SHORT COMMUNICATION EXPLORING THE RELATIONSHIP BETWEEN THE NORTH ATLANTIC OSCILLATION AND RAINFALL PATTERNS IN BARBADOS

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 6: (6) Published online in Wiley InterScience ( DOI:./joc. SHORT COMMUNICATION EXPLORING THE RELATIONSHIP BETWEEN THE NORTH ATLANTIC OSCILLATION AND RAINFALL PATTERNS IN BARBADOS J. CHARLERY a, *, L. NURSE b and K. WHITEHALL a a Dept. of Computer Science, Mathematics & Physics, University of the West Indies, Cave Hill Campus, Barbados b Centre for Resource Management and Environmental Studies (CERMES), University of the West Indies, Cave Hill Campus, Barbados Received ust 5 Revised 7 ruary 6 Accepted 9 ruary 6 ABSTRACT The North Atlantic oscillation (NAO) is a large-scale seesawing movement of atmospheric mass between the North Atlantic subtropical high-pressure system and the Icelandic low-pressure system. This phenomenon has been acknowledged as the dominant mode of winter climate variability in the temperate latitudes of the North Atlantic region. In this study, an attempt is made to identify the effect of the NAO on the rainfall pattern in the Eastern Caribbean, based on data from a representative station in that subregion. Rainfall data taken from a southeastern coastal station in Barbados are used to explore the relationship between the behavior of the NAO (using standardized monthly indexes) and the rainfall pattern at that station. In the period considered (95 ), the NAO is shown to have a very significant effect on the monthly variability of rainfall in Barbados during both El Niño and La Niña periods. Copyright 6 Royal Meteorological Society. KEY WORDS: North Atlantic oscillation; El Niño; La Niña; Caribbean; Barbados; rainfall variability. INTRODUCTION The North Atlantic oscillation (NAO) has been identified as the dominant mode of winter climate variability in the North Atlantic region ranging from central North America to Europe and into much of Northern Asia (Hurrell, 995). Many studies have already been conducted on the effects of other large-scale systems such as El Niño Southern Oscillation (ENSO) and the North Atlantic high-pressure system on the rainfall patterns in the Caribbean (e.g. Chen and Taylor, ; Giannini et al., ). This study attempts to explore the possible association, which may exist, between the NAO s behavior and the rainfall patterns in the tropical region of the Eastern Caribbean. This exploration is limited mostly to a qualitative analysis with the understanding that if a perceived association is indeed detected, then additional work in the future would seek to quantify the relationship. Barbados is the easternmost island of the Caribbean archipelago. Centered at latitude N and longitude 59 W with an area of only 66 square miles, its atmospheric conditions are almost exclusively controlled by the variations in the North Atlantic high-pressure system (hereafter referred to as the Atlantic High (AH)). As a relatively flat and small island and completely unobstructed by other physical features which could affect the airflow from the AH, Barbados geography and topography provide an unparalleled opportunity to investigate the effects of the NAO on the rainfall pattern in that island and by extension, the Eastern Caribbean as well. The geographical location of Barbados and the rainfall station are provided in Figure. * Correspondence to: J. Charlery, Dept. of Computer Science, Mathematics & Physics, University of the West Indies, Cave Hill Campus, Barbados; jcharlery@uwichill.edu.bb Copyright 6 Royal Meteorological Society

2 8 J. CHARLERY, L. NURSE AND K. WHITEHALL 5N 5N N 5N N 5N N Rainfall Station 5N N 5N EQ 9W 8W 7W 6W 5W W W W Figure. Location of Barbados W In the western and northern Caribbean islands, the rainfall patterns are produced by both the westward moving tropical systems (e.g. tropical disturbances, tropical waves, and so on) during the summer, and eastward moving extratropical systems (such as the tail-end of cold fronts) during the winter and early spring. However, unlike the Western and Northern Caribbean, the rainfall pattern in Barbados is produced almost exclusively by the westward moving tropical systems, the occasional upper atmospheric low-pressure troughs and the periodic migration of the intertropical convergence zone during the summer months of the Northern Hemisphere.. METHODOLOGY The rainfall data available for analysis were monthly values for the period uary 9 to ember, for Grantley Adams International Airport, a station located on the southeastern coast of the island. The full dataset was used to compute the mean monthly rainfall. Since the NAO indices available did not predate 95, the period that was investigated was 95 to. For that period, the deviation from the mean rainfall (hereinafter referred to as RRDev) was computed for each month. Fluctuations in the rainfall deviation could then be compared with the variations in the NAO signal to investigate whether any coupling existed. The monthly standardized NAO index computed by the National Center for Environmental Prediction (NCEP) and made available on its website, was used as the NAO dataset. The standardized NAO index here is obtained by projecting the NAO loading pattern on the daily 5 mb height anomaly field over latitudes 9 N. This loading pattern is defined as the first mode of a Rotated Empirical Orthogonal Function (REOF) analysis using monthly mean 5 mb height anomaly data from 95 to over latitudes 9 N. (For more information, see Halpert and Bell, 997). The NAO dataset used included monthly index values from 95 to 5. Some researchers (such as Malmgrem et al., 998; Chen and Taylor, ) have postulated that the rainfall variability in the Caribbean region bears some direct relation to the evolution of the ENSO events. Building upon that observation, this work therefore seeks to examine the influence of the NAO on the rainfall pattern at different phases of ENSO. Although the descriptions weak, moderate and strong ENSO events are used widely (e.g. NCEP website (); Chen and Taylor, ; Giannini et al., ; Intergovernmental Panel on Climate Change (IPCC), ), there appears to be no consensus on the quantitative thresholds for those classifications. Consequently, for this work, the index values for the events that are being classified as weak, moderate or strong are based on the assessment of the thresholds for the different groups and are approximated. A weak event is therefore defined as one in which the absolute value of the ENSO index is.5 or less but Copyright 6 Royal Meteorological Society Int. J. Climatol. 6: (6)

3 RELATIONSHIP BETWEEN THE NORTH ATLANTIC OSCILLATION AND RAINFALL PATTERNS 8 not zero. A moderate event is defined as one in which the absolute value of the ENSO index is between.5 and.75 and a strong event is defined as one in which the absolute value of the index is.75 or greater. The stages examined were: () weak to moderate events, () moderate to strong events, () weakening events and () strengthening events. Both phases of ENSO (El Niño or warm events and La Niña or cold) were considered. The ENSO dataset used was made available by NCEP on its website and contained monthly index values on the state of ENSO. This dataset also covered the period between 95 and 5. The period during which a given event occurred was then observed through graphical representations of the quantities along with the corresponding index values for the NAO and RRDev and a qualitative assessment was made. The three variables, which are therefore being considered, are the standardized NAO monthly index values (referred to as NAO), the mean monthly index for ENSO (referred to as ENSO) and the deviation from the mean monthly rainfall of Barbados (referred to as RRDev)... El Niño phases. OBSERVED RESULTS During the El Niño phase, the RRDev was observed to be generally positively correlated with the NAO index. However, the progressively weakening or strengthening states of the El Niño events seem to suggest an even more refined relationship between the NAO and the RRDev. During the periods when El Niño is weakening, irrespective of intensity, and when a weak to moderate El Niño event is strengthening, RRDev maintains an almost faithful positive correlation with NAO for each given month. Examples of these relationships are illustrated in Figures and. During moderate to strong El Niño events the positive correlation between NAO and RRDev is still being maintained; however, RRDev is sometimes observed to periodically lag the NAO by approximately month. Figure illustrates this observation Figure. Relationship between RRDev and NAO, during weakening El Niño events Copyright 6 Royal Meteorological Society Int. J. Climatol. 6: (6)

4 8 J. CHARLERY, L. NURSE AND K. WHITEHALL N Figure. Relationship between RRDev and NAO, during the strengthening stage of weak to moderate El Niño events Figure. Relationship between RRDev and NAO, during moderate to strong El Niño events Copyright 6 Royal Meteorological Society Int. J. Climatol. 6: (6)

5 RELATIONSHIP BETWEEN THE NORTH ATLANTIC OSCILLATION AND RAINFALL PATTERNS Figure 5. Relationship between RRDev and NAO, during persistently weak El Niño events However, during periods of five or more months of persistently weak El Niño events, the relationship between RRDev and NAO did not indicate any clear coupling. Figure 5 illustrates some of these patterns and associations... La Niña phases DuringtheLaNiña phases, the correlation between the two variables is observed to be weakly positive in general, with the RRDev occasionally lagging the NAO by one or two months. It was observed that during weak La Niña events the RRDev showed a strong positive correlation with the NAO index. This relationship is illustrated in Figure 6. During moderate to strong La Niña events, the relationship tended to be somewhat chaotic, although on a few occasions there was a weakly positive association with the RRDev lagging the NAO index by roughly month. Figure 7 provides an example of this observation. Strengthening La Niña events on the other hand, revealed a positive correlation between the two variables as illustrated in Figure 8. However, during periods of weakening La Niña events, irrespective of the intensity, the RRDev s behavior tended to be somewhat chaotic. This is further explained by Figure 9. Additionally, if the La Niña event persisted for longer than 6 months, even at various strengths, the RRDev behavior lagged the NAO pattern by month until the end of the event. Consistently, during the first two months of La Niña, the RRDev was directly proportional to the pattern of the NAO. In the case of the offset of La Niña, the RRDev was generally synchronized except for the incidences where the event persisted for more than six months. In such cases, the relationship was observed to be inversely proportional... Statistical correlation To further test the strength of the relationship between the NAO and RRDev during different phases of ENSO, the Pearson s Product Moment Correlation was applied. A summary of the results is provided in Copyright 6 Royal Meteorological Society Int. J. Climatol. 6: (6)

6 8 J. CHARLERY, L. NURSE AND K. WHITEHALL Figure 6. Relationship between RRDev and NAO, during weak La Niña events Figure 7. Relationship between RRDev and NAO, during moderate to strong La Niña events Copyright 6 Royal Meteorological Society Int. J. Climatol. 6: (6)

7 RELATIONSHIP BETWEEN THE NORTH ATLANTIC OSCILLATION AND RAINFALL PATTERNS N N Figure 8. Relationship between RRDev and NAO, during the strengthening stage of La Niña Table I. Correlation coefficients for NAO and rainfall deviation during ENSO phases ENSO Phase Pearson correlation coefficient Significance (-tailed) Correlation significance level (N) Moderate to strong El Niño Moderate to strong La Niña.9.8 N.S.S. Weak to moderate El Niño strengthening La Niña strengthening...5 Weak La Niña Persistently weak El Niño..85 N.S.S. 5 El Niño weakening La Niña weakening.6.8 N.S.S. 7 N.S.S., Correlation not statistically significant (i.e. at least at the.5 level.). Table I. In all, with the exception of three development phases of ENSO (i.e. moderate to strong La Niña, La Niña weakening and persistently weak El Niño), there was a strong statistical correlation at least at the.5 confidence level. In fact, the level of significance in four of the remaining five development phases was.. Even though, the sign of the variables values are sometimes opposite (even when the trends were synchronized) and the rainfall response sometimes lagged the NAO signal by a month or two, it is noteworthy that the relationships are still statistically significant. It should be noted further that in the cases in which the relationship is not statistically significant, the association between the NAO and RRDev is also not clearly discernible from the graphs. Copyright 6 Royal Meteorological Society Int. J. Climatol. 6: (6)

8 86 J. CHARLERY, L. NURSE AND K. WHITEHALL Figure 9. Relationship between RRDev and NAO, during the weakening stage of La Niña. DISCUSSION AND CONCLUSIONS On the basis of the aforementioned observations, it can be concluded that the NAO appears to exert some influence on the pattern of rainfall variability in Barbados. The analysis further leads to the conclusion that this influence may provide some degree of rainfall pattern predictability during an El Niño event and even during certain stages of La Niña as well. In addition, the data suggest that whenever an El Niño event occurred (with the exception of a persistently weak episode), the RRDev and the NAO were in phase. In contrast, it would appear that during a La Niña event, the relationship between the NAO and the RRDev showed a greater tendency to be less defined. However, during weak La Niña events a positive synchronized relationship exists between the two variables. Moreover, the onset and ending ofeither a La Niña or El Niño event also showed a positive association between the NAO and RRDev lasting for approximately two months, beginning from the month of commencement or ending of the event. From the analysis, it was observed that when the ENSO index is persistently weak, there is no discernible association between the RRDev and the NAO index. This observation was somewhat startling as the expectation was that the NAO would have been truly dominant. However, this finding strongly suggests that the influence of the NAO on the pattern of rainfall variability is therefore not only dependent on the phase of ENSO, but also on the intensity of the event. Finally, the study supports the conclusion that the effects of the NAO are not restricted to the temperate latitudes, but a clear and very discernible influence can also be identified on rainfall variability in the low latitudes as well. As the rainfall data for Barbados have shown, this influence is very significant (possibly even dominant) during certain stages of ENSO. One is fully aware of the dangers of generalizing this observation to include the entire Eastern Caribbean. However, the focus of this discussion is on the pattern of rainfall deviation, and not actual rainfall amounts that are influenced by many factors including geography and topography. Furthermore, since the Eastern Caribbean is a relatively small area and Barbados is located in the center of that region, a reasonable assumption can be made that the rainfall deviation exhibited in the Barbados data may be fairly representative of the wider subregion. 5. FUTURE WORK We are satisfied that the results from this preliminary study, although almost entirely qualitative, indicate a basis for future work in quantitatively assessing the influence of the NAO index on the rainfall pattern experienced over the island of Barbados. We are of the view that a quantitative assessment of the relationships identified (i.e. between NAO, ENSO and RRDev) could lead to the creation of a model for the seasonal forecasting of rainfall patterns in Barbados and by extension, the islands of the Eastern Caribbean as well. Copyright 6 Royal Meteorological Society Int. J. Climatol. 6: (6)

9 RELATIONSHIP BETWEEN THE NORTH ATLANTIC OSCILLATION AND RAINFALL PATTERNS 87 REFERENCES NOAA/National Weather Service, National Centers for Environmental Prediction, Climate Prediction Center, Camp Springs, yland NOAA/National Weather Service, National Centers for Environmental Prediction, Climate Prediction Center, Camp Springs, yland monitoring/ensostuff/ensoyears 877-present.shtml. Chen AA, Taylor MA.. Investigating the link between early season Caribbean rainfall and the El Niño + year. International Journal of Climatology : Giannini A, Kushnir Y, Cane MA.. Seasonality in the Impact of the ENSO and the North Atlantic high on Caribbean rainfall. Physics and Chemistry of the Earth, Part B: Hydrology, Oceans and Atmosphere 6(): 7. Halpert MS, Bell GD Climate assessment for 996. Bulletin of the American Meteorological Society 78(5): S S9. Hurrell JW adal trends in the North Atlantic oscillation: Regional temperatures and precipitation. Science 69: Intergovernmental Panel on Climate Change (IPCC),. Climate Change Impacts, Adaptation and Vulnerability. Contribution of Working group II to the Third Assessment Report. Cambridge University Press, Cambridge, UK:. Malmgrem BA, Winter A, Chen D El Niño Southern oscillation and North Atlantic oscillation control of climate in Puerto Rico. Journal of Climate (): Copyright 6 Royal Meteorological Society Int. J. Climatol. 6: (6)

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