The effects of the North Atlantic Oscillation, Atlantic Meridional Mode, and Sea Surface Temperature on Puerto Rico rainfall pattern Flavia Dias de Souza Moraes*, Thomas L. Mote, and Paul W. Miller Climate Research Laboratory, Department of Geography, University of Georgia, Athens, GA, USA
Introduction
Results
The North Atlantic Oscillation (NAO) influences Northern Hemisphere climate variability. The NAO index can be calculated as the difference between the normalized sea level pressure (SLP) near the Azores (subtropical high) and Iceland (polar low). The meridional pressure gradient affects the direction and strength of the westerly and trade winds and sea surface temperatures (SST) in the tropical Atlantic. The Atlantic Meridional Mode (AMM) also influences the climate of the tropics. AMM is calculated from the meridional gradient in SST near the Intertropical Convergence Zone (ITCZ). The AMM phase is related to tropical Atlantic SST anomalies and near-surface winds, as well as the location of the ITCZ. This study analyzes the relationship between NAO, AMM, and SST and eastern versus western Puerto Rico (PR) rainfall in Apr-Jul. Eastern PR has a different annual average rainfall when compared to western PR (Figure 1).
Eastern and western PR Early Rainfall Season (ERS, Apr-Jul) rainfall anomalies are negatively correlated with NAO for the same season and when lagged by one season (not shown). However, the correlation is significant only when precipitation is lagged. The wintertime NAO (Jan-Mar) results in more years with negative ERS precipitation anomalies (31%) in eastern PR than in western PR (25%) (Table 1). The AMM has a positive association with eastern and western PR precipitation both in the coincident season and when precipitation is lagged by one season.
SST composite map (Figure 2b) shows a pool of slightly cooler SST anomalies near PR for the years when western PR has a negative precipitation anomaly than for years when eastern PR experiences the same conditions. However, SST anomalies are the same near PR when both regions have positive precipitation anomalies (Figure 2). The SST composites of the preceding season (Jan-Mar) do not show spatially coherent anomalies near PR (not shown). a)
b)
c)
d)
Table 1. Frequency (% of years) with positive, negative and normal precipitation anomalies stratified by NAO and AMM phase during the preceding season (Jan-Mar) and coincident Early Rainfall Season (Apr-Jul).
*Values in blue, underlined and bolded are the significant at p=0.05. Neutral years include NAO ±0.2, AMM ±0.5 and precipitation anomalies ±10%.
Figure 1. Puerto Rico 30-year annual average rainfall (1985-2014). The dashed line corresponds to the 66.5°W meridian, dividing the island into eastern and western regions (Mote et al. 2017).
Data and Methods Daymet daily precipitation data was used to calculate the monthly precipitation for eastern and western PR. Monthly precipitation anomaly data from each region was correlated with monthly NAO and AMM indices (NOAA Earth System Research Laboratory). Months with significant correlations were averaged by season and used to create contingency tables. SST anomalies composites were created based on the same seasons identified in the contingency tables (NOAA Extended Reconstructed Sea Surface Temperature (SST) V5).
The magnitude of the ERS precipitation anomaly (mm) is greater in eastern than in western PR when stratified by phase of the wintertime NAO (Table 1). Although both regions have the same frequency of years with positive precipitation anomalies (19%) during negative NAO, the magnitude of the precipitation anomaly is 126 mm greater in eastern PR (Table 2). The same effect is evident with the AMM. The difference in the precipitation anomaly between eastern and western PR (Table 2) is greater than the difference in the frequency years with above or below normal rainfall (Table 1). Table 2. Precipitation anomaly (in mm) during the Early Rainfall Season (Apr-Jul) stratified by NAO and AMM phase during the preceding season (Jan-Mar) and coincident Early Rainfall Season.
*Values in blue, underlined and bolded are the significant at p=0.05. Neutral years include NAO ±0.2, AMM ±0.5 and precipitation anomalies ±10%.
References Charlery, J., Nurse, L., & Whitehall, K. (2006). Exploring the relationship between the North Atlantic Oscillation and rainfall patterns in Barbados. International Journal of Climatology, 26, 819–827. George, S. E., & Saunders, M. A. (2001). North Atlantic Oscillation impact on tropical North Atlantic winter atmospheric variability. Geophysical Research Letters, 28, 1015–1018. Jury, M., Malmgren, B. A., & Winter, A. (2007). Subregional precipitation climate of the Caribbean and relationships with ENSO and NAO. Journal of Geophysical Research: Atmospheres, 112, D16107. Malmgren, B. A., Winter, A., & Chen, D. (1998). El Nino–Southern Oscillation and North Atlantic Oscillation control of climate in Puerto Rico. Journal of Climate, 11, 2713–2717.
Figure 2. SST anomalies during the Early Rainfall Season (Apr-Jul) when precipitation anomaly is negative in eastern PR (a) and western PR (b), and when precipitation anomaly is positive in eastern PR (c) and western PR (d).
Conclusions The analysis of the association of the NAO, AMM and SST with eastern and western PR rainfall is discussed for the first time in this study. The results confirm studies that suggest the ERS is the season most influenced by NAO in PR (Malmgren et al. 1998). This is important because the ERS accounts for the majority of interannual variability in annual rainfall in PR (Miller et al. 2018). Results indicate that eastern PR is more strongly associated with NAO, AMM, and SST than western PR. SST composites indicate that western PR requires slightly cooler SST anomalies than eastern PR for a comparable change in precipitation. *Contact information:
[email protected]
Miller, P. W., Mote, T. L., and Ramseyer, C. A. (2018). A 15-yr climatology of thunderstorm activity and contemporaneous precipitation in Puerto Rico and their relationship to the thermodynamic environment. Weather and Forecasting, in preparation. Mote, T. L., Ramseyer, C. A., & Miller, P. W. (2017). The Saharan Air Layer as an early rainfall season suppressant in the eastern Caribbean: The 2015 Puerto Rico drought. Journal of Geophysical Research: Atmospheres, 122, 10,966–10,982. Vimont, D. J., and Kossin, J. P. (2007). The Atlantic Meridional Mode and hurricane activity. Geophysical Research Letters, 34, 1–5. Rugg, A., Foltz, G. R., and Perez, R. C. (2016). Role of Mixed Layer Dynamics in Tropical North Atlantic Interannual Sea Surface Temperature Variability. Journal of Climate, 29, 8083–8101.
