Principal component analysis of the seasonal cycle of rainfall in Caribbean
The first three principal components (PCs) explain 78% of the total variance (54%, 16%, 8%, respectively) of the pentad rainfall climatological seasonal cycle (Fig. 1a–c). The first principal component (Fig. 1a) displays the known seasonal characteristics dominant in the Caribbean: the Early-Rainy Season (ERS) beginning in late April and lasting through June, the MSD from June through August, the Late-Rainy Season (LRS) from August through late October, and the Winter Dry Season (WDS) from mid-November to April. In addition, the maximum rainfall occurs during the LRS and the MSD is not necessarily a dry season but rather a period of less intense rainfall than the ERS and LRS. Nearly all stations in the Caribbean have high positive correlation coefficients with PC1 (Fig. 1d; Table 2), with the exception of the four Guiana stations.
Table 2 Correlation coefficients between the station annual cycle of rainfall and PC1, 2, and 3
With its negative peak in June and positive peak in November, PC2 indicates a modification of the intensity of the ERS compared to the LRS rainfall (Fig. 1b). PC2 also shows a gradual ascent of rainfall towards the LRS in positively correlated stations. A notable contrast in spatial correlation pattern (Fig. 1e; Table 2) is seen with PC2 from NW to the SE portion of the Caribbean stations. Stations in the southeast, from the mid to upper portion of the Lesser Antilles, the U.S. Virgin Islands, and some in Puerto Rico have positive correlations with PC2, while stations in Florida, Cuba, and the Bahamas have negative correlations. This implies that the ERS peaking in June has a stronger signature in the NW Caribbean than in the SE Caribbean while the opposite is seen during the climatological LRS. Meanwhile, stations in the Guianas have high negative correlation coefficients, indicating a complete inverse of PC2: a dominant ERS and little to no rainfall during the climatological LRS. Finally, PC3 denotes, in stations positively correlated with it, the absence of the MSD and a late-LRS peaking in late November and extending into the WDS (Fig. 1c). The correlation coefficients in PC3 show a notable meridional contrast (Fig. 1f; Table 2). Stations in the NW Caribbean (i.e. Cuba, Jamaica, Bahamas, and Florida) have negative correlations whereas some stations in Central America, the lower tier of the Lesser Antilles, and Guyana, have positive correlations. Given the negative correlations seen in PC2 in the NW Caribbean, and a later peak of the LRS seen in PC3, the negative correlations in PC3 in the NW Caribbean suggest this region has a stronger signature of the MSD and an earlier LRS in comparison to the eastern Caribbean. Meanwhile, the positive correlations in Guyana is a combination of the absence of the climatological MSD and a late-LRS given their inverted relationship seen in PC2.
Overall, the PCA suggests three distinct sub-regions of the rainfall cycle in the Caribbean: the NW Caribbean, the Central and Eastern Caribbean (Puerto Rico and the Lesser Antilles), and the Guianas. However, this rough division of the Caribbean regions paints an incomplete picture. Findings from total moisture budget provide important insights on the regional precipitation patterns across the Caribbean.
Moisture budget seasonal cycle
The reanalysis’ spatial moisture budget climatology (1979–2012) for the Caribbean is divided into the four climatological precipitation seasons: the WDS (Fig. 2), ERS (Fig. 3), MSD (Fig. 4), and LRS (Fig. 5).
Winter dry season (December–April)
Given P-E (Fig. 2a) equals the TMF (Fig. 2b) in the moisture budget equation, both terms resemble each other well in the WDS. In the TMF, convergence is found over the continental United States and south of the Caribbean domain (except Guianas) where the ITCZ is situated. Divergence is seen throughout nearly all of the Caribbean, and the tropical North Atlantic coinciding with NASH. In addition, SSTs in the Caribbean have values between 24 and 27 °C (Fig. 2b). When the convergence of the TMF is broken down into its mean (Fig. 2c) and transient (Fig. 2d) components, the latter is seen to dominate the TMF convergence north of 20 N, and the mean flow dominates the TMF south of 20 N. Therefore, the convergence band in the continental U.S. and divergence band in the Gulf of Mexico and NW Caribbean are due primarily to the transient flow. The divergence across the tropical north Atlantic, SE Caribbean, and convergence bands in the Guianas/ITCZ regions are due to the mean flow. As for the transient flow, the transient mean moisture transport vectors are meridional, that is the land–ocean convergence and divergence couplet are a result of high-frequency transient eddies that transport moisture poleward.
The breakdown of the mean flow into its mass convergence (Fig. 2e) and moisture advection components (Fig. 2f) show that mass convergence is the dominant term of the mean flow impact on the moisture budget while moisture advection acts as a modifier. For instance, mean flow divergence in the tropical North Atlantic is due to mass divergence by northeasterly winds carrying lower values of specific humidity (q) into the region. This is as associated with the circulation imposed by the NASH along its southern flank. In addition, strong 925mb easterly winds are seen across the central CS, these are associated with the wintertime CLLJ (Wang 2007). The Gulf of Mexico is the only region where moisture advection dominates by acting to overcome the effect of weak mean mass divergence. Here, strong moisture advection by southeasterly winds carries higher q from the CS into the Gulf of Mexico.
Early rainy season (May–June)
During the ERS, the mean flow (Fig. 3e, f) is the dominant contributor of the TMF (Fig. 3c, d). In May, two bands of convergence in the Caribbean appear in the TMF (Fig. 3c): one in the western flank of NASH from the Dominican Republic/Puerto Rico to the mid-latitudes, and the other across western portion of the CS. Convergence also appears in the Guianas as the Atlantic ITCZ convergence band moved northward. Convergence is also seen where the northward migrating Eastern Pacific ITCZ and SAMS are located. Three zones of divergence are found: one in the Gulf of Mexico and NW Caribbean, the southern flank of NASH, and a divergence tongue from the Lesser Antilles to the Central CS. SSTs in the CS are between 27 °C-28 °C and the Gulf of Mexico is at or below 27 °C (Fig. 3c). Areas such as the Eastern Pacific ITCZ and the Caribbean shoreline of Central America have SSTs at or above 28.5 °C, denoting the warm pool and its Caribbean branch. The regional maximum of warm waters coincides with the largest values of convergence seen in the Caribbean, which is indicative of the WHWP enhancing the Eastern Pacific ITCZ and the convection in the western CS.
In the convergence band in the western flank of NASH, mass convergence (Fig. 3g) and advection of moister air (Fig. 3i) are found. The 925mb wind and mean moisture transport vectors in this convergence band have a stronger southerly component than during the WDS. Therefore, the convergence band in the western flank of NASH is a result of weakened trade winds and their associated mass divergence and higher values of q from the tropics being advected into the subtropics. The convergence band in the western CS is mainly associated with mass convergence that connects with the Eastern Pacific ITCZ. In addition, the 925mb winds in the CS have a stronger southerly component than during the WDS. Mass divergence in the central CS is weaker than during the WDS. This implies that the western CS band is receiving moisture from the Eastern Pacific ITCZ as a result of enhanced southerly flow which weakens the divergent trade winds in the CS. The Gulf of Mexico and far NW Caribbean mean flow divergence is due to mass divergence; however, positive advection of moisture via southeasterly winds in the region modifies the magnitude of the overall mean flow moisture flux divergence. Finally, the ITCZ convergent bands are a result of mass convergence by the trade winds that converge towards the ITCZ.
In June, the TMF and mean flow convergence bands associated with the western flank of NASH shifts westward to the eastern U.S. coastline and to portions of the Gulf of Mexico/NW Caribbean (Fig. 3d, f). This observation is consistent with recent studies that investigate the ERS (i.e. WT5 in Moron et al. 2015; Allen and Mapes 2017). The westward shift of the western flank of NASH, mean flow convergence band is due to the westward shift of mass convergence (Fig. 3h) and northwestward shift of positive moisture advection (Fig. 3j). In addition, the 925mb southeasterly winds shift westward as southeasterly winds in the Gulf of Mexico and NW Caribbean have strengthened while the southeasterly winds north of the Dominican Republic/Puerto Rico have become easterly. SSTs warm throughout the Caribbean domain and the AWP (28.5 °C) expands to regions where convergence across the western CS and Gulf of Mexico is found (Fig. 3d). The convergence band in the western CS shifts slightly northwestward as mass convergence from the Eastern Pacific ITCZ migrated further northward. The Atlantic ITCZ convergence band is also shifts northward, but the SAMS convergence band is stagnant. For the TMF/mean flow divergence, the divergence band in the Gulf of Mexico disappears, the divergence band in the southern flank of NASH shifts northward, and the divergence tongue in the CS extends westward as negative moisture advection (Fig. 3j) is enhanced across the CS. In addition, the 925 mb easterly winds and mean moisture divergence vectors in the CS strengthen (Fig. 3f), similarly to what is found during the WDS. The CS mass convergence-negative moisture advection couplet found in the WDS also returns (Fig. 3h, j).
Mid-summer drought (July–August)
The MSD TMF shows three convergent bands and one large divergent band in the Caribbean domain (Fig. 4b). The convergence bands coincide where precipitation exceeds evaporation the most (Fig. 4a) and vice versa with the seen divergence bands. The first convergence band is associated with the northwestern flank of NASH and is seen from Florida to the mid-latitude Atlantic Ocean. The second convergence band is seen in the western CS, south of the Eastern Pacific ITCZ convergence band. The third convergence band is seen in the Lesser Antilles in association with the Atlantic ITCZ. The divergence band is seen in the southern flank of NASH, portions of the Greater and Lesser Antilles, and central CS. In comparison to the ERS, the NASH convergence band shifts northwestward and the Eastern Pacific ITCZ/Atlantic ITCZ bands move northward. The mean flow is responsible for these convergent and divergent bands (Fig. 4c). Upon the breakdown of the mean flow into its mass convergence (Fig. 4e) and moisture advection (Fig. 4f) components, mass convergence is largely responsible for the convergence bands, whereas mass divergence and negative advection of specific humidity are largely responsible for the divergent band. The enhanced negative advection across most of the Caribbean is due to marginally lower values of q across the southern flank of NASH being zonally advected into the Caribbean. Mean flow convergence in the ITCZ regions and northwestern flank of NASH are a consequence of converging winds associated with the trade winds, and converging southeasterlies on the western flank of NASH which advect lower values of q into the mid-latitudes, respectively. Similarly, to the WDS and ERS-June, a large convergence-divergence couplet is seen between mass convergence and moisture advection in the CS.
The western CS alongside the Nicaragua shoreline has mass convergence and advection of drier air, whereas the central CS north of the Bay of Venezuela has mass divergence and positive moisture advection. Similar during the WDS, a pocket of lower q and strong divergent winds are seen north of the Bay of Venezuela, whereas larger values of q and strong convergent winds are seen to its east and west. The SSTs at and around the CS divergence pocket are cooler than the surrounding region (Fig. 4b), which is likely a result of coastal upwelling (Inoue et al. 2002). This upwelling is forced by the low-level easterly winds, or CLLJ, that are parallel to the South American coastline (Fig. 4c, e). In addition, the AWP (28.5C) extends eastward into the Lesser Antilles but does not reach the North South American coastline (Fig. 4b). This induces an inverted meridional SST gradient from the northern South American coast to the Greater Antilles. Based on thermal wind balance, the negative meridional temperature gradient enhances low-level winds from the east (Angeles et al. 2010; Wang 2007) in a positive feedback loop. Given that the CLLJ peaks during the MSD, it drives the moisture convergence-divergence couplet, an observation that is consistent with previous studies (Muñoz et al. 2008; Hidalgo et al. 2015; Herrera et al. 2015).
Finally, transient flow impact on the moisture budget is relatively weak across the Caribbean (Fig. 4d). That said, transient convergence modifies the extent of mean flow divergence as values of transient convergence are seen off the coast of West Africa and the Caribbean. The mean transient transport vectors over the tropical North Atlantic and off of West Africa are meridional; therefore, the transient convergence is a response to easterly moving high-frequency eddies (i.e. AEWs). However, the negligible transient convergence values seen in the Caribbean suggests their influence is marginal.
Late-rainy season (September–November)
The LRS TMF in September shows convergence throughout most of the Caribbean domain (Fig. 5a) as precipitation well exceeds evaporation (not shown). Convergence is seen throughout the NW Caribbean, eastern Central America, the Greater Antilles, and the Lesser Antilles. Some divergence is seen in the central CS and in the Guianas. The AWP covers the entire Caribbean domain (Fig. 5a). When breaking down the TMF into its mean (Fig. 5d) and transient (Fig. 5g) components, it is found that TMF convergence is due to convergence from the mean flow. Mass convergence (not shown) is the dominate component of the LRS mean flow as moisture advection (not shown) is negative but negligible across the Caribbean. In the mean flow, three bands of convergence are found: one located on the climatological Atlantic ITCZ, one located on the climatological Eastern Pacific ITCZ, and one located on the western flank of NASH. Both ITCZ convergence bands are seen in the TMF and only the southern portion of the western flank of NASH convergence band is seen in the TMF. Transient divergence is found across the Gulf of Mexico and where the northwestern flank of NASH is located. Therefore, transient divergence overtakes the mean flow convergence northeast of Florida, which results in TMF divergence. The mean transient transport vectors are mainly meridional and increasing in magnitude poleward; thus, extratropical high-frequency transient eddies are likely responsible for transient divergence seen in the Gulf of Mexico and U.S. eastern coastline.
When comparing the LRS-September to the seasonal MSD, it is found that the mean flow NASH convergence band migrates southward as the Eastern Pacific and Atlantic ITCZ convergence bands migrate northward. SAMS mean flow convergence band weakens and remains stagnant. The mean flow divergence pocket seen in the central CS is diminished. The southern flank of NASH divergence band is also diminished. Low-level winds shift from easterly to southeasterly across most of the Caribbean domain. Therefore, the migration of large-scale convergence bands alongside winds that favor mass convergence may explain the enhanced convergence across the entire Caribbean. As for transience, the convergence seen in the Caribbean and off of West Africa is diminished as divergence from the mid-latitudes extends southward into the Gulf of Mexico and portions of the NW Caribbean. This suggests mid-latitude high-frequency transient eddies extending southward as tropical easterly high-frequency transient eddies, i.e. AEWs, diminish.
In October, TMF convergence moves southward in the Caribbean (Fig. 5b) Convergence is seen across the Lesser Antilles, Central America, and Puerto Rico/Dominican Republic. Divergence infiltrates the NW Caribbean and Gulf of Mexico, as evaporation strengthens and exceeds precipitation (not shown) across the NW Caribbean and Gulf of Mexico. The mean flow (Fig. 5e) shows Atlantic ITCZ and Eastern Pacific ITCZ bands shifting slightly southward. The NASH mean flow convergence band shifts southeastward and is at similar position seen during the ERS-May. However, most of the NASH convergence band vanishes. Transient divergence by U.S. mainland high-frequency mid-latitude eddies (Fig. 5h) shifts southward and strengthens; therefore, the transient divergence overtakes the NASH convergence band. Also, seen in the TMF is the contraction of the AWP (Fig. 5b). SSTs at or below 27 °C in the Gulf of Mexico and portions of the NW Caribbean while the rest of the Caribbean is at or above 28.5C. Notably, areas in the Caribbean that have larger mean flow convergence than transient divergence, have SSTs at or greater than 28.5 °C, whereas the opposite is true where transient divergence is greater. Finally, low-level winds remain southeasterly across the eastern half of the Caribbean domain, whereas the western half sees an easterly shift. The easterly shift in the western half of the Caribbean is also on the southern flank of an emerging anticyclonic feature seen in the SE United States, which is likely the continental High.
TMF convergence zone moves further southward in November (Fig. 5c) as evaporation strengthens (not shown) and precipitation weakens (not shown) across most of the Caribbean. Convergence is seen across the Caribbean coast of Central America and the southern half of the Lesser Antilles in correspondence to the southward shift of the Atlantic ITCZ and Eastern Pacific ITCZ convergence bands (Fig. 5f). The Atlantic ITCZ shift results in moisture convergence to return to the Guianas. SAMS shifts southward. A pocket of convergence is seen in Dominican Republic/Puerto Rico and is a result of enhanced surface term convergence on top of mass convergence (not shown). The Caribbean is now under the influence of the southern flank of a broad anticyclonic circulation consisting of NASH and the continental U.S. anticyclone. The shift of low-level winds to easterly across the entire Caribbean is likely responsible for the pocket of convergence via orographic lifting/zonal convergence across the Caribbean side of Central America. The mean flow NASH convergence band disappears and is replaced by mean flow divergence. Mean flow divergence is also seen in the NW Caribbean and Gulf of Mexico. Therefore, mean flow divergence enhances TMF divergence in the Gulf of Mexico and NW Caribbean that is dominated by transient divergence (Fig. 5i). SSTs continue to cool across the Caribbean (Fig. 5c) and the AWP contracts further and is concentrated in the CS.
Caribbean climate regions and pentad climatologies
Based on the findings from the PCA, three sub-regions are seen in the Caribbean. However, with the results of the moisture budget analysis, five distinct regions are found, each with unique rainfall cycle characteristics (Fig. 6): Central Caribbean (Region 1: Dominican Republic, Puerto Rico, the Virgin Islands, and Northern Lesser Antilles), the Western Caribbean (Region 2: Caribbean coast of Central America), the Northwest Caribbean (Region 3: Florida, Bahamas, Cuba, Cayman Islands, and Jamaica), the Central and Southern Lesser Antilles (Region 4), and Trinidad & Tobago (T&T) / Guianas (Region 5). Furthermore, the TRMM-based results support the multi-seasonality of the different regions based on the station data (Fig. 7). Although most of the regional pentad climatologies in TRMM have lower magnitudes than the station data, this is possibly due to TRMM containing some ocean grids in its pentad calculations. Another possible source of difference could be due to the different temporal coverage of the station vs. TRMM data as shown in Fig. 7. However, when the station data temporal range matches with TRMM the station pentad climatologies’ from 1969 to 2017 vs. 1998–2014 matched well; therefore, there are no systematic differences between the two different sets of climatological periods. In sum, the station dataset is robust to represent the regional climatology.
Central Caribbean
The Central Caribbean rainfall cycle is influenced by moisture convergence associated with the expansion and contraction of the western flank of the NASH. At the beginning of the ERS (late-April/early May) convergence from the western flank of NASH emerges anywhere from the northern Lesser Antilles to Puerto Rico, and progresses northwestward into the NW Caribbean as NASH expands westward. The pentad climatologies (Fig. 7a, b) show an ERS in the Northern Lesser Antilles, and its magnitude increases over the U.S. Virgin Islands and Puerto Rico. However, their ERS peaks in May and diminishes in June, hence, the positive correlations seen in PC2 (absence of ERS in June) in this region. Moisture divergence from the southern flank of NASH returns in the Central Caribbean, which causes the MSD. During the LRS, NASH contracts eastward and its western flank moves back into the Central Caribbean on a similar path to what is seen during the ERS. The western flank stalls and dissipates in the Central Caribbean as NASH merges with the continental U.S. High; therefore, convergence remains in the region and is why the Central Caribbean LRS has a longer duration than its ERS (Fig. 7a, b).
Western Caribbean
The Western Caribbean rainfall cycle is dominated by the Eastern Pacific ITCZ with some interactions between ITCZ and NASH and regional modifications by the AWP and the CLLJ. The ERS peaks later than the Central Caribbean, starting in late May through early July (Fig. 7c, d). The northward migration of the Eastern Pacific ITCZ provides moisture for the Western Caribbean during the ERS to LRS. During the ERS, P-E is enhanced in this region from two convergence bands in the western CS: one on the Nicaraguan to Costa Rican Caribbean coast, and the other extending from Jamaica to the Yucatan/Belize Caribbean coast. Both are enhanced by the AWP; however, the former receives convergence from orographic lifting and zonal convergence by easterly winds, and the latter receives convergence from the transport of moisture from NASH induced southeasterly winds. During the MSD easterly winds are enhanced by the expansion of NASH, which (1) enhances orographic lifting and zonal convergence by the CLLJ and (2) cuts off penetration of the Eastern Pacific ITCZ into the Caribbean. Therefore, most of the Western Caribbean experiences an MSD but not to the extent seen in other regions. The pentad climatologies and PC3 support these findings. Stations in the western Caribbean (Fig. 7c, d) have a less prominent MSD and no significant correlation with PC3. The exception is the Nicaraguan to Costa Rican Caribbean coastlines where a lack of the MSD is observed due to enhanced convergence by the CLLJ (Herrera et al. 2015; Hidalgo et al. 2015). During the LRS, the easterly winds weaken as NASH contracts, and the moisture regime and processes described during the ERS return. The slow southward migrating Eastern Pacific ITCZ stays in the Western Caribbean. which is likely why the pentad climatologies and PC3 show a late-end and start to the LRS and WDS, respectively.
Northwest Caribbean
The NW Caribbean rainfall cycle is similar to the Western Caribbean, except it is influenced more by the expansion–contraction of the western flank of NASH, and migration patterns of U.S. mainland mid-latitude features. During the second half of the ERS, the convergence band associated with the western flank of NASH migrates from the Central Caribbean to the NW Caribbean. This is consistent with the negative correlation between PC2 and rain gauge stations on the eastern half of the NW Caribbean. In addition, the NASH southeasterly winds in the CS provide moisture to extend the Eastern Pacific ITCZ into the southern portion of the NW Caribbean. During the MSD, this ITCZ penetration is marginal as divergence associated with trade winds on the southern flank of NASH infiltrates the CS. The pentad climatologies (Fig. 7e, f) and the phase of PC3 support these findings in the moisture budget. The NW Caribbean has negative correlations with PC3, which is largest in its southern half, consistent with the absence of the MSD. During the LRS, NASH contracts and its western flank moves southeast on a similar path seen during the ERS. However, transient divergence from U.S. mainland mid-latitude features quickly strengthens in the continental U.S. and propagates into the Gulf of Mexico and eastern Atlantic. Simultaneously, the AWP begins its contraction in the Caribbean over the NW Caribbean first. Therefore, convergence from the western flank of the NASH dissipates as a result of transient divergence overtaking mean flow convergence in the NW Caribbean and the disappearance of the AWP. This supports what is seen in the pentad climatologies (Fig. 7e, f) and in PC3 in the NW Caribbean. PC3 shows that these stations have negative correlations to a late-LRS, and the pentad climatologies show these stations have the earliest demise of the LRS than any other region in the Caribbean.
Central and Southern Lesser Antilles/T&T/Guianas
The Central and Southern Lesser Antilles and the Trinidad and Tobago (T&T)/Guianas rainfall cycles are affected by competing influences from the Atlantic ITCZ and the diverging trade winds on the southern flank of NASH. During its northward migration, Atlantic ITCZ convergence moves through the Guianas and T&T during the late-ERS and into MSD and slows down while reaching its northern most extent at the central Lesser Antilles by the mid LRS. While the Atlantic ITCZ migrates north from the southern to central Lesser Antilles during the MSD to mid LRS, divergence is seen across the Guianas. During the late-LRS, convergence returns in T&T and the Guianas as the Atlantic ITCZ migrates south and divergence associated with the trade winds returns in the central and southern Lesser Antilles. The Atlantic ITCZ and its migration pattern explains what is seen in the Lesser Antilles and Guianas in the PCA analysis and pentad climatologies (Fig. 7g–j). This is consistent with numerous rainfall studies in the Guianas (i.e. Shaw 1987; Bovolo et al. 2012; Durán-Quesada et al. 2012); however, no study has mentioned the Atlantic ITCZ as the major source of moisture convergence for the Lesser Antilles. The lack of an ERS seen in the Lesser Antilles (Fig. 7g, h) is in response to not only divergence by NASH (Gamble et al. 2007) and trade winds, but also due to the absence of the Atlantic ITCZ in the region during the ERS. This also explains the Guianas’ negative correlations in PC1, and the positive correlations with PC3 across T&T and Guianas. The bimodal rainfall pattern in the T&T and Guianas (Fig. 7i, j) is distinct from the climatological bimodal pattern seen in most of the Caribbean.