ENSO diversity in the logbook-based reconstruction methods and predictor grid boxes, 1979-2013 predictor grid boxes, 1979-2013

Firstly, hit rates were calculated using the same methodology as explained in Chapter 3. The EP and CP El Niño events from the NINO method using HadISST-derived

105 Niño indices were compared to those identified from the PCR and CPS fitting reconstructions (Table 4.1). This was carried out using the full fitting data, and the sub-sampled data which aims to more directly replicate the logbook data availability within ERA-Interim reanalysis as explained in Chapter 3 (Section 3.2.1).

Table 4.1 Hit rates for EP and CP El Niño events from PCR, CPS and the instrumental DJF SOI, compared to the NINO method, 1979-2013. PCR and CPS full fitting and the mean of five sub-sampled iterations are presented.

EP Skill % CP Skill %

Instrumental DJF SOI 100 33

PCR fitting 100 66

PCR sub-sampled (5 it.) 66 66

CPS fitting 100 33

CPS sub-sampled (5 it.) 100 33

The NINO method identifies three EP events (1982/83, 1991/1992 and 1997/98) and three CP events (1994/95, 2002/03 and 2009/10) since 1979. All three EP events were identified as El Niño events in the instrumental DJF SOI, whereas only one of the three CP events were found in this record, a hit rate of 33% (Table 4.1). This immediately suggests that the instrumental DJF SOI classification may identify EP events better than CP events. The PCR reconstruction identified 100% of the EP events, and two of the three (66%) CP events, while the CPS reconstruction identified all three (100%) EP events, but only one of the three (33%) CP events (Table 4.1). This suggests that the two methods used in the logbook-based reconstruction also capture EP events more successfully than CP events, with a stronger bias towards EP events in the CPS-based reconstruction compared to PCR-based. These conclusions are also drawn from the sub-sampled reconstructions. However, the hit rates for the PCR five iterations is 66%

for both CP and EP events (Table 4.1).

To further investigate this potential bias, the spatial wind patterns of individual ENSO events were analysed, specifically within the nine predictor grid boxes, highlighted in Fig. 4.1 (see Appendix Table A.1 and Fig. A.2 for labels and exact locations).

Composite plots of the DJF mean seasonal zonal wind anomalies for EP and CP events

106 were constructed using ERA-Interim Reanalysis data based on the 1979-2013 climatology (Fig. 4.1) as well as anomaly plots for individual EP and CP El Niño events (Fig. 4.2 and Fig. 4.3 respectively). Stronger anomalies are seen in a higher number of the predictor grid boxes during EP events compared to CP events. Positive wind anomalies are found during EP events in grid boxes 2, 3, 4 (South African grid boxes), 5 and 6 (Indian Ocean) and negative anomalies in grid boxes 1, 8 and 9. In CP events, the only visible signal in the predictor grid boxes is a negative one in grid boxes 3, 4 and 9. It should be noted that the sign of the anomaly in grid boxes 3 and 4 is the opposite in EP compared to CP events. Therefore, the differences in the ENSO-wind teleconnection in this region around South Africa could be a useful indictor or ENSO flavours within future studies using wind as a key variable to inform ENSO reconstructions.

The El Niño composite presented in Chapter 3 (Fig. 3.7) was calculated based upon events defined from the one standard deviation of the DJF SOI threshold, and thus is informed by a different collection of events compared to the EP and CP composites presented here (Fig. 4.1). The zonal wind anomalies within the predictor grid boxes in Fig. 3.7 has higher similarity to that in the EP composites compared to the CP composites. This suggests that EP El Niño events have stronger similarities to more traditionally defined El Niño events (i.e. using the SOI) than CP El Niño events.

Looking at the individual events (Fig. 4.2; Fig. 4.3), the wind patterns are more varied due to the inter-event diversity of ENSO. Nevertheless, when considering the composite wind anomaly patterns (Fig. 4.1) the higher number of predictor grid boxes with stronger wind anomalies during EP compared to CP events, and the increased skill of the two methodologies at capturing EP events over CP events suggests that the logbook-based reconstructions would favour reconstruction of EP over CP events.

This has implications for the ability of the historical reconstruction to capture CP events and is considered in section 4.4.2 when interpreting the historical logbook-based reconstructions. The zonal wind patterns associated with EP and CP El Niño events globally and within the predictor grid boxes are also investigated further in Chapter 7 using a range of historical reanalysis and historical climate model simulations.

107 Fig. 4.1 DJF zonal wind composite anomalies (ms^-1) during EP El Niño events (1982/83, 1991/92 and 1997/98) and CP El Niño events (1994/95, 2002/03 and 2009/10), relative to the 1979-2013 climatology. Zonal wind data are taken from the ERA-Interim reanalysis. Predictor grid boxes used in the reconstruction are outlined in black, numbered 1-9 from west to east. Brackets indicate the number of events used to calculate each composite.

108 Fig. 4.2 DJF zonal wind anomalies (ms^-1) during EP El Niño events 1982/83, 1991/92 and 1997/98 relative to the 1979-2013 climatology. Predictor grid boxes used in the reconstruction are outlined in black.

109 Fig. 4.3 DJF zonal wind anomalies (ms^-1) during CP El Niño events 1994/95, 2002/03 and 2009/10, relative to the 1979-2013 climatology. Predictor grid boxes used in the reconstruction are outlined in black.

4.4.2 ENSO diversity within the logbook-based reconstructions, 1815-1854 The zonal wind anomalies in the predictor grid boxes were stronger in a higher number of grid boxes for EP events than CP events (Fig. 4.1). Using these different wind patterns, speculative analysis of El Niño events in the logbook-reconstruction was carried out, in an attempt to categorise the historical events into EP and CP El Niño.

PCR B and CPS B reconstructions are analysed in this section. Chapter 3 found CPS B to have the best agreement with the multi-proxy ENSO reconstructions (Table 3.11).

PCR B is also analysed as the PCR method was found to have a lower EP-bias than the CPS method (Table 4.1).

110 4.4.2.1 CPS logbook-based reconstruction, 1815-1854

An attempt to categorise the El Niño events identified by the logbook-based reconstructions was then carried out. In the ERA-Interim composites (Fig. 4.1), eight of the nine grid boxes show negative (grid box 1, 8 ,9) or positive (grid box 2-6) wind anomalies in EP events, whereas only three (3, 4, 9) display visible negative zonal wind anomalies in CP events. Fig. 4.4 shows the DJF zonal wind anomalies from logbook data for the five years defined as El Niño events in the CPS B reconstruction, 1824/25, 1830/31, 1833/34, 1838/39 and 1851/52, as identified in Fig. 3.14. Compared to the El Niño zonal wind anomalies in the fitting period, there is a less consistent pattern in the direction and strength of the anomalies in the logbook-derived winds.

An attempt to classify these events as EP and CP is very much an experimental one, however a simple agreement methodology was attempted in which the spatial wind patterns from modern composites were compared to those in historical individual events.

111 Fig. 4.4 DJF Zonal wind anomalies (ms^-1) from logbook observations for CPS B El Niño years with 1824/25 and 1830/31 anomalies relative to the mean of 1815-1833 and 1833/34, 1838/39 and 1851/52 relative to the mean of 1834-1854.

112 Table 4.2 Classification of CPS B El Niño events into EP and CP El Niño.

% EP Grid

Four of five the historical events from CPS B are suggested to be EP El Niño events and only one to be a CP event (Table 4.2). As discussed in Chapter 3, some of the multi-proxy reconstructions also identify El Niño events within four of these five years, however there is far from universal agreement upon these within the records.

The event chronology from this reconstruction suggests a lower frequency of CP El Niño events in this period compared to the reported rate of increase in CP El Niño in recent decades (Lee and McPhaden 2010).

4.4.2.2 PCR logbook-based reconstruction, 1815-1854

The PCR method was found to have a lower EP-bias than the CPS method over the modern period, 1979-2013 (Table 4.1), with both methods capturing all the EP events in this period, but PCR capturing two of three CP events while CPS only captured one.

Therefore, it is suggested that the CPS method has a stronger bias towards EP than the PCR method. This would suggest that the CPS method would identify less CP events.

During the logbook period, PCR B reconstruction found six El Niño events. Three of these events (1824/25, 1830/31, 1851/52) are also identified by CPS B reconstruction.

These are events are all classified as EP events (Table 4.2). The three remaining events from PCR B which are not found in CPS B, are in the years 1835/36, 1842/43 and 1847/48 and are all found in both PCR A and CPS A, as well as PCR B. The zonal wind anomalies for these additional events are shown in Fig. 4.5, and again are compared to the modern patterns from EP and CP El Niño composite analysis (Fig.

4.1).

113 Fig. 4.5 DJF Zonal wind anomalies from logbook observations for PCR B El Niño years, 1835/36, 1842/43 and 1847/48 relative to the1834-1854 mean. See Fig. 4.3 for the additional three PCR B events, which are also found in CPS B (1824/25, 1830/31, 1851/52).

114 Table 4.3 Classification of PCR B El Niño events into EP and CP El Niño, Bold indicates events unique to PCR B, thus not already presented in Table 4.2 for CPS B.

% EP Grid

Of the three additional El Niño events from PCR B, two of these are suggested CP El Niño years. Only W10 from the multi-proxy reconstructions indicate an El Niño event in 1835/36, whereas none of the multi-proxies indicate an El Niño during the remaining two years (1842/43 and 1847/48). Overall, four out of the six El Niño events identified by PCR B are classified as EP events, suggesting more EP than CP El Niño events during this historical period (1815-1854), with a CP-EP ratio of 0.5. This is also supported by the higher frequency of EP than CP events in CPS B-defined events, with a CP-EP ratio of 0.25. Further work is needed in order to fully understand the reasons for the different event-capture skill of the logbook-based reconstructions. This is discussed further in Section 4.5.

4.4.3 Documentary records: Event chronology comparison to EP and CP El