Atmospheric Response · El Niño Monitor

Observed Equatorial Pacific Surface Wind

Copernicus Marine WIND_GLO_PHY_L4_NRT (scatterometer / ASCAT) · ~1–2 day latency · updated daily · drag the slider for recent days

Observed Equatorial Zonal Wind: Strip / Hovmöller

Longitude × time strip of the daily-mean surface zonal wind averaged 5°S–5°N, 150°E–90°W, from the same gap-filled scatterometer (ASCAT) L4 product as the map above. Red is westerly (El Niño-favorable), blue is easterly (the trade winds). Westerly-wind bursts (red) over the warm pool force the downwelling Kelvin waves and eastward surface-current surges seen on the subsurface page. Newest day at the bottom.

Equatorial Pacific surface zonal-wind Hovmöller (observed ASCAT)

Copernicus Marine WIND_GLO_PHY_L4_NRT (scatterometer / ASCAT) · 5°S–5°N daily mean · rolling ~9 months · updated daily

Hourly SOI Estimate

The Southern Oscillation Index is a Troup standardisation of the Tahiti-minus-Darwin sea-level-pressure difference. It is published only monthly, and even LongPaddock's daily version lags 1–2 days. This estimates it in near-real time from the hourly QNH in the Darwin (YPDN) and Tahiti / Faa’a (NTAA) airport METARs, updated hourly by a GitHub Action.

The exact formula (Troup, 1965):

SOI = 10 × ( P_diff − P_diff ) ÷ σ_diff

where P_diff = mean-sea-level pressure at Tahiti minus Darwin (monthly mean), P_diff is the long-term average of that difference for the given calendar month, and σ_diff its standard deviation, both taken over the 1887–1989 base period. The factor of 10 is Troup’s scaling (so the index mostly falls within ±35). Crucially, σ_diff is the spread of monthly values (≈1.3–2.1 hPa), not the much wider day-to-day spread; the exact monthly mean and σ are recovered by regressing LongPaddock’s published SOI against its own Tahiti−Darwin pressure record, then applied to the live METAR pressures.

The grey line is the published LongPaddock daily SOI; the bold red line is the 24-hour-mean estimate, nudged onto the LongPaddock scale, which runs a day or two ahead of it. The faint blue line is the raw hourly SOI, very noisy (the semidiurnal pressure tide, passing weather, and 1-hPa METAR rounding). Negative is El Niño-favorable.

Hourly SOI estimate from Darwin and Tahiti METAR

Darwin (YPDN) + Tahiti (NTAA) METAR via aviationweather.gov · Troup normals & calibration from LongPaddock (BoM 1887–1989 base) · updated hourly

Southern Oscillation Index Forecast

Extending the observed SOI forward: the same Troup SOI (the standardized Tahiti−Darwin sea-level-pressure difference), here forecast from the combined AIFS-ENS + IFS-ENS ensemble (about 100 members). Observed values are from LongPaddock (Queensland Govt / BoM); the forecast is the Tahiti−Darwin MSL from the ensembles, bias-corrected to the recent observed level. Bold lines are the 30-day running SOI; the faint daily series and shaded 10–90% band show the day-to-day spread. Sustained negative SOI (below −7) is El Niño-favorable; sustained positive (above +7) is La Niña-favorable.

Southern Oscillation Index: observed (LongPaddock/BoM) plus AIFS-ENS/IFS-ENS forecast

LongPaddock daily SOI (BoM, 1933–1992 base) · forecast: ECMWF AIFS-ENS + IFS-ENS MSL · updated each 00/12Z cycle

Westerly-Wind-Burst Monitor: Tarawa

Westerly wind bursts (WWBs) over the west-central equatorial Pacific are a key El Niño trigger: the easterly trades briefly reverse to westerly, pushing warm water and convection eastward and forcing downwelling Kelvin waves. Tarawa / Bonriki (NGTA, 1.4°N 173°E) sits in that zone, so its hourly airport wind is a live proxy, with Christmas Island (PLCH, far to the east) shown for contrast. The arrows show the latest wind at each station (red is westerly, blue is easterly); the chart tracks the zonal wind component at Tarawa, where a sustained red excursion is an active WWB.

Tarawa equatorial zonal-wind time series, westerly-wind-burst monitor

Tarawa / Bonriki (NGTA) + Christmas Is. / Kiritimati (PLCH) METAR via aviationweather.gov · updated hourly

WWB Activity vs Past El Niño Onsets

Tarawa’s daily airport wind from 1 April onward through recent El Niño onset years, 2015–16 (very strong) and 2023–24 (weaker), against the current year, from the Iowa Environmental Mesonet METAR archive. Both panels are a 7-day running mean of the hourly obs: top is zonal wind (westerly positive); bottom is the fraction of each day with westerly winds. In a developing El Niño the easterly trades weaken and westerly bursts become more frequent, so both climb from the cold-state baseline.

Tarawa monthly westerly-wind activity across the 2015, 2023 and current El Niño onsets

Tarawa (NGTT/NGTA) METAR · Iowa Environmental Mesonet ASOS archive · monthly zonal wind & westerly fraction · refreshed daily

Equatorial Pacific 10 m Wind Forecast

Forecast 10 m zonal-wind anomaly (5°S–5°N), longitude × forecast day, from the AIFS-ENS (AI) and ECMWF IFS-ENS (physics) ensemble means versus the ERA5 1991–2020 climatology. Westerly (red) anomalies along the equator favor El Niño development; easterly (blue) anomalies favor La Niña. Agreement between the AI and physics ensembles raises confidence.

Equatorial Pacific 10 m zonal-wind anomaly Hovmöller forecast (AIFS-ENS and IFS-ENS)

ECMWF AIFS-ENS & IFS-ENS (open data) · anomaly vs ERA5 1991–2020 · updated each 00/12Z cycle

Super-Ensemble MSLP & 10 m Wind Forecast

The full-field Pacific surface forecast behind the equatorial diagnostics: combined AIFS-ENS + IFS-ENS super-ensemble-mean mean-sea-level pressure (contours, with H/L centres) and 10 m wind (speed shaded in knots, plus barbs), animated day 1–15 over the tropical and subtropical Pacific. An equatorial westerly push between the Maritime Continent and the dateline is a wind-burst signature.

ECMWF AIFS-ENS + IFS-ENS (open data, CC BY 4.0) · member-weighted super-ensemble mean · updated each 00/12Z cycle

200 hPa Velocity Potential & Irrotational Wind

The large-scale divergent circulation aloft from the AIFS-ENS ensemble mean: the 200 hPa velocity-potential anomaly shaded, with the irrotational (divergent) wind as vectors. Green is upper-level divergence (the outflow above deep convection); orange is convergence (subsidence). The vectors point out of the divergence centres, tracing the rising and sinking branches of the Walker and Hadley circulations; in El Niño the divergence shifts east over the central Pacific. Anomaly is versus the ERA5 1991–2020 climatology. The first frame is the analysis, the rest is the forecast to day 14.

ECMWF AIFS-ENS 200 hPa u/v, 50-member mean (open data, CC BY 4.0) · velocity potential via spherical-harmonic inversion · anomaly vs ERA5 1991–2020 · updated each 00/12Z cycle

Atmospheric Angular Momentum

Relative AAM is the angular momentum carried by the winds, the atmosphere's total westerly momentum, ∫ u·cos²φ over the whole globe. It is dominated by the upper-tropospheric subtropical jets and the deep tropics (largest moment arm), shown here for the globe and each hemisphere from the AIFS-ENS 13-level winds. The top panel is this year's observed AAM (black) and the 15-day forecast (red) on the climatological annual cycle; the hemispheres are strongly out of phase (each peaks in its own winter). The bottom panel is the anomaly (departure from the 1991–2020 normal), which removes the seasonal cycle. El Niño strengthens the subtropical jets and weakens the trades, so the atmosphere holds more westerly momentum; AAM also ties to length of day.

Forecast: ECMWF AIFS-ENS (open data, CC BY 4.0) · climatology & this-year observed: ERA5 (Copernicus C3S / ECMWF) · updated daily

AAM Forecast Trend (run-to-run)

Run-to-run trend of AIFS-ENS ensemble-mean AAM anomaly forecasts, global and hemispheric

ECMWF AIFS-ENS (open data, CC BY 4.0) · anomaly vs ERA5 1991–2020 (Copernicus C3S) · archived each cycle

What Drives AAM: the Torque Budget

Atmospheric angular momentum only changes through torques the Earth exerts on the atmosphere. With $M$ the relative (wind) AAM, the budget has two resolved terms, friction (surface wind stress) and mountain (pressure on topography):

$$M=\frac{a^3}{g}\iiint u\cos^2\!\phi\;d\lambda\,d\phi\,dp,\qquad \frac{dM}{dt}=T_{\text{fric}}+T_{\text{mtn}}\;(+\,T_{\text{gw}})$$

$$T_{\text{mtn}}=-a^2\!\iint p_s\,\frac{\partial h_s}{\partial\lambda}\,\cos\phi\;d\lambda\,d\phi,\qquad T_{\text{fric}}=-a^3\!\iint \tau_\lambda\,\cos^2\!\phi\;d\lambda\,d\phi$$

The two panels show the torque anomalies (versus the ERA5 1991–2020 climatology, by day-of-year), so the standing El Niño departure shows alongside the day-to-day synoptic systems. The grey contours are the surface-pressure anomaly, the field the torque uses (solid positive, dashed negative), so colours and contours read together. Top, friction ($\tau_\lambda\!\approx\!\rho\,C_d\,|V_{10}|\,u_{10}$): trade-wind and jet stress anomalies such as western-Pacific westerly bursts. Bottom, mountain (form drag $h_s\,\partial p_s/\partial\lambda$, the same net torque as $-p_s\,\partial h_s/\partial\lambda$): synoptic pressure systems crossing the Andes, Rockies, Himalaya/Tibet and Greenland. Insets are the ±60° cosφ-weighted zonal-mean anomaly; AIFS-ENS ensemble mean of all 51 members, ~5°. The sub-grid gravity-wave-drag term $T_{\text{gw}}$ appears only as the residual.

Hemispheric AAM torque budget time series (friction, mountain, sum) for NH and SH

Mountain-torque anomaly broken down by range (Himalaya, Rockies, Andes, Greenland, Antarctica, Alps)

Seasonal cycle of the three AAM surface-torque terms (friction, mountain, GWD) by latitude, ERA5 1991-2020

Seasonal cycle of the integrated AAM torque terms (friction, mountain, GWD, sum) for Global/NH/SH, ERA5 1991-2020

ECMWF AIFS-ENS surface pressure (open data, CC BY 4.0) · ERA5 orography + 1991–2020 torque climatology (Copernicus C3S, seasonal-cycle panel) · daily means · updated daily

Where the AAM Lives, and Where It’s Changing

The torques above change the global AAM; this shows where that angular momentum sits and how the forecast moves it, as a latitude–pressure cross-section of the relative-AAM density whose integral over latitude and pressure is the global AAM. Top panel, absolute: where the AAM resides, dominated by the subtropical jets (the winter hemisphere’s is strongest). Bottom, change versus the 0-h analysis: where the forecast is adding (red) or removing (blue) angular momentum, with the per-latitude breakdown of the global ΔAAM along the top strip. Black contours are the zonal-mean zonal wind (bold = zero). The bottom panel integrates to the ΔAAM in the title, tying it back to the torque budget above. AIFS-ENS 50-member ensemble mean; the slider steps the forecast Day 0–15.

ECMWF AIFS-ENS 13-level zonal wind + surface pressure (open data, CC BY 4.0) · ensemble mean · updated daily

Meridional Overturning: the Hadley Cell Response

The zonal-mean meridional mass streamfunction $\Psi(\phi,p)$ traces the overturning cells (Hadley, Ferrel, polar) from the AIFS-ENS 0-h analysis:

$$\Psi(\phi,p)=\frac{2\pi a\cos\phi}{g}\int_0^p [v]\,dp',\qquad [v]=\text{zonal-mean }v$$

Shown as the anomaly $\Psi'$ from the ERA5 1991–2020 harmonic climatology (mean + annual + semiannual, by day-of-year), how the overturning departs from its normal seasonal state. The black contours are the absolute $\Psi$ (the actual cells) and the arrows show the vertical motion (up is ascent, scaled by strength); the strong equatorial signal tracks the El Niño Walker/Hadley response. Each frame is a 7-day running mean (the 0-h analyses are stashed into a rolling history and trailing-week-averaged), which smooths the day-to-day wobble so the evolution of the Hadley-cell strength is legible. Fixed colour scale.

ECMWF AIFS-ENS 0-h analysis meridional wind (open data, CC BY 4.0) · anomaly vs ERA5 1991–2020 Ψ climatology (Copernicus C3S) · 7-day running mean · updated each cycle