Climate and Weather

Status of El Niño Southern Oscillation (ENSO)

On Guam and throughout Micronesia, all of the climate variables—including air temperature, rainfall, and typhoon distribution—exhibit large fluctuations on inter-annual and inter-decadal time periods.  The dominant driver of Guam’s inter-annual variation is the El Niño/Southern Oscillation or ENSO cycle, the average period of which is 3.6 years with a range of 2 to 7 years.  The canonical ENSO cycle and the newly described ENSO Modoki explain 57% of the variance of Pacific SST.  Guam and all of Micronesia are located in an ENSO core region. Global and regional scale precipitation patterns associated with the El Niño/Southern Oscillation.

The ENSO cycle is divided into three main states:

  1. El Niño (~25% of the time)
  2. La Niña (~25% of the time), and
  3. ENSO-Neutral (~50% of the time)

The U.S. Climate Prediction Center (CPC) monitors ENSO with an index – the Oceanic Niño Index (ONI) — based on the sea surface temperature in the central equatorial Pacific.  The climate state is:

El Niño when the values of the ONI are at-or-above +0.5°C;
La Niña when the values of the ONI are at-or-below -0.5°C; and
ENSO-neutral for ONI values between -0.5°C and +0.5°C.

El Niño or La Niña are designated as “strong” when the values of the ONI are higher or lower than +1.5° C and -1.5° C, respectively (Fig. 1).  During the past 20 years there have been six El Niño events, with the El Niño events of 1997-98 and 2015-16 considered “strong”.

Figure 1. A time series of the CPC’s Oceanic Niño Index (ONI) for the period 1996 to present. On Guam, it is the climatic disruptions of El Niño that garner the most attention.  A typical El Niño event lasts about 18 months, with various impacts distributed unevenly across the event (Fig. 2). Oceanic indices used to diagnose El Niño, such as the ONI, reach their peak long after the atmosphere begins to exhibit wild weather patterns typical of El Niño Onset.  Whereas wild weather patterns (e.g., extreme rains and early season typhoons) arrive in the first half of the El Niño year, the oceanic response to El Niño peaks late in the El Niño year (~ December). During El Niño, Guam’s rainfall is abundant at first, becoming very dry in the post-Peak phase.

Figure 2. Timing of climatic hazards associated with El Niño. The depicted timing of hazards is a generic average for Micronesia, and individual island groups may vary somewhat.

History of the Recent Epic El Niño of 2015-16

The 2015-16 El Niño event was one of the strongest in recorded history. Adverse weather conditions on Guam and throughout Micronesia unfolded as one might expect during El Niño. Large changes to rainfall, sea level and the typhoon distribution were observed. 

July 2015

During the first half of 2015, substantial warming of the equatorial Pacific sea surface and sub-surface waters clearly and unambiguously signaled the arrival of El Niño. Wild weather patterns typical of El Niño onset were observed across the region. These included noteworthy extremes of rainfall and an abundance of early-season tropical cyclones. Strong oceanic responses included the aforementioned oceanic warming and dramatic lowering of the sea level across much of Micronesia. Strong westerly wind bursts accompanied by twin (northern hemisphere-southern hemisphere) tropical cyclone formation were noted at roughly 30-day intervals, with quiet periods in between. This is the hallmark signature of the Madden Julian Oscillation (MJO) amplified by the background El Niño climate state. Through mid-July, twelve named topical cyclones were observed in the western North Pacific basin (see the tropical cyclone discussion). One of these — Typhoon Halola — entered from the central Pacific. All but one of the 12 (TS Kujira formed and stayed in the South China Sea) tracked within the bounds of Micronesia, where many islands took a beating. The most damaging of the early season typhoons was super typhoon Maysak, which (at the end of March 2015) left a trail of destruction from Chuuk State westward through Yap State. There was considerable damage and four deaths in Chuuk State, and Ulithi Atoll experienced a devastating direct strike by this super typhoon. During mid-May 2015, Typhoon Dolphin passed between Guam and Rota with typhoon force winds and very high surf observed on portions of both islands. This was the first time since 2002 (during the passage of Pongsona) that typhoon-force sustained wind was observed on Guam, and the first time since 2004 (during the passage of Chaba) that typhoon-force sustained wind was observed on Rota. [Note: Chaba may have produce minimal typhoon force winds on the extreme northern part of Guam (AAFB northward).] From January through mid-July 2015, adverse weather across Micronesia included several occurrences of damaging wind, very heavy rainfall and damaging high surf. The wild weather of the first half of 2015, which featured many high rainfall extremes throughout the region, managed even to include a prolonged localized drought across Palau. A selection of extraordinary weather and climate highlights from January through mid-July of 2015 includes:

  1.  RMI — record-setting heavy daily and monthly rainfall on some atolls;
  2. RMI – damaging sea inundation on more than one occasion;
  3. RMI – unusual tropical cyclone activity;
  4. Western North Pacific — abundant early season tropical cyclones (12 through mid-July);
  5. Pohnpei Island – record May rainfall;
  6. FSM – damaging typhoons (Maysak, Noul, and Dolphin),
  7. Western Pacific — two occurrences of named twin tropical cyclones (Bavi and Pam; Chan-hom and Raquel);
  8. Western Pacific — several major westerly wind bursts;
  9. Guam – February 2015 was the driest month of record;
  10. Guam – large waves (20-25 feet) on the east and northeast coasts during the passage of TS Bavi and again during the passage of TY Dolphin;
  11. Guam and CNMI – damaging wind from tropical cyclones (Bavi, Dolphin, Chan-hom and Nangka)
  12. Pacific Basin – major oceanic surface and subsurface warming signaling the onset of a potentially strong El Niño!

December 2015

The 2015 El Niño event became strong, rivaling the strong El Niño events of 1982-83 and 1997-98. During the first half of 2015, many of the atmospheric effects of the current El Niño event were already exhibiting substantial deviations from average conditions. These included noteworthy extremes of rainfall and an abundance of early-season tropical cyclones. Early oceanic responses portending strong El Niño included a rapid oceanic surface and sub-surface warming and a dramatic lowering of the sea level across much of Micronesia. Oceanic indices used to diagnose El Niño, such as the SST anomaly in the Nino 3.4 region, reach their peak long after the atmosphere begins to exhibit wild weather patterns typical of El Niño onset. Whereas wild weather patterns (e.g., extreme rains and early season typhoons) arrive in the first half of the El Niño year, the oceanic response to El Niño peaks late in the El Niño year (~ December). Through mid-October, the whole North Pacific Basin has seen a very high number of tropical cyclones (see the tropical cyclone discussion), with Hawaii and most of the islands of Micronesia experiencing multiple threats and various effects from the passages of these cyclones. During the 3rd Quarter, the island of Saipan in the CNMI was impacted by two tropical cyclones: (1) very intense Typhoon Soudelor in early August; and (2) a lesser storm (Tropical Storm Champi) in mid-October. Persistent gusty westerly surface winds had mostly nuisance effects at many Micronesian islands, such as disruption of local fisheries and minor inundation on west-facing shores. With the exit of Typhoon Champi from the western North Pacific during the last week of October, a dry spell ensued across most of the region that marked the early stages of the anticipated post-El Niño widespread severe drought.

July 2016

During the spring of 2016, the epic El Niño of 2015/16 entered its post-Peak phase, with the CPC’s Oceanic Niño Index falling away from its peak value reached in December 2015. Very dry conditions developed across much of Micronesia and into Hawaii. Prolonged and widespread dry conditions are typical during the post-Peak phase of a strong El Niño. Several records for low rainfall have were set1:

  1. Koror, Palau — driest Oct-Mar, driest Apr-Mar;
  2. Yap Island — driest Oct-Mar;
  3. Pohnpei Island — 3rd driest Oct-Mar, 4th driest Apr-Mar;
  4. Nukuoro (Pohnpei State) — driest Apr-Mar, 3rd driest Oct-Mar;
  5. Ulithi (Yap State) — 2nd driest Oct-Mar, driest March;
  6. Woleai (Yap State) — driest Oct-Mar, driest Apr-Mar;
  7. Alingalapalap (RMI) — 2nd driest Oct-Mar;
  8. Kwajalein (RMI) — 5th driest Oct-Mar;
  9. Majuro (RMI) — driest Oct-Mar;
  10. Jaluit (RMI) — driest Oct-Mar, driest Apr-Mar;
  11. Mili (RMI) — 2nd driest Oct-Mar, driest March;

1 Information compiled by R.Heim, NOAA NCEI 

In response to the impact of drought conditions on water supply, local governments have issued proclamations concerning drought: the governments of Palau and of the FSM have declared drought emergencies for portions of their jurisdictions, and the government of the Republic of the Marshall Islands has gone even further with a declaration of drought disaster. (Chip: is this accurate ??) With very dry conditions ongoing, information about the nature and extent of impacts is still being gathered and assessed. Substantial draw-down and degradation of municipal water supplies is widespread, streamflow on high islands is very low, and vegetation has suffered yellowing, wilting, and destruction by wildfire. Reports of ecological impacts have been received, such as the loss of perhaps a few million jellyfish in Palau’s world famous Jellyfish Lake.

December 2016

The CPC’s Oceanic Niño Index (ONI) continued its dramatic decline from its high-stand at strong El Niño in the first three months of the year. Indeed, the falling values of the ONI and other recent behaviors of the atmosphere and ocean indicate that the climate system has entered La Niña. For this reason, the CPC upgraded its La Niña Watch to a La Niña Advisory in its November 10th monthly ENSO diagnostics discussion. During the 3rd Quarter, rainfall continued below average at a majority of reporting sites across the U.S.-Affiliated Pacific Islands. High rainfall amounts at reporting sites on the island of Saipan were a notable exception. Several daily extreme rainfall events, and persistent wet conditions associated with the western Pacific monsoon trough helped to push the 3rd Quarter rainfall at Saipan locations to the top of the list for the entire region! 

Guam Weather Elements (2015-2016)

Temperature (2015-2016)

Since the end of the 2nd World War, the island of Guam has had many sites for observing rainfall and other climate parameters (e.g. MAX and MIN temperature) (Fig. 3). All of the sites have monthly rain records, and a few have daily rain records. 

Figure 3. Guam rain gauge site locations.  The sites with the longest and most complete daily climate records are shown by the large circles, smaller circles have shorter periods of record, are more incomplete, or have only monthly readings. The most complete and cleanest daily climate record available is from Andersen Air Force Base (AAFB).  Two other sites with long records and few missing data points are the Naval Air Station (NAS) (now the NOAA Weather Service Forecast Office (WSFO), Tiyan) and the NOAA Weather Service Meteorological Observatory (WSMO) in Taguac, Guam.  The United States Geological Survey (USGS) also has maintained a network of rain gages on Guam, which have some long and relatively complete historical records.  The official NOAA climate record for Guam is derived from observations at the NOAA Weather Service Meteorological Observatory (WSMO) and the WSFO Tiyan and the Guam Naval Air Station (NAS).  An official 63-year record (1953-present) of climate data for these three Guam sites is available on-line at the web site of the National Climatic Data Center (NCDC —  It is not recommended that the official NOAA data for these sites be used for studying Guam’s climate: in the data base, the NAS record is erroneously concatenated with the WSMO record, and the WSMO record has NAS data erroneously appended to it after 1995. This leaves the climate record at Andersen Air Force Base (AAFB) as the only station with a long-term, clean and complete (99.9%) daily climate record for Guam.  The record of AAFB on file at the NCDC has many missing days (2.5%) and several missing months (15) over its period of record (1953-2002), but the complete record is available locally, and is used in this study. Consistent with the ongoing rise of global atmospheric temperature, the AAFB record of air temperature from 1953 to present shows warming from the 1950s through the mid-1990s (Fig. 4).  The linear trend is roughly 1° C over the period of record.  However, the AAFB temperature trend is perceptibly more complex than what a simple linear trend would indicate.  There are large inter-annual variations (primarily associated with ENSO), and an obvious long-term 3rd-order behavior that manifests as a rapid rise of temperature to a peak in 1998 with a general cooling thereafter.  The temperature during 2015 was slightly cooler than average (a typical response to cloudy and wet El Niño weather).  The temperature during 2016 was slightly above average (a typical response to the sunny drier conditions of the post-Peak phase of El Niño). 

Figure 4. The time series of the MAX and MIN temperature at AAFB from 1953 to present.  Data plotted is a 12-month moving average of the departure from the period mean of the raw monthly values. Units are °C.

Rainfall (2015-2016)

The calendar year 2015 featured some wild weather across Guam and the CNMI.  It was a typical El Niño year, with an early start to typhoon activity (Tropical Storm Bavi in March and Typhoon Dolphin in May), and then a continuation of a series of close passages of tropical cyclones through November.  The final list of tropical cyclones passing within 180 n mi of Guam, Rota, Tinian, or Saipan includes: 

  1. Tropical Storm Bavi (March) — (gales and high waves, Guam, Rota, Tinian and Saipan);
  2. Typhoon Dolphin (May) — (typhoon winds on northern Guam and Rota);
  3. Tropical Storm Chan-hom (July) — (gales on Rota, Tinian and Saipan);
  4. Super Typhoon Nangka (July) — (gales on Saipan, direct hit of Alamagan, CNMI);
  5. Typhoon Soudelor (August) — (devastating strike of Saipan);
  6. Typhoon Goni (August) – (gales on Tinian and Saipan);
  7. Typhoon Champi (October) – (gales, extreme rainfall, Tinian and Saipan) and,
  8. Typhoon In-fa (November) – (brief gales and moderate waves on Guam). 
During November and December 2015, and continuing into January 2016, the weather became quiet.  A trade-wind regime dominated, with typical gusty east-northeast winds and several episodes of hazardous surf (at or just above 8-10 ft) on north and east exposed locations on all islands.  After the passage of In-fa safely to the south of Guam in mid-November, no further tropical cyclones affected the region.  Rainfall began to lessen, and by late January 2016, wildfire activity on Guam began to increase.  Annual rainfall amounts — boosted by several 24-hour extremes associated with passing tropical cyclones — were well above average at most locations on Guam and in the CNMI.  For example, the annual total of 129.90 inches at AAFB was the 5th highest annual rainfall total in its 63-year modern climate record. The first half of 2016 was very dry throughout Guam and the islands of the CNMI (Fig. 5a). This dryness was anticipated several months in advance since it is usually dry during the post-Peak phase of El Niño. The dryness was not anywhere near a record, and the total rainfall of 77.01 inches during the 12-month period July 2015 to August 2016 was the 16th driest such period in the 63-year climate record at AAFB.   Rainfall amounts recovered in the 2nd half of 2016.  A very active monsoon trough brought abundant rainfall throughout the region during August. The year ended with a wet December, but it was not enough to overcome earlier dryness so that the 2016 annual total rainfall was below average at most locations. There is a small trend of increasing annual rainfall on Guam during the period of historical record, but if one removes the very dry decade of the 1950s, the long-term trend of rainfall on Guam becomes zero (Fig. 5b). Using a 5-year moving sum, the very large inter-annual and inter-decadal variations are revealed (Fig. 5c).  The 1950s was a very dry decade.  The wettest period occurred in the first half of the 2000s, with another low point reached in the 5-year period 2006 through 2010.  During this latter dry period, there were few extremes of rainfall, partly a result of a very low typhoon activity. 

Figure 5. (a) The monthly rainfall during 2015 and 2016 (percent of average) at AAFB and WFO Guam.  (b) A time series of the 12-month running sum of rainfall at AAFB.  Blue shading indicates 12-month rainfall at-or-below the 77.01 inch total during July 2015 to August 2016.  (c) A time series of the 5-year running sum of rainfall at AAFB, with period extremes indicated by large yellow and blue dots.

Typhoon Distribution (2015 and 2016)

Figure 6. Tropical cyclone tracks of 2015 and 2016 in the western North Pacific.  A cluster of TCs in the subtropics to the north of Guam and Saipan formed during a strong far northward displaced monsoon trough during August.  During September and October, a persistent focus of TC development was seen to the west of Guam.  Clustering of TC activity and persistence of general track behavior are noted in most typhoon seasons, with each year possessing its own unique suite of characteristics.  The blue “El Niño Box” is a region in which tropical cyclone occurrences are very strongly affected by ENSO.

The 2016 annual total of 30 TCs numbered by the JTWC during 2016 was near average.  This belies some low-end statistics such as the late start and a below average number of typhoons (14 versus an average of 18).  The number of tropical storms (10) was near average, but there was an abundance of tropical depressions (6) that helped to inflate the annual total.  The number of named tropical cyclones passing through the bounds of Micronesia was greatly reduced from the high number of such TCs during 2015 (see Fig. 6).  The only common statistic of TC activity that was above average during 2016 was the high number of super typhoons: 6 versus the average of 4. As anticipated by the Pacific ENSO Applications Center, the 2016 typhoon season of the western North Pacific had a very late start, with the first named storm (Super Typhoon Nepartak) reaching tropical storm intensity on the 3rd of July. Through the remainder of July, the basin was quiet.  There was a burst of TC activity during August when a very active monsoon system formed in the subtropics of the western North Pacific. The TCs of August form the wide cluster of tracks northward of Guam and Saipan (Fig. 6).  During September, the monsoon trough collapsed to the west and south, and was the site of a highly concentrated cluster of TC formations between Palau and Guam. TC activity continued in the basin right up to the last week of December, with the final TC of year (Super Typhoon Nock-ten — TC 30W) making a devastating strike of the central Philippines.  In stark contrast to the TC activity of 2015, which was displaced eastward into Micronesia, the TCs of 2016 were displaced westward and northward away from most of Micronesia.  Two TCs (Super Typhoons Haima and Nock-ten) affected portions of Yap State on their way to devastating strikes of Luzon.  During the passage of Nock-ten through Yap State (while it was still a tropical storm) during 22-23 December, an unusual high-wind event occurred on one of the islets of Ulithi Atoll that was perceived by local residents to be a tornado! Particularly because of the unusual TC behavior in the subtropics during August, the number counts and behavior of TCs of 2016 as described by the Japan Meteorological Agency (JMA) versus the JTWC is somewhat different.  As per the JTWC, there were 30 numbered TCs during 2016, with a distribution of 14 typhoons, 10 tropical storms and 6 tropical depressions.  Six of the JTWC typhoons achieved super typhoon status (i.e., maximum one-minute sustained wind at-or-above 130 kt).  As per the JMA, there were 26 named TCs during 2016, with a distribution of 13 typhoons and 13 tropical storms.  The JMA does not include lesser TCs (i.e., tropical depressions) in its best track archive.  Of the 13 typhoons warned on by the JMA, five reached the high-end ranking bin of 105-140 kt (10-minute average).  One of the JTWC typhoons (Nida) was only a tropical storm as per JMA reckoning.  One of the JMA typhoons (Mindulle) was only a tropical storm by JTWC reckoning.  One of the JMA named tropical storms (Malou) was not warned on by the JTWC, and one of the JTWC TDs was deemed to be a tropical storm (Rai) by the JMA.

Guam Sea Level

The sea level has rebounded sharply from its low stands at the peak of El Niño in late 2015/early 2016 (Fig. 7).  A sharp rise of sea level typically occurs in the first few months of the post-Peak year of an El Niño event.  As anticipated by the Pacific ENSO Applications Center, the sea level across Micronesia returned to above average values during the 3rd Quarter of 2016.  Note that the rise and fall of sea level closely tracks the strength of the low-latitude trade winds. 

Figure 7. Time series of sea level at Guam (NOAA Sumay tide gauge) from January 2000 through July 2016.  Note the steady decline that reaches a low point at the end of 2015, and the subsequent sharp rise during 2016. Black line is a 12-month moving average of Guam’s sea level, and white line is a 12-month running sum of NOAA’s trade wind index (5°S-5°N ; 135°E to 180°).

Latest ENSO Diagnostic Statement

CLIMATE PREDICTION CENTER/NCEP/NWS and the International Research Institute for Climate and Society 09 February 2017 ENSO Alert System Status: La Niña Advisory ENSO-neutral conditions have returned and are favored to continue through at least the Northern Hemisphere spring 2017. “La Niña conditions are no longer present, with slightly below-average sea surface temperatures (SSTs) observed across the central equatorial Pacific and above-average SSTs increasing in the eastern Pacific. The latest weekly Niño index values were -0.3°C in the westernmost Niño-4 and Niño-3.4 regions, and +1.5°C in the easternmost Niño-1+2 region. The upper-ocean heat content anomaly increased during January and was slightly positive when averaged across the eastern Pacific, a reflection of above-average temperatures at depth. Atmospheric convection remained suppressed over the central tropical Pacific and enhanced over Indonesia. The low-level easterly winds were slightly enhanced over the western tropical Pacific, and upper-level westerly winds were near average.

Overall, the ocean and atmosphere system is consistent with ENSO-neutral conditions. Most models predict the continuation of ENSO-neutral (3-month average Niño-3.4 index between -0.5°C and 0.5°C) through the Northern Hemisphere summer. However, a few dynamical model forecasts, including the NCEP CFSv2, anticipate an onset of El Niño as soon as the Northern Hemisphere spring (March-May 2017).  Because of typically high uncertainty in forecasts made at this time of the year for the upcoming spring and summer, and the lingering La Niña-like tropical convection patterns, the forecaster consensus favors ENSO-neutral during the spring with a ~60% chance.  Thereafter, there are increasing odds for El Niño toward the second half of 2017 (~50% chance in September-November).

In summary, ENSO-neutral conditions have returned and are favored to continue through at least the Northern Hemisphere spring 2017 (click CPC/IRI consensus forecast for the chance of each outcome for each 3-month period:”