (v7) dataset (Becker et al., 2013). The precipitation changes associated with opposite anomalies of the CHOCO-D index extend over large areas of southern Mexico and Central America and spread southward into northern Colombia. The largest positive precipitation anomalies are found between July and September, when the Chocó jet is fully developed, and they are especially noticeable in the western coast of Central America from Guatemala to Panama, where precipitation anomalies exceed 5 mm day-1 during positive phases of CHOCO-D in relation to the negative ones. The connection of these rainfall anomalies with the moisture advection has been assessed by computing the vertically integrated moisture transport through the 1000–850 hPa levels and the corresponding moisture convergence (Fig. 8). We limited the latter analysis to the 1979–2014 period because of the large uncertainties in the vertical distribution of the specific humidity in the NCEP-NCAR reanalysis over the equatorial Pacific prior to the satellite era. In order to attain a large enough number of cases for this shorter period, we relaxed the threshold for including positive/negative phases by considering the years above/below 0.75 standard deviation of the CHOCO-D index. Despite the different periods considered and the lower spatial resolution of the reanalysis, the agreement between Figs. 7 and 8 is remarkable. As expected, large CHOCO-D values appear related to an enhanced moisture transport (anomalies of up to 100 kg m-1 s-1) from the Pacific into Central America in a latitude band extending from around 4 to 15∘ N. The higher values of moisture convergence related to this enhanced transport are located over Panama and the pacific coast of Colombia, westward of the Cordillera Central. It is also interesting to note that the Pacific is a relevant moisture source for the Caribbean as well. Over this area, large values of moisture convergence occur, resulting in significant changes in the precipitation of the Greater Antilles, especially in Jamaica and large areas of Cuba (see Fig. 7d, for example).

During the last years, a number of studies have dealt with climate reconstructions based on historical wind direction measurements (see Garcia-Herrera et al., 2018, for a recent review). However, due to the low number of historical meteorological records over the eastern Pacific, most of these works correspond to reconstructions over the North Atlantic or the Indian Ocean. The few exceptions considering the Pacific are limited to the study of the climatic implications in the changes in the duration of a particular shipping route (Garcia et al., 2001), focus on the westernmost North Pacific (Vega et al., 2018) or make use of indirect approaches, by estimating the climate in the tropical Pacific through the use of present-day teleconnection patterns using data from other oceanic basins (e.g. Barrett et al., 2017a, b). In this work, we have found that the strength of the Chocó jet can be estimated through an index starting in the 1850s by using in situ wind direction measurements contained in ICOADS. To the best of our knowledge, this is the first time that a quantitative instrumental climate index over the eastern tropical Pacific has been built for such a long period.

This reconstruction has been possible because the signature of the Chocó jet variability in the distribution of the wind direction at the surface is sought in a large area where the aggregated number of observations available in ICOADS is currently enough for computing a meaningful index. As described graphically by Poveda (2018), in the tropical eastern Pacific, the wind field configuration is the result of a “tug of war” between two low-level jets blowing from opposite directions: the CLLJ blowing from the Caribbean into the Pacific and the Chocó jet originating in the Southern Hemisphere trade winds belt and recurving to the east as it crosses the Equator (Poveda et al., 2001, 2006 and 2014). While the location of the core of both jets is almost stationary (Poveda and Mesa, 2000; Sakamoto et al., 2011; Sierra et al., 2018), their relative strength is quite dependent on the SST anomalies over the equatorial Pacific. During the Northern Hemisphere warm season, the modulation of the SST anomalies over both jets is opposite (Wang, 2007), and this results in a significant change in the distribution of the westerlies/easterlies in a large oceanic area. In this way during years of a strong Chocó jet, at the surface the westerlies can reach the 15∘ N latitude limit even as far west as 120∘ W, while in years of a weak Chocó jet, the westerlies are restricted to 80∘ W. We found that, the proportion of days with wind blowing from the southwest in the areas 4–15∘ N, 120–84∘ W plus 4–9∘ N, 84–77.5∘ W is significantly related to the relative strength of the Chocó jet as measured by the average wind speed at its core.

It must be pointed out that our index has some caveats. On the one hand, the strength of the Chocó jet is not exclusively dependent on the relative strengths of the CLLJ and the Chocó jet, as it is implicitly assumed in our approach. In fact, the strength of the Chocó jet is partly dependent on the CLLJ dynamics. From the Caribbean, the CLLJ divides in two branches, one passes through Central America toward the Pacific (the one entering our study area), but the other one crosses the Panama Isthmus and then merges with the Chocó jet (see Fig. 1a), enhancing it (Poveda et al., 2014). On the other hand, the Chocó jet suffers from positive feedbacks not necessarily related to the CLLJ, as the Chocó jet core is enhanced by the latent heat released as it enters the continent generating precipitation (Velasco and Fritsch, 1987; Poveda and Mesa, 2000). Additionally, while the Chocó jet is present in the region throughout the whole year, our index can be only computed between May and November. Between December and April, the southward migration of the ITCZ allows the northeasterly Northern Hemisphere trade winds to blow as far south as 4∘ N (Wodzicki and Rapp, 2016), and during these months easterlies are essentially dominant in our study area, and the CHOCO-D index is near zero regardless of the strength of the Chocó jet.

These limitations explain both the absence of estimations of the Chocó jet strength between December and April and the moderate correlation between CHOCO-D and the Chocó jet strength for the rest of the year (r=+0.59 p<0.01 for the May–October average; see Fig. 5b). However, during these months, this relation is significant, peaking in May and August, with correlations above r=+0.68 (p<0.01). Additionally, we have found that CHOCO-D is strongly representative of the moisture advection from the Pacific into Central America and northern South America and, therefore, of a significant part of the precipitation in this area related to the moisture transported from the Pacific.

According to the CHOCO-D record, the Chocó jet has experienced changes at decadal scales at least since 1880. They are quite dependent on the month. Interestingly the July series shows a tendency to be above its long-term average value from 1840 to 1910 and below it from 1910 to 1990. September exhibits an opposite behaviour, being below average up to the 1920s and above it from that decade onwards. As the reversal in the trends for July and September occurred at the beginning of the 20th century, the evaluation of the consequent changes in independent precipitation records is rather uncertain because of the low number of precipitation data in this part of the world during the first two decades of the 20th century (Becker et al., 2013). Notwithstanding this, the analysis of the GPCC dataset suggests that throughout the 20th century, there was a discernible change in the seasonal distribution of the precipitation related to the intra-annual variability of the Chocó jet. Finally, it is worth mentioning that Carmona and Poveda (2014) found an increasing trend in the precipitation of the Pacific coast of Colombia starting in the last decades of the 20th century. Our results support this finding, as the strength of the Chocó jet has been steadily increasing for May, June, July and August since the last years of the 20th century (Fig. 9).

Since their conception, one of the main applications of directional indices has been the analysis of the stability of teleconnection patterns (Gomez-Delgado et al., 2019). For instance, directional indices have been used to prove that the relation between the strength of the West African monsoon and the ENSO or the Atlantic Multidecadal Oscillation have been unstable (Gallego et al., 2015). Similarly, instabilities have been described in the relation between the ENSO and the strength of the western North Pacific summer monsoon (Vega et al., 2018). By contrast, we have found that the relation between the Chocó jet strength during JJA and the El Niño3.4 index (JJA and DJF yr +1) has been remarkably stable at least since the 1880s. This is particularly relevant because the stability of this relation is usually the basis of the hydrologic reconstruction or prediction in northern South America (Gutiérrez and Dracup, 2001; Prange et al., 2010; Córdoba-Machado et al., 2015). However, our results suggest that the intensity of these relations involving the SON-averaged CHOCO-D has changed throughout the 20th century, being clearly weaker prior to the 1930s. We do not currently have an explanation for this finding, and it must be considered cautiously as the uncertainty of CHOCO-D during this early period is the largest (Fig. 9), but it could be related to the shift from negative to positive anomalies around the 1920s–1930s discussed for the September and October CHOCO-D series (see Fig. 9). The results are also found when the ENSO cycle is represented by the Southern Oscillation Index (Ropelewski and Jones, 1987) based on sea level pressure instrumental measurements (not shown).

In this paper, we present a new example of how climate indices based exclusively on wind direction measurements over the ocean have a large potential to provide a better understanding of the long-term climate variability, in this case in the eastern equatorial Pacific, a region with scarce inland observational records. The low-level circulation in this region is dominated by the Chocó jet, driving a large amount of moisture toward Central America and northern South America, which as a result is one of the rainiest areas in the world. With the data currently available on ICOADS, it has been possible to assess the long-term variability of this jet since the late 19th century and some years of the 1850s, revealing a complex but significant variability at multidecadal scales. However, there are still some important gaps in the data coverage such as the World War II period and the years between 1860 and 1880, while prior to the 1920s, the low number of available observations results in uncertainties of the CHOCO-D index around 15 %. We expect the results of this research will stimulate future efforts in data rescue aimed at improving the data coverage in this part of the world for years prior to the 1920s.